Eddy Current Testing (ET) NDT31 Training and Examination Services Granta Park, Great Abington Cambridge CB21 6AL United Kingdom Copyright © TWI Ltd Eddy Current Testing (ET) NDT31 Contents Section Subject Preliminary pages Contents Standards and Associated Reading COSHH, H&S, Cautions and Warnings Introduction to NDT Methods NDT Certification Schemes 1 1.1 1.2 1.3 1.4 2 2.1 2.2 2.3 2.4 3 3.1 3.2 3.3 3.4 3.5 3.6 4 4.1 4.2 5 5.1 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 NDT31-50316b Contents Introduction The SI units of measurement History of eddy current testing Definition of non-destructive testing (NDT) Choice of method Principles Electricity Magnetism Alternating current theory Eddy currents Equipment Circuits Simple circuits Instruments Adjustments Probes Calibration blocks Practices Documentation Applications AC Theory Capacitive reactance Phase Analysis Signal/noise separation Phase analysis Idealised impedance diagram Normalised impedance Conductivity Magnetic permeability Thickness Frequency Probe diameter Characteristic parameter Characteristic frequency Skin effect Phase discrimination Suppression of undersired effects Copyright © TWI Ltd 6.15 7 7.1 7.2 7.3 7.4 7.5 8 8.1 8.2 8.3 8.4 8.5 8.6 9 Multifrequency testing Instrumentation Cathode ray oscilloscopes Send-receive coils Hall effect probes Dynamic testing Frequency response Material Sorting Conductivity meters Conductivity effects Electromagnetic sorting bridges Bridge sorting variables Automatic gates Standards Crack Detection 9.1 9.2 9.3 9.4 9.5 Universal crack detectors Surface coils Crack detection Weld testing Rotating probes 10 Tube Testing 11 Eddy Current for Welding Inspection 10.1 10.2 10.3 10.4 10.5 10.6 10.7 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 11.14 Manufactured tube testing Condenser tube inspection Probes Test frequency Coil size Signal patterns Reference standards Introduction Eddy current application overview Basic eddy current theory Generation of eddy currents Principles governing the generation of eddy currents Fundametal properties of eddy current flow Electrical circuits and probe impedance Resistance and reactance Inductive reactance Capactive reactance Impedance Inductance (L) Eddy current weld testing Probe/coil arrangements Appendix 1 Appendix 2 Appendix 3 NDT31-50316b Contents Copyright © TWI Ltd Preface These notes are provided as training reference material and to meet the study requirements for examination on the NDT course to which they relate. They do not form an authoritative document, nor should they be used as a reference for NDT inspection or used as the basis for decision making on NDT matters. The standards listed are correct at time of printing and should be consulted for technical matters. NOTE: These training notes are not subject to amendment after issue. NDT31-50316b Prelims Copyright © TWI Ltd Standards and Associated Reading EN ISO 1330-1 Non Destructive Testing – Terminology Part 1: List of general terms EN ISO 1330-2 Non Destructive Testing – Terminology Part 2: Terms common to NDT methods EN ISO 12718 Non Destructive Testing – Eddy Current Testing - Terminology EN ISO 15549 Non Destructive Testing – Eddy Current Testing – General Principles EN ISO 15548-1 Non Destructive Testing – Equipment for Eddy Current examination – Part 1 – Instrument characteristics and verification EN ISO 15548-2 Non Destructive Testing – Equipment for Eddy Current examination – Part 2 Probe characteristics and verification EN ISO 15548-3 Non Destructive Testing –Equipment for Eddy Current Examination– Part 3 System characteristics and verification EN ISO 17643 Non Destructive Examination of Welds – Eddy Current examination of welds by complex plane analysis EN ISO 17635 Non-destructive testing of welds. General rules for metallic materials M 38 Guide to compilation of instructions and reports for the inservice and non-destructive testing of aerospace products ISO 27831-1 Metallic and other inorganic coatings – cleaning and preparation of metallic surfaces. Part 1. Ferrous Metals and alloys ISO 27831-2 Metallic and other inorganic coatings – cleaning and preparation of metallic surfaces. Part 1. Non-Ferrous Metals and alloys ISO 9712 Non-destructive testing. Qualification and certification of personnel EN 4179 Aerospace series. Qualification and approval of personnel for non-destructive testing Special Techniques EN ISO 2360 Non-conductive coatings on non-magnetic electrically conductive basis materials – measurement of coating thickness: Amplitude sensitive eddy-current method EN ISO 21968 Non-magnetic metallic coatings on metallic and non-metallic basis materials – measurement of coating thickness: Phase sensitive eddy-current method NDT31-50316b Prelims Copyright © TWI Ltd Aerospace prEN 2002 Aerospace Series – Test methods for metallic materials – Part 20: Eddy Current test on pipes under pressure Welds EN ISO 17643 Non-destructive examination of welds – Eddy current examination of welds by complex plane analysis Tubes & Pipes EN 1971 Copper & Copper Alloys – Eddy current test for measuring defects on seamless round copper and copper ally tubes – Part 1: Testing with an encircling coil on the outer surface – Part 2: Test with an internal probe on the inner surface EN ISO 10893 NDT of Steel Tubes – Part 1: Automated electromagnetic testing of seamless and welded (except submerged arc-welded) steel tubes for the verification of hydraulic leak tightness – Part 2: Automated eddy current testing of seamless and welded (except submerged arc-welded) steel tubes for the detection of imperfections NDT31-50316b Prelims Copyright © TWI Ltd Associated Reading IAEA. Training Course Series 48. Training Guidelines for Non Destructive Testing Techniques: Eddy Current Testing at Level 2. http://www.ndted.org/EducationResources/CommunityCollege/EddyCurrents/cc_ec_index.htm Mathematics and Formulae in NDT. Edited by Dr. R Halmshaw. Obtainable from the British Institute of Non-Destructive Testing NDT31-50316b Prelims Copyright © TWI Ltd COSHH, H&S, Caution and Warnings Relevant to TWI Training & Examination Services Introduction The use of chemicals in NDT is regulated by law under the Control of Substances Hazardous to Health (COSHH) Regulations 2005. These regulations require the School to assess and control the risk of health damage from every kind of substance used in training. Students are also required by the law to co-operate with the School’s risk management efforts and to comply with the control measures adopted. Hazard Data Sheets The School holds Manufacturers Safety Data Sheets for every substance in use. Copies are readily available for students to read before using any product. The Data Sheets contain information on: The trade name of the product; eg Magnaglo, Ardrox, etc. Hazardous ingredients of the products. The effect of those ingredients on peoples health. The hazard category of the substance; eg irritant, harmful, corrosive or toxic, etc. Special precautions for use; eg the correct Personal Protective Equipment (PPE) to wear. Instructions for First Aid. Advice on disposal. EH40 – Occupational Exposure Limits What is Exposure? Exposure to a substance is uptake into the body. The exposure routes are by: Breathing fume, dust, gas or mist. Skin contact. Injection into the skin. Swallowing. Many thousands of substances are used at work but only about 500 substances have Workplace Exposure Limits (WELs). Until 2005 it had been normal for HSE to publish a new edition of EH40, or at least an amendment, each year. However with increasing use of the website facilities the HSE no longer always publishes a revised hardcopy edition, or amendment. The web based list which became applicable from 1st October 2007 can now be found at http://www.hse.gov.uk/coshh/table1.pdf NDT31-50316b Prelims Copyright © TWI Ltd Introduction to Non-Destructive Testing Non-destructive testing (NDT) is the ability to examine a material (usually for discontinuities) without degrading it or permanently altering the article being tested, as opposed to destructive testing which renders the product virtually useless after testing. Other advantages of NDT over destructive testing are that every item can be examined with no adverse consequences, materials can be examined for conditions internally and at the surface and, most importantly, parts can be examined whilst in service, giving a good balance between cost effectiveness and quality control. NDT is used in almost every industry with the majority of applications coming from the aerospace, power generation, automotive, rail, oil & gas, petrochemical and pipeline markets, safety being the main priority of these industries. When properly applied, NDT saves money, time, materials and lives. NDT as it is known today has been developing since around the 1920s, with the methods used today taking shape later and vast technological advancements being made during the Second World War. The basic principal methods are: Visual testing (VT). Penetrant testing (PT). Magnetic particle testing (MT). Eddy current testing (ET). Ultrasonic testing (UT). Radiographic testing (RT). In all NDT methods, the interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency. Visual testing (VT) With sufficient lighting and access, visual techniques provide simple, rapid methods of testing whilst also being the least expensive. Close visual testing (CVT) refers to viewing directly with the eye (with or without magnification) whereas remote visual inspection (RVI) refers to the use of optical devices such as the boroscope and the fibrescope. Visual testing begins with the eye; however, the first boroscopes used a hollow tube and a mirror with a small lamp at the end to investigate the bores of rifles and cannons for problems and discontinuities. In the 1950s, the lamps were replaced by glass fibre bundles which were used to transmit the light. These became known as fibrescopes which were also less rigid, increasing the capabilities of testing. With usage expanding, many users began to suffer from eye fatigue which led to the development of video technology. This was first used in the 1970s and relies on electronics to transmit the images rather than fibreoptics. Further enhancements to video technology include pan, tilt and zoom lenses, and mounting cameras to platforms and wheels, all allowing more parts to be tested and better images for improved inspection. Video devices also allow recordings of inspections to be taken, meaning permanent records can be kept. This has a number of advantages such as enabling other inspectors to observe the test as it was performed and allowing further review and evaluation. Penetrant testing (PT) Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of excess penetrant, the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used. NDT31-50316b Prelims Copyright © TWI Ltd Advantages Disadvantages Applicable to non-ferromagnetics Only detects defects open to the surface Able to test large parts with a portable kit Careful surface preparation required Batch testing Not applicable to porous materials Applicable to small parts with complex geometry Temperature dependent Simple, cheap, easy to interpret Cannot retest indefinitely Sensitivity Compatibility of chemicals History of penetrant testing A very early surface inspection technique involved the rubbing of carbon black on glazed pottery. The carbon black would settle in surface cracks, rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the oil and whiting method. In this method, heavy oil, commonly available in railway workshops, was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. A different (though related) method was introduced in the 1940s. The surface under examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not for finding defects. Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with Switzer Bros, Cleveland, OH, USA. More effective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or colour contrast dye penetrant method, is still used quite extensively today. In the 1940s, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as black light), rendering indications from cracks and other surface flaws more readily visible to inspectors. UV lights have become increasingly portable with hand held UV torches now readily available. NDT31-50316b Prelims Copyright © TWI Ltd Magnetic particle testing (MT) Magnetic particle testing is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages Disadvantages Will detect some sub-surface defects Ferromagnetic materials only Rapid and simple to understand Requirement to test in two directions Pre-cleaning not as critical as with dye penetrant testing (PT) Demagnetisation may be required Will work through thin coatings Oddly-shaped parts difficult to test Cheap equipment Not suited to batch testing Direct test method Can damage the component under test History of magnetic particle testing The origins of MT can be traced to the 1860s when cannon barrels were tested for defects by first magnetising the barrel and then running a compass down the length of the barrel. By monitoring the needle of the compass, defects within the barrel could be detected. This form of NDT became much more common after the First World War, in the 1920s, when William Hoke discovered that flaws in magnetised materials created distortions in the magnetic field. When a fine ferromagnetic powder was applied to the parts, it was observed that they built up around the defects, providing a visible indication of their location. Magnetic particle testing superseded the oil and chalk method in the 1930s as it proved far more sensitive to surface breaking flaws. Today it is still preferred to the penetrant method on ferromagnetic material and much of the equipment being used then is very similar to that of today, with the only advances coming in the form of fluorescent coating to increase the visibility of indications and more portable devices being used. In the early days, battery packs and direct current were the norm and it was some years before alternating current proved acceptable. Magnetism The phenomenon called magnetism is said to have been discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. The use of magnets in navigation goes back to Viking times or maybe earlier, where it was found that rods of magnetised material, when freely suspended, would always point in a north-south direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, followed by Michael Faraday (1791-1867), whose experiments revealed that magnetic and electrical energy could be interchanged. NDT31-50316b Prelims Copyright © TWI Ltd Historical perspective Electromagnetic testing – the interaction of magnetic fields with circulating electrical currents - had its origin in 1831 when Michael Faraday discovered electromagnetic induction. He induced current flow in a secondary coil by switching a battery on and off. D E Hughes performed the first recorded eddy current test in 1879. He was able to distinguish between different metals by noting a change in excitation frequency resulting from effects of test material resistivity and magnetic permeability. Introduction to electromagnetic testing Many electromagnetic induction or eddy current comparators were patented in the period from 1952. Innumerable examples of comparator tests were reported in the literature and in patents. Many involved simple comparator coils into which round bars or other test objects were placed, producing simple changes in the amplitudes of test signals, or unbalancing simple bridge circuits. In nearly all cases, particularly where ferromagnetic test materials were involved, no quantitative analyses of test objects dimensions, properties, or discontinuities were possible with such instruments. Often, difficulties were encountered in reproducing test results. Some test circuits were adjusted or balanced to optimise signal differences between a known good test object and a known defective test object for each group of objects to be tested. Little or no correlation could then be obtained between various types of specimens, each type having been compared to an arbitrarily selected specimen of the same specific type. Developments in electromagnetic induction tests Rapid technological developments in many fields before and during the Second World War (1939-45) contributed both to the demand for NDT and to the development of advanced test methods. Radar and sonar systems allowed the viewing of test data on the screens of cathode-ray tubes or oscilloscopes. Developments in electronic instrumentation and magnetic sensors used both for degaussing ships and for actuating magnetic mines brought a resurgence of activity. Eddy current testing (ET) Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages Disadvantages Sensitive to surface defects Very susceptible to permeability changes Can detect through several layers Only on conductive materials Can detect through surface coatings Will not detect defects parallel to surface Accurate conductivity measurements Not suitable for large areas and/or complex geometries Can be automated Signal interpretation required Little pre-cleaning required No permanent record (unless automated) Portability NDT31-50316b Prelims Copyright © TWI Ltd History of eddy current testing The principles of eddy currents arose in 1831 with Faraday’s discovery of electromagnetic induction; eddy current testing methods have their origins in a period just after the First World War, when materials with a high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering – they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in magnetic materials. The first eddy current testing devices for NDT were in 1879 by Hughes, who used the principles of eddy currents to conduct metallurgical sorting tests and the stray flux tube and bar tests. It was left to Dr Friedrich Förster in the late 1940s to develop the modern day eddy current testing equipment and formulate the theories which govern their use. The introduction by Förster of sophisticated, stable, quantitative test equipment and of practical methods for analysis of quantitative test signals on the complex plane was by far the most important factor contributing to the rapid development and acceptance of electromagnetic induction and eddy current testing. Förster is rightly identified as the father of modern eddy current testing. By 1950, he had developed a precise theory for many basic types of eddy current tests, including both absolute and differential or comparator test systems and probe or fork coil systems used with thin sheets and extended surfaces. Continued advances in research and development, advanced electronics and digital equipment have led to eddy currents becoming one of the most versatile of the surface methods of inspection. Eddy current methods have developed into a wide range of uses and are recognised as being the forerunner of NDT techniques today. From the mid1980s, microprocessor-based eddy current testing instruments were developed which had many advantages for inspectors. Modern electronics have made instruments more user friendly, providing reduced noise levels which made certain test applications very difficult, but also improving methods of signal presentation and recording capabilities. Applications for microcomputer chips abound, from giving lift-off suppression in simple crack detection to providing signal processing for immediate analysis of condenser tube inspection. As with other testing methods, improvements to the equipment have been made to increase its portability and computer-based systems now allow easy data manipulation and signal processing. Eddy current testing is now a widely used and understood inspection method for flaw detection as well as for thickness and conductivity measurements. Ultrasonic testing (UT) Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will return to the probe in a time period other than that expected of a faultfree specimen. NDT31-50316b Prelims Copyright © TWI Ltd Advantages Disadvantages Sensitive to cracks at various orientations No permanent record (unless automated) Portability Not easily applied to complex geometries and rough surfaces Safety Unsuited to coarse grained materials Able to penetrate thick sections Reliant upon defect orientation Measures depth and through-wall extent History of ultrasonic testing In Medieval times craftsmen casting bells for churches were aware that a properly cast bell rang true when struck and that a bell with flaws would give out a false note. This principle was used by wheel-tappers inspecting rolling stock on the railways; they struck wheels with a hammer and listened to the note given out. A loose tyre sounded wrong. The origin of modern ultrasonic testing (UT) is the discovery by the Curie brothers in 1880 that quartz crystals cut in a certain way produce an electric potential when subjected to pressure - the piezo-electric effect, from the Greek piedzein (to press or strike). In 1881 Lippman theorised that the effect might work in reverse, and that quartz crystals might change shape if an electric current was applied to them. He found that this was so and experimented further. Crystals of quartz vibrate when alternating currents are applied to them. Crystal microphones in a modern stereo rely on this principle. When the Titanic sank in 1912, the Admiralty tried to find a way of locating icebergs by sending out sound waves and listening for an echo. They experimented further with sound to detect submarines during the First World War. Between the wars, marine echo sounding was developed and in the Second World War ASDIC (Anti-Submarine Detection Investigation Committee) was extensively used in the Battle of the Atlantic against the U-boats. In 1929, the Russian physicist Sokolov experimented with through-transmission techniques, passing vibrations through metals to find flaws; this work was taken up by the Germans. In the 1930s the cathode ray tube was developed and miniaturised in the Second World War to fit small airborne radar sets into aircraft. It made the UT set as we know it possible. Around 1931 Mulhauser obtained a patent for a system using two probes to detect flaws in solids and following this Firestone (1940) and Simons (1945) developed pulsed UT using a pulse-echo technique. In the years after the Second World War, researchers in Japan began to experiment on the use of ultrasound for medical diagnostic purposes. Working largely in isolation until the 1950s, the Japanese developed techniques for the detection of gallstones, breast masses, and tumours. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation. The first flaw detector was made by Sproule in 1942 while he was working for the Scottish firm Kelvin & Hughes. Similar work was carried out by Firestone in the USA and by German physicists. Sproule went on to develop the shear-wave probe. NDT31-50316b Prelims Copyright © TWI Ltd Initially UT was limited to testing aircraft, but in the 1950s it was extensively used in the building of power stations in Britain for examining thick steel components safely and cheaply. UT was found to have several advantages over radiography in heavy industrial applications: No health hazards were associated with radiography, and a UT technician could work next to welders and other employees without endangering them of holding up work. It was efficient in detecting toe cracks in boilers – a major cause of explosions and lack of fusion in boiler tubes. It could find planar defects, like laminations, which were sometimes missed by radiography. A UT check on a thick component took no more time than a similar check on a thin component as opposed to long exposure times in radiography. Over the next twenty years, improvements focused on accurate detection and sizing of the flaws with limited success, until 1977 when Silk first discovered an accurate measurement and display of the top and bottom edges of a discontinuity with the timeof-flight diffraction (TOFD) technique. Advances in computing technology have now expanded the use of TOFD as real time analyses of results are now available. It was also during the 1970s that industries focused on reducing the size and weight of ultrasonic flaw detectors and making them more portable. This was achieved by using semiconductor technology and during the 1990s microchips were introduced into the devices to allow calibration parameters and signal traces to be stored. LCD display panels and digital technology have also contributed to reducing the size and weight of ultrasonic flaw detectors. With the development of ultrasonic phased array and increased computing power, the future for ultrasonic inspection is very exciting. Ultrasound used for testing The main use of ultrasonic inspection in the human and the animal world is for detecting objects and measuring distance. A pulse of ultrasound (a squeak from a bat or a pulse from an ultrasonic source) hits an object and is reflected back to its source like an echo. From the time it takes to travel to the object and back, the distance of the object from the sound source can be calculated. That is how bats fly in the dark and how dolphins navigate through water. It is also how warships detected and attacked submarines in the Second World War. Wearing a blindfold, you can determine if you are in a very large hall or an ordinary room by clapping your hands sharply; a large hall will give back a distinct echo, but an ordinary room will not. A bat’s echo location is more precise: the bat gives out and can sense short wavelengths of ultrasound and these give a sharper echo than we can detect. In UT a sound pulse is sent into a solid object and an echo returns from any flaws in that object or from the other side of the object. An echo is returned from a solid-air interface or any solid-non-solid interface in the object being examined. We can send ultrasonic pulses into material by making a piezo-electric crystal vibrate in a probe. The pulses can travel in a compression, shear or transverse mode. This is the basis of ultrasonic testing. However, the information from the returning echoes must be presented for interpretation. It is for this purpose that the UT set, or flaw detector as it is frequently called, contains a cathode ray tube. In the majority of UT sets, the information is presented on the screen in a display called the A Scan. The bottom of the CRT screen is a time base made to represent a distance say 100mm. An echo from the backwall comes up on the screen as a signal, the amplitude of which represents the amount of sound returning to the probe. By seeing how far the signal comes along the screen we can measure the thickness of the material we are examining. NDT31-50316b Prelims Copyright © TWI Ltd If that material contains a flaw, sound is reflected back from the flaw and appears on the screen as a signal in front of the backwall echo (BWE) as the sound reflected from the flaw has not had so far to travel as that from the backwall. BWE BWE BWE BWE Defect Defect Ultrasonic signals Anything that sends back sound energy to a probe to cause a signal on the screen is called a reflector. By measuring the distance from the edge of the CRT screen to the signal, we can calculate how far down in the material the reflector lies. Radiographic testing (RT) Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface-breaking defects. Advantages Disadvantages Gives a permanent record, the radiograph Radiation health hazard Detects internal flaws Can be sensitive to defect orientation and so can miss planar flaws Detects volumetric flaws readily Limited ability to detect fine cracks Can be used on most materials Access is required to both sides of the object Can check for correct assembly Skilled radiographic interpretation is required Gives a direct image of flaws Relatively slow method of inspection Fluoroscopy can give real time imaging High capital cost High running cost NDT31-50316b Prelims Copyright © TWI Ltd History of radiographic testing X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wϋrzburg University in Germany. Whilst performing experiments in which he passed an electric current through a Crookes tube (an evacuated glass tube with an anode and a cathode), he found that when a high voltage was applied, the tube produced a fluorescent glow. Roentgen noticed that some nearby photographic plates became fogged. This caused Roentgen to conclude that a new type of ray was being emitted from the tube. He believed that unknown rays were passing from the tube and through the plates. He found that the new ray could pass through most substances. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife. Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896 French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with photographic plates. Later when he developed these plates, he discovered that they were fogged (indicating exposure to light). Becquerel wondered what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light; in addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. Becquerel did not pursue his discovery of radioactivity, but others did. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitch-blende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radio-active element in pitchblende, and named it polonium in honour of Marie’s native homeland. Later that year, the Curies discovered another NDT31-50316b Prelims Copyright © TWI Ltd radioactive element which they named ‘radium’, or shining element. Both polonium and radium were more radioactive than uranium. Due to her lifelong research in this field, Marie Curie is widely credited with the discovery of gamma radiation and the introduction of the new term: radio-active. Since these discoveries, many other radioactive elements have been discovered or produced. Radiography in the form of NDT took shape in the early 1920s when H H Lester began testing on different materials. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During the Second World War, industrial radiography grew tremendously as part of the Navy's shipbuilding programme. In 1946, man-made gamma ray sources from elements such as cobalt and iridium became available. These new sources were far stronger than radium and much less expensive. The man-made sources rapidly replaced radium, and the use of gamma rays increased quickly in industrial radiography. William D Coolidge's name is inseparably linked with the X-ray tube popularly called the Coolidge tube. This invention completely revolutionised the generation of X-rays and remains the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents. Although the theories and practices have changed very little, radiographic equipment has developed. These developments include better images through higher quality films and also lighter, more portable equipment. In addition to conventional film radiography, digital radiographic systems are now widespread within the NDT industry. The use of photostimulable phosphor (PSP) bearing imaging plates with photomultipliers to capture image signals and analogue-to-digital converters (ADC) are used extensively in computed radiography (CR). Direct radiography (DR) systems are also used based upon complementary metal oxide sensor (CMOS) technology and TFT (thin film transistors). These systems have the ability to directly convert light into digital format; additionally, they may be coupled with a scintillator which coats CMOS and charged couple device (CCD) sensors. The scintillator converts photon energy to light before the sensor and ADC converts to digital format. Systems which use scintillators in this way are often referred to as indirect systems. Quality issues of any digital system are based upon the effective pixel size and the signal-to-noise ratio (SNR). The benefits of using digital systems are the speed of inspection and the absence of chemical processing requirements and wet film; however, the initial equipment costs will be high. NDT31-50316b Prelims Copyright © TWI Ltd NDT Certification Schemes CSWIP – Certification Scheme for Personnel Managed by TWI Certification Ltd (TWICL), a TWI Group company formed in 1993 to separate TWI’s activities in the field of personnel and company certification thus ensuring continued compliance with international standards for certification bodies and is accredited by UKAS to BS EN ISO 17024. TWICL establishes and implements certification schemes, approves training courses, and authorises examination bodies and assessors in a large variety of inspection fields, including; non-destructive testing (NDT), welding and plant inspectors, welding supervisors, welding coordination, plastic welders, underwater inspectors, integrity management, general inspection of offshore facilities, cathodic protection, heat treatment. TWI Certification Ltd Granta Park, Great Abington, Cambridge CB21 6AL, United Kingdom Tel: +44 (0) 1223 899000 Fax: +44 (0) 1223 892588 Email: twicertification@twi.co.uk Website: www.cswip.com NDT31-50316b Prelims Copyright © TWI Ltd PCN – Personal Certification in Non-destructive testing Managed and marketed by the British Institute of Non-Destructive Testing (BINDT) which owns and operates the PCN Certification Scheme, it offeres a UKAS accreditied certification of competence for NDT and condition monitoring in a variety of product sectors. The British Institute of Non-Destructive Testing Certification Services Division, Newton Building, St. Georges Avenue, Northampton, NN2 6JB, United Kingdom Tel: +44 (0)1604 893811 Fax: +44 (0)1604 892868 Email: pcn@bindt.org Website: http://www.bindt.org/Certification/General_Information Both schemes offer NDT certification conforming to BS EN ISO 9712; Qualification and Certification of NDT personnel, this superseding EN473. The PCN Scheme What follows is a summary of the general requirements for qualification and PCN certification of NDT personnel as described in PCN/GEN Issue 5 Revision R. PCN Certification is a scheme which covers the qualification of NDT inspection staff to meet the requirements of European and International Standards. Typically a standard or procedure will call for the Inspector to be certified in accordance with BS EN ISO 9712 and/or PCN requirements. The PCN Gen Document describes how the PCN system works. The points below cover extracts from this document which are major items, the full document can be viewed on the BINDT website – www.bindt.org/certification/PCN. NDT31-50316b Prelims Copyright © TWI Ltd References PCN documents PSL/4 PSL/8A PSL/30 PSL/31 PSL/42 PSL/44 PSL/49 PSL/51 PSL/57C PSL/67 PSL/70 CP9 CP16 CP17 CP19 CP22 CP25 CP27 Examination availability PCN documents – issue status Log of pre-certification experience Use of PCN & UKAS logo Log of pre-certification on-the-job training Vision requirements Examination exemptions for holders of certification other than PCN Acceptable certification for persons supervising PCN candidates gaining experience prior to certification Application for certification, experience gained post examination Supplementary 56 day waiver Request for L2 certificate issue to a L3 holder Requirements for BINDT authorised qualifying bodies Renewal and recertification of PCN Levels 1 & 2 certificates Renewal and recertification of PCN Level 3 certificates Informal access to authorised qualifying bodies by third parties Marking and grading PCN examinations Guidelines for the preparation of NDT procedures and instructions in PCN examinations Code of ethics for PCN certificate holders PCN/GEN Appendix Z1 – NDT Training Syllabi Levels of PCN certification Level 1 personnel are qualified to carry out NDT operations according to written instructions under the supervision of appropriately qualified Level 2 or 3 personnel. Within the scope of the competence defined on the certificate, Level 1 personnel may be authorised by the employer to perform the following in accordance with NDT instructions: Set up equipment. Carry out the test. Record and classify the results in terms of written criteria. Report the results. Level 1 personnel have not demonstrated competence in the choice of test method or technique to be used, nor for the assessment, characterisation or interpretation of test results. NDT31-50316b Prelims Copyright © TWI Ltd Level 2 personnel have demonstrated competence to perform and supervise nondestructive testing according to established or recognised procedures. Within the scope of the competence defined on the certificate, Level 2 personnel may be authorised by the employer to: Select the NDT technique for the test method to be used. Define the limitations of application of the testing method. Translate NDT standards and specifications into NDT instructions. Set up and verify equipment settings. Perform and supervise tests. Interpret and evaluate results according to applicable standards, specifications. Prepare written NDT instructions. Carry out and supervise all Level 1 duties. Provide guidance for personnel at or below Level 2. Organise and report the results of non-destructive tests. codes or Level 3 personnel are qualified to direct any NDT operation for which they are certificated and may be authorised by the employer to: Assume full responsibility for a test facility or examination centre and staff. Establish, review for editorial and technical correctness and validate NDT instructions and procedures. Interpret codes, standards, specifications and procedures. Designate the particular test methods, techniques and procedures to be used. Within the scope and limitations of any certification held carry out all Level 1 and 2 duties and; Provide guidance and supervision at all levels. Level 3 personnel have demonstrated: Competence to interpret and evaluate test results in terms of existing codes, standards and specifications. Possession of the required level of knowledge in applicable materials, fabrication and product technology sufficient to enable the selection of NDT methods and techniques and to assist in the establishment of test criteria where none are otherwise available. General familiarity with other NDT methods. Level 3 certificated personnel may be authorised to carry out, manage and supervise PCN qualification examinations on behalf of the British Institute of NDT. Where Level 3 duties require the individual to apply routine NDT by a method(s) within a particular product or industry sector, the British Institute of NDT strongly recommends that industry demand that this person should hold and maintain Level 2 certification in the applicable methods and sectors. NDT31-50316b Prelims Copyright © TWI Ltd Training Table 1 Minimum required duration of training. NDT method Level 1 hours Level 2 hours1 Level 3 hours ET 40 40 40 PT 16 24 24 MT 16 24 32 RT 40 80 72 RI N/A 56 N/A UT 40 80 72 VT 16 24 24 BRS 16 N/A N/A RPS N/A 24 N\A Basic knowledge (Direct access to Level 3 examination parts A- C) 80 Note 1. Direct access to Level 2 requires the total number of hours shown in Table 1 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 1 for Levels 1-3. Up to one third of the total specified in this table may take the form of OTJ training documented using form PSL/42 provided it is verifiable and covered practical application of the syllabus detailed in CEN ISO/TR 25107:2006. Industrial NDT experience Industrial NDT experience in the appropriate sector may be acquired prior to or following success in the qualification examination. In the event that the experience is sought following successful examination, the results of the examination shall remain valid for up to two years. Documentary evidence (in a form acceptable to the British Institute of NDT, ie. on PCN form PSL/30) of experience satisfying the following requirements shall be confirmed by the employer and submitted to BINDT AQB prior to examination, or directly to BINDT prior to the award of PCN certification in the event that experience is gained after examination. NDT31-50316b Prelims Copyright © TWI Ltd Table 2 Minimum duration of experience for certification. Experience, months NDT method Level 1 Level 2 Level 3 ET 3 9 18 MT 1 3 12 PT 1 3 12 RT 3 9 18 UT 3 9 18 RI N/A 6 N/A VT 1 3 12 Work experience in months is based on a nominal 40-hour week or the legal week of work. When an individual is working in excess of 40h/week, he may be credited with experience based on the total hours, but he shall be required to produce evidence of this experience. Direct access to Level 2 requires the total number of hours shown in Table 2 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 2 for Levels 1-3 Qualification examination Table 3 Numbers of general questions. NDT method Level 1 Level 2 ET 40 40 PT 30 40 MT 30 40 RT 40 40 RI N/A 40 UT 40 40 VT 30 40 BRS 30 N/A RPS N/A 20 plus 4 narrative Note: All Level 1 specific theory papers have 30 questions. All Level 2 specific theory papers have 36 questions. NDT31-50316b Prelims Copyright © TWI Ltd Re-examination a A candidate who fails to obtain the pass grade for any examination part (general, specific or practical) may be re-examined twice in the failed part(s), provided the reexamination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination. b A candidate who achieves a passing grade of 70% in each of the examination parts (general, specific or practical) but whose average score is less than the required 80% may be re-examined a maximum of two times in any or all of the examination parts in order to achieve an overall average score of 80%, provided the re-examination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination. c A candidate who fails all permitted re-examinations shall apply for and take the initial examination according to the procedure established for new candidates. d A candidate whose examination results have not been accepted for reason of fraud or unethical behaviour shall wait at least twelve months before re-applying for examination. Summary The PCN scheme is managed and administered by the British Institute of NDT (BINDT) on behalf of its stakeholders. It meets or exceeds the criteria of BS EN ISO 9712. There are 6 appendices covering various industry and product sectors, 1 2 3 4 5 6 Aerospace. Castings. Welds. Wrought Products and Forgings. Pre and in-service inspection (multi sector). Railway. There are many additional supporting documents varying from vision requirements PSL44 to renewal and recertification (Levels 1 and 2 – CP16; Level 3 – CP17) and so on. The document defines many terms used in certification of NDT personnel (PCN Gen Section 3) The certification body (BINDT) meets the requirements of BS EN ISO 17024 (PCN Gen section 5) NDT31-50316b Prelims Copyright © TWI Ltd BINDT approves authorised qualifying bodies (AQBs) to carry out the examinations (PCN Gen Section 5) a b c d e f g h i j k l The document sets out the Levels of PCN certification and what each level of personnel is qualified to do (PCN Gen section 6). There are 3 Levels of PCN certification. Candidates for examination must have successfully completed a BINDT validated course of training at a BINDT authorised training organisation (PCN Gen Section 7). Table 1 shows the minimum required duration of training for all Levels and methods plus a section of notes. Table 2 gives the minimum duration of experience for each Level and method. A candidate is required to have a vision test of colour perception and a near vision test (Jaeger Number 1 or N4.5). PCN Gen Section a – the near vision test to be taken annually. Examination applications are made directly with the AQB. PCN Level 1s and 2 initial exams comprise general; specific and practical parts. Table 3 shows the number of general questions at Levels 1 and 2 examinations. There are 30 specific questions on the Level 1 papers. There are 36 questions on the Level 2 specific papers. A variety of practical samples are tested depending on the method and sector. A Level 3 examination comprises a basic and a method examination – however the basic examination needs to be passed only once. Table 4 shows the number of basic examination questions. Table 5 shows the number of Level 3 examination questions. Table 4 Number of basic examination questions. Part Examination Number of questions A Materials technology and science, including typical defects in a wide range of products including castings welds and wrought products. 30 B Qualification and certification procedure in accordance with this document 10 C 15 general questions at Level 2 standard for each of four NDT methods chosen by the candidate, including at least one volumetric NDT method (UT or RT). 60 NDT31-50316b Prelims Copyright © TWI Ltd Table 5 Main method examination. Part Subject Number of questions D Level 3 knowledge relating to the test method applied 30 E Application of the NDT method in the sector concerned, including the applicable codes, standards, and specifications. This may be an open book examination in relation to codes, standards, and specifications. 20 F Drafting of one or more NDT procedures in the relevant sector. The applicable codes, standards, and specifications shall be available to the candidate. m A pass is obtained where each part is 70% or over with an average grade of 80% or over. n A PCN certificate is valid for 5 years. o Renewal and recertification requirements are covered in CP16 for Level 1 and Level 2 and CP17 for Level 3. NDT31-50316b Prelims Copyright © TWI Ltd Section 1 Introduction 1 Introduction This section covers the syllabus for PCN approval in eddy current testing of aircraft components and structures. It also provides a basis for the more advanced concepts used in tube testing, material sorting and weld testing which are covered in section 2. The text for this course is laid out in a manner which it is hoped will make it easier to follow than conventional course texts. In general, right hand pages are used for text and left hand pages for flow charts, diagrams and tables. Looking across the page to the right of a particular diagram you should find the relevant text. We have left plenty of space on the pages to encourage you to add notes from the lectures. The flow charts, we hope, you will find useful in following the progress of the course lectures. In eddy current methods there are many concepts and models that are difficult to comprehend unless they can be put into the context of the subject as a whole. Because we are using flow charts there is no index. Each flow chart splits a subject title into several subheadings, given with a decimal notation for the paragraph number. Therefore the number 2.2.31 means paragraph number 31, under subheading number 2 of subject title 2. This makes it easier for us to change the text. We hope it does not confuse you. 1.1 The SI units of measurement Before we start you may care to study the units of measurement on the facing page. The United Kingdom adheres to a treaty signed at the General Conference on Weights and Measures, which has established a Systèmes Internationales of units. Eventually these units will replace all existing Imperial and CGS units. Certainly not all of these units are of relevance to this course but the Table will be a useful reference. We shall also be using scientific notation, which is useful shorthand for writing numbers with a great number of zeros. For example: 7.0 x 10³ = 7000 7.0 x 10ˉ³ = 0.0007 But m.sˉ¹ = m/s m.sˉ² = m/s m.s² = m x s² But don’t worry, if in doubt write the numbers out in full. NDT31-50316b Introduction 1-1 Copyright © TWI Ltd 1.2 History of eddy current testing Eddy current testing methods have their origins in a period just after the First World War, when materials with high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering - they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in the magnetic materials. The first eddy current testing devices for NDT were by Huges in 1879. It was left to Frederick Forster in the late 1940s to develop the modern eddy current testing equipment and formulate the theories which govern their use. Since then, eddy current methods have developed into a wide range of uses and are recognised as being the front-runner in NDT techniques today. Modern electronics have not only reduced the noise levels which made certain test applications very difficult but they have also improved the methods of signal presentation. Microcomputer chips abound, from giving lift-off suppression in simple crack detectors to providing signal processing for immediate analysis of condenser tube inspections. 1.3 Definition of non-destructive testing (NDT) Non-destructive testing includes physical testing methods for detecting flaws in a material or component in a manner which does not in any way harm the service life of the material or component. The basic principal methods are: Visual testing (VT). Penetrant testing (PT). Magnetic particle testing (MT). Eddy current testing (ET). Ultrasonic testing (UT). Radiographic testing (RT). In all the NDT methods, results can be misinterpreted easily. For example, MT may reveal strong indications along the weld toe that are impossible to distinguish from toe cracks. Much depends on the skill of the operator, although properly formulated test techniques and procedures will improve test accuracy and consistency. Some NDT methods can be destructive. There are, for example, many corrosive liquids used in penetrants and contrast aids. NDT31-50316b Introduction 1-2 Copyright © TWI Ltd Table 1.1 SI units of measurement Base quantities Length Mass Time Electric current Thermodynamic temperature Luminous intensity Amount of substance metre kilogram second ampere kelvin candela mole Symbol m kg sec A K cd mol Derived units Frequency Force Pressure and stress Work and energy Power Quantity of electricity e.m.f. and potential difference Electric capacitance Electric resistance Electric conductance Magnetic flux Magnetic flux density Inductance Luminous flux Illumination hertz newton pascal joule watt coulomb volt Hz N Pa J W C V 1Hz=1secˉ¹ 1N= 1kg.m/sec² 1Pa=1N/m² 1J=1N/m 1W=1J/sec 1C=1A/sec 1V=1W/A farad ohm siemens weber tesla henry lumen lux F Ω S Wb T H lm lx 1F=1A.sec/V 1Ω=1V/A 1S=1Ωˉ¹ 1Wb=1V/sec 1T=1Wb/m ² 1H=1V.sec/A 1lm=cd/sec 1lx=1lm/m ² Other accepted units Volume Mass Energy litre tonne electron volt l t eV 1l=1dm³ 1t=10³kg Approx 1.60219 x 10ˉ¹9 Prefixes 10¹² 10 10 10³ 10² 10 10ˉ¹ 10ˉ² 10ˉ³ 10ˉ6 10ˉ9 10ˉ¹² 10ˉ¹5 10ˉ¹8 tera giga mega kilo hector deca deci centi milli nicro nano pico femto atto Symbol T G M k h d d c m µ n p f a NDT31-50316b Introduction 1-3 Copyright © TWI Ltd Penetrant testing (PT) Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of the excess penetrant the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used. Advantages Disadvantages Applicable to non-ferromagnetics Will only detect defects open to the surface Careful surface preparation required Able to test large parts with a portable kit Batch testing Not applicable to porous materials Applicable to small parts with complex geometry Simple, cheap, easy to interpret Temperature dependant Good Sensitivity to surface defects Compatibility of chemicals Cannot retest indefinitely Magnetic particle testing (MT) Magnetic particle testing is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages Disadvantages Will detect some sub-surface defects Ferromagnetic materials only Rapid and simple to understand Requirement to test in two directions Pre-cleaning not as critical as with dye penetrant inspection (DPI) Will work through thin coatings Demagnetisation may be required Odd shaped parts difficult to test Cheap rugged equipment Not suited to batch testing Direct test method Can damage the component under test NDT31-50316b Introduction 1-4 Copyright © TWI Ltd Eddy current testing (ET) Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages Disadvantages Sensitive to surface defects Very susceptible to permeability changes Can detect through several layers Only on conductive materials Can detect through surface coatings Will not detect defects parallel to surface Accurate conductivity measurements Can be automated Not suitable for large areas and/or complex geometries Signal interpretation required Little pre-cleaning required No permanent record (unless automated) Portability Radiography testing (RT) Radiography testing monitors the varying transmission of ionising radiation through a material with the aid of photographic film, fluorescent screens or digitally using (a) Computed Radiography with phosphor photostimulable screens or (b) Direct Radiography with Digital Detector Devices and Arrays, to detect changes in density and thickness. It will locate internal and surfacebreaking defects. Advantages Disadvantages Gives a permanent record, the radiograph There is a radiation health hazard Detects internal flaws Can be sensitive to defect orientation and so can miss planar flaws Has limited ability to detect fine cracks Detects volumetric flaws readily Can be used on most materials Gives a direct image of flaws Access is required to both sides of the object Skilled radiographic interpretation is required Is a relatively slow method of inspection Fluoroscopy can give real time imaging Has a high capital cost Can check for correct assembly Has a high running cost NDT31-50316b Introduction 1-5 Copyright © TWI Ltd Ultrasonic testing (UT) - pulse echo Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will be reflected to the probe in a time period other than would be expected of a fault free specimen. Advantages Disadvantages Sensitive to cracks at various orientations No permanent record (unless automated) Portability Safety Not easily applied to complex geometries and rough surfaces. Unsuited to coarse grained materials Able to penetrate thick sections Reliant upon defect orientation Measures depth and through-wall extent 1.4 Choice of method Before deciding on a particular NDT inspection method it is advantageous to have certain information: Reason for inspection. (To detect cracks, to sort between materials, to check assembly, etc.). Likely orientation of planar discontinuities, if they are the answer to the above question. Type of material. Likely position of discontinuities. Geometry and thickness of object to be tested. Accessibility. This information can be derived from: Product knowledge. Previous failures. Accuracy of critical sizing of indications varies from method to method. Liquid penetrant testing The length of a surface-breaking discontinuity can be determined readily but the depth dimensions can only be assessed subjectively by observing the amount of bleed out. Magnetic particle testing The length of a discontinuity can be determined from the indication but no assessment of discontinuity depth can be made. Eddy current testing The length of a discontinuity can be determined. The depth of a discontinuity or material thinning can be determined by amplitude measurement, phase measurement or both but the techniques for critical sizing are somewhat subjective. NDT31-50316b Introduction 1-6 Copyright © TWI Ltd Ultrasonic testing The length and position of a discontinuity can be determined. Depth measurements are more difficult but crack tip diffraction or time-of-flight techniques can give good results. Radiography testing The length and plan view position can be determined. Through-thickness positioning requires additional angulated exposures to be taken. The throughthickness dimension of discontinuities cannot readily be determined. NDT31-50316b Introduction 1-7 Copyright © TWI Ltd Section 2 Principles 2 Principles 2.1 Electricity Electricity refers to the flow of electrons through simple materials and devices. The name is derived from the Greek word Elektra, the name given to an exotic mineral, which when rubbed with a cloth, builds up a static charge which creates sparks. It was Benjamin Franklin and his hazardous experiments with flying a kite into thunder clouds, who hit on the idea that electricity could be described as something flowing through a conductor from positive to negative electrodes. We now assign the phenomenon of electricity to the flow of electrons which is of course from the negative to the positive electrode, but Franklin’s concept still remains. In fact the flow of electricity through semiconductors is somewhat different in manner from the flow of electricity through metals, where free electrons exist. We say that the flow of current is a semiconductor and is due to the displacement of positive ‘holes’, which of course is the director of Franklin’s electric current. Electricity is very dangerous to life. Currents of only a few fractions of an amp can set the heart muscles into fibrillation; a condition which stops the circulation of blood due to irregular and shallow heartbeats. Fortunately we are covered in a skin of very high electrical resistance and quite high voltages are needed to break down the barrier. However, eddy current testing instruments are electrical instruments and if they run off the mains power supply they will carry 240 volts. So be careful and for goodness’ sake do not poke around near cathode ray tubes when they are switched on. Some of those coloured bits on the circuit board may be capacitors charge with two or three thousand volts. NDT31-50316b Principles 2-1 Copyright © TWI Ltd a b c Figure 2.1: a Hydrogen atom; b Copper atom; c Experiment with pith balls and glass rod. 2.1.1 Electrons The basic building block of all matter is the atom. The nature of the atom and the electromagnetic forces within it determine the characteristics of matter. There are 118 different elements known to make up matter and each one has a characteristic atom. The simplest atom is hydrogen, which has a nucleus of one proton, or positively charged particle and one neutron, a neutral particle, orbited by one electron, a negatively charged particle (Figure 2.1a). The angular momentum of the orbiting electron is exactly balanced by the electrostatic forces between its negative charge and the positive charge on the nucleus. NDT31-50316b Principles 2-2 Copyright © TWI Ltd As the atoms become larger and the number of charged particles increases, so the electrons arrange themselves in fixed orbits or shells called K,L,M,N,O and P. The outer shell is the valence shell and it is the number of electrons in this shell which determines the electrical and chemical properties of an atom. Copper has only one valance electron. This can be lost easily and for this reason copper is a good conductor of electricity (Figure 2.1b). 2.1.2 Electrostatics Electrostatics is the study of electrical forces which exist between charge particles. In their most fundamental form these forces hold the electron in orbit around the nucleus of an atom. The origin of these forces is a mystery but we do know what their effects are. For example, we know that like charges repel and unlike charges attract. By convention the electrostatic force lines are drawn pointing away from the positive charge and towards the negative charge (Figure 2.1c). The effects of electrostatic fields can be demonstrated using pith balls and a glass rod. The glass rod is first charged positive by rubbing it with a silk cloth. This removes the electrons by friction. The rod is then brought close to the balls, which although initially of neutral charge, will become polarised so that they are both attracted to the rod. As soon as the rod touches the balls, electrons are removed from both so that they become positive and repel each other. Electrostatic charge is caused by electrons. An excess of electrons will create a negative charge. A deficiency of electrons will create a positive charge. The amount of electrostatic charge is measured in coulombs. One coulomb = 6.25 x 1018 electrons. Figure 2.2 A DC circuit. NDT31-50316b Principles 2-3 Copyright © TWI Ltd Figure 2.3 Dry cell. 2.1.3 Direct current If a positive charge is placed at one end of the conductor and a negative charge at the other, then electrons will flow along the conductor creating an electric current. This current will continue only until the charges have been neutralised (Figure 2.2). An electric circuit is a complete path around which electrons can flow. If the circuit is broken, then the electrons cannot flow and the circuit becomes an open circuit. To generate a continuous supply of electrons, a battery is needed. The battery relies on the chemical action between two different metals called electrodes immersed in a salt or acid solution called an electrolyte. The conductor provides a supply of electrons to conduct the current. The load provides the pressure against which the electromotive force of the battery must push the electrons, otherwise the circuit will short. 2.1.4 Battery A battery is a means of applying a potential difference across a circuit to push electrons around it. The simple battery shown is a primary cell and cannot be recharged. Other types including lead acid and nickel-cadmium batteries can be recharged and are therefore secondary cells (Figure 2.3). 2.1.5 Ampere (A) The ampere is the unit of measurement of current flow. 1 ampere = 1 coulomb of electrons passing any point in one second. NDT31-50316b Principles 2-4 Copyright © TWI Ltd In SI units it is a base quantity and therefore defined in absolute terms as that current which when flowing along two infinitely long parallel conductors, one metre apart in free space, exert any attraction of 2 x 10ˉ7 Newtons per metre. 2.1.6 Volt (V) The volt is a measure of pressure, forcing electrons around a circuit. A potential difference is created between opposite charges at either end of a conductor. The greater the difference, the greater the pressure which forces the electrons along. The voltage can occur without current flow in what we call an open circuit. The supply voltage is called the electromotive force. In SI units, the potential difference is one volt between two points of a conducting wire carrying a constant current of one ampere, when the power dissipated between them is one watt. 2.1.7 Resistance (R) The opposition to current flow in a DC circuit is called the resistance. It is rather like friction in mechanics. It opposes the flow of electrons and generates heat. Figure 2.4 Ohm’s law. NDT31-50316b Principles 2-5 Copyright © TWI Ltd Figure 2.5 Power formulae. Figure 2.6 series circuit. 2.1.8 Figure 2.7 parallel circuit. Ohm’s law Ohm discovered that the amount of current flowing through a material varies directly with the applied voltage and inversely with the resistance of the material. R is in Ohms (Ω). V is in volts. I is in amps. A simple way of remembering Ohm’s law is to draw it in circular form (Figure 2.4). Quantities on either side of the vertical line are multiplied, while quantities below the horizontal line are divided into quantities above it. NDT31-50316b Principles 2-6 Copyright © TWI Ltd To use the circle, simply cover the segment you want to find and the position of the remaining letters tells you the procedure to follow. 2.1.9 Power formula Power is the rate at which work is done. In a DC circuit, work is done whenever electrons are set in motion. Therefore in an open circuit, where electrons cannot flow, no work is done even through there is an electromotive force applied from the battery. P = I X V. P is in watts. I is in amps. V is in volts. By using Ohm’s law to substitute the variables, the power formulae (Figure 2.5) can also be written as: OR 2.1.10 Series circuits A series circuit (Figure 2.6) contains only one path along which the current can flow. It is governed by three laws: Individual resistances in a series circuit add up to the total circuit resistance: R = R1 + R2…RN Current has the same value at any point within a series circuit. Individual voltages across resistances in a series circuit add up to the total applied voltage. 2.1.11 Parallel circuits A parallel circuit (Figure 2.7) has two or more paths for the current to flow along. It is also governed by three laws: 1 2 3 Total voltage of parallel circuit is the same across each branch of that circuit. Total current in a parallel circuit is equal to the sum of the individual branch circuits. Total resistance in a parallel circuit is always less than the value of the smallest resistive branch. 1 1 1 1 R R1 R2 RN NDT31-50316b Principles 2-7 Copyright © TWI Ltd Figure 2.8 Series string of lights. Figure 2.9 Parallel string of lights. Figure 2.10 Systematics showing examples of an electrics circuit in a car. NDT31-50316b Principles 2-8 Copyright © TWI Ltd 2.1.12 Parallel and series circuits. Parallel circuits are an advantage in lighting a Christmas tree with several bulbs. When they are connected in series and one filament is blown then all the lights will go out. When connected in parallel, current will continue to flow to the bulbs even if one of the filaments is blown (Figures 2.8 and 2.9). 2.1.13 Car electrics circuits Three parallel branches of a car electrics circuit are shown (Figure 2.10), feeding current to the head-lamp, spark plugs and fan. The twelve volt battery consisting of six two volt cells in series supplies a voltage against the car chassis. We can analyse the circuits by taking measurements with a universal meter where circuits are accessible and calculating voltages or amperages to give information about circuit components which are not accessible. For example, to find the resistance of the head-light filament we could measure the current by connecting an ammeter across the open switch and dividing this value into the voltage across the bulb. We know this is twelve volts as there are no other loads in this branch. To find the voltage across the coil in the spark plug branch, the voltage across the dropping resistor could be measure and subtracted from twelve volts. Similarly, when the fan is not accessible, the current in the fan motor branch could be measured by connecting an ammeter across the open switch and measuring the voltage across the speed control. The fan voltage could then be calculated by subtracting the speed control voltage from twelve volts. The speed motor has three switches, the one without a resistance corresponding to the fastest fan speed. The spark plug branch is designed so that when one of the set of points in the distributor is closed, current rapidly builds up in the coil creating a strong magnetic field. When the points open, this field collapses suddenly, creating a high voltage and therefore arc in the spark plug. The buffer capacitor is placed across the points to prevent similar spark occurring there since it prevents the coil’s inductive voltage reaching the points. NDT31-50316b Principles 2-9 Copyright © TWI Ltd Figure 2.11 Meter controls. Figure 2.12 Capacitor. NDT31-50316b Principles 2-10 Copyright © TWI Ltd 2.1.14 Resistor Resistors are used to control the amount of current in a circuit. Two variable resistors, usually called potentiometers or ‘pots’ are shown which set the zero and control the sensitivity of the meter (Figure 2.11). As the resistance in the parallel potentiometer increases so a greater proportion of the circuit current will flow through the meter decreasing its sensitivity. The series potentiometer will zero the meter. 2.1.15 Capacitor A capacitor or condenser is a device for storing electric charge (Figure 2.12). It consists of two parallel plates separated by a dielectric material. If the plates are connected to the terminals of a battery, the positive terminal will take electrons from one plate and the negative terminal will push electrons onto the other plate. A voltage will build up across the capacitor which will eventually equal the electromotive force of the battery and the capacitor will be fully charged. The amount of charge that a capacitor can take is measured by a quantity called capacitance: C= Q V C is the capacitance in farads. Q is the amount of charge in coulombs. V is the voltage. The farad is a very large unit and so common capacitors are rated in microfarads or picofarads. The capacitance depends on three factors: 1 2 3 The size of the capacitor plates. The greater areas of the plates facing each other, the more charge they can hold. The distance between the plates. The closer they are together, the greater the capacitance. The nature of the dielectric material that separates the capacitor plates. Not only does the dielectric prevent charge breaking down the barrier between the plates, but also the dielectric helps the capacitor to store charge. For example, glass will allow the capacitor to store eight times more charge than air, when it is placed between the plates. Capacitors have a very wide range of uses where a large transient current is needed, for example, in spot welders, flash guns, ignitions systems and dc magnetic particle inspection equipment. NDT31-50316b Principles 2-11 Copyright © TWI Ltd For eddy current testing we are interested in variable capacitors which are used in alternating circuits to adjust the phase between voltage and current or create resonance. 2.1.16 Conductance (G) Conductance is a measure of the ability of a material to conduct electricity and is the inverse of resistance: G= 1 R G is in Siemens. R is in ohms. 2.1.17 Resistivity (ρ) Resistivity is a measure of how easy current will flow through a material. If the resistivity is very high then there are few free electrons available to conduct the current and the electrons have difficulty in passing obstacles such as atoms, discontinuities and impurities in the material. A great deal of heat will be generated depending upon the voltage pushing the electrons along (materials of this nature are called insulators). Conversely a very low resistivity allows more current to flow and is a characteristic of copper and aluminium. Materials of this nature are called conductors: 10 ρ is in micro-ohms • cm. is the length in cm. A is the cross-sectional area of the circuit in cm². R is in ohms. 2.1.18 Conductivity (ợ) Conductivity is the inverse of resistivity: 1 x 10 8 ợ is in Siemens/m. ρ is in micro-ohms •·cm. 2.2 Magnetism The phenomenon called magnetism was discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. NDT31-50316b Principles 2-12 Copyright © TWI Ltd The use of magnets in navigation goes back to the eleventh century, where it was found that rods of magnetised material, when freely suspended, would always point in a North-South direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, to be followed by Michael Faraday (1791-1867) whose experiments revealed that magnetic and electrical energy could be interchanged. The region surrounding a permanent magnet or electric current will deflect a small magnet or compass in curved lines known as the lines of magnetic force or flux. By convention, in the case of permanent magnets, the magnetic flux flows from south to north internally and north to south externally. In the case of a conductor, the direction of flux flow is determined by the right hand rule. This study of magnetism is of vital importance in electrical engineering, electronics and computers. In eddy current testing we are interested in magnetism both in the way that it couples the current in the test coils to the eddy current field in the testpiece and in the dramatic test signals created in ferromagnetic materials that can obliterate defect signals. Figure 2.13 Magnetic domains. NDT31-50316b Principles 2-13 Copyright © TWI Ltd Figure 2.14 Magnetisation curve. 2.2.1 Paramagnetism and diamagnetism All matter is made up of magnetic as well as electrical forces. These forces make up the atoms which are the fundamental building blocks of matter and give matter its substance. Within the atom there are magnetic forces which are the result of the spinning and orbiting motions of the electrons. If an external field is applied, then according to Lenz’s law, any magnetic moments should align themselves to oppose the applied field. This is the case with diamagnetic elements, a group which includes copper. The effect is so very slight as to be negligible. In a paramagnetic element, the balance of magnetic moments which exist in a diamagnetic is offset because there are more electrons spinning or orbiting in one direction than there are in another. This gives rise to a resultant magnetic moment with a north and South Pole which will align itself parallel with any external magnetic field. The effect is very weak because the thermal excitation in the atoms prevent anything but a very weak alignment. Paramagnetics, a group of elements which includes aluminium, can therefore be regarded as nonmagnetic. 2.2.2 Ferromagnetism Ferromagnetism is a term used to describe certain materials which exhibit strong magnetic behaviour. We say that they have high magnetic permeability. The three most common ferromagnetic elements are iron, cobalt and nickel, but there are others, for example, gadolinium, which is important for electronics. Within the crystal lattices of ferromagnetics there exist magnetic domains. Within each domain, the magnetic dipoles, as we call a north-south pole pair, are in parallel alignment with the same pole pointing in one direction. NDT31-50316b Principles 2-14 Copyright © TWI Ltd One end of the domain will therefore have a strong north pole and the other a south pole. The magnetic circuits created by the domains are aligned to reduce flux leakage to a minimum. They therefore adopt a parallel but opposing arrangement or lay across each other. Between the domains exists a domain wall across which the magnetic dipoles twist like a corkscrew. In the ground state, the domains have no preferred orientations and the ferromagnetic is unmagnetised. If an external magnetic field is applied in the direction of the magnetic dipoles in domain A, then this domain will grow at the expense of domain B by twisting over the dipoles in the domain wall (Figure 2.13). This change is elastic. If the external field is removed the domains will return to the ground state. As the external field continues to increase, the domains walls become detached from dislocations which they tend to follow in the crystal lattice and will latch themselves to achieve eventually a state of magnetic saturation. The shape of the magnetisation curve is a characteristic of a ferrogmagnetic element or material. The steeper the curve, the easier it is to magnetise. Figure 2.15 Magnetic hysteresis. NDT31-50316b Principles 2-15 Copyright © TWI Ltd Figure 2.16 Hysteresis loops for iron and steel. 2.2.3 Magnetic permeability (µ) The ease with which a material conducts magnetic flux is called its magnetic permeability: B H µ is the absolute magnetic permeability in Henry/metre. B is the magnetic flux density in teslas. H is the magnetic field strength in A/m. For air and non-magnetic materials, µ is constant and denoted by µo. µo = 4π x 10ˉ7 teslas or Henries/metre. For ferromagnetic materials it varies considerably according to the value of H. For convenience we use relative permeability µr: r 0 Relative permeability is therefore a dimensionless ratio which relates the permeability of the material to that of air. NDT31-50316b Principles 2-16 Copyright © TWI Ltd 2.2.4 Magnetic hysteresis When a ferromagnetic is placed in an alternating magnetic fiend (H), the variation in the density of flux lines (B) through it, gives rise to magnetic hysteresis (Figure 2.15). The word hysteresis is derived from the Greek for delayed and is used to describe one thing lagging behind another. The variation of B with H follows a hysteresis loop and is a characteristic of the ferromagnetic material. Let us take it from the point where all the domains in the ferromagnetic are aligned with the applied field. This is called the saturation point. As H falls to zero, B is reduced to a value given by the remanence point. This is due to a degree of plasticity in the domain alignment which prevents them from returning to random orientations. It gives ferromagnetics their permanent magnetism. H has to be applied in the opposing direction to a value given by the coercive force to knock the magnetic domains out of alignment and reduce B to zero. Further increases in H in this direction will then take the domains to saturation once more, but with the polarity reversed. The shape of the hysteresis loop is an important characteristic of ferromagnetic materials and can be used to grade sort them on the basis of their hardness’s. Steels are magnetically hard, while irons are magnetically soft (Figure 2.16). The slope of the axes of the two hysteresis loops show that the steel is more difficult to magnetise than the iron, but that once magnetised, it is more difficult to demagnetise. The steel would therefore make a better permanent magnet despite having a smaller remanence. It is much larger coercive force will make steel much more difficult to demagnetise by shaking and knocking. On the other hand the much smaller magnetisations and demagnetisation forces operating in the iron reduce the energy losses called hysteresis losses and make it more suitable for the cores of coils and transformers. Figure 2.17 Right hand rule. NDT31-50316b Principles 2-17 Copyright © TWI Ltd Figure 2.18 Magnetic field through a coil. Figure 2.19 Car motor ignition. 2.2.5 Electromagnetism Whenever electric current flows along a conductor, a magnetic field is set up around the conductor in a plane with its axis parallel with the flow of electrons. The magnetic field is there only when electrons are flowing. The direction of the flow of magnetism is given by the right hand rule (Figure 2.17). If the thumb of the right hand is extended in the direction in which the conventional current is flowing, then the direction of the magnetic flow is given by the fingers. Remember that the electron flow is in the opposite direction to the conventional current flow. NDT31-50316b Principles 2-18 Copyright © TWI Ltd The larger the current, the stronger the magnetic flow. The further away from the conductor, the weaker the magnetic flow. The magnetic flow can be detected by placing a compass needle near the conductor. It will align itself with the North Pole pointing in the direction of the magnetic flow. 2.2.6 Coils If a current-carrying wire is looped into several turns, the magnetic field around each turn link together, giving rise to a strong magnetic field through what is now a coil (Figure 2.18). This magnetic field behaves like a bar magnet and will attract ferromagnetic objects. The polarity of the coil ends is determined by a rule which shows that when the coil is viewed end-on, if the conventional current is flowing clockwise, that will be the South Pole. If the conventional current is flowing anti-clockwise, that end of the coil will be the North Pole. The intensity of the magnetic field through the coil is a product of the coil current and the number of coil turns. Coils can be used to control electrical switches called relays. One typical application is the ignition switch of a car engine (Figure 2.19). Here a small control current passes through a coil. When it is switched on the ferromagnetic plunger moves through the coil and closes the contacts of the starter motor circuit. This carries an extremely high load, of perhaps one hundred amperes and it would be extremely hazardous to connect this directly to the key switch. Figure 2.20 Magnetic circuit. 2.2.7 Magnetic circuits A magnetic circuit (Figure 2.20) can be made by analogy to an electric circuit by replacing the battery with a coil and the conductor with a ferromagnetic. The electromotive force then becomes the magnetomotive force and is measure by multiplying the coil current by the number of turns. The amperage becomes the magnetic flux or linkage and is measured in webers. NDT31-50316b Principles 2-19 Copyright © TWI Ltd The ration between magnetomotive force and flux is constant as is the ration of electromotive force to current in an electric circuit. This magnetic equivalent of Ohm’s law is called Bosanquet’s law and the magnetic equivalent of resistance is called reluctance, and is given by: Where F = Magnetomotive force (Mmf). = Magnetic flux. 2.2.8 Magnetic flux density (B) This is also called the magnetic induction and the SI unit of measurement is the tesla: B A B is in the flux density in teslas. Φ is the magnetic flux in webers. A is the cross-sectioned area of the magnetic circuit in m². One tesla is the magnetic flux density of a uniform field that produces a torque of 1N/m on a plane current loop carrying one ampere and having a projected area of 1m² in a plan perpendicular to the field. 2.2.9 Magnetic field strength (H) The SI unit of magnetic field strength is the ampere per metre: H m.m.f. D H is in A/m. mmf is in A • turns (Mmf is the magnetomotive force, also known as magnetic potential and is analogous to emf or voltage in electricity). D is the axial distance in metres. It is the magnetic field strength in the interior of an elongated, uniformly would solenoid which is excited with a linear current density in its winding of one ampere per metre of axial distance. 2.2.10 Inductors Coils have an effect on the current which is passing through them and are therefore called inductors. The magnetic field which they create acts as a store of energy, which has been taken from the electrical current. As long as the current is not changing, the magnetic field is in a steady state and it has no effect on the current. If the current is building up, the current finds itself building up the magnetic field as well and its flow is opposed. This results in the current taking a longer time to build up in a circuit containing an inductor than in a circuit containing a resistor only. NDT31-50316b Principles 2-20 Copyright © TWI Ltd As the current in the inductor decreases, the magnetic field is reconverted to electrical energy and so slows the rate at which the current in the inductor decays. It can be said therefore, that inductors act to oppose any change in the current through them. 2.2.11 Inductance (L) The ability of a coil to store magnetic energy and oppose changes in the current is called inductance: L ϒ N A l = = = = = Inductance in henrys. A geometric factor. Number of coil turns. Coil’s planar surface area in mm². Coil’s axial length. The henry is a very large unit. Eddy current coils have inductances of a few microhenrys. Inductance is a property of only those electrical circuits where the current is varying. The opposition to current flow generates a voltage or self-inductance in the circuit, but it can also generate a voltage in a neighbouring circuit through mutual-inductance. The latter is the transformer principle. 2.3 Alternating current theory Alternating currents are continually reversing. The electrons will be flowing along a circuit in one direction, slowing down until they are stationery then flowing in the opposite directions until they reach a maximum velocity (current) before slowing down again and reversing once more. This alternation occurs in a regular period termed at the frequency. The current from the mains supply alternates approximately 50 times a second or at 50 hertz. In eddy current testing, the frequency of the currents is of vital importance and may range from 10Hz-10MHz (10 megahertz or 10 million cycles per second). The change in the current with time can be represented by a sine wave model. When the capacitors or inductors are placed in an AC circuit we find that the voltage and current waves do not coincide. We say they are out of phase. To analyse these phase differences we use vector diagrams. NDT31-50316b Principles 2-21 Copyright © TWI Ltd Figure 2.21 Sine waves. Figure 2.22 Root mean square. 2.3.1 Sine waves If a pen rotates in a circle around a sheet of paper which is moving under it at a constant velocity, we find that the pen describes a sine wave (Figure 2.21). The characteristics of a sine wave are: 1 2 3 4 5 2.3.2 One cycle is equivalent to one revolution of the circle or 360° or 2 π radians. The amplitude of the current (Φ) is proportional to sine of the included angle (ợ). The rate of change in the current is at a maximum where it crosses the datum and zero where it reaches the peak values. The positive and negative peak values are equal but opposite. Each cycle has a constant period determined by the frequency. Root mean square Since alternating currents are reversing between equal but opposite peak values it is not possible to measure their mean value. In practice the root mean square (RMS) (Figure 2.22) value is measured, which is defined as that value of steady current which would dissipate heat at the same rate in a given resistance. NDT31-50316b Principles 2-22 Copyright © TWI Ltd The power is dissipated in heat is given by: P = l²R If the resistance is constant, the average power (Pa) will be given by 2. By plotting the squares of the current values we can find an average, since negative as well as positive values become positive. To measure the square of the current we use a moving iron ammeter. This type of ammeter consists of two iron rods which are forced apart as they are magnetised. Their level of magnetisation is proportional to the current and therefore the force between them is roughly proportional to the square of the current. The meter is calibrated to read the root of the mean to the square values and is therefore non-linear. Figure 2.23 Faraday’s experiment. NDT31-50316b Principles 2-23 Copyright © TWI Ltd Figure 2.24 Reactance and resistance. 2.3.3 Faraday’s laws Faraday discovered the inductive effects of rapid changes in the magnetic field. When current is abruptly switched off in an electrical circuit it will induce an electromotive force which, if magnetically coupled to another electrical circuit, will create a current in that circuit. When the battery is disconnected in circuit A, the light in circuit B flashes for an instant. Similarly when the battery is reconnected and the current is building up in circuit A, so the bulb in circuit B flashes. While current is flowing steadily in circuit A, the light in B is off (Figure 2.23). The two circuits are not linked electrically but the magnetic field around circuit A does link through circuit B. Faraday went on to define two laws: Whenever a magnetic field linking a circuit is changed, it sets up an electromotive force. The amplitude of this induced electromotive force is proportional to the rate of change of magnetic flux. NDT31-50316b Principles 2-24 Copyright © TWI Ltd 2.3.4 Lenz’s law This law states that the electromotive force induced by the variation in magnetic flux is always in such a direction that if it produces a current, the magnetic effect of that current opposes the flux variation responsible for both the electromotive force and the current. 2.3.5 Resistance and reactance The resistance in an AC circuit represents a loss of electrical energy as heat, as it does in a dc circuit. In an AC however, there are two other components which oppose the flow of current and these are called reactances (Figure 2.24). One is the capacitive reactance, which creates a voltage across a capacitor and the other is the inductive reactance which creates a voltage across an inductor. The capacitor converts current into electrostatic energy and the inductor converts current into magnetic energy. As the energy is reconverted to current when the polarity of the circuit current reverses, neither of the reactances represents an actual loss in electrical energy. Ohm’s law can be applied to the reactances. The ratio of voltage to current across each component is constant: XC Xc Vc XL VL I Vc I = = = = = XL VL I Capacitive reactance in ohms. Voltage across the capacitor. Inductive reactance in ohms. Voltage across the inductor. Circuit current. There is a complication. The voltages and currents in an AC circuit are sinusoidal waves and therefore have a phase as well as amplitude. Across the resistor, voltage and current are in phase. Across a capacitor the current leads the voltage by a quarter cycle (π/2). This can be explained as follows: When the voltage in the circuit is at maximum, so is the charge in the capacitor. It is therefore not charging and the current is zero. When the voltage starts to fall the capacitor is completely discharged and the voltage is zero. Across an inductor, the voltage leads the current by a quarter cycle (π/2). This can be explained as follows: When the current in the circuit is at a maximum, the rate of change in the magnetic flux in the coil is zero and therefore the self-induced voltage is zero. When the current in the circuit is at zero so the rate of change in the magnetic flux is at a maximum and so therefore is the self-induced voltage. When the current is building up in the positive direction, so the induced voltage will be slowing down in the positive directions. When the current is building up in the negative direction, so the voltage is slowing down in the negative direction. NDT31-50316b Principles 2-25 Copyright © TWI Ltd Figure 2.25 Coil’s equivalent circuit. Figure 2.26 Impedance diagram. 2.3.6 Impedance (Z) The application of Ohm’s law to an AC circuit gives the formula: Z V I Z is the circuit impedance in ohms. V is the voltage. I is the current. The impedance is a vector quantity, which is described by an amplitude and a phase. In eddy current testing, the most important impedance is that which exists across a test coil (Figure 2.25). The coil can be regarded as an indicative reactance and resistance in series. The capacitive reactance of the coil is negligible. The impedance (Figure 2.26) in this case has two components which are vectors that are at right angles to each other. NDT31-50316b Principles 2-26 Copyright © TWI Ltd Let us say that we know that the inductive reactance of the coil is 4ohms and its resistance is 3ohms. If the two quantities were scaler we would simply add them together to find the impedance. However, they are vectors and must be added together vectorily as described in Section 2.3.8. To add the two vectors we can draw an impedance diagram as shown, from which we find that the impedance is 5ohms. Alternatively we could find the impedance vector by using Pythagoras’s theorem and solving the equation: You may sometimes see this written in the form Z = R + j XL Where j = √-1 and is the mathematical operator which rotates the XL vector through 90°. The angle ợ gives the angle between the voltage and current phases in the coil. This is because the voltage is in phase with the current in resistance and 90° lead of the current in the inductive reactance. The voltage vector can therefore be substituted for the impedance vector. The angle ợ can be solved from: arctan XL R Let us now say that the inductive reactance of the coil is reduced to 3ohms because of the increased eddy currents in the coil’s magnetic field which dissipate more heat and therefore increase the coil’s resistance to 4ohms. The coil’s impedance is still 5ohms, but the phase angle between voltage and current has changed by 160°. There has been a phase change but no amplitude change. A simple meter reading circuit would miss this change. Similar hypothetical changes in the coil’s impedance could be used to show changes in the amplitude of the impedance but not in the phase between voltage and current. Normally of course we are dealing with combinations of inductive and resistive components that can be described by movements in a point at the end of the impedance vector to any position on the impedance diagram. A diagram of this sort can be displayed on a cathode ray tube and is called a vector point or more colloquially, a flying dot or spot display. A complete phase and amplitude analysis can then be made of the coil impedance. Other facts which we have already realised can be described on the impedance diagram. If the coil has no resistance, then it is a pure inductor, the impedance equals the inductive reactance and the voltage leads the current by 90°. If the coil has no inductive reactance, then the coil is a pure resistor, the impedance will equal the resistance and the voltage and current are in phase. As described in section 2.3.7 this occurs when dc is passed through the coil. NDT31-50316b Principles 2-27 Copyright © TWI Ltd 2.3.7 Frequency The inductive reactance and the capacitive reactance depend upon the frequency of the ac current, as can be seen from the following equations: 1 2 XL Xc f L c = = = = = 2 Inductive reactance. Capacitive reactance. Test ac frequency. Circuit inductance. Circuit capitance. Figure 2.27 Vectors. 2.3.8 Vectors Some physical quantities are described by a single number. These are scalar quantities. Examples of scalar quantities are speed, temperature and weight. Others have a directional quantity as well and cannot be described by a single number. These are vector quantities (Figure 2.27). Examples include velocity, force and coil impedance. If we represent a vector as a point in space and it moves to another point in space we say it undergoes a displacement. Displacement is a vector quantity, because it is to be described completely we must know its magnitude and its direction. Another feature of vectors is that if two vectors are to be equal, they must have the same magnitude and the same direction. Vectors which have the same magnitude but not the same direction cannot be equal. A typical vector problem is shown. An aeroplane flies 20km in a direction 60°N of east, then 30km straight east then 10km straight north. Where will it end up? NDT31-50316b Principles 2-28 Copyright © TWI Ltd We can plot the vectors as components in a rectangular (cartesian) co-ordinate system on a scaled diagram. The resultant vector R and its direction can be measured from the diagram or calculated. For eddy current testing we use vector diagrams to describe impedance in a coil. Let’s simulate the vector problem we have just solved in an impedance diagram. To begin with the equivalent circuit for a coil, we have an inductive reactance and resistance in series. The voltage across the resistance is in phase with the current so we shall replace the x co-ordinate with this. The voltage across the inductive reactance leads the current by 90° and we shall replace the y coordinate with this. Initially the voltage across the whole coil is 20 millivolts and leads the current by 60°. The voltage across the resistance then increases by 30 millivolts and across the inductive reactance increases by 10 millivolts. The voltage across the whole coil now becomes 48.4 millivolts and it leads the current by 34.3°. 2.4 Eddy currents Eddy currents are electrical currents induced in metals by alternating magnetic fields. They are closed loops of current which circulate in a plane perpendicular to the magnetic flux except at the surface, where they will flow parallel with that surface. For eddy current testing, the magnetic fields are generated by a coil carrying high frequency AC. When the coil is brought into close proximity with a metal, the alternating magnetic field induces the eddy currents. The eddy currents are encircled by their own magnetic fields which are in a direction to oppose the field from the coil which is generating them. They therefore have a choking effect on the coil current. The choking effect, which is reflected in the coil’s impedance, is monitored by the eddy current instrument. Changes in the eddy current field due to changes in the metals properties near the surface, cause changes in the coil’s impedance. These are the test signals. It is difficult to understand the process without the conceptual models of the physicist. These are enshrined in the classical laws of Faraday and Lenz and in Maxwell’s equations. The following sections describe the factors which affect the eddy current field. NDT31-50316b Principles 2-29 Copyright © TWI Ltd Table 2.1 Conductivities. Materials Silver Copper Gold Aluminium Al-6101 Al-5052 7075-T6 Magnesium Phosphor bronze Cartridge brass Admiralty brass Tungsten Nickel 98Cu-2Ni 70Cu-30Ni 70Cu-22Ni Iron Platinum Tantalum Carbon steel Chromium steel Cobalt steel Stainless steel 501 Stainless steel 410 Stainless steel 304 Lead Monel Zirconium Titanium 2.4.1 IACS% 105 100 75 61 56 35 32 37 15 28 24 30 23 35 4.5 5.7 18 16 14 9.5 6.1 6.3 4.5 3 2.5 8.4 3.6 3.4 3.1 µΩ•cm 1.6 1.7 2.35 5.3 3.1 4.93 5.3 4.6 10.5 6.2 7.0 5.65 7.98 4.99 37 30 9.7 10.6 12.45 18 29 28 40 57 70 20.6 48.2 50 54.8 Resistivity Electrical conductivity (ợ) Conductivity is a measure of the ease with which electrons flow in a material and will therefore determine the eddy current density. Conductivity is the inverse of resistivity. Some tables of material properties will list one, some tables will list the other and this can be very confusing. Resistivity is usually given in µΩ•cm and conductivity in mΩ•mm² or siemens/m. To add to the confusion, in eddy current testing, conductivities are usually measured in IACS. This is the International Annealed copper standard which ranks pure annealed copper as 100% IACS and air as 0%. The conversion factors are: 100%IACS = 58m/Ω mm² = 5.8 10 / / 1m/Ω mm² = 106 siemens/m 1siemen / m 10 8 1 cm Conductivity depends on a number of material properties. It will depend upon its composition, temperature, hardness, temperature history and cold working. Any discontinuity within the material matrix which obstructs the free flow of electrons will reduce the conductivity. This is why increasing alloy composition will reduce conductivity. Eddy current testing therefore makes a useful sorter of mixed alloys, particularly of aluminium-magnesium alloys. NDT31-50316b Principles 2-30 Copyright © TWI Ltd Conductivity is affected by heat treatment of the material. This feature can be used in assessing the fitness-for-purpose of aluminium aircraft components damaged in engine burn-outs and tyre bursts. The hardening of the aluminium alloy increases its conductivity. However, it must be remembered that at very high temperatures this effect can be reversed. Table 2.2 Magnetic permeabilities. Material 0.1%C steel 0.34% C steel Annealed Normalised Cast Max.rel. µ 2420 1950 2100 Annealed Normalised Cast 1200 970 840 Mn steel 2.4.2 1300 Spheroidal graphite Pearlitic Annealed 290 1150 Grey iron As-cast Annealed 315 1560 Magnetic permeability Permeability has a dominant effect on eddy currents. The noise created by permeability changes in ferrous welds makes the eddy current technique a difficult method to apply to weld inspection. Another problem lies in the inspection of non-magnetic condenser tubes, where ferrous baffle plates can often give a noise level high enough to obliterate defect signals from the tube wall. Recent advances in eddy current testing do seem to be overcoming these problems. As well as introducing high levels of noise to the eddy current test, permeability also reduces the depth of penetration of the eddy currents to the extent that only surface discontinuities can be detected. The permeability effects can magnetically. Beyond saturation This can only be accomplished in current test coil sits between two be removed by saturating the material a ferromagnetic behaves as a paramagnetic. pipe and bar testing systems, where the eddy powerful DC coils that encircle the pipe or bar. The magnetic permeability is reduced to unity if the ferromagnetic is heated above its curie point. For mild steel, this lies at about 720°C. Saturationmagnetisation is not necessary therefore when testing hot bar and billet with eddy currents. Magnetic and Relative Permeability are further described in Section 2.2.3 with maximum values of relative permeabilities given in Table 2.2 above. NDT31-50316b Principles 2-31 Copyright © TWI Ltd Measurement of magnetic permeability does provide useful information about ferromagnetic materials. This is the basis of ferrous segregators and electromagnetic sorting bridges. Permeability is affected by: Thermal processing history. Mechanical working. Internal stresses. Temperature. Chemical composition. The equipment used in these material sorters is based upon the principles of eddy current testing, but because it is the inductive effects of magnetic materials in the test coil and not the eddy current effect which dominates over any given test signal, the methods are referred to as electromagnetic testing and not eddy current testing. See section 8 for further details. A typical use for the instrument is to sort out forgings in a batch which have been case-hardened. The method is surprisingly sensitive to even minor changes in the case depth. Figure 2.28 Standard depth of penetration. NDT31-50316b Principles Figure 2.29 Skin of currents around a slot. 2-32 eddy Copyright © TWI Ltd Figure 2.30 Standard depths of penetration. 2.4.3 Frequency The most important test variable is the frequency of the current sent through the test coil. Eddy current testing is done at frequencies from a few hertz to several megahertz. The most important effect of test frequency is upon the depth of penetration (Figure 2.28) of the eddy current field. As the frequency increases so the depth of penetration decreases. This is known as skin effect (Figure 2.29) and it can be defined by the formula: 500 f.. Where is the standard depth of penetration in mm. f is the frequency in hertz. σ is the conductivity in m/ mm2. is the permeability. When Eddy Currents flow in a Conducting material magnetic fields are produced that oppose the primary magnetic field, thus reducing the resultant magnetic flux and causing a reduction in current flow as the depth below the surface increases. This is known as the ‘skin effect’. The standard depth of penetration is defined as the depth below the surface (Figure 2.30) at which the intensity of the eddy current field has been reduced 1 to a value of e of its intensity at the surface. The function of e is the base of the natural logarithms. It is equal to 2.718 when taken to three decimal places. Therefore at the standard depth of penetration, the eddy current field intensity is at approximately one third of its surface value (36.8%). NDT31-50316b Principles 2-33 Copyright © TWI Ltd What a bizarre way of setting a standard, you may say. Well the intensity of eddy current field falls exponentially with increasing depth. The equation for a curve describing this decay is of the form: Intensity = 8 -depth The intensity never actually reaches zero, so we take value beyond which the effect of eddy currents on the test coil is small. The standard depth of penetration acts as a good reference point to base frequencies used for finding subsurface discontinuities. Remember however, that there are eddy currents at greater depths that may affect coil impedances and that with thin wall tubes and solid bars it is their geometry that determines the depth of penetration, not the formula. Always use calibration blocks with discontinuities at known depths when setting sensitivities for low frequency testing. The reason for the exponential decay of eddy current intensity with increasing depth is that each layer of eddy current partially shields the next deeper layer from the coil’s magnetic field. The rate of decay in intensity falls as the depth increases and the eddy current intensity decreases, so that in theory the intensity reaches zero only at infinity. The formula only applies to the skin depth. High frequency eddy current testers are made sensitive to surface breaking slots well in excess of the standard depth of penetration because the eddy currents flow around the slot sides and tip. The standard depth of penetration is also dependent upon the conductivity and permeability of the material. An increase in the conductivity increases the intensity of the eddy currents at the surface, creating a greater shield against the coil’s magnetic field. The rate of decay therefore increases. Permeability has a very strong effect. Unless it can be removed from a ferromagnetic by magnetic saturation, the eddy currents are going to be contained to within a few microns of the surface. NDT31-50316b Principles 2-34 Copyright © TWI Ltd 2.4.4 Figure 2.31 Edge effect. Figure 2.32 Lift-off. Figure 2.33 Fill factor. Figure 2.34 Discontinuities. Edge effect Edge effect (Figure 2.31) is the name given to the eddy current test noise caused by contours and edges to the test surface. Signals from cracks emanating from an edge can be difficult to detect unless the edge effect can first be cancelled or zeroed out on the meter. NDT31-50316b Principles 2-35 Copyright © TWI Ltd Surface probes are often held in fixtures or jigs that will keep the probe at a fixed distance from the edge, as it is scanned parallel with the edge. The edge effect is therefore kept constant. Where a ferromagnetic material abuts the edge, the edge effect is much stronger and it is necessary to use shielded probes where the coil’s external magnetic field can be constrained within a ferrite housing. For tube testing with different encircling coils, a few millimetres at each end of the tube cannot be tested because of the edge effect. 2.4.5 Lift-off Lift-off (Figure 2.32) is the term given to the eddy current test response to lifting a surface coil from the test surface. As the coil moves away, the magnetic coupling to the eddy current field weakens very rapidly. Small movements give a pronounced effect. The noise generated by the test coil as it scans a round surface would be too high unless measures are taken to lessen lift-off. These measures include tuning the coil with a capacitor and rotating the lift-off plane in the impedance diagram in a manner which reduces the lift-off effect. On the other hand, the lift-off effect can be used to measure the thickness of non-conductive paint coatings on a metal substrate. 2.4.6 Fill factor Fill factor (Figure 2.33) is the lift-off equivalent when using encircling coils. It is a measure of magnetic coupling between tube and coil. For an internal bobbin coil, the fill factor is measured as the square of the ratio of the coil diameter over tube diameter. for example: ɳ For an encircling coil, the fill factor is the square of the ration of the tube diameter over the coil diameter. for example: ɳ The fill factor can never exceed 1.0 and is more usually about 0.7. At high test speeds, large fill factors will inevitably result in damage to the coil. A fill factor below 0.6 will result in a low sensitivity. There is an exception in the case of electromagnetic sorting bridges. The test frequencies are low and the test signals are caused by the inductive effects of the ferromagnetic testpieces. Fill factor is then of less importance. 2.4.7 Discontinuities Only cracks and laminations which distort the eddy current field will give rise to eddy current test signals. Laminations parallel with the test surface will not be detected (Figure 2.34). A surface crack will increase the resistive path of the eddy currents and deflect them downwards so that their magnetic fields have less effect on the coil. Changes in both the inductive reactance and resistance of the coil can then be expected. NDT31-50316b Principles 2-36 Copyright © TWI Ltd Section 3 Equipment 3 Equipment 3.1 Circuits The circuits used in the eddy current test instruments are designed to amplify the very small changes in the coil current while keeping noise to a minimum. Although it is not necessary to know of the complexities of modern electronics it is both useful and interesting to know something of the principles. Early high frequency crack detectors have much in common with radio receivers. The coil is analogous to the radio aerial. The bridge circuit however, has always formed the basis of low frequency equipment. NDT31-50316b Equipment 3-1 Copyright © TWI Ltd a b Figure 3.1 Series resonance curve: a Simple circuit; b Double circuit. NDT31-50316b Equipment 3-2 Copyright © TWI Ltd 3.2 Simple circuits A very simple circuit for detecting changes in the coil impedance would consist of an oscillator to supply high frequency sinusoidal currents to the coil and a voltmeter connected across the coil, Figure 3.1a. The meter would be zeroed with the probe on the test surface so that the eddy current field affecting the coil’s impedance is in a steady state. As the probe crosses a crack, the eddy currents flow around the crack tip. The coil impedance changes creating a deflection in the voltmeter. The probe coil is in an absolute arrangement with the instrument circuit. Alternatively there could be a double arrangement, Figure 3.1b. The oscillator feeds current to a separate driver coil. In the steady state an E.M.F. is induced in the receiver coil by the driver coil and the eddy current field. A change in the eddy current field will again cause a change in the impedance of the receiver coil that will be recorded by the meter. These circuits would not make practical eddy current test instruments because the voltage changes due to quite major cracks would only be of the order of 0.1%. 3.2.1 Resonance circuits Resonance circuits are tuned circuits in which the coil’s inductive reactance is in resonance (Figure 3.3) with the capacitive reactance of a capacitor placed in the circuit. Small changes in the coil impedance can then be made to create large changes in the coil voltage. Resonance occurs in an AC circuit when the capacitive reactance equals the inductive reactance: XL = XC 2fL f 1 2fC 1 2 LC f is frequency. C = capacitance (farads, F). L = Inductance (henrys, H). Resonance occurs at a unique frequency and for most practical purposes; this is done in the kHz-MHz range. NDT31-50316b Equipment 3-3 Copyright © TWI Ltd Taking the example of an L-C-R series circuit, the variable capacitor XC is adjusted so that the oscillator frequency is the resonance frequency of the circuit: Z R2 ( X L X C )2 Z R XL Xc = = = = Impedance of circuit (ohms). resistance (ohms). Inductive reactance (ohms). Capacitive reactance (ohms). At resonance Z = R. By plotting XL, XC and R for various frequencies, it can be seen that at resonance, Z is at a minimum and therefore the voltage is at a minimum (Figure 3.4). A slight change in the coil impedance will displace the resonance frequency from the oscillator frequency and the circuit voltage will increase dramatically. High frequency eddy current testers usually have one absolute coil tuned in parallel with a fixed capacitor and do not have selectable frequencies. To maintain a reasonably constant coil impedance, the frequency for testing materials of low conductivity, for example austenitic stainless steel, may be up to 2MHz whereas for materials of too high conductivity, for example aluminium, may be no more than 500kHz. To test ferrous materials which have low conductivity but high permeability, the higher test frequency is used but with a probe at lower inductance. Figure 3.2 Wheatstone bridge. NDT31-50316b Equipment 3-4 Copyright © TWI Ltd Figure 3.3 Eddy current test AC bridge circuits. Figure 3.4 Phase sensitive circuits. 3.2.2 Bridge circuits Most eddy current test instruments use AC bridge circuits to detect the very slight changes in the impedance of the test coil. These are modified forms of the Wheatstone Bridge (Figure 3.2) which is a classroom instrument used to measure resistances to a high degree of accuracy. Resistor R is adjusted until the meter reads zero. If current is not flowing through the meter then the potential at A equals the potential at C: V AB VCB V AD VCD VAB = VCB. VAD = VCD. NDT31-50316b Equipment 3-5 Copyright © TWI Ltd Since the meter is zero, the current through P must be the same as the current through R and the current through Q must equal the current through X: VBC I c Q V I P and AB A VCD I c X VAD I A R Q X=RX P Where: X is the unknown. R is adjustable. P and Q are the ratio arms that set the resolution of the bridge. In AC bridges (Figure 3.3), the resistors are replaced with impedances. These introduce voltage phases as well as amplitudes into the balancing. A typical crack detector may have an absolute test coil in one arm and a load in the other, or alternatively, one half of a differential coil in one arm and the other half of the differential coil in the other. For the sorting bridge, one arm contains a coil with the reference standard, the other arm the coil with the testpiece of unknown properties. The X and R controls are used to bring the bridges into balance by affecting both the amplitude and phase of the voltage through the meter. If the meter is replaced by a cathode ray tube, the sinusoidal voltages from the standard and test coils are adjusted until they are exactly 180° out of phase. The trace then appears as a horizontal line. 3.2.3 Phase-sensitive circuits Meters normally detect only changes in the amplitude of the coil voltage. They can however be made sensitive to changes in the phase of the voltage as well as by using a double bridge arrangement, Figure 3.4. The primary bridge circuit containing the test coils shown in this case as differential coils, are connected to a second phase-sensitive bridge which also receives the reference voltage. The meter is so arranged that it only receives current through the diodes which is in phase with the reference voltage. The signal voltage may for example change in phase only without a change in amplitude by moving to the right on the A-scan. The meter will respond because the proportion of current now entering the meter is increased. Phase-sensitive instruments are essential in low frequency work because of the effect of subsurface discontinuities upon the eddy current phase. NDT31-50316b Equipment 3-6 Copyright © TWI Ltd 3.3 Instruments The instruments used in eddy current testing range from pocket-sized paint thickness gauges to computer-controlled automated test systems. We shall concentrate on the meter reading and cathode ray tube display types. Figure 3.5 Moving coil ammeter. Figure 3.6 Lift off compensation. 3.3.1 Meter reading instruments Most eddy current testing instruments are meter reading. They are simple to use and the meter can be calibrated to measure conductivity, crack severity, paint thickness or many other test variables. For the level of sensitivity required, meters have to be of moving coil type (Figure 3.5). These measures mean values of the current. Since the mean value of an AC current is zero, the current has first to be rectified before measurement. NDT31-50316b Equipment 3-7 Copyright © TWI Ltd The moving coil is rotated inside a magnetic field by the interaction between the current in the coil and the magnetic field between the magnets. The direction of the mechanical force is given by Fleming’s right hand rule and is against the coil spring. The great the current in the coil, the greater the force. Moving coil ammeters have a slow response due to the inertia in the spring. The meter will not respond fully to short eddy current signals generated as the probe scans the surface. For this reason, light-emitting diodes are incorporated, set to illuminate at predetermined levels. The diodes respond immediately. 3.3.2 Lift-off control Meter reading instruments that are used for crack detection have a lift-off control to deaden the effect of probe movement when scanning. Lift-off compensation (Figure 3.6) can be accomplished in a number of ways, which are best understood with reference to the impedance diagram. A simple sequence for setting the lift-off compensation is as follows: Figure 3.6 is the impedance diagram for an eddy current test circuit containing a coil, a variable capacitor and a variable resistor in series. When the coil is placed on the test surface the impedance meter reads 6 (OA). When the coil is placed on a thin sheet of cardboard the impedance meter reads 8(OB). A locus AB produced therefore represents the lift-off plane. The second Figure in Figure 3.6 shows the changes in impedance with adjustments to the lift-off control (variable capacitor) and zero control (variable resistor). Adjustments 1 2 3 4 5 With coil on the cardboard, increase LIFT-OFF until meter reads 6(OC). With coil on the test surface, the meter will now read 6.2(OD). Decrease ZERO until meter reads 6 once more (OE). With coil on the cardboard, the meter will read 5.8(OF). Decrease LIFT-OFF until meter reads 6 once more (OG). With coil on the test surface, the meter will read 5.9(OH). Increase ZERO until meter reads 6 once more (OI). Repeat the sequence of adjustments until the meter reads 6 both on the test surface and on the cardboard. The locus AB will have moved to A1B1 on the circle of radius 6 as shown in the third Figure in Figure 3.6. NDT31-50316b Equipment 3-8 Copyright © TWI Ltd Figure 3.7 Cathode ray storage scope. Figure 3.8 CRT displays. Figure 3.9 Vector point display. NDT31-50316b Equipment 3-9 Copyright © TWI Ltd 3.3.3 Cathode ray tubes Cathode ray tubes (Figure 3.7) can be used to analyse changes in the phase and amplitude of the eddy current test signal. An electron gun fires a beam of electrons between electrostatic plates, the X and Y plates. The electrons, which carry a negative charge, can be deflected upwards by putting a positive charge on the upper Y plate or to the left by putting a positive charge on the left hand X plate. The point written onto the phosphor screen by the electrons can therefore be moved to any position on the screen. In storage scopes (Figure 3.7) the illuminated spot on the phosphor screen can be retained when the electron beam is moved or switched off by flooding the screen with low speed electrons. These do not illuminate the screen but only continue to excite the phosphors, which have been hit by the high speed electrons from the electron gun. To erase the screen, the flood current is switched off. 3.3.4 A-scan display A-scan display shown on the left in Figure 3.8. A time base can be created between the X plates by applying a sawtooth-shaped pulse. This sends the electron beam from left to right and then almost instantly back to left to begin another sweep. If this is repeated one hundred times each second, then a continuous horizontal line will appear across the screen. Its length corresponds to 1/100th of a second. Time base sweeps of as little as one microsecond can be achieved so that extremely short transient signals can be seen. These signals are sent to the Y plates. 3.3.5 Ellipsoid display Ellipsoid display shown on the right in Figure 3.8. If two unrectified sinusoidal voltages are sent simultaneously to the X and Y plates, then an ellipsoid is formed on the cathode ray tube screen. The two voltages must have the same frequency. The phase and amplitudes of the two voltages will affect the shape of the ellipsoid. It can vary from a straight line when the voltages are in phase to a circle if the voltages are 90o out of phase. The tilt of the ellipsoid is affected by the relative amplitudes of the two voltages. 3.3.6 Vector point display Modern eddy current test instruments use a cathode ray tube display which simulates the impedance diagram, Figure 3.9. The signal voltage is first rectified and then split into sine and cosine components about an arbitrarily selected phase angle. The sine and cosine functions are arrived at electronically and of course give two components to the signal voltage which are 90o to each other. These can be regarded as the XL and R axes of the impedance diagram although their actual vector directions are controlled by the phase rotation control. With the cathode ray tube connected across a bridge circuit, an absolute coil is one arm and a load coil is the other, the variable capacitor (X) and variable resistor (R) are adjusted to bring the bridge to a state of balance. In this state, no current is entering the CRT and rotation of the phase control will have no effect on the vector point. Most instruments allow automatic balancing of the bridge circuit. NDT31-50316b Equipment 3-10 Copyright © TWI Ltd The bridge should be balanced with the probe down on the test surface. Lifting the probe up will show the lift-off plane. This is usually rotated until it moves off to the left of the screen. The probe can then be moved over slots and towards the sides of a slotted testblock to give the crack and edge effect signals. The sensitivity control is used to alter the amplitudes of the signals. The frequency control will alter the phase angle between the signals. 3.4 Probes Eddy current test probes come in many forms. When selecting a probe there is the coil arrangement to consider and its effect on sensitivity. The coil size is constrained by high inductive reactances at high frequencies. Surface probes may need to be shaped to reach confined spaces. Encircling probes and internal bobbin probes should fit the tube as closely as possible. Finally, the probe has to match the circuitry of the instrument. There is not the ability to interchange like that is found in ultrasonic test equipment. Often it is necessary to make special probes and a probe-making facility becomes necessary where eddy current testing is used on a wide range of component shapes. Figure 3.10 Coil arrangements. 3.4.1 Coil arrangements The coil arrangements (Figure 3.10) can be classified into four types. Single coils have the same coil both to drive the eddy currents and receive signals due to changes in the eddy current flow. The meter or cathode ray tube monitors the voltage across the coil. The circuit is suitable for the simple high frequency crack detectors where signals are confined to amplitude changes and noise from the subsurface eddy current field is negligible. The double coil arrangement has one coil to drive the eddy currents and another coil to receive the test signals. The voltage in the receiver coil is induced by eddy currents and the current in the driver coil. It is much less than the voltage in the driver coil alone and there is a higher signal to noise ratio. NDT31-50316b Equipment 3-11 Copyright © TWI Ltd Differential coils are commonly used in tube testing. The coil is in two different halves, wound in opposition. The inductive reactance in one half is equal but opposite to the inductive reactance in the other half. Bipolar signals are produced when a discontinuity comes through the coil. The wavelength of the signal is dependent upon the separation of the coil halves and the speed of travel of the discontinuity. Signals are therefore suitable for modulation analysis, where only signals of a certain wavelength are allowed through the filters. Differential coils do not respond to gradual changes in tube dimension that would generate unacceptable levels of noise in absolute coils. However, they detect only the ends of continuous uniform defects lying parallel with probe travel. If the defect ends correspond with tube ends, differential coils will miss the defect entirely. By having a separate drive coil from the differential receiver coil in a double differential coil arrangement, noise levels are further reduced. The choice of differential or absolute coils for tube testing is a difficult one. Differential coils are less prone to temperature drift and ignore gradual changes in tube dimensions. Absolute coils give signals that are easier to interpret and do not miss longitudinal defects throughout the tube length. A combination of the two may be necessary: differential coils for a primary tester that will detect defective tubing and absolute coils for a fuller analysis. Figure 3.11 Surface probe. NDT31-50316b Equipment 3-12 Copyright © TWI Ltd Figure 3.12 Encircling shape. Figure 3.13 Internal bobbine probe. 3.4.2 Surface probes Surface probes (Figure 3.11) induce an eddy current field which is parallel with the test surface. The field circulates about the probe and so there is good sensitivity to planar discontinuities in any plane except the one which is parallel with the surface. Laminar defects remain undetected. The simplest surface probes are pencil probes. These are used at high frequencies to detect surface breaking flaws. The coil is only a few millimetres long and is wrapped around a ferrite core to increase the flux density. In shielded pencil probes the coil is in a ferrite housing that pulls in the coil’s external field to reduce edge effects. The ferrite tip may be protected by stick PTFE tape. The bolt-hole probe is designed for insertion into a fastener or bolthole. The coil lies perpendicular to the hole bore and the split end will accommodate a small change in the hole diameter. The holder for the probe allows rotation at fixed depths within the hole. Manual manipulation of a probe in fastener holes using static eddy current testers is laborious and has largely been superseded by rotating probes and dynamic testers. Lower frequency probes have larger coil diameters and usually double differential coil arrangements. A ferrite housing is essential if the field width is to be kept reasonable. These larger probes are called pancake probes. Ring or doughnut probes are low frequency probes designed for testing around steel fasteners in a wing skin without the need of removing the fastener for a bolt-hole probe inspection. The ferrite core of the pancake probe has been removed and the ferromagnetism of the fasteners is relied upon to draw the coil flux down into the skin. NDT31-50316b Equipment 3-13 Copyright © TWI Ltd 3.4.3 Encircling probes For tube, rod and wiring testing, the coil is wrapped around the aperture of the probe and as close to the surface as possible (Figure 3.12). The fill factor (See Section 2.4.6) should be no less than 0.7 if sensitivity is to be kept high. Guiding the tube or bar through the coil at high speeds is difficult and this restricts the maximum fill factor that is attainable without danger of damage to the probe aperture. Electromagnetic sorting bridges have large coils. Fill factor is not critical as it is the inductive effects of the ferromagnetic testpiece inside the coil that dominates the coil impedance and not the eddy current flow. 3.4.4 Internal bobbin probes To inspect condenser tubes in heat exchangers, the probe (Figure 3.13) must be inserted into the tube as there is no access to the outside. Low fill factors of the order of 0.65 are necessary because of probe jams in dented tubes. The probe is first fired with compressed air to the tube end and then retrieved at a constant speed of 200-300mm/sec while the test signals are recorded. 3.4.5 Remote Field Eddy Current (RFET) RFET uses an Eddy Current send-receive type probe technique for tube testing (usually from the tube inner) that operates in both differential and absolute modes simultaneously such that localised defects can be detected with the differential mode and gradual defects with the absolute mode. The detector coils are separated by the equivalent of two or three times the tube diameter and are equally sensitive to internal and external indications with tube wall loss being measured through variations in phase. 3.5 Calibration blocks Calibration blocks are a vital part of eddy current testing. The tests rely on the appropriate design of calibration blocks and reference standards to an extent greater than any other NDT method. Eddy current fields are too complex for any quantitative assessments of signals. Signals can only be compared with those from known discontinuities. Cracks must be compared with slots thinning with stepped wedges, tube wall defects with through drilled holes and conductivity measurements with IACS testblocks. Figure 3.14 HF slotted calibration block. NDT31-50316b Equipment 3-14 Copyright © TWI Ltd Figure 3.15 Ring probe calibration block. 3.5.1 Slotted calibration blocks High frequency surface crack detectors are calibrated on blocks of the test material which contain 0.5 and 1mm deep spark eroded slots. Aluminium, mild steel and austenitic stainless steel blocks (Figure 3.14) are readily supplied. For meter reading instruments, the zero and lift-off are set with the probe on the block, away from the edge or slots. The probe is then scanned over the 0.5mm slot to obtain the greatest meter deflection and then held steady while the meter sensitivity is adjusted to give a 40% deflection of full scale. Proceeding to the 1mm slot, the sensitivity is adjusted to give an 80% deflection. Signal deflections from the testpiece can now be compared with those from the slots. A threshold may be set at 25% of full scale deflection and signals above this investigated. On no account should measurements of crack depth be based on comparisons with the reference deflections. Crack morphology will differ greatly from that of the slot. Low frequency eddy current instruments for detecting subsurface cracks must be calibrated with the slots at the required depth below the surface. This may be accomplished by placing a plate of the test material over the slotted block. The frequency should be set to give a standard depth of penetration which is about 110% of the thickness of the cover plate. Depth of penetration has to be traded off against test sensitivity. It should be just enough to reach the subsurface slots but not so great as to give poor signal to noise separation. Moreover, if the eddy current field penetrates too far, noise may be picked up from features below the layer of interest. Special blocks have been designed for calibrating ring probes for detecting cracks emanating from steel fasteners in a wing spar (Figure 3.15). The frequency is first set to give a standard depth of penetration greater than the skin thickness. The lift-off and zero of the instrument are set with the ring probe over a flaw-free fastener. Then the ring probe is moved to a fastener with one slot and the sensitivity adjusted to give a 50% full scale meter deflection. Then finally to a fastener with two slots to give a 100% full scale meter deflection. When moved to the testpiece, noise levels due to variations between the permeabilities of the fasteners are very high. This makes inspection difficult. In all cases, the calibration block sets the sensitivity level only. Lift-off and zero have to be reset when the probe is moved to the testpiece. NDT31-50316b Equipment 3-15 Copyright © TWI Ltd Figure 3.16 Step wedges. Figure 3.17 Tube standards. 3.5.2 Step wedges The meter deflection can be sent to indicate wall thinning in a thin metal plate. The frequency is set to give a standard depth of penetration just beyond the plate thickness but not so great as to be affected by deeper metal substrates. The illustrated step wedge (Figure 3.16) could be used to indicate 50% thinning in a 2mm thick wing skin by setting the zero, 50 and 100% full scale meter deflections on the 2.0, 1.5 and 1.0mm steps. Remember that the eddy current fields respond to volume changes rather than changes in the residual wall thickness. A deep conical shaped pit may give no greater meter deflection than a shallow but flat area of thinning. To assess the depth of thinning, two methods can be used that both involve the construction on graph paper of calibration curves that note the response of the meter to known changes in thickness. In the first, a curve is constructed at a fixed frequency that gives field penetration just below the wall being measured. This is suited to larger areas of thinning. In the second, the frequency is adjusted and its value noted at which thinning to known depths get a response on the meter. This is more suited to pitting. In either case, the method, although providing useful information, cannot give a reliable level of accuracy. NDT31-50316b Equipment 3-16 Copyright © TWI Ltd 3.5.3 Tube standards Manufactured tube is usually tested for through defects that may cause leaks (Figure 3.17). The through drilled hole therefore gives a suitable reference signal. For condenser tube inspection, corrosion on the inner tube surfaces has to be distinguished from corrosion on the outer tube surface. This is done by setting up the instrument on tubes containing machined slots or flats. The frequency is set to give a 90° phase difference between the two surfaces as they appear on the cathode ray tube display. This can be done because an internal groove will appear as a change in fill factor to an internal bobbin probe, while an external groove will appear as a change in wall thickness. The f90 frequency as it is called can be found by analysis of the impedance diagram. It is approximately 110% of one standard depth of penetration. The use of Impedance diagrams is covered in further detail in Section 6, Phase Analysis. The use of calibration blocks in tube testing is covered in greater detail within Section 10 of the course notes. NDT31-50316b Equipment 3-17 Copyright © TWI Ltd Section 4 Practices 4 Practices 4.1 Documentation Proper documentation of non-destructive tests is essential if they are to have a meaningful role in quality control. For eddy current testing this is even more important because the specifications and procedures which do exist tend to be ambiguous and the tests must be tied down to more specific requirements, including applications to products, manufacturing processes and in-service inspection. NDT31-50316b Practices 4-1 Copyright © TWI Ltd Eddy current methods Technique sheet Technique no: Component identification: Area of examination: Purpose of examination: Equipment required: Instrument: Probes: Calibration blocks: Preparation of component: Examination procedure: a) Instrument calibration 1. Initial setting 2. Sensitivity setting 3. Alarm threshold setting b) Test procedure Probe position Setting-up procedure c) Acceptance standard Reporting procedure: Additional information: Prepared by: Qualification: Date: Approved by: Qualification: Date: Figure 4.1 Technique sheet. NDT31-50316b Practices 4-2 Copyright © TWI Ltd Techniques An NDT technique is a way of using an NDT method within the constraints of a procedure. It is definitive in approach and does not allow the operator to exercise choice in carrying out the test. The NDT procedure is a more general document, which describes how, where, when and why NDT is to be applied. The technique is prepared according to the procedure, in the light of past experience and a knowledge of the defects sought. A good technique will provide coverage, be concise and give clear instructions. It may have to be modified if experience indicates improvements, even to the extent of changing the test method. The document must therefore allow for subsequent amendments and be part of a system in which amendments can be released to all concerned. The technique must be approved by someone in authority who is suitably qualified and experienced in the specific NDT technique and who will have a sound working knowledge of NDT and product technology including the product application and defects sought by the test. A suitably qualified and experienced Eddy Current NDT Technician (Level 2) may prepare the Technique sheet and also carry out the test as required. Technique writing requires discipline. The blank form (Figure 4.1) shows the main subject areas to be covered but the actual document may need to be extended to several pages to include diagrams of calibration blocks and special probes, and the extent of probe test coverage. NDT31-50316b Practices 4-3 Copyright © TWI Ltd Eddy current methods Test report - Practical exercises report sheet Course no: Date: Test operator(s): Component: Number Equipment: Instrument: Probes: Test frequency(ies): Sensitivity setting: Defect threshold level: Lift-off setting: Scanning procedure: DIAGRAM SHOWING LOCATION AND LENGTH OF DISCONTINUITIES Name: Qualification: Signature: Figure 4.2 Test report. NDT31-50316b Practices 4-4 Copyright © TWI Ltd 4.1.1 Test reports Any NDT report should: 1 2 3 4 5 Be properly documented with a report number and date (Figure 4.2). Refer to a technique (Figure 4.1) which will give details of the test operation. Contain enough information for the test to be repeated under identical conditions. It should give details of the equipment used, calibration standards and where possible instrument serial numbers. Record the results of the test. Where a diagram is used this should show the datum used to locate flaws. Major defects such as cracks should be measured and their lengths given with perhaps the maximum signal amplitude as an indication of crack depth. Spurious and non-relevant indications must not be recorded. Show the signature of the test operator, as well as his name and qualifications. If no defects are present, then words should be chosen carefully. Phrases such as no significant defects are ambiguous. All defects are significant because they are defined as those flaws which create a substantial risk of failure. They are therefore outside of specification. Phrases like acceptable to specification or no indications are preferable. 4.2 Applications Eddy current testing has an ever-expanding repertoire of applications. The problem lies in isolating the discontinuities which may be signals in one application but noise in another. 4.2.1 Crack detection Eddy current crack detection equipment can be divided into high frequency instruments for finding surface breaking cracks in ferrous and non-ferrous materials and low frequency instruments for finding cracks in non-ferrous materials. Detection of subsurface cracks in ferrous materials in possible but only when it has been saturated magnetically to remove permeability effects. This is a complex affair and is only practicable in automated tube testing systems. Eddy current test are the most sensitive of all NDT methods to surface cracks. High frequencies of the order of 2MHz give high resolution, but the probes are small and covering large surface areas takes a long time. Low frequency crack detectors need larger probes to accommodate for suitable coil inductances. The frequency setting is critical and is in the range 100100kHz depending on the depth of penetration that is required. Subsurface eddy current fields are influenced more by phase changes than amplitude changes and therefore phase sensing circuits are essential. Although traditionally they were meter reading instruments, the trend in crack detectors is towards instruments with cathode ray tube displays for their added versatility. NDT31-50316b Practices 4-5 Copyright © TWI Ltd 4.2.2 Tube and wire testing Automated eddy current test systems have been developed for the inspection of tube, bar and wire at speeds of up to 3 metres per second. Once the operator has calibrated the instrument using a tube or wire with known flaws, the test installation runs automatically, ejecting defective pieces from the production line or marking them with paint. The mechanical handling equipment for the test pieces becomes so complex that the actual eddy current test instrumentation may appear an insignificant part. Facilities for magnetic saturation and demagnetisation of ferrous tubes and wires increase the capital costs considerably. The constant test speeds and differential coils allow for modulation of the test signal with the speed and then filtering to remove noise. Unfortunately, when using differential coils, it is possible to pass through tubes with consistent defects throughout their whole length, without detection. Because of the edge effect, tube ends cannot be detected. Extrusion defects along the centre of bar cannot be detected either because the eddy current field from an encircling coil is at zero intensity at the centre of a solid cylinder. 4.2.3 Condenser tube inspection This application is currently receiving a great deal of attention in connection with the heat exchangers of pressurised water reactors. Tube thinning is the main defect and by selecting what is known as the f90 frequency, signals from thinning on the outside surface can be set 90o out of phase from signals from thinning on the inside surface. By recording the X and Y signals from the impedance diagram on a two-channel strip chart recorder, the extent of thinning can be ascertained at test speeds of 200-300mm per second. A major problem is caused by the baffle plates which separate the condenser tubes. The tubes are non-magnetic, stainless steel, cupro-nickel or more recently, titanium. The baffle plates are ferrous and the permeability signal is enough to obliterate signals from thinning between the tube and baffle plate. To alleviate this problem, instruments have been developed to operate with two frequencies simultaneously. The separate signal phases are then mixed in a manner which removes unwanted permeability effects. Inspection is usually done with differential coils because they do not drift with temperature changes. The signal interpretation is more difficult and it is often necessary to do supplementary tests with absolute coils. Other recent developments include the use of computers to analyse X and Y channels for defect signals. The inspection can then be done in real time. 4.2.4 Material sorting Ferrous segregators and electromagnetic sorting bridges are useful tools in sorting steels which have been hardened. Conductivity meters can be used to sort aluminium and copper alloys, both for compositional variations and hardness variations. NDT31-50316b Practices 4-6 Copyright © TWI Ltd Great care has to be taken to ensure that the variation being detected is the relevant one. For example, the change in the conductivity of an aluminium may be due to a change in composition or a change in its hardness. Because eddy current fields penetrate below the surface of the test material, the method does provide a better sample of material properties than many other material sorting methods and more importantly it is very rapid. 4.2.5 Weld testing Simple high frequency eddy current testers have been used for some time to detect toe cracks in ferrous welds. The method has the advantage in being able to detect cracks through paint layers. The disadvantages lie in the high noise levels caused by permeability changes in the weld and lift-off noise from rough cap surfaces. Recent devices have to some extent overcome these problems. They are being used to supplement magnetic particle inspections under water, to distinguish strong spurious indications from toe cracks. The equipment uses a cathode ray tube with a vector point display and special coil arrangements. 4.2.6 Coating thickness measurement The high near surface resolution of eddy current tests makes it useful for measuring coatings, metallic and paint, on metal substrates. 4.2.7 Ferrite Testing Ferrite testing is undertaken to determine the ferrite content (usually as a percentage) in austenitic stainless steel, duplex steel welds and cladding to ensure that the residual ferrite content is within a specific range that is compatible with the mechanical strength requirements and corrosion resistant properties needed. The ferrite meter is an eddy current conductivity meter typically used on welded and clad vessels used in the petrochemical and process plant industries. NDT31-50316b Practices 4-7 Copyright © TWI Ltd Table 4.1 Logarithms of numbers 10-49. NDT31-50316b Practices 4-8 Copyright © TWI Ltd Table 4.2 Logarithms of numbers 50-99. NDT31-50316b Practices 4-9 Copyright © TWI Ltd Table 4.3 Antilogarithms 00-49. NDT31-50316b Practices 4-10 Copyright © TWI Ltd Table 4.4 Antilogarithms 50-99. NDT31-50316b Practices 4-11 Copyright © TWI Ltd Table 4.5 functions of angles 1-45°. NDT31-50316b Practices 4-12 Copyright © TWI Ltd Section 5 AC Theory 5 AC Theory Alternating current theory was introduced during section 1 of this course. The reason for phase leads or lags between voltage and current in an AC circuit is now explained. For those who are used to mathematical concepts, equations are introduced for eddy current flow, as is the j notation as a shorthand way of operating vector quantities. The effects of inductors and capacitors in AC networks are investigated. Figure 5.1 Voltage (A, B, C, D, E and F) and current across a capacitor. Figure 5.2 Voltage (A, B, C, D, E and F) and current across an inductor. NDT30-50316b AC Theory 5-1 Copyright © TWI Ltd 5.1 Capacitive reactance Across a capacitor we are concerned with the build-up of electric charge on the plates of two electrodes which face each other. The rate of build-up of charge on the two plates will depend upon the voltage in the circuit. We therefore use the voltage as our reference and construct the current wave onto it. In the diagram of an alternating current (Figure 5.1), the voltage is changing at its greatest rate at A, C and E. At these points the current away from the electrodes of the capacitor will be at its greatest. At B, D and F the voltage change is zero and therefore the current will be zero. From A to B the voltage increasing in the positive direction and therefore the current will be positive. From B to C the voltage is decreasing in the positive direction and therefore the current will be negative. We can therefore draw the current wave on to the voltage wave and show that in an AC circuit which has only a capacitive reactance, the current leads the voltage by 90o. 5.1.1 Inductive reactance In an inductor (Figure 5.2), when the current changes there is a self-induced EMF which by Lenz’s law acts in opposition EMF is at a maximum. At B, D and F the rate of change of current is zero and therefore the induced EMF is zero. According to Lenz’s law, at A the current is going to positive and therefore the induced EMF will be negative. Similarly at C the current is going to negative and therefore the induced EMF will be positive. The induced EMF opposes the applied EMF and therefore the voltage. The current lags the voltage by 90o. In real AC circuits, there are combinations of capacitive and inductive reactances and of resistances. The resistances in effect loses electrical energy as heat. The capacitance temporarily stores energy in an electrostatic field and the inductance temporarily stores it is a magnetic field. Both reactances return the energy into electricity but cause a displacement between the voltage and current. In eddy current testing we are mainly interested in coils. These have an inductance and resistance and even a very small capacitive reactance can become quite significant at high frequencies. NDT30-50316b AC Theory 5-2 Copyright © TWI Ltd Figure 5.3 Eddy current induction. 5.1.2 Equations for eddy current flow Equations for eddy current induction is shown in Figure 5.3. For a proper analysis of eddy current effects it is necessary to express the variables as mathematical equations. In this course we are interested in the practical effects, for which only a superficial knowledge of the mathematical analyses is necessary. We can start with a look at Faraday’s laws. The value from current that is varying sinusoidally with time at any instant given by: = o sin ( t) Where: = instantaneous value of the current at time, t. o = peak value of the current. = angular velocity = 2πx frequency in hertz. Oersted discovered that the amount of magnetic flux in a current carrying coil is given by: = N Where: = flux. N = number of turns in the coil. = coil current. Faraday’s laws state that there is a voltage induced whenever the flux changes and that its magnitude is dependent upon that rate of change. Since the voltage opposes the change, according to Lenz’s law: V=– d dt Where V = induced voltage. NDT30-50316b AC Theory 5-3 Copyright © TWI Ltd Figure 5.4 AC series circuit. Figure 5.5 AC Parallel L-R circuit. 5.1.3 AC Series circuit The voltage and current phase relationships in an AC circuit (Figure 5.4) in which the capacitor, inductor and resistor are in series have been dealt with in section 1. Since NDT30-50316b AC Theory 5-4 Copyright © TWI Ltd The impedance (Z) is given by: Z= R 2 ( XL X C )2 and arctan Where is the phase angle between the voltage and current. It XL is greater and XC then the circuit is effectively inductive and current lags the voltage. If XC is greater than XL then the circuit is effectively capacitive and the current leads the voltage. If XL = XC then the circuit is effectively resistive and is in a state of resonance. We dealt with series resonance in Section 1. To reiterate, at resonance the impedance of the circuit falls to a minimum and is equal to the resistance. The energy oscillating between electrostatic energy and magnetic energy in the capacitor and inductor respectively. Series resonance circuits act as acceptor filters. Frequencies outside the bandwidth of the filter are rejected. This is used in radio communications to tune the radio to a particular frequency. 5.1.4 Parallel L–R circuits Unlike the series circuit, the supply voltage is now taken as common to all branches and it is the current which is divided into the networks (Figure 5.5). and the voltage and current are in phase in the resistive branch. and the voltage leads the current by 90o in the inductive branch. From the vector diagram. = + Since Z = 1 1 NDT30-50316b AC Theory 1 5-5 Copyright © TWI Ltd Figure 5.6 Parellel L-C circuit. Figure 5.7 Parallel resonance. NDT30-50316b AC Theory 5-6 Copyright © TWI Ltd 5.1.5 Parallel L-C circuit Current in the capacitive branch is given by Current in the inductive branch is given by and is lagging the voltage by 90°. From the vector diagram (Figure 5.6), the current voltage may be leading or lagging the voltage, depending upon whether the inductive reactance or the capacitive reactance is the greater. 1 1 1 When XL = XC and the circuit is in resonance, then Z becomes infinite. 5.1.6 Parallel circuit resonance In the case of a coil, which has both inductance and resistance and is in parallel with a capacitance, we can see what happens as the frequency of the supply current increases. When the frequency is zero (DC condition) the coil reactance is zero so that only the coil resistance limits the current. The capacitive reactance is infinite and therefore lC is zero. As the frequency increase so the coil reactance increases and the current decrease, lagging at a progressively greater angle from the voltage. On the other hand, the capacitive reactance decreases so that increases but always remains 90° leading the voltage. At some frequency XL = XC and resonance will occur, the circuit impedance will reach a maximum and the circuit current a minimum. At resonance, current is oscillating between the inductance and capacitance. Only a small current is needed from the supply to make good resistive losses in the coil. A parallel resonance network (Figure 5.7) acts as a rejecter of resonance frequencies in the circuit. NDT30-50316b AC Theory 5-7 Copyright © TWI Ltd Figure 5.8 Addition of vectors. 5.1.7 j Operator j is a mathematical operator which rotates a vector clockwise through 90° without changing its magnitude. For example, we can split the impedance vector into the inductive reactance and resistance components such that: XL = Zsin Φ and R = Zcos Φ the where is the phase angle between impedance and resistance. For phase angle has been rotated through 90 degrees and this can be denoted by j. Thus a commonly used shorthand version of the formula|: Z= R 2 X L2 is Z = R + j Where the underline indicates that we are dealing with vector and not scalar quantities. From the diagram it can be seen that two operations of j (=j2) rotate the phasor through 180o and it in effect becomes -1 j2 = -1 1 An application of the j operator is shown in Figure 5.8. The resulting vector of adding a+jb and c+jd is given by (a+c) + j (b+d). Similarly the resulting vector from subtracting the vectors is given by (c-a) + j (d-b). NDT30-50316b AC Theory 5-8 Copyright © TWI Ltd Section 6 Phase Analysis 6 Phase Analysis The signals generated in an eddy current test are vector and not scalar quantities. That is to say, they are properly described by two quantities, their amplitude and phase and not just by one quantity, their amplitude. Phase analysis of the test signal using a cathode ray oscilloscope instead of mere amplitude measurements with a meter; allow a greater level of differentiation between relevant signals and unwanted noise. The effect of eddy currents on the coil impedance is described on the impedance plane diagram and various analytical standards are introduced. Methods of suppressing undesired noise are described. Figure 6.1 Signal and noise separation. 6.1 Signal/noise separation One of the most difficult problems in an eddy current test is the separation of signals from non-relevant noise. Many tests are impossible because signals from flaws cannot be distinguished from background noise. This is particularly true of testing ferrous welds, where magnetic permeability effects can obliterate crack signals. NDT31-50316b Phase Analysis 6-1 Copyright © TWI Ltd There are three conventional approaches to the problem: Firstly, amplitude analysis uses the amplitude of the incoming signal. It may give a deflection on a meter or strip-chart recorder. There are severe limitations to this method. Non- relevant signals from lift-off and edge effect may exceed in amplitude those from the crack. Secondly, phase analysis uses the phase as well as the amplitude of the signal. The phase displacement between the output or reference signal and the input or incoming signal is analysed with a cathode ray oscilloscope in an A-scan, ellipsoid, or vector point display. The A-scan displays the signal on a time base. The ellipsoid is created by the interference of the output signal across the Xplates with the input signal across the Y-plates. The vector point display divides the signal into real and imaginery components that are sent to the X- and Yplates of the oscilloscope. Discrimination is still not possible if the non-relevant signal has both the same phase and amplitude as the relevant one. Finally, the frequency of signal may form the basis of an analysis. It must first be modulated and this is done with the test speed. Either the testpiece passes the probe at a fixed speed as is usually the case in tube testing or the probe scans the test surface at a constant speed, as is the case in rotating probes for bolt hole inspection. Non-relevant signals due to minor dimensional changes will have a long wavelength and low frequency which can be filtered out from relevant, relatively high frequency signals from flaws. The test speed must be chosen carefully for the desired discrimination between signal and noise and must be constant. Dynamic testing using signals modulated with the test speed combined with phase analysis of the filtered signals provides the most sensitive method of eddy current testing (Figure 6.1). 6.2 Phase analysis Eddy currents have surprisingly well ordered effects on the amplitude and phase of coil voltages. Thanks mainly to the efforts of Dr Forster in the immediately post-war years; these effects have been rationalised using mathematics. Computers can now be used with a good level of accuracy to predict the eddy current test responses to simulated defects. Although well beyond the scope of this text and arguably often taken beyond the needs of NDT, impedance analysis is a fascinating subject, allowing greater insight into the nature of eddy currents and will be dealt with in a simplified version here. NDT31-50316b Phase Analysis 6-2 Copyright © TWI Ltd Figure 6.2 Idealised impedance. Figure 6.3 Normalised impedance. NDT31-50316b Phase Analysis 6-3 Copyright © TWI Ltd 6.3 Idealised impedance diagram The reduction of a complex electrical circuit into a simple equivalent circuit is a common method of analysis. We can, for example, regard the test coil as a primary winding of a transformer and the eddy current field as the secondary winding. The eddy currents therefore load the current in the test coil as the output voltage loads the input voltage of a transformer and simple electrical analysis can be done (Figure 6.2). in the testpiece. This can be transferred The eddy currents meet a resistance back to the primary circuit by multiplying by the turns ratio squared. If we regard the eddy current field as a one turn coil only, then the new resistance will appear in parallel with the coil as a shunt. Ignoring any other resistance or capacitance in the circuit, the impedance Z is given by: 1 Z= 1 NP R E 2 1 X L 2 When the coil is in air, When = and Z will equal XL. = O and the testpiece is a perfect conductor, then Z = O. If the conductance of the shunt and are equal then The impedance therefore describes a semicircle. 6.4 √ Normalised impedance The circuit impedance depends upon probe parameters as well as frequency and the eddy current field. The coil diameter, coil length, number of turns and coil material all have an effect. Therefore impedance analysis could only be done for individual coils and separate diagrams would have to be constructed for each coil. To overcome this problem, normalised impedance diagrams are used (Figure 6.3). The effect of the coil parameters is removed by dividing the impedance components by the inductive reactance of the coil in air when it is away from any eddy current field. The coil wire and cable resistance must also be subtracted from the resistance component, RL=R-RDC and and are dimensionless and independent of the coil inductance and resistance. NDT31-50316b Phase Analysis 6-4 Copyright © TWI Ltd Figure 6.4 Impedance diagram for a surface coil. Figure 6.5 Impedance diagram for a surface coil. NDT31-50316b Phase Analysis 6-5 Copyright © TWI Ltd 6.5 Conductivity Figure 6.5 shows how the lift-off impedance planes on a normalised impedance graph come from unity in the empty coil condition down to the test surface along a locus of points which show increasing conductivity in a downward direction. With the coil in air, the relative impedance is at 1. As the coil comes down onto the metal the impedance moves along one of the lift-off impedance planes until it meets the locus of increasing conductivity. Metals of different conductivity can therefore be identified according to the direction of their lift-off impedance plane. Notice that the separation between lift-off and conductivity is greatest near the knee of the curve. Lift-off effects and conductivity changes are almost inseparable at the top of the curve. The length of the lift-off planes is greater on good conductors than on poor conductors. The lift-off planes may be calibrated to measure paint thickness on metal substrates. A high degree of resolution will be attainable on good conductors. 6.6 Magnetic permeability Very small increases in permeability send the (Figure 6.5). Permeability variations of only 1-5 found in austenitic stainless steels but for low likely to be 500-1000. It is evident therefore dominate changes in the coil impedance. coil impedance up the graph (relative permeability) may be carbon steels the variation is that permeability effects will On ferromagnetics which are good conductors, the permeability and conductivity impedance planes are almost parallel whereas on ferromagnetics of low conductivity they are at right angles to each other. NDT31-50316b Phase Analysis 6-6 Copyright © TWI Ltd Figure 6.6 Impedance diagram for a surface coil. Figure 6.7 Lift-off plane at two frequencies. NDT31-50316b Phase Analysis 6-7 Copyright © TWI Ltd Figure 6.8 Impedance diagram for a surface coil. 6.7 Thickness As the thickness of the test material decreases, so its resistance will increase and the impedance will be expected to move up the curve (Figure 6.6). However, we have to take into account skin effect and therefore only a finite thickness will have an effect. This thickness will depend upon the frequency of the alternating magnetic field. On a material of a particular conductivity and at a particular test frequency, we can expect the impedance to leave the conductivity plane as the thickness approaches the standard depth which will affect the coil impedance. As the thickness decreases, the impedance moves in a characteristic spiral that it in consequence of skin depth and phase lag. The lift-off and thickness impedance planes are at 180o apart on good conductors and nearly parallel on poor conductors. 6.8 Frequency Frequency has a similar effect to conductivity. As it increases so the impedance moves down the graph. At low frequencies, the depth of penetration is greater and the resistance of the testpiece has a more significant effect on coil impedance. Generally, frequencies are selected to operate near the knee of the curve. Frequency is the most important variable that can be controlled in the test. It determines the phase angles between different impedance planes and therefore the ease with which different effects can be discriminated. NDT31-50316b Phase Analysis 6-8 Copyright © TWI Ltd The example (Figure 6.7 and 6.8) shows how the lift-off planes at different conductivities are more easily discriminated at high frequencies than at low frequencies. However, at high frequencies there is more noise due to pronounced lift-off signals and increased skin effect. (Figure 6.8). Figure 6.9 Impendance diagram for a surface coil. Figure 6.10 Impedance diagram for a surface coil. NDT31-50316b Phase Analysis 6-9 Copyright © TWI Ltd Figure 6.11 Impedance diagram for an encircling coil. 6.9 Probe diameter Probe diameter has the same effect as frequency (Figure 6.9). As it increases so the impedance moves down the curve. Therefore when working at low frequencies it helps to use a large diameter probe to bring the impedance close to the knee of the curve. 6.10 Characteristic parameter Various methods have been developed to combine all the effects of conductivity, permeability, thickness, frequency and coil parameters in one impedance diagram (Figure 6.10). One method uses the characteristic parameter PC. Where: i.d. o.d. 4 r = mean coil radius in metres = = angular velocity = 2 radians. = absolute permeability (for non-magnetics = 4 = electrical conductivity in siemens/metre. 10 henry/metre). For non-magnetic the equation may be written as 7.9 10 Where: = resistivity in cm. r = mean radius in mm. The solid lines represent PC values increasing from zero to infinity, while holding the coil at a constant lift-off from the surface. NDT31-50316b Phase Analysis 6-10 Copyright © TWI Ltd Test conditions with the same characteristic parameter will operate at the same point on the impedance diagram. It shows, for example, that lift-off can be best discriminated from other effects at high test frequencies. 6.11 Characteristic frequency Similar in function to the characteristic parameter, the characteristic frequency was derived by Forster for the setting of test parameters in testing tubes and cylinders. The characteristic frequency is the frequency for which the Bessel function solutions to Maxwell’s equations equals one. Maxwell’s equation describe electromagnetic induction and the Bessel functions are a way of solving them (Figure 6.11). For thick wall tubes and cylinders For thin wall tubes 5066 5066 ° Where: Do = external diameter, cm. t = wall thickness. Where: Do = external diameter, cm. = conductivity in m/ mm2 The equation for thick wall tubes applies when the wall thickness is much greater than δ. For thin wall tubes it applies when the wall thickness is much smaller than δ. (ie standard depth of penetration, mm). Forster’s similarity law states that two eddy current changes will have a similar effect on the coil impedance if their f/fg ratios are the same. Figure 6.12 Eddy current standard depth of penetration. NDT31-50316b Phase Analysis 6-11 Copyright © TWI Ltd Figure 6.13 Eddy current distribution in solid cylinders. , Figure 6.14 Eddy current phase lag with depth. 6.12 Skin effect This term is used to describe the concentration of eddy currents at the surface of the metal, just beneath the test coil. Eddy currents are closed loops of current that flow perpendicular to magnetic flux from the coil but are deflected parallel with the surface contours. NDT31-50316b Phase Analysis 6-12 Copyright © TWI Ltd Each layer of eddy current shields the layer of eddy currents below it, thus reducing the strength of the magnetic field. The intensity of the eddy currents at any particular depth will depend upon the intensity of the eddy current above it, which leads to an exponential decay that can be expressed as: sin Where: lo = eddy current intensity at the surface. ld = eddy current intensity at depth d. δ = depth at which intensity of eddy currents is reduced to 1 of its value at the e surface ie standard depth of penetration (Figure 6.12). d = eddy current depth; e=constant 2.718 (Natural logarithm base). For infinitely thick material and where fields are planar, that is to say induced by large diameter long coils, then the depth at which the intensity of the eddy 1 (approximately 37%) of its value at the surface is given by: currents is e 500 f Where: δ = standard depth of penetration, mm. = conductivity, m/ mm2. = permeability. f = frequency, Hz. (See also Section 2.4.3 – Frequency). At 2δ the intensity is surface. 1 1 (13.5%) and at 3δ (5%) of the intensity at the 2 e e3 This formula is applicable only under strict conditions. For thin wall tubes the current intensity drops less quickly and for solid cylinders inside encircling coils it is always zero at the centre (Figure 6.13). Although the eddy currents may be restricted within thin strata in a component, the magnetic field may extend beyond to generate further skins of eddy current flow. This effect has significant application in, for example, testing wing spars underneath the skin of an aircraft. NDT31-50316b Phase Analysis 6-13 Copyright © TWI Ltd The phase angle of the eddy currents also changes with depth (Figure 6.14). ∝ sin or thick material phase lag d d radians x 57o. At the phase lag is 57°. At 2 it is 114°. This phenomenon allows us to distinguish defects on the inside from those on the outside surfaces of a tube by selecting frequencies at which there is a 90° separation. Figure 6.15 Phase discrimination between lift off and thickness. Figure 6.16 Suppression of undesired effects. NDT31-50316b Phase Analysis 6-14 Copyright © TWI Ltd 6.13 Phase discrimination Successful eddy current test rely to a large extent on adjusting the test frequency to get as wide a phase angle as possible between impedance planes due to unwanted noise and impedance planes due to relevant signals. The impedance diagrams show how a particular frequency has been chosen so that lift-off and thickness variations (Figure 6.15) are at almost 90° to each other. This occurs when t is approximately 0.8. The sensitivity is increased until the vector point display covers the relevant part of the impedance diagram that includes the nominal thickness being measured. The whole diagram is then rotated on the display so that the lift-off plane is horizontal and moves off to the left of the screen. Y movements now correspond to changes in thickness and X movements to unwanted lift-off effects. 6.14 Suppression of undesired effects In eddy current tests using meters and bridge circuits, the undesired effects (Figure 6.16) can be suppressed by selecting a null point for the bridge on the impedance diagram which is at the centre of curvature of a curved impedance plane. Of course impedance planes are rarely circular and so the suppression can only work for a limited number of impedance changes. Another problem is that only one undesired effect can be suppressed when two or even three effects may be contributing noise to the test. The diagrams show bridge null points that have been selected in the right hand diagram to suppress lift-off effects to measure conductivity and in the left hand diagram to suppress conductivity effects in order to measure permeability. A preset lift-off of this nature is used in conductivity meters. Notice however that the lift-off will work satisfactorily only over a limited range of conductivity values. Very high values of conductivity or very low values will require a different null point. NDT31-50316b Phase Analysis 6-15 Copyright © TWI Ltd Figure 6.17 Impedance signals at different frequencies. Figure 6.18 Dual frequency mixing. 6.15 Multifrequency testing The impedance plane diagrams (Figure 6.17) show that the phase angles between impedance planes vary with frequency. The phase angles between liftoff, cracks and permeability may be at one set of values at one frequency and at another set of values at a different frequency. If two test frequencies are used simultaneously in the test coil and displayed separately on the vector point display, then it may be possible to subtract unwanted signals. This is the basis of multifrequency tests. Tests with two frequencies are now common but even five frequencies can be processed and analysed with the help of a computer. NDT31-50316b Phase Analysis 6-16 Copyright © TWI Ltd Firstly two frequencies are selected to give good phase discrimination. The one frequency is likely to be about ten times the other. The phase and sensitivity of the impedances at each frequency are then adjusted independently on the vector point display. In the example shown (Figure 6.18), it is the permeability effect that is to be removed. When the permeability plane at one frequency is 180° out of phase with the permeability plane at the other, then they can be mixed to give a third vector point which has only lift-off (L) and crack planes (C). This can then be adjusted in the usual way so that the lift-off plane is horizontal and off to the left of the screen. In summary, in single frequency tests, one variable can be suppressed using phase analysis. With two frequencies, two variables can be suppressed. With three frequencies, three variables and so on (Figure 6.18). The more frequencies there are, the more complex the electronics and the greater the possibility of extraneous non-relevant signals due to cross-talk between the frequencies. NDT31-50316b Phase Analysis 6-17 Copyright © TWI Ltd Section 7 Instrumentation 7 Instrumentation The instruments used for phase analysis have developed quickly in recent years. Solid state display instruments are improving the portability of eddy current testing equipment. Cathode Ray Tube (CRT) instruments are still in common use and are described in Section 7.1. In the probes, innovative coil arrangements are helping to improve signal to noise separation and induce eddy current fields with improved sensitivity to defects with certain orientations. Dynamic test systems in which signals are modulated by the test speed provide a means of filtering unwanted noise from the test. Figure 7.1 Cathode ray oscilloscope schematic. NDT31-50316b Instrumentation 7-1 Copyright © TWI Ltd Figure 7.2 Lissajous Figure. 7.1 Cathode ray oscilloscopes Phase analysis is carried out with cathode ray oscilloscopes (Figure 7.1). The instruments used in eddy current testing are adapted from those used to investigate networks in electronic and telecommunication engineering. Special purpose instruments in rugged cases and with battery operation are available and because they are easily portable are used for site or field applications. Cathode ray oscilloscopes can be used to measure voltage, current, time, frequency and phase. They have a very high impedance and only a slight loading effect on the circuits to which they are connected. They are capable of resolving signals from a few millivolts to a few hundred volts at frequencies up to 1GHz. The cathode ray tube was dealt with in Section 3.3.3. A schematic design of a cathode ray oscilloscope to give an A-scan vector point or ellipsoid display is shown. For an A-scan the input signal from the bridge circuit that contains the test coil comes in through the Y-amplifier. The attenuator is present to reduce the input signals in discrete measured amounts if measurements are to be made of signal amplitude. To the X-amplifier is fed a saw-tooth waveform from the time-base generator. This drives the beam horizontally across the screen at a fixed repetition rate. To synchronise the flyback of the electron beam to zero, with the input signal so that the next input signal superimposed on the next time base sweep, the pulse generator is also connected to the Y-amplifier. For vector point displays, the time base generator is not needed and the Xamplifier is driven from another signal source. Both the X and Y signals are rectified and averaged. They are derived by splitting the bridge signal into sine and cosine components electronically using the phase angle between the coil voltage and a reference voltage. The sine and cosine components are of course at 90° to each other and form the axes of the impedance diagram display. NDT31-50316b Instrumentation 7-2 Copyright © TWI Ltd If the X and Y signals are both sine waves of the same frequency, a Lissajous Figure is obtained (Figure 7.2). The sensitivities of the amplifiers are adjusted so that the height of the variation of the beam in the Y-direction is equal to the width of the variation of the beam in the X-direction. If both amplifiers drive the beam positive at the same time and negative at the same time, they are in phase and a straight line is seen. If they are 90o out of phase, a circle forms and if 180o out of phase a straight line across the screen in the other direction is formed. If Vx sine ( t ) is the voltage applied to the X-amplifier and Vy sin (t ) is the voltage applied to the Y-amplifier, then when the voltage at the Y-amplifier is zero, t 0 and the voltage across the Y-amplifier is Vx and is given by OB. OA Vx sin sin OB Vx The ratio would therefore give the phase angle between the two signals. Lissajous figures were commonly used in tube and wire testing, where test parameters can be selected so that the ellipse would only open up in the presence of defects. Figure 7.3 Send-receive coils. NDT31-50316b Instrumentation 7-3 Copyright © TWI Ltd Figure 7.4 Hall effect transducer. 7.2 Send-receive coils Send-receive or double coil arrangements are used particularly at the lower test frequencies to reduce temperature drift (Figure 7.3). They can be found in both encircling and surface probes. The magnetomotive force generated by the oscillator in the primary winding . The current is relatively constant despite temperature changes equal because of the high resistance in the circuit. The receiver coils are two coils wound in opposite directions. The secondary circuit is connected to a high impedance amplifier and therefore the induced voltage (proportional to Np ocos t) is not affected by coil resistance either. In the presence of a conductor the receiver coils receive two signals. One is transmitted from the primary coil and the other is reflected from the eddy currents in the conductor. The reflected signal is picked up by the receiver coil near the conductor, but not by the other coil which is too far away. This state of imbalance creates the signal. 7.3 Hall effect probes Detector-coils have the disadvantage of being affected by the frequency as well as intensity of the eddy currents. Hall effect probes will detect eddy current fields and will give a voltage signal which is not frequency dependent. Moreover they can be made very small in size. The hall effect is caused when a magnetic field passes through a special semiconductor material transducer across which a current is flowing (Figure 7.4). The current trajectory is deflected so that electrons tend to build up to one side of the element creating a voltage. This voltage is proportional to the vertical component of the magnetic field. NDT31-50316b Instrumentation 7-4 Copyright © TWI Ltd Figure 7.5 Filters. 7.4 Dynamic testing Many eddy current tests are carried out at a constant test speed so that signals can be modulated and then filtered to improve the signal to noise separation. Applications include tube and wire testing and the rotating probes used in fastener hole inspection. They normally employ differential coils that will generate bipolar signals that can be filtered. The filter arrangement illustrated will only accept one frequency. The input signal first meets an acceptor filter which is a series circuit of capacitance and inductance set to resonate at the required frequency. It therefore has low impedance at this frequency, whereas other frequencies are rejected or diminished depending upon the bandwidth of the filter. The bandwidth is defined between the signal intensities to which they fall to 1 2 of peak and is dependent upon the Q factor (ratio of reactive power to active power) of the series circuit. The second rejector filter, where the capacitance and inductance are resonating in parallel at the required frequency, prevents more of the unwanted frequencies reaching the amplifier. At resonance, the parallel resonance circuit is at maximum impedance and therefore effectively forces the signal into the amplifier (Figure 7.5). 7.5 Frequency response Measures the time required to respond to a signal and is particularly important when testing at high speeds. It is defined as the frequency at which the output signal fall to 1 2 of the maximum. Therefore if the two halves of an encircling differential coil are d mm wide and d the test speed is s mm/sec then the duration of each signal is sec and the s s Hz if sensitivity is to be maintained. This frequency response will need to be d should be regarded as a minimum. A frequency response of twice the signal time is preferable. NDT31-50316b Instrumentation 7-5 Copyright © TWI Ltd Cathode ray tubes have a very high frequency response but it can be reduced drastically with high levels of input signals. Meters give a poor frequency response and should therefore be supplemented with a light emitting diode to give a visible alarm if signals exceed the threshold levels. The frequency response of chart records varies from 100Hz for ink pen types to 1kHz for ultraviolet marker types. NDT31-50316b Instrumentation 7-6 Copyright © TWI Ltd Section 8 Material Sorting 8 Material Sorting There are two properties of metals that can be used for material sorting with eddy current test equipment. The first is electrical conductivity and the second is magnetic permeability. The measurements are purely relative however, relative to some standard for calibration, which must be chosen carefully. Drawing conclusions about the composition or metallurgical characteristics of a testpiece from an eddy test is very difficult because of the diversity of variables which affect conductivity and permeability. NDT31-50316b Material Sorting 8-1 Copyright © TWI Ltd Figure 8.1 Circuit diagram for a conductivity meter. Figure 8.2 Resistivity variations with aluminium alloy contents. Figure 8.3 Cold age hardened Al alloys. NDT31-50316b Material Sorting 8-2 Figure 8.4 Heat damage to aluminium. Copyright © TWI Ltd 8.1 Conductivity meters Conductivity measurements are quite straightforward and do not normally require a phase analysis. A typical bridge circuit with a meter is shown. The bridge is unbalanced as previously described to suppress the effects of lift-off. There are number of causes of error however. If the frequency is low and the material section being tested is thin, then the measurement will be affected by thickness variations and the presence of a different conductor in the substrate. The thickness should be at least 3 . If the frequency is high then surface inhomogeneities, for example thin oxide layers, will interfere with the measurement. Although the double probes used in most conductivity meters are insensitive to temperature changes, the conductivity of the test material and the reference standards will be sensitive to ambient temperature (Figure 8.1). 1 1 (1 T ) 2 Where is the thermal coefficient of resistivity. The conductivities of aluminium reference standards used for calibration have been known to drift over a period of years due to ageing. Surface curvature, edge conductivity readings. effect and other discontinuities will all affect A ferrite meter is an example of a conductivity meter and its application is described in Section 4.2.7. 8.2 Conductivity effects Conductivity can vary with a number of factors. Some of the more useful are shown (Figures 8.2, 8.3 and 8.4). In particular, conductivity measurements can be useful in detecting heat damage in aluminium. Cold working tends to decrease conductivity by introducing dislocations into the metal lattice. Cold working has a very marked effect on measurements taken on austenitic stainless steels, but this is due to increasing permeability rather than changes in conductivity. NDT31-50316b Material Sorting 8-3 Copyright © TWI Ltd Figure 8.5 Frequency selection. Figure 8.6 Fundamental and harmonic spead bands. Figure 8.7 Bainte formation in austenitic. NDT31-50316b Material Sorting 8-4 Copyright © TWI Ltd 8.3 Electromagnetic sorting bridges Electromagnetic (EM) sorting bridges offer a rapid method of sorting ferromagnetic materials. Although the magnetic permeability effects predominate in these tests there is some sensitivity to conductivity changes as well. A bridge circuit is used with two test coils. One contains a standard, the other a testpiece. A CRT with A-scan display shows a sine wave that is the resultant from summing vectorily the sine waves generate in the coils containing the standard and testpiece respectively. If the standard and testpiece are identical then the resultant is a straight line. A degree of difference will produce a fundamental wave the shape of which will be affected by amplitude and phase differences in the two waves. Moreover, the presence of harmonics over and above the fundamental frequency can also be an important distinguishing feature. The sensitivity is adjusted to give the required level of distinction between the grades of material that are being sorted. This is attained by experimentation. Too high a sensitivity will show up differences in every testpiece. Ideally the largest spread bands should be symmetrical about a vertical line on the screen. However, harmonics can be as equally important in distinguishing spread bands as the fundamental waves (Figures 8.5 and 8.6). Better resolution of permeability from conductivity is obtained at low test frequencies. At high frequencies, conductivity and permeability effects may cancel each other out. Conductivity effects are generally the result of compositional changes and can be important. At low frequencies they tend to change the harmonics rather than the fundamental. The permeability of the test material is dependent upon the applied magnetic field strength. On the magnetisation curve, at high levels of (H) the permeability falls and this should be avoided in bridge sorting. Among the variables which affect the EM bridges are: 1 2 3 4 5 Thermal processing. Mechanical processing. Chemical composition. Internal stresses. Temperature. Internal stresses reduce the permeability as a result of magnetostriction. The notable exception is austenitic stainless steel, where working leads to the formation of magnetic bainite (Figure 8.7) which increases the permeability. NDT31-50316b Material Sorting 8-5 Copyright © TWI Ltd Figure 8.8 Automatic bridge sorter. 8.4 Bridge sorting variables Analysis of the wave forms produced, with attention given to the harmonics as well as the fundamental waves, can differentiate materials on the basis of chemical composition, hardness, structure and dimension. In carbon steels, increasing carbon content decreases the conductivity and the permeability but compositional changes are generally overshadowed by heat treatment. In low alloy steels there is a significant fall in permeability with increase in alloy composition. The assessment of case hardening depth is a very important application, but the properties of the core material must remain constant. It is easier to quantify the measurements on induction or flame hardened testpieces than on carburised or nitrided cases, because the former involve changes in the metal structure only and not the composition. 8.5 Automatic gates Bridge sorters can be automated (Figure 8.8) to receive test components on a conveyor and sort them into receiver bins. High test speeds can be attained and because every component can be inspected, the system, when built into a production line, provides a very useful tool in quality control. NDT31-50316b Material Sorting 8-6 Copyright © TWI Ltd In the system illustrated, a standard is held stationary in the reference standard coil while the testpieces move continuously through the test coil. A reading is taken when the testpiece is in the centre of the coil. This corresponds to the point of inversion of the sine waves on the CRT. The test speed must be slow enough for at least 2-3 cycles of the magnetising current to give an impedance signal. For this reason long coils have been developed for systems in which the testpiece drops through the coil. Many automated systems use a vector point display, with the screen divided into quadrants and monitored. An electronic counter counts the number of impedances which occur in each quadrant. At the end of the batch of testpieces the Figures can be analysed statistically to indicate the quality of the batch. 8.6 Standards The choice of suitable standards for material sorting is vital. They should be of the same size and shape, with identical composition and heat treatment and similar surface finish. They should be demagnetised and attention should be given to any stresses which may be set up due to cold working. Allowance should be made for temperature increases due to the induced currents. Finally, play safe and have two or three standards available. NDT31-50316b Material Sorting 8-7 Copyright © TWI Ltd Section 9 Crack Detection 9 Crack Detection Eddy current tests provide the most sensitive of all the non-destructive methods for detecting cracks. However, there are considerations of crack orientation to the eddy current flow to be taken into account and there is very little penetration, particularly in ferrous materials, below the surface. Moreover, it is very easy to misinterpret signals. Two recent developments in weld testing and bolt hole inspection show how eddy current test techniques can be developed to overcome problems with test noise. NDT31-50316b Crack Detection 9-1 Copyright © TWI Ltd Figure 9.1 CRT with flying dot display. Figure 9.2 Surface coil. Figure 9.3 Directional properties of a pancake probe. Figure 9.4 Coil arrangement for scanning rivets. NDT31-50316b Crack Detection 9-2 Copyright © TWI Ltd 9.1 Universal crack detectors For use over a wide range of applications a CRT is used with a vector point display (Figure 9.1). It is connected to a bridge circuit with the test probe containing an absolute coil in one of the arms or a differential coil in two of the arms. The test frequency has to be varied to accommodate different test conditions. Not only is depth of penetration a factor to be considered but also the phase discrimination between relevant impedance planes and unwanted effects have to be maximised. A frequency range of 1kHz-10MHz is common. Most instruments have automatic bridge balancing. The sensitivity control affects the bridge output signal and not the coil current. It may be calibrated in decibels, a scale in which an increase of 6dB is equivalent to doubling the signal amplitude. Some instruments do have controls for the coil current. This compensates for gross imbalances in the bridge when using coils at very low or very high frequencies. The quadrature components of the bridge are generated as sine and cosine phasors of the voltage using the current as datum and dialling in a value for the phase angle from the phase rotation control. The phase rotation control does not give an absolute value therefore. It needs a reference which is often taken as the lift-off plane set horizontally off to the right of the screen. 9.2 Surface coils Bridge circuits require two similar coils. If one senses the testpieces, then it is an absolute coil. If both sense the testpiece it is a differential coil. Differential coils are not sensitive to gradual changes and temperature drift (Figure 9.2). Ferrite cores are needed to increase the inductances of very small coils; they provide a small surface contact area and resist wear. The increase in inductance increases the XL ratio of the coil and therefore reduces the relative importance R of the temperature effects upon R. When using the flatter pancake probes (Figure 9.3), the directional properties of the coil become important. As well as being insensitive to laminar flaws, there is zero sensitivity at the coil centre. There is little sensitivity to flaws parallel with the coil winding, but there is maximum sensitivity to flaws across the coil winding. Gap probes have a magnetic field which shapes the eddy current flow to cross laminar defects. Many ingenious coil arrangements have been developed for special applications. The one shown has four receiver coils dispersed equidistantly around a central send coil. In the balanced defect-free condition, a uniform eddy current field is set up around the send coil. When the coils pass along rivets in an aircraft skin (Figure 9.4), distortions are created which form a regular pattern of signals with the receive coils. When a crack is present, however, an increased distortion is created. By suitable selection of test parameters the vector point movement on the impedance diagram due to normal rivet distortions can be clearly distinguished from a movement due to crack distortions. NDT31-50316b Crack Detection 9-3 Copyright © TWI Ltd Figure 9.5 Slot depth. Figure 9.6 Impedance display of weld toe. Figure 9.7 Rotating probes. NDT31-50316b Crack Detection 9-4 Copyright © TWI Ltd 9.3 Crack detection Surface breaking cracks are not affected by skin depth. They deflect the eddy currents down and around the tip of the crack. They therefore increase the resistive path of the eddy currents and change the orientation of the magnetic field generated by the eddy currents. These have an effect on both the coil’s inductive reactance and resistance. A phase shift can be detected which rotates the impedance plane slightly in a clockwise direction as the cracks become deeper. An interesting phenomenon is observed when comparing edge effect with a slot signal. The edge effect is along a different impedance plane. As another edge is brought up to the coil, therefore simulating a very wide slot which gradually gets narrower, so the edge effect plane moves up towards the slot original. Subsurface cracks rotate the impedance plane clockwise. This phase rotation can be used to assess crack depth below the surface (Figure 9.5). Very tight cracks may leak the current but when all is said and done, of all the NDT methods, eddy current test are the most sensitive to cracks. 9.4 Weld testing The testing of ferrous welds is made difficult by the roughness of the weld cap and changes in the magnetic permeability along the heat affected zone. Eddy current methods offer significant advantages over magnetic particle inspection, however, because paint layers do not have to be removed and troublesome spurious indication in the weld toe can be identified. The permeability variations in the weld toe can be very great due to hardening in the heat affected zone. Conventional vector point displays drift to such an extent therefore, that crack signals become impossible to identify. A recently developed eddy current testing device, however, uses a special coil arrangement to balance out permeability effects. It displays impedance signals from cracks in a manner that is readily discernible from noise due to probe movement to and fro across the weld toe (Figure 9.6). 9.5 Rotating probes Inspection of fastener holes with bolt hole probes is a laborious process. Manual rotation of the probe gives rise to high levels of noise due to wobble. A much more efficient method uses a differential coil that creates a signal that is modulating with the probe rotating (Figure 9.7) at constant high speed inside the hole. An A-scan is created on the CRT for every rotation of the coil inside the hole. By marking it off in degrees from a datum coinciding with a marker on the probe, the angular position of the crack is indicated. By modulating the signal much unwanted noise can be filtered away. The two halves of the differential coil are wrapped in a Figure eight over the forked ferrite core. Cracks often propagate right through the fastener hole and if the halves of the differential coil were opposite each other they would both pass through crack simultaneously and there would not be a signal. NDT31-50316b Crack Detection 9-5 Copyright © TWI Ltd To adjust test frequency and set the phase so that the impedance plane of one unwanted signal is horizontal and does not appear on the A-scan, a vector point is used as an alternative display on the CRT. NDT31-50316b Crack Detection 9-6 Copyright © TWI Ltd Section 10 Tube Testing 10 Tube Testing Eddy current testing has been used to inspect tubes from the very early days in the development of the method. It is rapid, can be made sensitive to a wide range of defect conditions and the equipment can be fully automated. There are two distinct applications; one is for manufactured tube on line and the other is for condenser (and heat exchangers) tubes in situ. The latter in particular has captured much attention of late because of its importance in the maintenance of nuclear (and petrochemical) plant. Figure 10.1 Tube tester with DC saturation. NDT31-50316b Tube Testing 10-1 Copyright © TWI Ltd Figure 10.2 Condenser tube tester. 10.1 Manufactured tube testing Eddy currents have been used in the testing of tube since the 1930s. Most of Dr Forster’s pioneering work was done on the theoretical analysis of eddy current fields in tubes, cylinders and wires. His mathematical solutions were proven by experimenting with glass tubes filled with mercury containing plastic inserts to simulate discontinuities. The equipment used today is highly automated, often computer-controlled and capable of test speeds of up to 6m/sec. The block diagram shows a circuit plan for eddy current test system for inspecting ferrous tubes. The permeability effects of the ferrous material have to be overcome to make the tests sensitive to subsurface flaws. This is accomplished by including DC magnetic saturation coils in the test head,Figure 10.1. The test probe has a double differential coil arrangement. The primary coils also generate a reference voltage that is used to discriminate the phase of the secondary coil voltage. NDT31-50316b Tube Testing 10-2 Copyright © TWI Ltd A Lissajous display is used to calibrate the instrument on a standard tube with reference flaws. Adjustments are made to test frequency, signal amplitude and phase to give the best discrimination between signals and noise. When a defect signal is detected it appears on a strip chart recorder to provide a permanent record. The signal also triggers the paint gun to mark the position of the defect and operate the sorting gate. 10.2 Condenser tube inspection To test condenser tubes (Figure 10.2) in heat exchangers, an internal probe is fired to the end of the tube and retracted at a constant speed of about 200mm/sec. Signal analysis is more complex, particularly in view of the presence of ferrous baffle plates that separate the condenser tubes. Their ferromagnetic properties give a level of noise that may obliterate test signals from corrosion between the plate and tube. Efforts have been made to design multi frequency tests that overcome this problem. The test results of a condenser tube inspection have in the past been recorded as X and Y deflections on a two channel strip chart recorder. The several hundred metres of recorded chart from one heat exchanger are then inspected visually. There can then be a problem in identifying the defective tube in the heat exchanger are then inspected visually. There can then be a problem in identifying the defective tube in the heat exchanger when inspecting the results of several hundred tube tests. This has led to the development of a real time testing system that uses a computer to analyse the test results. A backup recording on a video provides a permanent record. 10.3 Probes For testing manufactured tubes up to 50mm in diameter, encircling coils are used. Beyond 50mm the sensitivity to all but gross defects are reduced and surface coils are needed that orbit the tube. Encircling coils are not sensitive to purely circumferential planar flaws or laminar flaws. The depth of penetration is determined by test frequency except in the case of solid cylinders, where the intensity of eddy current is always zero at the centre despite the test frequency. Most tube tests are carried out with the differential coil arrangements. These are not sensitive to temperature drift and gradual insignificant changes in the tube dimension and are less affected by tube wobble than absolute coils. However, they only detect the ends of longitudinal flaws and will miss entirely uniform longitudinal flaws that extend the whole tube length. This problem has exercised the minds of equipment developers for many years. Where continuous defects can arise, for example in seam welded tubes, then it is evidently not worth the risk in having differential encircling coils. Absolute encircling coils and surface coils, however, are much noisier and test speeds are greatly reduced. To test ferrous tubes for internal defects the tube wall must be saturated magnetically. This can only be done with dc energised fields that are superimposed upon the AC fields of the test coils. Furthermore, the tube has to be demagnetised afterwards. The conventional diminishing AC field will only demagnetise the surface because of skin effect and so a slowly reversing DC field is necessary. This reduces test speeds considerably. NDT31-50316b Tube Testing 10-3 Copyright © TWI Ltd Figure 10.3 Forster curve for tube. Figure 10.4 f90 frequency. Figure 10.5 Phase angle changes at f90 frequency. NDT31-50316b Tube Testing 10-4 Copyright © TWI Ltd 10.4 Test frequency The test frequency is the most important variable used in controlling the eddy current test. It determines the depth of penetration of the eddy current field and the phase discrimination between noise and signals. For setting the frequency when testing manufactured tube for through defects the Forster curves are often employed (Figure 10.3). These were derived by plotting the f ration on a normalised impedance diagram. f is the test fg frequency and fg is the characteristic frequency defined by: 5066 Where: = Relative permeability. Do t = Conductivity, m / mm 2. = External diameter, cm. = Wall thickness, cm. A similar formula is used to define the characteristic frequency of solid cylinders. fg 5066 d 2 Where d is the diameter of the cylinder, cm. According to Forster’s similarity law, geometrically similar defects result in the same eddy current effect if their f ratios are the same. If for example, a fg particular defect in one tube gives a particular eddy current signal, it will give the same signal in a tube of different diameter if the f ratio is adjusted to the fg same value. Balanced sensitivity to defects, conductivity changes and dimensional changes are obtained on the knee of the f curve, where the ratio is approximately fg equal to six for solid cylinders. To distinguish ferromagnetic inclusions, then an f of about two provides an fg almost 90o separation between permeability effects and conductivity changes. NDT31-50316b Tube Testing 10-5 Copyright © TWI Ltd For testing condenser tubes with internal bore probes, it is important to distinguish between thinning of the tube wall due to corrosion on the inside surface, from that due to corrosion on the outside surface. This is done at the f90 frequency, Figure 10.4 and 10.5. To an internal coil, thinning from the outside of the tube is seen as a reduction in wall thickness, while thinning from the inside of the tube is seen as a reduction in fill factor. A frequency can be selected where these two effects have impedance planes at 90o to each other. It occurs when the nominal wall thickness is approximately 1.1 of the standard depth of penetration: f90 3 kHz t2 Where: = Resistivity in t .cm. = Wall thickness in mm. As can be seen in the normalised impedance diagram, the phase angle between the internal and external slots varies from a few degrees at low frequencies, to also 180o at high frequencies. The impedance plane for a hole occurs between the slots. It coincides with the impedance plane, when thinning from both the inside and outside surfaces meet and the tube wall disappears. There is a relationship between amount of thinning and the phase angle that can be used to determine the residual wall thickness. Figure 10.6 Coil dimensions. NDT31-50316b Tube Testing 10-6 Copyright © TWI Ltd Figure 10.7 Tube inspection signal patterns. 10.5 Coil size The closer the coil fits the tube, the higher will be the magnetic coupling between coil and tube and therefore the greater the sensitivity of the test. A tight fit cannot be used because either the tube must be free to move inside the coil or the coil inside the tube. The fill factor is used as a measure of coupling. 2 D t Dc for encircling probes D c Dt for internal probes 2 Where: Dc = coil diameter. Dt = tube diameter. =fill factor. Probe damage is a constant problem where tubes have to be fed through encircling coils at high speed. Internal probes often get stuck inside condenser tubes due to dents. For these applications, fill factors no better than 0.7 are used. Ideally as in the diagram shown (Figure 10.6), the fill factor should be such that the gap between coil and tube should be approximately half the wall thickness. With differential coils, the distance between the differentially wound halves of the coils should be considered because along with the test speed, it will determine the frequency of signals. These must not exceed the frequency response of the instrument and will determine the bandwidths of the filters. NDT31-50316b Tube Testing 10-7 Copyright © TWI Ltd 10.6 Signal patterns The diagram illustrates the signal patters which may be derived from passing an absolute and a differential coil probe through a stainless steel condenser tube (Figure 10.7). The signals are derived form an internal slot, an external slot, a through drilled hole, the ferrous baffle plate, a magnetite deposit and a dent. The differential coil gives characteristic petal-shaped impedance patterns. As the leading coil passes the defect, the vector point extends around one petal, coming back to the origin when the defect is between the coils, before extending around the petal in the opposite quadrant. Although differential signals are more difficult to analyse, there is no drift in the balanced vector point that can be expected when using absolute coils. Condenser tube inspection is conducted at the f90 frequency. The impedance diagram is rotated so that the impedance planes for slots on the inside surface are horizontal and slots on the outside surface are vertical. X and Y movements in the vector point are then recorded on a two channel strip chart recorder. By comparing the pen movements on the X and Y channels, the different flaws can be distinguished. 10.7 Reference standards Eddy current instruments for testing tubes must be calibrated with tubes containing reference flaws. These are usually machined flats, longitudinal EDM (electrodischarge machined) slots, circumferential EDM notches and drilled holes. These reference standards should be easy to fabricate, reproducible, precisely sized and should closely resemble the natural flaw. Drilled holes are more commonly used where through defects that will cause leaking are sought. Machined flats are more suitable for detecting thinning. If used to set the f90 frequency, they will need to be on both the inside and outside surfaces. NDT31-50316b Tube Testing 10-8 Copyright © TWI Ltd Section 11 Eddy Current for Welding Inspection 11 Eddy Current for Welding Inspection 11.1 Introduction Traditionally surface crack detection in ferritic steel welds with eddy current techniques has been difficult due to the change in material properties in the heat affected zone. These typically produce signals much larger than crack signals. Sophisticated probe design and construction, combined with modern electronic equipment, have largely overcome the traditional problems and now enable the advantages of eddy current techniques to be applied to in-service inspection of ferritic steel structures in the as-we!ded conditions. Specifically, the advantage of the technique is that under quantifiable conditions an inspection may now be carried out through corrosion protection systems. This means the costly removal and replacement of the protective coating is now not necessary. An additional advantage is that, on detection of surface breaking defects, the amplitude of the signals obtained, given the appropriate corrections for coating thickness, geometry etc. can be compared directly with the slots in the calibration block, therefore enabling decisions on appropriate remedial action to be taken immediately. The general principles of Eddy Current, Non Destructive Testing are described in BS EN ISO 15549. NDT31-50316b Eddy Current for Welding Inspection 11-1 Copyright © TWI Ltd 11.2 Eddy current application overview Eddy current testing is based on inducing electrical currents in the material to be inspected and observing the interaction between these currents and the material. The process is as follows: Figure 11.1 Test material – conductor. 1 2 3 4 5 When a changing magnetic field intersects an electrical conductor, eddy currents are induced according to Faraday’s and Ohm’s Laws. Consider this to be the excitation or primary magnetic field. The induced electrical currents (known as eddy currents because of their closed circulatory path) generate their own magnetic field. Consider this to be the secondary magnetic field. This secondary magnetic field opposes the primary magnetic field and an equilibrium results. The primary field is changed – therefore the electrical properties of the coil are changed – specifically the property known as the Electrical Impedance. By monitoring the changes in coil impedance the electrical, magnetic and geometric properties of the component can be measured. NDT31-50316b Eddy Current for Welding Inspection 11-2 Copyright © TWI Ltd Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux, Figure 11.1. They normally travel parallel to the coils windings and parallel to the surface. The shape of the induced eddy currents reflects the shape of the coils. Coils parallel to the surface will induce circular eddy currents. In the weld probe the coils sit on their rim resulting in an oval shape eddy current field. Eddy current flow is restricted to the area affected by the primary magnetic field. The depth of penetration of the induced eddy currents depends on a number of variables; Electrical resistivity or electrical conductivity (electrical resistivity and electrical conductivity are reciprocal of each other). Magnetic permeability. Test Frequency. Phase lag is a key parameter in eddy current testing. Phase lag depends on the same material properties as that governing standard depth of penetration. Phase lag β X 50√ρ/fμ radians Where x is the distance below the surface in mm. At one standard depth of penetration the phase lag is 57° at two standard depths of penetration the phase lag would be 114°. NDT31-50316b Eddy Current for Welding Inspection 11-3 Copyright © TWI Ltd 11.3 Basic eddy current theory The basic equipment required to produce eddy currents consists of: Source of an alternating current (AC) called an oscillator. Probe containing a coil – usually of insulated copper wire. Volt meter to measure the voltage (potential drop) across the coil. Figure 11.2 Basic Eddy current Test Equipment The oscillator usually is capable of generating a time varying (Alternating in direction – usually sinusoidal) current at frequencies ranging from about 1,000 cycles per second (1kHz) to about 2,000,000 cycles per second (2MHz). Special applications may generate higher or lower frequencies or even use pulsed currents. The probe coil has many variables and must be specific to the application. These variables include: Wire diameter. Number of turns. Coil diameter. Length of coil. NDT31-50316b Eddy Current for Welding Inspection 11-4 Copyright © TWI Ltd There are several configurations of surface probe. These, again, must be considered specific to the application. In general terms, surface probes may be one of the following: Single coil (Self-Inductance). An excitation coil with a separate receiving (sensing) coil. (Send-Receive). An excitation coil with a Hall-Effect sensing detector. (Magnetic Reaction). These are illustrated below: Voltmeter Voltmeter Voltmeter Oscillator Oscillator Oscillator Excitation coil Excitation coil Hall Effect Sensor Coil Test Material Self-Inductance Sensing coil Test Material Send-Receive Test Material Magnetic Reaction Figure 11.3 Surface probe types. The voltmeter measures changes in the voltage across the coil. These changes may be the result of: Changes in electrical conditions and material properties such as: Electrical conductivity (resistivity). Magnetic permeability. Geometry of the component. Material dimensions. Relative position between the coil and the material being tested. This voltage change consists of both an amplitude variation and a phase variation relative to the current passing through the coil. 11.4 Generation of eddy currents Magnetic field around a coil When an electric current flows through a conductor, a magnetic field is set up around the conductor in a direction at 90 to the electric current. This is explained by maxwell’s right hand rule. If the thumb of the right hand is extended in the direction in which the current is flowing, then the direction of the magnetic field is represented by the fingers. NDT31-50316b Eddy Current for Welding Inspection 11-5 Copyright © TWI Ltd Figure 11.4 Right hand rule. When the conductor is ferromagnetic, strong magnetic flux lines are created, also in the direction of the fingers, this is called circular magnetism. Circular magnetism is not polar and cannot be detected externally on a round symmetrical specimen. Now, if the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction. Associated with the magnetic field is the magnetic flux density. This is the number of lines of force or maxwells, as they are called in cgs units, per unit area. The unit of flux density in SI units is the Tesla (T). A Tesla is 1weber per square metre (Wb/m2). 1 weber is 100,000,000 maxwells or lines of force. The Tesla replaces the Gauss. 1 Gauss is 1 magnetic line of force per cm2. There are 10,000 (10kG) Gauss in 1 Tesla. Or 10 Gauss = 1 Tesla. Flux density is in the same direction as the magnetic field and its magnitude depends on its position and amplitude of the current flowing through the conductor. Flux density is therefore a field vector quantity and is given the symbol B. NDT31-50316b Eddy Current for Welding Inspection 11-6 Copyright © TWI Ltd Figure 11.5 Current flow along a straight conductor. NDT31-50316b Eddy Current for Welding Inspection 11-7 Copyright © TWI Ltd Figure 11.6 Magnetic flux distribution – single turn coil. Flux density B varies linearly with electric current in the coil ie. if coil current doubles the flux density doubles everywhere. The total magnetic flux Φp contained within the loop is the product of B and the area of the coil. The unit of magnetic flux in the SI system is the weber (Wb). NDT31-50316b Eddy Current for Welding Inspection 11-8 Copyright © TWI Ltd Figure 11.7 Longitudinal magnetic flux generated from a current carrying coil. The field within the loop has direction and one side will be a north pole and the other a south pole. By increasing the number of loops, a coil, or solenoid, is created and the strength of the field passing through the coil is proportional to the current passing through the conductor in amperes multiplied by the number of turns in the solenoid. When a ferromagnetic material is placed in an energised coil, the magnetic field is concentrated in the specimen. One end of the specimen is a north pole and the other south pole. This is called longitudinal magnetism. Longitudinal magnetism has polarity and is therefore readily detectable. Only one type of field can exist in a material at one time; the stronger will wipe out the weaker. Normally in magnetic particle inspection, circular tests are carried out before longitudinal ones. NDT31-50316b Eddy Current for Welding Inspection 11-9 Copyright © TWI Ltd Current Flow N – North Pole – Anti-clockwise Current Flow S – South Pole – Clockwise Figure 11.8 Looking at the ends of the coil – direction of current flow. 11.5 Principles governing the generation of eddy currents The three major principles or laws governing the generation of eddy currents are: Ohm’s Law. Faraday’s Law. Lenz’s Law. Ohm’s law Ohm discovered that the amount of current flowing through a material varies directly with the applied voltage and inversely with the resistance of the material. I V R Where: R is in Ohms () V is in volts (V) I is in amps (A) A simple way of remembering Ohm’s law is to draw it in circular form. Quantities on either side of the vertical line are multiplied, while quantities below the horizontal line are divided into quantities above it. To use the circle, simply cover the segment you want to find and the position of the remaining letter tells you the procedure to follow. NDT31-50316b Eddy Current for Welding Inspection 11-10 Copyright © TWI Ltd V I R Figure 11.9 Ohm’s law in circular form. In any electrical circuit, current flow is governed by Ohm’s Law and is equal to the driving (primary circuit) voltage divided by primary circuit impedance. In an electrical circuit, Impedance is defined as the total opposition to flow of alternating current (AC). Impedance represents the combination of those electrical properties that affect the flow of current through the circuit. The application of Ohm’s Law to an alternating current (AC) circuit gives the formula: Z V I Where: Z is the circuit impedance in Ohms (). V is the voltage in volts (V). I is the current in amps (A). NDT31-50316b Eddy Current for Welding Inspection 11-11 Copyright © TWI Ltd Figure 11.10 Test material – conductor. The eddy current coil is part of the primary circuit. From Oersted’s discovery, a magnetic flux Φp exists around a current carrying coil proportional to the number of turns in the coil (Np) and the current (Ip). Faraday’s laws Faraday discovered the inductive effects of rapid changes in the magnetic field. When current is abruptly switched off in an electrical circuit it will induce an electromotive force which, if magnetically coupled to another electrical circuit, will create a current in that circuit. In Figure 11.11 - When the battery is disconnected in circuit A, the light in circuit B flashes for an instant. Similarly when the battery is reconnected and the current is building up in circuit A, so the bulb in circuit B flashes. While current is flowing steadily in circuit A, the light in B is off. The two circuits are not linked electrically but the magnetic field around circuit A does link through circuit B. NDT31-50316b Eddy Current for Welding Inspection 11-12 Copyright © TWI Ltd Faraday went on to define two laws: 1 Whenever a magnetic field linking a circuit is changed, it sets up an electromotive force. 2 The amplitude of this induced electromotive force is proportional to the rate of change. Figure 11.11 Faraday’s experiment. Lenz’s law This law states that the electromotive force (emf or voltage) induced by the variation in magnetic flux is always in such a direction that if it produces a current (Is) the magnetic effect of that current opposes the flux variation (Φp) responsible for both the electromotive force and the current. Summary Magnetic flux is created by passing alternating current (AC) through the test coil. When this coil is brought into close proximity to a conducting material, eddy currents are induced. The flow of eddy currents results in resistive (Ohmic) losses. In addition, the magnetic flux associated with the eddy currents opposes the coil’s magnetic flux, thereby decreasing the net magnetic flux. This results in a change of coil impedance and subsequent voltage drop across the coil. It is the opposition between the primary (coil) and secondary (eddy currents) fields that provide the basis for extracting information during eddy current testing. 11.6 Fundamental Properties of eddy current Flow Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux generated by the probe coil. They normally travel parallel to the coil’s windings and parallel to the surface of the component being tested. Eddy current flow is limited to the area of influence of the inducing magnetic field, see Figue 11.12. Frequency The most important test variable is the frequency of the current sent through the test coil. Eddy current testing is conducted at frequencies from a few hertz (Hz) to several megahertz (MHz). NDT31-50316b Eddy Current for Welding Inspection 11-13 Copyright © TWI Ltd The most important effect of test frequency is upon the depth of penetration of the eddy current field. As the frequency increases so the depth of penetration decreases. The phenomenon known as skin effect is described as follows: Figure 11.12 Standard depth of penetration. Eddy currents induced by a changing magnetic field concentrate near the surface of the test material adjacent to the excitation coil. The depth of penetration decreases with increasing test frequency and is a function of electrical conductivity (σ) and magnetic permeability (µ) of the test material. The eddy currents flowing in the test material at any depth produce magnetic fields, which oppose the primary magnetic field produced by the excitation coil. The net magnetic field is therefore reduced thus decreasing the current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil’s magnetic field thereby weakening the magnetic field at greater depths and reducing induced eddy currents. Skin Effect can be defined by the formula: Standard depth of penetration: δ = 50 / mm or δ = mm Where: ρ is electrical resistivity. Units are microhm-centimetres (µΩ.cm). σ is the electrical conductivity in siemens/metre. f is the test frequency in hertz (Hz). µr is relative magnetic permeability, no units – dimensionless. µ is the absolute permeability of the material, Henry/metre. NDT31-50316b Eddy Current for Welding Inspection 11-14 Copyright © TWI Ltd For air and non-magnetic materials, µ is constant and is denoted by µ0. µ0. = 4π x 10-7 teslas or Henries/metre For ferromagnetic materials it varies considerably according to the value of H, the magnetic field strength. For convenience we use relative permeability µr Where: µr = µ / µ0. Relative permeability is therefore a dimensionless permeability of the material to that of air. ratio, relating the Or use the following formula: 660 Where: is the standard depth of penetration in mm. f is the frequency in hertz. is the conductivity in IACS (International Annealed Copper Standard). is the relative permeability. / . IACS = 5.8 10 Iron = ~ 18% IACS. Low Alloy Steel = ~ 11% IACS. BS EN ISO 12718 gives an alternative formula for . 1 f Where: is in cm. = 3.14. f is the frequency in hertz. is the conductivity in % IACS. = 4 x 10-7 H/m. The standard depth of penetration is defined as the depth below the surface at which the intensity of the eddy current field has been reduced to a value of of its intensity at the surface. The function e is the base of natural logarithms. It is equal to 2.718 when taken to three decimal places. Therefore at the standard depth of penetration, the eddy current field intensity is at approximately one third of its surface value. (37%). Phase lag is a key parameter in eddy current testing. Phase lag depends on the same material properties as that governing standard depth of penetration. Phase lag β √ / NDT31-50316b Eddy Current for Welding Inspection 11-15 Copyright © TWI Ltd Where x is the distance below the surface in mm. At one standard depth of penetration, the phase lag is 570° at two standard depths of penetration the phase lag would be 1140°. Standard Depths of Penetration as a function of frequency for various test material are illustrated below: Figure 11.13 Standard depths of penetration. Sensitivity to defects depends on eddy current density at the expected defect location. Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (2), eddy current density has decreased to (1/e)2 or 13.5% of the current density at the surface. At three standard depths of penetration (3), the eddy current density is down to 5% of the surface density. However, keep in mind that these values only apply to thick materials (thickness greater than 5) and planar magnetic excitation fields. Please also note that the magnetic flux is attenuated across the test material but not completely. Although the currents are restricted to flow within specimen boundaries, the magnetic field extends into the air space beyond. This allows the inspection of multi-layer components such as aircraft wings. The sensitivity to a sub-surface defect depends on the current density at that depth. It is therefore important to know the effective depth of penetration. The effective depth of penetration is arbitrarily defined as the depth at which eddy current density decreases to 5% of the surface density. For large probes and thick samples this depth is about three standard depths of penetration (3). Unfortunately for most components and practical probe sizes, this depth will be less than 3, the eddy currents being attenuated more than predicted by the skin depth equations. NDT31-50316b Eddy Current for Welding Inspection 11-16 Copyright © TWI Ltd 11.7 Electrical circuits and probe impedance Eddy current testing applications consist of monitoring the flow and distribution of eddy currents in test material. This is achieved indirectly by monitoring probe (coil) impedance during the inspection. It is therefore necessary that an appreciation of impedance and associated electrical factors Is gained. Resistance (R) The opposition to current flow in direct (DC) and alternating (AC) circuits is called the resistance. It is rather like friction in mechanics. It opposes the flow of electrons and generates heat. Where: R = Resistance in Ohms (). = Resistivity in micro-ohms cm. = Length of conductor. A = Cross-sectional area of conductor. Ohm’s Law may be applied: R V I Where: V is the voltage drop across the resistor (Volts). I is the current through the resistor (Amps). 900 Voltage Current 0 0 1800 0 3600 2700 Figure 11.14 Voltage and current through a resistor. NDT31-50316b Eddy Current for Welding Inspection 11-17 Copyright © TWI Ltd 11.8 Resistance and reactance The resistance in an AC circuit represents a loss of electrical energy as heat, as it does in a DC circuit. In an AC circuit however, there are two other components which oppose the flow of current and these are called reactances. One is the capacitive reactance, which creates a voltage across a capacitor and the other is the inductive reactance which creates a voltage across an inductor (coil). The capacitor converts current into electrostatic energy and the inductor converts current into magnetic energy. As the energy is reconverted to current when the polarity of the circuit current reverses, neither of the reactances represents an actual loss in electrical energy. The effect of the capacitance and inductance in the circuit is to push the voltage and current out of phase with each other, either lagging or leading as follows: a) b) c) In an AC circuit with only resistance, current and voltage are in phase (Figure 11.14). In an AC circuit with only inductance, current and voltage are out of phase by 90, with voltage leading current (Figure 11.15). In an AC circuit with only capacitance, current and voltage are out of phase by 90 with voltage lagging current (Figure 11.16). An aid to memorising these is: C I Capacitance – current leads voltage 11.9 V I L Voltage leads current in Inductance Inductive reactance Opposition to changes in alternating current (AC) flow through a coil is called inductive reactance. The symbol for Reactance is (X). For inductive reactance the symbol is (XL). Inductive reactance is calculated using one of the following formulae: XL = ωL OR XL = 2πfL - unit is ohms (Ω) Where: f is the frequency of alternating current (Hz). ω is the angular frequency in radians/second. NDT31-50316b Eddy Current for Welding Inspection 11-18 Copyright © TWI Ltd Figure 11.15 Voltage Current across an Inductor. 11.10 Capacitive reactance Opposition to changes in alternating current (AC) across a capacitor is called capacitive reactance. The symbol for Reactance is (X) for capacitive reactance the symbol is (Xc). Eddy current coil capacitive reactance is normally negligible, however, capacitance can be important when considering the impedance of probe cables. NDT31-50316b Eddy Current for Welding Inspection 11-19 Copyright © TWI Ltd Capacitive reactance is calculated using: XC 1 unit is ohms 2fC Where: f is the frequency of alternating current (Hz). C is the capacitance – unit is the farad. Figure 11.16 Voltage and current across a capacitor. NDT31-50316b Eddy Current for Welding Inspection 11-20 Copyright © TWI Ltd 11.11 Impedance The total opposition to alternating current (AC) flow is called Impedance. The symbol for impedance is Z. For a coil impedance is calculated using: X R 2 XL 2 XL Z XT = (XL- XC) R XC Figure 11.17 Impedance may be represented in a vector diagram. 11.12 Inductance (L) The ability of a coil to store magnetic energy and oppose changes in the current is called inductance: L R N2 A I Where: L is the inductance in henrys. R is the geometric factor. N is the number of coil turns. A is the coil’s planar surface area in mm2. I is the coil’s axial length. The henry is a very large unit. Eddy current coils have inductances of a few micro-henrys (µH). Inductance is a property of only those electrical circuits where the current is varying. The opposition to current flow generates a voltage or self-inductance in the circuit but it can also generate a voltage in a neighbouring circuit through mutual-inductance. The latter is the transformer principle. NDT31-50316b Eddy Current for Welding Inspection 11-21 Copyright © TWI Ltd The self-inductance of a coil is proportional to the square of the coil windings ( ) and planar surface area (A) and inversely proportional to coil length (l). 11.13 Eddy current weld testing When considering the use of Eddy current Techniques for coated welds there are a number of variables to assess prior to choosing specific pieces of equipment. These are as follows: Suitable Eddy current probes/coils. Material. Coatings. Weld Geometry caused by the weld profile. With reference to Figure 11.18 coated weld section, the variables are reasonably self evident. The coating thickness varies considerably, the thickest section being on the bottom toe of the weld, exactly where we would expect our in-service defects such as fatigue cracks to occur. 1 2 3 4 5 6 8 7 'Lift-off' signal corresponding with coating thickness. 3&6 4 7&8 1&2 'Lift-off' signal horizontal 5 0 Figure 11.18 Coated weld section – variation in sensitivity dure to application of protective coatings. NDT31-50316b Eddy Current for Welding Inspection 11-22 Copyright © TWI Ltd The thickness will also change along the length of the weld as the geometry changes. The K-Node found Offshore is used as a typical example, see Figure 11.19. Figure 11.19 Typical K node configuration. It is therefore necessary to ensure that the technique chosen is capable of the following: 11.14 Evaluating the material to be tested. Measuring the coating thickness in order that the full extent of the problem is quantified and evaluating the constituents of the coating. The sensitivity of the equipment is capable of being adjusted in order to compensate for the maximum coating thickness noted in the previous exercise. The resolution of the equipment is sufficient to distinguish between the signals generated by the defects sought and the background noise caused by the surface conditions (profile and/or roughness) of the weld and adjacent areas. Probes/coil arrangements The first consideration must be access. Is it possible to get to the area of interest? Let us look at the vast range of coils used in everyday applications and try to work our way through them until suitable probes/coil arrangements are identified. NDT31-50316b Eddy Current for Welding Inspection 11-23 Copyright © TWI Ltd Types of inspection probes/coils In general, we can categorise probes into two distinct applications: The surface probe, see Figure 11.20. Encircling or through probe, see Figure 11.21. Inside coil, see Figure 11.22. Test coil arrangements For our purposes we can sub-divide these as follows: Single coil (Absolute), see Figure 11.23. Differential coil, see Figure 11.24. The relative advantages or disadvantages and applications of each type of coil arrangement is dealt with elsewhere in the notes so for the purpose of this exercise we shall only consider surface probes. We are immediately drawn to the pencil probe. It is very versatile. It may be formed into numerous shapes and sizes to meet most weld configurations. The basic components of the Pencil Probe are as follows: Single Coil, Absolute Arrangement. In this arrangement the same coil is used to induce eddy currents in the component and to sense the component's reaction on the eddy currents. The single coil will test only the area under the coil and does not compare itself with a reference standard. These probes generally have small coils and operate at relatively high frequencies. The pencil probes we shall assess have ferrite cores. These are used to induce a greater magnetic flux and eddy current field. Let us draw up a specification and/or check list for evaluation of the probes: Material Evaluation: Is the probe suitable for this purpose? Coating Thickness Measurements: Is the probe capable of measuring the coating thickness to be found on components to be tested? The varying constituents of the coatings must also be subject to some thought. Are any of the layers which make up the coating conductive? What effect, if any, will these conductive layers have on the coating thickness checks? Surface Crack Detection: Are we capable of detecting surface breaking defects in carbon steel? What size of defect are we capable of detecting and under what circumstances? Are we capable of detecting these defects under typical coatings found in the field? In other words we must define the limitations of the instrumentation/probe coil combination and systematically build up a reasonable specification and testing procedure for the instrumentation/probe coil combination which will allow reproducibility of test and results. We have developed a series of practical exercises to try and demonstrate the various topics discussed previously. It may also be possible to quantify some of the limitations of the system by completing the exercises. NDT31-50316b Eddy Current for Welding Inspection 11-24 Copyright © TWI Ltd Figure 11.20: a Schematic of eddy current surface probe; b Surface probe and the effect of non-conductive coating thickness on eddy current distribution in the test material. Figure 11.21 Schematic of eddy current encircling coil probe showing the primary excitation coil and the secondard pick up coil. Figure 11.22 Schematic of eddy current inside coil bobbin probe showing defect detection in a non-ferrous tube. NDT31-50316b Eddy Current for Welding Inspection 11-25 Copyright © TWI Ltd Figure 11.23 Schematic of eddy current single coil probe showing the effect of a crack on eddy current distribution (right) compared to a defect free distribution (left). TEST INSTRUMENT Figure 11.24 Schematic of eddy current single coil self-comparison differential probe. NDT31-50316b Eddy Current for Welding Inspection 11-26 Copyright © TWI Ltd Appendix 1 Sample Instruction - Amplitude Analysis, Full Length, Internal Defects. Scope: This instruction is documented as the process by which Condenser Tubes are inspected over their Full Length for Internal Defects using the Eddy Current Amplitude Analysis method with a differential coil. The signals are recorded/printed out and the defects classified. Document reference and status: TWI/ET/Tubes/AAFLID/1/UK Component identification number: Training Bundle, AAFLID/TB1/UK Description (incl material and dimensions): 90/10 Copper Nickel Tubes 1.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect Internal defects Area to be tested: Full Length of 1.5m tubes Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Differential bobbin probe, 11mm diameter. Part No. PID110N05R20K. This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.42m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with thru holes to simulate internal metal loss. Part No. CEGB, ESI Type A. See Figure A1-1. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper width. Tools - Flexible measuring tape. NDT31-50316b Sample Instructions A1-1 Copyright © TWI Ltd Pre-test: Ppreparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency: 20KHz. X Y gain: Set 1:1. X Y position: Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence: As required. Phase: Set to 0° initially. Lo pass filter: 50. Balance: Off. Flaw detector initial calibration Phase: Set so that defects along Y axis are initially negative going. Sensitivity: Set to achieve an 80% FSH vertical deflection from simulated defect No. 6 (8 x 0.65mm holes). Chart recorder settings – channel 1 only Chart speed 15mm/sec. Pen position Pen is to be positioned centrally (five main-scale division from top and five main-scales from bottom). Amplitude 5v up and 5v down. Chart recorder final sensitivity Phase Check defect signals are initially negative going. Amplitude Set to achieve an 80% deflection from simulated defect No. 6 (8 x 0.65mm holes). NDT31-50316b Sample Instructions A1-2 Copyright © TWI Ltd Detailed instructions for application of test (continued) 1 Calibration recording: With calibration as above, make a recording/print-out of the 6 sets of simulated defects in Calibration tube type A. Probe should be withdrawn at a steady rate and the trace should show outlet and inlet signals at either end of calibration trace. Identify trace as ‘Calibration In’. 2 Inspection: Carry out an inspection of first tube in a similar manner, ensuring that scan speed is constant and that both outlet and inlet signals are produced at either end of calibration trace. 3 Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded. 4 Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position. 5 Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... NDT31-50316b Sample Instructions A1-3 Copyright © TWI Ltd Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: Cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-conformance statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed. NDT31-50316b Sample Instructions A1-4 Copyright © TWI Ltd Recording and classifying results: Action to be taken when defects are detected or no fault found: Classify calibration tube - Using the report sheet attached, enter the signal amplitudes obtained from the calibration tube. The smallest amplitude, (obtained from the single hole) should be classified as class limit 1 and the others as class limit 2, 3A, 3B, 4 and 5 respectively. Defect classification - All defect indications appearing on the chart recorder paper trace, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Assess class of defect by referring to calibration tube class limits. Defects > 8 cross drilled holes = class 6. All tubes with defects equal to or greater than class 4 are to be retested and reported separately. Ensure ‘calibrations ‘in’ and ‘out’ are repeated for this retest. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, and defect amplitude, classification and positional information with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe PCN/Level 2/ET/Wrought Tubes Tubby Pipe 23rd July 2013 Authorising person's details: Ted Brass PCN/Level 3/ET/Wrought Tubes Ted Brass 23rd July 2013 8 6 3 4 2 1 D=0.6 14.2mm OD 0.42m Figure A1.1 Calibration tube CEGB ESI Type A. NDT31-50316b Sample Instructions A1-5 Copyright © TWI Ltd Eddy Current Tubes Inspection Results Recording Sheet Full Length Test – Differential Mode – Internal Defects Name: Date: EQUIPMENT USED Flaw Detector: Serial No: Class Limit Sample: CALIBRATION Amplitude Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle: Reference Tube: DEFECT SIGNAL TUBE No. Amp. mm. COMMENTS Class Location RETESTS NDT31-50316b Sample Instructions A1-6 Copyright © TWI Ltd Appendix 1B – Sample Instruction - Amplitude Analysis, Inlet End, Internal Defects Scope: This instruction is documented as the process by which Condenser Tubes are inspected at the Inlet end for Internal Erosion/Thinning using the Eddy Current Amplitude Analysis method with an absolute coil. The signals are recorded/printed out and assessed against a calibration tube graph of amplitude and thinning. Document reference and status: TWI/ET/Tubes/AAIEID/2/UK Component identification number: Training Bundle, AAIEID/TB2/UK Description (incl material and dimensions): 90/10 Copper Nickel Tubes 0.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect internal material loss in the tubes, coincident with the ends of the inserts, due to erosion. Area to be tested: Inlet ends only, in vicinity of where 6" or 7.5" venture inserts may have been fitted. Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Absolute bobbin probe, 11mm diameter. Part No. PID110N05R20K This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.7m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with tapered external annuli to simulate internal metal loss. Part No. CEGB, ESI Type D, see Figure A1.2. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper width. Tools - Flexible measuring tape. NDT31-50316b Sample Instructions A1-7 Copyright © TWI Ltd Pre-test: preparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency: 10KHz. X Y gain: Set 1:1. X Y position: Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence: As required. Phase: Set initially to 0°. Hi pass filter: Off. Lo pass filter: 30. Balance: 120μΗ. Flaw detector initial calibration Phase and Set to achieve a 50% FSH vertical deflection from simulated defect Sensitivity: 50% material loss. Chart recorder settings Chart speed: 15mm/sec. Pen position: Pen is to be positioned centrally (five main-scale division from top and five main-scales from bottom. Amplitude: 5v up and 5v down. Chart recorder final sensitivity Set to achieve an 50% deflection from simulated defect 50% material loss. NDT31-50316b Sample Instructions A1-8 Copyright © TWI Ltd Detailed instructions for application of test (continued) 1 Calibration recording: With calibration as above, make a recording/print-out of the 5 sets of simulated defects in Calibration tube type D. Probe should be withdrawn at a steady rate and the trace should show outlet and inlet signals at either end of calibration trace. Identify trace as ‘Calibration In’. 2 Inspection: Ensure null balance obtained in each tube prior to scan. Carry out an inspection of first tube in a similar manner, ensuring that scan speed is constant and that both outlet and inlet signals are produced at either end of calibration trace. 3 Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded. 4 Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position. 5 Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... NDT31-50316b Sample Instructions A1-9 Copyright © TWI Ltd Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: Cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-Conformance Statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed. NDT31-50316b Sample Instructions A1-10 Copyright © TWI Ltd Recording and classifying results: Action to be taken when defects are detected or no fault found: Calibration tube – Using the report sheet shown in Figure A1.2, enter the signal amplitudes obtained from 10, 20, 30, 40 and 50% material losses. Produce a graph of the calibration tube’s amplitude against percentage thinning. Inspection defects - All defect indications appearing on the chart recorder paper trace, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Assess the amount of material loss for each defect using the graph produced from the calibration tube. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, and defect amplitude, characterisation, classification and positional information, with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe PCN/Level 2/ET/Wrought Tubes Tubby Pipe 23rd July 2013 Authorising person's details: Ted Brass PCN/Level 3/ET/Wrought Tubes Ted Brass 23rd July 2013 10% 20% 30% 40% 50% 0.7m Figure A1.2 Calibration Tube CEGB ESI Type D. NDT31-50316b Sample Instructions A1-11 Copyright © TWI Ltd Eddy Current Tubes Inspection Results Recording Sheet Inlet End Test – Absolute Mode – Internal Defects Name: Date: EQUIPMENT USED Sample: CALIBRATION Flaw Detector: % Thinning Amplitude Serial No: Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle: Reference Tube: TUBE No. DEFECT SIGNAL Amp. mm. NDT31-50316b Sample Instructions % Thinning COMMENTS Location A1-12 Copyright © TWI Ltd Appendix C – Sample instruction - phase analysis, full length, external defects Scope: This instruction is documented as the process by which Condenser Tubes are inspected over their Full Length for External Defects using the Eddy Current Phase Analysis method with a differential coil. The defect signals are assessed against a graph of the calibration tube, phase angle and thinning. Printer is used to give positional information. Document reference and status: TWI/ET/Tubes/PAFLED/1/UK Component identification number: Training Bundle, PAFLED TB1/UK Description (incl material and dimensions): 90/10 Copper Nickel tubes 1.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect external defects. Area to be tested: Full length of 1.5m tubes. Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Differential bobbin probe, 11mm diameter. Part No. PID110N05R20K. This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.7m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with External annuli to simulate external metal loss. Part No. CEGB, ESI Type B, see Figure A1.3. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper. Tools - Protractor and Flexible measuring tape. NDT31-50316b Sample Instructions A1-13 Copyright © TWI Ltd Pre-test: preparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency 30KHz. X Y Gain Set 1:1. X Y Position Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence Permanent. Phase Set initially to 0°. Hi Pass Filter Off. Lo Pass Filter 50. Balance Off. Flaw detector calibration Phase: Set to achieve a 90degree phase display from the 100% thinning simulated defect. Sensitivity: Set to achieve an 80% FSH vertical deflection from the 100% defect. Chart recorder settings Chart speed 15mm/sec. Pen position Central. Amptiude 5v up and 5v down. Chart recorder final sensitivity Set to achieve 80% FSH from the 100% defect. NDT31-50316b Sample Instructions A1-14 Copyright © TWI Ltd Detailed instructions for application of test (continued) 1 Calibration recording - With calibration as above, obtain signals from the 10, 30 and 50% simulated defects and measure the phase angles of each defect in turn. The phase angle is measured from the extrpulated peak signal using the flaw dectors phase control see figure A1.4. Adjust amplitude to approximately 80% FSH in turn in order to make a correct assessment of phase angles. 2 Inspection - Carry out an inspection of first tube in a similar manner. Defect signals should be maximised and phase angles recorded, noting defect position, from chart recorder. 3 Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded. 4 Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position. 5 Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... NDT31-50316b Sample Instructions A1-15 Copyright © TWI Ltd Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-Conformance Statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed. NDT31-50316b Sample Instructions A1-16 Copyright © TWI Ltd Recording and classifying results: Action to be taken when defects are detected or no fault found: Calibration tube - Using the report sheet attached, enter the signal phase angles obtained from the calibration tube. Draw a graph of the calibration tube defects – phase angle against percentage thinning. Inspection defects - All indications, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Phase angles are to be measured and the amount of thinning assessed by using the calibration tube graph. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, defect phase angles and percentage thinning. Give positional information with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe PCN/Level 2/ET/Wrought Tubes Tubby Pip 23rd July 2013 Authorising person's details: Ted Brass PCN/Level 3/ET/Wrought Tubes Ted Brass 23rd July 2013 NDT31-50316b Sample Instructions A1-17 Copyright © TWI Ltd Calibration Tube Type B – Cupro-Nickel 90/10 External Annuli and Thru Holes to simulate OD defects. 8x0.85mm thru holes 10% 10% 50% 30% 70% 100% 50% 490mm Figure A1.3 Calibration Tube CEGB ESI Type B. 0° 90° Figure A1.4 showing typical phase measurement. NDT31-50316b Sample Instructions A1-18 Copyright © TWI Ltd Eddy Current Tubes Inspection Results Recording Sheet Full length Test – Differential Mode - External Defects Name: Date: EQUIPMENT USED Sample: CALIBRATION Flaw Detector: % Thinning Phase Angle Serial No: Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle: TUBE No. Reference Tube: Phase Angle NDT31-50316b Sample Instructions DEFECT SIGNAL % Thinning Location A1-19 COMMENTS Copyright © TWI Ltd Appendix 2 ESTestMaker Questions 1 An eddy current test system closely approximates a transformer. In this approximation, what would the second coil be represented by? a b c d 2 By convention, the direction of a magnetic line of force is represented by an arrow on a line. The arrow would point in the direction: a b c d 3 Electrical contact. Specimen conductivity. An alternating magnetic field. Induced electrical current. Which of the following is not a mandatory component in a basic eddy current test apparatus? a b c d 7 A mythical quantity. An imaginary but useful concept. Equal to 1gh mass when converted by Einstein’s equation. 1 micron diameter and 10 microns long. Which of the following conditions is not necessary for eddy current testing? a b c d 6 A dry cell battery. A generator or alternator. A microphone. An electric motor. The magnetic line of force is: a b c d 5 In which a unit north pole would be moved. In which a unit south pole would be moved. Perpendicular to the plane of the line. Indicated by the thumb in the left hand rule. Which of the following is not an example of electromechanical energy conversion devices? a b c d 4 The induced eddy currents. The eddy current probe. The test sample. A Hall detector used as a receiver. An AC source. A coil (probe). An impedance plane. A volt meter. Which of the following is not a probe configuration used in eddy current testing? a b c d Self inductance (single coil). Send-receive (2 coils). Magnetic reaction (coil and hall detector). Semi-conductor reaction (2 hall detectors). NDT31-50316b ESTestMaker Questions A2-1 Copyright © TWI Ltd 8 The sense or direction of a magnetic field around a conductor is most commonly determined using: a b c d 9 Tesla or Webers per square metre (Wb/m2) are units of: a b c d 10 Core permeability. Number of coil turns. Current in the coil. All of the above. A voltage is induced in a region of space when there exists a changing magnetic field. This is a statement of: a b c d 14 25T. 5T. 2.5T. 2.25Wb/m2. An increase in which of the following would result in the increase of magnetic flux density (B) in a solenoid? a b c d 13 Halves. Remains unchanged. Doubles. Quadruples. If the magnetic flux density for a given location and orientation near a current carrying conductor is 5 Wb/m2, what is it when the current is cut by half? a b c d 12 Eddy current. Impedance. Reluctance. Magnetic flux density. If the electric current in a coil is doubled the magnetic flux density: a b c d 11 Lenz’s Law. Ohm’s Law. A Rowland Ring. The right hand rule. Faraday’s Law. Oersted’s Law. Helmholtz’s Theorem. Ohm’s Law Lenz’s Law states: a b c d An alternating magnetic field induces an alternating voltage. The magnitude of induced current is a function of magnetic flux through a circuit. The induced EMF is opposite to the change causing it. I = B A cos where B=flux density, A = circuit area and = the angle between B and the circuit area A. NDT31-50316b ESTestMaker Questions A2-2 Copyright © TWI Ltd 15 The back EMF opposing the inducing EMF is a result of: a b c d 16 The principal cause of magnetism in a naturally magnetic substance is: a b c d 17 1 kHz. 1 standard depth of penetration (e). 3 standard depths of penetration (3e). It is not possible to estimate. When gap between plates of the same material is being measured, the probe should be placed on the thinner of the two plates when possible. Why? a b c d 21 No current flow in the test piece. Dc being induced in the test piece. AC being induced in the test piece. A short circuit. When performing thickness or gap testing, what should the operating frequency be? a b c d 20 Permeability. Flux density. Pole strength. Field intensity. Moving a direct current carrying conductor up and down near a conductive test piece will result in: a b c d 19 Hysteresis. The weak nuclear force. Uncompensated electron spin. Graviton concentration in the Domain wall. The number of lines of magnetic flux divided by a unit area is the: a b c d 18 The Hall effect. Eddy current flow. Geo-magnetic reversals. Weak nuclear forces. The frequency needed would be too low otherwise. To minimize depth of penetration problems. So results are linear. To increase signal to noise ratio. The relationship between electric current flow, electromotive force and resistance to electric current flow is described by: a b c d Lenz’s law. Ohm’s law. Faraday’s rule. The ampere-ohm equation. NDT31-50316b ESTestMaker Questions A2-3 Copyright © TWI Ltd 22 Another term for voltage is: a b c d 23 When determining resistivity of a sample of an aluminium alloy, why is it recommended you do not tough the sample with your fingers? a b c d 24 A V block is used to maintain parallelism. Curved calibration standards are used. Lower operating frequencies are used. Both a and b. When eddy current probes used for restivitiy readings are required to be used on small surfaces (eg bolt heads), what can be done to overcome edge effects? a b c d 28 Variations in alloy. Variations in heat treatment time/temperature. Variations in fabrication stresses. All of the above. What is done to correct for reduced field coupling when making conductivity measurements on curved surface? a b c d 27 Linear. Logarithmic. Exponential. Zero, that is why it is chosen as the standard. In field applications, specific conductivity values are not used; instead a range of conductivities can be expected from a finished product. Why is this so? a b c d 26 Oil of the skin increases resistivity. Oil of the skin decreases resistivity. Sample temperature can be changed. Oils on the test surface from the fingers will produce an unwanted lift-off. Conductivity changes for annealed copper (100 IACS) as a function of temperature change are: a b c d 25 Electromotive force. Magnetomotive force. Potential drop. Both a and c. Use field collimators. Use correction factors from a pre-made edge-distance curve. Both a and b. Use higher frequencies. When does material thickness affect the results of a conductivity test? When: a b c d Eddy current effective penetration is greater than material thickness. Conductivity is very high. The material is backed by a higher conductivity material. Lift-off is a result of a surface roughness. NDT31-50316b ESTestMaker Questions A2-4 Copyright © TWI Ltd 29 If temperature of a test piece increases what other eddy current parameter will likely increase? a b c d 30 Lift-off compensating probes place a compensating coil around the sensing coil. The purpose of this is: a b c d 31 Localised heating caused by eddy currents. Skin depth effect. Decrease in magnetic flux. Permeability of the test piece. To eliminate probe wobble using a two frequency multifrequency set up, what function listed below would be incorrect? a b c d 35 Gap. Pencil. Spring. Spinning. The main factor limiting sensitivity to subsurface defects is: a b c d 34 Smaller than the drive coils. Wound in opposition to each other. Arranged to provide a zero net voltage in air. All of the above. Laminations or disbanding would most likely require you use a (n) probe. a b c d 33 To rotate the defect signal relative to the lift-off signal. Allow shallow defects to be detected on rough surfaces. Both a and b. None of the above. In the reflection type send-receive coil, the receive coils are: a b c d 32 Conductivity. Resistivity. Frequency. Lift-off. Adjust signal amplitudes at the two frequencies to be equal. Adjust phase at the two frequencies to be 90ø apart. Add the two signals together. Both a and c are incorrect. Eddy current information is often digitized for transmission and processing. What is the best resolution possible using 8 bit conversion? a b c d 0.5%. 1.0%. 5.0%. 8.0%. NDT31-50316b ESTestMaker Questions A2-5 Copyright © TWI Ltd 36 Characterising eddy current responses by patterns rather than specific signal responses is termed: a b c d 37 What are the charge carriers used by hall effect devices? a b c d 38 Skin depth. Effective depth of penetration. Stand depth of penetration. Saturation depth. Nonlinear distortion characterised by the appearance of harmonics of the fundamental output when the input wave was sinusoidal is called: a b c d 42 Depth of penetration. Critical distance. Exponential distance. Coating thickness. The depth beyond which a test system can no longer detect further increase in specimen thickness is the: a b c d 41 Lift-off. Wobulation. Coil clearance. Shimmy. The distance in a test specimen that eddy current intensity has decreased 37% of their surface value is the: a b c d 40 Electrons. Positrons. Holes. Both a and c. The effect that produces signal variations due to variation in coil spacing due to lateral motion of test specimen when passing through an encircling coil is? a b c d 39 Spectrum analysis. Signature analysis. Waveform analysis. Pattern recognition. Harmonic distortion. Amplitude distortion. RF noise. Both a and b. Conductance is an electrical quantity which can also be defined as the reciprocal of: a b c d Inductance. Resistance. Resistivity. Reluctance. NDT31-50316b ESTestMaker Questions A2-6 Copyright © TWI Ltd 43 Resistivity of a material is a function of: a b c d 44 A change in signal voltage resulting from EMF produced by the relative motion between test piece and coil is a result of the: a b c d 45 ohm. ohms. ohms. ohms. R = Ro + T. R = Ro + dT. R = Ro (1 + a dT). None of the above. Given copper at 20oC. With resistivity 5.9 ohm-cm and thermal coefficient of resistivity of 0.0039, what is the resistivity when the copper is warmed to 40°C.? a b c d 49 1 2 4 8 Which equation would be used to calculate the resistance of a length of conductor at room temperature other than standard temperature? a b c d 48 Resistor, series. Resistor, parallel. Capacitor, series. Capacitor, parallel. If the resistance in a 1cm long wire is 2 ohms when it has 0.1cm diameter, what will the resistance be in a wire of the same length and material but only 0.05cm diameter? a b c d 47 Edge effect. Speed effect. Harmonic distortion. Fill factor. In order to use a galvanometer (which normally measures currents in the range of milliamps) as an ammeter measuring 10-20 amps you would put put a in with the galvanometer: a b c d 46 A material’s cross-sectional area. A material’s length. Overall resistance. None of the above. 2.90 ohm-cm. 5.80 ohm-cm. 6.25 ohm-cm. 11.60 ohm-cm. When an eddy current probe is brought near a conductive sample the net magnetic flux in the system: a b c d Increases. Decreases. Remains unchanged. Drops to zero when the part is contacted. NDT31-50316b ESTestMaker Questions A2-7 Copyright © TWI Ltd 50 Eddy current density in a sample is: a b c d 51 Strictly speaking, the standard skin depth equation; J/Jo = (e^- β) sin (wt- β), is true for only: a b c d 52 Rods with diameters greater than 2δ. Rods with radius greater than 2 δ. All conditions. No condition, a slight current density will always exist. Xδ. x/δ. δ /x. 57 x/δ. Phase lag of eddy currents in a sample is dependent on: a b c d 56 66%. 37%. 14%. 9%. Phase lag in degrees would be represented by (where x - depth, δ = standard depth of penetration). a b c d 55 that at When inspecting a rod with an encircling coil the eddy current density at the centre of the rod is zero for δ = standard depth of penetration). a b c d 54 Thick material and planar magnetic fields. Tubular products. Thin plate inspection. All of the above. At 2 standard depths of penetration, eddy current density is about the surface: a b c d 53 Proportional to the conductivity of the sample. Proportional to the permeability of the sample. Inversely proportional to the depth from the surface of the sample. All of the above. Depth into the sample. Resistivity of the test piece. Relative magnetic permeability of the sample. All of the above. Eddy current flow in a test sample is accomplished indirectly by monitoring: a b c d Current changes in the sample. Resistivity changes in the sample. Impedance changes in the coil. Coil resonance. NDT31-50316b ESTestMaker Questions A2-8 Copyright © TWI Ltd 57 The equation 2πfL =: a b c d 58 The equation 1/2πfC =: a b c d 59 Pulse-echo. Impedance. Send-receive. Sing-a-round. When the eddy current test system is represented by the transformer the sample can be considered the secondary winding with: a b c d 63 Simple addition. Simple subtraction. Vector addition. A weighted average. The method of eddy current testing that uses a dedicated coil to induce eddy currents in a test piece and another coil to detect eddy current variations in the test piece is the method. a b c d 62 Impedance. Resistance. Reactance. Reluctance. In an AC circuit the total voltage across a resistor and an inductor in series is found by: a b c d 61 Inductive reactance. Capacitance. Capacitive reactance. Total impedance (electrical). The vector sum quantity of resistance and reactance in an AC current is: a b c d 60 Inductive reactance. Inductance. Capacitive reactance. Total impedance (electric). A single turn. 10 turns. Zero turns. None of the above, it is not possible to determine. On a normalised impedance curve which of the following parameters would move the operating point up the curve when increased? a b c d Resistivity of sample. Operating frequency. Sample conductivity. Lift off. NDT31-50316b ESTestMaker Questions A2-9 Copyright © TWI Ltd 64 An increase in tube wall or plate thickness will move the operating point on the impedance curve: a b c d 65 An increase in test frequency will move the operating point on the impedance curve: a b c d 66 Upward. Downward. To a point inside the curve. To a point outside the curve. Increasing which of the following parameters will move the operating point up on the impedance curve? a b c d 70 The primary coil. The secondary coil. The receive coil. Both b and c. An increase in electrical resistivity of a sample will move the operating point on the impedance curve: a b c d 69 Changes in voltage across the primary coil. Changes in current across the primary coil. Two separate coils. A single multiplexed coil. In the send-receive method of eddy current testing the variations in eddy current flow due to flaws in the test piece are monitored by: a b c d 68 Up. Down. To a point inside the original curve. To a point outside the original curve. The send-receive method of eddy current testing uses: a b c d 67 Up. Down. Inside the curve. Outside the curve. Resistivity. Thickness (of tube or plate). Frequency. Diameter of a surface probe. In the impedance method of eddy current testing the impedance phase Ө (in degrees) is calculated from (w is the angular frequency, L is inductance, R is resistance): a b c d Ө Ө Ө Ө = = = = Arcsin (wL/R). Arccos (R/Lw). Arctan (wL/R). Arcsin (R2+L2)^½. NDT31-50316b ESTestMaker Questions A2-10 Copyright © TWI Ltd 71 The effect of sample and test parameters can be illustrated using: a b c d 72 Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50kHz; what is the coil inductive reactance? a b c d 73 20.2 ohms. 20.2 microhenries. 44.7 ohms. Not possible to determine with information given. Given a probe with 50 ohms resistance and 40 H inductance, when operated next to a copper sample at 20kHz the probe impedance is 55 ohms and impedance phase Ө is 40o, what is the inductive reactance of the probe when operating on the sample? a b c d 76 1.59 ohms. 2.51 ohms. 6.3 ohms. 10 ohms. Given a coil with 20ohms and 60 microhenries inductance in air and operated at 50 kHz, when brought next to an inconel sample the probe impedance is 28.5 ohms and impedance phase Ө is 45o, what is the probe’s inductive reactance? a b c d 75 0.4 ohms. 1.6 ohms. 3.9 ohms. 15.7 ohms. Given a coil with 2ohms resistance and 20 H inductance and operated at 20kHz, what is the coil’s inductive reactance? a b c d 74 Magnetographs. Impedance diagrams. Polar projections. Polarised light. 55 ohms. 42.1 ohms. 35.3 ohms. 5 ohms. If given total impedance of a probe operating on a test sample and know the impedance phase angle, what equation is used to determine the inductive reactance of the probe? a b c d Xp Xp Xp Xp = = = = 2πfL. Zp cos Ө Zp tan Ө Xp sin Ө NDT31-50316b ESTestMaker Questions A2-11 Copyright © TWI Ltd 77 Voltage changes across the probe due to a defect in most eddy current inspections are on the order of: a b c d 78 Balancing is required in the eddy current instrument to: a b c d 79 Nonlinear voltage output with change in probe impedance. Increased sensitivity. Reduced lift-off effects. All of the above. In the L-C circuit used by simple meter crack detectors, the circuit is operated: a b c d 83 Amplitude. Phase. Both a and b. No form. The result of operating an eddy current test instrument at a point other than balance point is: a b c d 82 Resistivity. Lift-off. Resonance. Permeability. When a simple bridge made up of 4 impedance arms, the voltage in adjacent arms of the bridge must be equal in: a b c d 81 Allow resonance. Avoid resonance. Set meter type instruments to zero. Eliminate the voltage difference between two coils. The most troublesome parameter in eddy current testing is: a b c d 80 1%. 10%. 100%. 1000%. Independent of operating frequency. At the resonance frequency. Very near resonance frequency. Both b and c. The reactive power of inductance and capacitance in a tuned L-C circuit are: a b c d Equal. Maximum. Minimum. Zero. NDT31-50316b ESTestMaker Questions A2-12 Copyright © TWI Ltd 84 Crack detector type ECT instruments based on resonant circuits detecting surface defects on low resistivity materials such as aluminium would have operating frequencies in the range. a b c d 85 The purpose of multifrequency ECT technique is: a b c d 86 c d Have very low response rates (8Hz). Are usually used as a playback recording instrument for hardcopies of specific signals. Do not have the ability to locate defects and provide length information about the defect. All of the above. Instrument frequency response is limited by: a b c d 90 Be impedance matched to ECT instrument. Be phase locked to the ECT instrument. Have an equal or higher frequency response. Have a lower frequency response that the ECT instrument. X-Y recorders: a b 89 Mixing modules. Filters. Phasors. Frequency selectors. Recording of eddy current signals from ECT instruments requires that the recording instrument: a b c d 88 To increase frequency response of instruments. Elimination of the effects of undesirable parameters. Increase sensitivity to non-surface breaking defects. To allow inspection with phased array probes. Multifrequency instruments have the same controls and functions as general purpose ECT instruments with the addition of: a b c d 87 DC. 60-100Hz. 10-100kHz. 10-100MHz. Probe size. Operating frequency of the probe. Probe motion (inspection speed). None of the above. Most eddy current instruments use some form of options are available for lift-off compensation. a b c d for balancing but several Inductor. AC bridge. DC bridge. Potentiometer. NDT31-50316b ESTestMaker Questions A2-13 Copyright © TWI Ltd 91 The impedance of probes used in eddy current testing can vary over a range. Instruments must be able to balance over this range. Most instruments can handle prove impedances between: a b c d 92 A parallel L-C circuit used in crack detectors has an inductive of 150 ohms. The capacitive reactance would be about under normal operating conditions. a b c d 93 One coil. Two coils. More than two coils. A DC magnetic field. Absolute probe. Differential probe. Spring probe. Pencil probe. The purpose of spring loading an eddy current probe against the test material is: a b c d 97 interact(s) with the test material. When two similar coils on the AC bridge of the eddy current instrument sense with the test material the probe is a (n): a b c d 96 Using a resonance test frequency. Multiple coil probes. Testing under liquid nitrogen. None of the above. An absolute probe requires a b c d 95 0.1 ohms. 75 ohms. 150 ohms. Not possible to know. Compensation for undesirable material and coupling variations can be achieved by: a b c d 94 50-75 ohms. 0.1-100k ohms. 10-200 ohms. 5-500 ohms. Greater wear protection. To maintain constant capacitance. To minimise lift-off. To prevent bearkhausen noise. The purpose of the ferromagnetic core used in a gap probe is to: a b c d Shape the magnetic field. Saturate the test piece with magnetism. Compensate for lift-off. Reduce heating effects caused by eddy current. NDT31-50316b ESTestMaker Questions A2-14 Copyright © TWI Ltd 98 In the send-receive probe arrangement where the driver and receiver coil are on opposite sides of a plate, signal variation will result from: a b c d 99 Maximum response to defects detected by eddy currents are obtained when: a b c d 100 Allow operators to set lift-off horizontal. Avoid resonance on impedance graph displays. Allow study of test specimen variations without concern from probe variations. Establish a common ground for international discussions of eddy current testing. Decrease in sensitivity resulting from increasing lift-off is more pronounced for: a b c d 104 Phase angel increases. Amplitude increases. Both a and b. None of the above, no significant change occurs to the defect signal. The reason for normalising probe impedance is to: a b c d 103 r. 1/r. r^/½. No relationship exists. For a given sized defect, what significant defect signal change occurs when testing a plate using the through transmission (send-receive) method and the defect occurs first 25% of the wall thickness from the transmit coil, then 50% and 75%? a b c d 102 Eddy current flow is parallel to the maximum dimension of the defect. Eddy current flow is perpendicular to the maximum dimension of the defect. The defect cause probe resonance. Lift-off is eliminated. Eddy current flow and its associated magnetic flux are a function of position under the coil. The relationship could best be described as being proportional to (r=radial distance from coil centre): a b c d 101 Material variations (eg Voids) in the test material. Coil-to-coil spacing. Proximity variations of test piece to coils (lift-off). Both a and b. Large diameter probes. Small diameter probes. Deeper defects. Both b and c. As a general rule, probe diameter should be selected so that it is: a b c d Greater than or equal to the expected defect length. Less than or equal to the expected defect length. Less than or equal to the expected defect depth. Twice the minimum allowable defect length. NDT31-50316b ESTestMaker Questions A2-15 Copyright © TWI Ltd 105 At high operating frequencies, the effective coil diameter (sensing diameter) is approximately equal to: a b c d 106 Permeability changes are of greater concern in eddy current testing because: a b c d 107 Depth. Size. Orientation. None of the above (ie all are frequency dependant). Using a typical impedance type EC machine with storage monitor, electrical resistivity determinations are made by: a b c d 111 Lift-off vector and the defect to sound specimen vector. Total voltage vector and the resistive voltage vector. About double the phase leg. Both a and c. Which defect parameter will not affect the probe frequency you select to locate a defect? a b c d 110 Skin depth and phase lag effects. Resonance effect. Test specimen capacitance effect. All of the above. The phase angle used to estimate defect depth is the angle between the: a b c d 109 They can cause parts to fail but cannot be detected. Small changes in permeability cause large impedance changes. Small changes in permeability can obscure other test variables. Both b and c. The reversal swirl that is observed on a normalised impedance graph showing the effects of decreasing thickness is a result of: a b c d 108 0.5 coil diameters. The actual coil diameter. 2 coil diameter. The skin depth. Observing resonance effects. Comparison to reference samples. Taking measurements at two different frequencies. None of the above, impedance instruments cannot be used for resistivity measurements. When given a plate sample for resistivity determination, test frequency should be selected such that skin depth is at least: a b c d Equal to the plate thickness. One third the plate thickness. One tenth the plate thickness. Twice the plate thickness. NDT31-50316b ESTestMaker Questions A2-16 Copyright © TWI Ltd 112 Frequency for plate thickness determinations of thin sections can be approximated by ; where resistivity ( ohm-cm), t=thickness (mm) and δ=standard depth of penetration (mm). a b c d 113 Operating at about 80% of the resonant frequency. Using a lower inductance probe. Reduce cable length. Any or all of the above. 1mm. 6mm. 12mm. 18mm. Which of the following is not an advantage of the eddy current test method? a b c d 118 Operating at about 80% of the resonant frequency. Using a lower inductance probe. Reduce cable length. Any or all of the above. A practical depth limit for flaw detection and location using eddy current test methods is about: a b c d 117 That is as low as possible. That is as high as is practical. Giving only one skin depth of penetration. One half the resonant frequency. Most impedance eddy current instruments will not operate at resonance. This situation is remedied by: a b c d 116 1.6 /t2 (kHz). t/ (Hz). 3t2/ (kHz). t (kHz). Measuring the thickness of conductive layer on another conductor (neither being magnetic) requires: a b c d 115 = = = = Thickness determination of a non-conductive coating on a conductive (nonmagnetic) material is done using a frequency: a b c d 114 f f f f 100% volumetric inspection is possible (within limits). Speed. Clean smooth surfaces not required. No couplant required. When performing an eddy current test and you encountered a signal that could be a crack, permeability change or restivity change, you would: a b c d Change the frequency. Rotate the phase to put the lift-off vertical. Increase gain and look for roughness of signal. Use MPI instead of eddy current testing. NDT31-50316b ESTestMaker Questions A2-17 Copyright © TWI Ltd 119 The f90 for tubing and plate are found using similar but different equations. These equations were determined: a b c d 120 Problems with ferromagnetic indications occurring in material that is not ferromagnetic can be overcome by: a b c d 121 Defect depth. Wall thickness. Both a and b. Not important. Long gradual defects can be missed by using a b c d 125 Reduced frequency range. Increased probe-cable capacitance. Decreasing sensitivity to the far surface defects. Bobbin breakdown. Coil spacing on differential probes for general inspection purposes of tubing is usually: a b c d 124 Encircling probes cannot be made bigger. Fill factor becomes too difficult to regulate for large encircling probes. Higher defect sensitivity can be achieved using surface probes. Both b and c. To increase sensitivity to near surface defects using a bobbin style probe coil length and thickness are reduced. This however results in: a b c d 123 Using a saturating permanent magnet. Retesting at a lower frequency. Retesting at a higher frequency. Both a and b. Encircling probes (or internal probes) are likely to be replaced by surface probes for tubing with a diameter greater than 50mm. The reason for this is: a b c d 122 Empirically. By computer simulations. From characteristic frequency (fg). From the phase lag equation. probes. Encircling. Differential. Bobbin. Absolute. Which of the following is an advantage of the differential probe compared to the absolute? a b c d Sensitive to gradual dimensional changes. Low sensitivity to probe wobble. Easily interpreted signals. All of the above. NDT31-50316b ESTestMaker Questions A2-18 Copyright © TWI Ltd 126 Effects of temperature drift are reduced by using: a b c d 127 The main reason an eddy current coil can detect support plates in heat exchangers when testing tubes from the inside diameter is: a b c d 128 12.2. 11.6. 11.1. 10.9. An encircling coil is used on a 12mm diameter solid rod. What is the fill-factor if the average coil diameter is 13mm? a b c d 132 25Hz. 250Hz. 250kHz. 250MHz. If a probe for internal tube testing has an average coil diameter of 11mm, what size would the tube inside diameter be to give a 0.9 fill-factor? a b c d 131 Decreased signal to noise ratio. Decreased signal amplitude. Both a and b. None of the above, probe impedance matching to instrument impedance is not important. Assuming resistance is negligible and probe inductance is 80 henries, for a cable with 5 x 10^-9 farads capacitance, what is resonance frequency? a b c d 130 Support plates are always ferro-magnetic. Support plates are always the same material as the tube. Magnetic flux is not restricted by the tube wall. Support plates act as resonance amplifiers in the circuit. A probe whose operating impedance is not between 20-200 ohms will most likely in: a b c d 129 Differential probes. Probe pre-heat. Liquid nitrogen baths. Gap probes. 0.80. 0.85. 0.92. 1.08. Impedance diagrams for cylinders are not the simple semi circular shapes used for plate. This is a result of: a b c d Skin effect. Phase lag. Leakage fields. Both a and b. NDT31-50316b ESTestMaker Questions A2-19 Copyright © TWI Ltd 133 A tube being tested by an internal probe has an ID to OD ration of 0.8. Under what conditions does this appear to be a thin wall tube? a b c d 134 Test frequency for solid cylinders, maximum sensitivity to defects, resistivity and dimensions is obtained when f/fg=: a b c d 135 1630Hz. 2.3kHz. 70kHz. 128kHz. What is the f90 for an encircling coil used on aluminium tubing, P =5.1 ohm-cm, wall thickness 5mm, diameter 40mm? a b c d 139 Internal coil inspection of tubing. External coil inspection of tubing. Pancake coil inspection of plate. Both a and b. Given a brass tube 20mm diameter (OD) with a 3mm wall and the resistivity of brass is 7.0 ohm-cm, what is the f90 for testing this tubing? a b c d 138 0.08. 0.9. 1.1. 3. The equation f90 = 3 /t2 applies to: a b c d 137 2. 6. 100. 400. The f90 frequency has been found empirically from the ratio of thickness and skin depth. For testing tubing this ratio is: a b c d 136 Higher operating frequency. Lower operating frequency. When fill factor is 1. When fill factor is reduced. 612Hz. 3.1kHz. 14.7kHz. 61.2kHz. When tube testing at f90 (internal absolute probe), if ID wall loss moves the operating point for an absolute coil in a negative X direction, a shallow OD defect would move the operating point: a b c d +X. –Y. +Y. Both -X and -Y in equal proportions. NDT31-50316b ESTestMaker Questions A2-20 Copyright © TWI Ltd 140 When tube testing (internal absolute probe) at f90 and setting OD wall loss to move +Y on the scope, what is the probably source of a +X moving signal? a b c d 141 When interpreting eddy current signals by quadrature components on strip charts the X channel information is used for: a b c d 142 Tooling or handling equipment. Impurities in the melt. Working below the curie temperature. Oxidation. Ferromagnetic deposits and inclusions are usually: a b c d 146 Signals are too large making small defects hard to see. No magnetite occurs to use as a reference. Elimination frequencies are too high. Elimination frequencies are too low. Ferromagnetic inclusions on or in normally non magnetic aluminium will arise due to: a b c d 145 You re-inspect the area at 2f90. You re-inspect the area at 4f90. You re-inspect the area at 0.1f90. Both a and b. Vectorial addition of signals at conductive non-magnetic support plates is not usually viable because: a b c d 144 Analysing defect type. Analysing defect depth. An analysis threshold. Both a and b. To eliminate magnetic deposits as a possible cause of defect signals (ie a nonrelevant indication) it is recommended that: a b c d 143 ID wall loss. Through hole. Dent. Support plate. Non detectable. Non-relevant or false indications. More critical than their signals indicate. Eliminated by small saturating magnets within the coil. In multifrequency instruments 2 or more operating frequencies are input to a probe simultaneously. What output must be adjusted to permit effective vectorial addition? a b c d Gain. Phase. Frequency difference. Both a and b. NDT31-50316b ESTestMaker Questions A2-21 Copyright © TWI Ltd 147 What condition can be eliminated using multifrequency eddy current technique? a b c d 148 Metal hardness can be indicated by eddy current testing. This is accomplished by: a b c d 149 Sample curvature. Ambient temperature variation. Coatings. All of the above. Relative permeability is measured in which units? a b c d 153 Austenitic stainless steel. Titanium. Tungsten. Annealed aluminium. Which of the following can cause variability in resistivity readings taken for the purpose of sorting? a b c d 152 Brass à = 0.0046. Copper à = 0.0050. Titanium à = 0.0400. Platinum à = 0.0040. Degree of cold working of which material can be determined by eddy current methods monitoring for permeability changes instead of resistivity changes? a b c d 151 Indirect measurement of effects on restivity. Amplitude measurement. Multifrequency technique. Both b and c. Which of the following will have the largest resistivity change with change in temperature (à = thermal coefficient): a b c d 150 Denting and pilgering. Magnetic deposits. Support plates. All of the above. No units (dimensionless ratio). Webers/Ampere-metre. Webers/metre2. Amperes/metre. The amount of reverse magnetising force required to eliminate the residual magnetic flux in a ferromagnetic material is: a b c d 5.5 kilgauss. The coercive force. The de-saturating force. Hysteresis. NDT31-50316b ESTestMaker Questions A2-22 Copyright © TWI Ltd 154 Which of the following series of stainless steels is not likely to exhibit an increase in relative permeability with increasing cold working? a b c d 155 In order to facilitate testing of magnetic materials without the interference of permeability changes you would: a b c d 156 100 Hz. 50 kHz. 250 kHz. 320 kHz. What test frequency has a standard dept of penetration of 1mm for a plate material with resistivity of 130 ohm-cm and relative magnetic permeability of 500? a b c d 159 20mm. 10mm. 0.1mm. 0.05mm. If a plate material has a resistivity of 65 ohm-cm and relative magnetic permeability of 50, what test frequency should you use to achieve f90 at a depth of 0.2mm? a b c d 158 Heat and hold the part over the curie temperature for testing. Use saturating magnets as part of the probe. Both a and b. Stress relieve the part prior to testing. If testing a material and you have set up acceptable conditions for phase separation of 90o for 1mm sample depth when relative magnetic permeability is 1, what depth would the 90° separation occur at if relative magnetic permeability changed to 20? a b c d 157 301. 302. 304. 316. 650Hz. 1.20kHz. 240khz. 320kHz. Magnetic saturation techniques for EC testing that use DC saturation coils are limited to the amount of saturation achieved by: a b c d Test frequency. Heating of the saturation coil. The size of battery used. The voltage that can be safely used. NDT31-50316b ESTestMaker Questions A2-23 Copyright © TWI Ltd 160 What is a resistivity of 6.2 ohm-cm as a % IACS? a b c d 161 Given the permeability of free space is 4πX10^-7 Wb/A/m and the permeability of an iron bar is 7X10^-4 Wb/A/m, what is the relative permeability of the iron? a b c d 162 Potential differences. Eddy currents. Electron flow. Strawberry fields. Two insulated wires are wound on a plastic rod such that they are positioned close to each other but not touching. The ends of one wire are connected battery; the ends of the other are connected to a galvanometer. If the connected to the battery has 1 amp flowing through it, what will galvanometer read? a b c d 166 Heat. Magnetic field strength. Mechanical force or torque. All of the above Electric fields are the same as: a b c d 165 Precise crack length determinations. Crack extension rate determination. Crack width determination. Both a and b. Eddy current generation to determine material properties use detection of variations in: a b c d 164 12.56. 87.9. 280. 557. Small eddy current sensors in the vicinity of cracks could be used for: a b c d 163 27.7. 13.1. 9.8. 6.2. very to a wire the 0 A. 1 A. Just a little less than 1 amp. Just a bit more than 1 amp. The time constant of the circuit is a ratio inductance to resistance (L/R). This accounts for: a b c d Generation of eddy currents. Phase lag of induced currents. Voltage amplitude. Self inductance. NDT31-50316b ESTestMaker Questions A2-24 Copyright © TWI Ltd 167 In an eddy current test set-up, magnetic lines of flux from the probe which fail to couple the test piece: a b c d 168 Complex numbers are often used in the analysis of eddy current test systems. Complex numbers have 2 components, they are: a b c d 169 0°. 45°. 90°. 180°. is plotted on the ordinate (vertical axis). The imaginary component. Inductive reactance. Resistance. Both a and b. Surface coil eddy current transducers are: a b c d 173 Pure inductance. Pure resistance. All conditions. No conditions. In an R-L circuit a b c d 172 in an AC circuit. The phase angle between applied voltage and resultant current in an AC circuit of pure inductance is: a b c d 171 Real and imaginary. Whole and natural. Absolute and integer. Real and unreal. Voltage and current will be in phase for a b c d 170 Carry no information. Cause self inductance in the magnetising coil. Are responsive to the spacing of coil and test piece. Both b and c. Always used in the absolute mode. Always flat. Always used on flat surfaces. None of the above. Measurement of the thickness of a non conductive coating would utilise the effect. a b c d Skin. Lift-off. Hall. Bassel. NDT31-50316b ESTestMaker Questions A2-25 Copyright © TWI Ltd 174 The inductance in the excitation coil is proportional to the diameter square (D2) and the number of turns squared (N2). The voltage induced in the pickup coil is proportional to: a b c d 175 The purpose of small diameter and high frequency probes for determining thickness of thin coatings on conducting substrates is to: a b c d 176 Increased penetrating ability. Decreased coupling ability. A path of low magnetic reluctance. Both a and c. Shielding obtained by eddy current skin effect differs from magnetic methods of shielding in which way? a b c d 180 Maintain constant lift-off. Ensure the coil axis is perpendicular to the test surface. Prevent the probe from scratching the test piece. Shape the magnetising field. Magnetic shielding technique provides the magnetic field lines of the eddy current probe with: a b c d 179 Test piece conductivity and thickness. Test frequency. Proximity of coil to test piece. All of the above. The purpose of curved wear pieces (shoes) to guide surface probe assemblies is to: a b c d 178 Minimise the eddy current field in the substrate. Maximise the eddy current field in the substrate. Maximise the field in the non-conductive coating. Minimise lift-off effect The magnetic flux density around an empty test coil is reduced by increases in when testing non-magnetic materials: a b c d 177 N and D. N2 and D. N2 and D2. N and D2. Skin effect methods amplify the magnetic fields. Skin effect methods attenuate the field rather than change the path. Magnetic methods only work on ferromagnetic test pieces. Skin effect methods are the same as magnetic methods. Maximum test sensitivity is obtained at which point on the signal locus of the complex plane? a b c d Maximum displacement to the right. Maximum displacement to the left. Maximum vertical displacement. Minimum vertical displacement. NDT31-50316b ESTestMaker Questions A2-26 Copyright © TWI Ltd 181 What f/fg ration is recommend for testing thin wall non-magnetic tubing for cracks, alloy variations or wall thickness variations? a b c d 182 When using an external encircling coil the frequency ration f/fg to obtain maximum sensitivity to all test variables will be greatest for which variety of heavy wall tube? a b c d 183 Eddy current density on the inner wall is too low for crack detection. There is no sensitivity to ferromagnetic inclusions. No discrimination between inner and outer wall is possible. Variation in wall thickness and cracks look the same. Which of the following is the direct cause of eddy currents in a test piece placed in an encircling transducer? a b c d 187 The energising coil. The pickup coil. Both a and b. The surrounding air. Phase angle differences of eddy currents greater than about 100° is not recommended for tube testing with encircling coils because: a b c d 186 Increases near surface sensitivity. Reduce magnetic permeability’s of ferromagnetic test materials. Increase magnetic permeability’s of ferromagnetic test materials. Eliminate probe wobble signals. When both a primary (energising) and secondary (pickup) coil are used as an encircling coil probe, the time varying flux in the test piece induces an AC voltage in: a b c d 185 Solid bars. Wall thickness to outside tube radius = 0.5. Wall thickness to outside tube radius = 0.01. None of the above, f/g is constant for all encircling coil tests. What is the purpose of DC magnetic bias in eddy current testing? a b c d 184 0.1. 1.0. 3.6. 10. Induced voltages form the AC magnetic field. Back EMF within the transducer. Resistivity of the test piece. The magnetic field opposing the transducer’s field. The limit frequency is: a b c d The optimum test frequency. The maximum limit test frequency. The minimum limit test frequency. None of the above. NDT31-50316b ESTestMaker Questions A2-27 Copyright © TWI Ltd 188 All other conditions being equal for a bar tested in an encircling coil system, an increase in relative permeability of the bar tested would result in: a b c d 189 Locus curves for diameter changes on the test piece are not straight lines on the normalised impedance plane. Why is this so? a b c d 190 d Real and imaginary components are interchanges. Real component is rotated by 180°. Vertical and horizontal scales are increased by the magnitude of the relative permeability. Complex impedance plane presentation cannot be used when testing ferromagnetic material. When testing ferromagnetic bars with an encircling coil, the effects of changes in are reduced or eliminated by DC magnetic saturation. a b c d 193 Greater penetration afforded permits better determination of bulk properties. The angle between diameter and conductivity locii is greater. The angle between diameter and conductivity locii is 90°. None of the above, frequency ratio should be less than 4 for such work. The complex impedance plane presentation for testing a ferromagnetic bar should be changed in what way from the same test on a non-ferromagnetic bar? The: a b c 192 Due to changes in the Bessel function constant. Diameter changes affect the test frequency ratio. Because of the skin effect. Relative permeability of air change. Separation of diameter and conductivity effects is better carried out at frequency ratios greater than 4 because: a b c d 191 Decreased secondary coil voltage. Increased secondary coil voltage. No change in secondary coil voltage. None of the above, the premise of the question is incorrect as testing of bars with relative permeability over 1 is not possible. Resistivity. Diameter. Relative magnetic permeability. Fill factor. Defect effects from tests in the mercury cylinder can be applied to ferromagnetic materials for practical applications provided: a b c d Phase is rotated 90°. Mercury resistance is subtracted from the results. Voltages from the mercury tests are multiplied by the relative magnetic permeability of the ferromagnetic material. All of the above. NDT31-50316b ESTestMaker Questions A2-28 Copyright © TWI Ltd 194 The through-transmission technique is used for testing of sheet and foil under certain conditions: a b c d 195 For two separate objects with different relative permeabilities and resistivities, equivalent eddy current tests can be performed by adjusting test frequencies. This is explained by: a b c d 196 Suitably shaped insulators inside a mercury filled tube. Suitably shaped conductors inside a water filled tube. Saw cuts in the material to be tested. EDM notches in the material to be tested. The curve traced on X-Y storage monitor as an active coil is brought up to a sample of 1100 aluminium (100% pure) is called the: a b c d 199 f/fg ratios are equal. Fill factors are equal. Both a and b. None of the above, signals could never look the same for magnetic and nonmagnetic materials. The best method of measuring the effects of a specific discontinuity totally within a test specimen but at different depths and orientations is by using: a b c d 198 The similarity law for eddy current testing. Maxwell’s Law. Lenz’s Law. Newton’s First Law of Electromagnetics. Eddy current tests using encircling coils would provide similar test coil impedances or voltage signals for tests on 100mm diameter aluminium rod and 2mm diameter steel wire if: a b c d 197 When test surfaces are not excessively large. When both surfaces are accessible. When the sheet is not multi-layered. Both a and b. Reference curve. Coil lift-off locus. Aluminium standard arc (ASA). Eddy current curve. When a metal sheet is inserted into a through transmission probe arrangement, the transmission coefficient phasor: a b c d Remains unchanged. Changes in magnitude and phase. Changes in real and imaginary values. Both b and c. NDT31-50316b ESTestMaker Questions A2-29 Copyright © TWI Ltd 200 In a through transmission test of sheet products, why might a metered output monitor the product of thickness and conductivity (absolute measurement method)? a b c d 201 For a non-magnetic foil thickness D, conductivity σ, the effective coil distance is found from Aeff = (253,000/fg σ D). Effective coil distance will decrease if: a b c d 202 Increase coil to part spacing. Increase coil to diameter. Decrease coil to part spacing. Decrease coil to diameter. Sensitivity of conductivity measurement with the probe coil is: a b c d 206 A straight line connecting the zero lift-off point to the empty coil value. Bent slightly left towards increasing f/fg values. Bent slightly right towards increasing f/fg values. Bent slightly right towards decreasing f/fg values. In plate testing, to minimise effects of lift-off variations you would: a b c d 205 To increase effective coil distance. To decrease effective coil distance. Unpredictable. Not noticeable. The lift-off locus is: a b c d 204 Thickness decreases. Resistivity increases. Both a and b. None of the above. The effect of increasing coil diameter on the effective coil distance is: a b c d 203 Changes in either parameter results in the same change in transmission coefficient. Conductivity of a sheet can be assumed to always be constant. Thickness of a sheet can always be assumed to be constant. Because it is not possible to arrange the frequency ratio to provide maximum sensitivity. Proportional to the coil’s geometric field gradient. A function of the specimen thickness. A function of the effective coil distance. All of the above. The apparent impedance curve for two different metals of the same thickness will be the same if: a b c d Different probe diameters are used. Frequencies are adjusted so σ f is equal (where σ is conductivity and f frequency). Lift-off is adjusted to compensate for skin effects. All of the above. NDT31-50316b ESTestMaker Questions A2-30 Copyright © TWI Ltd 207 A practical f/fg ratio for thickness measurements would be in the range of 1-7.5. This would provide maximum sensitivity to: a b c d 208 In eddy current tests to determine non-conductive coating thicknesses, probe diameter and operating frequency are selected to minimise the effects of what parameter? a b c d 209 = = = = σ1D1 + σ2D2. σ1σ2 + D1D2. σ1D1)(σ2D2). σ1D1)2 + (σ2D2)2. A difference in conductivities between the two materials. Use of a lift-off compensating probe. A resonance circuit be used. All of the above. Angle between crack direction and lift-off effect increases. Magnitude of crack effect decreases. Lift-off effect increases. All of the above. When inspecting spheres with an encircling coil, what is the equivalent effect of increasing the coil length? a b c d 213 D D D D To discern very shall cracks using a surface coil you would use a relatively high frequency-conductivity product (σ f). Which of the following would then be true? a b c d 212 σ σ σ σ Determining plating thickness of a conducting non-magnetic material on another conducting non-magnetic material requires: a b c d 211 Conductivity. Density. Lift-off. Both a and b. If a sheet was composed of 2 metallic layers with thicknesses D1 and D2 and conductivities σ1 and σ2, what would the equivalent product be when tested by through transmission? a b c d 210 Conductivity. Resistivity. Lift-off. Both a and b. A frequency increase. A frequency decrease. An increase in material conductivity. A decrease in fill factor. A part with a length to diameter ration to 1 tested in an encircling coil: a b c d Cannot be tested by eddy current methods. Results in demagnetisation effects. Gives greatly reduced apparent magnetic permeability. Both b and c. NDT31-50316b ESTestMaker Questions A2-31 Copyright © TWI Ltd 214 When using surface coils for crack detection, shallow cracks and lift-off cannot be separated unless: a b c d 215 The result of using a longer encircling test coil to test a spherical object as compared to a short coil or hemispherical coil would be: a b c d 216 The magnetic field due to skip off the opposite wall phase lagged. The electric field due to skip off the opposite wall phase lagged. The exciter passing the defect. Mode conversion. In the range of about 3-13mm wall thickness, what frequency range would be used for low frequency remote field eddy current testing of ferromagnetic tubing? a b c d 220 Ferromagnetic tubes. Laminates sheets of tungsten carbide. Paint coatings on aluminium boat hulls. Riveted joints on aircraft fuselage. Signals received in the remote field eddy current set-up give two response off large defects, one occurs due to the receiver coil passes the defect. What causes the other signal? a b c d 219 0.1 coil diameter. 1/2 the inside pipe diameter. 2 inside pipe diameters. There is no direct coupling zone in remote field eddy current testing. Remote field eddy current testing is a technique commonly used on: a b c d 218 Increased sensitivity. Reduced fill factor. Improved phase discrimination of cracks and conductivity changes. All of the above. In remote field eddy current testing, how far does the direct coupling zone extend from the exciter coil? a b c d 217 Lift-off compensating probes are used. Frequency is high enough. Frequency is low enough. Both a and b. 10-300Hz. 500Hz-2kHz. 2-10kHz. 10-50kHz. When eddy currents are used for sorting techniques it is usual to establish impedance values from: a b c d Probe characteristics. Samples of known materials. Published information. Trial and error methods. NDT31-50316b ESTestMaker Questions A2-32 Copyright © TWI Ltd 221 Applying a DC electric field to a ferromagnetic coil is done for what purpose? a b c d 222 Sorting of materials by impedance values of an eddy current probe require: a b c d 223 Altered lattice structure inhibits electron flow. Electrons move to lower energy states when alloyed. Electrons move to neutral energy states when alloyed. Both a and b. Work hardened aluminium has a higher resistivity than annealed aluminium for what reason? a b c d 227 Non-magnetic coatings applied to magnetic bases. Alloys. Superconductors. Isotopic variations of the metal. Alloying metals added to pure base metals result in decreasing conductivity of the initial value of the pure base metal. Why does this occur even with alloying metals having higher conductivity than the base metal? a b c d 226 Its curved shape. Its vertical direction of movement. Both a and b. Edge effect on the magnetic material would follow the lift-off trace exactly. Substitutional solid solutions and interstitial solid solutions of metals are forms of: a b c d 225 Relative permeability of all parts to be fixed at 1. Specimen thickness exceeds depth of eddy current penetration. Conductivity of all parts tested be within 10% of each other. Use of the characteristic frequency for test frequency. In general, the edge effect seen as a probe is moved towards edge of a magnetic test piece as compared to a non-magnetic test piece would be recognised by what feature? a b c d 224 Reduce background noise. Improve signal to noise ratio. Eliminate permeability variations that might affect eddy current coil response. All of the above. Changes in alloy content. Disruptions in lattice structure. Excitation states of electrons are higher. None of the above, the premise of the question is wrong, resistivity is a constant for a given alloy content regards of worked state. Which of the following will increase conductivity of an alloy? a b c d Solution heat treating. Precipitation or aging. Annealing. Cold working. NDT31-50316b ESTestMaker Questions A2-33 Copyright © TWI Ltd 228 What would the effect on conductivity signal be as radius of curvature of the test piece is decreased? a b c d 229 Although specimen and standard may be within the recommended 5oC temperature difference for resistivity measurement, why might the value determined still be incorrect? a b c d 230 A minute increase. A significant increase. A decrease. No change. Resistivity measurements standards: a b c d 234 Increased resistivity. Decreased resistivity. Both a and b. None of the above (no effect). What is the effect of ferromagnetic materials on the inductance of an eddy current test coil? a b c d 233 50o below melting point At TC (critical temperature). Room temperature. In a range from -50 to + 15oC. What is the effect on eddy current determined properties of aluminium alloys that have been annealed for an excessive amount of time? a b c d 232 Measurement temperature and the temperature the standard was originally established at are different. A different probe is used that was used to establish the standard. Test frequency is too high. The specimen is work hardened. Natural aging of aluminium alloys occurs at what temperature? a b c d 231 Signal amplitude increases. Conductivity measured would decrease from the true value. Conductivity measured would increase above the true value. No effect would be noticed. made on bulk material and without reference Cannot be made by any methods known. Use very high frequency eddy current probes. Do not use eddy current methods. Use magnetostrictive effects. When a ferromagnetic material has a magnetising force applied to it, the magnetic flux that builds within the material lags the applied force. The same lag occurs upon the reduction in magnetising force. What is the lag called? a b c d Permeability. Hysteresis. Barkhausen effect. Phase shift. NDT31-50316b ESTestMaker Questions A2-34 Copyright © TWI Ltd 235 Which of the following is not a method used to manufacture notches in calibration standards used for eddy current tests of tubing? a b c d 236 What is main advantage of foil calibration standards over affixed coatings calibration pieces? a b c d 237 Simplicity. Economic. Behaves like a crack. Provided a good indication of sensitivity. What is the advantage of artificial defects made by the EDM process? a b c d 241 Electric discharging machining. Ion milling. TEM (tunnelling electron microscopy). Saw cuts. What is not one of the advantages of drilled holes being used as reference standard? a b c d 240 Establish acceptance criteria. Verify accuracy of a test. Provide traceability of a test. All of the above. The most common and reliable method of manufacturing artificial cracks for eddy current standard is by: a b c d 239 Robustness. Accuracy. Resilience. Calibration on curved surfaces. What is the purpose of calibration reference standard? a b c d 238 Electric discharge machining. Electrophoresis. Milling. Saw cuts. Speed. Cost. Accuracy. None of the above, EDM is not used to make artificial defects. A significant disadvantage of using a natural crack as a calibration standard is accurately sizing it. What is the only reliable direct sizing method to determine nature crack depth? a b c d Time of flight diffraction. X-ray. Potential drop. Cutting the specimen open and optically sizing it under a microscope. NDT31-50316b ESTestMaker Questions A2-35 Copyright © TWI Ltd 242 What is used to regulate the consistency of the manufacturing of calibration standards? a b c d 243 Which of the following is not a means of suppressing an undesired eddy current test signal? a b c d 244 Image stability. Resolution. Cost. Size and power consumption. How many thresholds must be set on the CRT display of an eddy current instrument in a box gate alarm system? a b c d 248 Operator response. Meter movement (rise time). Operating frequency. Defect type or coating thickness. What is the most significant drawback of dot matrix displays of EC signals compared to CRT displays? a b c d 247 External, stray magnetic and electric fields. Electrical noise generated within the EC instrument. Mechanical vibrations of test coil or material. All of the above. What limits the scanning speed when using meter display eddy current instruments? a b c d 246 Varying phase rotation. Reducing receiver gain. Varying bridge balance point. Tuning reactive components in the probe bridge circuit. Which of the following noise sources can be filters with the appropriate electronics in an eddy current instrument? a b c d 245 Eddy current instruments calibrated to national standards. Codes and specifications. Licensed metrology labs. Level 3 technicians. 2. 3. 4. 8. What would a polar co-ordinate based phase-gate look like? a b c d Single line. Box. Pie-slice. Sinusoid. NDT31-50316b ESTestMaker Questions A2-36 Copyright © TWI Ltd 249 Multifrequency can discriminate signals at the same depth because: a b c d 250 Compared to single frequency units, multifrequency eddy current instrument circuits are: a b c d 251 Magnitude of magnetic fields. Direction of magnetic fields. Magnitude and direction of electric fields. Both a and b. N-type semi conductors use what form of charge carrier? a b c d 255 D/A converter. A/D converter. Motherboard. Parallel interface. Hall detectors are used to sense magnetic fields. They detect: a b c d 254 CPU. Analogue-to-digital converter. Digital-to-analogue converter. Retro-virus. A circuit block that uses an analogue voltage as an input and outputs, a proportional binary value is a (n): a b c d 253 The same except for signal separation circuitry. The same except for signal separation and combining circuitry. The same in every way. Equipped with better filters and signal averaging circuits. A circuit block that accepts a binary number and translates it to an analogue voltage or current proportional to the binary number is a (n): a b c d 252 Sensitivities are greater at lower frequencies. The phase will be different at different frequencies. Ferrites are independent of frequency. Sensitivity is the same for defects but reduced for geometry changes as frequency increases. Electrons. Positrons. Holes. Quarks. P-type semiconductors use a b c d as charge carriers. Electrons. Holes. Protons. Positrons. NDT31-50316b ESTestMaker Questions A2-37 Copyright © TWI Ltd 256 The ideal signal voltage in a Hall detector element in the absence of a magnetic field is: a b c d 257 The magnitude of the Hall voltage is: a b c d 258 Stabilised DC supplies are needed. Excitation AC current must be constant for all frequencies. Both a and b. None of the above, Hall detectors can be used on any EC instrument. Eddy current test systems using Hall detectors can accomplish differential tests by: a b c d 262 Magnitude of magnetic field. Direction of magnetic field. Rate of change of total flux linkage. Both a and b. Instrumentation for systems using Hall detectors instead of pickup coils are different in what respect? a b c d 261 Single pass inspections of large surfaces. Improving depth resolution. Increased depth of penetration. Increasing frequency response. Which of the following are Hall effect detectors not sensitive to? a b c d 260 Proportional to the external magnetic field. Proportional to the content current in the element. Both a and b. Fixed only by the direction of the magnetic field. Linear multichannel Hall detector arrays are ideal for: a b c d 259 Zero. Maximum. A loc minimum. Determined by ambient temperature. Using two Hall detectors. Using two superimposed excitation frequencies. Having the excitation coil double as a pickup coil. No method presently known. When using Hall detectors, how are sensitivities to relatively great depths achieved? a b c d Increasing Hall detector size. Increasing test frequency. Increasing excitation coil size. Both a and b. NDT31-50316b ESTestMaker Questions A2-38 Copyright © TWI Ltd 263 What are slip rings used for in eddy current inspection systems? a b c d 264 To avoid rotating parts, probes or test piece, what system would be used to inspect round bar stock? a b c d 265 The probe is water cooled. The probe is set back from the test surface at least 3cm. Extra windings and diameter are used in probe construction. Both b and c. Seamless pipe and tubing are often made from billets made from continuous cast blooms. The rounds, as the billets are called, are test by eddy current to detect what types of defect? a b c d 269 Ultrasonic probes will depolarise test hot surfaces. No stream of water coupling is needed for ECT. UT cannot be done on steels above the curie temperature. UT mechanical waves cannot penetrate the surface scale. Eddy current testing of hot billets (1,100oC) can be done provided what precautions are taken? a b c d 268 Orthogonal winding transducer. Multi-pancake probe. Zig-zag probe. Transverse compensating probe. In what way is eddy current testing more suitable to high speed production tests on hot metals than ultrasonics? a b c d 267 Encircling probes. Circumferential array of probes. Hall effect exciters. Both a and b. A differential transducer with the two windings around perpendicular to each other used to detect both longitudinal and transverse cracks is called a (n): a b c d 266 Electrical contacts in rotating heads. Clutch mechanisms in probe pushers. To allow ease of motion by encircling probes. To allow ease of motion by bobbin probes. Cracks. Ovalities. Laps. All of the above. When ECT is used to test thickness of coatings having a tolerance range, what is the minimum number of calibration specimens required to calibrate the instrument? a b c d 2. 3. 4. 2 or 4 depending on if the coating is conductive or not. NDT31-50316b ESTestMaker Questions A2-39 Copyright © TWI Ltd 270 Truly effective sorting of aluminium alloys by eddy current determination of resistivity is not possible because: a b c d 271 When online testing of ERW welds in steel pipe using eddy current testing, what problem occurs if the inspection is performed too far from the induction heating coils used for normalising the weld? a b c d 272 The paint is conductive. Edge effects are causing wrong readings. Different metals are causing wrong readings. The lighter one has a void defect. What is the most effective way of assessing heat or fire damage to heat-treatable aluminium on aircraft? a b c d 275 Increased conductivity. Decreased hardness. Both a and b. No effect. You are given 2 plates of identical size (50x50x10mm) both painted with a thin coating of black acrylic paint of the same thickness. Eddy current test indicate both have a conductivity of 37% IACS, yet one is nearly twice as heavy as the other. How is this possible? a b c d 274 Noise results as the metal cools below the curie point. The induction coils cannot be used as primary coils the inspection. Too much warpage occurs and lift-off is excessive. Near surface defects are masked by the spherodizing effects within the grain structure. What is the effect of over-aging on aluminium heat treatable alloys? a b c d 273 Variations due to heat treatment overlap ranges of conductivity. Grain realignments result when eddy currents flow. Defect free areas can never be found in aluminium. Differences in meters used are never standardised. Eddy current conductivity tests. Ultrasonic velocity tests. Brinnel hardness tests. Thermography. Alpha-case forms on titanium and its alloys at elevated temperatures. Eddy currents are used to establish the depth of case. What is the cause of the formation of alpha-case? a b c d Oxygen diffusion from the heated surface. Carbide migration. Active cathodic protection by-products. Passive anodic protection by-products. NDT31-50316b ESTestMaker Questions A2-40 Copyright © TWI Ltd 276 When a single channel strip charge recorder is used with eddy current testing of bolt holes using spinning probes, what 2 parameters are recorded? a b c d 277 Why are cadmium plated steel bolts used as fasteners on aircraft? a b c d 278 Both surface and subsurface defects are found. Even automotive engineers can perform the tests. The speed at which tests can be performed. The application to determine heat treatment quality. Eddy current test methods are more sensitive than x-rays for detection of aircraft structures. a b c d 282 Paint thickness determinations. Subsurface corrosion detection in multilayer structures. Conductivity determination for alloy sorting. All of the above. What is the biggest advantage eddy current test methods have that make them the most frequently used NDT method in the automotive industry? a b c d 281 Elimination of frequency dependent phase angle. Improved crack detection by suppressing lift-off output. Increased signal to noise ratio. Frequency control without affecting balance. Low frequency eddy current (100Hz to 5kHz) is commonly used in aircraft inspections for: a b c d 280 To ensure maximum shear strength. To provide corrosion resistance to the steel bolt. To provide a galvanically similar surface next to any aluminium to reduce corrosion of aluminium. Both b and c. Some CRT display eddy current instruments allow X and Y gains to be adjusted independently. Increasing Y gain and reducing X (eg Y = 0.2 V/div, = 2.0 V/div) accomplishes what? a b c d 279 Y (vertical) output of signal vs. depth along hole axis. X (horizontal) output of signal vs. depth along hole axis. Y (vertical) output of signal vs. time. Phase of signal vs. position of probe along the hole axis. in Corrosion. Missing fasteners. Fatigue cracks. Overloading cracks. Finned copper tubing used in air conditioning units has smooth land areas at regular intervals along the tube. What is the purpose of these land areas? a b c d Increase tube rigidity. The locations at which the tube is roll expanded into the tube supports. Increase heat transfer rate by wall thinning. Calibration points for differential coil eddy current inspections. NDT31-50316b ESTestMaker Questions A2-41 Copyright © TWI Ltd 283 During evaluation of an indication in a heat exchanger tube, the probe is moved back and forth over the defect. It is noted that the indication has changed position along the length of the tube. What is the likely source? A: a b c d 284 In eddy current inspections of chiller tubes, freeze cracks located at freeze bulges are often not possible to detect using conventional differential probes because: a b c d 285 c d A single probe operating at more than one frequency. A single probe operated at one frequency then rescanning the flaw at a different frequency with the same probe. Two or more probes in tandem each at a different frequency. Any of the above constitutes multifrequency ECT. Multifrequency instruments may be one of two types; simultaneously frequency or alternate frequency. Which is not an advantage of the simultaneous frequency systems? a b c d 289 Physics of a testing media. Characteristics of the test object. Geometry of the test. All of the above. Multifrequency eddy current testing utilises: a b 288 Wobble, electrical noise. Dimensional variables, material variables. Internal variables, external variables. External variables, internal variables. In order to do computer modelling of eddy current fields you must provide: a b c d 287 Bulges and cracks have the same phase. The bulge signal is so large it masks the crack. The crack only occurs under the support plate. Both a and c. Generally in multifrequency techniques for in-situ boiler tube inspections, high frequencies are used to suppress while low frequencies are used to suppress . a b c d 286 Magnetic deposit. Spiral fret. Active cracking. Probe wire has loosened at the connector. No unnecessary saturation in separation stages. Wide passband of x and y outputs. Low cost of equipment. Permits high inspection speeds. Increasing temperature of a dielectric (insulating) materials has what effect? a b c d Increased resistivity. Increased conductivity. Destabilisation of isotypes. No effect on any properties (that’s why they are called insulators). NDT31-50316b ESTestMaker Questions A2-42 Copyright © TWI Ltd 290 What is the advantage of eddy current testing over the potential drop method for sizing surface cracks? a b c d 291 For practical applications of surface probes on curved surfaces: a b c d 292 Over the shallowest notch. Over the deepest notch. Over a defect free area. In air away from the test piece. Why are eddy current coils not made using iron wire? a b c d 296 Negative Y spike. Positive Y spike. Ellilpse. Angulated line (ie Not horizontal). When an eddy current is balanced for surface testing for flaws, where is the probe placed? a b c d 295 High resistivity indications. High penetration of eddy currents. Cyclic variations in magnetic permeability. Electrical noise. In the 1960s a non-storage type oscilloscope was used for eddy current tests. The defect free specimen gave a horizontal line. A defective specimen gave a (n): a b c d 294 Curvature should be small within the region directly below the cross-sections of the coil. Frequency of operation should be as low as possible. Perspex supports should be arranged to fit the curvature inspected. All of the above. How does hysteresis manifest itself when testing ferromagnetic materials? a b c d 293 Accuracy. It is non-contacting. Cost. There is no advantage. To avoid hysteresis effects. To make mathematical calculations easier. To prevent excessive heat build-up. For cathodic breakdown considerations. The higher the value of inductance for a given frequency the greater the degree of: a b c d Balance ability. Sensitivity. Q factor. Capacitive reactance. NDT31-50316b ESTestMaker Questions A2-43 Copyright © TWI Ltd 297 The transmit-receive or transformer style probe provides: a b c d 298 Inductance increases improve eddy current sensitivity. Why is increasing coil area not a preferred method of increasing sensitivity even though inductance is increased? a b c d 299 Lift-off does not decrease sensitivity. Conductivity and thickness can be measured simultaneously. Temperature changes do not affect conductivity readings. All of the above. What degree of accuracy can be expected when using eddy currents to determine paint thickness 10 m thick? a b c d 303 Thin samples. Thick samples. Rough surfaces. Ferromagnetic samples. The through transmission method has the advantage that: a b c d 302 Coils are wound in opposition to each other. Coils are operated at cancelling frequencies. One coil has an air core and the other has an iron core. A subtractive circuit is incorporate into the eddy current instrument. Phase adjustment on simple conductivity meter instruments is especially useful for what conditions? a b c d 301 It makes the coil too bulky. Resolution of defects is decreased. Solid cores cannot be used. Iron cores must be used. How does the differential (or auto-comparator) coil provide insensitivity to gradual changes? a b c d 300 Improved s/n ratio. Increased sensitivity to deeper defects. Both a and b. No advantage over single coil probes. 0.01 m. 0.1 m. 1.0 m. 5.0 m. Tubes with a diameter of more than about 50mm are more effectively tested using than encircling probes. a b c d Internal axial coils. Surface probes or arrays. Differential probes. Forked probes. NDT31-50316b ESTestMaker Questions A2-44 Copyright © TWI Ltd 304 Multifrequency techniques are performed using: a b c d 305 What is the purpose of pulsed saturation eddy current testing? a b c d 306 Insulators. Semi-conductors. Superconductors. Gold. On the normalised impedance plane showing the effects of changing conductivity (σ) the coil’s normalised resistance is zero under what condition? a b c d 310 2 δ. 3 δ. 5 δ. 10 δ. As conductivity of a material approaches infinity its resistive losses approach zero. What type of material exhibits such extremes? a b c d 309 Pulsed eddy current testing. Remote field eddy current testing. Multifrequency eddy current testing. Both a and b. Using a shielded ferrite coil and the pulsed eddy current technique, penetration of measurable currents in a metal sample can be increased to (where δ the standard depth of penetration): a b c d 308 To allow sequenced multifrequency application. To achieve greater penetration in ferromagnetic materials. Synchronization of gates. It allows time for ring to die down and so improves far wall resolution. Large DC saturation units for eddy current inspection of ferromagnetic tubing are often required. What technique can be used to avoid use of these heaving DC saturation units? a b c d 307 Absolute coils. Differential coils. Both a and b. Special multifrequency coils only. σ is zero. σ is infinite. Both a and b. When it equal the normalised inductive reactance. What does an increase in operating frequency do to the probe coil inductance? a b c d It increases. It decreases. It may increase or decrease depending where you are on the locus. None of the above. NDT31-50316b ESTestMaker Questions A2-45 Copyright © TWI Ltd 311 The heating of a ferromagnetic part that occurs when the AC field works to align the magnetic domains into a preferred magnetic orientation is reduced by: a b c d 312 The coil to specimen impedance Z can be defined by (where Zc is coil impedance and Zs is specimen impedance): a b c d 313 Monitor effects of any temperature changes. Monitor instrument drift. Monitor probe degradation. All of the above. In what way does computer acquisition and analysis of eddy current signals (particularly heat exchanger tubing) out-perform humans? a b c d 317 Coil width. Number of turns. Intended operating frequency. Both a and b. During an eddy current inspection of heat exchanger tubing, what is the purpose of recording a calibration signal with each tube inspected? a b c d 316 Multifrequency techniques. Magnetic focusing probes. Spring loaded probes. Using remote field eddy current techniques What are the main limiting parameters for a single coil probes dimensions? a b c d 315 (Zc X Zs)/(Zc + Zs). (Zc + Zs)/(Zc X Zs). Zc – Zs. None of the above. The only way to reduce or eliminate the edge effect is by: a b c d 314 Performing eddy current tests under water. Performing eddy current tests where air temperature is below 0øc. Pre-aligning the domains with DC saturation. Multifrequency eddy currents. Detection. Reproducibility. Accuracy. All of the above. The analytical method that consists in correlating changes in amplitude, phase and/or quadrature components of a complex test signal voltage to electromagnetic conditions in the test piece is: a b c d Phase analysis. Impedance analysis. Differential analysis. Absolute analysis. NDT31-50316b ESTestMaker Questions A2-46 Copyright © TWI Ltd 318 An instrumentation technique that discriminates between variables in the test piece by different phase-angle changes these variables produce in the test signal is: a b c d 319 The ration of the square of the diameter of a cylindrical test piece to the square of the average diameter of the test coil is the: a b c d 320 Skin effect. Doppler shift. Edge effect. Phase shift. The property of a test system that allows separation of signals from defects on close proximity to each other is: a b c d 324 Absolute probe. Axial probe. Differential probe. Multipancake probe. The phenomenon whereby depth of penetration decreases with increasing frequency is called: a b c d 323 Optimum frequency. Test frequency. Limit frequency. Characteristic frequency. Two or more coils in electrical series opposition arranged so EM conditions not common to the areas of the specimen being tested produce a bridge imbalance is a (n): a b c d 322 Flux ratio. Fill factor. Physical impedance. Test ratio. The frequency providing the highest signal-to-noise ratio for detection of an individual property of the test piece is the: a b c d 321 Impedance analysis. Phase analysis. Modulation analysis. Frequency analysis. Phase separation. Amplitude discrimination. Defect resolution. Multifrequency demodulation. The time required for a test system to return to its original state after it has received a signal is the: a b c d Dead time. Recovery time. Recoil time. System delay. NDT31-50316b ESTestMaker Questions A2-47 Copyright © TWI Ltd 325 Current flow that is time constant in both direction and amplitude is: a b c d 326 The method whereby desirable frequency signals are separated from undesirable frequency signals from the modulating envelope of the carrier frequency signal is called: a b c d 327 Acceptance limits. Reject level. Test criteria. Group level options. Differential coils are, in some areas, also called a b c d 331 Difficulty in interpreting signals. Over sensitivity to wobble. Reduced sensitivity to outside wall defects. Insensitivity to circumferential cracks. Test levels used in ECT that establish the group into which a material under test belongs are termed: a b c d 330 Only one or two parameters are subject to change. It must be ferromagnetic. Composition must be uniform throughout. Size and shape must always be small with simple geometric symmetry. What is the disadvantage of the multi-pancake probe used for internal tube inspections as compared to the axial bobbin type probe? a b c d 329 Phase analysis. Modulation analysis. Filtering. Fast fourier transformer. In order that useful results be obtained from an eddy current test, what must be true about the test specimen? a b c d 328 Direct current. Eddy current. Induced current. Boring. ID coils. Bucking coils. Annular coils. Tandem coils. A test level above or below which test specimens are found to be unacceptable is called? a b c d The cut-off level. The rejection level. The acceptance threshold. Both a and b. NDT31-50316b ESTestMaker Questions A2-48 Copyright © TWI Ltd 332 A network that passes electromagnetic wave energy over a described range of frequencies and attenuates energy at all other frequencies is a (n): a b c d 333 The slope of the induction curve at zero magnetising force as the test piece is being taken from its demagnetised state is the: a b c d 334 0.735. 0.819. 0.907. 0.956. Given a tube with a 15mm OD and 1.5mm wall, what size (average diameter) coil is used to obtain an 85% fill factor for an internal inspection? a b c d 338 A two way sort. A three way sort. Threshold sorting. Standard deviation testing. Given an encircling coil with an average coil diameter of 10.5mm and testing a tube 10mm OD with a 1mm thick wall, what is the fill factor of this set up? a b c d 337 Flux density. Flux leakage. Magnetic history. Permeability. An electromagnetic sorting based on a signal response from the material under test above or below a level established by two or more calibration standards is: a b c d 336 Virgin permeability. Initial permeability. Maximum permeability. Effective permeability. The magnetic condition of a ferromagnetic part based on its previous exposures to magnetic fields if the part’s: a b c d 335 Filter. Gate. Inductor. Grate. 14mm. 13mm. 12mm. 11mm. What is the standard depth of penetration of 304 stainless steel (68.96 ohmcm) having 60% cold work applied ( rel=2) tested at 20kHz? a b c d 1.1mm. 2.1mm. 3.1mm. 4.4mm. NDT31-50316b ESTestMaker Questions A2-49 Copyright © TWI Ltd 339 What is the standard depth of penetration for 301 stainless steel having been 25% cold worked (71 a b c d 340 1.3mm. 2.3mm. 3.4mm. 4.6mm. Given a standard depth of penetration of 1.3mm exists for a 10kHz test of navelbrass (6.63 a b c d 341 Ampere. Faraday. Förster. Linqvist. Phase relative to current in the coil. Amplitude. Both a and b. None of the above. The right hand rule for determining magnetic field direction around a current carrying conductor assumes: a b c d 345 Resistance. Resistivity. Probe electrical impedance. Specimen thickness. The voltage changes used to determine various parameters in eddy current testing consist of changes in: a b c d 344 1.3mm. 3.9mm. 5.2mm. 6.5mm. Electromagnetic induction, on which ECT has its foundations, was first discovered by a b c d 343 ohm-cm), what is the effective depth of penetration? The quantity actually monitored by an eddy current probe is: a b c d 342 ohm-cm, rel=10) tested at 10kHz? Conventional current flow. Modern theory current flow. Only alternating current flow. Non-geomagnetic The left hand rule for determining the magnetic field around a current carrying conductor assumes: a b c d Conventional current flow. Modern theory current flow. The conductor is in the shape of a helix. An antiparellel universe. NDT31-50316b ESTestMaker Questions A2-50 Copyright © TWI Ltd 346 The product of the magnetic flux density in a loop of a current carrying coil times the area of that coil gives: a b c d 347 Magnetic induction or the force per unit pole in a magnetic field is the magnetic analog of: a b c d 348 Electric fields. Magnetic fields. Both a and b. None of the above, energy conversion by electromechanics is not possible. Faraday’s Law states that the magnitude of the induced voltage in a circuit is: a b c d 352 Decrease its impedance. Wobble. Resonate. Increase its operating frequency. Electromechanical energy conversion is possible due to: a b c d 351 Probe. Probe and a generator combination. Test sample. None of the above. As an operating eddy current probe (a coil) is brought near a conductive sample the induction of eddy currents in the sample causes the probe to: a b c d 350 Electric intensity. Electric impedance. Electric resistance. Electromotive force. An eddy current test system can be considered a form of transformer. As such, the secondary side would be the: a b c d 349 Eddy current intensity. A dimensionless value equal to infinity. Total magnetic flux outside the coil. Total magnetic flux inside the coil. Equal to the rate of change of the magnetic flux through it. Inversely proportional to the rate of change of the magnetic flux through it. Opposite in sign to the inducing field. Of the same sign as the inducing field. An alternating voltage in a coil brought near a sample that has a finite impedance will result in: a b c d A counter EMF. Induced eddy current flow. Both a and b. None of the above. NDT31-50316b ESTestMaker Questions A2-51 Copyright © TWI Ltd 353 The intensity of a magnetic field that a unit magnetic pole experiences of a force of one dyne is one: a b c d 354 A single magnetic line of flux is given the unit: a b c d 355 Abvolts. Coulombs. Electro-stats. Hertz. If 20 coulombs of charge passes a point in 5 seconds, the electric current value would be: a b c d 360 Proportional to pole strength. Inversely proportional to the square of the distance separating them. Both a and b. The opposite of a and b. The product of current in amperes times time in seconds gives units of: a b c d 359 Hysteresis. Eddy currents. Magnetisation. Permeability. The force between point magnetic poles is: a b c d 358 Wb/m2. Gauss. Maxwell’s/cm2. All of the above. Alignment of the magnetic domains in iron by an external field result in: a b c d 357 Dyne. Oersted. Maxwell. Tesla. Magnetic flux density is expressed in: a b c d 356 Oersted. Telsa. Ohm-com. Gauss. 4 amperes. 100 amperes. 0.8 amperes. 20 amperes. The purpose of using a radial magnetic field around the current carrying coil in a galvanometer instead of a parallel magnetic field is: a b c d To reduce resistance. To increase heat dissipation. To maintain a simple direct proportionality between current and coil rotation. For ease of construction of the instrument. NDT31-50316b ESTestMaker Questions A2-52 Copyright © TWI Ltd 361 Given a wire made of copper with resistivity 1.724 ohm-cm, that is 1cm in length, and has a cross-sectional area of 1cm2, what is the resistance of this section of wire? a b c d 362 Resistance of a piece of wire is a function of: a b c d 363 Negative. Positive. Zero. Unity. A negative thermal coefficient of resistivity would be characteristic of: a b c d 367 Inherent resistivity. Length and cross-sectional area. Temperature. All of the above. The temperature coefficient of resistance of a pure metallic conductor is always: a b c d 366 AC power transformers. Carbon composite materials. Eddy current testing. Ultrasonic testing. Which of the following will have an effect on the electrical resistance of a wire? a b c d 365 Wire length. Cross sectional area of the wire. Resistivity of the material the wire is made of. All of the above. Eddy currents are an undesirable feature in: a b c d 364 1 ohm. Micro-ohm. 1.724 ohms. 2.972 ohms. All pure metals. Some semi-conductors. Insulators. Materials conductivity > 100% IACS. In a nonmagnetic material the back EMF produced by the induced eddy currents has what effect on the probe? a b c d Reduced coil impedance. Reduced coil current. Increase coil current. Both a and b. NDT31-50316b ESTestMaker Questions A2-53 Copyright © TWI Ltd 368 The decrease in eddy current density with increasing depth from the surface is: a b c d 369 The time dependent component of the skin depth equation indicates: a b c d 370 Large diameter. Long. Zig-zag. Either a or b depending on whether plate or tube testing is being done. 57o. 90o. 114o. 180o. Phase lag in the test sample for a void at 1 standard depth of penetration is: a b c d 374 probes are needed. The phase lag, in units of degrees, for an eddy current signal displayed on a typical impedance plane scope for a void originating 1 standard depth of penetration below the surface would be: a b c d 373 0.5. 2. 5. 25. To ensure planar shaped magnetic field a b c d 372 Flux density decreases with depth. Current density decreases with depth. Phase lag of the signal with depth. All of the above. For the calculation for eddy current density to apply, a sample should be relatively thick. The minimum thickness to allow the simple equation to apply is about δ (where δ is the standard depth of penetration). a b c d 371 Linear. Exponential. Logarithimic. Sinusoidal. 1 radian. 90o. Both a and b. None of the above, it cannot be determined from the given information. For the purpose of determining electrical characteristics of a coil/sample combination, capacitance can be an important factor in: a b c d The sample. The probe cables. The probe coil. All of the above. NDT31-50316b ESTestMaker Questions A2-54 Copyright © TWI Ltd 375 The inductive reactance component of an eddy current probe coil’s impedance will with increasing AC frequency: a b c d 376 In the eddy current probe circuit the capacitive component of its impedance is degrees out of phase with its inductive component: a b c d 377 Move up the curve. Move down the curve. Trace smaller semi-circles. Trace larger semi-circles. The impedance method of eddy current testing uses: a b c d 381 Voltage amplitude and phase representation. Repairing broken solder joints. Fusing near surface defects. Terminating technicians who make incorrect evaluations. On the ideal impedance diagram the effect of reducing mutual coupling between probe and sample would be to have the impedance point: a b c d 380 Arcsin (R/x). Arccos (R/x). Arctan (R/x). Arctan (x/R). In eddy current terminology phasors are used for: a b c d 379 0o. 90o. 180o. 270o. The phase of the impedance in an AC circuit is found from: a b c d 378 Increase. Decrease. Remain unchanged. React unpredictably. Two coils. Changes in voltage across the primary coil. Changes in voltage across the secondary coil. Spring loaded probes only. As the diameter of the eddy current probe increases, the operating point on the normalised impedance curve moves (for a surface probe ie not for tube testing). a b c d Up. Down. In. Out. NDT31-50316b ESTestMaker Questions A2-55 Copyright © TWI Ltd 382 Variations in the flow of eddy currents caused by flaws in the test piece are monitored as voltage fluctuations in the secondary coil in the: a b c d 383 When a probe/sample combination is modelled as an equivalent circuit, the secondary circuit load equivalent would be considered a (n): a b c d 384 L L L L D2. D. 1/D. 1/D2 An increase in probe diameter will move the operating point on the impedance curve: a b c d 388 Up. Down. Inside the original curve. Outside the original curve What best describes probe inductance as a function of probe diameter? ( indicates proportional to): a b c d 387 Short circuit. Open circuit. Resonance circuit. None of the above. All other factors constant, increasing lift-off will move the operating point on the impedance curve: a b c d 386 Resistive load in parallel with the coil’s inductive reactance. Inductive load in parallel with the coil’s inductive reactance. Capacitive load in series with the coil’s inductive reactance. Short circuit. Using the analogy of the coil/sample as a transformer circuit, when the coil is held far from the sample we can approximate a (n): a b c d 385 Send-receive method of ECT. Impedance method of ECT. Resonance method of ECT. Potential drop method. Up. Down. To a point inside the original curve. To a point outside the original curve. An inductive and a resistance impedance change in the test coil resulting when an operating eddy current probe is moved near a conductive test sample is represented on a (n): a b c d Standard penetration chart. Phase correction graph. E meter. Impedance graph display. NDT31-50316b ESTestMaker Questions A2-56 Copyright © TWI Ltd 389 The decrease in semicircle radius of the impedance curve display when lift-off increases indicates: a b c d 390 Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50 kHz; what is the impedance phase angle? a b c d 391 The AC signal is too difficult to analyse. DC is more energy efficient. To allow phase rotation. So both electronic and mechanical balancing can be used. H inductance and operated at 20 Given a coil with 2 ohms resistances and 20 kHz, what is the impedance phase angel (in degrees)? a b c d 394 Balance button. Video filter. AC to DC converter. Amplifier. Conversion of the AC unbalance voltage signal to a DC signal retaining amplitude and phase characteristics is done for what reason? a b c d 393 0o. 5.6o. 17.4o. 90o. The most significant instrument component required to detect the small variation in probe impedance or voltage caused by detecting defects in eddy current testing is the . a b c d 392 A smaller change in coil impedance. Quantum effects. Increased resistivity. An approximate short circuit. -14.2°. 38.6°. 44.4°. 51.4°. The impedance phase angle of a probe operating next to a copper test sample is 40o. What is the inductive reactance of the probe in this situation if the total impedance measured is 30 ohms? a b c d 19.3 ohms. 22.9 ohms. 25.2 ohms. Not possible to determine with information given. NDT31-50316b ESTestMaker Questions A2-57 Copyright © TWI Ltd 395 Given a probe operating at 0.5MHz next to a brass sample, total probe impedance is measured at 47.2 ohms, if the impedance phase angel is 45o what is the resistive load of the sample? a b c d 396 Given the resistive load of a probe/sample circuit as 5.1 ohms and the resistance of the probe when operated in air as 15 ohms, what would the impedance phase angle be if total impedance of this circuit was 24.5 ohms? a b c d 397 c d 45o. 90o. 180o. 270o. Internal filtering to decrease instrument or system noise results in: a b c d 401 Phase change in the bridge circuit. An impedance change in the bridge circuit. Current flow in the previously balance bridge circuit. All of the above. Quadrature components of the bridge AC output are generated by sampling the sinusoidal signal at two positions apart on the waveform: a b c d 400 Detecting impedance changes between coils. Detecting impedance changes between a single coil and a reference impendance. Both a and b. None of the above. The typical figure 8 pattern that occurs with a differential probe moving over a defect is a result of: a b c d 399 22o. 35o. 55o. 68o. In eddy current instruments, bridge circuits are used for: a b 398 Same as the inductive reactance in the probe. 33.3 ohms. 47.2 ohms. Not possible to determine from information given. Decreasing frequency response of the instrument. Decreasing maximum inspection speed. Increased s/n ration. All of the above. Most eddy current instruments can tolerate an impedance mismatch in the AC bridge on the order of: a b c d 0%. 5%. 50%. Any amount. NDT31-50316b ESTestMaker Questions A2-58 Copyright © TWI Ltd 402 In the L-C bridge circuit used by simple meter crack detectors, the capacitor is connected in parallel with the in the bridge circuit. a b c d 403 At the resonant frequency of an L-C circuit, output voltage for a given measurement: a b c d 404 Gain, lift-off, balance. Gain, lift-off, frequency. Gain, lift-off, filter. Lift-off, balance, frequency. In resonant circuit crack detectors, the lift-off control actually varies: a b c d 408 Not selectable. Infinitely variable. Limited to the khz range. Limited to the MHz range. Resonant circuit crack detectors have a meter output and 3 controls: a b c d 407 5-10 ohms. 40-60 ohms. 100-200 ohms. 10-200 ohms. Test frequencies for crack detectors operating at or close to resonant frequency are: a b c d 406 Zero. Minimum. Maximum. Not useful. On most eddy current instruments using the impedance method, the AC bridge circuits can usually balance coils having impedances in a range of: a b c d 405 Probe coil. Resistor in the arm adjacent to the probe coil. Resistor in the arm opposite the probe coil. Oscillator generator. Amplifier gain. Operating frequency (by less than 25%). Bridge resistance. None of the above. Which of the following systems has the advantage of being unaffected by temperature variations? a b c d General purpose instruments. Send-receive instruments. Resonant circuit instruments. None of the above can eliminate temperature drift. NDT31-50316b ESTestMaker Questions A2-59 Copyright © TWI Ltd 409 In send-receive ECT systems, probes with 2 receive coils have those coils would in opposition. The purpose for this to: a b c d 410 Now obsolete, the ellipse and slit methods of eddy current testing: a b c d 411 c d 0.9. 0.866. 0.707. 0.5. Given a parallel L-C circuit with a probe inductance of 80 x 10^-6 Henries and operated at resonance frequency, 252kHz, what is the cable capacitance? a b c d 415 Lower than general purpose ECT instruments. Proportional to recording speed (length of tape past the record head per unit time). Inversely proportional to recording speed. Based on tape thickness. Frequency response of an instrument is based on the fact that the output signal of an instrument will be less than the input signal as inspection speed increases. Instrument frequency response is defined as the frequency where output signal is -3dB from the input. This would relate to a volt signal out for a 1 volt signal input: a b c d 414 FM shielding. AM shielding. Relative motion between the coil and sample. Two send and two receive coils. FM tape recorders have often been used to store eddy current signals for subsequent retrieval. Frequency response for these instruments is: a b 413 Used the AC signal, without conversion to DC, for analysis. Were mainly for sorting materials. Were used to measure large (>5%) coil impedance variations. All of the above. Modulation analysis is a specialised ECT method that requires: a b c d 412 Eliminate thermal draft. Permit phase discrimination. Allow no net voltage in the receive coils when both sense the same material. Both and b. 126.5 ohms. 80 x 10^-6 farads. 5 x 10^-9 farads. Cannot be determined. Given a parallel L-C circuit with cable capacitance 5 x 10^-9 farads and operating at a resonance frequency of 2252kHz, what is the inductive reactance of the probe? a b c d 5 x 10^-9 henries. 126.5 ohms. 253 ohms. Cannot be determined. NDT31-50316b ESTestMaker Questions A2-60 Copyright © TWI Ltd 416 When selecting an eddy current instrument for a particular project you need to know: a b c d 417 Resonance frequency can be determined for a parallel L-C circuit by: a b c d 418 c d Lift-off compensation. Temperature compensation. Test frequency not affecting relative impedance of the coils. Both a and b. Band pass filters. Ferrite cups. Annular arrays. Lift-off compensating coils. In an absolute probe configuration, a second coil, apart from the sensing coil, is required for: a b c d 422 Provides greater inductance from a given coil size. Provides increased field coupling for small surface area in contact with test material. Temperature compensation. To increase distance from coil to test surface to allow wear protection. To reduce the effective sensing diameter of surface probes operating at relatively low frequencies, the use of is recommended: a b c d 421 =1/2(π)(LC)^½. =(2I(π)LC)^ ½. =L/C. =2(π)L/C. Mounting a disc of metal, having similar properties to the test material, next to the reference coil in an absolute probe has the advantage of: a b c d 420 fr fr fr fr Which of the following is not a reason for using a ferrite core on the sensing coil of a pencil probe? a b 419 Test frequency and type of lift-off compensation. Type of output signals (eg meter or scope). Instrument type (impedance, send-receive, crack detector, etc.). All of the above. Bridge nulling. Lift-off compensation. Temperature compensation. All of the above. The effective probe diameter extends to about a b c d beyond the coil diameter. 1mm. 1 coil radius. 1 coil diameter. 4 skin depths. NDT31-50316b ESTestMaker Questions A2-61 Copyright © TWI Ltd 423 Ferrite cups can be used to obtain a b c d 424 b c d ½ the standard depth of penetration. The skin depth (δ). Twice the skin depth. The effective depth of penetration. Varying frequency for a probe on a given specimen will move the operating point down the impedance graph with increasing frequency. If the specimen is not thick, a reversal swirl occurs forming a knee. This is a result of: a b c d 429 Up the curve. Down the curve. Horizontally left. Horizontally right. For a thick specimen, test frequency should be selected to provide good separation from lift-off variations. This is facilitated by setting frequency so that the greatest expected defect depth is at: a b c d 428 Dividing the inductive reactance component by the coil’s inductive reactance in air (XL/Xo). Subtracting the coil/cable resistance in air. Both a and b. None of the above. All other parameters constant, an increase of permeability in the test piece causes the operating point on a normalised impedance curve to move: a b c d 427 Frequency. Coil diameter and core materials. Number of turns. Length of coil. Normalising probe impedance for impedance graph displays is accomplished by: a 426 A concentrated field. Right angle current changes. Reduced lift-off noise. Higher frequencies. Which of the following is not a probe parameter affecting impedance results? a b c d 425 without affecting depth of penetration. Skin depth and phase lag effects. Instruments instability. Capacitive effect. None of the above. The characteristic parameter, Pc, used by Deeds and Dodd is primarily a modelling tool. Test conditions with the same characteristic parameter have the same: a b c d Probe parameters. Material parameters. Operating point on the normalised impedance graph. Probe and instrument parameters. NDT31-50316b ESTestMaker Questions A2-62 Copyright © TWI Ltd 430 If lift-off is arranged on the eddy current storage monitor so the signal moves from right to left as the probe is moved away from the sample, an increase in sample thickness would conventionally move: a b c d 431 Maximum frequency you would use for determining thickness of a non-conductive coating on a conductor would be: a b c d 432 d provides equal discrimination for resistivity Ensure test sample and standards are at a uniform temperature. Perform all such tests in liquid nitrogen. Ensure the probe used has large inductive reactance compared to coil resistance (xl/rc>50). Both a and c. The most significant difficulty in determining thickness of conductive coatings on conductors is that: a b c d 435 At the top of the curve. At the bottom of the curve. Near the knee of the curve. Anywhere on the curve measurements. To prevent error in resistivity determinations caused by temperature, you should: a b c 434 1MHz. 500kHz. 1000Hz. Limited by probe to instrument impedance matching, cable resonance and cable noise. When making resistivity measurements on unknown samples, the frequency used is selected such that the operating point on the impedance graph is: a b c d 433 Down. Up. Right. Left. Variations in base material as well as coating material will affect the signal. Probes must be specially designed. The test cannot be done if an air gap exists between the two conductive materials. Both b and c. The problem with overcoming probe-cable resonance by operating above 1.2fg (fr-resonance frequency) is: a b c d Phase discrimination. Greatly reduced sensitivity. Arcing from probe to test piece. None of the above, operating at 1.2 times the resonance frequency is the preferred option. NDT31-50316b ESTestMaker Questions A2-63 Copyright © TWI Ltd 436 What is the effective diameter of a surface probe with a 5mm diameter coil used on a sample with p = 72 ohm-cm and operated at 2MHz. (p is resistivity): a b c d 437 The ration of thickness to skin depth t/δ that provides a 90o separation between lift-off and thickness change is empirically derived. It is found to be about for plate testing: a b c d 438 Its approach signal. Amplitude. The rougher signal quality. All of the above provide evidence of flaws. The best way to distinguish between localised resistivity changes and a real defect is: a b c d 442 ½ Ө. Ө. 2 Ө. 3 Ө. A very shallow surface defect can be distinguished from lift-off by: a b c d 441 Work hardened 7075-T6 (AL alloy). Fe304 deposits on heat exchanger tubing. EDM notches in 304 stainless steel. Segregation in Austenitic stainless steel. The phase angle (as measured from the lift-off signal) of a shallow surface or sub-surface defect is related to the eddy current phase lag á=x/δ (radius), where x = flaw depth and δ = skin depth. The phase angle seen on the storage monitor is approximately: a b c d 440 0.1. 0.8. 1.6. 4. Which is not a source of ferromagnetic indications? a b c d 439 5.5mm. 6.2mm. 7.5mm. 10mm. Retest the area with a smaller probe at the same frequency. Retest the area at 1.3 of the test frequency. Retest the area at 3 times the test frequency. All of the above. Encircling or bobbin style probes used for tube testing require careful design of coil size to optimise sensitivity and coupling. Coil length and coil depth should be about: a b c d Equal Equal A 3-1 Equal to wall thickness. to the shortest allowable defect. ratio. to 1 skin depth at the f90 frequency. NDT31-50316b ESTestMaker Questions A2-64 Copyright © TWI Ltd 443 The reference coil in a bobbin style probe can be mounted concentrically inside the test coil and the probe still be considered and absolute probe because: a b c d 444 When using a bobbin type differential probe, sensitivity to near surface defects can be improved by: a b c d 445 Sufficient wall loss has occurred at the point of maximum deterioration. The leading and trailing edges are abrupt. A sufficiently low frequency is used. A fill factor of greater than 0.9 is used. Ferromagnetic materials can affect probe impedance. These ferrogmagnetic materials: a b c d 449 Gap probes. Absolute probes. Differential probes. Bobbin style internal probes. If a defect is longer than the spacing between the coils on a differential coil, the defect can only be recognised as such if: a b c d 448 Shape of the defect. Length of the defect. Coil configuration of the probe. All of the above. Insensitivity to gradual changes in dimensions or properties is both an advantage and disadvantage, depending on the situation. This feature is exhibited by: a b c d 447 Wrapping coils in opposition. Decreasing coil spacing. Increasing coil spacing. Turning up the gain. Symmetry of a differential signal as the probe is moved over a defect will depend on: a b c d 446 The AC bridge doesn’t know the difference. It is used in conjunction with an external reference coil inside a calibration tube. The fill factor for the reference coil is <<1. All of the above. Need not form closed paths for eddy currents. Need not be electrical conductors. Most only within the coil’s magnetic field. All of the above. Probe operational impedance between 20-200 ohms is usually accommodated by most ECT instruments unless: a b c d Test frequency is too close to probe-cable resonance. Coil material is aluminium instead of copper. Operated at the characteristic frequency. The instrument is operated in the send-receive mode. NDT31-50316b ESTestMaker Questions A2-65 Copyright © TWI Ltd 450 Operating at frequencies above resonant frequency will result in: a b c d 451 Eddy current flow in a cylinder, using an encircling probe, changes with radial distance r from the centre of the cylinder. Eddy current flow is proportional to for cylinder testing. a b c d 452 The same as increasing frequency. Similar to decreasing fill factor. The same as increasing. No effect at all. The characteristic frequency, fg, is the frequency for which the Bessel function solution to Maxwell’s magnetic field equation is equal to: a b c d 456 21.8 ohms. 237.5 ohms. 475 ohms. 21.8 ohms. The effect on the operating point on the impedance diagram of decreasing coil length for a bobbin type internal probe would be: a b c d 455 Phase lag across the tube wall. Resonance effects. Fill-factor changes (field coupling). Increased capacitive reactance components. Given that a probe operated at 300kHz has an inductive reactance of 475 ohms, what is the cable’s capacitive reactance if this frequency results in resonance? a b c d 454 R2. R. 1/r. 1/r2. The curl in the impedance locus that results when increasing test frequencies for inspecting tubing is a result of: a b c d 453 Decreased sensitivity. Current short circuits across the cable instead of going through the coil. Both a and b. None of the above, sensitivity actually increases above fr. A local maximum. A local minimum. 1. 0. (5.0p) a b c d D2 is the general equation to find (p=resistivity). f90. Characteristic frequency for tubes. Locus operating point. Forster’s fixed frequency. NDT31-50316b ESTestMaker Questions A2-66 Copyright © TWI Ltd 457 The characteristic frequency ratio, f/fg, is not used for determining a frequency for phase discrimination in tube testing because the ration is: a b c d 458 Not a function of phase lag. A function of tube diameter. Both a and b. Only used for plate testing. Given a brass tube to be tested with an internal bobbin probe, resistivity of the brass is 7.0 ohm-cm. If an operating frequency of 2.3kHz gives 90° phase separation between ID and OD defects, what is nominal wall thickness of the tubing? a b c d 459 The main disadvantage of multipancake coil probes used as internal tube inspection probes is: a b c d 460 OD defect. Dent. External magnetite. Internal magnetite. If an absolute probe is used, defect depth is estimated from: a b c d 463 ID defects. OD defects. Dents. Permeability changes. When testing brass tubing (internal absolute probe) at f60 a signal moves off to the right on the scope (+X). If the 5% ID wall loss is set to move –X, what is the probable source of this signal? a b c d 462 Their insensitivity to external defects. Their insensitivity to circumferential cracks. Cost. All of the above. When tube testing at operating frequencies at 2f90 and higher it is difficult to discriminate probe wobble and: a b c d 461 12mm. 3mm. 4.8mm. 7mm. Y amplitude. X amplitude. Fly-back angle. Tangent angle from balance to defect signal tip. When using differential probes, defect depth can be estimated from the: a b c d X-amplitude. Y amplitude. Fly-back angle. Angle difference between f90 and NDT31-50316b ESTestMaker Questions f90. A2-67 Copyright © TWI Ltd 464 Circumferential stress corrosion cracking can be detected by normal bobbin style probes during in-service inspection of heat exchanger tubing because: a b c d 465 Defects at non magnetic support plates are detected by using: a b c d 466 468 a b Magnesium 37% IACS. Iron 1.03 x 10^7 siemens per metre. c d Zinc 5.9 ohm-cm. Lead 0.49 x 10^7mhos/m. A conductive deposit (copper) is suspected of being on the OD of a heat exchanger tube being inspected with an absolute internal bobbin probe. The evaluation of this signal is best made by: Observing the amplitude and direction of signals at f90. Retesting at between 2-5 times f90. Retesting at between 0.5-0.1 f90. Retesting using a different probe. Separation of defect signals from insignificant parameters is the function provided by multifrequency ECT units. What condition could not be separated by multifrequency technology? a b c d 470 Internal erosion. Internal pitting. External fretting. Circumferential cracking. Which of the following is the most conductive? a b c d 469 Special probes. Multifrequency units. Vectorial addition. Both b and c. When using multifrequency techniques for tube inspection with an internal probe, the most effective results are had for: a b c d 467 Two frequencies are used in in-service inspections. Of the branching nature of the cracks. Tubes are inspected in two directions, once going in and once pulling out of the tube. None of the above, SCC cannot be detected by bobbin probes. Fretting under nonmagnetic support plates. Pitting under magnetic support plates. SCC in finned copper tubing. Magnetite deposits causing dentin. The induced magnetic flux (B) divided by the applied magnetising force (H) gives what quantity? a b c d Relative permeability. Magnetic permeability. Recoil permeability. All of the above (ie different name for same value). NDT31-50316b ESTestMaker Questions A2-68 Copyright © TWI Ltd 471 For ferromagnetic materials the relative permeability is: a b c d 472 Which of the following metals, when alloyed with pure aluminium will result in the alloy having resistivity less than the aluminium? a b c d 473 Release of bond energy. Formations of a martensite phase. Reduction of chromium content. The fourier transformation process. Changes in permeability with applied stress below the elastic yield strength of iron are due to: a b c d 477 Paramagnetised. Diamagnetised. Saturated. Unretentive. The primary cause of increased permeability in initially nonmagnetic stainless steels with increased cold working is: a b c d 476 The increase of lattice defects. Re-crystallisation. The change from metallic to ionic bonding. The change from metallic to covalent bonding. When the induced magnetic flux in a ferromagnetic material increase linearly with increasing applied magnetising force the material is: a b c d 475 Manganese. Magnesium. Copper. None of the above. The primary cause for the increase in resistivity with increase in cold working is: a b c d 474 >1. 1. 0. -1. Formation of martensite. Formation of pearlite. Magnetostriction. Geomagnetic reversals. If a sample’s permeability changes up by a factor of 2, the standard depth of penetration will: a b c d Increase by 2. Increase by 1.414. Decrease by 2. Decrease by 1.414. NDT31-50316b ESTestMaker Questions A2-69 Copyright © TWI Ltd 478 Pulsed saturation techniques used by EF testing to overcome magnetic permeability superimpose an AC signal and sampling of the eddy current is done: a b c d 479 Given a material with resistivity of 65 ohm-cm a relative magnetic permeability of 50 and testing at 100kHz, what is the standard depth of penetration? a b c d 480 2.4. 6.7. 7.2. 72. 0.11mm. 0.22mm. 0.33mm. 0.4mm. In order to respond to steady-state magnetic flux conditions eddy current probes should use: a b c d 484 ohm-cm as a %IACS? Given a sample with 5 ohm-cm resistivity, and a relative magnetic permeability of 4.1, what is the standard depth of penetration if it is tested at magnetic saturation at a test frequency of 250 kHz? a b c d 483 12.5kHz. 25kHz. 100kHz. 250kHz. What is a resistivity of 72 a b c d 482 1mm. 0.8mm. 0.18mm. 0.05mm. If an acceptable f90 is achieved with a probe on a slightly magnetic ( r=4) plate when operating at 50kHz, what frequency must be used to maintain that same f90 if relative permeability was to drop to 2? a b c d 481 Between pulses. At peak maximum DC pulse. At peak minimum DC pulse. Continuously during DC pulses. Differential coils. Magnetodiodes. Hall detectors. Both b and c could be used. W = Q(V2-V1) describes the work done moving a charge within an electric field. If W is positive then: a b c d Energy must be used to move the charge. Energy is released by the move. The charge moved is a negative charge. Both b and c. NDT31-50316b ESTestMaker Questions A2-70 Copyright © TWI Ltd 485 Amperes traversing a cross-sectional area is a useful concept in eddy current studies. The term used for this measure is: a b c d 486 Relative magnetic permeability for a magnetic material is: a b c d 487 Tangent. Sine. Cosine. Both b and c. As the abscissa value. As the ordinate value. As a vector sum representing resultant amplitude. In the imaginary plane projecting out of the paper. Current leads voltage in an AC circuit of pure: a b c d 492 Resistance. Reactance. Inductance. Capacitance. The imaginary component on the complex plane is plotted: a b c d 491 Current to voltage. Inductance to resistance. Resistance to voltage. Voltage to inductance. The time variations of current, voltage and magnetic fields in AC circuits can be best described by which trigonometric functions(s)? a b c d 490 in a series circuit. Current lags voltage in an AC circuit of pure: a b c d 489 A function of flux density (not constant). A constant for a given material. Greater for small parts than for larger parts of the same material. Not possible to determine. The time constant of a circuit, Tc, is the ration of a b c d 488 Flux modulus. Current density. AM-meters. All of the above. Resistance. Reactance. Inductance. Capacitance. An eddy current transducer whose impedance or induced voltage is measured directly is considered a (n): a b c d Absolute probe. Differential probe. Array transducer. Forked transducer. NDT31-50316b ESTestMaker Questions A2-71 Copyright © TWI Ltd 493 The empty coil impedance of an eddy current probe is determined by: a b c d 494 When using an eddy current technique to determine the thicknesses on a large nonconductive plastic sheet you would require a: a b c d 495 diameter, high frequency. diameter and low frequency. diameter and high frequency. diameter and low frequency. Minimum. Maximum. Zero. Unaffected. When orbiting eddy current probes are used lift-off may need to be increased to ensure clearance from moving test pieces, the effects of lift-off are reduced by: a b c d 499 Small Small Large Large When testing ferro-magnetic materials, coil inductance and inductive reactance are when lift-off is minimum. a b c d 498 Spacing pieces on the conveyor. Signal suppression circuits. Both a and b. None of the above, nothing can be done about this effect. For measurement of thickness of a conductive coating on a conductive substrate where the coating conducting is higher than the substrate, you would use a probe with: a b c d 497 Ferrite cup. Conductive non-magnetic backing sheet. Differential array probe. Saturating magnet. In systems employing automatic feed of test pieces through the test coil, end effects are limited by: a b c d 496 Test piece temperature. Test piece material. Probe design. All of the above. Larger coil diameters. Increased current to the probe coil. Increasing the number of coil turns. Any or all of the above. Shielding effects used in shielded eddy current probes is provided by which method? a b c d Magnetic. Active. Eddy current. All of the above. NDT31-50316b ESTestMaker Questions A2-72 Copyright © TWI Ltd 500 The active shielding technique used to shield eddy current probes uses which of the following principles? a b c d 501 Test frequency ratios less than 0.1 or greater than 10 would be inappropriate for thin wall tube testing. This is because: a b c d 502 30-60°. 40-100°. 89-91°. 90-180°. When testing a ferromagnetic tube with an encircling coil at a frequency ration of 1, what is the ration of magnetic field strengths inside to outside (ie Hi/Ho)? a b c d 506 0. 1. A maximum value. A value not possible to determine. Phase angle between eddy currents on the inside and outside tube wall should lie between to provide sensitivity to cracks. a b c d 505 0. 1. A minimum value. A value not possible to determine. When placed on the normalised impedance plane, the operating point for the coil impedance (empty coil) has a real component to: a b c d 504 Depth of penetration would be too great. Depth of penetration would be too little. Sensitivity would be greatly reduced. Both a and b. When placed on the normalised impedance plane, the operating point for the coil impedance (empty coil) has the imaginary component equal to: a b c d 503 Other coils. Ferrite cups. Ferrite cores. Highly conductive metal housings. 0. 1. <<1. Not possible to know. The voltage induced in the secondary winding of an encircling probe (sendreceive): a b c d Opposes the externally applied voltage. Influences the inductive reactance of the primary winding. Is considered the test signal. All of the above. NDT31-50316b ESTestMaker Questions A2-73 Copyright © TWI Ltd 507 Characteristic frequency can be given by a) 50p/µd2 or b) 1353.8µăd2. What is the difference? (p=resistivity σ =conductivity). a b c d 508 What is the difference between limit frequency and characteristic frequency? a b c d 509 Conductivity variations are well separated from diameter changes. Skin effect reduces the influence of internal properties. Both a and b. None of the above, low test frequencies are preferred. Fill factor, affects the secondary coil voltage. If n is not too small, the correction term 1-n can be ignored for what conditions? a b c d 513 To determine bulk properties in highly conductive materials. When both diameter and conductivity must both be determined accurately. To determine variations in permeability and diameter. Both a and b. High test frequencies are preferred for bar diameter measurements when using encircling coils, why? a b c d 512 Secondary coil voltage for a smaller diameter bar in an encircling coil. Defect depth. The ration of relative to effect permeability. Primary coil turns ratio. When using eddy current encircling coils for sorting, low frequency rations would be used for which conditions? a b c d 511 2π. 0.5π. fc=2fg. Units used. Fill factor, n, is a useful parameter that can be used in determining which quantity? a b c d 510 The Bessel function value solved for. Units being used. Whether paramagnetic or ferromagnetic materials are being tested. Whether European 50Hz current is used or North American 60Hz. High permeability test pieces. Inspection of wire. Low conductivity test pieces. All of the above In testing of ferromagnetic bars with an encircling coil selection of the appropriate frequency ration can permit detection of changes in conductivity independent of: a b c d Diameter. Permeability. Both a and b. None of the above, conductivity cannot be determined in ferromagnetic materials. NDT31-50316b ESTestMaker Questions A2-74 Copyright © TWI Ltd 514 The effective permeability’s, as well as the geometrical distributions of the magnetic field strength, and the eddy current densities, are the same for two different test objects if the frequency ration f/fg is the same for each test object. a b c d 515 Geometrically similar defects will result in the same eddy current effects and in the same variations in effective permeability’s, coil impedance or voltage if the f/fg ration is the same for each test. This principle is explained by: a b c d 516 Thin-wall tubing. Wire. Rod. Heavy bar stock. When a sheet of metal is inserted between a transmitting coil and a receiving coil the voltage in the secondary coil (receiving coil) changes from its empty coil value. The ratio of the new voltage to the empty coil voltage is: a b c d 520 The similarity law. Ohms Law. The Hering/Breuer Reflex. Maxwell’s principle. Coil impedance variations for inspection of sheet products would be most similar to encircling coil inspections of: a b c d 519 Noise elimination. Signal enhancement. Establishing the effects of defect’s shape and orientation. Operating at resonance frequencies. Test results found using the mercury model to establish effects of various shapes and orientations of defects can be applied to bar, rod or wire of any metal because of what principle? a b c d 518 Snell’s law. The similarity law for eddy current testing. The bessel function. Bridge circuitry. Use of a mercury filled glass cylinder in eddy current testing is ideal for: a b c d 517 This applies to nonmagnetic material only. This applies to ferromagnetic material only. The statement is the similarity law for eddy current testing. The premise of the statement is incorrect. Always greater than 1. The transmission coefficient. Independent of the thickness of the sheet. Independent of the conductivity of the sheet. In through transmission testing of nonmagnetic metallic sheet products, the empty coil value of the transmission coefficient is: a b c d Only real (no imaginary component). A maximum. 1. All of the above. NDT31-50316b ESTestMaker Questions A2-75 Copyright © TWI Ltd 521 For through transmission testing of sheet products maximum sensitivity to conductivity and thickness changes occur at what f/fg ration? a b c d 522 The transmission coefficient used in describing phasors in eddy current tests of sheets and foils is analogous to which quantity in cylinder testing? a b c d 523 Geometrical decrease in field intensity. Increased effective coil distance. Decreased effective conductivity. All of the above. What is the effect of increased lift-off on the frequency ration (f/fg)? a b c d 527 Increase coil to part spacing. Increase f/fg ration to 200. Decrease the coil diameter. Decrease the coil to part spacing. The more sharply curved impedance locus traced by a given probe set-up as foil thickness increases is best explained by what aspect of eddy current theory? a b c d 526 Added to the effective coil distance. Subtracted from the effective coil distance. Subtracted from the test piece thickness. Added to the test piece thickness. To increase sensitivity to non-conductive coating thicknesses you would: a b c d 525 Lift-off. Fill factor. Effective permeability. Frequency ratio. When calculations are made for f/fg for a single coil testing of sheet products where the probe is held away from the sheet by prove configuration or nonconductive coating, what must be done with this lift-off component? It is: a b c d 524 1. 2.7. 48. 100. No effect. f/fg increases. f/fg decreases. f/fg is reversed and becomes fg/f for any lift-off greater than zero. To measure foil conductivity independent of thickness effects for sheets in the range of 1-2mm thick, the σf product should be about (conductivity σ in m/ohm-mm2 and f in kHz): a b c d 10-20. 100-200. 1000-2000. 10,000-20,000. NDT31-50316b ESTestMaker Questions A2-76 Copyright © TWI Ltd 528 A dual frequency probe coil system has been developed to determine sheet thickness. If the lower frequency is used to measure the product of conductivity and thickness, what is the higher frequency used for: a b c d 529 In a resonance circuit setup to suppress effects of conductivity and maximise sensitivity to lift-off, an increase in resistivity would result in a signal amplitude decrease but this is compensated by: a b c d 530 Clockwise towards pure copper. Counterclockwise towards pure copper. Clockwise towards pure zinc. Counterclockwise towards pure zinc. Adding increasing thicknesses of zinc (5.9µohm-cm) to a thick copper base (1.79µohm-cm) will cause the operating point on the normalized impedance plane to move: a b c d 533 Plating thickness increases. Base metal thickness increases. Resistivity of plating metal approaches that of the base metal. All of the above. Adding increasing thickness of copper to a thick zinc base (Cu=1.7 µohm-cm Zn=5.9µohm-cm) will cause the operating point on the normalised impedance plane to move: a b c d 532 Reactance being increased. Reactance being decreased. A sudden change in frequency. An effective probe short-circuit. The influence of plating metal on the apparent impedance of the test coil is reduced as: a b c d 531 Conductivity determination. Absolute thickness. Lift-off. Effective coil distance. Clockwise towards pure zinc. Counterclockwise towards pure zinc. Clockwise towards pure copper. Counterclockwise towards pure copper. At some point, the improved signal separation from lift-off for a given crack is lost or overshadowed by what drawback when increasing test frequency? a b c d Loss of lift-off sensitivity. Signal amplitude decrease. Both a and b. None of the above, no quality of signal deteriorates with increased frequency. NDT31-50316b ESTestMaker Questions A2-77 Copyright © TWI Ltd 534 When inspecting spheres of very high relative permeability increasing test frequency (f/fg ratio) will result in: a b c d 535 The demagnetised factor: a b c d 536 d On the outer surface of the tube. In the direct coupling zone. In the remote field zone. In phase with the primary exciter. Eddy currents give austenite it ferromagnetic state. Heating and cooling during welding can change magnetic state. Stresses from the magnetic fields of the eddy currents cause magnetostrictive effect which causes magnetic permeability changes. All of the above. the The most difficult aspect of material sorting as compared to discontinuity by eddy current testing arises from what problem? a b c d 540 Ds/Dc. (Ds/Dc)2. (Ds/Dc)^3. Ds2/Dc2. Austenitic stainless steel is not considered ferromagnetic; however permeability changes often plague inspection of austenitic tubing with welded seams. Why? a b c 539 where Remote field eddy current testing when used on tubular products with an internal probe set-up utilises a secondary exciter effect from currents occurring: a b c d 538 Is caused by free magnetic poles. Increases as length-to-diameter ratio decreases towards 1. Accounts for apparent magnetic permeability decrease. All of the above. Fill factor for spherical objects tested in spherical test coils is found by Ds=diameter of sphere tested and Dc=diameter of the test coil: a b c d 537 Very large phase changes for small frequency changes. Increased signal amplitude. Both a and b. None of the above, virtually no apparent impedance change occurs. Selecting the appropriate test frequency. Unpredictable appearance of unwanted metallurgical factors. Encircling probes cannot be used for sorting. Conductive materials coated with nonconductive layers cannot be sorted. The magnetic flux moving along the tube outer wall, in remote eddy current testing, is the amplitude of the inner wall flux at the same distance from the primary exciter. a b c d About ½. 0.1. 0.01. 10 times. NDT31-50316b ESTestMaker Questions A2-78 Copyright © TWI Ltd 541 In the remote field zone of a remote field eddy current test, the relationship between phase lag and depth is approximately: a b c d 542 Which of the following is not true of remote field eddy current testing? a b c d 543 Resonance. Lift-off. Skin effect. Hysteresis. What effect does annealing have on eddy current tests of nonmagnetic alloys? a b c d 547 Interstitial sold solutions. Substitutional solid solutions. Sintered composites. Both a and b. The edge effect for nonmagnetic material is similar to what other eddy current phenomenon? a b c d 546 The change in inductive reactance in the test piece. The change in inductive reactance in the test coil. Magnetic field distortions within the test piece. Both a and b. Which of the following would be a form of an alloy? a b c d 545 Relatively low frequencies must be used. Dirt, scale and probe lift-off limit effectiveness. Records do not indicate if flaws are internal or external. Inspection speeds are limited by low test frequencies. As a test probe is moved towards the edge of a ferromagnetic test piece the locus traced on the impedance plane is an arc unlike the straighter lift-off trace. What AC counts for the arc shape? a b c d 544 Linear. Exponential. Logarithmic. Inverse square related. It introduces ferromagnetic properties. Increases conductivity. Increases resistivity. It has no effect on the results of eddy current tests. Solution heat treating of an alloy results in: a b c d Increasing conductivity. Decreasing internal stresses. Increasing metal strength. Both a and c. NDT31-50316b ESTestMaker Questions A2-79 Copyright © TWI Ltd 548 Which of the following would have a similar result on conductivity of an aluminium alloy as does annealing? a b c d 549 What effect does natural aging of aluminium alloys have on the conductivity of specimen? a b c d 550 10°C. 15°C. 20°C. 25°C. What is the maximum temperature difference that could be tolerated between standard and specimen when making resistivity measurements? a b c d 553 Lift-off measurement of the oxide layer. Resistivity measurement. Acoustic velocity determination. Remote field eddy current technique. At what temperature are resistivities of most metals stated? a b c d 552 A significant increase. A significant decrease. An unpredictable result. No effect or a slight decrease. 7073-T73 aluminium alloys is specially tempered to resist intergranular corrosion and stress corrosion cracking. What would be used as a process control method for ensuring the adequacy of its aging? a b c d 551 Quench hardening. Cooling the test specimen. Cold working. Aging treatment. 1°C. 5°C. 10°C 20°C. A probe is made using 4 in-line copper contacts. The contacts are placed on a sample and current passed through the outer pair of contacts while voltage is monitored by the inner pair. What application does this have to eddy current tests? a b c d This is used to calibrate the eddy current impedance meter. This provides an absolute measurement of resistivity and can be used for establishing standards. The eddy currents that result are used to test thin foils. This is the potential drop method and has no eddy current applications or relevance. NDT31-50316b ESTestMaker Questions A2-80 Copyright © TWI Ltd 554 Comparing two identically shaped samples of the same grade of carbon steel, one annealed the other quench hardened, which statement would not be correct concerning hysteresis loop tests? a b c d 555 What is the effect of a paramagnetic material on the inductance of an eddy current test coil? a b c d 556 Conductivity changes. Machining burrs. Tube distortion. Permeability changes. The use of 2 calibration foils, one on top the other, to calibrate for checking coating thickness should be avoided except for what conditions? a b c d 560 Depths cannot be duplicated. Lengths cannot be duplicated. Fatigue cracks are more conductive. Both a and b. Drilled holes are often used when making calibration standards for eddy current tube testing. What is the most significant potential problem with production of this artificial defect? a b c d 559 Minimum allowed. Maximum allowed. An average or nominal size. Standards must be made for all possible variations. Why is a fatigue crack a poor simulation for a quench crack? a b c d 558 A minute increase. A significant increase. A decrease. No change. When manufacturing a test standard for parts that are allowed a tolerance in parameter such as size, what size should the standards be: a b c d 557 Maximum flux density for the annealed sample is higher. Residual magnetism for the hardened sample is higher. Coercive force for the annealed sample is less. Retentivity for the hardened sample is greater. The proper thickness is not available. Flexibility is needed on curved surfaces. The substrate is multiple layered. The substrate is ferromagnetic. If too large a drill size is used when making a drilled hole standard for eddy current testing, what happens to the response signal? a b c d Amplitude is unpredictable. It resembles the response from edge effect. Phase must be rotated 45ø clockwise. It is confused with lift-off. NDT31-50316b ESTestMaker Questions A2-81 Copyright © TWI Ltd 561 Which method produces the narrowest slot simulating a crack in a test standard? a b c d 562 Which of the following methods used for machining longitudinal notches are reference standards would be used for making transverse notches? a b c d 563 Bandpass. Low pass. High pass. Low cut. What advantage does the digital bar graph display have over analogue meter display EC instruments? a b c d 567 Amplitude of the undesired signal. Shape of the signal path in the impedance plane. Frequency of the demodulated signal. Both a and b. Which of the following filter types would most likely be used to enhance (eliminate noise) from demodulated DC signals on an eddy current instrument? a b c d 566 AM radio. Ultrasonic testing. Electro-cardiograms. Star wars. The degree of suppression of undesired eddy current test signals depend on: a b c d 565 EDM. Jet abrasives. Planning/milling. All of the above. An eddy current signal that changes in amplitude only is similar to what other common technology? a b c d 564 EDM. Planer fabrication. Jet abrasives. Diamond saw blade. Ease of interpretation. Defect discrimination. Signal response time (allows faster scanning speeds). Expense. What is the most significant advantage of dot matrix displays of EC signals of CRT displays? a b c d Resolution. Contrast. Size and power consumption. Viewing angle. NDT31-50316b ESTestMaker Questions A2-82 Copyright © TWI Ltd 568 When is a gate output indication generated? a b c d 569 Sequential actuation of multiple box gates is used for what purpose in eddy current instruments with computer controlled gating with complex impedance plane displays? a b c d 570 V V V V is is is is proportional proportional proportional proportional to to to to cos Ө. tan Ө. sin Ө. cot Ө. Response to temperature effects is minimised in Hall detectors by: a b c d 574 Eddy current generation and increased temperature. Eddy current generation and decreased temperature. The hall effect and magnetoresistive effect. The pyro-electric and magnetostrictive effects. How is the magnitude of the Hall voltage related to the angle the element normal makes to the magnetic field? a b c d 573 Is constant for all frequencies. Varies linearly with depth at a given frequency. Varies logarithmically with depth at a given frequency. Varies exponentially with depth at a given frequency. What two phenomena occur when a semiconductor is placed in a magnetic field? a b c d 572 Instrument internal calibration checks. Bar graph displays. To detect direction of signal motion. Allow colour display of signals. At intermediate depths, multifrequency EC methods take advantage of the fact that phase angle: a b c d 571 Whenever a gate threshold is set. When a signal enters a gate region. When signals have sufficient negative voltage. When signals have sufficient change in inductive reactance without altering resistive component. Probe orientation with respect to the magnetic field. Using two detectors at right angles to each other. Using three detectors at right angles to each other. Selecting the semiconductor materials used in the probe to be least sensitive to temperature changes. External correction circuits are used to reduce the voltage across the Hall element to zero in the absence of a magnetic field. Why are these circuits needed? a b c d The control current causes a negative bias. The control current causes a positive bias. To provide temperature compensation. Either a or b depending on the applied electromotive force. NDT31-50316b ESTestMaker Questions A2-83 Copyright © TWI Ltd 575 What is the advantage of use 3 Hall effect detectors mounted at mutual right angles to each other? a b c d 576 Response of an inductive pickup coil is not uniform for what waveform? a b c d 577 Not used, it is electronically subtracted. Used as the lift-off reference for phase adjustment. Part one of a two part multifrequency technique. Used to compensation for permeability changes within the test specimen. Which of the following is not a magnetic field vector measured by a magnetic reaction analyser? a b c d 581 To prevent cross-talk due to mutual coupling. To utilise sub-harmonic frequencies from interference. To utilise overtone beat frequencies. Both b and c. When a Hall detector is used it is usually within the magnetic field of the excitation coil. How is this signal used? It is: a b c d 580 Increase coil diameter. Increase hall detector size. Decrease test frequency. Increase ampere-turns of the coil. Some EC inspection systems have 2 or more probes operating independently of each other but in close proximity. Why would these probes be operated at slightly different frequencies? a b c d 579 Square waves. Pulsed waves. Sinusoidal waves. Both a and b. Which is not a method used to generate and measure eddy currents at greater depths using Hall detectors: a b c d 578 Insensitivity to lift-off. Insensitivity to permeability changes. Field magnitude and direction determination independent of probe orientation. Increased sensitivity to long gradual property changes. Ho the magnetising coil field. Hh the magnetising field made by the Hall detector material. Hn the net magnetic field from primary coil and test material. Hr the reaction field in the test material. What is the main advantage of an orthogonal winding transducer? a b c d Elimination of lift-off. Locates longitudinal and transverse cracks. No requirement for a balancing bridge. Allows easier display on isometric plots of amplitude. NDT31-50316b ESTestMaker Questions A2-84 Copyright © TWI Ltd 582 Hot billets are possible to inspect with eddy current methods using: a b c d 583 Above the curie point (δ is standard depth of penetration, σ is conductivity, µ is permeability): a b c d 584 Initialising. Fast fourier transform. Averaging. Standard deviation. Which of the following methods is used to determine coating thickness? a b c d 588 Filtering. Shock absorbers. Spinning probes. Gyroscopic mounted probes. Computer analysis of test results and signals are now common. Which process would most likely be used to separate periodic defect signals from noise to determine periodicity of repetitive signals? a b c d 587 Transverse cracking can be detected. Process control is made feasible. Sensitivity is improved at the elevated temperatures involved. Wield tracking is simplified at higher temperatures. At the high inspection speeds (100m/s) during the production of steel rod, the rod often has a significant vertical vibration as it moves horizontally along and through the eddy current encircling coil. How are defects detected through the resulting shaking noise? a b c d 586 δ increases due to a decrease in σ. δ increases due to a decrease in μ. δ decreases due to a decrease in μ. Both a and b. The advantage of inline eddy current inspection of continuous butt welded pipe is: a b c d 585 Differential probes. Absolute probes. Hall detector probes. Essentially any probe provided it is adequately cooled. X-Ray and radioactive. Optical and magnetic. Ultrasonic and electromagnetic. All of the above. When the gap between two sheets of aluminium increases to a point past where no further change is seen on the eddy current instrument, what is being measured? a b c d Maximum lift-off. Maximum gap. Metal thickness of upper plate. Nothing is being indicated. NDT31-50316b ESTestMaker Questions A2-85 Copyright © TWI Ltd 589 When gap between two plates is to be determined the probe should be placed on a b c d 590 How was the 100% IACS value for annealed pure copper determined? a b c d 591 Conductivity will appear greater if the surface is concave. Conductivity will appear less if the surface is concave. Conductivity will appear greater if the surface is convex. Both a and c. 1/(πfσµ)^½, 26/(πfσµ)^½ and 1980(p/µf)^ ½ are equations used in eddy current testing (p is used here as resistivity, σ conductivity and µ permeability and f frequency), what do they calculate: a b c d 595 Power supply fluctuations. Internal temperature variations. Tidal effects. Both a and b. Having calibrated a flat eddy current probe on a flat conductivity standard you now move to a radiused surface. What will the effect be on conductivity reading if we already know the standard and test specimen have identical conductivities? a b c d 594 Zinc. Phosphorous. Iron. Aluminium. Instruments used for conductivity testing must be checked to ensure they are free from drift. Drift can be a result of: a b c d 593 It was arbitrarily assigned. By conversion from si resistivity. By conversion from si conductivity. From imperial units of resistivity. Which of the following, when added as an alloy of only 0.1% to copper will provide the greatest decrease in conductivity? a b c d 592 The thinner of the two plates. The plate with higher resistivity. The plate with higher conductivity. Both a and b must be considered. Phase lag, effective coil diameter and standard depth of penetration. Effective coil diameter, effective lift-off, standard depth of penetration. Effective coil diameter, effective depth of penetration, standard depth of penetration. All are forms of standard depth of penetration (units vary). In order that a specimen increase its resistivity as its temperature decreases what must hold true? The: a b c d Probe must be thermally insulated from the part. Temperature coefficient must be negative. Temperature coefficient must be positive. Part must be paramagnetic. NDT31-50316b ESTestMaker Questions A2-86 Copyright © TWI Ltd 596 Given resistivity of pure annealed copper is 1.72µohm-cm and pure aluminium is 2.78µohm-cm (both at 20°C.), what is the conductivity % IACS of the aluminium at 55°C if the thermal coefficient of aluminium is 0.0038? a b c d 597 Nonconductive coatings that are less than or slightly more than 0.08mm will result in less than variation of 0.5% IACS in conductivity using a standard conductivity meter. How is this accomplished? a b c d 598 d As quenched. Artificially aged. Naturally aged. Annealed. What is the difference between 2024-T3 and 2024 –T6 aluminium alloy? a b c d 601 Divide the readings by the conductivity of pure copper. From a conversion chart you make using standards. By frequency adjustment so the product of frequency and current provides the desired conductivity reading. The inverse value of current will give conductivity. Which condition of aluminium heat treatment will provide the maximum resistivity? a b c d 600 Internal lift-off compensation. Multi-frequency operation. Ferrite core probes are used. Ferrite cup probes are used. Indirect conductivity meters provide readings in µA (mircoAmperes). How do you convert this to % IACS readings? a b c 599 49. 52. 55. 61. Tempering process. Hardness. Conductivity. All of the above. Oxygen diffusion from the surface of titanium and its alloys at elevated temperatures is of concern in aircraft industry because: a b c d It prevents inspection by eddy current methods. It causes embrittlement. It accelerates corrosive attack. Lift characteristics at high speeds is reduced. NDT31-50316b ESTestMaker Questions A2-87 Copyright © TWI Ltd 602 How are real cracks placed in standards used for calibration of bolt hold inspection by spinning eddy current probes? a b c d 603 A severe form of intergranular corrosion, whereby thin layers of aluminium delaminate parallel to the plate surface is: a b c d 604 Increase the effective depth of penetration. Act as a core and concentrate the electromagnetic field. Cause the eddy currents to bend upwards and move along as surface waves. Disperse the eddy currents and make such inspections impossible. The off-null balance technique is used only on meter type phase analysis instruments. It cannot be used on CRT type instruments because: a b c d 607 6mm. 10mm. 15mm. 25mm. What is the effect of a steel fastener when inspecting multilayer aluminium in the region of the fasteners? They: a b c d 606 Peeling. Exfoliation. Fretting. Pitting. When using low frequency eddy currents to inspect multiple layers of aluminium corrosion or cracking, what is the maximum thickness of outer layer that can be tested? a b c d 605 Fatigue cracks are grown off EDM notches which are later machined away. Pre-drilled holes are made in plates that are clamped and dipped in corrosive liquids to make SCC’s. A bolt is placed through a hole and a nut tightened on the bolt creates a compression crack. All of the above. The flying dot would usually be off screen. CRT instruments cannot be adjusted to off-null balance. The principle works in the 3rd plane, ie out of the screen. It is not a time based function. In the early 1960s what limited the use of eddy current testing to detect subsurface cracks in aircraft structures? a b c d The lack of flying dot phase analysis instruments. The inability of instruments to operate at low frequencies (100hz to 10khz). The absence of small diameter probes. All of the above. NDT31-50316b ESTestMaker Questions A2-88 Copyright © TWI Ltd 608 When performing an eddy current test on finned copper tubing (as in air conditioning units) single frequency instruments in conjunction with differential coil probes are used. A 1.3mm fin pitch requires you use a coil space of 5mm. Why? a b c d 609 Inside diameter pitting on heat exchanger tubing can be a result of: a b c d 610 Cost of training. Independence from probe speed. Amplitude range of application. Freedom from non-linearity effects. In selecting a mixing frequency to suppress internal variables the mix frequency should be the primary frequency. a b c d 614 Phase of the variable changes. Amplitude of the variable changes. The ratio of desired signals and undersized signals changes. All of the above. An advantage of multifrequency ECT for eliminating undesirable signals over monofrequency filtering is: a b c d 613 Forced shutdowns. Slow (low volume) leaks. Reduced power generating ability from plugging. All of the above. In a multifrequency setup, simple subtraction of the mix signal, which is at four times the primary frequency, will not result in zero output of the undesirable variable. Why not? a b c d 612 l fits at tube supports. Poor water chemistry. Operating at too low a temperature. Improper pre-inspection cleaning. Signal analysis of eddy current signals is an important aspect of testing. Of particular concern is its use in establishing depth of cracks or corrosion. What is the result of oversizing defect depths in boiler tube inspections? a b c d 611 To reduce pilger noise. To ensure coils are simultaneously at a finned and unfinned area. To ensure any tight bends in the tube can be passed through. Both a and b. No No No No less than twice. more than twice. less than four times. more than four times. In selecting a mixing frequency to suppress external variables the mix frequency should be the primary technique: a b c d No No No No greater than twice. greater than half. less than four times. less than twice. NDT31-50316b ESTestMaker Questions A2-89 Copyright © TWI Ltd 615 Which of the following is not a multifrequency eddy current system for defining and eliminating a given parameter? The: a b c d 616 Wire rope testing by electromagnetic methods utilises: a b c d 617 To To To All increase inductance for a given coil length. increase resistance for a given coil length. increase capacitance for a given coil length. of the above. Coil cores used for eddy current probes are: a b c d 622 Vertical displacement of the minor axis. Horizontal displacement of the major axis. Orientation of the major axis and the axis ration. The vector sum of the major and minor axis. What is the purpose of multiple layer windings in an inductive coil? a b c d 621 Generalised corrosion and wear. Internal corrosion. Broken wires. Both b and c. On the old sigmaflux instruments which indicated defective parts by displaying ellipses, how were phase and amplitude indicated? a b c d 620 Stretched wires. Broken wires. Internal wire corrosion. Both b and c. Alternating field excitation inspection of steel wire ropes is used to detect: a b c d 619 Alternating field excitation. Direct field excitation. Microwaves. Both a and b can be used. Direct field excitation inspection of steel wire ropes is used to detect: a b c d 618 Algebraic method. Elemental analysis method. Co-ordinate transformation method. Combination method. Iron. Air. Solid dieletrics. All of the above. Current through an eddy current probe coil should be: a b c d As low as possible. At as low a frequency as possible (DC). Grounded to the external protective shell. All of the above. NDT31-50316b ESTestMaker Questions A2-90 Copyright © TWI Ltd 623 What is the effect of too high a current to the eddy current probe: a b c d 624 How does the use of increasing current increase coil inductance? a b c d 625 Coating thickness is constant. Effective lift-off is zero. Conductivity of the substrate is constant. All of the above. What do the side drilled holes used for ultrasonic testing, and the round bottom transverse notch on the OD of a tube for eddy current testing have in common? a b c d 629 The instruments’ wave shape. The signal to noise ratio in the instrument. Conductivity of the test piece. All of the above. What assumption must be made when using eddy currents to determine thickness of a nonconductive coating on a conductive (non-magnetic) substrate? a b c d 628 Uses one primary and two secondary coils. Has two coils wound in opposition. Is used for determining metal thicknesses and detecting subsurface defects. All of the above. Although 3δ is usually accepted as the maximum depth of eddy current that can be detected. It has been noted that in some cases depths of 5δ can be achieved. What determines the increase depth sensitivity? (δ is standard depth of penetration): a b c d 627 Increasing temperature causes coil expansion. Core permeability changes. Resistivity increases causes coil inductance to increase. By the hall effect. The reflection probe: a b c d 626 Increased coil temperature. Increased coil inductance. Magnetic hysteresis. All of the above. Both are artificial defects for calibration. Neither relates their dimensions to real defects. Both are provided to establish acceptance standards. All of the above. What type of crack would cause an absolute surface probe to give a figure-eight display on the storage monitor? a b c d A fatigue crack. A bent crack (major facets in opposite direction). A stress corrosion crack. No crack can provide such an indication with an absolute probe. NDT31-50316b ESTestMaker Questions A2-91 Copyright © TWI Ltd 630 Multifrequency techniques using absolute coils are: a b c d 631 Multifrequency techniques using differential coils are: a b c d 632 The size of coils used. The shape of coils used. The parameter being measured. There is no difference; it is a different name for the same test. The point where increasing operating frequency does not increase ohmic losses in the test material is the: a b c d 636 Keeps heating of the sample to a low level. Aligns the magnetic domains. Ionizes the outer layers making it more conductive. Converts metallic bonds to covalent bonds. What is the main difference between eddy current and flux leakage testing? a b c d 635 An internal axial probe. An array of surface coil probes. Both a and b. The same coil as used for excitation. The pulsed eddy current technique has the advantage of producing high magnetic peak power but still maintaining low average power. This has what effect on the test piece? a b c d 634 Best for detecting large volume defects. Best for detecting small cracks and pits. Used to size dents. Never successful. When access for inspection of a pipe is from the inside in the remote field eddy current technique, the receiver coil is: a b c d 633 Best for detecting large volume defects. Best for detecting pits. Best for detecting small cracks. Not possible (only different coils can be used). Reluctance point. Limit frequency. Terminal point. Fmax. Every part of an electric circuit is acted upon by a force that tends to move it in such a direction as to enclose the maximum amount of magnetic flux. This statement is known as: a b c d Maxwell’s law. Len’s law. Newton’s fifth law of electric action. Planck’s law. NDT31-50316b ESTestMaker Questions A2-92 Copyright © TWI Ltd 637 Calibration standards are used in eddy current test to: a b c d 638 ACPD (alternating current potential drop) and ECT (eddy current testing) both use alternating currents to size surface breaking cracks. ECT uses induction to generate currents in the piece. What does ACPD use? a b c d 639 Information redundancy reduces changes of missing defects. So spare parts are readily available. To save time. Because multifrequency units are cheaper than single frequency. When a digital output is available on an eddy current instrument, why should the digitalising rate be at a reasonably high rate? a b c d 643 Defect depth is not indicated by phase. Flaw geometry affects phase angle. Depth is indicated by amplitude of signal only. It just cannot be explained. Multifrequency eddy current techniques should be used whenever possible, even if the mixing capability is not needed. Why? a b c d 642 It is easy to produce. It does not modify phase of signals. It is always 90ø from OD defects. Both a and b. Why do holes of different diameter and the same through wall depths have different calibration phase angles (eg flyback angle for a differential coil)? a b c d 641 Capacitive discharge. Mutual induction. Physical contact (electrodes). Microwaves. Lift-off is used as a reference signal in many eddy current test applications. Why? a b c d 640 Set the instrument to produce indications similar to the depth of expected flaws. Check the instrument for drift. Check the prove coil for damage. All of the above. To ensure proper depth of penetration. To match the ac oscillator. To allow variation in scanning speed without degrading the signal. This is only needed for applications where determining metallic coating thickness on metallic substrates. A material with a permeability less than that of a vacuum is a a b c d material. Diamagnetic. Paramagnetic. Ferromagnetic. Nonmagnetic. NDT31-50316b ESTestMaker Questions A2-93 Copyright © TWI Ltd 644 An acceptable ratio between defect signal amplitude and non-relevant indications is usually considered to be as a minimum: a b c d 645 External magnetic forces causing an increase in the normal number of electrons with the same spin, thereby increasing the number of uncompensated spins results in what property? a b c d 646 Bridge. L-C circuit. Resonance circuit. Short circuit. In tubing inspection a tube used to establish acceptance levels with artificial discontinuities as specified in applicable product standards is a (n): a b c d 650 Threshold magnetisation. Saturation magnetisation. Initial magnetisation. Critical magnetisation. An electrical circuit incorporating four impedance arms is a (n): a b c d 649 Ferromagnetic material. Paramagnetic material. Diamagnetic material. Both b and c. The degree of magnetisation produced in a ferromagnetic material for which incremental permeability has decreased to unity is: a b c d 648 Magnetism. Paramagnetism. Ferromagnetism. Antimagnetism. Which of the following is not considered to be magnetisable? a b c d 647 1:1. 3:1. 5:1. 10:1. Test piece. Acceptance standard. Comparator tube. Short tube. A wave filter with a single transmission band and neither of the cut-off frequencies being zero or infinity is a: a b c d Bandpass filter. Highpass filter. Lowpass filter. Half wave filter. NDT31-50316b ESTestMaker Questions A2-94 Copyright © TWI Ltd 651 What is the disadvantage of zig-zag coil probes compared to axial bobbin type probes used for internal tube inspections? a b c d 652 If two or more coils are electrically connected in series such that there is no mutual inductance between them and no electric or magnetic condition (or both) that is not common to the test standard and test specimen, will produce an unbalance and yield an output, this arrangement is called: a b c d 653 Absolute signal. Differentiated signal. Harmonic signal. Modulation signal. A standard is: a b c d 656 Effective. Relative. Absolute. Initial. An output signal that is proportional to the rate of change of the input signal is a (n): a b c d 655 Bucking coils. Differential coils. Comparator coils. Circumferential coils. Which permeability is described as a hypothetical quantity magnetic permeability experienced under a given set of physical conditions eg a cylinder in an encircling coil at a specific test frequency? a b c d 654 Non-uniformity of sensitivity. Decreased far surface sensitivity. Unpredictable phase differences with increasing flaw depth. Insensitivity to circumferential cracks. A physical reference used for calibration. A concept established by authority to serve as a rule in measurement of quality. Both a and b. None of the above. An electromagnetic sorting based on a signal response from the material under test above or below two levels established by three or more calibration standards is: a b c d A two way sort. A three way sort. Threshold sorting. Standard deviation testing. NDT31-50316b ESTestMaker Questions A2-95 Copyright © TWI Ltd 657 Given the requirement to test tubing, OD 10mm and wall thickness 1mm, using an encircling coil, what is the average coil diameter if you need to maintain a 90% fill factor? a b c d 658 What is the fill factor of the test using 1.1mm diameter encircling coil to test wire with a diameter of 1.02mm? a b c d 659 900Hz. 15kHz. 22kHz. 31kHz. Given a sample of titanium (54.8µohm-cm) what test frequency must be used to obtain a 1mm standard depth of penetration? a b c d 661 0.90. 0.86. 0.84. 0.81. Given a sample of 50% cold worked 304 stainless steel (68.96µohm-cm, µrel=2) what test frequency would provide a 2mm standard depth of penetration? a b c d 660 10.25. 10.50. 10.7. 11.00. 35kHz. 70kHz. 140kHz. 280kHz. Given a sample of cold worked stainless steel (71 µohm-cm, µrel=10) tested at 10kHz, what is the effective penetration? a b c d 1.3mm. 3.9mm. 5.2mm. 6.5mm. NDT31-50316b ESTestMaker Questions A2-96 Copyright © TWI Ltd Appendix 3 ESTestMaker – Answers 1 An eddy current test system closely approximates a transformer. In this approximation, what would the second coil be represented by? c 2 By convention, the direction of a magnetic line of force is represented by an arrow on a line. The arrow would point in the direction: a 3 Doubles. If the magnetic flux density for a given location and orientation near a current carrying conductor is 5 Wb/m2, what is it when the current is cut by half? c 12 Magnetic flux density. If the electric current in a coil is doubled the magnetic flux density: c 11 The right hand rule. Tesla or Webers per square metre (Wb/m2) are units of: d 10 Semi-conductor reaction (2 hall detectors): The sense or direction of a magnetic field around a conductor is most commonly determined using: d 9 An impedance plane. Which of the following is not a probe configuration used in eddy current testing? d 8 Electrical contact. Which of the following is not a mandatory component in a basic eddy current test apparatus? c 7 An imaginary but useful concept. Which of the following conditions is not necessary for eddy current testing? a 6 A dry cell battery. The magnetic line of force is: b 5 In which a unit north pole would be moved. Which of the following is not an example of electromechanical energy conversion devices? a 4 The test sample. 2.5T. An increase in which of the following would result in the increase of magnetic flux density (B) in a solenoid? d All of the above. NDT31-50316b ESTestMaker Answers A3-1 Copyright © TWI Ltd 13 A voltage in induced in a region of space when there exists a changing magnetic field. This is a statement of: a 14 Lenz’s law states: c 15 Both a and c. When determining resistivity of a sample of an aluminium alloy, why is it recommended you do not tough the sample with your fingers? c 24 Ohm’s law. Another term for voltage is: d 23 To minimize depth of penetration problems. The relationship between electric current flow, electromotive force and resistance to electric current flow is described by: b 22 3 standard depths of penetration (3e). When gap between plates of the same material is being measured, the probe should be placed on the thinner of the two plates when possible. Why? b 21 AC being induced in the test piece. When performing thickness or gap testing, what should the operating frequency be? c 20 Flux density. Moving a direct current carrying conductor up and down near a conductive test piece will result in: c 19 Uncompensated electron spin. The number of lines of magnetic flux divided by a unit area is the: b 18 Eddy current flow. The principal cause of magnetism in a naturally magnetic substance is: c 17 The induced EMF is opposite to the change causing it. The back EMF opposing the inducing EMF is a result of: b 16 Faraday’s law. Sample temperature can be changed. Conductivity changes for annealed copper (100 IACS) as a function of temperature change are: a Linear. NDT31-50316b ESTestMaker Answers A3-2 Copyright © TWI Ltd 25 In field applications, specific conductivity values are not used; instead a range of conductivities can be expected from a finished product. Why is this so? d 26 What is done to correct for reduced field coupling when making conductivity measurements on curved surface? d 27 Adjust phase at the two frequencies to be 90ø apart. Eddy current information is often digitized for transmission and processing. What is the best resolution possible using 8 bit conversion? a 36 Decrease in magnetic flux. To eliminate probe wobble using a two frequency multifrequency set up, what function listed below would be incorrect? b 35 Gap. The main factor limiting sensitivity to subsurface defects is: c 34 all of the above Laminations or disbanding would most likely require you use a (n) probe. a 33 Both a and b. In the reflection type send-receive coil, the receive coils are: d 32 Resistivity. Lift-off compensating probes place a compensating coil around the sensing coil. The purpose of this is: c 31 Eddy current effective penetration is greater than material thickness. If temperature of a test piece increases what other eddy current parameter will likely increase? b 30 Both a and b. When does material thickness affect the results of a conductivity test? When: a 29 Both a and b. When eddy current probes used for restivitiy readings are required to be used on small surfaces (eg bolt heads), what can be done to overcome edge effects? c 28 All of the above. 0.5%. Characterising eddy current responses by patterns rather than specific signal responses is termed: b Signature analysis. NDT31-50316b ESTestMaker Answers A3-3 Copyright © TWI Ltd 37 What are the charge carriers used by Hall effect devices? d 38 The effect that produces signal variations due to variation in coil spacing due to lateral motion of test specimen when passing through an encircling coil is: b 39 Resistor, parallel. If the resistance in a 1cm long wire is 2 ohms when it has 0.1cm diameter, what will the resistance be in a wire of the same length and material but only 0.05cm diameter? d 47 Speed effect. In order to use a galvanometer (which normally measures currents in the range of milliamps) as an ammeter measuring 10-20 amps you would put a in with the galvanometer: b 46 None of the above. A change in signal voltage resulting from EMF produced by the relative motion between test piece and coil is a result of the: b 45 Resistance. Resistivity of a material is a function of: d 44 Both a and b. Conductance is an electrical quantity which can also be defined as the reciprocal of: b 43 Effective depth of penetration. Nonlinear distortion characterised by the appearance of harmonics of the fundamental output when the input wave was sinusoidal is called: d 42 Depth of penetration. The depth beyond which a test system can no longer detect further increase in specimen thickness is the: b 41 Wobulation. The distance in a test specimen that eddy current intensity has decreased 37% of their surface value is the: a 40 Both a and c. 8 ohms. Which equation would be used to calculate the resistance of a length of conductor at room temperature other than standard temperature? c R = Ro (1 + a dT). NDT31-50316b ESTestMaker Answers A3-4 Copyright © TWI Ltd 48 Given copper at 20oC. With resistivity 5.9 ohm-cm and thermal coefficient of resistivity of 0.0039, what is the resistivity when the copper is warmed to 40oC.? c 49 When an eddy current probe is brought near a conductive sample the net magnetic flux in the system: b 50 Impedance changes in the coil. Inductive reactance. The equation 1/2πfC =: c 59 All of the above. The equation 2πfL =: a 58 57 x/σ. Eddy current flow in a test sample is accomplished indirectly by monitoring: c 57 All conditions. Phase lag of eddy currents in a sample is dependent on: d 56 14% Phase lag in degrees would be represented by (where x - depth, σ = standard depth of penetration): d 55 that at When inspecting a rod with an encircling coil the eddy current density at the centre of the rod is zero for δ = standard depth of penetration): c 54 Thick material and planar magnetic fields. At 2 standard depths of penetration, eddy current density is about the surface: c 53 All of the above. Strictly speaking, the standard skin depth equation; J/Jo = (e^-β) sin (wt- β), is true for only: a 52 Decreases. Eddy current density in a sample is: d 51 6.25 ohm-cm. Capacitive reactance. The vector sum quantity of resistance and reactance in an AC current is: a Impedance. NDT31-50316b ESTestMaker Answers A3-5 Copyright © TWI Ltd 60 In an AC circuit the total voltage across a resistor and an inductor in series is found by: c 61 The method of eddy current testing that uses a dedicated coil to induce eddy currents in a test piece and another coil to detect eddy current variations in the test piece is the method: c 62 Upward. Increasing which of the following parameters will move the operating point up on the impedance curve? a 70 Both b and c. An increase in electrical resistivity of a sample will move the operating point on the impedance curve: a 69 Two separate coils. In the send-receive method of eddy current testing the variations in eddy current flow due to flaws in the test piece are monitored by: d 68 Down. The send-receive method of eddy current testing uses: c 67 Down. An increase in test frequency will move the operating point on the impedance curve: b 66 Resistivity of sample. An increase in tube wall or plate thickness will move the operating point on the impedance curve: b 65 A single turn. On a normalised impedance curve which of the following parameters would move the operating point up the curve when increased? a 64 Send-receive. When the eddy current test system is represented by the transformer the sample can be considered the secondary winding with: a 63 Vector addition. Resistivity. In the impedance method of eddy current testing the impedance phase Ө (in degrees) is calculated from (w is the angular frequency, L is inductance, R is resistance): c Ө = Arctan (wL/R). NDT31-50316b ESTestMaker Answers A3-6 Copyright © TWI Ltd 71 The effect of sample and test parameters can be illustrated using: b 72 Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50kHz; what is the coil inductive reactance? d 73 Lift-off. When a simple bridge made up of 4 impedance arms, the voltage in adjacent arms of the bridge must be equal in. c 81 Eliminate the voltage difference between two coils. The most troublesome parameter in eddy current testing is: b 80 1%. Balancing is required in the eddy current instrument to: d 79 Xp = Xp sin Ө. Voltage changes across the probe due to a defect in most eddy current inspections are on the order of: a 78 35.3 ohms. If given total impedance of a probe operating on a test sample and know the impedance phase angle, what equation is used to determine the inductive reactance of the probe? d 77 20.2 ohms. Given a probe with 50 ohms resistance and 40 H inductance, when operated next to a copper sample at 20kHz the probe impedance is 55 ohms and impedance phase Ө is 40°, what is the inductive reactance of the probe when operating on the sample? c 76 2.51 ohms. Given a coil with 20ohms and 60 microhenries inductance in air and operated at 50kHz, when brought next to an inconel sample the probe impedance is 28.5ohms and impedance phase Ө is 45o, what is the probe’s inductive reactance? a 75 15.7 ohms. Given a coil with 2 ohms resistance and 20 H inductance and operated at 20kHz, what is the coil’s inductive reactance? b 74 Impedance diagrams. Both a and b. The result of operating an eddy current test instrument at a point other than balance point is: a Nonlinear voltage output with change in probe impedance. NDT31-50316b ESTestMaker Answers A3-7 Copyright © TWI Ltd 82 In the L-C circuit used by simple meter crack detectors, the circuit is operated: c 83 The reactive power of inductance and capacitance in a tuned L-C circuit are: a 84 AC bridge. 10-200 ohms. A parallel L-C circuit used in crack detectors has an inductive of 150ohms. The capacitive reactance would be about under normal operating conditions. c 93 for balancing but several The impedance of probes used in eddy current testing can vary over a range. Instruments must be able to balance over this range. Most instruments can handle prove impedances between: c 92 None of the above. Most eddy current instruments use some form of options are available for lift-off compensation. b 91 All of the above. Instrument frequency response is limited by: d 90 Have an equal or higher frequency response. X-Y recorders: d 89 Mixing modules. Recording of eddy current signals from ECT instruments requires that the recording instrument: c 88 Elimination of the effects of undesirable parameters. Multifrequency instruments have the same controls and functions as general purpose ECT instruments with the addition of: a 87 10-100kHz. The purpose of multifrequency ECT technique is: b 86 Equal. Crack detector type ECT instruments based on resonant circuits detecting surface defects on low resistivity materials such as aluminium would have operating frequencies in the range. c 85 Very near resonance frequency. 150 ohms. Compensation for undesirable material and coupling variations can be achieved by: b Multiple coil probes. NDT31-50316b ESTestMaker Answers A3-8 Copyright © TWI Ltd 94 An absolute probe requires a 95 Both b and c. As a general rule, probe diameter should be selected so that it is: b 105 Allow study of test specimen variations without concern from probe variations. Decrease in sensitivity resulting from increasing lift-off is more pronounced for: d 104 None of the above, no significant change occurs to the defect signal. The reason for normalising probe impedance is to: c 103 r. For a given sized defect, what significant defect signal change occurs when testing a plate using the through transmission (send-receive) method and the defect occurs first 25% of the wall thickness from the transmit coil, then 50% and 75%? d 102 Eddy current flow is perpendicular to the maximum dimension of the defect. Eddy current flow and its associated magnetic flux are a function of position under the coil. The relationship could best be described as being proportional to (r=radial distance from coil centre): a 101 Both a and b. Maximum response to defects detected by eddy currents are obtained when: b 100 Shape the magnetic field. In the send-receive probe arrangement where the driver and receiver coil are on opposite sides of a plate, signal variation will result from: d 99 To minimise lift-off. The purpose of the ferromagnetic core used in a gap probe is to: a 98 Differential probe. The purpose of spring loading an eddy current probe against the test material is: c 97 One coil. When two similar coils on the AC bridge of the eddy current instrument sense with the test material the probe is a (n): b 96 interact(s) with the test material. Less than or equal to the expected defect length. At high operating frequencies, the effective coil diameter (sensing diameter) is approximately equal to: b The actual coil diameter. NDT31-50316b ESTestMaker Answers A3-9 Copyright © TWI Ltd 106 Permeability changes are of greater concern in eddy current testing because: d 107 The reversal swirl that is observed on a normalised impedance graph showing the effects of decreasing thickness is a result of: a 108 Any or all of the above. A practical depth limit for flaw detection and location using eddy current test methods is about: b 117 Using a lower inductance probe. Most impedance eddy current instruments will not operate at resonance. This situation is remedied by: d 116 That is as high as is practical. Measuring the thickness of conductive layer on another conductor (neither being magnetic) requires: b 115 f = 1.6 /t2 (kHz). Thickness determination of a non-conductive coating on a conductive (nonmagnetic) material is done using a frequency: b 114 One third the plate thickness. Frequency for plate thickness determinations of thin sections can be approximated by ; where resistivity ( ohm-cm), t=thickness (mm) and δ =standard depth of penetration (mm). a 113 Comparison to reference samples. When given a plate sample for resistivity determination, test frequency should be selected such that skin depth is at least: b 112 Orientation. Using a typical impedance type EC machine with storage monitor, electrical resistivity determinations are made by: b 111 Both a and c. Which defect parameter will not affect the probe frequency you select to locate a defect? c 110 Skin depth and phase lag effects. The phase angle used to estimate defect depth is the angle between the: d 109 Both b and c. 6mm. Which of the following is not an advantage of the eddy current test method? c Clean smooth surfaces not required. NDT31-50316b ESTestMaker Answers A3-10 Copyright © TWI Ltd 118 When performing an eddy current test and you encountered a signal that could be a crack, permeability change or restivity change, you would: a 119 The f90 for tubing and plate are found using similar but different equations. These equations were determined: a 120 Low sensitivity to probe wobble. Differential probes. The main reason an eddy current coil can detect support plates in heat exchangers when testing tubes from the inside diameter is: c 128 Differential. Effects of temperature drift are reduced by using: a 127 probes. Which of the following is an advantage of the differential probe compared to the absolute? b 126 Both a and b. Long gradual defects can be missed by using b 125 Decreasing sensitivity to the far surface defects. Coil spacing on differential probes for general inspection purposes of tubing is usually: c 124 Higher defect sensitivity can be achieved using surface probes. To increase sensitivity to near surface defects using a bobbin style probe coil length and thickness are reduced. This however results in: c 123 Both a and b. Encircling probes (or internal probes) are likely to be replaced by surface probes for tubing with a diameter greater than 50mm. The reason for this is: c 122 Empirically. Problems with ferromagnetic indications occurring in material that is not ferromagnetic can be overcome by: d 121 Change the frequency. Magnetic flux is not restricted by the tube wall. A probe whose operating impedance is not between 20-200 ohms will most likely in: c Both a and b. NDT31-50316b ESTestMaker Answers A3-11 Copyright © TWI Ltd 129 Assuming resistance is negligible and probe inductance is 80 henries, for a cable with 5 x 10^-9 farads capacitance, what is resonance frequency? c 130 If a probe for internal tube testing has an average coil diameter of 11mm, what size would the tube inside diameter be to give a 0.9 fill-factor? b 131 2.3 kHz. What is the f90 for an encircling coil used on aluminium tubing, P =5.1 ohm-cm, wall thickness 5mm, diameter 40mm? a 139 Both a and b. Given a brass tube 20mm diameter (OD) with a 3mm wall and the resistivity of brass is 7.0 ohm-cm, what is the f90 for testing this tubing? b 138 1.1. The equation f90 = 3 /t2 applies to: d 137 6. The f90 frequency has been found empirically from the ratio of thickness and skin depth. For testing tubing this ratio is: c 136 Lower operating frequency. Test frequency for solid cylinders, maximum sensitivity to defects, resistivity and dimensions is obtained when f/fg=: b 135 Both a and b. A tube being tested by an internal probe has an ID to OD ration of 0.8. Under what conditions does this appear to be a thin wall tube? b 134 0.85. Impedance diagrams for cylinders are not the simple semi-circular shapes used for plate. This is a result of: d 133 11.6. An encircling coil is used on a 12mm diameter solid rod. What is the fill-factor if the average coil diameter is 13mm? b 132 250 kHz. 612 Hz. When tube testing at f90 (internal absolute probe), if ID wall loss moves the operating point for an absolute coil in a negative X direction, a shallow OD defect would move the operating point: c +Y. NDT31-50316b ESTestMaker Answers A3-12 Copyright © TWI Ltd 140 When tube testing (internal absolute probe) at f90 and setting OD wall loss to move +Y on the scope, what is the probably source of a +X moving signal? c 141 When interpreting eddy current signals by quadrature components on strip charts the X channel information is used for: d 142 Titanium à = 0.0400. Degree of cold working of which material can be determined by eddy current methods monitoring for permeability changes instead of resistivity changes? a 151 Indirect measurement of effects on restivity. Which of the following will have the largest resistivity change with change in temperature (à = thermal coefficient): c 150 All of the above. Metal hardness can be indicated by eddy current testing. This is accomplished by: a 149 Both a and b. What condition can be eliminated using multifrequency eddy current technique? d 148 Non-relevant or false indications. In multifrequency instruments 2 or more operating frequencies are input to a probe simultaneously. What output must be adjusted to permit effective vectorial addition? d 147 Tooling or handling equipment. Ferromagnetic deposits and inclusions are usually: b 146 Signals are too large making small defects hard to see. Ferromagnetic inclusions on or in normally non-magnetic aluminium will rise due to: a 145 You reinspect the area at 0.1f90. Vectorial addition of signals at conductive non-magnetic support plates is not usually viable because: a 144 Both a and b. To eliminate magnetic deposits as a possible cause of defect signals (ie a nonrelevant indication) it is recommended that: c 143 Dent. Austenitic stainless steel. Which of the following can cause variability in resistivity readings taken for the purpose of sorting? d All of the above. NDT31-50316b ESTestMaker Answers A3-13 Copyright © TWI Ltd 152 Relative permeability is measured in which units? a 153 The amount of reverse magnetising force required to eliminate the residual magnetic flux in a ferromagnetic material is: b 154 27.7. Given the permeability of free space is 4πX10^-7 Wb/A/m and the permeability of an iron bar is 7X10^-4 Wb/A/m, what is the relative permeability of the iron? d 162 Heating of the saturation coil. What is a resistivity of 6.2 ohm-cm as a % IACS? a 161 650Hz. Magnetic saturation techniques for EC testing that use DC saturation coils are limited to the amount of saturation achieved by: b 160 50 kHz. What test frequency has a standard dept of penetration of 1mm for a plate material with resistivity of 130 ohm-cm and relative magnetic permeability of 500? a 159 0.05mm. If a plate material has a resistivity of 65 ohm-cm and relative magnetic permeability of 50, what test frequency should you use to achieve f90 at a depth of 0.2mm? b 158 Both a and b. If testing a material and you have set up acceptable conditions for phase separation of 90° for 1mm sample depth when relative magnetic permeability is 1, what depth would the 90° separation occur at if relative magnetic permeability changed to 20? d 157 316. In order to facilitate testing of magnetic materials without the interference of permeability changes you would: c 156 The coercive force. Which of the following series of stainless steels is not likely to exhibit an increase in relative permeability with increasing cold working? d 155 No units (dimensionless ratio). 557. Small eddy current sensors in the vicinity of cracks could be used for. d Both a and b. NDT31-50316b ESTestMaker Answers A3-14 Copyright © TWI Ltd 163 Eddy current generation to determine material properties use detection of variations in: d 164 Electric fields are the same as: a 165 Both b and c. Real and imaginary. Voltage and current will be in phase for b 170 Phase lag of induced currents. Complex numbers are often used in the analysis of eddy current test systems. Complex numbers have 2 components, they are: a 169 0 A. In an eddy current test set-up, magnetic lines of flux from the probe which fail to couple the test piece: d 168 in an AC circuit. Pure resistance. The phase angle between applied voltage and resultant current in an AC circuit of pure inductance is: c 90°. 171 In an R-L circuit d Both a and b. 172 Surface coil eddy current transducers are: d 173 very to a wire the The time constant of the circuit is a ratio inductance to resistance (L/R). This accounts for: b 167 Potential differences. Two insulated wires are wound on a plastic rod such that they are positioned close to each other but not touching. The ends of one wire are connected battery; the ends of the other are connected to a galvanometer. If the connected to the battery has 1 amp flowing through it, what will galvanometer read? a 166 All of the above. is plotted on the ordinate (vertical axis). None of the above. Measurement of the thickness of a non-conductive coating would utilise the effect: b Lift-off. NDT31-50316b ESTestMaker Answers A3-15 Copyright © TWI Ltd 174 The inductance in the excitation coil is proportional to the diameter square (D2) and the number of turns squared (N2). The voltage induced in the pickup coil is proportional to: c 175 The purpose of small diameter and high frequency probes for determining thickness of thin coatings on conducting substrates is to: a 176 Wall thickness to outside tube radius = 0.01. What is the purpose of DC magnetic bias in eddy current testing? b 184 1.0. When using an external encircling coil the frequency ration f/fg to obtain maximum sensitivity to all test variables will be greatest for which variety of heavy wall tube? c 183 Maximum displacement to the right. What f/fg ration is recommend for testing thin wall non-magnetic tubing for cracks, alloy variations or wall thickness variations? b 182 Skin effect methods attenuate the field rather than change the path. Maximum test sensitivity is obtained at which point on the signal locus of the complex plane? a 181 A path of low magnetic reluctance. Shielding obtained by eddy current skin effect differs from magnetic methods of shielding in which way? b 180 Ensure the coil axis is perpendicular to the test surface. Magnetic shielding technique provides the magnetic field lines of the eddy current probe with: c 179 All of the above. The purpose of curved wear pieces (shoes) to guide surface probe assemblies is to: b 178 Minimise the eddy current field in the substrate. The magnetic flux density around an empty test coil is reduced by increases in when testing non-magnetic materials: d 177 N2 and D2. Reduce magnetic permeability’s of ferromagnetic test materials. When both a primary (energising) and secondary (pickup) coil are used as an encircling coil probe, the time varying flux in the test piece induces an AC voltage in: c Both a and b. NDT31-50316b ESTestMaker Answers A3-16 Copyright © TWI Ltd 185 Phase angle differences of eddy currents greater than about 100° is not recommended for tube testing with encircling coils because: a 186 Which of the following is the direct cause of eddy currents in a test piece placed in an encircling transducer? a 187 Voltages from the mercury tests are multiplied by the relative magnetic permeability of the ferromagnetic material. The through-transmission technique is used for testing of sheet and foil under certain conditions: d 195 Relative magnetic permeability. Defect effects from tests in the mercury cylinder can be applied to ferromagnetic materials for practical applications provided: c 194 The vertical and horizontal scales are increased by the magnitude of the relative permeability. When testing ferromagnetic bars with an encircling coil, the effects of changes in are reduced or eliminated by DC magnetic saturation. c 193 The angle between diameter and conductivity locii is greater. The complex impedance plane presentation for testing a ferromagnetic bar should be changed in what way from the same test on a non-ferromagnetic bar? c 192 Diameter changes affect the test frequency ratio. Separation of diameter and conductivity effects is better carried out at frequency ratios greater than 4 because: b 191 Increased secondary coil voltage. Locus curves for diameter changes on the test piece are not straight lines on the normalised impedance plane. Why is this so? b 190 None of the above. All other conditions being equal for a bar tested in an encircling coil system, an increase in relative permeability of the bar tested would result in: b 189 Induced voltages form the AC magnetic field. The limit frequency is: d 188 Eddy current density on the inner wall is too low for crack detection. Both a and b. For two separate objects with different relative permeabilities and resistivities, equivalent eddy current tests can be performed by adjusting test frequencies. This is explained by: a The similarity law for eddy current testing. NDT31-50316b ESTestMaker Answers A3-17 Copyright © TWI Ltd 196 Eddy current tests using encircling coils would provide similar test coil impedances or voltage signals for tests on 100mm diameter aluminium rod and 2mm diameter steel wire if: c 197 The best method of measuring the effects of a specific discontinuity totally within a test specimen but at different depths and orientations is by using: a 198 Increase coil to diameter. Sensitivity of conductivity measurement with the probe coil is: d 206 Bent slightly left towards increasing f/fg values. In plate testing, to minimise effects of lift-off variations you would: b 205 To increase effective coil distance. The lift-off locus is: b 204 None of the above. The effect of increasing coil diameter on the effective coil distance is: a 203 Changes in either parameter results in the same change in transmission coefficient. For a non-magnetic foil thickness D, conductivity σ, the effective coil distance is found from Aeff = (253,000/fg σ D). Effective coil distance will decrease if: d 202 Both b and c. In a through transmission test of sheet products, why might a metered output monitor the product of thickness and conductivity (absolute measurement method)? a 201 Coil lift-off locus. When a metal sheet is inserted into a through transmission probe arrangement, the transmission coefficient phasor: d 200 Suitably shaped insulators inside a mercury filled tube. The curve traced on X-Y storage monitor as an active coil is brought up to a sample of 1100 aluminium (100% pure) is called the: b 199 Both a and b. All of the above. The apparent impedance curve for two different metals of the same thickness will be the same if: b Frequencies are adjusted so σ f is equal (where σ is conductivity and f frequency). NDT31-50316b ESTestMaker Answers A3-18 Copyright © TWI Ltd 207 A practical f/fg ratio for thickness measurements would be in the range of 1-7.5. This would provide maximum sensitivity to: c 208 In eddy current tests to determine non-conductive coating thicknesses, probe diameter and operating frequency are selected to minimise the effects of what parameter? a 209 Reduced fill factor. In remote field eddy current testing, how far does the direct coupling zone extend from the exciter coil? c 217 Both a and b. The result of using a longer encircling test coil to test a spherical object as compared to a short coil or hemispherical coil would be: b 216 Both b and c. When using surface coils for crack detection, shallow cracks and lift-off cannot be separated unless: d 215 A decrease in fill factor. A part with a length to diameter ration to 1 tested in an encircling coil: d 214 All of the above. When inspecting spheres with an encircling coil, what is the equivalent effect of increasing the coil length? d 213 A difference in conductivities between the two materials. To discern very small cracks using a surface coil you would use a relatively high frequency-conductivity product (σ f). Which of the following would then be true? d 212 σD = σ1D1 + σ2D2. Determining plating thickness of a conducting non-magnetic material on another conducting non-magnetic material requires: a 211 Conductivity. If a sheet was composed of 2 metallic layers with thicknesses D1 and D2 and conductivities σ1 and σ 2, what would the equivalent product be when tested by through transmission? a 210 Lift-off. 2 inside pipe diameters. Remote field eddy current testing is a technique commonly used on: a Ferromagnetic tubes. NDT31-50316b ESTestMaker Answers A3-19 Copyright © TWI Ltd 218 Signals received in the remote field eddy current set-up give two response off large defects, one occurs due to the receiver coil passes the defect. What causes the other signal? c 219 In the range of about 3-13mm wall thickness, what frequency range would be used for low frequency remote field eddy current testing of ferromagnetic tubing? a 220 Disruptions in lattice structure. Which of the following will increase conductivity of an alloy? c 228 Altered lattice structure inhibits electron flow. Work hardened aluminium has a higher resistivity than annealed aluminium for what reason? b 227 Alloys. Alloying metals added to pure base metals result in decreasing conductivity of the initial value of the pure base metal. Why does this occur even with alloying metals having higher conductivity than the base metal? a 226 Both a and b. Substitutional solid solutions and interstitial solid solutions of metals are forms of: b 225 Specimen thickness exceeds depth of eddy current penetration. In general, the edge effect seen as a probe is moved towards edge of a magnetic test piece as compared to a non-magnetic test piece would be recognised by what feature? c 224 All of the above. Sorting of materials by impedance values of an eddy current probe require: b 223 Samples of known materials. Applying a DC electric field to a ferromagnetic coil is done for what purpose? d 222 10-300Hz. When eddy currents are used for sorting techniques it is usual to establish impedance values from: b 221 The exciter passing the defect. Annealing. What would the effect on conductivity signal be as radius of curvature of the test piece is decreased? b Conductivity measured would decrease from the true value. NDT31-50316b ESTestMaker Answers A3-20 Copyright © TWI Ltd 229 Although specimen and standard may be within the recommended 5oC temperature difference for resistivity measurement, why might the value determined still be incorrect? a 230 Natural aging of aluminium alloys occurs at what temperature? c 231 and without reference Do not use eddy current methods. Hysteresis. Electrophoresis. Calibration on curved surfaces. Verify accuracy of a test. The most common and reliable method of manufacturing artificial cracks for eddy current standard is by: a 239 material What is the purpose of calibration reference standard? b 238 bulk What is main advantage of foil calibration standards over affixed coatings calibration pieces? d 237 on Which of the following is not a method used to manufacture notches in calibration standards used for eddy current tests of tubing? b 236 made When a ferromagnetic material has a magnetising force applied to it, the magnetic flux that builds within the material lags the applied force. The same lag occurs upon the reduction in magnetising force. What is the lag called? b 235 A significant increase. Resistivity measurements standards: c 234 None of the above (no effect). What is the effect of ferromagnetic materials on the inductance of an eddy current test coil? b 233 Room temperature. What is the effect on eddy current determined properties of aluminium alloys that have been annealed for an excessive amount of time? d 232 Measurement temperature and the temperature the standard was originally established at are different. Electric discharging machining. What is not one of the advantages of drilled holes being used as reference standard? c Behaves like a crack. NDT31-50316b ESTestMaker Answers A3-21 Copyright © TWI Ltd 240 What is the advantage of artificial defects made by the EDM process? c 241 A significant disadvantage of using a natural crack as a calibration standard is accurately sizing it. What is the only reliable direct sizing method to determine nature crack depth? d 242 The phase will be different at different frequencies. Compared to single frequency units, multifrequency eddy current instrument circuits are: b 251 Pie-slice. Multifrequency can discriminate signals at the same depth because: b 250 4. What would a polar co-ordinate based phase-gate look like? c 249 Resolution. How many thresholds must be set on the CRT display of an eddy current instrument in a box gate alarm system? c 248 Meter movement (rise time). What is the most significant drawback of dot matrix displays of EC signals compared to CRT displays? b 247 All of the above. What limits the scanning speed when using meter display eddy current instruments? b 246 Reducing receiver gain. Which of the following noise sources can be filters with the appropriate electronics in an eddy current instrument? d 245 Codes and specifications. Which of the following is not a means of suppressing an undesired eddy current test signal? b 244 Cutting the specimen open and optically sizing it under a microscope. What is used to regulate the consistency of the manufacturing of calibration standards? b 243 Accuracy. The same except for signal separation and combining circuitry. A circuit block that accepts a binary number and translates it to an analogue voltage or current proportional to the binary number is a (n): c Digital-to-analogue converter. NDT31-50316b ESTestMaker Answers A3-22 Copyright © TWI Ltd 252 A circuit block that uses an analogue voltage as an input and outputs, a proportional binary value is a (n): b 253 Hall detectors are used to sense magnetic fields. They detect: d 254 Using two Hall detectors. Increasing excitation coil size. What are slip rings used for in eddy current inspection systems? a 264 Both a and b. When using Hall detectors, how are sensitivities to relatively great depths achieved? c 263 Rate of change of total flux linkage. Eddy current test systems using Hall detectors can accomplish differential tests by: a 262 Single pass inspections of large surfaces. Instrumentation for systems using Hall detectors instead of pickup coils are different in what respect? c 261 Both a and b. Which of the following are Hall effect detectors not sensitive to? c 260 Zero. Linear multichannel Hall detector arrays are ideal for: a 259 Holes. The magnitude of the Hall voltage is: c 258 as charge carriers. The ideal signal voltage in a hall detector element in the absence of a magnetic field is: a 257 Electrons. P-type semiconductors use b 256 Both a and b. N-type semi-conductors use what form of charge carrier? a 255 A/D converter. Electrical contacts in rotating heads. To avoid rotating parts, probes or test piece, what system would be used to inspect round bar stock? d Both a and b. NDT31-50316b ESTestMaker Answers A3-23 Copyright © TWI Ltd 265 A differential transducer with the two windings around perpendicular to each other used to detect both longitudinal and transverse cracks is called a (n): a 266 In what way is eddy current testing more suitable to high speed production tests on hot metals than ultrasonics? b 267 Both a and b. You are given 2 plates of identical size (50x50x10mm) both painted with a thin coating of black acrylic paint of the same thickness. Eddy current test indicate both have a conductivity of 37% IACS, yet one is nearly twice as heavy as the other. How is this possible? c 274 Noise results as the metal cools below the curie point. What is the effect of over-aging on aluminium heat treatable alloys? c 273 Variations due to heat treatment overlap ranges of conductivity. When online testing of ERW welds in steel pipe using eddy current testing, what problem occurs if the inspection is performed too far from the induction heating coils used for normalising the weld? a 272 3. Truly effective sorting of aluminium alloys by eddy current determination of resistivity is not possible because: a 271 All of the above. When ECT is used to test thickness of coatings having a tolerance range, what is the minimum number of calibration specimens required to calibrate the instrument? b 270 The probe is water cooled. Seamless pipe and tubing are often made from billets made from continuous cast blooms. The rounds, as the billets are called, are test by eddy current to detect what types of defect? d 269 No stream of water coupling is needed for ECT. Eddy current testing of hot billets (1,100°C) can be done provided what precautions are taken? a 268 Orthogonal winding transducer. Different metals are causing wrong readings. What is the most effective way of assessing heat or fire damage to heat-treatable aluminium on aircraft? c Eddy current conductivity tests. NDT31-50316b ESTestMaker Answers A3-24 Copyright © TWI Ltd 275 Alpha-case forms on titanium and its alloys at elevated temperatures. Eddy currents are used to establish the depth of case. What is the cause of the formation of alpha-case? a 276 When a single channel strip charge recorder is used with eddy current testing of bolt holes using spinning probes, what 2 parameters are recorded? c 277 The locations at which the tube is roll expanded into the tube supports. Magnetic deposit. In eddy current inspections of chiller tubes, freeze cracks located at freeze bulges are often not possible to detect using conventional differential probes because: b 285 Fatigue cracks. During evaluation of an indication in a heat exchanger tube, the probe is moved back and forth over the defect. It is noted that the indication has changed position along the length of the tube. What is the likely source? A: a 284 in Finned copper tubing used in air conditioning units has smooth land areas at regular intervals along the tube. What is the purpose of these land areas? b 283 The speed at which tests can be performed. Eddy current test methods are more sensitive than x-rays for detection of aircraft structures. c 282 Subsurface corrosion detection in multilayer structures. What is the biggest advantage eddy current test methods have that make them the most frequently used NDT method in the automotive industry? c 281 Improved crack detection by suppressing lift-off output. Low frequency eddy current (100Hz to 5kHz) is commonly used in aircraft inspections for: b 280 Both b and c. Some CRT display eddy current instruments allow X and Y gains to be adjusted independently. Increasing Y gain and reducing X (eg Y = 0.2V/div, = 2.0V/div) accomplishes what? b 279 Y (vertical) output of signal vs. time. Why are cadmium plated steel bolts used as fasteners on aircraft? d 278 Oxygen diffusion from the heated surface. The bulge signal is so large it masks the crack. Generally in multifrequency techniques for in-situ boiler tube inspections, high frequencies are used to suppress while low frequencies are used to suppress . c Internal variables, external variables. NDT31-50316b ESTestMaker Answers A3-25 Copyright © TWI Ltd 286 In order to do computer modelling of eddy current fields you must provide: d 287 Multifrequency eddy current testing utilises: a 288 To avoid hysteresis effects. The higher the value of inductance for a given frequency the greater the degree of: b 297 Over a defect free area. Why are eddy current coils not made using iron wire? a 296 Ellilpse. When an eddy current is balanced for surface testing for flaws, where is the probe placed? c 295 Cyclic variations in magnetic permeability. In the 1960s a non-storage type oscilloscope was used for eddy current tests. The defect free specimen gave a horizontal line. A defective specimen gave a (n): c 294 Curvature should be small within the region directly below the cross-sections of the coil. How does hysteresis manifest itself when testing ferromagnetic materials? c 293 It is non-contacting. For practical applications of surface probes on curved surfaces: a 292 Increased conductivity. What is the advantage of eddy current testing over the potential drop method for sizing surface cracks? b 291 Low cost of equipment. Increasing temperature of a dielectric (insulating) materials has what effect? b 290 A single probe operating at more than one frequency. Multifrequency instruments may be one of two types; simultaneously frequency or alternate frequency. Which is not an advantage of the simultaneous frequency systems? c 289 All of the above. Sensitivity. The transmit-receive or transformer style probe provides: c Both a and b. NDT31-50316b ESTestMaker Answers A3-26 Copyright © TWI Ltd 298 Inductance increases improve eddy current sensitivity. Why is increasing coil area not a preferred method of increasing sensitivity even though inductance is increased? b 299 How does the differential (or auto-comparator) coil provide insensitivity to gradual changes? a 300 Rough surfaces. The through transmission method has the advantage that: b 302 Coils are wound in opposition to each other. Phase adjustment on simple conductivity meter instruments is especially useful for what conditions? c 301 Resolution of defects is decreased. Conductivity and thickness can be measured simultaneously. What degree of accuracy can be expected when using eddy currents to determine paint thickness 10 c 303 Both a and b. Using a shielded ferrite coil and the pulsed eddy current technique, penetration of measurable currents in a metal sample can be increased to (where δ the standard depth of penetration): d 308 To achieve greater penetration in ferromagnetic materials. Large DC saturation units for eddy current inspection of ferromagnetic tubing are often required. What technique can be used to avoid use of these heaving DC saturation units? d 307 Both a and b. What is the purpose of pulsed saturation eddy current testing? b 306 Surface probes or arrays. Multifrequency techniques are performed using: c 305 m. Tubes with a diameter of more than about 50mm are more effectively tested using than encircling probes. b 304 1.0 m thick? 10 δ. As conductivity of a material approaches infinity its resistive losses approach zero. What type of material exhibits such extremes? c Superconductors. NDT31-50316b ESTestMaker Answers A3-27 Copyright © TWI Ltd 309 On the normalised impedance plane showing the effects of changing conductivity (σ) the coil’s normalised resistance is zero under what condition? c 310 What does an increase in operating frequency do to the probe coil inductance? d 311 Phase analysis. The ration of the square of the diameter of a cylindrical test piece to the square of the average diameter of the test coil is the: b 320 Impedance analysis. An instrumentation technique that discriminates between variables in the test piece by different phase-angle changes these variables produce in the test signal is: b 319 All of the above. The analytical method that consists in correlating changes in amplitude, phase and/or quadrature components of a complex test signal voltage to electromagnetic conditions in the test piece is b 318 All of the above. In what way does computer acquisition and analysis of eddy current signals (particularly heat exchanger tubing) out-perform humans? d 317 Both a and b. During an eddy current inspection of heat exchanger tubing, what is the purpose of recording a calibration signal with each tube inspected? d 316 Magnetic focusing probes. What are the main limiting parameters for a single coil probes dimensions? d 315 (Zc X Zs)/(Zc + Zs). The only way to reduce or eliminate the edge effect is by: b 314 Pre-aligning the domains with DC saturation. The coil to specimen impedance Z can be defined by (where Zc is coil impedance and Zs is specimen impedance): a 313 None of the above. The heating of a ferromagnetic part that occurs when the AC field works to align the magnetic domains into a preferred magnetic orientation is reduced by: c 312 Both a and b. Fill factor. The frequency providing the highest signal-to-noise ratio for detection of an individual property of the test piece is the: a Optimum frequency. NDT31-50316b ESTestMaker Answers A3-28 Copyright © TWI Ltd 321 Two or more coils in electrical series opposition arranged so EM conditions not common to the areas of the specimen being tested produce a bridge imbalance is a (n): c 322 The phenomenon whereby depth of penetration decreases with increasing frequency is called: a 323 Acceptance limits. Differential coils are, in some areas, also called: b 331 Reduced sensitivity to outside wall defects. Test levels used in ECT that establish the group into which a material under test belongs are termed: a 330 Only one or two parameters are subject to change. What is the disadvantage of the multi-pancake probe used for internal tube inspections as compared to the axial bobbin type probe? c 329 Modulation analysis. In order that useful results be obtained from an eddy current test, what must be true about the test specimen? a 328 Direct current. The method whereby desirable frequency signals are separated from undesirable frequency signals from the modulating envelope of the carrier frequency signal is called: b 327 Recovery time. Current flow that is time constant in both direction and amplitude is: a 326 Defect resolution. The time required for a test system to return to its original state after it has received a signal is the: b 325 Skin effect. The property of a test system that allows separation of signals from defects on close proximity to each other is: c 324 Differential probe. Bucking coils. A test level above or below which test specimens are found to be unacceptable is called? d Both a and b. NDT31-50316b ESTestMaker Answers A3-29 Copyright © TWI Ltd 332 A network that passes electromagnetic wave energy over a described range of frequencies and attenuates energy at all other frequencies is a (n): a 333 The slope of the induction curve at zero magnetising force as the test piece is being taken from its demagnetised state is the: b 334 12mm. What is the standard depth of penetration of 304 stainless steel (68.96 cm) having 60% cold work applied ( rel=2) tested at 20kHz? b 339 0.907. Given a tube with a 15mm OD and 1.5mm wall, what size (average diameter) coil is used to obtain an 85% fill factor for an internal inspection? c 338 A two way sort. Given an encircling coil with an average coil diameter of 10.5mm and testing a tube 10mm OD with a 1mm thick wall, what is the fill factor of this set up? c 337 Magnetic history. An electromagnetic sorting based on a signal response from the material under test above or below a level established by two or more calibration standards is: a 336 Initial permeability. The magnetic condition of a ferromagnetic part based on its previous exposures to magnetic fields if the part’s: c 335 Filter. 2.1mm. What is the standard depth of penetration for 301 stainless steel having been 25% cold worked (71 a 340 Given a standard depth of penetration of 1.3mm exists for a 10kHz test of navel- b ohm-cm), what is the effective depth of penetration? 3.9mm. The quantity actually monitored by an eddy current probe is: c 342 ohm-cm, rel=10) tested at 10kHz? 1.3mm. brass (6.63 341 ohm- Probe electrical impedance. Electromagnetic induction, on which ECT has its foundations, was first discovered by: b Faraday. NDT31-50316b ESTestMaker Answers A3-30 Copyright © TWI Ltd 343 The voltage changes used to determine various parameters in eddy current testing consist of changes in: c 344 The right hand rule for determining magnetic field direction around a current carrying conductor assumes: a 345 Both a and b. The intensity of a magnetic field that a unit magnetic pole experiences of a force of one dyne is one: a 354 Equal to the rate of change of the magnetic flux through it. An alternating voltage in a coil brought near a sample that has a finite impedance will result in: c 353 Both a and b. Faraday’s law states that the magnitude of the induced voltage in a circuit is: a 352 Decrease its impedance. Electromechanical energy conversion is possible due to: c 351 Test sample. As an operating eddy current probe (a coil) is brought near a conductive sample the induction of eddy currents in the sample causes the probe to: a 350 Electric intensity. An eddy current test system can be considered a form of transformer. As such, the secondary side would be the: c 349 Total magnetic flux inside the coil. Magnetic induction or the force per unit pole in a magnetic field is the magnetic analog of: a 348 Modern theory current flow. The product of the magnetic flux density in a loop of a current carrying coil times the area of that coil gives: d 347 Conventional current flow. The left hand rule for determining the magnetic field around a current carrying conductor assumes: b 346 Both a and b. Oersted. A single magnetic line of flux is given the unit: c Maxwell. NDT31-50316b ESTestMaker Answers A3-31 Copyright © TWI Ltd 355 Magnetic flux density is expressed in: d 356 Alignment of the magnetic domains in iron by an external field result in: c 357 Positive. A negative thermal coefficient of resistivity would be characteristic of: b 367 All of the above. The temperature coefficient of resistance of a pure metallic conductor is always: b 366 AC power transformers. Which of the following will have an effect on the electrical resistance of a wire? d 365 All of the above. Eddy currents are an undesirable feature in: a 364 1.724 ohms. Resistance of a piece of wire is a function of: d 363 To maintain a simple direct proportionality between current and coil rotation. Given a wire made of copper with resistivity 1.724 ohm-cm, that is 1cm in length and has a cross-sectional area of 1cm2, what is the resistance of this section of wire? c 362 4 amperes. The purpose of using a radial magnetic field around the current carrying coil in a galvanometer instead of a parallel magnetic field is: c 361 Coulombs. If 20 coulombs of charge passes a point in 5 seconds, the electric current value would be: a 360 Both a and b. The product of current in amperes times time in seconds gives units of: b 359 Magnetisation. The force between point magnetic poles is: c 358 All of the above. Some semi-conductors. In a nonmagnetic material the back EMF produced by the induced eddy currents has what effect on the probe? d Both a and b. NDT31-50316b ESTestMaker Answers A3-32 Copyright © TWI Ltd 368 The decrease in eddy current density with increasing depth from the surface is: b 369 The time dependent component of the skin depth equation indicates: c 370 180°. Arctan (x/R). In eddy current terminology phasors are used for: a 379 Increase. The phase of the impedance in an AC circuit is found from: d 378 The probe cables. In the eddy current probe circuit the capacitive component of its impedance is degrees out of phase with its inductive component: c 377 1 radian. The inductive reactance component of an eddy current probe coil’s impedance will with increasing AC frequency: a 376 114°. For the purpose of determining electrical characteristics of a coil/sample combination, capacitance can be an important factor in: a 375 either a or b depending on whether plate or tube testing is being done. Phase lag in the test sample for a void at 1 standard depth of penetration is: a 374 probes are needed. The phase lag, in units of degrees, for an eddy current signal displayed on a typical impedance plane scope for a void originating 1 standard depth of penetration below the surface would be: c 373 5. To ensure planar shaped magnetic field d 372 Phase lag of the signal with depth. For the calculation for eddy current density to apply, a sample should be relatively thick. The minimum thickness to allow the simple equation to apply is about δ (where δ is the standard depth of penetration): c 371 Exponential. Voltage amplitude and phase representation. On the ideal impedance diagram the effect of reducing mutual coupling between probe and sample would be to have the impedance point: c Trace smaller semi-circles. NDT31-50316b ESTestMaker Answers A3-33 Copyright © TWI Ltd 380 The impedance method of eddy current testing uses: b 381 As the diameter of the eddy current probe increases, the operating point on the normalised impedance curve moves (for a surface probe ie not for tube testing). b 382 Down. Impedance graph display. The decrease in semicircle radius of the impedance curve display when lift-off increases indicates: a 390 D2. An inductive and a resistance impedance change in the test coil resulting when an operating eddy current probe is moved near a conductive test sample is represented on a (n): d 389 L An increase in probe diameter will move the operating point on the impedance curve: b 388 Inside the original curve. What best describes probe inductance as a function of probe diameter? ( indicates proportional to): a 387 Open circuit. All other factors constant, increasing lift-off will move the operating point on the impedance curve: c 386 Resistive load in parallel with the coil’s inductive reactance. Using the analogy of the coil/sample as a transformer circuit, when the coil is held far from the sample we can approximate a (n): b 385 Send-receive method of ECT. When a probe/sample combination is modelled as an equivalent circuit, the secondary circuit load equivalent would be considered a (n): a 384 Down. Variations in the flow of eddy currents caused by flaws in the test piece are monitored as voltage fluctuations in the secondary coil in the: a 383 Changes in voltage across the primary coil. A smaller change in coil impedance. Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50kHz; what is the impedance phase angle? c 17.4o. NDT31-50316b ESTestMaker Answers A3-34 Copyright © TWI Ltd 391 The most significant instrument component required to detect the small variation in probe impedance or voltage caused by detecting defects in eddy current testing is the : d 392 Conversion of the AC unbalance voltage signal to a DC signal retaining amplitude and phase characteristics is done for what reason? a 393 90o. Internal filtering to decrease instrument or system noise results in: d 401 All of the above. Quadrature components of the bridge AC output are generated by sampling the sinusoidal signal at two positions apart on the waveform. b 400 Both a and b. The typical figure 8 pattern that occurs with a differential probe moving over a defect is a result of: d 399 35o. In eddy current instruments, bridge circuits are used for: c 398 Not possible to determine from information given. Given the resistive load of a probe/sample circuit as 5.1 ohms and the resistance of the probe when operated in air as 15 ohms, what would the impedance phase angle be if total impedance of this circuit was 24.5 ohms? b 397 19.3 ohms. Given a probe operating at 0.5MHz next to a brass sample, total probe impedance is measured at 47.2 ohms, if the impedance phase angel is 45o what is the resistive load of the sample? d 396 51.4°. The impedance phase angle of a probe operating next to a copper test sample is 40o. What is the inductive reactance of the probe in this situation if the total impedance measured is 30 ohms? a 395 The AC signal is too difficult to analyse. Given a coil with 2 ohms resistances and 20 H inductance and operated at 20kHz, what is the impedance phase angel (in degrees)? d 394 Amplifier. All of the above. Most eddy current instruments can tolerate an impedance mismatch in the AC bridge on the order of: b 5%. NDT31-50316b ESTestMaker Answers A3-35 Copyright © TWI Ltd 402 In the L-C bridge circuit used by simple meter crack detectors, the capacitor is connected in parallel with the in the bridge circuit. a 403 At the resonant frequency of an L-C circuit, output voltage for a given measurement: c 404 All of the above. Modulation analysis is a specialised ECT method that requires: c 412 allow no net voltage in the receive coils when both sense the same material. Now obsolete, the ellipse and slit methods of eddy current testing: d 411 Send-receive instruments. In send-receive ECT systems, probes with 2 receive coils have those coils would in opposition. The purpose for this to: c 410 Operating frequency (by less than 25%). Which of the following systems has the advantage of being unaffected by temperature variations? b 409 Gain, lift-off, balance. In resonant circuit crack detectors, the lift-off control actually varies: b 408 Not selectable. Resonant circuit crack detectors have a meter output and 3 controls: a 407 10-200 ohms. Test frequencies for crack detectors operating at or close to resonant frequency are: a 406 Maximum. On most eddy current instruments using the impedance method, the AC bridge circuits can usually balance coils having impedances in a range of. d 405 Probe coil. Relative motion between the coil and sample. FM tape recorders have often been used to store eddy current signals for subsequent retrieval. Frequency response for these instruments is. b Proportional to recording speed (length of tape past the record head per unit time). NDT31-50316b ESTestMaker Answers A3-36 Copyright © TWI Ltd 413 Frequency response of an instrument is based on the fact that the output signal of an instrument will be less than the input signal as inspection speed increases. Instrument frequency response is defined as the frequency where output signal is -3dB from the input. This would relate to a volt signal out for a 1 volt signal input. c 414 Given a parallel L-C circuit with a probe inductance of 80 x 10^-6 Henries and operated at resonance frequency, 252kHz, what is the cable capacitance? c 415 Bridge nulling. The effective probe diameter extends to about d 423 Ferrite cups. In an absolute probe configuration, a second coil, apart from the sensing coil, is required for: a 422 Test frequency not affecting relative impedance of the coils. To reduce the effective sensing diameter of surface probes operating at relatively low frequencies, the use of is recommended: b 421 Temperature compensation. Mounting a disc of metal, having similar properties to the test material, next to the reference coil in an absolute probe has the advantage of: c 420 fr =1/2(π)(LC)^½. Which of the following is not a reason for using a ferrite core on the sensing coil of a pencil probe? c 419 All of the above. Resonance frequency can be determined for a parallel L-C circuit by: a 418 126.5 ohms. When selecting an eddy current instrument for a particular project you need to know: d 417 5 x 10^-9 farads. Given a parallel L-C circuit with cable capacitance 5 x 10^-9 farads and operating at a resonance frequency of 2252kHz, what is the inductive reactance of the probe? b 416 0.707. 4 skin depths. Ferrite cups can be used to obtain a beyond the coil diameter. without affecting depth of penetration. A concentrated field. NDT31-50316b ESTestMaker Answers A3-37 Copyright © TWI Ltd 424 Which of the following is not a probe parameter affecting impedance results? a 425 Normalising probe impedance for impedance graph displays is accomplished by: c 426 Near the knee of the curve. To prevent error in resistivity determinations caused by temperature, you should: d 434 Limited by probe to instrument impedance matching, cable resonance and cable noise. When making resistivity measurements on unknown samples, the frequency used is selected such that the operating point on the impedance graph is: c 433 Down. Maximum frequency you would use for determining thickness of a non-conductive coating on a conductor would be: d 432 Operating point on the normalised impedance graph. If lift-off is arranged on the eddy current storage monitor so the signal moves from right to left as the probe is moved away from the sample, an increase in sample thickness would conventionally move: a 431 Skin depth and phase lag effects. The characteristic parameter, Pc, used by Deeds and Dodd is primarily a modelling tool. Test conditions with the same characteristic parameter have the same: c 430 The skin depth (δ). Varying frequency for a probe on a given specimen will move the operating point down the impedance graph with increasing frequency. If the specimen is not thick, a reversal swirl occurs forming a knee. This is a result of: a 429 Up the curve. For a thick specimen, test frequency should be selected to provide good separation from lift-off variations. This is facilitated by setting frequency so that the greatest expected defect depth is at: b 428 Both a and b. All other parameters constant, an increase of permeability in the test piece causes the operating point on a normalised impedance curve to move: a 427 Frequency. Both a and c. The most significant difficulty in determining thickness of conductive coatings on conductors is that: a Variations in base material as well as coating material will affect the signal. NDT31-50316b ESTestMaker Answers A3-38 Copyright © TWI Ltd 435 The problem with overcoming probe-cable resonance by operating above 1.2fg (fr-resonance frequency) is: b 436 What is the effective diameter of a surface probe with a 5mm diameter coil used on a sample with p = 72 ohm-cm and operated at 2MHz. (p is resistivity): b 437 The fill factor for the reference coil is <<1. When using a bobbin type differential probe, sensitivity to near surface defects can be improved by: c 445 Equal to wall thickness. The reference coil in a bobbin style probe can be mounted concentrically inside the test coil and the probe still be considered and absolute probe because: c 444 All of the above. Encircling or bobbin style probes used for tube testing require careful design of coil size to optimise sensitivity and coupling. Coil length and coil depth should be about: a 443 Its approach signal. The best way to distinguish between localised resistivity changes and a real defect is: d 442 2Ө. A very shallow surface defect can be distinguished from lift-off by: a 441 Work hardened 7075-T6 (AL alloy). The phase angle (as measured from the lift-off signal) of a shallow surface or sub-surface defect is related to the eddy current phase lag á=x/δ(radius), where x = flaw depth and δ = skin depth. The phase angle seen on the storage monitor is approximately: c 440 0.8. Which is not a source of ferromagnetic indications? a 439 6.2mm. The ration of thickness to skin depth t/δ that provides a 90° separation between lift-off and thickness change is empirically derived. It is found to be about for plate testing: b 438 Greatly reduced sensitivity. Decreasing coil spacing. Symmetry of a differential signal as the probe is moved over a defect will depend on: d All of the above. NDT31-50316b ESTestMaker Answers A3-39 Copyright © TWI Ltd 446 Insensitivity to gradual changes in dimensions or properties is both an advantage and disadvantage, depending on the situation. This feature is exhibited by: c 447 If a defect is longer than the spacing between the coils on a differential coil, the defect can only be recognised as such if: b 448 Similar to decreasing fill factor. The characteristic frequency, fg, is the frequency for which the Bessel function solution to Maxwell’s magnetic field equation is equal to: c 456 475 ohms. The effect on the operating point on the impedance diagram of decreasing coil length for a bobbin type internal probe would be: b 455 Phase lag across the tube wall. Given that a probe operated at 300kHz has an inductive reactance of 475 ohms, what is the cable’s capacitive reactance if this frequency results in resonance? c 454 R. The curl in the impedance locus that results when increasing test frequencies for inspecting tubing is a result of: a 453 None of the above, sensitivity actually increases above fr. Eddy current flow in a cylinder, using an encircling probe, changes with radial distance r from the centre of the cylinder. Eddy current flow is proportional to for cylinder testing. b 452 Test frequency is too close to probe-cable resonance. Operating at frequencies above resonant frequency will result in: d 451 All of the above. Probe operational impedance between 20-200 ohms is usually accommodated by most ECT instruments unless: a 450 The leading and trailing edges are abrupt. Ferromagnetic materials can affect probe impedance. These ferrogmagnetic materials: d 449 Differential probes. 1. (5.0p) b D2 is the general equation to find (p=resistivity). Characteristic frequency for tubes. NDT31-50316b ESTestMaker Answers A3-40 Copyright © TWI Ltd 457 The characteristic frequency ratio, f/fg, is not used for determining a frequency for phase discrimination in tube testing because the ration is: c 458 Both a and b. Given a brass tube to be tested with an internal bobbin probe, resistivity of the brass is 7.0 ohm-cm. If an operating frequency of 2.3kHz gives 90° phase separation between ID and OD defects, what is nominal wall thickness of the tubing? b 459 The main disadvantage of multipancake coil probes used as internal tube inspection probes is: a 460 Special probes. When using multifrequency techniques for tube inspection with an internal probe, the most effective results are had for: c 467 Of the branching nature of the cracks. Defects at non magnetic support plates are detected by using: a 466 Fly-back angle. Circumferential stress corrosion cracking can be detected by normal bobbin style probes during in-service inspection of heat exchanger tubing because: b 465 Tangent angle from balance to defect signal tip. When using differential probes, defect depth can be estimated from the: c 464 External magnetite. If an absolute probe is used, defect depth is estimated from: d 463 OD defects. When testing brass tubing (internal absolute probe) at f60 a signal moves off to the right on the scope (+X). If the 5% ID wall loss is set to move –X, what is the probable source of this signal? c 462 Their insensitivity to external defects. When tube testing at operating frequencies at 2f90 and higher it is difficult to discriminate probe wobble and: b 461 3mm. External fretting. Which of the following is the most conductive? c Zinc 5.9 NDT31-50316b ESTestMaker Answers ohm-cm. A3-41 Copyright © TWI Ltd 468 A conductive deposit (copper) is suspected of being on the OD of a heat exchanger tube being inspected with an absolute internal bobbin probe. The evaluation of this signal is best made by: C 469 Separation of defect signals from insignificant parameters is the function provided by multi frequency ECT units. What condition could not be separated by multifrequency technology? a 470 Magnetostriction. If a sample’s permeability changes up by a factor of 2, the standard depth of penetration will: d 478 Formations of a martensite phase. Changes in permeability with applied stress below the elastic yield strength of iron are due to: c 477 Saturated. The primary cause of increased permeability in initially nonmagnetic stainless steels with increased cold working is: b 476 The increase of lattice defects. When the induced magnetic flux in a ferromagnetic material increase linearly with increasing applied magnetising force the material is: c 475 None of the above. The primary cause for the increase in resistivity with increase in cold working is: a 474 >1. Which of the following metals, when alloyed with pure aluminium will result in the alloy having resistivity less than the aluminium? d 473 Magnetic permeability. For ferromagnetic materials the relative permeability is: a 472 Fretting under nonmagnetic support plates. The induced magnetic flux (B) divided by the applied magnetising force (H) gives what quantity? b 471 Retesting at between 0.5-0.1 f90. Decrease by 1.414. Pulsed saturation techniques used by EF testing to overcome magnetic permeability superimpose an AC signal and sampling of the eddy current is done: b At peak maximum DC pulse. NDT31-50316b ESTestMaker Answers A3-42 Copyright © TWI Ltd 479 Given a material with resistivity of 65 ohm-cm a relative magnetic permeability of 50 and testing at 100kHz, what is the standard depth of penetration? c 480 If an acceptable f90 is achieved with a probe on a slightly magnetic ( r=4) plate when operating at 50kHz, what frequency must be used to maintain that same f90 if relative permeability was to drop to 2? c 481 in a series circuit. Inductance to resistance. Current lags voltage in an AC circuit of pure: Inductance. The time variations of current, voltage and magnetic fields in AC circuits can be best described by which trigonometric functions(s)? d 490 Function of flux density (not constant). The time constant of a circuit, Tc, is the ration of c 489 Current density. Relative magnetic permeability for a magnetic material is: b 488 Energy must be used to move the charge. Amperes traversing a cross-sectional area is a useful concept in eddy current studies. The term used for this measure is: a 487 Both b and c could be used. W = Q(V2-V1) describes the work done moving a charge within an electric field. If W is positive then: b 486 0.22mm. In order to respond to steady-state magnetic flux conditions eddy current probes should use: a 485 2.4. d 484 ohm-cm as a %IACS? Given a sample with 5 ohm-cm resistivity and a relative magnetic permeability of 4.1, what is the standard depth of penetration if it is tested at magnetic saturation at a test frequency of 250 kHz? b 483 100kHz. What is a resistivity of 72 a 482 0.18mm. Both b and c. The imaginary component on the complex plane is plotted: b As the ordinate value. NDT31-50316b ESTestMaker Answers A3-43 Copyright © TWI Ltd 491 Current leads voltage in an AC circuit of pure: d 492 An eddy current transducer whose impedance or induced voltage is measured directly is considered a (n). a 493 Other coils. Test frequency ratios less than 0.1 or greater than 10 would be inappropriate for thin wall tube testing. This is because: c 502 All of the above. The active shielding technique used to shield eddy current probes uses which of the following principles? a 501 Any or all of the above. Shielding effects used in shielded eddy current probes is provided by which method? d 500 Maximum. When orbiting eddy current probes are used lift-off may need to be increased to ensure clearance from moving test pieces, the effects of lift-off are reduced by: d 499 Small diameter, high frequency. When testing ferro magnetic materials, coil inductance and inductive reactance are when lift-off is minimum. b 498 Both a and b. For measurement of thickness of a conductive coating on a conductive substrate where the coating conducting is higher than the substrate, you would use a probe with: a 497 Conductive non-magnetic backing sheet. In systems employing automatic feed of test pieces through the test coil, end effects are limited by: c 496 Probe design. When using an eddy current technique to determine the thicknesses on a large nonconductive plastic sheet you would require a: b 495 Absolute probe. The empty coil impedance of an eddy current probe is determined by: c 494 Capacitance. Sensitivity would be greatly reduced. When placed on the normalised impedance plane, the operating point for the coil impedance (empty coil) has the imaginary component equal to: b 1. NDT31-50316b ESTestMaker Answers A3-44 Copyright © TWI Ltd 503 When placed on the normalised impedance plane, the operating point for the coil impedance (empty coil) has a real component to: a 504 Phase angle between eddy currents on the inside and outside tube wall should lie between to provide sensitivity to cracks. b 505 Both a and b. Fill factor, affects the secondary coil voltage. If n is not too small, the correction term 1-n can be ignored for what conditions? a 513 To determine bulk properties in highly conductive materials. High test frequencies are preferred for bar diameter measurements when using encircling coils, why? c 512 Secondary coil voltage for a smaller diameter bar in an encircling coil. When using eddy current encircling coils for sorting, low frequency rations would be used for which conditions? a 511 Units used. Fill factor, n, is a useful parameter that can be used in determining which quantity? a 510 Units being used. What is the difference between limit frequency and characteristic frequency? d 509 All of the above. Characteristic frequency can be given by a) 50p/µd2 or b) 1353.8µăd2. What is the difference? (p=resistivity σ =conductivity): b 508 1. The voltage induced in the secondary winding of an encircling probe (sendreceive): d 507 40-100°. When testing a ferromagnetic tube with an encircling coil at a frequency ration of 1, what is the ration of magnetic field strengths inside to outside (ie Hi/Ho)? b 506 0. High permeability test pieces. In testing of ferromagnetic bars with an encircling coil selection of the appropriate frequency ration can permit detection of changes in conductivity independent of: c Both a and b. NDT31-50316b ESTestMaker Answers A3-45 Copyright © TWI Ltd 514 The effective permeability’s, as well as the geometrical distributions of the magnetic field strength and the eddy current densities, are the same for two different test objects if the frequency ration f/fg is the same for each test object. c 515 Geometrically similar defects will result in the same eddy current effects and in the same variations in effective permeability’s, coil impedance or voltage if the f/fg ration is the same for each test. This principle is explained by: b 516 Effective permeability. When calculations are made for f/fg for a single coil testing of sheet products where the probe is held away from the sheet by prove configuration or nonconductive coating, what must be done with this lift-off component? It is a 524 2.7. The transmission coefficient used in describing phasors in eddy current tests of sheets and foils is analogous to which quantity in cylinder testing? c 523 All of the above. For through transmission testing of sheet products maximum sensitivity to conductivity and thickness changes occur at what f/fg ration? b 522 The transmission coefficient. In through transmission testing of nonmagnetic metallic sheet products, the empty coil value of the transmission coefficient is: d 521 Thin-wall tubing. When a sheet of metal is inserted between a transmitting coil and a receiving coil the voltage in the secondary coil (receiving coil) changes from its empty coil value. The ratio of the new voltage to the empty coil voltage is: b 520 The similarity law. Coil impedance variations for inspection of sheet products would be most similar to encircling coil inspections of: a 519 Establishing the effects of defect’s shape and orientation. Test results found using the mercury model to establish effects of various shapes and orientations of defects can be applied to bar, rod or wire of any metal because of what principle? a 518 The similarity law for eddy current testing. Use of a mercury filled glass cylinder in eddy current testing is ideal for: c 517 The statement is the similarity law for eddy current testing. Added to the effective coil distance. To increase sensitivity to non-conductive coating thicknesses you would: c Decrease the coil diameter. NDT31-50316b ESTestMaker Answers A3-46 Copyright © TWI Ltd 525 The more sharply curved impedance locus traced by a given probe set-up as foil thickness increases is best explained by what aspect of eddy current theory? a 526 What is the effect of increased lift-off on the frequency ration (f/fg)? b 527 Signal amplitude decrease. When inspecting spheres of very high relative permeability increasing test frequency (f/fg ratio) will result in: d 535 Clockwise towards pure zinc. At some point, the improved signal separation from lift-off for a given crack is lost or overshadowed by what drawback when increasing test frequency? b 534 Clockwise towards pure copper. Adding increasing thickness of zinc (5.9µohm-cm) to a thick copper base (1.79µohm-cm) will cause the operating point on the normalised impedance plane to move: a 533 Resistivity of plating metal approaches that of the base metal. Adding increasing thickness of copper to a thick zinc base (Cu=1.7µohm-cm Zn=5.9µohm-cm) will cause the operating point on the normalised impedance plane to move: a 532 Reactance being increased. The influence of plating metal on the apparent impedance of the test coil is reduced as: c 531 Conductivity determination. In a resonance circuit setup to suppress effects of conductivity and maximise sensitivity to lift-off, an increase in resistivity would result in a signal amplitude decrease but this is compensated by: a 530 1000-2000. A dual frequency probe coil system has been developed to determine sheet thickness. If the lower frequency is used to measure the product of conductivity and thickness, what is the higher frequency used for: a 529 f/fg increases. To measure foil conductivity independent of thickness effects for sheets in the range of 1-2mm thick, the σf product should be about (conductivity σ in m/ohm-mm2 and f in kHz): c 528 Geometrical decrease in field intensity. None of the above, virtually no apparent impedance change occurs. The demagnetised factor: d All of the above. NDT31-50316b ESTestMaker Answers A3-47 Copyright © TWI Ltd 536 Fill factor for spherical objects tested in spherical test coils is found by Ds=diameter of sphere tested and Dc=diameter of the test coil: c 537 Lift-off. What effect does annealing have on eddy current tests of nonmagnetic alloys? b 547 Both a and b. The edge effect for nonmagnetic material is similar to what other eddy current phenomenon? b 546 Magnetic field distortions within the test piece. Which of the following would be a form of an alloy? d 545 Dirt, scale and probe lift-off limit effectiveness. As a test probe is moved towards the edge of a ferromagnetic test piece the locus traced on the impedance plane is an arc unlike the straighter lift-off trace. What AC counts for the arc shape? c 544 Linear. Which of the following is not true of remote field eddy current testing? b 543 10 times. In the remote field zone of a remote field eddy current test, the relationship between phase lag and depth is approximately: a 542 Unpredictable appearance of unwanted metallurgical factors. The magnetic flux moving along the tube outer wall, in remote eddy current testing, is the amplitude of the inner wall flux at the same distance from the primary exciter. d 541 Heating and cooling during welding can change magnetic state. The most difficult aspect of material sorting as compared to discontinuity by eddy current testing arises from what problem? b 540 On the outer surface of the tube. Austenitic stainless steel is not considered ferromagnetic; however permeability changes often plague inspection of austenitic tubing with welded seams. Why? b 539 (Ds/Dc)^3. Remote field eddy current testing when used on tubular products with an internal probe set-up utilises a secondary exciter effect from currents occurring: a 538 where Increases conductivity. Solution heat treating of an alloy results in: c Increasing metal strength. NDT31-50316b ESTestMaker Answers A3-48 Copyright © TWI Ltd 548 Which of the following would have a similar result on conductivity of an aluminium alloy as does annealing? b 549 What affect does natural aging of aluminium alloys have on the conductivity of specimen? d 550 An average or nominal size. Why is a fatigue crack a poor simulation for a quench crack? c 558 A decrease. When manufacturing a test standard for parts that are allowed a tolerance in parameter such as size, what size should the standards be: c 557 Coercive force for the annealed sample is less. What is the effect of a paramagnetic material on the inductance of an eddy current test coil? c 556 This provides an absolute measurement of resistivity and can be used for establishing standards. Comparing two identically shaped samples of the same grade of carbon steel, one annealed the other quench hardened, which statement would not be correct concerning hysteresis loop tests? c 555 10°C. A probe is made using 4 in-line copper contacts. The contacts are placed on a sample and current passed through the outer pair of contacts while voltage is monitored by the inner pair. What application does this have to eddy current tests? b 554 20°C. What is the maximum temperature difference that could be tolerated between standard and specimen when making resistivity measurements? c 553 Resistivity measurement. At what temperature are resistivities of most metals stated? c 552 No effect or a slight decrease. 7073-T73 aluminium alloys is specially tempered to resist intergranular corrosion and stress corrosion cracking. What would be used as a process control method for ensuring the adequacy of its aging? b 551 Cooling the test specimen. Fatigue cracks are more conductive. Drilled holes are often used when making calibration standards for eddy current tube testing. What is the most significant potential problem with production of this artificial defect? c Tube distortion. NDT31-50316b ESTestMaker Answers A3-49 Copyright © TWI Ltd 559 The use of 2 calibration foils, one on top the other, to calibrate for checking coating thickness should be avoided except for what conditions? b 560 If too large a drill size is used when making a drilled hole standard for eddy current testing, what happens to the response signal? b 561 When a signal enters a gate region. Sequential actuation of multiple box gates is used for what purpose in eddy current instruments with computer controlled gating with complex impedance plane displays? c 570 Size and power consumption. When is a gate output indication generated? b 569 Signal response time (allows faster scanning speeds). What is the most significant advantage of dot matrix displays of EC signals of CRT displays? c 568 Low pass. What advantage does the digital bar graph display have over analogue meter display EC instruments? c 567 Both a and b. Which of the following filter types would most likely be used to enhance (eliminate noise) from demodulated DC signals on an eddy current instrument? b 566 AM radio. The degree of suppression of undesired eddy current test signals depend on: d 565 All of the above. An eddy current signal that changes in amplitude only is similar to what other common technology? a 564 EDM. Which of the following methods used for machining longitudinal notches are reference standards would be used for making transverse notches? d 563 It resembles the response from edge effect. Which method produces the narrowest slot simulating a crack in a test standard? a 562 Flexibility is needed on curved surfaces. To detect direction of signal motion. At intermediate depths, multifrequency EC methods take advantage of the fact that phase angle. b Varies linearly with depth at a given frequency. NDT31-50316b ESTestMaker Answers A3-50 Copyright © TWI Ltd 571 What two phenomena occur when a semiconductor is placed in a magnetic field? c 572 How is the magnitude of the Hall voltage related to the angle the element normal makes to the magnetic field? a 573 of probe Both a and b. Increase Hall detector size. To prevent cross-talk due to mutual coupling. Not used, it is electronically subtracted. Hh the magnetising field made by the Hall detector material. What is the main advantage of an orthogonal winding transducer? b 582 independent Which of the following is not a magnetic field vector measured by a magnetic reaction analyser? b 581 determination When a Hall detector is used it is usually within the magnetic field of the excitation coil. How is this signal used? It is: a 580 direction Some EC inspection systems have 2 or more probes operating independently of each other but in close proximity. Why would these probes be operated at slightly different frequencies? a 579 and Which is not a method used to generate and measure eddy currents at greater depths using Hall detectors: b 578 Field magnitude orientation. Response of an inductive pickup coil is not uniform for what waveform? d 577 To provide temperature compensation. What is the advantage of use 3 Hall effect detectors mounted at mutual right angles to each other? c 576 Selecting the semiconductor materials used in the probe to be least sensitive to temperature changes. External correction circuits are used to reduce the voltage across the Hall element to zero in the absence of a magnetic field. Why are these circuits needed? c 575 V is proportional to cos Ө. Response to temperature effects is minimised in Hall detectors by: d 574 The Hall effect and magnetoresistive effect. Locates longitudinal and transverse cracks. Hot billets are possible to inspect with eddy current methods using: d Essentially any probe, provided it is adequately cooled. NDT31-50316b ESTestMaker Answers A3-51 Copyright © TWI Ltd 583 Above the curie point (δ is standard depth of penetration, σ is conductivity, µ is permeability): d 584 The advantage of inline eddy current inspection of continuous butt welded pipe is: b 585 Phosphorous. Instruments used for conductivity testing must be checked to ensure they are free from drift. Drift can be a result of: d 593 It was arbitrarily assigned. Which of the following, when added as an alloy of only 0.1% to copper will provide the greatest decrease in conductivity? b 592 Both a and b must be considered. How was the 100% IACS value for annealed pure copper determined? a 591 Metal thickness of upper plate. When gap between two plates is to be determined the probe should be placed on: d 590 All of the above. When the gap between two sheets of aluminium increases to a point past where no further change is seen on the eddy current instrument, what is being measured? c 589 Fast fourier transform. Which of the following methods is used to determine coating thickness? d 588 Shock absorbers. Computer analysis of test results and signals are now common. Which process would most likely be used to separate periodic defect signals from noise to determine periodicity of repetitive signals? b 587 Process control is made feasible. At the high inspection speeds (100m/s) during the production of steel rod, the rod often has a significant vertical vibration as it moves horizontally along and through the eddy current encircling coil. How are defects detected through the resulting shaking noise? b 586 Both a and b. Both a and b. Having calibrated a flat eddy current probe on a flat conductivity standard you now move to a radiused surface. What will the effect be on conductivity reading if we already know the standard and test specimen have identical conductivities? b Conductivity will appear less if the surface is concave. NDT31-50316b ESTestMaker Answers A3-52 Copyright © TWI Ltd 594 1/(πfσµ)^½, 26/(πfσµ)^½ and 1980(p/µf)^ ½ are equations used in eddy current testing (p is used here as resistivity, σ conductivity and µ permeability and f frequency), what do they calculate: d 595 In order that a specimen increase its resistivity as its temperature decreases what must hold true? b 596 Fatigue cracks are grown off EDM notches which are later machined away. A severe form of intergranular corrosion, whereby thin layers of aluminium delaminate parallel to the plate surface is: b 604 It causes embrittlement. How are real cracks placed in standards used for calibration of bolt hold inspection by spinning eddy current probes? a 603 All of the above. Oxygen diffusion from the surface of titanium and its alloys at elevated temperatures is of concern in aircraft industry because: b 602 As quenched. What is the difference between 2024-T3 and 2024 –T6 aluminium alloy? d 601 From a conversion chart you make using standards. Which condition of aluminium heat treatment will provide the maximum resistivity? a 600 Internal lift-off compensation. Indirect conductivity meters provide readings in µA (mircoAmperes). How do you convert this to % IACS readings? b 599 55. Nonconductive coatings that are less than or slightly more than 0.08mm will result in less than variation of 0.5% IACS in conductivity using a standard conductivity meter. How is this accomplished? a 598 The temperature coefficient must be negative. Given resistivity of pure annealed copper is 1.72µohm-cm and pure aluminium is 2.78µohm-cm (both at 20°C.), what is the conductivity % IACS of the aluminium at 55°C, if the thermal coefficient of aluminium is 0.0038? c 597 All are forms of standard depth of penetration (units vary). Exfoliation. When using low frequency eddy currents to inspect multiple layers of aluminium corrosion or cracking, what is the maximum thickness of outer layer that can be tested? a 6mm. NDT31-50316b ESTestMaker Answers A3-53 Copyright © TWI Ltd 605 What is the effect of a steel fastener when inspecting multilayer aluminium in the region of the fasteners? They: b 606 The off-null balance technique is used only on meter type phase analysis instruments. It cannot be used on CRT type instruments because: a 607 No less than twice. In selecting a mixing frequency to suppress external variables the mix frequency should be the primary technique. b 615 Independence from probe speed. In selecting a mixing frequency to suppress internal variables the mix frequency should be the primary frequency. a 614 All of the above. An advantage of multifrequency ECT for eliminating undesirable signals over monofrequency filtering is: b 613 Reduced power generating ability from plugging. In a multifrequency setup, simple subtraction of the mix signal, which is at four times the primary frequency, will not result in zero output of the undesirable variable. Why not? d 612 Poor water chemistry. Signal analysis of eddy current signals is an important aspect of testing. Of particular concern is its use in establishing depth of cracks or corrosion. What is the result of oversizing defect depths in boiler tube inspections? c 611 Both a and b. Inside diameter pitting on heat exchanger tubing can be a result of: b 610 All of the above. When performing an eddy current test on finned copper tubing (as in air conditioning units) single frequency instruments in conjunction with differential coil probes are used. A 1.3mm fin pitch requires you use a coil space of 5mm. Why? d 609 The flying dot would usually be off screen. In the early 1960s what limited the use of eddy current testing to detect subsurface cracks in aircraft structures? d 608 Act as a core and concentrate the electromagnetic field. No greater than half. Which of the following is not a multifrequency eddy current system for defining and eliminating a given parameter? The b Elemental analysis method. NDT31-50316b ESTestMaker Answers A3-54 Copyright © TWI Ltd 616 Wire rope testing by electromagnetic methods utilises: d 617 Direct field excitation inspection of steel wire ropes is used to detect: d 618 The signal to noise ratio in the instrument. What assumption must be made when using eddy currents to determine thickness of a nonconductive coating on a conductive (non-magnetic) substrate? c 628 All of the above. Although 3δ is usually accepted as the maximum depth of eddy current that can be detected. It has been noted that in some cases depths of 5δ can be achieved. What determines the increase depth sensitivity? (δ is standard depth of penetration): b 627 Increasing temperature causes coil expansion. The reflection probe: d 626 All of the above. How does the use of increasing current increase coil inductance? a 625 As low as possible. What is the effect of too high a current to the eddy current probe: d 624 All of the above. Current through an eddy current probe coil should be: a 623 To increase inductance for a given coil length. Coil cores used for eddy current probes are: d 622 Orientation of the major axis and the axis ration. What is the purpose of multiple layer windings in an inductive coil? a 621 Generalised corrosion and wear. On the old sigmaflux instruments which indicated defective parts by displaying ellipses, how were phase and amplitude indicated? c 620 Both b and c. Alternating field excitation inspection of steel wire ropes is used to detect: a 619 Both a and b can be used. Conductivity of the substrate is constant. What do the side drilled holes used for ultrasonic testing and the round bottom transverse notch on the OD of a tube for eddy current testing have in common? d All of the above. NDT31-50316b ESTestMaker Answers A3-55 Copyright © TWI Ltd 629 What type of crack would cause an absolute surface probe to give a figure-eight display on the storage monitor? b 630 Multifrequency techniques using absolute coils are: a 631 Physical contact (electrodes). Lift-off is used as a reference signal in many eddy current test applications. Why? d 640 All of the above. ACPD (alternating current potential drop) and ECT (eddy current testing) both use alternating currents to size surface breaking cracks. ECT uses induction to generate currents in the piece. What does ACPD use? c 639 Maxwell’s Law. Calibration standards are used in eddy current test to: d 638 Limit frequency. Every part of an electric circuit is acted upon by a force that tends to move it in such a direction as to enclose the maximum amount of magnetic flux. This statement is known as: a 637 The parameter being measured. The point where increasing operating frequency does not increase ohmic losses in the test material is the: b 636 Keeps heating of the sample to a low level. What is the main difference between eddy current and flux leakage testing? c 635 Both a and b. The pulsed eddy current technique has the advantage of producing high magnetic peak power but still maintaining low average power. This has what effect on the test piece? a 634 Best for detecting small cracks and pits. When access for inspection of a pipe is from the inside in the remote field eddy current technique, the receiver coil is: c 633 Best for detecting large volume defects. Multifrequency techniques using differential coils are: b 632 A bent crack (major facets in opposite direction). Both a and b. Why do holes of different diameter and the same through wall depths have different calibration phase angles (eg flyback angle for a differential coil)? b Flaw geometry affects phase angle. NDT31-50316b ESTestMaker Answers A3-56 Copyright © TWI Ltd 641 Multifrequency eddy current techniques should be used whenever possible, even if the mixing capability is not needed. Why? a 642 When a digital output is available on an eddy current instrument, why should the digitalising rate be at a reasonably high rate? c 643 Bridge. Acceptance standard. A wave filter with a single transmission band and neither of the cut-off frequencies being zero or infinity is a: a 651 Saturation magnetisation. In tubing inspection a tube used to establish acceptance levels with artificial discontinuities as specified in applicable product standards is a (n): b 650 Both b and c. An electrical circuit incorporating four impedance arms is a (n): a 649 Ferromagnetism. The degree of magnetisation produced in a ferromagnetic material for which incremental permeability has decreased to unity is: b 648 3:1. Which of the following is not considered to be magnetisable? d 647 Diamagnetic. External magnetic forces causing an increase in the normal number of electrons with the same spin, thereby increasing the number of uncompensated spins results in what property? c 646 material: An acceptable ratio between defect signal amplitude and non-relevant indications is usually considered to be as a minimum: b 645 To allow variation in scanning speed without degrading the signal. A material with a permeability less than that of a vacuum is a a 644 Information redundancy reduces changes of missing defects. Bandpass filter. What is the disadvantage of zig-zag coil probes compared to axial bobbin type probes used for internal tube inspections? a Non-uniformity of sensitivity. NDT31-50316b ESTestMaker Answers A3-57 Copyright © TWI Ltd 652 If two or more coils are electrically connected in series such that there is no mutual inductance between them and no electric or magnetic condition (or both) that is not common to the test standard and test specimen, will produce an unbalance and yield an output, this arrangement is called: c 653 Which permeability is described as a hypothetical quantity magnetic permeability experienced under a given set of physical conditions eg a cylinder in an encircling coil at a specific test frequency? a 654 22kHz. Given a sample of titanium (54.8µohm-cm) what test frequency must be used to obtain a 1mm standard depth of penetration? c 661 0.86. Given a sample of 50% cold worked 304 stainless steel (68.96µohm-cm, µrel=2) what test frequency would provide a 2mm standard depth of penetration? c 660 10.50. What is the fill factor of the test using 1.1mm diameter encircling coil to test wire with a diameter of 1.02mm? b 659 A three way sort. Given the requirement to test tubing, OD 10mm and wall thickness 1mm, using an encircling coil, what is the average coil diameter if you need to maintain a 90% fill factor? b 658 Both a and b. An electromagnetic sorting based on a signal response from the material under test above or below two levels established by three or more calibration standards is: b 657 Differentiated signal. A standard is: c 656 Effective. An output signal that is proportional to the rate of change of the input signal is a (n): b 655 Comparator coils. 140kHz. Given a sample of cold worked stainless steel (71µohm-cm, µrel=10) tested at 10kHz, what is the effective penetration? b 3.9mm. NDT31-50316b ESTestMaker Answers A3-58 Copyright © TWI Ltd Eddy Current Testing (ET) Magnetic Particle Testing (MT) Refresher Section 1 Copyright © TWI Ltd Magnetism Some natural materials strongly attract pieces of iron to themselves. Such materials were discovered near the Greek city of Magnesia and in China as early as 900BC. Early in the 19th century Oersted found a link between electricity and magnetism. Not long afterwards Faraday proved that electrical and magnetic energy could be interchanged. Copyright © TWI Ltd Domain Theory A domain is a minute internal magnet. Each domain comprises 1015 to 1020 atoms typically several million domains exist in each individual grain. Unmagnetised state Copyright © TWI Ltd Magnetic Particle Testing (MT) or Inspection (MPI) MT is a test method for the detection of surface and near surface defects in ferromagnetic materials. Magnetic field induced in component Defects disrupt the magnetic flux causing ‘flux leakage’. Flux leakage can be detected by applying ferromagnetic particles. Copyright © TWI Ltd Theory of Magnetism - Domains When an electric current flows there is an associated magnetic field. An electric current consists of a flow of electrons through a conductor. The electrons in any atom are in constant motion. This motion causes an associated magnetic field. In most materials this field is cancelled by the movement of electrons in opposing directions. Domains randomly orientated Copyright © TWI Ltd Copyright © TWI Ltd 1 Theory of Magnetism - Domains In any metal some electrons are shared between neighbouring atoms. In a ferromagnetic material small groups of atoms exist in which the magnetic field caused by movement of electrons is not cancelled by opposing movement. These small groups of atoms are called magnetic domains. They are, in effect, tiny electromagnets. Domain Theory Magnetising force Magnetising force Magnetised state Domains aligned in external field Copyright © TWI Ltd Copyright © TWI Ltd Domain Theory Domain Theory Magnetising force Magnetising force Magnetising force Magnetising force Saturated state All domains fully aligned with external field Copyright © TWI Ltd Domain Theory Magnetising force removed Residual magnetism remains Copyright © TWI Ltd Magnetic Fields In order to understand how magnets interact with one another the concept of a magnetic field is used. The idea of a magnetic field is based on the patterns made by magnetic particles when they are placed in a magnetic field. Unmagnetised Magnetised Saturated Residual Copyright © TWI Ltd Copyright © TWI Ltd 2 Magnetic Fields Magnetic field around a bar magnet Magnetic Fields Magnetic field North Pole - South Pole Copyright © TWI Ltd Magnetic Fields Magnetic field Copyright © TWI Ltd Magnetic Fields Magnetic fields are thought to consist of lines of flux. North Pole - South Pole Copyright © TWI Ltd Lines of Flux Copyright © TWI Ltd Properties of Lines of Flux Lines of flux: Flow from a North Pole to a South Pole outside a magnet. Flow from a South Pole to a North Pole inside a magnet. Form closed loops. Repel one another. Never cross. Copyright © TWI Ltd Copyright © TWI Ltd 3 Magnetic Flux Magnetic flux is defined as the total number of lines of flux in a magnetic field or circuit. Magnetic Flux Density Magnetic flux density is defined as the total number of lines of flux passing through each square metre in a cross section of the magnetic field. The S.I. unit of magnetic flux density is the tesla. The old CGS unit is the gauss. Copyright © TWI Ltd Electromagnetism Copyright © TWI Ltd Electromagnetism Oersted discovered that when an electrical current flows a magnetic field is produced. Faraday investigated the relationship between electricity and magnetism. The magnetic field produced is always at 90° to the direction of electrical current flow. The flux density produced is proportional to the magnitude of the electric current. Copyright © TWI Ltd Right Hand Rule Copyright © TWI Ltd Coil Magnetisation Changes circular field into longitudinal. Increases the strength of the field. Copyright © TWI Ltd Copyright © TWI Ltd 4 Coil Magnetisation Hysteresis Hysteresis comes from a Greek word that means lagging behind. Ferromagnetic materials resist being magnetised. But once magnetised, they resist being demagnetised. They oppose change. Copyright © TWI Ltd Copyright © TWI Ltd Hysteresis Saturation Virgin curve Now slowly decrease the magnetising force (H) to zero. Magnetic flux density (B) Tesla Magnetic flux density (B) Tesla Place an unmagnetised sample of ferromagnetic material in a slowly increasing magnetic field. Hysteresis Residual magnetism Magnetising force (H) ampere/metre Magnetising force (H) ampere/metre Copyright © TWI Ltd Copyright © TWI Ltd Hysteresis Now apply a slowly increasing negative magnetising force (H). Magnetic flux density (B) Tesla Magnetic flux density (B) Tesla Now continue to increase the negative magnetising force (H). Hysteresis Negative saturation Coercive force Magnetising force (H) ampere/metre Magnetising force (H) ampere/metre Copyright © TWI Ltd Copyright © TWI Ltd 5 Hysteresis Hysteresis Coercive force Magnetising force (H) ampere/metre Magnetising force (H) ampere/metre Copyright © TWI Ltd Magnetising force (H) ampere/metre Copyright © TWI Ltd Hard Versus Soft Ferromagnetics Soft: Typically low carbon steel. High permeability. Easy to magnetise. Low residual magnetism. Magnetically soft material Magnetic flux density (B) Tesla Residual magnetism Magnetically hard material Magnetic flux density (B) Tesla Magnetic flux density (B) Tesla Now slowly increase the magnetising force (H) back to the positive saturation point. Permeability (µ) Permeability can be defined as the relative ease with which a material may be magnetised. It is defined as the ratio of the flux density (B) produced within a material under the influence of an applied field to the applied field strength (H). μ =B/H From the hysteresis loops in the previous slides it can be seen that permeability is not a constant. Hard: Typically high carbon steel. Lower permeability. More difficult to magnetise. High levels of residual magnetism. Copyright © TWI Ltd Copyright © TWI Ltd Relative Permeability (µr) This is the permeability of any material relative to the permeability of free space. Free space is basically air. Permeability of free space = µ° = 1.0 Relative permeability (µr) = µ/µ° Absolute permeability is difficult to measure. Copyright © TWI Ltd Relative Permeability (µr) On the basis of relative permeability materials can be divided into three groups: 1. Diamagnetic. 2. Paramagnetic. 3. Ferromagnetic. Copyright © TWI Ltd 6 Relative Permeability (µr) Diamagnetic: Permeability slightly below 1, weakly repelled by magnets. Examples: Gold, copper, water. Paramagnetic: Permeability slightly greater than 1, weakly attracted by magnets. Examples: Aluminium, tungsten. Ferromagnetic: Very high permeability, strongly attracted by magnets. Permeability 240+. Examples: Iron, cobalt, nickel. Relative Permeability (µr) Paramagnetics: Diamagnetics: Ferromagnetics: Slightly > 1. Slightly < 1. 240+. Copyright © TWI Ltd Copyright © TWI Ltd Relative Permeability (µr) Permeability is affected by chemical composition. Permeability is affected by heat treatment. Permeability is affected by the shape of the component. The opposite of permeability is reluctance. Definitions Magnetic field: Region in which magnetic forces exist. Magnetic flux: The total number of lines of force in a magnetic circuit. Magnetic flux density: The number of lines of force passing through a unit area. Copyright © TWI Ltd Copyright © TWI Ltd Root Mean Square (RMS) Current2 (Amps2) Current (Amps) Magnetising current values are sometimes specified in amps peak. Standard moving iron and moving coil ammeters do not measure peak current. Usually they measure root mean square current or RMS. Root Mean Square (RMS) Mean square = 8 Root mean square = 2.828 = 4/1.414 Current (Amps) Copyright © TWI Ltd Copyright © TWI Ltd 7 Root Mean Square (RMS) Electromagnetism Converting RMS current to mean or peak current: Wave form AC HWAC FWAC RMS Mean Peak I 0 1.414 I 0.637 2 I 0.9 1.414 Copyright © TWI Ltd Copyright © TWI Ltd Coil Magnetisation Changes circular field into longitudinal. Increases the strength of the field. Copyright © TWI Ltd 8 Eddy Current Testing (ET) Flaw Detection Section 2 Copyright © TWI Ltd Copyright © TWI Ltd Magnetic Effect of an Electric Current Magnetic Effect of an Electric Current Conductor DC current DC current Magnetic field Copyright © TWI Ltd Copyright © TWI Ltd Magnetic Field of a Coil Magnetic Field of a Coil North + ve + ve DC DC - ve - ve Copyright © TWI Ltd South Copyright © TWI Ltd 1 Magnetic Field of a Coil Magnetic Field of a Coil South North AC AC Primary field South Copyright © TWI Ltd Probe Construction Ferrite core Copyright © TWI Ltd Probe Construction North Copyright © TWI Ltd Probe Construction Ferrite core Copyright © TWI Ltd Probe Construction Resin housing PTFE tape Copyright © TWI Ltd Copyright © TWI Ltd 2 Probe Construction Stainless steel shield Ferrite pot Magnetic Field of a Coil Electrical current produces an encircling magnetic field. Alternating electrical current produces an alternating magnetic field. The alternating magnetic field in the coil is called the primary field. Copyright © TWI Ltd Copyright © TWI Ltd If a conductor is within the influence of changing magnetic field or it moves within a constant magnetic field an EMF will be produced within the conductor. If the circuit is closed the EMF sets up a current within the conductor - this will be proportional to the rate of change of flux. Effect of a Magnetic Field S In Faraday’s Words: N Electrical conductor Copyright © TWI Ltd Copyright © TWI Ltd Effect of a Magnetic Field S S Effect of a Magnetic Field Electrical conductor N N Electrical conductor Current flow Current flow Copyright © TWI Ltd Copyright © TWI Ltd 3 Effect of a Magnetic Field S S Effect of a Magnetic Field Electrical conductor N N Electrical conductor Current flow Current flow Copyright © TWI Ltd Copyright © TWI Ltd Faraday’s Laws of Electromagnetic Induction Effect of a Magnetic Field Moving a magnetic field about the conductor will produce a current flow. An alternating current flow can be produced by using a moving (alternating) magnetic field. 1. An electro-motive force (EMF) is induced in a conductor when the magnetic field surrounding it changes. 2. The magnitude of the EMF is proportional to the rate of change of the magnetic field. Copyright © TWI Ltd Copyright © TWI Ltd Production of Eddy Currents AC Alternating (moving) primary field Production of Eddy Currents AC Conductive material Copyright © TWI Ltd Copyright © TWI Ltd 4 Production of Eddy Currents AC Production of Eddy Currents AC Copyright © TWI Ltd Copyright © TWI Ltd Production of Eddy Currents AC Production of Eddy Currents AC Copyright © TWI Ltd Copyright © TWI Ltd Production of Eddy Currents AC Production of Eddy Currents AC Localised alternating electrical current Copyright © TWI Ltd Copyright © TWI Ltd 5 Lenz’s Law Production of Eddy Currents An induced electric current always flows in such a direction that it opposes the change producing it. AC Eddy currents Copyright © TWI Ltd Copyright © TWI Ltd Lenz’s Law Eddy currents produce a flux of their own which is in opposition to the change of flux which caused them. Production of Eddy Currents AC Primary field Secondary field Copyright © TWI Ltd Eddy Current Flaw Detection Fault Copyright © TWI Ltd Copyright © TWI Ltd Eddy Current Flaw Detection Fault Copyright © TWI Ltd 6 Eddy Current Flaw Detection Copyright © TWI Ltd Eddy Current Flaw Detection Copyright © TWI Ltd Eddy Current Flaw Detection Copyright © TWI Ltd Eddy Current Flaw Detection Copyright © TWI Ltd Eddy Current Flaw Detection Copyright © TWI Ltd Eddy Current Flaw Detection Copyright © TWI Ltd 7 Any questions? Summary Copyright © TWI Ltd Eddy currents, are local electrical currents in a conductive body, produced by electromagnetic induction. Electro-magnetic induction, is caused by lines of magnetic flux cutting a conductor and inducing an EMF (electro motive force). It has to be a changing magnetic flux. Copyright © TWI Ltd Eddy currents, produced in a conductive material, generate their own magnetic field. This is known as the secondary magnetic field. The secondary magnetic field, opposes the one that induced it, i.a.w. Lenz’s law. Copyright © TWI Ltd Copyright © TWI Ltd The current produced, will be proportional to the rate of change of flux, i.a.w. Faraday’s law. The changing magnetic flux, is created by passing an AC current through the probe coil. Called Primary magnetic field. AC is used to give a constantly changing field to induce constantly changing eddy currents. The coil, is wound around a ferrite core to concentrate the flux field. Copyright © TWI Ltd A change in the eddy current path will result in the following: 1. A change in the magnitude of the secondary magnetic field. 2. Any change in the secondary magnetic field is felt back on the primary magnetic field (the one that induced it). Copyright © TWI Ltd 8 3. This changes the coils impedance (opposition to current flow in an AC circuit). 4. Any change in the coils impedance, is then displayed on the instrument, either visually or as an audible warning. Copyright © TWI Ltd Any Questions? Copyright © TWI Ltd 9 Eddy Current Testing (ET) Factors Affecting Eddy Currents Section 3 Copyright © TWI Ltd Basic Principle of Eddy Currents Copyright © TWI Ltd Basic Principle of Eddy Currents Ferrite core Primary inducing field Opposing secondary field Eddy currents Copyright © TWI Ltd Basic Principle of Eddy Currents Copyright © TWI Ltd Basic Principle of Eddy Currents D.C. AC Opposing field No induction + _ Induction creates secondary AC Primary AC Current flow (right hand rule) Alternating magnetic field Static magnetic field Copyright © TWI Ltd Copyright © TWI Ltd 1 Eddy Current Inspection The size of the current is affected by: Electrical conductivity. Permeability. Frequency. Edge effect. Lift-off/stand off distance. Fill factor. Specimen dimensions. Flaws. Eddy Current Inspection Electrical conductivity The ease of electron flow. Inverse of resistivity. Symbol is . Units are: I.A.C.S. Siemens/m. m/·mm². 100% I.A.C.S. = 58 m/·mm². 1 m/·mm² = 106 siemens/m. Copyright © TWI Ltd Copyright © TWI Ltd Permeability Has a dominant effect on eddy currents. The noise created by permeability changes in ferrous materials make eddy current inspection of welds etc difficult. Magnetic saturation can negate the effect of permeability. Measurement of permeability is the basis of sorting bridges. Frequency Affects depth of penetration (skin effect). The standard depth of penetration () = 1e-1 x surface intensity of eddy-currents. Standard depth of penetration = 660 ƒ - SDP (mm). ƒ - frequency (Hz). - conductivity (% IACS). Note: If m/·mm² use 500. - relative permeability. Copyright © TWI Ltd Copyright © TWI Ltd Standard Depth of Penetration Fill Factor Equivalent to lift off when using encircling coils: Fill Factor = Coil diameter DC² (internal coil) Tube diameter DT² δ Or I/e = Tube diameter DT² (external coil) Coil diameter DC² must be less than 1.0. is usually about 0.7. I Standard Depth of Penetration Copyright © TWI Ltd Copyright © TWI Ltd 2 Flaws Planar discontinuities - cutting eddy currents. Planar discontinuities - parallel to eddy currents. Depth of crack cannot be accurately measured. Factors Affecting Eddy Currents Conductivity. Permeability. Frequency. Geometry. Thickness. Edge. Mass. Ferrous effect. Lift off. Probe handling. Copyright © TWI Ltd Copyright © TWI Ltd Specimen Dimensions Thickness Effect Material thickness. Component geometry. AC Eddy currents Copyright © TWI Ltd Thickness Effect Copyright © TWI Ltd Copyright © TWI Ltd Thickness Effect Copyright © TWI Ltd 3 Thickness Effect Copyright © TWI Ltd Thickness Effect Can Be Used: For approximate thickness measurement. To detect blind side corrosion. Thickness Effect Copyright © TWI Ltd Edge Effect The effect that the components edge or sharp changes in geometry have on the eddy currents. Can be negated by balancing probe near to edge and scanning at that distance. Material loss/corrosion Copyright © TWI Ltd Edge Effect Copyright © TWI Ltd Edge Effect AC Copyright © TWI Ltd Copyright © TWI Ltd 4 Edge Effect Copyright © TWI Ltd Edge Effect Copyright © TWI Ltd Mass Effect Edge Effect Copyright © TWI Ltd Edge Effect Copyright © TWI Ltd Mass Effect AC Copyright © TWI Ltd Copyright © TWI Ltd 5 Mass Effect Mass Effect Copyright © TWI Ltd Copyright © TWI Ltd Mass Effect Ferrous Effect AC Ferrous fastener Copyright © TWI Ltd Ferrous Effect Copyright © TWI Ltd Copyright © TWI Ltd Ferrous Effect Copyright © TWI Ltd 6 Ferrous Effect Ferrous Effect Copyright © TWI Ltd Copyright © TWI Ltd Lift-off/Stand-off Distance The term used for the distance between a surface coil and the test surface. Small lift off gives pronounced effects. Most high frequency sets employ lift off compensation. Lift off can be used to measure non-conductive coating thickness. Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) AC Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd 7 Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd Lift Off (Proximity) Copyright © TWI Ltd 8 Probe Handling Probe Handling AC Copyright © TWI Ltd Copyright © TWI Ltd Probe Handling Probe Handling Copyright © TWI Ltd Copyright © TWI Ltd Review Factors Affecting Eddy Currents Discontinuities AC Conductivity. Permeability. Frequency. Geometry. Thickness. Edge. Mass. Ferrous effect. Lift off. Probe handling. Copyright © TWI Ltd Copyright © TWI Ltd 9 Review Factors Affecting Eddy Currents Can be used to detect: Flaws and discontinuities. Material thickness. Thickness of non-conductive coatings. Material specification. Copyright © TWI Ltd 10 Eddy Current Testing (ET) Impedance Plane Display Section 4 Copyright © TWI Ltd Copyright © TWI Ltd Information Presentation The information received by the instrument is displayed as a flying working spot on an LCD display screen. This form of information representation is known as impedance plane. Impedance Plane Signal Generation Changes in the reaction between primary and secondary fields cause changes in the coils impedance. Impedance can be plotted as a graph known as the A-Y curve. Copyright © TWI Ltd Copyright © TWI Ltd The A-Y Curve The A-Y Curve Air Ti Reactance Reactance Al Cu Resistance The A-Y curve therefore directly represents material conductivity at the point of contact. Changes in the conductivity therefore cause a point to move along the A-Y curve. Resistance Copyright © TWI Ltd Copyright © TWI Ltd 1 The A-Y Curve The A-Y Curve Air 0.0% IACS Air Titanium 3.3% IACS Reactance Reactance Aluminium 35% IACS Al Lift off changes the conductivity towards the probe in air point. A fault changes conductivity momentarily at a tangent to the curve. The direction of point movement indicates the reason for the change in conductivity. The impedance plane display is a window into the A-Y curve. The window can be rotated and zoomed to provide a display screen. Copper 100% IACS Resistance Resistance Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd Typical indications: Copyright © TWI Ltd Copyright © TWI Ltd 2 Typical indications: Typical indications: Copyright © TWI Ltd Typical indications: Edge effect Copyright © TWI Ltd Typical indications: Edge effect Fault Lift-off Fault Ferrous Lift-off Copyright © TWI Ltd Copyright © TWI Ltd Impedance Plane Typical indications: Edge effect XL Fault Z Ferrous XT (XL- XC) R Lift-off XC Mass Copyright © TWI Ltd Copyright © TWI Ltd 3 Applications: Crack detection. Tube and wire testing. Condenser tube inspection. Material sorting. Weld testing. Coating thickness measurement. Copyright © TWI Ltd 4 Eddy Current Testing (ET) Basic Electrical Theory Section 5 Copyright © TWI Ltd Primary Cell Negative terminal Positive terminal Copyright © TWI Ltd Circuit Made and Current Flowing Positive terminal Negative terminal Electrolyte Electrolyte Cell plates Cell plates Copyright © TWI Ltd Copyright © TWI Ltd Electrical Theory Electrical Theory The atom -ve +ve nucleus +ve nucleus Copyright © TWI Ltd electron Copyright © TWI Ltd 1 Electrical Theory Electrical Theory Angular momentum -ve electron -ve electron +ve nucleus +ve nucleus Electrostatic field Electrostatic field Copyright © TWI Ltd Electrical Theory Copyright © TWI Ltd Copper Atom Electron Transfer Valence-1 M-18 L-8 Valence=1 M=18 L=8 K-2 K=2 Copper atom +29 The copper atom has lots of space in the outer valence shell for electron transfer. Copyright © TWI Ltd Copyright © TWI Ltd Electrical Theory - Units Potential difference Current Resistance Resistivity Conductance Conductivity - Electrical Theory Battery PD (volt [V]). I (ampere [A]). R (ohm []). ( m). G (siemens [S]). (S/m). - + Conductor V I Load R Copyright © TWI Ltd Copyright © TWI Ltd 2 Electrical Theory – Ohm’s Law Electrical Theory – Ohm’s Law Battery V - + I Conductor R V V=IxR I I= V R Load R V=IxR R= V I Copyright © TWI Ltd Copyright © TWI Ltd Electrical Theory Electrical Theory Copyright © TWI Ltd Copyright © TWI Ltd Resistances in Series Total resistance in a series circuit is equal to all resistances added together. Series Circuit – Voltage Divider In a series circuit current remains the same throughout, but voltage is divided between the resistances. Current (I) in circuit = V/R R1 12v R2 R3 I = 12/12 R1 I = 1amp throughout R1 = 3 R2 = 4 R3 = 5 RT = R1 = R2 = R3 RT = 3 + 4 + 5 RT = 12 12v R2 R3 R1 = 3 R2 = 4 R3 = 5 RT = 12 Voltage in R1 = 1 x 3 = 3v Voltage in R2 = 1 x 4 = 4v Voltage in R3 = 1 x 5 = 5v 12v Copyright © TWI Ltd Copyright © TWI Ltd 3 Resistances in Parallel In a parallel circuit total resistance is always less than the value of the lowest single resistor. Parallel Circuit – Voltage Constant In a parallel circuit it is the voltage which remains constant, whilst the current is drawn at varying rates depending on the value of each resistance. R1 = 3 1/RT = 1/R1 + 1/R2 + 1/R3 R1 = 3 R2 = 4 R3 = 5 R2 = 4 1/RT = 1/3 +1/ 4 + 1/5 12v R3 R2 R3 = 5 1/RT = 20/60 + 15/60 + 12/60 R1 1/RT = 47/60 12v R3 R R1 2 RT/1 = 60/47 Current in R1: I = V/R = 12/3 = 4 Amps. Current in R2: I = V/R = 12/4 = 3 Amps. RT = 1.28 Current in R3: I = V/R = 12/5 = 2.4 Amps. Copyright © TWI Ltd Copyright © TWI Ltd Potential Divider 3Ω 6Ω A B 12Ω 24Ω Ratio in top arm 3 : 12 or 1 : 4 Ratio in lower arm 6 : 24 or 1 : 4 12v When ratios are the same, voltage at A will be the same as voltage at B. When one load changes, the balance of ratios is upset and a potential difference will exist across A-B current will now flow. Wheatstone Bridge Wheatstone bridge R1 R3 R2 R4 Wheatstone bridge with a probe circuit R1 R3 R2 R4 ‐ Probe Circuit Changes in any arm will Changes in probe circuit will create a PD across the bridge. create a PD across bridge. Copyright © TWI Ltd Copyright © TWI Ltd Direct Current (DC) Direct Current in a Wire Amplitude Peak Current building at switch on Current collapsing at switch off D.C. _ Current flow (right hand rule) Time Copyright © TWI Ltd Copyright © TWI Ltd 4 Alternating Current in a Wire Amplitude Current Primary AC current a b 45° c 90° 135° d 225° 180° e 270° 315° 360° Time Alternating magnetic field Copyright © TWI Ltd Copyright © TWI Ltd Alternating Current AC Sine Wave 90° Peak Rate of change fastest here Positive cycle Amplitude Amplitude 45° 135 ° +4 180 ° 0 45° 90° 135° 180° 225° 270° Negative cycle 315° 360° 45° 90° 135° 180° 225° 270° 360° 315° 45° 90° 135° Time 180° 225° 270° 315° 360° Time 225 ° -4 315 ° 270 ° One Cycle Copyright © TWI Ltd AC Sine Wave Copyright © TWI Ltd Copyright © TWI Ltd AC Sine Wave Copyright © TWI Ltd 5 AC Sine Wave Alternating Current in a Wire Opposing field Induction creates secondary current Primary AC current Alternating magnetic field Copyright © TWI Ltd Copyright © TWI Ltd Inductance D.C. A.C. Opposing field No induction + _ Induction creates secondary AC Alternating magnetic field Static magnetic field Inductance (L) The ability of a device to store magnetic energy and oppose changes in the current is called inductance and is calculated as follows: L Primary AC Current flow (right hand rule) Inductance Where: L is the Inductance in Henries. R is a geometric factor to do with the shape of the coil and gap between windings etc. N is the number of turns. A is the coil’s planar surface area in mm². l is the coil’s axial length. From the formula, it can be seen that factors affecting inductance in a coil are mostly its physical attributes. Copyright © TWI Ltd Copyright © TWI Ltd Inductive Reactance Capacitor/Condenser Definition: Inductive reactance is the opposition to current flow offered by the inductance. X 2ΠfL L Where XL is the inductive reactance in ohms. F is the frequency in Hertz. L is the Inductance in Henries. Example: What is the inductive reactance in a coil where the inductance is 15µH and it is operated at a frequency of 100kHz? X L X L RN 2 A l Capacitor plates Dielectric material 2 x 3 .142 x100 ,000 x 0 .000015 9.43 Copyright © TWI Ltd Copyright © TWI Ltd 6 Capacitor/Condenser Capacitors store electric charge. Capacitance is the amount of stored charge measured in farads. Note; Farad is a very large unit and most capacitors are rated in microfarads. Capacitance is dependent on: Plate size – large plates = large capacity. Distance between plates – the closer the plates the larger the capacitance. Nature of dielectric material. Capacitor/Condenser Used where a large transient current is needed – ie: spot welders, flash guns, ignition systems etc. In ET instruments, variable capacitors are used in AC circuits to adjust the phase between voltage and current to create resonance. Copyright © TWI Ltd Capacitive Reactance Definition: The action of capacitance in opposing the flow of AC, and in causing the current to lead the voltage. Measured in ohms. XC 1 2ΠfC Where Xc is the capacitive reactance in ohms. F is the frequency in Hertz. C is the Capacitance in Farads. 1 X c 2 x3.142 x5,000 x0.000020 Example: What is the capacitive reactance in a circuit where the frequency is 5kHz and the capacitance is 20 µF? Copyright © TWI Ltd Inductive Reactance Effect on Voltage Current a b c d e At (a) current changes rapidly and goes positive. At (b), the current is slow and changes from positive to negative. Rate of change of current a b c d e Back EMF induced by the alternating current is at maximum when the rate of current change is at a maximum and that it always opposes the change of current. Back emf a b c d e Applied voltage a b c d 1 X c 0.6284 X c 1.59 e Current For current to flow, back EMF must be overcome by the applied EMF. Therefore the applied EMF must always be of opposite phase and of greater amplitude. If we show the current for comparison, you see that the current reaches its maximum 90 degrees later than the voltage. Copyright © TWI Ltd Copyright © TWI Ltd Timing Relationship of Current to Voltage in a Circuit A Purely Resistive Circuit In a purely inductive circuit, voltage leads the current by 90°. 90 90 In a purely capacitive circuit, current leads the voltage by 90°. However, in a purely resistive circuit, voltage and current are in phase. Copyright © TWI Ltd Amplitude CIVIL Voltage Current 45° 90° 135° 180° 225° 270° 315° 360° Time In a purely resistive circuit, voltage and current are in phase. Copyright © TWI Ltd 7 A Purely Capacitive Circuit A Purely Inductive Circuit Voltage Voltage Current 45° 90° Amplitude Amplitude Current 135° 180° 225° 270° 315° 360° Time In a purely capacitive circuit, current leads the voltage by 90°. Time Copyright © TWI Ltd Timing Relationship of Current to Voltage in a Circuit RMS Values from Peak 16 Step 2: Square peak value. I² = 16. 12 CIVIL 90 90° 135° 180° 225° 270° 315° 360° In a purely inductive circuit, voltage leads the current by 90°. Copyright © TWI Ltd In a purely inductive circuit, voltage leads the current by 90°. 45° Step 3: Take mean of square I²/2 = 8. 8 90 In a purely capacitive circuit, current leads the voltage by 90°. However, in a purely resistive circuit, voltage and current are in phase. 4 2.828 0 Step 1: Take peak value I = 4. Step 4: Take root of mean = 2.828. -4 Copyright © TWI Ltd Copyright © TWI Ltd Converting RMS to Peak to RMS In order to obtain RMS values from peak: RMS = peak ⁄ √2 Example: What is the RMS value for 100 amps peak? RMS = 100/1.414 = 70.71 amps RMS. In order to obtain peak values from RMS: Peak = RMS X √2 Example: What is the peak value for 100 amps RMS? Peak = 100 x 1.414 = 141.4 amps peak. Copyright © TWI Ltd 8 Eddy Current Testing (ET) Equipment Circuits Section 6 Copyright © TWI Ltd Copyright © TWI Ltd Equipment Circuits Simple circuits. Resonance circuits. Bridge circuits. Phase sensitive circuits. Simple circuits: Oscillator Meter Single absolute circuit Probe Copyright © TWI Ltd Copyright © TWI Ltd Basic Eddy Current Equipment Equipment Circuits Simple circuits: Meter Oscillator Current Equipment Circuits Probe Driver Double absolute circuit (reflection probe) Receiver Copyright © TWI Ltd Copyright © TWI Ltd 1 Equipment Circuits Resonance circuits: Equipment Circuits Resonance circuits: Z Resonance occurs when: XL = XC V 2fL = XC XL R f = XL Amplitude R 1 . 2 f C 1 . 2 LC XC fR f At resonance Z = R Copyright © TWI Ltd Copyright © TWI Ltd Equipment Circuits Bridge circuits - based on the Wheatstone bridge: Equipment Circuits Bridge circuits - based on the Wheatstone bridge: 4 2 Meter 2 12 v 12 v Potential divider 4 4 8 Copyright © TWI Ltd Copyright © TWI Ltd Equipment Circuits Bridge circuits - based on the Wheatstone bridge: Equipment Circuits Bridge circuits - based on the Wheatstone bridge: Meter Meter Copyright © TWI Ltd Copyright © TWI Ltd 2 Equipment Circuits Phase sensitive bridge circuits: Instruments Ref.Volts Meter reading instruments. Lift-off control. Cathode ray tubes. A-scan display. Ellipsoid display. Vector point display. Primary bridge Copyright © TWI Ltd Copyright © TWI Ltd 3 Eddy Current Testing (ET) Probe Coils Section 7 Copyright © TWI Ltd Copyright © TWI Ltd Coil Arrangements There are four basic coil arrangements: 1. Single absolute. 2. Double absolute. 3. Single differential. 4. Double differential (which can be self or external comparative). Probe Coil Arrangements Absolute Differential Single Double Copyright © TWI Ltd Copyright © TWI Ltd Probe Coil Arrangements Probe Coil Arrangements The single absolute has one coil covering one area. Small coils can be very sensitive, but a large coil will suffer from loss of sensitivity. The coil will detect an absolute change in the eddy currents induced. Absolute Single Copyright © TWI Ltd Copyright © TWI Ltd 1 Probe Coil Arrangements Probe Coil Arrangements Most, but not all, hand held probes used in the aerospace industry have single absolute coil arrangements. They are extremely sensitive. There are a range of probes to suit virtually every application. Copyright © TWI Ltd Copyright © TWI Ltd Probe Coil Arrangements Probe Coil Arrangements Shielded probes have the coil wound within a ferrite pot. This reduces the magnetic footprint and helps alleviate the problems of mass, edge and ferrous effect. Copyright © TWI Ltd Copyright © TWI Ltd Probe Coil Arrangements Probe Coil Arrangements The double absolute has two coils, a driver and a pick-up. The driver produces the eddy currents, the pick-up detects any changes. This arrangement allows for high sensitivity. Absolute Double Copyright © TWI Ltd Copyright © TWI Ltd 2 Probe Coil Arrangements The single differential self comparative has one coil acting as both driver and pick up. The coil is wound in both directions and compares one area of the component to an adjacent area. It allows relatively gradual changes in eddy currents to pass but still pick up faults. It can suffer from a lack of sensitivity if a large coil is used. Copyright © TWI Ltd Probe Coil Arrangements Probe Coil Arrangements Differential Single Copyright © TWI Ltd Probe Coil Arrangements The double differential self comparative has a separate driver and pick up coil. The pick up is wound in both directions. It allows gradual eddy current changes to pass but will detect sudden changes such as a fault. The separate pick up allows for good sensitivity. Copyright © TWI Ltd Probe Coil Arrangements Differential Copyright © TWI Ltd Probe Coil Arrangements The double differential external comparative has a driver and a pick-up. They are both wound around the component under test and a reference standard. This arrangement is usually quite sensitive and can be set to give an alarm to very exacting tolerances. Double Copyright © TWI Ltd Copyright © TWI Ltd 3 Probe Coil Arrangements Probe Coil Arrangements The double differential coil arrangement is used in the external and self-comparative modes. The low frequency and donut probe are examples of external comparative arrangements. (They compare to air/perspex). The rotary probe is an example of a selfcomparative winding. Copyright © TWI Ltd Probe Coil Arrangements Copyright © TWI Ltd Probe Coil Arrangements Copyright © TWI Ltd Copyright © TWI Ltd Probe Coil Arrangements Surface probes. Encircling probes. Internal bobbin probes. Calibration Blocks Slotted calibration blocks. Step wedges. Tube standards. Copyright © TWI Ltd Copyright © TWI Ltd 4 Eddy Current Testing (ET) Probes Section 8 Copyright © TWI Ltd Eddy Current Probes Copyright © TWI Ltd Eddy Current Probes Due the large variety of probes in eddy current testing there are many different systems of classification. Three of the most common classifications are: 1. Surface probes. 2. Inside diameter (ID) or bobbin probes. 3. Outside diameter (OD) or encircling probes. Copyright © TWI Ltd Eddy Current Probes Surface probes are coils that are typically mounted close to one end of a plastic housing. As the name implies, the technician moves the coil end of the probe over the surface of the test component. Copyright © TWI Ltd Copyright © TWI Ltd Eddy Current Probes Some surface probes are specifically designed for crack detection of fastener holes. These include sliding probes, ring probes and hole probes. Copyright © TWI Ltd 1 Eddy Current Probes Surface probes can be very small in size to allow accessibility to confined areas. Eddy Current Probes Inside diameter (ID) probes, also known as bobbin probes, are coils that are usually wound circumferentially around a plastic housing. These probes are primarily designed for inspection inside of tubular materials. Finger probe Copyright © TWI Ltd Eddy Current Probes Outside diameter (OD) probes are coils that are wound around the circumference of a hollow fixture. The coil is designed such that the test part is ran through the middle of the coil. These probes can be used to inspect bars, rods as well as tubes. Copyright © TWI Ltd Reference Standards Copyright © TWI Ltd Reference Standards In order to give the eddy current inspector useful data while conducting an inspection, signals generated from the test specimen must be compared with known values. Reference standards are typically manufactured from the same or very similar material as the test specimen. Many different types of standards exist due to the variety of eddy current inspections performed. The following slides provide examples of specific types of standards. Copyright © TWI Ltd Reference Standards Material thickness standards used to help determine such things as material thinning caused by corrosion or erosion. Copyright © TWI Ltd Copyright © TWI Ltd 2 Reference Standards Crack standards: Reference Standards ASME tubing pit standard: Copyright © TWI Ltd Copyright © TWI Ltd Reference Standards Nonconductive coating (paint) standard with various thickness of paint on aluminum substrate. Copyright © TWI Ltd 3 Eddy Current Testing (ET) Tube Testing Section 9 Copyright © TWI Ltd Copyright © TWI Ltd Tube Testing Can detect: Cracks. Erosion – large amounts of suspended solids within the water. Corrosion – low flow rate, stagnant water (blocked tubes). Corrosion - dissimilar metals – support plates. Mechanical damage – vibration between tube and support – temperature changes high water velocity. Copyright © TWI Ltd Tube Material Commercial – Brass 70% copper 30% zinc Trace of arsenic – prevents dezincification – cause leaks and loss of strength. Poor resistance to corrosion. Admiralty – Brass Similar to above with addition of tin – slight improvement on corrosion resistance. Tubes made by drawing over mandrel. Copyright © TWI Ltd Tube Manufacture Choice of condenser tube materials depend mainly on type of cooling water and the amount of suspended solids within the water. Six main materials used. Commercial – Brass. Admiralty – Brass. Aluminium – Brass. Cupro – Nickels. Titanium. Stainless steel. A typical 500MW steam turbine can have up to 20,000 tubes either together or in blocks of 5000, with a length of each tube up to 70 feet. Copyright © TWI Ltd Tube Material Aluminium – Brass 70% copper 28% zinc 2% aluminium Small amount of arsenic. High erosion rate – rarely crack. Developed for use with salt water – better corrosion rate than other brasses. Poor resistance to erosion when water contains suspended solids. Copyright © TWI Ltd 1 Tube Material Cupro - Nickels 70% - 90% copper 30% - 10% nickel Small amounts of manganese and iron. Developed where water contains large amounts of suspended solids and is highly corrosive. Poor heat transfer compared to brasses – higher cost – larger condensers required. Tube Material Titanium Good corrosion and erosion resistance. Mainly used in coastal power stations. Steam impingement on outer diameter can cause erosion. Can be seamed or seamless tubes. Stainless steel Good against erosion. Susceptible to localised pitting corrosion – salt water environment. Susceptible to stress corrosion cracking. Copyright © TWI Ltd Copyright © TWI Ltd Erosion Prevention Tube Testing Cleaning High pressure air (14000 psi). Low pressure water. Scraper bullets. Component and system design. Choice of material. Protective coatings. Additives to the water. Correct commissioning procedures. Calibration Start of inspection. End of inspection. Any changes to equipment. Copyright © TWI Ltd Copyright © TWI Ltd Tube Testing Scan speed Manual < 0.6m/sec. Automated up to 1.2m/sec. (constant speed – changes in signal amplitude – incorrect assessment). Chart speed Full length inspections 25mm per metre of tube. Inlet scans minimum 50mm of chart per tube. Tube Testing Amplitude analysis Mainly used for detection of internal defects. Frequency range 5 – 30 kHz. No phase information required. Phase analysis Detect internal and external defects. Phase separation between signals is normally 90°. Carried out at the f90 frequency. 90° External defect Internal defect Copyright © TWI Ltd Copyright © TWI Ltd 2 f90 Frequency Defect amplitude is a function of its surface area size and depth. Defect phase is mainly a function of depth. f90 frequency – gives good phase lag between defects and good signal amplitude. Fill Factor Equivalent to lift off when using encircling coils. Fill Factor = Coil diameter DC² (internal coil) Tube diameter DT² Or f90 (kHz) = 3 t2 f90 = recommended driving frequency (kHz). = resistivity (µΩcm). t = tube thickness (mm). Copyright © TWI Ltd = Tube diameter DT² (external coil) Coil diameter DC² must be less than 1.0. is usually about 0.7. Copyright © TWI Ltd 3 Eddy Current Testing (ET) In Service Inspection of Coated Steel Welds Using Eddy Current Techniques Copyright © TWI Ltd Copyright © TWI Ltd Introduction Traditionally surface crack detection in ferritic steel welds with eddy-current techniques has been difficult due to the change in material properties in the heat affected zone. These typically produce signals much larger than crack signals. Introduction Sophisticated probe design and construction, combined with modern electronic equipment, have largely overcome the traditional problems and now enable the advantages of eddy-current techniques to be applied to inservice inspection of ferritic steel structures in the as-welded conditions. Copyright © TWI Ltd Copyright © TWI Ltd Introduction Specifically, the advantage of the technique is that under quantifiable conditions an inspection may now be carried out through corrosion protection systems. This means the costly removal and replacement of the protective coating is now not necessary. Basic Principle of Eddy Currents DC AC Opposing field No induction + _ Induction creates secondary AC Current flow (right hand rule) Static magnetic field Copyright © TWI Ltd Primary AC Alternating magnetic field Copyright © TWI Ltd 1 Basic Principle of Eddy Currents Basic Principle of Eddy Currents Ferrite core Primary inducing field Opposing secondary field Eddy currents Copyright © TWI Ltd Copyright © TWI Ltd Advantages of Eddy Currents Sensitive to surface defects. Can detect through several layers. Can detect through surface coatings. Accurate conductivity measurements. Can be automated. Little pre-cleaning required. Portable. Disadvantages of Eddy Currents Very susceptible to permeability changes. Only on conductive materials. Will not detect defects parallel to surface Not suitable for large areas and/or complex geometries. Signal interpretation required. No permanent record (unless automated). Copyright © TWI Ltd Copyright © TWI Ltd 2 Eddy Current Testing (ET) Practical Exercises Copyright © TWI Ltd Practical Exercises Copyright © TWI Ltd Practical Exercises The profile will also change along the length of the weld as the geometry changes. The K-Node found offshore is used as a typical example (figure number 02). 1 2 3 3&6 4 4 7&8 1&2 Lift-off signal horizontal 6 5 7 8 5 0 Lift-off signal corresponding with coating thickness. Figure number 01: Coated weld section. Variation in sensitivity due to application of protective coatings. Figure No. 02, Typical K Node Configuration. Copyright © TWI Ltd Practical Exercises It is therefore necessary to ensure that the technique chosen is capable of the following: Evaluating the material to be tested. Measuring the coating thickness in order that the full extent of the problem is quantified and evaluating the constituents of the coating. Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises Approach the material samples on the conductivity block in turn. Repeat the balance and erase. Approach the ferrite sample, adjust the phase control until the signal generated is in the vertical direction. Repeat the lift-off on each of the sample pieces in turn. The screen presentation should now be the same as that described in figure number 03. Copyright © TWI Ltd 1 Practical Exercises 50D steel Increasing magnetic permeability (µ) mu Stainless steel Aluminium alloys Increasing electrical conductivity (σ) sigma Figure number 03 Lift-off signals obtained from standard conductivity block. Copyright © TWI Ltd Practical Exercises If you now join up the ends of the signals (phasor points) two comma shaped curves are produced. Please note that the top curve represents changes in magnetic permeability (µ) and the bottom curve represents changes in electrical conductivity (IACS) or resistivity (p). Please also note that the changes in magnetic permeability and resistivity trace out the curve in an anti-clockwise direction whereas increasing conductivity changes in a clockwise direction. Copyright © TWI Ltd Practical Exercises Repeat the exercise at the various frequencies using samples of carbon steel and compare these to the response from the 50D steel calibration block. Using the 50D block as the reference, measure the amplitudes and angles of the responses from the other samples. Please note whether the responses are clockwise/anti-clockwise relative to the 50D sample. Copyright © TWI Ltd Practical Exercises This-screen presentation represents a vector diagram showing how the coil’s impedance is affected by liftoff, conductivity and permeability changes. Keeping the screen as is do a lift-off on the 50D carbon steel calibration block. The signal from the steel should be approximately 30 degrees from the ferrite signal. Please measure the angle accurately using the protractor. Please reproduce the phasor diagram onto the graph paper. Please pay particular attention to the angles between the signals and the lengths (amplitudes) of the signals. Copyright © TWI Ltd Practical Exercises Repeat the exercise changing the frequency first to 60kHz, then to 100kHz and finally 25OkHz. Reproduce each phasor diagram on the graph paper and compare each with respect to: Amplitude of signal. Angle between signals. Relative position of the material points. Copyright © TWI Ltd Practical Exercises Summary: The impedance-plane response to varying nonmagnetic alloys trace out a comma shaped curve with conductivity increasing in a clockwise direction. When ferromagnetic materials are tested the magnetising coil reactance changes in a far different way than with non-magnetic materials. When a high permeability material is tested the magnetising-coil inductance and inductive reactance increase dramatically because of an increase in flux density. Copyright © TWI Ltd 2 Practical Exercises When testing ferro-magnetic material an increase in permeability will move the spot in an anti-clockwise direction up the comma curve. Please note that a probe with a ferrite core yields better magnetic coupling and hence a larger impedance diagram than a probe with a similar air-core coil. Copyright © TWI Ltd Practical Exercises 1.2. Defining the limiting parameters affecting the use of a specific calibration block. When contemplating the use of eddy-current techniques it is imperative that a calibration block representative of the component to be tested in the three main areas is available. These factors are compensated for in practical terms. The main areas are: Material. Coatings and geometry. Practical Exercises Please also note that magnetic permeability has the same effect as resistivity and therefore these two parameters usually cannot be separated when a surface probe (coil) is used. As a result of the above we conclude that it is possible to quickly evaluate the material of the component to be tested by carrying out a series of lift-offs on the component and comparing the responses to a known standard such as the 50D calibration block. The limiting parameters have yet to be evaluated ie when is the use of the 50D block acceptable and when is it necessary to use a calibration block manufactured from material closer to the component to be tested? Copyright © TWI Ltd Practical Exercises When comparing impedance changes due to permeability variations in the previous exercise we did not consider the possible effect cracks and other discontinuities produce. In this exercise we are using spark eroded slots to represent surface breaking defects, the theory being that the signal generated from similar slots in near matching materials will be reasonably similar both in amplitude (length) and phase (angle from a known reference point). This exercise will address material variations only. Copyright © TWI Ltd Practical Exercises Start by carrying out lift-offs on the material, taking care not to wander close to the slots and rotate the phase adjustment until the lift-off signal is horizontal. Please note that the initial balance point is very important. Balance in air then approach the material, setting the lift-off signal horizontal right to left. Keep the probe on the material and try to run over the slots starting with the 0.5mm deep slot. As you can see it is extremely difficult to see the slots on the CRT. Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises lf we now balance on the material equidistant from two slots and carry out a series of lift-offs you will recognise that the horizontal has shifted 180 degrees from the original lift-off position. Now please try and run over the slots in turn, commencing with the 0.5mm deep slot. A signal is generated approximately at right angles to the horizontal and the amplitude of the signal increases with increasing depth of slot. However, the ratio is not uniform. ln order to try and make the screen as clear as possible we would recommend the following procedure is followed: Copyright © TWI Ltd 3 Practical Exercises Balance the equipment in air. Adjust the horizontal until it reads +30 and the vertical until it reads 0. Place the probe onto the material taking care to keep the probe at right angles to the material under test. Balance the equipment. The spot should return to the same position ie +30 and O. Commence lift-offs and whilst doing so adjust the phase control until the lift-off signal is horizontal going from the balance point to the left hand side of the CRT. Practical Exercises Repeat this exercise at the mid-point between the end of the block and the 0.5mm deep slot. When the lift-off is set as described above run the probe over the 0.5mm deep slot. Note the vertical displacement of the signal and the angle the slot signal makes with the lift-off signal. Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises Repeat the process with the 1.0mm, 2.0mm, 3.0mm, 4.0mm and 5.0mm deep slots. Produce a graph with the x-axis the slot depths and the y-axis the vertical displacement of the signals obtained from the slots. With reference to the graph produced are there any patterns apparent that may be useful? Practical Exercises Are the responses uniform? Are they linear? Is any part of the graph linear, eg from 0-3.0mm or 3.0-5.0mm? Please make your comments to the above on the graph paper under the graph. Consider the variables in each case. Jot some thoughts down! What are the main parameters affecting the eddy-current responses to the slots? Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises One of the main factors influencing the amplitude of the response from the slots is choice of frequency. Repeat all of the exercise but in turn use 40kHz, 100kHz and 200kHz as the frequency. It may be necessary to de-sensitise the probe by applying insulating tape over the end of the probe. lf this is so, it is necessary to carry out the previous exercise again, this time with the tape on. The exercise should therefore commence with the highest frequency. Please note the responses on graph format as before in order that a ready comparison may be made. Copyright © TWI Ltd Practical Exercises Summary of results: Lift-off = frequency 10kHz. Lift-off = frequency 40kHz. Lift-off = frequency 100kHz. Lift-off = frequency 200kHz. Figure number 04 Typical responses from spark eroded slots using the absolute coil. Copyright © TWI Ltd 4 Eddy Current Testing (ET) Practical Exercises 2 Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises Practical Exercises 1.3. Effect of increasing non-conductive coating thickness on defect detectability. As the welded components to be examined will, in general terms, be coated it is now necessary to try and estimate the effect of that coating. For the purpose of this exercise only non-conductive coatings will be used. Procedure: Using the 50D steel calibration block. Initial equipment settings: Frequency 10kHz. Gain Phase 20.5dB Lift-off horizontal, left to right. Copyright © TWI Ltd Copyright © TWI Ltd Practical Exercises Practical Exercises Increasing layers of aluminium Responses from multiple layers of aluminium foil Figure number 06(a) Thickness measurement of aluminium foil. 2.0mm coatin g 1.5mm coa ting ting Aluminium foil 1.0mm coa 0.5mm coating No Coating Relative Lift-off signal amplitude Balance point 0 Copyright © TWI Ltd Copyright © TWI Ltd 1 Practical Exercises Increasing thickness of aluminium Lift-off aluminium Lift-off 50D steel Layers of aluminium 50 D calibration block Figure number 06 Typical responses from aluminium foil and aluminium/carbon steel combined using the absolute coil. Practical Exercises 1.6 Material and heat affected zone evaluation, effect of weld geometry. Up until now we have been using a flat plate as a calibration block and therefore have not taken the possible changes in material properties due to the heat affected zone of the weld into consideration. Neither have we considered the possible effects of the various geometries made by the plate/pipe and the weld profile(s) as the weld configurations change. Copyright © TWI Ltd Practical Exercises Copyright © TWI Ltd Practical Exercises 6 5 4 Figure number 07 Weld section calibration block. Copyright © TWI Ltd Practical Exercises Balance point Lift-off 50D calibration block 1 2 4 3&5 Figure number 08 Responses from component parts of weld section using the absolute coil. Copyright © TWI Ltd Lift-off points 3 2 1 Figure number 08(a) Lift-off responses from weld zones. Copyright © TWI Ltd Practical Exercises Section 2.0: Practical exercises - evaluation of weld probe (tangential, orthogonal, differential coils). Exercises to be completed: 2.1. Material evaluation. 2.2. Defining the limiting parameters affecting the use of a specific calibration block. 2.3. Effect of increasing non-conductive coating thickness on defect detectability. 2.4. Measure the distribution (cross section) of the induced eddy-currents on the surface of the 50d carbon steel calibration block. 2.5. Additional tests to be carried out on weld probes prior to use. Copyright © TWI Ltd 2 Web Probe Coil Configurations Web Probe Coil Configurations 2.6 Coil 1 Plan view Coil 2 Coil 1 Coil 2 Tangential view Figure number 09 Coil configurations (weld probe). Quantify the thickness of conductive coatings eg TSA flame spray aluminium on 50d carbon steel substrate. 2.7. Material and heat affected zone evaluation. Effect of weld geometry. 2.8. Weld surface examination in addition to weld configuration 2.9. Defect detection using appropriate scanning procedures. 2.10. Defect evaluation. 2.11. Defect depth capability. 2.12. Effect of orientation of defect to coils on defect detectability. 2.13. Summary review of eddy-current equipment and inspection procedures. Copyright © TWI Ltd Copyright © TWI Ltd Web Probe Coil Configurations Protective coverings for eddy-current probes. As we shall be running the probes over steel surfaces it is necessary to try and ensure we do not damage the coils. We try and protect the probes by adding two layers of standard electrical insulating tape preferably of a contrasting colour to the probe in order that wear on the tape can be monitored before reaching and therefore damaging the coils. It is imperative that the calibration procedures are conducted with the two layers of tape on the probe as the tape will obviously increase the stand-off between the probe and the material under test thereby decreasing the relative sensitivity setting. Web Probe Coil Configurations Directional field: The effect of the orientation of the defect relative to the coils is illustrated in figure number 11. ln the meantime, we shall continue to repeat the exercises conducted using the pencil probe but substituting the weld probe for the pencil probe. Copyright © TWI Ltd Copyright © TWI Ltd Web Probe Coil Configurations Web Probe Coil Configurations 1 + + 2 3 Tangential side view Tangential front and rear view Figure number 10 Outline of weld probes showing direction of movement. Copyright © TWI Ltd 2 1 5 3 4 4 5 Figure number 11 Phase angle of defect signals relative to the orientation of defect to coils. Copyright © TWI Ltd 3 Web Probe Coil Configurations +1.5 Graticules 50D Steel Balance point - 0 -1.5 Graticules Ferrite and non-magnetic material Relative Sensitivity Level What is the maximum coating thickness through which a relative sensitivity level can be maintained for each frequency? What are the limiting parameters affecting the detection of defects under non-conductive coatings? 50D Steel Figure number 12 Typical material lift-off responses using the weld probe on the standard conductivity block. Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks 2.4. Measure the distribution (cross section) of the induced eddy-currents on the surface of 50d carbon steel calibration block. ln order to try and estimate the extent of the eddy-currents induced in the material under test, it is necessary to have a reference point to work to. In ultrasonic testing this reference point would be known as the index point. Weld Probe Checks Figure number 15 Outline of eddy-currents induced in carbon steel calibration block. α α X mm β β X mm Y mm Y mm Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks The index point coincides with the maximum concentration of eddy-currents in the material. This is the point where the two coils cross each other. The probe should have two layers of electrical insulating tape on the testing face to protect the coils. We suggest that one layer is wrapped round the probe in order that the index points may be marked on the sides of the probe. Once the tape is in place balance the probe on the first section of the 50D calibration block central between the 0.5mm deep slot and the end/edges of the block. Run the probe back and forth over the 0.5mm and 1.0mm deep slots. Copyright © TWI Ltd Weld Probe Checks + Mark index points on the block Copyright © TWI Ltd 4 Weld Probe Checks Weld Probe Checks Signal amplitude from 1.0mm deep slot to 1 Graticule vertical. Balance point Figure number 13 Plotting eddy-current distribution on 50d steel block. Mark index point on the block Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks 50% reduction level Signal amplitude from 1.0mm deep slot to 6 Graticules vertical. Weld Probe Checks Scanning procedures: For this purpose it is necessary to once again divide the weld to be tested into component parts. Material and heat affected zone examination. Weld cap surface examination. Balance point Figure number 14 Plotting eddy-current distribution on 50d steel block. Copyright © TWI Ltd Weld Probe Checks 2.5. Additional tests to be carried out on weld probes prior to use. Experience has shown that due to the complex manufacturing process the weld probes are subject to variations in performance that may affect defect detectability. In addition to exercise 2.4 a few areas have been identified as being critical and should be assessed prior to use. Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks These are: Coil symmetry. Variation in signal amplitude from 0.5, 1.0 and 2.0mm deep slots. Plotting the induced field in 50d steel calibration block. Variation in amplitude from slots due to lift-off from coils. Evaluation of lift-off. Copyright © TWI Ltd 5 Weld Probe Checks Weld Probe Checks Not less than one vertical graticule difference between the responses. 0.5mm 1.0mm 2.0mm Balance level Individual responses from 0.5, 1.0 and 2.0mm deep slots. Balance level Figure number 16 Variation in amplitude from slots in 50D steel calibration block. Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks α Weld Probe Checks β The central coil has come off the surface of the metal. 0.5mm 1.0mm 2.0mm Balance point Figure number 17 Variation in signal amplitude in the vertical plane. Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Checks Weld Probe Checks Full screen height of 7 graticules vertical Lift-off signal Balance point 0.5mm 1.0mm h Figure number 18 Evaluation of lift-off signal. 2.0mm Copyright © TWI Ltd Copyright © TWI Ltd 6 Weld Probe Summary Weld probe Orthogonal, tangential, differential coils. The lift-off is dramatically reduced and therefore is of no practical use for coating thickness and/or material evaluation. Defect detectability is reasonably consistent providing the following limitations are addressed: Weld Probe Summary Limitations: Relatively large diameter therefore access must be considered. Directional field. Minimum size of defect detectable is 1.0mm deep x 5.0mm long. Maximum non-conductive coating thickness is 2.0mm. Typical maximum conductive coating (TSA) is 0.8mm. Copyright © TWI Ltd Copyright © TWI Ltd Weld Probe Summary Defects As this technique is to be used as an in-service tool the defect(s) to be detected and evaluated are in general terms fatigue cracks. Again in general terms, these defects are directional, initiating in the toe of the weld or heat affected zone. They may however take any route if and when they propagate from the toe of the weld or heat affected zone. Weld Probe Summary As you can see, the pencil probe and weld probe are complimentary. It is always necessary to use the pencil probe to evaluate the coating thickness and/constituents and material relative to the calibration block prior to using the weld probe for defect detection. By keeping to this sequence the relative sensitivity level will be maintained by ensuring that the appropriate compensation factors are added for geometry and coating to the basic calibration settings. Copyright © TWI Ltd Copyright © TWI Ltd Weld Testing Procedures Weld Testing Procedures Balance point Optimum angle of probe Figure number 19 Optimum angle of coils for weld toe examination. Copyright © TWI Ltd Copyright © TWI Ltd 7 Weld Testing Procedures Signal amplitude typically 1 to 1.5 graticules high Weld Testing Procedures Balance point Figure number 20 Additional scan of the toe of the weld. Copyright © TWI Ltd Copyright © TWI Ltd Weld Testing Procedures Weld Testing Procedures Crack depth profile Crack depth profile 0 1 0 4 1 3 2 2 Crack 1 2 3 No Crack Balance point Toe crack 0 The central coil angle must bisect the geometry angle under test. α Figure number 21 Typical screen presentation from geometry and crack in the toe of the weld. β Copyright © TWI Ltd Weld Testing Procedures Copyright © TWI Ltd Weld Testing Procedures 120° typical weld, stand-off 1mm, geometry compensation 1.5dB. 30° typical weld, stand-off 3mm, geometry compensation 4.5dB. Figure number 23 Compensation levels for stand-off due to varying weld configurations. Copyright © TWI Ltd Copyright © TWI Ltd 8 Weld Testing Procedures Weld Testing Procedures α α Signal amplitude indicative of depth of crack β β 90° typical weld, stand-off 2mm, geometry compensation 3dB. 90° small weld, stand-off 2mm, geometry compensation 3dB. Copyright © TWI Ltd Weld Testing Procedures 2.6. Weld surface examination in addition to weld configuration. We have not as yet considered the possible affects of the weld profile(s). It is now necessary to evaluate the weld probe(s) for weld surface examination in addition to the geometry effect. We require 1 x weld section as illustrated in figure number 7 plus a variety of weld profiles to run the probe over and quantify the effect. Is it possible to predict the signal(s) which will be generated by the variable profile of the weld? Copyright © TWI Ltd Weld Testing Procedures 2 1 Balance point Figure number 22 Typical screen presentation of crack using single pass technique. Copyright © TWI Ltd Weld Testing Procedures Why is it that the signal generated by the toe of the weld always moves towards seven o'clock? Likewise when the probe is run over the weld profile the signal envelope will, in general terms, move from seven o'clock to one o'clock. What is the reason for this? A general schematic for weld examination is illustrated in figure number 24. ln general terms the signal will follow the direction of the lift-offs. ln order to try and confirm this theory please follow the procedure described below. Copyright © TWI Ltd Weld Testing Procedures 4 Copyright © TWI Ltd Copyright © TWI Ltd 9 Weld Testing Procedures 2.7 Component parts of the weld For the purposes of this exercise the weld components are: 1.0mm deep slot. Defect signals Weld Testing Procedures Weld profile or weld cap responses. Geometry signals caused by the heat affected zones. Transverse defects or defects parallel to the positive direction of movement. Heat affected zone and material adjacent to the heat affected zone. Toe of the weld (geometry effect). Weld run. Inter-weld run. Figure number 24 General schematic for weld inspection. Copyright © TWI Ltd Copyright © TWI Ltd Weld Testing Procedures Place the probe on the material adjacent to the heat affected zone at a distance approximately twice the material thickness away from the weld toe. Balance the equipment. Lift the probe from the material and place the probe into and/or onto the component parts of the weld in sequence ie toe of the weld, weld run, inter-weld run and then the opposite toe of the weld. The screen presentation should be similar to that illustrated in figure number 25. Weld Testing Procedures Summary: The impedance plane display from a weld should be predictable. The signals generated should be re-producible from the component parts of a weld providing the reference levels remain constant. Copyright © TWI Ltd Copyright © TWI Ltd Weld Testing Procedures 2.8. Defect detection using appropriate scanning procedures. It is necessary to summarise the equipment limitations and try to predict the type of defect to be detected in order to compile relevant scanning procedures. Weld Testing Procedures Equipment summary: Pencil probe Absolute coil with ferrite core. Reference coil of 82uH used. The lift-off signal is used to measure coating thickness and the relative phase is used to evaluate conducting coating thickness and component material relative to a standard calibration block. Not to be used for defect detection. Copyright © TWI Ltd Copyright © TWI Ltd 10 Weld Testing Procedures Material and heat affected zone examination This procedure has been described in detail in exercise number 2.7, material and heat affected zone evaluation, effect of weld geometry. ln addition to that described in exercise number 2.7 it is imperative that the material is scanned in different directions looking for defects occurring and/or propagating in paths out-with the toe of the weld. This procedure is illustrated in figure numbers 21, 22 and 26. Copyright © TWI Ltd Weld Testing Procedures Weld cap surface examination It is necessary to ensure full coverage of the weld surface and take into account the likely direction of the fatigue cracks. In practice this involves the scanning of the weld surface in two directions ie transverse across the weld looking for longitudinal defects and longitudinal looking for transverse defects. ln the event of weld profiles being exceedingly rough and/or proud it may be necessary to carry out an additional single pass scan along each weld inter-run. Copyright © TWI Ltd Weld Testing Procedures Defect detectability Material and heat affected zone. lf we consider the fatigue crack to be the classic case ie the defect runs adjacent to the heat affected zone and through wall the mechanism of the defect signal is as follows: Copyright © TWI Ltd Weld Testing Procedures Toe cracks Figure number 26 Additional scans in the heat affected zone. Copyright © TWI Ltd Weld Testing Procedures It is common to find that the scanning sensitivity necessary to achieve a relative sensitivity level at the toe of the weld is excessive for weld cap examination. In general the reason for this is that the coating thickness is greatly reduced relative to the toe of the weld. If this is the case please adjust the gain in order to ensure the signal envelope obtained from the weld surface is contained within the screen. Copyright © TWI Ltd Weld Testing Procedures Turn your mind back to the calibration exercises. When you approach the 1.0mm deep slot in the calibration block the signal goes vertical, maximises and then returns to the balance point when the probe is over and going away from the slot. Similarly when we approached the toe of the weld (geometry effect) the signal would, depending on the severity of the weld configuration, maximise as the signal moved towards 7o'clock. Copyright © TWI Ltd 11 Weld Testing Procedures The signal would then re-trace it’s movement when the probe was returned to its original position. In each case we had an equal and opposite effect. When you introduce a defect in the toe of the weld (geometry) the defect signal produced is the vector addition of the signals generated by these two conditions. Weld Testing Procedures As the geometry remains reasonably uniform over a relatively short distance the main factor influencing the amplitude of the signal is obviously the cross section of the defect. For discussion purposes only let us consider if we had to implant a 1.0mm deep x 20mm long defect in exactly the same position in the heat affected zone of three typical weld configurations and monitor the defect signal generated. What would be the outcome? Copyright © TWI Ltd Weld Testing Procedures Which weld would produce the largest (longest) signal? Why? The weld configuration with the least geometry effect would have the least effect on the vertical going signal from the defect. It is therefore with some confidence that we can predict that the pipe/plate weld would produce the most obvious defect signal. So, from this exercise we learn that the defect signal depends on a number of variables when scanning the material adjacent to the weld into the weld. Copyright © TWI Ltd Weld Testing Procedures The single pass technique eradicates the majority of these variables however we must always remember to compensate for the geometry effect by adding the appropriate number of dBs gain. It is not possible to immediately carry out the single pass. The material and the heat affected zone must be examined in detail prior to conducting the single pass. The reasons are self evident. Copyright © TWI Ltd Copyright © TWI Ltd Weld Testing Procedures These include: Geometry of component under test. Stand-off of probe from area under test. Cross section of defect. Copyright © TWI Ltd Weld Testing Procedures Weld cap surface examination Having established the signal envelope for the weld cap examination (2.6) we should be able to distinguish abnormal signals and by further investigation establish these are defects and/or caused by other conditions such as scale etc. Copyright © TWI Ltd 12 Weld Testing Procedures Under normal circumstances the fatigue defects will occur in the weld cap in two locations: Inter-run. Weld run itself. Copyright © TWI Ltd Weld Testing Procedures Inter-run fatigue defect The most important consideration is the profile of the weld. If the weld consists of large runs and therefore has a rough profile then the defect will be hidden in the trough between weld runs. The geometry effect will be pronounced relative to a smooth contoured weld. The signal envelope will also be extensive compared to a relatively smooth weld. The normal signal envelope will be going between the geometry effect of the toe of the weld to the peak of the weld run then to the geometry effect caused by the trough between the weld runs until finally we have the geometry effect from the opposite toe of the weld. Copyright © TWI Ltd Weld Testing Procedures Weld Testing Procedures Please consider the locations and try to establish in your mind the variables existing for each. Are the variables the same for each location? Will the defect signal be the same for each? What shape will the defect be? We have tried to establish set patterns for each component part of the weld (figure number 24). Let us take each in turn and try to predict the change to the normal signal pattern and the probable shape of the defect. Copyright © TWI Ltd Weld Testing Procedures We can therefore expect the defect signal to look similar to that generated by a defect occurring in the toe of the weld ie the defect signal will be a vector addition of the vertical displacement from the defect itself and the geometry of the trough in which it occurs. Any abnormal signal can, of course, be confirmed by carrying out a single pass scan along the interrun. Copyright © TWI Ltd Additional Scanning Techniques The probe should be placed some distance from the area under consideration in the same trough as the suspect defect. The equipment should then be balanced thereby negating the geometry effect of the trough. Run the probe along the inter-run into the suspect area. The only out of balance condition should be the defect. The signal from the defect should of course, be in the vertical direction. The movement of the signal in the horizontal direction as it increases in the vertical direction shall be considered in future exercises. Copyright © TWI Ltd Copyright © TWI Ltd 13 Additional Scanning Techniques Copyright © TWI Ltd Additional Scanning Techniques Copyright © TWI Ltd 14 Eddy Current Testing (ET) Amplitude Analysis, Full length and Internal defects Practical Copyright © TWI Ltd Absolute – Amplitude Analysis, Full length, Internal defects Setup equipment as per Appendix A. Use calibration tube A1 or A2. Set indication from 8 x 0.65mm holes to 90° and 80% fsh (5 main scales) (this needs to be on the print out). Move probe through the rest of the 0.65mm holes to obtain printout. These amplitudes will be your class limits (1, 2, 3A, 3B, 4 and 5). Class limit 6 is when the amplitude of the signal is greater than 80% fsh. Copyright © TWI Ltd Copyright © TWI Ltd Absolute – Amplitude Analysis, Full length, Internal defects Carry out Calibration in. Carry out full length scans of tube bundle (all tubes). Using Calibration determine percentage wall loss. Carry out Calibration out. Fill out record sheet. Copyright © TWI Ltd 1 Eddy Current Testing (ET) CIVIL Explanation Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd Copyright © TWI Ltd 1 Copyright © TWI Ltd Copyright © TWI Ltd 2 Eddy Current Testing (ET) Differential Practical Copyright © TWI Ltd Copyright © TWI Ltd Differential – Full length Setup equipment as per Appendix C. Use calibration tube B1 or B2. Set 100% wall loss to 90° and 80% fsh (4 main scales) (8 drilled holes). Move probe to external wall loss grove (47%). Change phase angle to bring signal back to 90°. increase gain to bring signal to 80% fsh. Note phase angle. Repeat above for other external wall losses. Plot the phase angles against wall loss. percentage on graph. Draw line of best fit through the plots. Copyright © TWI Ltd Copyright © TWI Ltd Differential – Full length Carry out full length scans of tube bundle (all tubes). Identify areas of external wall loss - mark on print out. Rescan area of wall loss – changing phase angle to bring signal back to 90° and amplitude to 80% fsh. Using graph determine percentage wall loss. Fill out record sheet. Copyright © TWI Ltd 1 Eddy Current Testing (ET) Inlet End Practical Copyright © TWI Ltd Copyright © TWI Ltd Absolute – Amplitude Analysis, Inlet end, Internal defects Setup equipment as per Appendix B. Use calibration tube D1 or D2. Set 50% wall loss to 90° and 50% fsh (5 main scales) (this needs to be on the print out). Move probe through the rest of the external wall loss grove to obtain printout. Plot the amplitude against wall loss percentage on graph. Draw line of best fit through the plots. Copyright © TWI Ltd Copyright © TWI Ltd Absolute – Amplitude Analysis, Inlet end, Internal defects Carry out Calibration in. Carry out inlet end scans of tube bundle (all tubes). Using graph determine percentage wall loss. Carry out Calibration out. Fill out record sheet. Copyright © TWI Ltd 1