Eddy Current Testing (ET)
NDT31
Training and Examination Services
Granta Park, Great Abington
Cambridge CB21 6AL
United Kingdom
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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
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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
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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
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Contents
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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.
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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
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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
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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
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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:
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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.
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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
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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:
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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.
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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.
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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.
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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
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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.
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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.
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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:
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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.
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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
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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)
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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
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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.
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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.
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Introduction
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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.
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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
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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Figure 2.11 Meter controls.
Figure 2.12 Capacitor.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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.
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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.
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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%).
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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.
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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.
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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.
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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.
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a
b
Figure 3.1 Series resonance curve:
a
Simple circuit;
b
Double circuit.
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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
2fL 
f
1
2fC
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.
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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.
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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.
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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.
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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.
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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.
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Figure 3.7 Cathode ray storage scope.
Figure 3.8 CRT displays.
Figure 3.9 Vector point display.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Table 4.1 Logarithms of numbers 10-49.
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Table 4.2 Logarithms of numbers 50-99.
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Table 4.3 Antilogarithms 00-49.
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Table 4.4 Antilogarithms 50-99.
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Table 4.5 functions of angles 1-45°.
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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.
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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.
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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.
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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
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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
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Figure 5.6 Parellel L-C circuit.
Figure 5.7 Parallel resonance.
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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.
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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).
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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.
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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.
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Figure 6.2 Idealised impedance.
Figure 6.3 Normalised impedance.
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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.
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Figure 6.4 Impedance diagram for a surface coil.
Figure 6.5 Impedance diagram for a surface coil.
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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.
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Figure 6.6 Impedance diagram for a surface coil.
Figure 6.7 Lift-off plane at two frequencies.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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  
2fC
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’.
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NDT31-50316b
Sample Instructions
A1-3
Copyright © TWI Ltd
Detailed instructions for application of test (continued)
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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’.
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NDT31-50316b
Sample Instructions
A1-9
Copyright © TWI Ltd
Detailed instructions for application of test (continued)
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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’.
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NDT31-50316b
Sample Instructions
A1-15
Copyright © TWI Ltd
Detailed instructions for application of test (continued)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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A3-14
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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
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A3-15
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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
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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
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A3-17
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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A3-33
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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
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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
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Copyright © TWI Ltd
Domain Theory
Domain Theory
Magnetising force
Magnetising force
Magnetising force
Magnetising force
Saturated state
All domains fully aligned
with external field
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Domain Theory
Magnetising force
removed
Residual magnetism
remains
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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
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Copyright © TWI Ltd
2
Magnetic Fields
Magnetic field around a bar magnet
Magnetic Fields
Magnetic field
North Pole - South Pole
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Magnetic Fields
Magnetic field
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Magnetic Fields
 Magnetic fields are thought to consist of lines
of flux.
North Pole - South Pole
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Lines of Flux
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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.
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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.
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Electromagnetism
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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.
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Right Hand Rule
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Coil Magnetisation
 Changes circular field into longitudinal.
 Increases the strength of the field.
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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.
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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
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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
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5
Hysteresis
Hysteresis
Coercive
force
Magnetising force (H) ampere/metre
Magnetising force (H) ampere/metre
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Magnetising force (H) ampere/metre
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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.
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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.
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Relative Permeability (µr)
On the basis of relative permeability materials
can be divided into three groups:
1. Diamagnetic.
2. Paramagnetic.
3. Ferromagnetic.
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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+.
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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.
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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)
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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
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Coil Magnetisation
 Changes circular field into longitudinal.
 Increases the strength of the field.
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8
Eddy Current Testing (ET)
Flaw Detection
Section 2
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Magnetic Effect of an Electric Current
Magnetic Effect of an Electric Current
Conductor
DC current
DC current
Magnetic field
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Magnetic Field of a Coil
Magnetic Field of a Coil
North
+ ve
+ ve
DC
DC
- ve
- ve
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South
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1
Magnetic Field of a Coil
Magnetic Field of a Coil
South
North
AC
AC
Primary field
South
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Probe Construction
Ferrite core
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Probe Construction
North
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Probe Construction
Ferrite core
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Probe Construction
Resin housing
PTFE tape
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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.
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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
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Effect of a Magnetic Field
S
S
Effect of a Magnetic Field
Electrical conductor
N
N
Electrical conductor
Current flow
Current flow
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3
Effect of a Magnetic Field
S
S
Effect of a Magnetic Field
Electrical conductor
N
N
Electrical conductor
Current flow
Current flow
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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.
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Production of Eddy Currents
AC
Alternating
(moving)
primary
field
Production of Eddy Currents
AC
Conductive material
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4
Production of Eddy Currents
AC
Production of Eddy Currents
AC
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Production of Eddy Currents
AC
Production of Eddy Currents
AC
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Production of Eddy Currents
AC
Production of Eddy Currents
AC
Localised alternating
electrical current
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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
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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
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Eddy Current Flaw Detection
Fault
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Eddy Current Flaw Detection
Fault
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6
Eddy Current Flaw Detection
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Eddy Current Flaw Detection
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Eddy Current Flaw Detection
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Eddy Current Flaw Detection
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Eddy Current Flaw Detection
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Eddy Current Flaw Detection
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7
Any
questions?
Summary
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 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.
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 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.
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 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.
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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).
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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.
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Any Questions?
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9
Eddy Current Testing (ET)
Factors Affecting Eddy Currents
Section 3
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Basic Principle of Eddy Currents
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Basic Principle of Eddy Currents
Ferrite core
Primary inducing field
Opposing secondary
field
Eddy currents
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Basic Principle of Eddy Currents
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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
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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.
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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.
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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
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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.
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Specimen Dimensions
Thickness Effect
 Material thickness.
 Component geometry.
AC
Eddy currents
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Thickness Effect
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Thickness Effect
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3
Thickness Effect
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Thickness Effect Can Be Used:
 For approximate thickness measurement.
 To detect blind side corrosion.
Thickness Effect
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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
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Edge Effect
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Edge Effect
AC
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4
Edge Effect
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Edge Effect
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Mass Effect
Edge Effect
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Edge Effect
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Mass Effect
AC
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5
Mass Effect
Mass Effect
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Mass Effect
Ferrous Effect
AC
Ferrous fastener
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Ferrous Effect
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Ferrous Effect
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6
Ferrous Effect
Ferrous Effect
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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.
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Lift Off (Proximity)
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Lift Off (Proximity)
AC
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Lift Off (Proximity)
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7
Lift Off (Proximity)
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Lift Off (Proximity)
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Lift Off (Proximity)
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Lift Off (Proximity)
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Lift Off (Proximity)
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Lift Off (Proximity)
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8
Probe Handling
Probe Handling
AC
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Probe Handling
Probe Handling
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Review Factors Affecting
Eddy Currents
Discontinuities




AC
Conductivity.
Permeability.
Frequency.
Geometry.



Thickness.
Edge.
Mass.
 Ferrous effect.
 Lift off.
 Probe handling.
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9
Review Factors Affecting
Eddy Currents
Can be used to detect:
 Flaws and discontinuities.
 Material thickness.
 Thickness of non-conductive coatings.
 Material specification.
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10
Eddy Current Testing (ET)
Impedance Plane Display
Section 4
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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.
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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
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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
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Typical indications:
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2
Typical indications:
Typical indications:
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Typical indications:
Edge effect
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Typical indications:
Edge effect
Fault
Lift-off
Fault
Ferrous
Lift-off
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Impedance Plane
Typical indications:
Edge effect
XL
Fault
Z
Ferrous
XT
(XL- XC)
R
Lift-off
XC
Mass
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3
Applications:
 Crack detection.
 Tube and wire testing.
 Condenser tube inspection.
 Material sorting.
 Weld testing.
 Coating thickness measurement.
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4
Eddy Current Testing (ET)
Basic Electrical Theory
Section 5
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Primary Cell
Negative
terminal
Positive
terminal
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Circuit Made and Current Flowing
Positive
terminal
Negative
terminal
Electrolyte
Electrolyte
Cell plates
Cell plates
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Electrical Theory
Electrical Theory
The atom
-ve
+ve
nucleus
+ve
nucleus
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electron
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1
Electrical Theory
Electrical Theory
Angular
momentum
-ve electron
-ve electron
+ve nucleus
+ve nucleus
Electrostatic field
Electrostatic field
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Electrical Theory
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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.
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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
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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
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Electrical Theory
Electrical Theory
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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
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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.
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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.
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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
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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
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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
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AC Sine Wave
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AC Sine Wave
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5
AC Sine Wave
Alternating Current in a Wire
Opposing field
Induction creates
secondary current
Primary AC current
Alternating magnetic field
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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.
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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 
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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.
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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?
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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.
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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.
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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
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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°.
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 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
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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.
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8
Eddy Current Testing (ET)
Equipment Circuits
Section 6
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Equipment Circuits
Simple circuits.
Resonance circuits.
Bridge circuits.
Phase sensitive circuits.
Simple circuits:
Oscillator
Meter
Single absolute circuit
Probe
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Copyright © TWI Ltd
Basic Eddy Current Equipment
Equipment Circuits
Simple circuits:
Meter
Oscillator
Current




Equipment Circuits
Probe
Driver
Double
absolute
circuit
(reflection
probe)
Receiver
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Copyright © TWI Ltd
1
Equipment Circuits
Resonance circuits:
Equipment Circuits
Resonance circuits:
Z
Resonance occurs when:
XL = XC
V
2fL =
XC
XL
R
f =
XL
Amplitude
R
1 .
2 f C
1
.
2  LC
XC
fR
f
At resonance Z = R
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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
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Copyright © TWI Ltd
Equipment Circuits
Bridge circuits - based on the Wheatstone bridge:
Equipment Circuits
Bridge circuits - based on the Wheatstone bridge:
Meter
Meter
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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
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3
Eddy Current Testing (ET)
Probe Coils
Section 7
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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
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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
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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.
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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.
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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
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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.
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Probe Coil Arrangements
Probe Coil Arrangements
Differential
Single
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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.
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Probe Coil Arrangements
Differential
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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
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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.
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Probe Coil Arrangements
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Probe Coil Arrangements
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Copyright © TWI Ltd
Probe Coil Arrangements
 Surface probes.
 Encircling probes.
 Internal bobbin probes.
Calibration Blocks
 Slotted calibration blocks.
 Step wedges.
 Tube standards.
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Copyright © TWI Ltd
4
Eddy Current Testing (ET)
Probes
Section 8
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Eddy Current Probes
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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.
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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.
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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.
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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
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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.
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Reference Standards
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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.
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Reference Standards
Material thickness standards used to help
determine such things as material thinning
caused by corrosion or erosion.
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Copyright © TWI Ltd
2
Reference Standards
Crack standards:
Reference Standards
ASME tubing pit standard:
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Copyright © TWI Ltd
Reference Standards
Nonconductive coating (paint) standard with
various thickness of paint on aluminum
substrate.
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3
Eddy Current Testing (ET)
Tube Testing
Section 9
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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).
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 = Tube diameter DT² (external coil)
Coil diameter DC²
  must be less than 1.0.
  is usually about 0.7.
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3
Eddy Current Testing (ET)
In Service Inspection of Coated Steel Welds
Using Eddy Current Techniques
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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.
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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
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Primary AC
Alternating magnetic field
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1
Basic Principle of Eddy Currents
Basic Principle of Eddy Currents
Ferrite core
Primary inducing field
Opposing secondary
field
Eddy currents
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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).




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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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:
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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.
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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?
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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.
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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.
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4
Eddy Current Testing (ET)
Practical Exercises 2
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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.
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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
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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.
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Practical Exercises
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Practical Exercises
6
5
4
Figure number 07
Weld section calibration block.
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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.
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Lift-off points
3
2
1
Figure number 08(a)
Lift-off responses from weld zones.
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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.
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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.
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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.
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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.
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2
1
5
3
4
4
5
Figure number 11
Phase angle of defect signals
relative to the orientation of
defect to coils.
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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.
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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
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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.
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Weld Probe Checks
+
Mark index
points on
the block
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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Weld Testing Procedures
Weld Testing Procedures
Balance
point
Optimum angle
of probe
Figure number 19
Optimum angle of coils for weld toe examination.
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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.
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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
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Weld Testing Procedures
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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.
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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.
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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?
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Weld Testing Procedures
2
1
Balance
point
Figure number 22
Typical screen presentation of crack using single pass technique.
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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.
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Weld Testing Procedures
4
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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.
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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.
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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.
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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.
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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.
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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:
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Weld Testing Procedures
Toe cracks
Figure number 26
Additional scans in the heat affected zone.
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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.
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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.
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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?
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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.
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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.
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Weld Testing Procedures
These include:
 Geometry of component under test.
 Stand-off of probe from area under test.
 Cross section of defect.
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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.
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12
Weld Testing Procedures
Under normal circumstances the fatigue defects
will occur in the weld cap in two locations:
 Inter-run.
 Weld run itself.
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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.
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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.
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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.
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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.
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13
Additional Scanning Techniques
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Additional Scanning Techniques
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14
Eddy Current Testing (ET)
Amplitude Analysis, Full length
and Internal defects
Practical
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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.
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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.
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1
Eddy Current Testing (ET)
CIVIL Explanation
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1
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2
Eddy Current Testing (ET)
Differential
Practical
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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.
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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.
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1
Eddy Current Testing (ET)
Inlet End
Practical
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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.
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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.
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1