Lectures for Week 5
Prof. Qumrul Ahsan, PhD
Department of Engineering Materials
Faculty of Manufacturing Engineering

•
This module presents information on the NDT method of radiographic inspection or radiography.
•
Radiography uses penetrating radiation that is directed towards a component.
•
The component stops some of the radiation. The amount that is stopped or absorbed is affected by material density and thickness differences.
•
These differences in “absorption” can be recorded on film, or electronically.

•
Electromagnetic Radiation
•
General Principles of
Radiography
•
Sources of Radiation
– Gamma Radiography
– X-ray Radiography
•
Imaging Modalities
– Film Radiography
– Computed Radiography
– Real-Time Radiography
– Direct Digital Radiography
– Computed Radiography
•
Radiation Safety
•
Advantages and Limitations
•
Glossary of Terms

The radiation used in Radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see every day.
Visible light is in the same family as x-rays and gamma rays.

X-rays and gamma rays differ only in their source of origin.
 X-rays are produced by an x-ray generator and gamma radiation is the product of radioactive atoms.

They are both part of the electromagnetic spectrum. They are waveforms, as are light rays, microwaves, and radio waves.
 They can be diffracted (bent) in a manner similar to light.
Properties of X-Rays and Gamma Rays
They are not detected by human senses (cannot be seen, heard, felt, etc.).
They travel in straight lines at the speed of light.
Their paths cannot be changed by electrical or magnetic fields.
They can be diffracted to a small degree at interfaces between two different materials.
They pass through matter until they have a chance encounter with an atomic particle.
Their degree of penetration depends on their energy and the matter they are traveling through.
They have enough energy to ionize matter and can damage or destroy living cells.

X-ray film
The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation.
The film darkness (density) will vary with the amount of radiation reaching the film through the test object.
= less exposure
= more exposure
Top view of developed film

•
The energy of the radiation affects its penetrating power.
Higher energy radiation can penetrate thicker and more dense materials.
•
The radiation energy and/or exposure time must be controlled to properly image the region of interest.
Thin Walled Area
Low Energy Radiation High Energy Radiation

IDL 2001
Optimum
Angle
Radiography has sensitivity limitations when detecting cracks.
= easy to detect
= not easy to detect
Xrays “see” a crack as a thickness variation and the larger the variation, the easier the crack is to detect.
When the path of the x-rays is not parallel to a crack, the thickness variation is less and the crack may not be visible.

Since the angle between the radiation beam and a crack or other linear defect is so critical, the orientation of defect must be well known if radiography is going to be used to perform the inspection.
IDL 2001
0 o 10 o 20 o

Two of the most commonly used sources of radiation in industrial radiography are x-ray generators and gamma ray sources. Industrial radiography is often subdivided into
“X-ray Radiography” or “Gamma Radiography”, depending on the source of radiation used.

• Gamma radiation is one of the three types of natural radioactivity.
Gamma rays are electromagnetic radiation, like X-rays. The other two types of natural radioactivity are alpha and beta radiation, which are in the form of particles. Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nanometer
Alpha Particles
Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha particles (two neutrons and two protons each).
Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate
Beta Particles
A nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high speed electron called a beta particle.
This results in a net change of one unit of atomic number (Z).
Beta particles have a negative charge
Gamma-rays
A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies.
The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable).

• The quantity which expresses the degree of radioactivity or the radiation producing potential of a given amount of radioactive material is activity.
• The curie was originally defined as that amount of any radioactive material that disintegrates at the same rate as one gram of pure radium. (as a quantity of radioactive material in which 3.7 x 10 10 atoms disintegrate per second.)
• The International System (SI) unit for activity is the Becquerel (Bq), which is that quantity of radioactive material in which one atom is transformed per second.
• Radioactivity is expressed as the number of curies or becquerels per unit mass or volume.

• Each radionuclide decays at its own unique rate which cannot be altered by any chemical or physical process.
• Half-life is defined as the time required for the activity of any particular radionuclide to decrease to one-half of its initial value.
• one-half of the atoms have reverted to a more stable state material.
• Half-life of two widely used industrial isotopes are 74 days for iridium-192, and
5.3 years for cobalt-60.

• As penetrating radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters.
• The rate at which this energy loss occurs depends upon the type and energy of the radiation and the density and atomic composition of the matter through which it is passing.
• The term "excitation" is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state.
• The term "ionization" refers to the complete removal of an electron from an atom following the transfer of energy from a passing charged particle.

•
Gamma rays are produced by a radioisotope.
•
A radioisotope has an unstable nuclei that does not have enough binding energy to hold the nucleus together.
•
The spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter is known as radioactive decay.

•
Most of the radioactive material used in industrial radiography is artificially produced.
•
This is done by subjecting stable material to a source of neutrons in a special nuclear reactor.
•
This process is called activation.

Unlike X-rays, which are produced by a machine, gamma rays cannot be turned off. Radioisotopes used for gamma radiography are encapsulated to prevent leakage of the material.
The radioactive “capsule” is attached to a cable to form what is often called a
“pigtail.”
The pigtail has a special connector at the other end that attaches to a drive cable.

A device called a “camera” is used to store, transport and expose the pigtail containing the radioactive material. The camera contains shielding material which reduces the radiographer’s exposure to radiation during use.

A hose-like device called a guide tube is connected to a threaded hole called an “exit port” in the camera.
The radioactive material will leave and return to the camera through this opening when performing an exposure!

A “drive cable” is connected to the other end of the camera. This cable, controlled by the radiographer, is used to force the radioactive material out into the guide tube where the gamma rays will pass through the specimen and expose the recording device.

• X-ray tubes produce x-ray photons by accelerating a stream of electrons to energies of several hundred kilovolts with velocities of several hundred kilometers per hour and colliding them into a heavy target material.
• The abrupt acceleration of the charged particles (electrons) produces Bremsstrahlung photons.
• X-ray radiation with a continuous spectrum of energies is produced with a range from a few keV to a maximum of the energy of the electron beam.
• Target materials for industrial tubes are typically tungsten.

Unlike gamma rays, x-rays are produced by an X-ray generator system. These systems typically include an X-ray tube head, a high voltage generator, and a control console .

•
X-rays are produced by establishing a very high voltage between two electrodes, called the anode and cathode.
•
To prevent arcing, the anode and cathode are located inside a vacuum tube, which is protected by a metal housing.

•
The cathode contains a small filament much the same as in a light bulb.
•
Current is passed through the filament which heats it. The heat causes electrons to be stripped off.
•
The high voltage causes these “free” electrons to be pulled toward a target material (usually made of tungsten) located in the anode.
•
The electrons impact against the target. This impact causes an energy exchange which causes x-rays to be created.
High Electrical Potential
+
Electrons
-
X-ray Generator or Radioactive
Source Creates
Radiation
Radiation
Penetrate the Sample
Exposure Recording Device

• Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law.
• The intensity of the influence at any given radius (r) is the source strength divided by the area of the sphere.
• a point radiation source can be characterized by the diagram above whether you are talking about
Roentgens, rads, or rems.
All measures of exposure will drop off by the inverse square law. For example, if the radiation exposure is 100 mR/hr at 1 inch from a source, the exposure will be 0.01 mR/hr at 100 inches.

Interaction Between Penetrating Radiation and Matter
• When x-rays or gamma rays are directed into an object, some of the photons interact with the particles of the matter and their energy can be absorbed or scattered.
• This absorption and scattering is called attenuation.
• Other photons travel completely through the object without interacting with any of the material's particles.
• The number of photons transmitted through a material depends on the thickness, density and atomic number of the material, and the energy of the individual photons.
For a narrow beam of mono-energetic photons, the change in x-ray beam intensity at some distance in a material can be expressed in the form of an equation as:
:
Where
I
I
0 s
µ x
=
=
=
=
= the intensity of photons transmitted across some distance x the initial intensity of photons a proportionality constant that reflects the total probability of a photon being scattered or absorbed the linear attenuation coefficient distance traveled

• The thickness of any given material where 50% of the incident energy has been attenuated is know as the halfvalue layer (HVL).
The HVL is inversely proportional to the attenuation coefficient. If an incident energy of 1 and a transmitted energy is 0.5 is plugged into the equation it can be expressed as
Approximate HVL for Various Materials when Radiation is from a Gamma Source
Source
Iridium-192
Concrete
Half-Value Layer, mm (inch)
Steel Lead Tungsten Uranium
44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11)
Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)

• Geometric unsharpness refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup.
• It occurs because the radiation does not originate from a single point but rather over an area.
The three factors controlling unsharpness are source size, source to object distance, and object to detector distance.
The source size is obtained by referencing manufacturers specifications for a given Xray or gamma ray source.
Industrial x-ray tubes often have focal spot sizes of 1.5 mm squared but microfocus systems have spot sizes in the 30 micron range.

• For the case, such as that shown to the right, where a sample of significant thickness is placed adjacent to the detector, the following formula is used to calculate the maximum amount of unsharpness due to specimen thickness:
Ug = f * b/a
Where f = source focal-spot size a = distance from the source to front surface of the object b = the thickness of the object

• For the case when the detector is not placed next to the sample, such as when geometric magnification is being used, the calculation becomes:
Ug = f* b/a
Where, f = source focal-spot size.
a = distance from x-ray source to front surface of material/object b = distance from the front surface of the object to the detector

• At x-ray energies, filters to absorb the lower-energy x-ray photons emitted by the tube before they reach the target.
• The use of filters produce a cleaner image by absorbing the lower energy xray photons that tend to scatter more.
• The total filtration of the beam includes the inherent filtration (composed of part of the x-ray tube and tube housing) and the added filtration (thin sheets of a metal inserted in the x-ray beam).
• Filters are typically placed at or near the x-ray port in the direct path of the x-ray beam.
• Placing a thin sheet of copper between the part and the film cassette has also proven an effective method of filtration.
• For industrial radiography, the filters added to the x-ray beam are most often constructed of high atomic number materials such as lead, copper, or brass.
• The thickness of filter materials is dependent on atomic numbers, kilovoltage settings, and the desired filtration factor.
• Gamma radiography produces relatively high energy levels at essentially monochromatic radiation, therefore filtration is not a useful technique and is seldom used.

• Secondary or scattered photons create a loss of contrast and definition.
• Secondary radiation striking the film reflected from an object in the immediate area
• Control of side scatter can be achieved by moving objects in the room away from the film, moving the x-ray tube to the center of the vault, or placing a collimator at the exit port.
• Backscatter when it comes from objects behind the film.
• Industry codes and standards require a lead letter
"B" be placed on the back of the cassette to verify the control of backscatter.
– If the letter "B" shows as a "ghost" image on the film, a significant amount of backscatter radiation is reaching the film.
– The control of backscatter radiation is achieved by backing the film in the cassette with a sheet of lead that is at least 0.010 inch thick.
– It is a common practice in industry to place a
0.005" lead screen in front and a 0.010" screen behind the film.

• Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are not put in place.
• Undercut appears as a darkening of the radiograph in the area of the thickness transition.
 This results in a loss of resolution or blurring at the transition area.
 Undercut occurs due to scattering within the film.
 The faster the film speed, the more undercut that is likely to occur
 Masks are used to control undercut.
 Sheets of lead cut to fill holes or surround the part
 Metallic shot and liquid absorbers are often used as masks.

Several different imaging methods are available to display the final image in industrial radiography:
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Film Radiography
•
Real Time Radiography
•
Computed Tomography (CT)
•
Digital Radiography (DR)
•
Computed Radiography (CR)

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One of the most widely used and oldest imaging mediums in industrial radiography is radiographic film.
•
Film contains microscopic material called silver bromide.
•
Once exposed to radiation and developed in a darkroom, silver bromide turns to black metallic silver which forms the image.
 Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide, and thus increases the film speed.
 The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a reasonable time.
 A few of the films used for radiography only have emulsion on one side which produces the greatest detail in the image.

• Factors to be considered:
– Composition, shape, and size of the part being examined and, in some cases, its weight and location.
– Type of radiation used, whether x-rays from an x-ray generator or gamma rays from a radioactive source.
– Kilovoltages available with the xray equipment or the intensity of the gamma radiation.
– Relative importance of high radiographic detail or quick and economical results.
if high resolution and contrast sensitivity is of overall importance, a slower and finer grained film should be used in place of a faster film.

•
Film must be protected from visible light. Light, just like xrays and gamma rays, can expose film. Film is loaded in a
“light proof” cassette in a darkroom.
•
This cassette is then placed on the specimen opposite the source of radiation. Film is often placed between lead screens to intensify the effects of the radiation.

•
In order for the image to be viewed, the film must be
“developed” in a darkroom. The process is very similar to photographic film development.
•
Film processing can either be performed manually in open tanks or in an automatic processor.

1) Development - The developing agent gives up electrons to convert the silver halide grains to metallic silver. Grains that have been exposed to the radiation develop more rapidly, but given enough time the developer will convert all the silver ions into silver metal. Proper temperature control is needed to convert exposed grains to pure silver while keeping unexposed grains as silver halide crystals.
2) Stopping the development - The stop bath simply stops the development process by diluting and washing the developer away with water.
3) Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind.
4) Washing - The film is washed with water to remove all the processing chemicals.
5) Drying - The film is dried for viewing.

Once developed, the film is typically referred to as a “radiograph.”

• Subject contrast is the ratio of radiation intensities transmitted through different areas of the component being evaluated
– dependent on:
• the absorption differences in the component
• the wavelength of the primary radiation
• intensity and distribution of secondary radiation due to scattering
• The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast.
– low kilovoltage will generally result in a radiograph with high contrast
• low energy radiation is more easily attenuated
• Hence, the ratio of photons that are transmitted through a thick and thin area will be greater with low energy radiation
• In turn will result in the film being exposed to a greater and lesser degree in the two areas

• As contrast sensitivity increases, the latitude of the radiograph decreases.
• Radiographic latitude refers to the range of material thickness that can be imaged.
– more areas of different thicknesses will be visible in the image.
• The goal is to balance radiographic contrast and latitude so that there is enough contrast to identify the features of interest but also to make sure the latitude is great enough so that all areas of interest can be inspected with one radiograph.
• In thick parts with a large range of thicknesses, multiple radiographs will likely be necessary to get the necessary density levels in all areas.

• Film contrast:
– density differences that result due to:
• the type of film used
• how it was exposed, and
• how it was processed.
• Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film.
• Exposing a film to produce higher film densities will generally increase the contrast in the radiograph.

• A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown to the right.
• From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will result in a low film density) the slope of the curve is low.
• In this region of the curve, it takes a large change in exposure to produce a small change in film density. Therefore, the sensitivity of the film is relatively low.
• It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about 0.30. However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum.
• In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density.
• In general, the highest overall film density that can be conveniently viewed or digitized will have the highest level of contrast and contain the most useful information.

 Radiographic definition is the abruptness of change in going from one area of a given radiographic density to another.
 Like contrast, definition also makes it easier to see features of interest, such as defects,
Since radiographic contrast and definition are not dependent upon the same set of factors, it is possible to produce radiographs with the following qualities:
Low contrast and poor definition
High contrast and poor definition
Low contrast and good definition
High contrast and good definition

• Radiographic density (or film density) is a measure of the degree of film darkening.
• Technically it should be called
"transmitted density" when associated with transparent-base film since it is a measure of the light transmitted through the film.
• Radiographic density is the logarithm of two measurements: the intensity of light incident on the film (I
0
) and the intensity of light transmitted through the film
(I t
). This ratio is the inverse of transmittance.
Transmittance
(I t
/I
0
)
1.0
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
Percent
Transmittance
100%
10%
1%
0.1%
0.01%
0.001%
0.0001%
0.00001%
Inverse of
Transmittance
(I
0
/I t
)
Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers.
1
10
100
1000
10000
100000
1000000
10000000
Film
Density
(Log(I
0
/I t
))
0
1
2
3
4
5
6
7
Film density is measured with a densitometer.

•
Image quality is critical for accurate assessment of a test specimen’s integrity.
•
Various tools called Image Quality Indicators
(IQIs) are used for this purpose.
•
There are many different designs of IQIs. Some contain artificial holes of varying size drilled in metal plaques while others are manufactured from wires of differing diameters mounted next to one another.

•
IQIs are typically placed on or next to a test specimen.
•
Quality typically being determined based on the smallest hole or wire diameter that is reproduced on the image.

•
One of the newest forms of radiographic imaging is
“Digital Radiography”.
•
Requiring no film, digital radiographic images are captured using either special phosphor screens or flat panels containing micro-electronic sensors.
•
No darkrooms are needed to process film, and captured images can be digitally enhanced for increased detail.
•
Images are also easily archived (stored) when in digital form.

There are a number of forms of digital radiographic imaging including:
•
Computed Radiography (CR)
•
Real-time Radiography (RTR)
•
Direct Radiographic Imaging (DR)
•
Computed Tomography

Computed Radiography (CR) is a digital imaging process that uses a special imaging plate which employs storage phosphors.

X-rays penetrating the specimen stimulate the phosphors. The stimulated phosphors remain in an excited state.
CR Phosphor Screen Structure
X-Rays
Protective Layer
Phosphor Layer
Phosphor Grains Substrate

The imaging plate is read electronically and erased for reuse in a special scanner system.

As a laser scans the imaging plate, light is emitted where Xrays stimulated the phosphor during exposure. The light is then converted to a digital value.
Optical
Scanner
Photo-multiplier Tube
Laser Beam
A/D
Converter
Imaging
Plate 110010010010110
Motor

Digital images are typically sent to a computer workstation where specialized software allows manipulation and enhancement.

Examples of computed radiographs:

•
Real-Time Radiography (RTR) is a term used to describe a form of radiography that allows electronic images to be captured and viewed in real time.
•
Because image acquisition is almost instantaneous, X-ray images can be viewed as the part is moved and rotated.
•
Manipulating the part can be advantageous for several reasons:
– It may be possible to image the entire component with one exposure.
– Viewing the internal structure of the part from different angular prospectives can provide additional data for analysis.
– Time of inspection can often be reduced.

• The equipment needed for an RTR includes:
•
•
•
X-ray tube
Image intensifier or other real-time detector
Camera
•
Computer with frame grabber board and software
•
Monitor
•
Sample positioning system
(optional)

•
The image intensifier is a device that converts the radiation that passes through the specimen into light.
•
It uses materials that fluoresce when struck by radiation.
•
The more radiation that reaches the input screen, the more light that is given off.
•
The image is very faint on the input screen so it is intensified onto a small screen inside the intensifier where the image is viewed with a camera.

•
A special camera which captures the light output of the screen is located near the image intensifying screen.
•
The camera is very sensitive to a variety of different light intensities.
•
A monitor is then connected to the camera to provide a viewable image.
•
If a sample positioning system is employed, the part can be moved around and rotated to image different internal features of the part.

Comparing Film and Real-Time Radiography
Real-time images are lighter in areas where more X-ray photons reach and excite the fluorescent screen.
Film images are darker in areas where more X-ray photons reach and ionize the silver molecules in the film.

•
Direct radiography (DR) is a form of real-time radiography that uses a special flat panel detector.
•
The panel works by converting penetrating radiation passing through the test specimen into minute electrical charges.
•
The panel contains many microelectronic capacitors. The capacitors form an electrical charge pattern image of the specimen.
•
Each capacitor’s charge is converted into a pixel which forms the digital image.

Computed Tomography (CT) uses a real-time inspection system employing a sample positioning system and special software.

•
Many separate images are saved (grabbed) and complied into 2-dimensional sections as the sample is rotated.
•
2-D images are then combined into 3-dimensional images.
Real-Time
Captures
Compiled 2-D
Images
Compiled 3-D
Structure

•
– Energy of Penetration(kV)
– Exposure factors (mA x time)
– Radiographic coverage, which implies projecting every portion of the component on the film. The total number of exposures to be taken
– The selection of the most suitable condition of exposure

• Satisfactory Radiograph – Material and Geometric Considerations
• Knowledge of the source and film characteristics
• Factors are summerised in Exposure Chart
• Alternative method for exposure time
– Exposure time in minutes = fd 2 2 [x/HVL] x 60/ C(RHM)100 2
Where f = film factor i.e. Radiation dose in Roentgens to produce a certain film density ( can be obtained from the chac: curve of the film), x = thickness of the specimen in cm, HVL= half Value Layer, d= source to film distance (SFD) in cm, C= source strength in curies, RHM = radiation output in Roentgen per hour by one curie at 1 m.

• Single Wall Single Image Techniques
– Both sides of the specimen are accessible
– Used for plates, cylinders, shells and large diameters pipes
– Source outside and the film inside or vice versa
Panaromic Technique
Radiation source is kept in the centre of the pipe and the film is fixed around the weld on the outer surface of the pipe
Reduces the examination time, IQI can be placed either on source side or film side and SFD is sufficient enough to ensure the proper sensitivity

• Double Wall Penetration
Technique : Used when the inside surface of the pipe is not accessible
– Double Wall Single Image
– Double Wall double Image
– Superimposing Technique

• Multiple thickness recorded on the radiograph within the useful range of film density
– High contrast film-less latitude and vice versa
– Double Film Technique
• Selection of films and exposure conditions, the thicker sections will be recorded on the faster film and the thinner sections on the slower film
• Use with or without lead screens
• A centre screen between the two films may also be used

• Multiwall single image
• Multiwall double image
• For double envelope pipe of more than 90 mm OD and the interpretable length is ascertained by the radiographic weld density.
• For double envelope pipe of 90 mm OD or less.
• 4 exposures are taken for each weld
• Not wrapping the pipe

• Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material
(interpass cold lap).
• Porosity is the result of gas entrapment in the solidifying metal.
• Cluster porosity is caused when flux coated electrodes are contaminated with moisture

• Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal.
• Incomplete penetration (IP) or lack
of penetration (LOP) occurs when the weld metal fails to penetrate the joint.
• Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal.

• Internal or root undercut is an erosion of the base metal next to the root of the weld.
• External or crown undercut is an erosion of the base metal next to the crown of the weld.
• Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material.

• Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes.
• Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas welding.
• Oxide inclusions are usually visible on the surface of material being welded (especially aluminum).

• Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Use of radiation sources in industrial radiography is heavily regulated by state and federal organizations due to potential public and personal risks.

There are many sources of radiation. In general, a person receives roughly 100 mrem/year from natural sources and roughly 100 mrem/year from manmade sources.

X-rays and gamma rays are forms of ionizing radiation, which means that they have the ability to form ions in the material that is penetrated. All living organisms are sensitive to the effects of ionizing radiation (radiation burns, x-ray food pasteurization, etc.)
X-rays and gamma rays have enough energy to liberate electrons from atoms and damage the molecular structure of cells.
This can cause radiation burns or cancer.

Technicians who work with radiation must wear monitoring devices that keep track of their total absorption, and alert them when they are in a high radiation area.
Survey Meter Pocket Dosimeter Radiation Alarm Radiation Badge

There are three means of protection to help reduce exposure to radiation:

•
Technique is not limited by material type or density.
•
Can inspect assembled components.
•
Minimum surface preparation required.
•
Sensitive to changes in thickness, corrosion, voids, cracks, and material density changes.
•
Detects both surface and subsurface defects.
•
Provides a permanent record of the inspection.

•
Many safety precautions for the use of high intensity radiation.
•
Many hours of technician training prior to use.
•
Access to both sides of sample required.
•
Orientation of equipment and flaw can be critical.
•
Determining flaw depth is impossible without additional angled exposures.
•
Expensive initial equipment cost.