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RADIOGRAPHIC TESTING
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Introduction
This module presents information on theNDT method of radiographic inspection orradiography.
Radiography uses penetrating radiationthat is directed towards a component.
The component stops some of theradiation. The amount that is stopped orabsorbed is affected by material density
and thickness differences. These differences in absorption can berecorded on film, or electronically.
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Outline
ElectromagneticRadiation
General Principles ofRadiography
Sources of RadiationGamma RadiographyX-ray Radiography
Imaging ModalitiesFilm RadiographyComputed RadiographyReal-Time Radiography
Direct Digital Radiography
Computed Radiography
Radiation Safety Advantages and
Limitations
Glossary of Terms
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Electromagnetic Radiation
The radiation used in Radiography testing is a higher
energy (shorter wavelength) version of theelectromagnetic waves that we see every day. Visiblelight is in the same family as x-rays and gamma rays.
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General Principlesof Radiography
Top view of developed film
X-ray film
The part is placed between theradiation source and a piece of film.The part will stop some of theradiation. Thicker and more densearea will stop more of the radiation.
= more exposure
= less exposure
The film darkness(density) will vary withthe amount of radiationreaching the filmthrough the test object.
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General Principlesof Radiography
The energy of the radiation affects its penetratingpower. Higher energy radiation can penetratethicker and more dense materials.
The radiation energy and/or exposure time mustbe controlled to properly image the region of
interest.Thin Walled Area
Low Energy Radiation High Energy Radiation
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IDL 2001
Radiography
has sensitivitylimitations whendetectingcracks.
X-rays see a crack as a thickness variation and thelarger the variation, the easier the crack is to detect.
OptimumAngle
Flaw Orientation
= easy to
detect
= not easyto 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.
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IDL 2001
0
o
10
o
20
o
Since the angle between the radiation beam and a crack
or other linear defect is so critical, the orientation ofdefect must be well known if radiography is going to beused to perform the inspection.
Flaw Orientation (cont.)
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Radiation SourcesTwo of the most commonly used sources ofradiation in industrial radiography are x-raygenerators and gamma ray sources. Industrialradiography is often subdivided into X-rayRadiography or Gamma Radiography,depending on the source of radiation used.
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Gamma Radiography
Gamma rays areproduced by aradioisotope.
A radioisotope has anunstable nuclei that does
not have enough bindingenergy to hold thenucleus together.
The spontaneous
breakdown of an atomicnucleus resulting in therelease of energy andmatter is known asradioactive decay.
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Gamma Radiography (cont.)
Most of the radioactivematerial used inindustrial radiographyis artificially produced.
This is done bysubjecting stablematerial to a source ofneutrons in a special
nuclear reactor.This process is calledactivation.
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Gamma Radiography (cont.)
Unlike X-rays, which are
produced by a machine,gamma rays cannot be turnedoff. Radioisotopes used forgamma radiography areencapsulated to prevent
leakage of the material.
The radioactive capsule isattached to a cable to form
what is often called a pigtail.
The pigtail has a specialconnector at the other endthat attaches to a drive cable.
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Gamma Radiography (cont.)A device called a camera is used to store,
transport and expose the pigtail containing theradioactive material. The camera contains shieldingmaterial which reduces the radiographers exposureto radiation during use.
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Gamma Radiography (cont.)
A hose-like device
called a guide tube isconnected to athreaded hole calledan exit port in thecamera.
The radioactive
material will leave andreturn to the camerathrough this openingwhen performing an
exposure!
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Gamma Radiography (cont.)A drive cable is connectedto the other end of the camera.
This cable, controlled by theradiographer, is used to forcethe radioactive material outinto the guide tube where the
gamma rays will pass throughthe specimen and expose therecording device.
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X-ray RadiographyUnlike gamma rays, x-rays are produced by an X-raygenerator system. These systems typically includean X-ray tube head, a high voltage generator, and acontrol console.
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X-ray Radiography (cont.) 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 protectedby a metal housing.
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X-ray Radiography (cont.) The cathode contains a small
filament much the same as in alight bulb.
Current is passed through thefilament which heats it. The heatcauses electrons to be stripped
off. The high voltage causes these
free electrons to be pulledtoward a target material (usually
made of tungsten) located in theanode.
The electrons impact againstthe target. This impact causesan energy exchange which
causes x-rays to be created.
High Electrical Potential
Electrons
-+
X-ray Generator
or RadioactiveSource Creates
Radiation
Exposure Recording Device
RadiationPenetratethe Sample
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Imaging ModalitiesSeveral different imaging methods are
available to display the final image inindustrial radiography:
Film Radiography
Real Time Radiography Computed Tomography (CT) Digital Radiography (DR)
Computed Radiography (CR)
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Film Radiography
One of the most widely usedand oldest imaging mediumsin industrial radiography isradiographic film.
Film contains microscopicmaterial called silver bromide.
Once exposed to radiation anddeveloped in a darkroom,silver bromide turns to black
metallic silver which forms theimage.
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Film Radiography (cont.) Film must be protected from visible light. Light, just
like x-rays 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 oftenplaced between lead screens to intensify the effects
of the radiation.
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Film Radiography (cont.) 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.
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Film Radiography (cont.)Once developed, the film is typically referredto as a radiograph.
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Digital Radiography
One of the newest forms of radiographicimaging is Digital Radiography.
Requiring no film, digital radiographicimages are captured using either specialphosphor screens or flat panelscontaining 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.
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Digital Radiography (cont.)
There are a number of forms of digital
radiographic imaging including:
Computed Radiography (CR)
Real-time Radiography (RTR)
Direct Radiographic Imaging (DR)Computed Tomography
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Computed RadiographyComputed Radiography (CR) is a digital imagingprocess that uses a special imaging plate whichemploys storage phosphors.
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Computed Radiography (cont.)
CR Phosphor Screen Structure
X-rays penetrating the specimen stimulate the
phosphors. The stimulated phosphors remain inan excited state.
X-Rays
Phosphor Layer
Protective Layer
SubstratePhosphor Grains
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Computed Radiography (cont.)
After exposure:
The imaging plate is readelectronically and erased for re-use in a special scanner system.
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Motor
A/D
Converter
A/D
Converter
Imaging
Plate
Optical
ScannerPhoto-multiplier Tube
110010010010110110010010010110
Laser Beam
Computed Radiography (cont.)As a laser scans the imaging plate, light is emittedwhere X-rays stimulated the phosphor duringexposure. The light is then converted to a digital value.
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Computed Radiography (cont.)Digital images are typically sent to a computer
workstation where specialized software allowsmanipulation and enhancement.
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Computed Radiography (cont.)Examples of computed radiographs:
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Real-Time Radiography Real-Time Radiography (RTR) is a term used to
describe a form of radiography that allows
electronic images to be captured and viewed inreal time.
Because image acquisition is almostinstantaneous, 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 foranalysis.
Time of inspection can often be reduced.
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Real-Time Radiography (cont.) The equipment needed for an RTR
includes:
X-ray tube Image intensifier or
other real-time detector
Camera
Computer with framegrabber board andsoftware
Monitor Sample positioningsystem (optional)
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Real-Time Radiography (cont.) The image intensifier is a
device that converts the
radiation that passes throughthe specimen into light.
It uses materials that fluorescewhen struck by radiation.
The more radiation thatreaches the input screen, themore light that is given off.
The image is very faint on theinput screen so it is intensifiedonto a small screen inside theintensifier where the image isviewed with a camera.
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Real-Time Radiography (cont.) A special camera
which captures the
light output of thescreen is located nearthe image intensifyingscreen.
The camera is verysensitive to a varietyof different lightintensities.
A monitor is then connectedto the camera to provide a
viewable image. If a sample positioning
system is employed, the partcan be moved around and
rotated to image differentinternal features of the part.
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Real-Time Radiography (cont.)Comparing Film and Real-Time Radiography
Real-time images are lighterin areas where more X-rayphotons reach and excitethe fluorescent screen.
Film images are darker inareas where more X-rayphotons reach and ionizethe silver molecules inthe film.
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Direct Radiography Direct radiography (DR) is a
form of real-time radiography
that uses a special flat paneldetector.
The panel works by convertingpenetrating radiation passing
through the test specimen intominute electrical charges.
The panel contains many micro-electronic capacitors. Thecapacitors form an electricalcharge pattern image of thespecimen.
Each capacitors charge isconverted into a pixel which
forms the digital image.
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Computed TomographyComputed Tomography (CT) uses a real-timeinspection system employing a samplepositioning system and special software.
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Computed Tomography (cont.) Many separate images are saved (grabbed) and
complied into 2-dimensional sections as the
sample is rotated. 2-D images are them combined into 3-dimensional
images.
Real-Time
Captures
Compiled 2-D
Images
Compiled 3-D
Structure
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Image Quality Image quality is critical for accurate assessment of a
test specimens 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 metalplaques while others are manufactured from wires ofdiffering diameters mounted next to one another.
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Image Quality (cont.) IQIs are typically
placed on or next to atest specimen.
Quality typically beingdetermined based onthe smallest hole orwire diameter that isreproduced on theimage.
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Radiation SafetyUse of radiation sources in industrial
radiography is heavily regulated by stateand federal organizations due to potentialpublic and personal risks.
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Radiation Safety (cont.)There are many sources of radiation. In general, a personreceives roughly 100 mrem/year from natural sources androughly 100 mrem/year from manmade sources.
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Radiation Safety (cont.)X-rays and gamma rays are forms of ionizing radiation, whichmeans that they have the ability to form ions in the material that ispenetrated. All living organisms are sensitive to the effects of
ionizing radiation (radiation burns, x-ray food pasteurization, etc.)
X-rays andgamma rays have
enough energy toliberate electronsfrom atoms anddamage themolecularstructure of cells.
This can causeradiation burns orcancer.
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Technicians who work with radiation must wear monitoring devicesthat keep track of their total absorption, and alert them when theyare in a high radiation area.
Survey Meter Pocket Dosimeter Radiation Alarm Radiation Badge
Radiation Safety (cont.)
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Radiation Safety (cont.)There are three means of protection to help
reduce exposure to radiation:
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Radiographic Images
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Radiographic ImagesCan you determine what object was radiographedin this and the next three slides?
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Radiographic Images
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Radiographic Images
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Radiographic Images
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Advantages of Radiography 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 materialdensity changes.
Detects both surface and subsurfacedefects. Provides a permanent record of theinspection.
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Disadvantages of Radiography Many safety precautions for the use of
high intensity radiation. Many hours of technician training prior touse.
Access to both sides of sample required. Orientation of equipment and flaw can becritical.
Determining flaw depth is impossiblewithout additional angled exposures. Expensive initial equipment cost.
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Glossary of Terms Activation: the process of creating
radioactive material from stable materialusually by bombarding a stable material witha large number of free neutrons. This processtypically takes place in a special nuclear
reactor. Anode: a positively charged electrode. Automatic Film Processor: a machine
designed to develop film with very littlehuman intervention. Automatic processorsare very fast compared to manualdevelopment.
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Glossary of Terms Capacitor: an electrical device that stores an
electrical charge which can be released ondemand.
Cathode: a negatively charged electrode.
Darkroom: a darkened room for the purpose offilm development. Film is very sensitive toexposure by visible light and may be ruined.
Exposure: the process of radiation penetratingand object.
Gamma Rays: electromagnetic radiationemitted from the nucleus of a some radioactive
materials.
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Glossary of Terms Phosphor: a chemical substance that emits
light when excited by radiation. Pixel: Short for PictureElement, a pixel is asingle point in a graphic image. Graphicsmonitors display pictures by dividing the
display screen into thousands (or millions) ofpixels, arranged in rows and columns. Thepixels are so close together that they appearconnected.
Photo-multiplier tube: an amplifier used toconvert light into electrical signals.
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Glossary of Terms Radioactive: to give off radiation
spontaneously. Radiograph: an image of the internalstructure of and object produced using asource of radiation and a recording device.
Silver Bromide: silver and brominecompound used in film emulsion to form theimage seen on a radiograph.