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Radiographic Testing (RT):

 

Radiographic inspection or testing (RT) is a non-destructive inspection method based on using short wavelength electromagnetic radiation passing through the material. Materials with areas of reduced thickness or lower material density allow more, and therefore absorb less, radiation. The radiation, which reaches the film after passing through the material, forms a shadow image on a photographic film (radiograph).

 

Areas of low absorption (slag, porosity) appear as dark areas on the developed film (radiograph). Areas of high absorption (dense inclusions) appear as light areas on the developed film.

 

Lower energy radiation can be in the form of either gamma or X rays. Gamma rays are
the result of the decay of radioactive isotope. A common radioactive source is Iridium 192. A gamma source is constantly emitting radiation and must be kept in a shielded storage container when not in use. These containers often employ lead or depleted uranium.

 

X rays are produced when electrons, travelling at high speed, collide with matter. The conversion of electrical energy to X radiation is achieved in an evacuated tube. A low current (mA) is passed through a filament to produce electrons. Application of a high potential (kV) voltage between the filament and a target accelerates electrons across this voltage differential. The action of an electron stream striking the target produces X rays; these are produced only while voltage is applied to the X ray tube. Whether using gamma or X ray sources, the test object, e.g. weld, is not radioactive following the inspection.

 

Subsurface discontinuities that are readily detected by this method are voids, e.g. rounded flaws, metallic and non-metallic inclusions, and favourably aligned incomplete fusion and cracks. Voids, such as porosity, produce dark areas on the film because they represent a significant loss of material density. Metallic inclusions produce light areas if they are denser than the test object.

 

For example, tungsten inclusions in aluminium welds, which can be produced when using gas tungsten arc welding (GTAW) techniques, appear as light areas on the film. Nonmetallic inclusions, such as slag produce dark areas on the film. Cracks and incomplete fusion must be aligned such that the depth of discontinuities is nearly parallel to the radiation beam
for detection. Surface discontinuities, which may also be detected using VT, include undercut, excessive reinforcement, incomplete fusion, and heavy penetration. Radiographic testing can be used to inspect all common engineering materials and is used extensively for the inspection of welds in pressure equipment.

 

Among the equipment required to perform radiographic testing is some source of radiation. This source can be either an X ray machine, which requires some electrical input, or a radioactive isotope that produces gamma radiation. The isotope offers increased portability. Either type requires film in a light-tight film cassette. Lead letters or lead tape is used to identify the test object. Because of the high density of lead and the local increased thickness, these letters form light areas on the developed film. Penetrameters or image quality indicator “IQIs” are used to verify sensitivity of the radiograph. These are made of known material and known diameters or thicknesses. Sensitivity of usually 2% or better is verified by the ability to detect a given difference in film density (usually between 2–3) of the wire diameter or shim thickness.

 

Limitations

 

• Equipment can be bulky and heavy.
• Radiation hazards.
• Testing area needs to be controlled access.
• Equipment relatively time consuming and expensive.
• Access may be required to both sides of object.
• May not detect critical flaws.
• Results require interpreting by experienced person.
• Gamma results inferior to X ray results.
• Gamma less sensitive than X ray, especially on thin materials.
• Not suitable for certain configurations, e.g. tee joint.

 

Advantages

 

• Volumetric inspection.
• Can detect surface and subsurface flaws.
• Permanent records.
• Good quality control method.

 

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