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 7 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