Many internal characteristics of materials can be photographed and inspected by the radiographic process. Radiography is based on the fact that gamma and X-rays pass through materials at different levels and rates. Therefore, either X-rays or gamma rays can be directed through a test object onto a photographic film and the internal characteristics of the part can be reproduced and analyzed. Because of their ability to penetrate through the materials and disclose subsurface discontinuities, X-rays and gamma rays have been applied to the internal inspection of forgings, castings, welds, etc. for both metallic and non-metallic products.
For proper X-ray examination, adequate standards must be established for evaluating the results. A radiograph can show voids, porosity, inclusions, and cracks if they lie in the proper plane and are sufficiently large. However, radiographic defect images are meaningless, standards are used. A standard, acceptable for one application, may be inadequate for another.
Neutron Radiograph and Related Techniques
Neutron radiography is a fairly recent radiographic technique that has useful and unique applications. A neutron is a small atomic particle that can be produced when a material, such as beryllium, is bombarded by alpha particles. Neutrons are uncharged and move through materials unaffected by density. When X-rays pass through an object, they interact with electrons. Therefore, a material with a high electron density, such as lead, is nearly impenetrable. N-rays, on the other hand, are scattered or absorbed by particles in the atomic nuclei rather than by electrons. A metal that is opaque to X-rays is nearly transparent to N-rays. However, materials rich in hydrogen or boron, such as leather, rubber, plastics and many fluids are opaque to N-rays. The methods used to perform neutron radiography are fairly simple. The object is placed in a neutron beam in front of an image detector.
There have been new developments in the radiographic field of non-destructive testing, several common recent applications include fluoroscopy, gamma radiography, televised x-ray (TVX), microwave testing and holographic inspection.