Radiographic Tests on Metals ( NDT ) AND ITS PROCESS

 These tests are based on absorption and dispersion of X-rays or Gamma Rays passing through the material. By means of  a luminescent screen or photographic plate , points of varying radiation intensity that occur at faults can be detected. 

As radiation sources, X-rays produced by Betatron devices ( electron centrifuge ) or Gamma rays produces by radioactive decay processes are used. In these methods the radiation is passed through the metal being examined and is then allowed to impinge upon sensitive film. Thus, if an object has an internal defect , more radiation will be passed through the defective area than through the sound region and the defect will show up as dark area on the film. Unfortunately , scattering tends to obscure the defect, so that small defects may not be defected . In general , X rays are preferred for laboratory testing since they offer greater control over intensity and they produce sharper pictures. 

X-ray equipment is generally large and bulky , whereas gamma radiography depends upon the use of small, mobile sources of radioactivity, such as radioactive cobalt, iridium , samorium and other radio active elements. In addition gamma rays, though of the same nature as X-ray , have a generally shorter wave length, and therefore, more effective in the study of thick sections. 

In radiographic tests, the inclusions appear darker , while blow - holes, cavities and porosity appear lighter, than the surrounding metal.