RADIOGRAPHY 



831 



Industrial Radiographic Interpretation. — In the interpretation of industrial radio- 

 graphs, as in medical work, there can be no substitute for experience and observation. 

 It is necessary for the operator to be more or less familiar with industrial processes 

 such as welding, casting, molding, etc., and he should have at least a working knowl- 

 edge of the fundamentals of metallography. For the beginner a fairly safe rule is as 

 follows : if on the radiographic film of a weld, casting, mold, plate, or other supposedly 



Fig. 5. — Radiograph of welded steel plates showing gas pockets and cracks of faulty weld. 



homogeneous material, one or more dark spots or lines appear in an otherwise even 

 field, these areas represent defects. 



Example: Fig. 5 is an actual radiograph of two steel plates which have been placed 

 edge to edge and welded together. The dark areas are the plain steel and the light 

 center strip is the weld material which is denser than the surrounding metal. Within 

 this center strip numerous spots and one or more dark lines appear. The spots are 

 small gas pockets while the lines are cracks. This is a faulty weld and, if subjected to 



Fig. 6. — Diagram illustrating diffraction of X rays. 



high pressures or excessive strain, is likely to fail. This film is fairly representative of 

 the general run of industrial radiographs, the interpretation of which depends on a 

 knowledge of the relative densities involved in the object being inspected. 



X-ray Diffraction. — Although X-ray diffraction is not usually classed under radi- 

 ography, a strict interpretation of the word radiograph would require the inclusion of 

 this application of X rays. A simple explanation of this phenomenon is as follows: 



Reference to Fig. 6 shows that a single light beam L divided at pinholes A and B 

 will produce alternate light and dark areas on surface X — points like P, equidistant 

 from A and B will be brightly illuminated by reinforcing similar-phased waves, but 



