Table 6-13. Correlation between Beam Divergence and Crack Detectability.
Beam to Crack Angle (Degree)
Probability of Crack Detection
A 2 1/2° or 5° intersect angle X-ray beam will not project over the surface of a 14 inch by 17 inch film at normally used
focal spot to film distances (FFDs). The entire film will be exposed but only a small cone of radiation will be within
the desired or allowable intersect angle limits. Table 6-14 reflects the radiation cone coverage at various intersect
angles and FFDs. The table can be used to determine the necessary FFD when developing procedures. Example: A 12
inches long splice plate must be inspected for cracks. From Table 6-13, a 72-inch FFD is required to be within the 5°
intersect angle limit (6.3 inches on either side of the aiming point). Cracks occurring further than 6.3 inches from the
aiming point will produce indications with reduced film density change and contrast an there is a greater chance of not
detecting them. This emphasizes the need for information on crack location and orientation before developing an X-
ray procedure, plus the requirement for accurate tubehead alignment during equipment set-up.
Table 6-14. Radiation Cone Radii at Various Intersect Angles and FFDs.
Whenever X-rays interact with material, one or more of the following will occur, absorption, scattering or penetration.
In industrial radiography, scattered radiation can present a problem since it has the ability to expose the X-ray film
without contribution to image information. Exposure due to scatter is usually referred to as fog and it substantially
reduces the image contrast. Scattered radiation can have three different sources (see Figure 6-28). One source of