Figure 6-27. Density Changes Due to Varying Crack Widths
and Intersection Angles.
Obtaining parallelism between X-ray beam and crack plane is difficult to achieve. Cracks do not always initiate at the
expected origin and often are not perpendicular to the part surface. When the X-ray beam passes through a crack at
any angle other than directly along the crack plane, both the width of the crack and the intersect angle determine the
density change and indication contrast. Figure 6-27 shows two cracks of approximately the same width and depth but
with different angles of X-ray beam to crack plane intersection. As the angle between the x-ray beam and crack plane
increases, both film density change and contrast decreases. The film indication becomes broad, and more diffuse until
it blends into the background and is no longer discernible.
Detection of cracks depends upon crack width, depth, total metal thickness and angle of intersection. When only the
intersection angle varies, it becomes a matter of statistics or probability. Table 6-13 reflects the probability of detecting
a crack at various intersect angles. The table indicates the probability of detecting a crack with an intersect angle of 9'
is 75 percent. Conversely, the chances of missing a crack with a 9° intersect angle is 25 percent or 1 out of 4. When
developing X-ray procedures to be used for detection of cracks, the maximum angle of intersection is 5°, which
corresponds to an 85 percent probability of detection. The preferred limit is 2 1/2° corresponding to 90 percent