T.O. 33B-1-16-59Figure 6-27. Density Changes Due to Varying Crack Widthsand Intersection Angles.6.7.2.10.1.1Obtaining parallelism between X-ray beam and crack plane is difficult to achieve. Cracks do not always initiate at theexpected origin and often are not perpendicular to the part surface. When the X-ray beam passes through a crack atany angle other than directly along the crack plane, both the width of the crack and the intersect angle determine thedensity change and indication contrast. Figure 6-27 shows two cracks of approximately the same width and depth butwith different angles of X-ray beam to crack plane intersection. As the angle between the x-ray beam and crack planeincreases, both film density change and contrast decreases. The film indication becomes broad, and more diffuse untilit blends into the background and is no longer discernible.6.7.2.10.1.2Detection of cracks depends upon crack width, depth, total metal thickness and angle of intersection. When only theintersection angle varies, it becomes a matter of statistics or probability. Table 6-13 reflects the probability of detectinga 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. Whendeveloping X-ray procedures to be used for detection of cracks, the maximum angle of intersection is 5°, whichcorresponds to an 85 percent probability of detection. The preferred limit is 2 1/2° corresponding to 90 percentdetection probability.
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