An ultrasonic instrument calibrated to obtain a 2 inch indication from a 0.08 inch diameter flat bottom hole located 3 inches from the front surface. When testing a forging, a 2 inch indication is obtained from a discontinuity located 3 inches from the entry surface. The cross sectional area of this discontinuity is probably:

• A. The same as the area of the 0.08 in. flat bottom hole
• B. Slightly less than the area of the 0.08 in. flat bottom hole
• C. Greater than the area of the 0.08 in. flat bottom hole
• D. About 1/2 the area of the 0.08 in. flat bottom hole

Answer: (C)

The key idea is that the amount of energy a reflector sends back to the probe—and thus the size of the indication on the instrument display—depends on the reflector’s cross-sectional area. A larger area presents more boundary to the ultrasonic beam and returns more energy, producing a stronger signal that the instrument interprets as a longer or larger indication, assuming depth and coupling are similar.

Here, a 0.08 inch diameter flat bottom hole at the same depth (3 inches) gives a 2 inch indication. The discontinuity in the forging at the same depth also yields a 2 inch indication. For the same depth and transducer, obtaining the same indication length with the forging discontinuity is best explained by the discontinuity having a cross-sectional area larger than the FBH’s. A smaller or equal area would typically produce a smaller or equal indication, not the same length. Therefore, the cross-sectional area of the discontinuity is greater than that of the 0.08 inch flat bottom hole.