Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

5.3: Reflector Sizing
There are many sizing methods, these include:
5.3.1 Crack Tip Diffraction
When the geometry of the part is relatively uncomplicated and the orientation
of a flaw is well known, the length (a) of a crack can be determined by a
technique known as tip diffraction. One common application of the tip
diffraction technique is to determine the length of a crack originating from on
the backside of a flat plate as shown below. In this case, when an angle beam
transducer is scanned over the area of the flaw, the principle echo comes
from the base of the crack to locate the position of the flaw (Image 1). A
second, much weaker echo comes from the tip of the crack and since the
distance traveled by the ultrasound is less, the second signal appears earlier
in time on the scope (Image 2).