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

Monitoring of fatigue cracks in parts during laboratory tests and while in
service in the field has been extensively done using ultrasonic techniques.
Reference 13 describes the use of surface waves to detect the initiation of
cracks in cylindrical compression-fatigue test pieces having a circumferential
notch. The surface waves, which were produced by four angle-beam search
units on the circumference of each test piece, were able to follow the contour
of the notch and detect the cracks at the notch root.
Monitoring the crack-growth rate was accomplished by periodically removing
the cracked test piece from the stressing rig and measuring the crack size by
straight-beam, pulse-echo immersion inspection. It was found necessary to
break open some of the cracked test pieces (using impact at low temperature)
and visually measure the crack to establish an accurate calibration curve of
indication height versus crack size.