Crack depth measurements on fracture mechanics specimens by the ultrasonic amplitude modulation method

Crack depth measurements on a large number of fracture mechanics specimens of various shapes, sizes and materials loaded in fatigue were performed with some selected nondestructive examination methods. The experiences gained from these measurements can be summarized as follows: • - optical measurement of crack depth with microscopes yielded satisfying results as long as this was practicable and appropriate, • - the potential drop method does not allow any correlation between the potential measurement and the actual crack depth because of strong variability of the results, • - the ultrasonic amplitude modulation method was characterised by the high accuracy and by the low variability in its results.