Qualifying indentation fracture toughness testing by ultrasonics

A short review on applications of ultrasonic methods including acoustic emission, scanning acoustic microscopy, and surface wave scattering in the field of indentation fracture toughness testing of brittle materials is given. A problem not solved so far is the nondestructive testing of Hertzian cone cracks in opaque materials. The results of three experimental approaches are discussed: Scanning acoustic microscopy is capable of measuring the local cone angle, but there has been no indication of the crack tip. C-mode scanning permits the determination of the cone diameter, but a calibration is necessary. Time-of-flight measurements have the potential to provide both cone diameter and cone depth simultaneously.