Experimental observations of ultrasonic waves reflecting from and passing through a crack

A crack was introduced into a glass plate by applying thermal stress, and the propagation characteristics of longitudinal ultrasonic waves at this crack were observed using the photoelastic method. Waves incident on the closed crack passed through completely, and no flaw echo was observed on an A-scope display. Propagating waves with slightly open cracks were observed using a sensitive tint technique. The results indicate that the tensile phase of these waves was reflected at the crack, whereas the compressive phase was transmitted. This phenomenon is considered the principle behind the generation of harmonic waves from a crack by contact acoustic nonlinearity. Multi-cycle ultrasonic waves were visualized, and frequency analyses were performed based on the luminance distribution. Immediately after passing through the crack, a wave component with half the incident wave frequency was observed.