Surface-breaking fatigue crack detection using laser ultrasound

A laser technique is presented for the assessment of the depth of a tight fatigue crack in a metal alloy. The study involved an analysis of ultrasonic interaction with a crack using a laser time-of-flight diffraction (TOFD) method. Investigations exploited the use of a confocal Fabry-Perot interferometer as the ultrasonic sensor. TOFD measurements proved to be feasible. Exploiting laser beams on the same surface as the fatigue crack, ultrasonic wave forms were characterized to show that there was a feature arising from the bottom tip of the fatigue crack. A signal forms the basis of crack size evaluation.