Laser Ultrasonic Detection of Surface Breaking Discontinuities: Scanning Laser Source Technique

In this paper we propose a new approach for ultrasonic detection of small surface breaking cracks using laser based techniques. This approach does not monitor the interaction of a well established surface ultrasonic wave with an anomaly, as in the case of traditional pitch catch or pulse echo methods ,but rather monitors the changes in the generated ultrasonic signal as the laser source passes over a discontinuity. Changes in amplitude and frequency of the generated ultrasound are observed which result from the changed constraints under which the ultrasound is generated over uniform and discontinuous areas. These changes are quite readily detectable using existing laser detectors. The main advantages of the proposed technique are enhanced signal to ncise performance compared to the conventional pitch catch mode of operation, ability to detect surface breaking discontinuities of various orientations with respect to the scanning direction, that the procedure can be used with either a laser interferometer or a conventional PZT transducer, and ability to inspect surfaces of complex geometry. Results are presented for detection of fatigue cracks using a complete laser based ultrasonic system.