Sizing of micro cracks using laser-induced broad-band surface waves

A new method to estimate crack depth based on laser-induced surface acousti c wave (SAW) is proposed. Laser ultrasonic technique that uses laser beams for generation and detection of ultrasonic waves allows remote and nonconta cting inspection. Moreover, since laser-induced SAW has a wide frequency ba nd, it also offers crack-depth measurement using a frequency analysis. To c onfirm the performance, some feasibility tests using artificial slits were performed. Simulated surface cracks were machined on the type 304 stainless steel plates; each of them had a width of about 100 mum and a depth of fro m 0.2mm to 1.5 mm. The location and the depth of the slits were estimated f rom the detected waves by both the pulse-echo measurement and a frequency a nalysis. Results of the frequency analysis show that a surface slit behaves as the low pass filter to the laser-induced SAW and the step response time of the transfer function has linear relationship with the slit-depth. Cons equently, it is possible to detect small cracking having 0.2mm depth and go od agreement can be achieved between measured and nominal depths.