Sizing of Micro Cracks Using Laser-induced Broad-band Surface Waves.

A new method to estimate crack depth based on laser-induced surface acoustic wave (SAW) is proposed. Laser ultrasonic technique that uses laser beams for generation and detection of ultrasonic waves allows remote and non-contacting inspection. Moreover, since laser-induced SAW has a wide frequency band, it also offers crack-depth measurement using a frequency analysis. To confirm 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 100μm and a depth of from 0.2mm to 1.5mm. The location and the depth of the slits were estimated from the detected waves by both the pulse-echo measurement and a frequency analysis. 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. Consequently, it is possible to detect small cracking having 0.2mm depth and good agreement can be achieved between measured and nominal depths.