Nondestructive Evaluation of Plane Crack Tip in a Thin Plate Using Laser-Induced Pulse Wave and Symmetric Lamb Wave

A new measurement system for detecting microcrack tips was investigated for the nondestructive evaluation of thin plates. In this system, we combined two different techniques. One technique is the use of a low-power pulsed laser to induce ultrasonic pulse waves, which are generated by the thermoelastic effect. The other technique is the vibration of samples using a symmetric mode Lamb wave at low frequencies, which causes the fluctuation of crack tips. Taking account of the Lamb wave propagation in the plate sample, laser-induced waves were excited to pass through plane cracks under two different fluctuation conditions (compression and extension). The characteristic difference between the frequency spectra of these waves gave us information on fluctuating cracks. Using an acrylic sample, we confirmed that the difference in phase could be used for the detection of crack tips. (C) 2012 The Japan Society of Applied Physics