Laser ultrasonics, whereby an elastic wave of an ultrasonic frequency range is generated on the surface of a structural member using pulse laser irradiation, is applicable to noncontact and nondestructive testing. In this paper, the frequency is manipulated by switching the on/off status of the emission gate of a highrepetition-rate laser. The chirp signals are employed for gate control, thereby generating an ultrasonic wave in the range of tens to hundreds of kHz within the member. This technique is applied to measure residual thickness in metal plate members. The thickness can be estimated by detecting the cut-off frequencies of the non-zero order mode of a guided wave, especially zero-group velocity mode.
