Measurement on a thin plate of stepped thickness using zero-group velocity effect of an S1 mode Lamb wave induced by a moving laser source

Abstract

The desired narrowband mode of a Lamb wave can be generated efficiently, as long as the laser’s moving speed matches the mode’s phase velocity, which is a typical characteristic of the moving continuous wave (CW) laser excitation method. In this paper, S1 mode zero group velocity (ZGV) waves can be generated efficiently when the laser’s moving speed matches the phase velocity of the S1 mode at the ZGV point, while the two fundamental Lamb modes, i.e., A0 and S0 waves, can be avoided. Meanwhile, measurements on thin plates of stepped thickness were carried out by using the zero-group velocity effect of the S1 mode. Those simulation results demonstrated that the S1-ZGV resonance can be used to measure the thickness of thin plates by tracking the resonance peak as the sample was scanned, and good accuracy can be achieved, since the measurement was reduced to a simple frequency gauging of sample resonances in the ultrasonic domain, which provides a reliable method to measure a thin plate of varying thickness.