Group velocity measurement of various Lamb waves with two-beam laser interferometer

Multimode Lamb waves in a 1.0-mm-thick aluminum plate were excited by a lin e-focused Q-switched neodymium yttrium aluminum garnet (Nd:YAG) laser and t he waves were received at two different positions by a laser interferometer . Using the phase spectrum method and moving average, we measured the dispe rsion curves of the group velocities of various modes of the Lamb waves and compared the measured results with the theoretical dispersion curves. The measured dispersion curves of the modes (S-0, S-1, A(1). A(2), and A(3)) of the Lamb waves agree well with the theoretical dispersion curves in the ve locity range of 2000-4000 m/s and frequency x thickness range of 0.2-8.5 MH z mm. This method may be effective for material characterization of thin pl ates and pipes.