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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 line-focused Q-switched neodymium yttrium aluminum garnet (Nd:YAG) laser and the waves were received at two different positions by a laser interferometer. Using the phase spectrum method and moving average, we measured the dispersion 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 (S0, S1, A1, A2, and A3) of the Lamb waves agree well with the theoretical dispersion curves in the velocity range of 2000–4000 m/s and frequency x thickness range of 0.2–8.5 MHz mm. This method may be effective for material characterization of thin plates and pipes.