Abstract
Some Lamb modes exhibit a resonant behavior at frequencies where the group velocity vanishes while the phase velocity remains finite. Such a zero group velocity point exists in most isotropic materials for the first-order symmetric mode. Laser sources couple efficiently into this resonance and a sharp peak is observed with source and receiver on epicenter. Moving the detection point away from the source allows us to establish the dispersion curves of the plate and observe the negative phase velocity. We used a modulated diode laser source and a Michelson interferometer to observe the resonance at high frequency. Measurements were made on different material of thickness down to 4μm. The resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable for the measurement and mapping of nanoscale thickness variations. We also observed this behavior with a 20-ns pulsed YAG laser source and a heterodyne interferometer. The low attenuation in duralumin allows the excitation of a very sharp resonance in a single shot. Thickness variations as small as 0.1μm have been measured in a 0.50-mm-thick duralumin plate. Furthermore, the time decay of the signal provides an estimation of the local attenuation.
Published Version
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