Abstract
A Q‐switched, Nd:YAG laser has been used to generate elastic waves within aluminum and mild steel disks. A single 24‐ns pulse from the laser irradiated one face of each disk, and acoustic displacements were detected at the other face following propagation through the substrate. A capacitance transducer was used as the detector, providing undistorted displacement waveforms over a wide bandwidth. The displacement waveform depended markedly on the irradiation conditions. At low laser power densities, such that generation occurred primarily by thermoelastic expansion, a common waveform was produced whose amplitude varied linearly with optical energy. At higher power densities in the presence of material ablation, the longitudinal amplitude increased substantially whereas that of the shear decreased. Wave propagation theory in a plate has been applied to the problem, and theoretical waveforms in good agreement with experiment have been produced; this leads to conclusions concerning the nature of the acoustic source formed by laser irradiation.
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