LASER-GENERATED ULTRASOUND - A THERMOELASTIC ANALYSIS OF THE SOURCE

The purpose of this work is to study the behavior of laser ultrasound sources in isotropic metals using ultra-short laser pulses in the ther moelastic regime. Temporally Gaussian and high frequency modulated las er pulses are investigated, and numerical results show that the therma l response rates of both steel and aluminum are much faster than their mechanical response rates. This indicates that the temporal point sou rce limit in laser ultrasonics is a mechanical rather than thermal lim itation. Two temporal point source limits are identified. For waves ge nerated by the SCOE (surface center of expansion), it is shown that th e temporal point source limit occurs at a Gaussian rise time of 1 ns i n both aluminum and steer. However, the precursor is a result of therm al diffusion and does not display a temporal point source limit within the range of parameters considered here. A dimensional analysis of th e laser source is conducted, and it is found that a single dimensionle ss parameter defines thermal similarity for laser sources, but no sing le parameter can define acoustic similarity. The physical dimensions o f the laser source are estimated for steer and aluminum, and it is fou nd that for temporally and spatially Gaussian laser pulses the outer e dge of the source is 1.65 times the Gaussian spot radius over a wide r ange of rise times and spot sizes. Also, a dimensionless parameter is given for three dimensional problems which predicts minimum rise times and maximum modulation frequencies for which hyperbolic heat conducti on effects will be of no significance in the ultrasonic displacements. (C) 1997 Elsevier Science B.V.