DIRECTIVITY PATTERNS OF LASER THERMOELASTICALLY GENERATED ULTRASOUND IN METAL WITH CONSIDERATION OF THERMAL CONDUCTIVITY

摘要

Using the thermal diffusion equation with the consideration of the effect of thermal conductivity, we obtain two terms contributing to the directivity patterns respectively for the longitudinal wave and the transverse wave. The first term is exactly the same as the results obtained by earlier workers. The additional second term is due to the effect of thermal diffusion. In the transverse directivity pattern, the second term does not influence effectively the transverse directivity pattern. However, for the longitudinal directivity pattern, the second term has more and more influence as the spatial profile of the laser beam is widened or/and the detecting frequency increased. The theoretical predictions are qualitatively in agreement with our experimental results in aluminum. In the appendix, we start from the hyperbolic heat conduction equation so as to avoid the finite thermal propagation velocity. The results of directivity patterns for the longitudinal wave and the transverse wave are the same as those based as above on the classic parabolic equation until the detecting frequency is higher than similar to 10 GHz in aluminum. (C) 1997 Elsevier Science B.V. [References: 14]