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

Using the thermal diffusion equation with the consideration of the eff ect of thermal conductivity, we obtain two terms contributing to the d irectivity patterns respectively for the longitudinal wave and the tra nsverse wave. The first term is exactly the same as the results obtain ed by earlier workers. The additional second term is due to the effect of thermal diffusion. In the transverse directivity pattern, the seco nd term does not influence effectively the transverse directivity patt ern. However, for the longitudinal directivity pattern, the second ter m 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 resul ts in aluminum. In the appendix, we start from the hyperbolic heat con duction equation so as to avoid the finite thermal propagation velocit y. The results of directivity patterns for the longitudinal wave and t he transverse wave are the same as those based as above on the classic parabolic equation until the detecting frequency is higher than simil ar to 10 GHz in aluminum. (C) 1997 Elsevier Science B.V.