LASER GENERATION OF ANTISYMMETRIC LAMB WAVES IN THIN PLATES

A convenient method is described for optically exciting and monitoring Lamb waves in thin plates. The interference pattern of two high power laser beams on a sample surface produces periodic heating and thus coherent antisymmetric Lamb waves are excited. By varying the interface fringe spacing, the acoustic frequency is easily and continuously tunable from 2.5 to 23 MHz with the present experimental apparatus. The ultrasound velocities as a function of frequency are measured to determine a dispersion curve, from which the Young's modulus is deduced. Furthermore, the temperature dependence is estimated from the velocity changes. Experimental results are presented for several kinds of brittle plate and thin film materials.