The Thermoelastic Surface Strip Source for Laser-generated Ultrasound

The vertical ultrasonic surface displacement generated by a laser-induced, thermoelelastic, rectangular surface strip source is formulated. The main features of this source, ultrasonic rise times comparable to those of the generating light pulse derivatives, and a large amplitude, double-pulsed waveform, are advantageous for ultrasonic spectraland time-domain measurements for nondestructive material evaluation. The analysis shows that, for laterally symmetric sources in the thermoelastic regime, only one tangential thermoelastic stress component contributes to the vertical displacement. Therefore, the strip source is equivalent to two, tangential line forces acting outward at the strip's front and back edges. The leading-edge rise time of the signal is virtually independent of the lateral extent of the source, which mostly affects the trailing portions of the ultrasonic pulses. Consequently, a particularly simple expression, which compares favorably with experimental results, is obtained for short strips (subtending small angles at the observation point). In conjunction with this formulation, the thermoelastic strip source is an important ool for quantitative, laser-based, ultrasonic nondestructive material evaluation.