Study of Surface Acoustic Wave Dispersion Using Laser-Ultrasonics and Application to Thickness Measurement

摘要

Dispersion of surface acoustic waves (SAW) has been used to characterize subsurface anomalies [1], estimate physical property gradients [2], calculate the effective elastic constants of fiber composite materials [3] and evaluate thin film thickness [4] and microstructures [5]. In most cases, SAW were generated and detected using piezoelectric transducers either in direct contact with the sample [1], or coupled to it with a liquid couplant [3–5]. Coupling problems arise in the case of samples at elevated temperature or in motion. These problems are solved by laser-ultrasonics, which uses lasers to generate and detect ultrasound, without contact and at distance [6, 7]. This technique applied to SAW generation and detection has been used for the evaluation of material properties [8, 9], to near-surface flaw detection [8, 10] and to the measurement of the thickness of thin metal sheets [9, 11]. Most applications require conditions of generation, which do not affect the surface of the specimen, i.e. operation in the thermoelastic regime. In this case, rather weak ultrasonic displacement signals are generally produced, but considerable enhancement of the signal has been demonstrated by distributing the laser energy on a circle and detecting the displacement at the center of convergence of the generated SAW [8].