Thermoelastic effects on surface acoustic wave propagation

Abstract

The effect of thermal stresses on ultrasonic surface wave propagation was investigated. Quasi-static and dynamic thermal stresses and deformations were considered in materials with and without fatigue cracks. The perturbation of ultrasonic surface waves due to these thermal effects were studied both theoretically and experimentally. A long-duration infrared laser pulse was used to irradiate a series of aluminum and titanium specimens with fatigue cracks of known sizes between 0.5 and 1.0 mm. Preliminary numerical results from finite element simulation were found to be in good agreement with the experimentally observed laser-induced modulations of the ultrasonic surface wave. In particularly, both the numerical and experimental results indicated the same characteristic transient behaviors of the thermo-optical modulations for cracked and intact materials. The different behaviors suggest a new promising method for ultrasonic nondestructive evaluation, which cannot only effectively distinguish fatigue cracks from other artifact scatters, but potentially can also provide a method for quantitative evaluation of crack features. [This effort was sponsored by the Defense Advanced Research Project Agency (DARPA) Multidisciplinary University Research Initiatiative (MURI), under Air Force Office of Scientific Research Grant No. F49620-96-1-0442.]