Diffraction correction for precision SAW velocity measurements

Abstract

Surface acoustic wave (SAW) dispersion measurements can be used to nondestructively characterize shot-peened, laser shock-peened, and other surface-treated specimens. In recent years, there have been numerous efforts to separate the contribution of surface roughness from those of near-surface material variations, such as residual stress, texture, and increased dislocation density. This talk addresses the problem that a perceivable dispersive effect, similar to the one found on rough shot-peened specimens, is exhibited by untreated smooth surfaces as well. The dispersion measurements were performed using laser-ultrasonic scanning combined with special digital signal-processing methods. The observed dispersion effect is on the order of 0.1%, which is comparable to the expected velocity change produced by near-surface compressive residual stresses in metals below their yield strength. The cause of this apparent dispersion is the diffraction of the SAW as it travels over the surface of the specimen. It is demonstrated that a diffraction correction may be introduced to increase the accuracy of surface wave dispersion measurements. A simple diffraction correction model was developed for surface waves; this correction was subsequently validated by laser-interferometric surface wave velocity measurements on 2024-T351 aluminum specimens.