SAW dispersion

Nondestructive evaluation of the prevailing compressive residual stress in the shallow subsurface layer of surface-treated metals is greatly complicated by the accompanying surface roughness and cold work. A high-precision laser-ultrasonic technique has been developed to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on surface-treated metals in a wide frequency range from 1.5 MHz to 15 MHz. This paper presents experimental data obtained on samples that have been surface treated with shot peening, laser shock peening and low plasticity burnishing, which indicate that the dispersion of the surface wave is affected by surface roughness, compressive residual stress, and cold work. Although surface roughness induced scattering provides a significant contribution to the observed dispersion of the SAW, our experimental data indicate that it is feasible to observe a substantial and highly characteristic change in the velocity of the SAW when the specimen is heat treated at different annealing temperatures. These results also indicate that precise SAW dispersion measurements can be exploited to monitor thermal relaxation and recovery in surface-treated metals.