Application of Scanning Acoustic Microscopy to Residual Stress Analysis: Theory vs. Experiment

In this project, a new technique for the nondestructive evaluation of residual stress in manufactured components is proposed. Where most approaches to residual stress analysis have been based on monitoring the stress induced changes in sound velocity these techniques are limited in their utility due to use of contact, shear transducers (effectively precluding scanning large areas), the inability to resolve the dependence of residual stresses on depth (important for surface treatments) and the difficulty in making accurate time delay measurements due to the very small acoustoelastic effect observed for most practical materials. Here, we propose to develop a nondestructive test technique suitable for scanning plate structures. An aspherically focused, immersion transducer is used in a scan mode to generate an axially symmetric pulse. We utilize interference phenomena between two shear waves polarized in the directions of the in-plane principal stress axes to increase resolution of the small differences in transit time between the two waves. This technique may become a powerful tool to study actual residual stress distributions in practical engineering materials