| Résumé | A convenient non-destructive means of characterizing residual stresses would be highly desirable. Ultrasonics is one of very few techniques which might succeed at this. However, the effects of residual stresses on ultrasound propagation are small and can be hidden by other material properties, especially crystallographic texture. The traditional ultrasonic approach has been to compare the surface before and after surface processing, assuming that texture remains unchanged. However, surface processing does affect surface texture and the traditional approach often fails. In this paper, we present a novel method to measure surface residual stresses with ultrasound when the process also modifies surface texture. Then, we apply this method to a sample of aluminum 7075-T651 surface treated using low-plasticity burnishing (LPB). This technique can produce a very smooth surface, a stress gradient that penetrates relatively deeply into the material, and an anisotropy of the residual stresses. The velocity of surface acoustic waves (Rayleigh waves and surface skimming longitudinal waves) was measured as a function of propagation direction and frewquency on as-received and LPB-treated surfaces. The observed differences are used to estimate the magnitudes and directions of the two principal components of the residual stresses indiced by the LPB process, independently of surface texture modifications. |
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