Ultrasonic Measurement of Residual Stress in SMAT Metals

Surface mechanical attrition treatment (SMAT) consists of producing a thin surface layer of compressive residual stresses that prevents crack initiation and retards crack growth during service. The capability for a non-destructive measurement of the compressive stresses is very useful to assure fatigue life of components treated by SMAT in the industrial domain. In this paper, an ultrasonic method based on the longitudinal critically refracted (Lcr) wave is proposed for this purpose. The Lcr wave travels just below the surface of the material, so it may be used to detect subsurface stresses in the structure components. This wave has several advantages over conventional surface waves and bulk waves in detecting the residual stress. The Lcr technique has been applied on the SMAT aluminum alloy specimen to determine the residual stresses in different depths, as well as on the specimens before and after stress relaxation annealing treatment in different zones. The experimental results show that this Lcr technique is feasible for the evaluation of residual stresses and control of the state of the material during mechanical treatment.