Determination of compressive residual stresses using critically refracted longitudinal (LCR) waves

Surface treatment methods are widely used in the aerospace industry to increase the metal parts damage tolerance by producing a thin surface layer of compressive residual stresses that prevents crack initiation and retards crack growth during service. Nondestructive evaluation of the prevailing compressive residual stresses in the shallow subsurface layer is realized by the ultrasonic technique. The method is based on the measurement of ultrasonic wave velocity variation versus the stress state variation. The application of the Lcr technique (using longitudinal critically refracted wave) has been performed on the SMAT (surface mechanical attrition treatment) steel sample, for which the residual stresses are determined in different zones and depths. The results show that the ultrasonic technique is rather efficient for the evaluation of residual stresses and control of the state of the material after mechanical treatment. The experimental results are successfully compared with the hole drilling method.