Investigation on characteristics of tensile damage and microstructure evolution of steel AISI 316L by nonlinear ultrasonic Lamb wave

In view of the high sensitivity to microstructure evolution of higher harmonics generated by the nonlinear ultrasonic method, the tensile damage in the stainless steel AISI 316L was evaluated by nonlinear ultrasonic Lamb wave. The effect of different degree of tensile deformation on nonlinear ultrasonic response in 316L was analyzed by considering the microstructure evolution and the test results. First, the power-law relation of nonlinear ultrasonic parameter and plastic strain is proposed. Meanwhile, martensitic transformation and the increasing of dislocation density can be observed in the micrographs of specimens with the increase of plastic deformation. Then, the same variation tendency of intergranularly averaged Kernel Average Misorientation (KAM), as well as the relative nonlinear ultrasonic parameter, demonstrated that the nonlinearity due to tensile damage is controlled by the evolution of microstructure that directly affects higher order elastic constant of stainless steel 316L. Furthermore, based on continuous damage mechanics, the method quantitatively characterizes the degradation of mechanical properties was proposed with a functional relation of damage variable and relative nonlinear parameter. The method proposed in this study provides a theoretical basis for online monitoring of the damage status of in-service equipment. It is of great significance for residual life assessment, structure integrity assessment, and service life extension of equipment