Evaluation of yield strength by ultrasonic reconstruction of quadratic nonlinear Stress-Strain curve

Abstract

The ultrasonic nonlinearity parameter beta defined by the ratio of the second-order harmonic amplitude to the square of fundamental frequency amplitude, has been considered as a potential index to evaluate material degradation. However, the yield strength obtained from destructive tensile testing is still widely adopted for this purpose since it is a more intuitive concept to the field engineer than the parameter beta. Therefore, this study proposes a nondestructive method to evaluate the yield strength directly from the ultrasonic measurements. In this regard, the tensile stress-strain curve is represented in the form of a quadratic nonlinear stress-strain equation within the elastic range, which includes the linear elastic modulus and the second-order nonlinearity parameter beta(t). The linear elastic modulus is obtained by measuring the propagation velocity of longitudinal and transverse waves using a traditional ultrasonic pulse-echo method, and the second-order nonlinearity parameter is obtained by measuring the Murnaghan constants using acoustoelastic effects. Then, the tensile stress-strain curve is reconstructed to estimate the 0.01% offset yield strength. To demonstrate the application of the proposed algorithm, the experiments were performed for heat-treated SA508 specimens. The results indicate that the 0.01% offset yield strength obtained using the proposed algorithm exhibit a good agreement with that obtained via destructive tensile testing.