Evaluation of quadratic nonlinearity in tensile curve from ultrasonic linear and nonlinear measurements

Abstract

The nonlinear ultrasonic technique has been considered as a potential nondestructive method to evaluate material degradation, in which the ultrasonic nonlinearity parameter is usually measured. However, it does not tell intuitively about the degree of material degradation so that the yield strength obtained from the destructive tensile test is still widely used. This study proposes a method to evaluate the quadratic nonlinearity parameter of the tensile curve by ultrasonic linear and nonlinear measurements. For this, the correlation between the ultrasonic nonlinearity parameter and the quadratic nonlinearity parameter of the tensile curve is investigated first. Next, a method to estimate the quadratic nonlinearity parameter of the tensile curve is proposed, which is based on the acoustoelastic measurements. Finally, the tensile curve in the form of quadratic polynomial within the elastic range is reconstructed to estimate the 0.01% offset yield strength. In order for demonstration, heat-treated SA508 alloy specimens were tested, and results were compared with the tensile tests. Results showed that the reconstructed tensile curve and the 0.01% offset yield strength were in good agreement with those obtained from the destructive test. This method can be used for quantitative evaluation of material degradation.