Measurement of Acoustic Nonlinearity Parameter of Steel Alloy using Acoustoelastic Effect

Abstract

The nonlinear ultrasonic technique highly sensitive to the microstructural degradation of materials has been widely studied. In this technique, the acoustic nonlinearity parameter β is used as an index that quantitatively characterizes the microstructural features of materials. The acoustic nonlinearity parameter is a function of the second- and third-order elastic constants and can be measured using the acoustoelastic effect, a phenomenon in which the elastic wave velocity changes slightly owing to the changes in the microstructure and density of a material when stress is applied to. In this study, the acoustic nonlinearity parameter of an aluminum alloy, stainless steel, and carbon steel were measured using the acoustoelastic effect. First, the stress-dependent coefficients were measured from the velocity changes of ultrasonic waves under various compressive-stress conditions. Then, the third-order elastic constants were calculated using the measured Lamé constants and stress-dependent coefficients. Finally, the acoustic nonlinearity parameter β under the uniaxial stress condition was estimated using the calculated second- and third-order elastic constants. To verify the reliability of the experiment, the second- and third-order elastic constants, as reference values of the aluminum alloy, were confirmed to be similar to the experimental values. In the same way, the third-order elastic constants and acoustic nonlinearity parameter of four different steel alloys were measured. The acoustic nonlinearity parameters obtained in this study were measured under the uniaxial stress condition; thus, the results were compatible with the elastic nonlinearity in the stress-strain curve obtained from the tensile test. Therefore, it is expected that the measurement of acoustic nonlinearity parameter values can be used for the evaluation of tensile properties such as the yield strength.