Measurement of Acoustic Nonlinearity using Acoustoelastic Technique

Abstract

The nonlinear ultrasonic technique has been widely researched due to its high-sensitivity to microstructural degradation. To quantify it, the acoustic nonlinearity parameter has been regarded as an essential index for non-destructive evaluation. Conventionally, it has been measured using the harmonic generation technique. In harmonic generation, is measured from the ratio of fundamental and second-harmonic displacement amplitudes. On the other hand, is theoretically defined by the third-order elastic constants. And so that it can be measured using the acoustoelastic technique. Acoustoelastic technique is a phenomenon where the elastic wave velocity changes slightly due to the applied stress. However, there have been only few researches using acoustoelastic technique to investigate the measurement of . Therefore, in this study, the validity of the acoustoelastic technique to measure the parameter was confirmed experimentally. In addition, this research proposes a new acoustic nonlinearity parameter defined in uniaxial stress condition, which is compatible with elastic nonlinearity in the stress–strain curve obtained from tensile test. For experimental verification, the of heat-treated Al6061-T6 was measured by acoustoelastic technique and compared with the result measured by harmonic generation. Then, the was measured and the correlation with the yield strength was confirmed. The obtained by the two-different techniques showed similar variation according to the heat-treatment time and the showed higher correlation than with yield strength. From these results, it was possible to quantify the acoustic nonlinearity using the acoustoelastic technique, and the feasibility of for the evaluation of microstructural degradation was confirmed.