Experimental comparison of nonlinear parameters obtained from absolute measurement and relative measurement

Abstract

The second-order ultrasonic nonlinear parameter β is obtained from the displacement of fundamental and second-harmonic frequency components, so it needs to measure the absolute displacement of fundamental and second-harmonic frequency components. However, the displacement of high-frequency harmonic component is not easily measured because that is very weak. Therefore, many researchers have measured a relative ultrasonic nonlinear parameter β` using the detected signal instead of measuring absolute displacement. The parameter β` is more convenient than measuring the parameter β, which is effective to compare before and after damage of a material; however, the parameter β` has never been verified in comparison with absolute ultrasonic nonlinear parameter. This study compares the relative parameter with the absolute parameter, and we make it clear that the fractional ratio of the relative parameters is identical to that of the absolute parameters when the detected signal amplitude is linearly proportional to the actual displacement amplitude. For the experimental verification, the absolute and the relative parameters were measured for two different materials, fused silica and Al6061-T6. The experimental results showed that the ratio of absolute parameters and that of relative parameters were in good agreement.