Inversion of Acoustic Nonlinearity Parameter of a Solid Using Immersion Method

This article presents a new inversion technique to measure the acoustic nonlinearity parameter $\beta $beta of a solid using the immersion ultrasonics. This work uses the immersion ultrasonic method over contact-based piezoelectric detection method due to its advantages of consistent couplant and easier calibration of the piezoelectric transducer. In the immersion measurement, the acoustic wave propagates through three layers (water-solid-water) from transmitter to receiver. The second harmonic pressure measured at the receiver is generated by combined accumulation in water and the solid. To calculate $\beta $beta specifically for the solid layer, a theoretical model of nonlinear wave propagation in n-layer media with a new approach of calculating diffraction loss was developed. This developed model was first validated with experimentally measured harmonic pressures of a known solid; fused silica solid. The validation showed good agreement between theoretical and experimental results. Next, an inverse algorithm was developed to calculate $\beta $beta of solid by combining the theoretical model and the measurements from the nonlinear immersion testing. Finally, the inversion framework was applied to measure $\beta $beta of rolled aluminum. The obtained results align well with literature values and measurements of existing contact-based technique.