Characterization of air-coupled ultrasonic receivers for nonlinear Rayleigh wave nondestructive evaluation

Nonlinear Rayleigh wave measurements can be used to determine damage precursors and measure microstructural material changes while only requiring one-sided access to a specimen. These measurements are extremely sensitive to coupling conditions between the specimen surface and the generating and receiving transducers used in propagation and measurement of the Rayleigh waves. Air-coupled detection offers many advantages to traditional contact techniques because it mitigates these experimental coupling concerns with a reasonable cost as contrasted with other non-contact methods. However, to use these devices to measure absolute nonlinearity in a specimen requires a detailed understanding of the transducers and the experimental setup. This work provides a combined numerical and analytical approach to understanding the received signal and uses this information to determine a framework for calculation of absolute measures of material nonlinearity.