Nonlinear ultrasonic evaluation of damaged concrete based on mixed harmonic generation

Nonlinear wave mixing provides a potential method to evaluate localized micro-damage in structures. The wave mixing technique has some unique advantages over the nonlinear ultrasonic technique based on the higher harmonic generation, such as frequency selectivity, which can separate harmonic components generated due to instrumentation. In this paper, we numerically studied the frequency mixing response induced by the nonlinear interaction of two primary Rayleigh waves in a concrete material. The nonlinearity considered in the present study is due to crack-wave interaction, which is often called contact acoustic nonlinearity (CAN). A limited number of experimental studies were conducted on concrete prisms to evaluate surface cracks. Both numerical and experimental results illustrate the generation of the second- and third-order combined harmonics, which can be utilized to detect and localize subsurface cracks in concrete structures. The newly discovered third-order combined harmonics, which result from the mutual interaction between the primary Rayleigh waves and their induced second-harmonic waves, can be exploited to characterize micro-damage in various applications.