Amplitude dependence of the parametric interaction components inside the retrofocusing area with nonlinear time reversal acoustics

During the last few years, the combination of Time Reversal (TR) acoustics with Nonlinear Elastic Wave Spectroscopy (NEWS) techniques has been used as a new tool for local investigation of nonlinearity in complex media. TR process is used to retrofocus and reconstruct coherent signals in order to locally generate nonlinear stressed area. It is necessary to evaluate the exact signature of the nonlinear response i.e. classical nonlinearity, local or global nonlinearity or non-classical nonlinearity generated by cracks. This study deals with the investigation of the amplitude dependence of the nonlinear component versus the fundamental excitation level with numerical and experimental analyzes. Parametric interaction of TR synchronized acoustic waves (f(1) = 490 kHz and f(2) 860 kHz) has been chosen as the NEWS technique for local interrogation of a solid steel sample with and without cracks. Pulse inversion filtering has been used in order to extract nonlinear signature at frequencies f(L) = f(2) - f(1), kHz and f(H) = f(2) + f(1) kHz. A pseudo-spectral 3D time domain-TR method has been adapted to this amplitude dependence algorithm developed for the NEWS study. As done for experiments, the parametric component amplitude dependence is observed for various excitation parameters and comparison with the Westervelt theory is discussed.