Nonlinear dynamics and time reversed acoustic imaging in damaged solids

Our ultimate goal is locating and imaging nonlinear scatterers in solids (e.g., cracks) without a priori knowledge of their existence. Toward that goal, two methods have been devised that combine the spatial and temporal focusing abilities of TRA with nonlinear elastic wave spectroscopy’s (NEWS) sensitivity to mechanical damage. The first method uses TRA to create large amplitude signals and induce a nonlinear response in a highly localized region on a sample surface. Repeating the process in a step-scan approach provides the means to image the sample surface/near-surface with high resolution, distinguishing nonlinear features (cracks) from the linear background (undamaged material). The second method takes advantage of TRA to focus acoustic energy from two input frequencies onto an unknown, but nonlinear, scattering source, creating nonlinear wave modulation. The time reversal mirror (TRM) array records the primary and scattering source waveforms. Filtering the recorded signals for a modulation sideband, time reversing, and rebroadcasting through the TRM focuses the filtered signal onto the source of the modulation—the crack. Experimental results demonstrating each technique will be presented. [This work was supported by Institutional Support (LDRD) at Los Alamos National Laboratory.]