SYMMETRY ANALYSIS SIGNAL PROCESSING FOR LOCAL NONLINEAR SCATTERERS SIGNATURE OF COMPLEX MATERIALS STRUCTURAL INTEGRITY

Abstract. Over the last few decades, increasing interest has been bestowed on employing nonlinear wave propagation effects (which possess the intrinsic property of enlarging bandwidth frequency of analysis) to detect, at a sub-wavelength scale (below the ultrasonic wavelength), the early damage in the degradation process leading to mechanical fracture. Higher frequencies are used for resolution improvement and lower frequencies for aging monitoring. The NDT community today refers to methods based on analysis of nonlinear effects as Nonlinear Elastic Wave Spectroscopy (NEWS). Recent years have seen considerable development (both in numerical- and experimental aspects) of Time Reversal (TR)-based NEWS methods using TR invariance and reciprocity. The defined TR-NEWS methods were integrated into applications for improving identification of nonlinear scatterers, such as bubbles, landmines, cracks in complex aeronautic materials, and these techniques are now widely recognized as extremely reliable. The complexity of the experimental set-up has been analyzed with a recent signal processing analysis using symmetry analysis combined with localization methods using the Decomposition of the Time Reversal Operator (DORT), with the objective to detect and localize spatially the nonlinear scatterers responsible of the damaged properties of complex material. Several experimental configurations will be presented in the paper, with materials and medium coming from aeronautic industry (cracked aluminum samples, damaged composites, heated layere