Nonlinear time reversal tomography of structural defects

Abstract

Nonlinear Elastic Wave Spectroscopy (NEWS) comprises a new class of NDT techniques providing sensitive detecting and imaging of damages like micro-cracks, etc. The main advantage of nonlinear methods is their high sensitivity and ability to detect small defects relatively far from ultrasonic actuators and sensors. Some limitations of their use are due to difficulties with defect localization when global excitation of complicated structures is considered. To overcome these constraints, a modified TR-NEWS procedure is proposed in this paper. Procedure uses transducer arrays providing improvement of damaged zone location. One array transmits and another array records signals, which are then processed as to obtain nonlinear parameters corresponding to wave-paths between transducers. DORT method is used to derive Green's function of defects and TR-NEWS procedure is completed by the Excitation Symmetry Analysis Method (ESAM) as to extract nonlinear terms from acoustic responses along different wave-paths, approximated by straight lines. Test results, obtained by simulations and by experiments on metallic sample, illustrate performance of the proposed procedure. Extracted nonlinear parameters depend on wave-paths along damaged zones (higher when crossing area with scatterers). This allows pseudo-tomography imaging of defective zones as areas around the wave-paths with significantly enhanced nonlinear parameters.