Nonlinear Time Reversal Acoustics for Defect Localization: Numerical Study of Retrofocusing Properties

Nonlinearity based Time Reversal (NL-TR) for retrofocusing of ultrasound appears to be a new tool in nonlinear imaging because of its ability to efficiently concentrate ultrasound in heterogeneous media for defect localization. A 2D pseudospectral time domain algorithm has been developed with a Adams-Bashforth method for solving elastic wave equation in nonlinear hysteretic heterogeneous solids. A 200 kHz central frequency pulse with 50 kPa source pressure was used as initial excitation in a 28x50cm(2) plate with a 56.25 mm(2) defect area having nonlinear hysteretic behavior. Nonlinear Elastic Wave Spectroscopy pretreatment of Time Reversal, called NEWS-TR has been investigated in this work. After ultrasonic propagation in the plate, signal is recorded, filtered and time reversed and sent back in order to evaluate the ability to efficiently retrofocuse on the defect. A new imaging method of nonlinear zone with harmonic generation measurements is studied.