Time-reversed Lamb waves

Lamb waves are extensively involved in plate structure inspection because of their guided nature. However, their dispersive nature often limits their use in flaw detection. In this paper we show that the use of a time-reversal mirror (TRM) allows to automatically compensate for the dispersive nature of Lamb waves. Experiments showing the spatial and temporal behavior of time-reversed Lamb waves, demonstrate the ability of TRMs to self-focus and to recompress dispersive pulses. This is demonstrated in a set of experiments in which a broadband ultrasonic laser source is used to simulate a point Lamb wave source and an optical interferometer is used to map the time reversed elastic field. We also show that TRM may work in pulse echo mode and allows to detect and to focus along large 2-D plates on any flaws located in the inspected area.