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The paper studies the interaction of Lamb waves with the free edge of aplate. The reflection coefficients of a Lamb mode at a plate free edge arecalculated using a semi-analytical method, as a function of frequency and angleof incidence. The conversion between forward and backward Lamb modes isthoroughly investigated. It is shown that, at the zero-group velocity (ZGV)frequency, the forward $S_1$ Lamb mode fully converts into the backward$S_{2b}$ Lamb mode at normal incidence. Besides, this conversion is veryefficient over most of the angular spectrum and remains dominant at frequenciesjust above the ZGV-point. This effect is observed experimentally on a Duraluminplate. Firstly, the $S_1$ Lamb mode is selectively generated using a transducerarray, secondly the $S_{2b}$ mode is excited using a single circulartransducer. The normal displacement field is probed with an interferometer. Thefree edge is shown to retro-focus the incident wave at different depthsdepending on the wave number mismatch between the forward and backwardpropagating modes. In the vicinity of the ZGV-point, wave numbers coincide andthe wave is retro-reflected on the source. In this frequency range, the freeedge acts as a perfect phase conjugating mirror.

Negative reflection of Lamb waves at a free edge: tunable focusing and mimicking phase conjugation