Experimental observation of zero-group velocity combined harmonic generated by counter-directional Lamb wave mixing

In this paper, we present an experimental observation of the phenomenon known as zero-group velocity (ZGV) combined harmonic generation, which is induced by the mixing of counter-directional Lamb waves. We utilize internal resonant conditions to selectively choose the primary mode pair at specific frequencies for the purpose of combined harmonic generation. To detect the ZGV combined harmonic component, we propose a hybrid system that incorporates dual wedge-transducers for generation and a laser interferometric system for receiving. The appearance of the predicted S₁-ZGV combined harmonic at a specific mixing frequency is clearly observed in our experiments. Furthermore, we experimentally verify the controllability of the generated combined harmonics induced by the mixing of Lamb waves.