Experimental observation of non-collinear mixing of fundamental antisymmetric Lamb waves

Non-collinear mixing of guided elastic waves in an isotropic plate with material nonlinearity was experimentally investigated to validate key features predicted in previous theoretical and numerical studies. The sum-frequency components generated by the mixing of two fundamental antisymmetric (A0) Lamb waves were measured in a 2-mm-thick aluminum alloy plate for two primary frequency pairs, 0.6/0.6 and 0.55/0.65 MHz. Based on wavenumber-frequency spectra obtained from temporal waveforms at various propagation distances, the sum-frequency components at 1.2 MHz were identified as the fundamental symmetric (S0) Lamb waves, consistent with theoretical predictions. Non-collinear measurements were also performed for various intersection angles of the two primary A0 modes. The results demonstrated that the amplitude of the sum-frequency S0 mode increases near the angle satisfying the internal resonance condition, a theoretically derived necessary condition for resonant mutual interaction, where the wavenumber of the sum-frequency component matches the magnitude of the sum of the primary wavevectors. Notably, the location of the maximum amplitude was slightly shifted from the resonant intersection angle, in agreement with numerical results obtained from three-dimensional finite-element simulations.