Abstract
The dispersion characteristics of Lamb waves depend on the thickness and the elastic properties of a plate. At the cut-off frequencies (when the wavenumber goes to zero), Lamb waves are standing waves associated with plane longitudinal or shear waves reflecting up and down between the surfaces of the plate at normal incidence. In the general case, these thickness resonances have an infinite wavelength and do not carry energy along the plate. However, if the material properties are selected such that longitudinal and shear thickness mode resonances of the same symmetry coincide, then the elastic fields associated with the resonances couple to produce a wave with an infinite wavelength that propagates energy along the plate surface. Intriguing effects associated with this phenomenon can be observed in aluminum alloy plates where near-degeneracy between resonances occurs. We show that waves generated near the cutoff frequency in such cases spread from the excitation point and produce a spatially uniform oscillation over the plate surface. These infinite wavelength waves show angle independent mode conversion upon encountering the free edge of a plate or thickness step on the surface. Furthermore, we demonstrate that these peculiar waves flow around plate defects and are remarkably insensitive to scattering.
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