Decoherence of lamb waves by rough interface

Abstract

Propagation of Lamb waves in elastic plate with a periodic grating on one interface has shown an attenuation which can be interpreted as a phenomenon of decoherence. In order to verify this hypothesis, a simplified model is considered, where a fluid plate with a limited periodic grating on one interface is studied by the finite element method (ATILA). An antisymmetric Lamb mode is excited before the grating. The pressure is studied under the grating along the median plane of the plate, where it is equal to zero for the homogeneous plate. In the general case, when the wavelength of the Lamb wave is close to the grating spacing, reflected waves are observed and a phonon relation is written between the incident signal, the converted mode and the phonon related to the grating. The pressure in the median plane is particularly studied if the phonon relation is verified or not. A. Homogeneous fluid plate In this section, an homogeneous fluid plate is considered. Its thickness T is 5 mm. Its density is 2700 kg/m 3 and its wave velocity VF is 6320 m/s. It corresponds to a fictive fluid which properties are close to aluminium, in order to explain previous observations made with an elastic rough plate (3). The equations of the dispersion curves (4) can be obtained by writing the propagation of waves in the plate and using the boundary conditions at the surfaces ± T/2. The wave number of the propagating modes are given by: