FE modeling of Lamb mode diffraction by defects in anisotropic viscoelastic plates

A Finite Element code is used in a stationary mode to compute, at a single frequency, the stress and displacement fields in plates made of anisotropic, viscoelastic materials. An appropriated spatial load at one boundary of the plate is applied to generate a guided mode. This step is included in a frequency loop for which the number of iterations (≈ usually less than 50) is defined by the frequency spectrum of a temporal excitation. Then the temporal response at any location in the plate can be reconstructed with inverse Fourier transform. The phase velocity, attenuation and nature of one or several propagating modes can be identified in the classical frequency/wave number representation. The first example concerns the interaction of the symmetric Lamb mode S0 with an opening notch in a Perspex plate. The proportion of S0 and converted antisymmetric A0 mode, transmitted past the notch is evaluated by the FE method and validated by a comparison with measurements made on a real system using an air coupled transducer. The second example shows the effect of the viscoelasticity on the propagation of the symmetric Lamb mode S0 in a cross-ply, carbon-epoxy material plate. Focus is made on some difficulties that the attenuation causes on the detection of a delamination between plies.