5K-1 F.E.M. Simulation of Lamb Wave Propagation: Application to the Thermo-Oxidative Ageing of Carbon-Epoxy Plates

Thermo-oxidative ageing of carbon-epoxy plates has been done for up to 9000 hours at two temperatures (160 and 180degC). Non-destructive evaluation by means of non contact ultrasonic methods has been tested to characterize the aged plates. It is shown that the phase velocity of the A0 and S0 Lamb modes is decreasing when ageing duration increases. Micrographs reveal the presence of oxidation cracks propagating from the free surfaces of the plate. The plates thus become inhomogeneous in the thickness. The propagation of the Lamb waves, in the aged plates, is simulated using a finite element model of an inhomogeneous and anisotropic plate. The excitation of several modes in a frequency range (chirp excitation) or a single mode at a chosen frequency (burst windowed excitation) is applied. To simulate the ageing of carbon-epoxy composite plates (8 plies), the elastic parameters are decreased in the two external plies. The results are then compared with experiments. Using this FE model, we demonstrate that this phenomenon is related to an important decrease of the C₅₅ elastic parameter of the plate