Detection des fissures de fatigue par vibrothermographie

This thesis investigates the use of vibrothermography for the detection of fatigue cracks in steel compact tension coupons and in beam shaped aluminum specimens using combined experimental and finite element analyses.;First, modal analyses are carried out by finite element method. The resulting natural frequencies and associated mode shapes serve to define the optimal excitation parameters. Coupled thermo-mechanical models are then built to simulate the thermography inspection processes. The first coupled model related to the compact tension coupons includes a representation of frictional heating. The one associated to the beam shaped specimens represents, as well, heating due to plastic deformation. The model related to frictional heating predicts the detection of cracks as short as 0.1 mm that is also confirmed experimentally using a commercial infrared camera with an error about 9 % on the temperature evolution. The one related to plastic deformation heating predicts the detection of cracks as short as 1 mm that is also confirmed experimentally with an error about 8 % on the temperature evolution.;The model associated with friction heating shows that the stress at the crack tip is lower than the material's yield stress, which makes the test truly non-destructive. The one associated to cyclic plasticity shows that the stress at the crack faces exceeds the material's yield stress but not at the crack tip, confirming that the heat generated during plastic deformation leads to crack detection. Finally, the Fourier transform applied on the numerical thermal response shows that the specimen's temperature at the crack vicinity varies according to the excitation frequency and presents harmonics due to the nonlinearity induced by the crack.