Résumé
The DISFAT project is a ongoing French project financially supported by the French National Agency ANR. It aims at a deeper understanding of mechanisms leading to crack initiation in ductile metals in Very High Cycle Fatigue (VHCF). The VHCF regime is associated with stress amplitudes lower than the conventional fatigue limit and numbers of cycles higher than 109. Tests were conducted using an ultrasonic technique at loading frequency of 20 kHz. The mechanisms leading to crack initiation express via slip bands at the specimen surface and self-heating due to intrinsic dissipation. Thermal maps were used to estimate the mean dissipation and its change with number of cycles and stress amplitudes in case of pure copper polycrystals. At the same time, the surface relief changes due to plasticity were characterized using optical and scanning electronic microscopes. A good correlation was found between slip band initiation and dissipation. Dissipation and slip band amount always increased over the number of cycles. At very small stress amplitudes, no slip band appeared up to 108 cycles but the material was found to dissipate energy. These results reveal that the material never reached a steady state. Therefore it could break at higher number of cycles.