13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- at 106 cycles (Fig. 4d). The roughness and the quantity of slip bands kept rising over the cycles as dissipation (Fig. 4e and 4f). At 107 cycles, the narrowest zone of the specimen was covered with slip bands and the specimen self-heating is about 70°C. In addition to average values, the intrinsic dissipation distribution can be estimated along the specimen axis (Fig. 5a). The intrinsic dissipation is concentrated in the middle of the specimen and decreases toward the ends. As expected, it is related to the distribution of stress (Fig. 1). Optical micrographs revealed that this zone displays the highest quantity of slip markings (Fig. 5b). This result confirms that dissipation is related to microplasticity. 4. Conclusions The DISFAT project is an ongoing French project financially supported by the French National Agency ANR. It aims at a deeper understanding of mechanisms leading to crack initiation in metals and alloys in Very High Cycle Fatigue (VHCF). 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 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. They are higher with increasing stress amplitudes. The 1D dissipation distribution is in good agreement with the stress 1D profile. The strongest dissipation value was found at the place where the number of slip bands is the highest. 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. a) b) Figure 3. (a) Average temperature and (b) Average intrinsic dissipation during fatigue test at different constant stress amplitude fatigue test for CuOF 99.99%.
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