Résumé
This work aims at studying the mechanisms leading to crack initiation for ductile single-phase metallic materials when they are subjected to stress magnitudes lower than the conventional fatigue limit. In these loading conditions, the number of cycles to failure is higher than 109 and belongs to the so-called very high cycle fatigue (VHCF) range. The main challenge of this work results from the fact that the manifestations of the mechanisms of interest give rise to very low and localized signal owing to the very low stress magnitudes involved. To rapidly reach the VHCF regime, the ultrasonic fatigue technique has been used with the hourglass shaped plate specimen in commercial Cu 99.95% copper. Using infrared thermography techniques, the temperature field at the specimen surface was measured during the fatigue test up to 108 cycles. Then, the dissipation in the variable section part of the specimen was calculated using a diffusion model. Moreover, the other surface was observed using a Scanning Electron Microscope (SEM) after interrupted tests at 106, 107 and 108 cycles, respectively. At stress lower than 34 MPa, we did not observe any change of the specimen surface despite a significant self-heating induced by dissipation, whatever the number of cycles. For stress higher than 34 MPa, localized slip band appeared on the specimen surface and high dissipation zones were detected. The dissipation was found to increase higher and faster with increasing applied stress. The amount of slip bands observed on the specimen surface followed the same trend. In addition, two types of slip bands were observed: straight, concentrated, intensive slip bands which were often initiated at grain boundaries and fine, spreading slip bands which were often initiated inside the grains. These results suggested different plasticity behavior of material which were correlated to the work of S. E. Stanzl-Tschegg et al. in 2009 who determined VHCF Persistent Slip Bands threshold at 45 MPa.