13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Very high cycle fatigue for single phase ductile materials: microplasticity and energy dissipation Ngoc Lam Phung1, Antoine Blanche2, Nicolas Ranc1, Véronique Favier1*, André Chrysochoos2, Nicolas Marti1,3,5, Nicolas Saintier3, Chong Wang4, Danièle Wagner4, Claude Bathias4, Fabienne Grégori5 , Brigitte Bacroix5, Haël Mughrabi6, Guillaume Thoquenne7 1Arts et Métiers ParisTech, PIMM UMR 8006, 75013 Paris, France 2Montpellier II University, LMGC UMR 5506, France 3Arts et Métiers ParisTech, I2M UMR 8006, 33607 Bordeaux, France 4Paris 10 University, LEME, France 5Paris 13 University LSPM UPR 3407, France 6Erlangen University, Germany 7Cetim, Senlis, France * Corresponding author: veronique.favier@ensam.eu Abstract 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. Keywords copper, self-heating, slip bands, IR thermography, ultrasonic fatigue 1. Introduction Nowadays there is a growing demand for the development of fast and robust fatigue life prediction methods in the very high cycle fatigue (VHCF) domain. The VHCF regime is associated with stress amplitudes lower than the conventional fatigue limit and as a result, numbers of cycles higher than 109. Some mechanical components, such as pistons, rotating axes, have been designed previously using fatigue resistance data at lower numbers of cycles (<107 cycles ; the regime of High Cycle Fatigue, HCF) whereas they must endure oscillating loads for a number of cycles higher than >109 cycles and finally fail [1]. These requirements motivate the need to understand the fundamental mechanisms of fatigue in the VHCF regime, correspondingly, to explore novel methods for the characterization of fatigue behavior at these very long lifetimes. The DISFAT project is an ongoing French project financially supported by the French National Agency ANR. It aims at a deeper
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