A low-cycle fatigue life prediction model of ultrafine-grained metals

A (high strain) low-cycle fatigue (LCF) life prediction model of ultrafine-grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two-phase 'composite' consisting of the 'soft' matrix (all the grain interiors) and the 'hard' reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high-strain LCF conditions, the fatigue-damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding-off process within it. The fatigue crack growth rate is directly correlated to the range of the crack-tip opening displacement. The empirical Coffin-Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high-cycle fatigue (HCF) regime.