UCLA

The fatigue of an engineering structure originates from the nucleation of micro-size cracks. The accumulation of these micro-cracks ultimately results in the failure of the structure. In FCC materials, the localization of plastic strain into persistent slip bands (PSBs) during fatigue results in the initiation of micro-cracks at sites where the band emerges at the crystal surface. The main objective of this study is to explore the underlying dislocation mechanisms which significantly contribute to the fatigue deformation in FCC materials. In this context, a dislocation dynamics approach is employed in order to simulate the plasticity and fatigue evolution in FCC crystals.

We focus on the role of dipolar loops, which are the main ingredients of dislocation dense regions in FCC crystals, in the patterning of dislocation structures. The simulation results show that the interaction between glide dislocations and dipolar loops may lead to the refinement of dipolar loops. It is also likely that such an interaction results in a sessile dislocation configuration. We also investigate the clustering of a group of dipolar loops under the influence of a glide dislocation moving back and forth under fatigue. We identify a new dipolar loop dragging mechanism dominant in spontaneous clustering of relatively large dipolar loops. Furthermore, we explore the dislocation dipole mechanisms leading to the occurrence of slip avalanches in FCC single crystals.

We then present a persistent slip band model to investigate the evolution of dislocation structure and ultimately crack nucleation in fatigued FCC materials. The PSB model is cyclically deformed at two different plastic strain amplitudes corresponding to a partly formed PSB and a fully formed PSB. Comparing the stress strain responses of these two cases, it is shown that there is a correlation between the hardening behavior of a PSB and dislocation densities in channels of the PSB. The simulation results also reveal that, for the case of partly formed PSB, strain is substantially carried by the movement of screw dislocations, while, for the case of fully formed PSB, the edge dislocations moving in the direction of burgers vector plays a significant role in strain carrying.