AuthorJ. D. Hochhalter, D. J. Littlewood, M. G. Veilleux, J. E. Bozek, A. M. Maniatty, A. D. Rollett, and A. R. Ingraffea
TitleA geometric approach to modeling microstructurally small fatigue crack formation: III. Development of a semi-empirical model for nucleation
JournalModelling and Simulation in Materials Science and Engineering
AbstractIt has been observed during fatigue of AA 7075-T651 that a small percentage of Al7Cu2Fe particles crack during manufacturing or very early in the life. Some of the cracked particles eventually nucleate cracks into the surrounding microstructure, and among them the number of cycles required for nucleation varies widely. It is important to comprehend the mechanics underpinning the observed variation so that the subsequent propagation stage can be accurately modeled. To this end, finite element models of replicated grain and particle geometry are used to compute mechanical fields near monitored cracked particles using an elastic-viscoplastic crystal plasticity model that captures the effect of the orientation of the grains near each monitored particle. Nonlocal, slip-based metrics are used to study the localization and accumulation of slip near the cracked particles providing mechanics-based insight into the actuation of the nucleation event. A high slip localization and accumulation rate is found to be a necessary, but not sufficient, condition for nucleation from cracked particles. A sufficient local driving stress must also be present, which is strongly dependent on the local microstructure and accumulated slip. Furthermore, the simulation results elucidate a quantitative relationship between slip accumulation and a reduction of the local driving stress required for nucleation, providing a physical basis for the fatigue damage concept. The observed nucleation direction was orthogonal to the computed local maximum tangential stress direction, and did not coalign with the directions of slip localization and accumulation (i.e. low-index crystallographic system directions). The result is a semi-empirical model for the number of cycles to nucleation, and its direction.
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