Crystal plasticity finite element modeling of grain size and morphology effects on yield strength and extreme value fatigue response

Abstract

A computational framework is presented to include the effects of grain size and morphology in the crystal plasticity finite element (CPFE) method for simulations of polycrystals. The developed framework is used to investigate the effects of grain size and morphology on the yield strength and extreme value fatigue response using a new grain-level length scale. Each grain is approximated by a best-fit ellipsoid, whose information is used to modify the slip resistances based on a Hall-Petch type relation extended to each slip system. The grain-level length scale is computed for each slip system using a shape factor proposed in an earlier work based on discrete dislocation dynamics simulations. This is incorporated into a rate-dependent CPFE model with kinematic and isotropic hardening within the PRISMS-Plasticity open-source software. CPFE simulations are conducted on Al 7075-T6 microstructure models with different textures, grain sizes, and grain morphologies which relate qualitative trends in yield strength to a parameter constructed from the power-law flow rule. Incorporating grain morphology in the model reveals a notable influence on the computed extreme value fatigue response which may be critical in simulations of polycrystalline microstructure models with significant grain morphology anisotropy, for instance in components produced by large deformation rolling or additive manufacturing. The developed framework is available to the community as part of the open-source software PRISMS-Plasticity and PRISMS-Toolbox.