Phase-Field Modeling of Ductile Fracture Across Grain Boundaries in Polycrystals

In this study, we address damage initiation and microcrack formation in ductile failure of polycrystalline metals. We show how our recently published thermodynamic framework for ductile phase-field fracture of single crystals can be extended to polycrystalline structures. A key feature of this framework is that it accounts for size effects by adopting gradient-enhanced (crystal) plasticity. Gradient-enhanced plasticity requires the definition of boundary conditions representing the plastic slip transmission resistance of the boundaries. In this work, we propose a novel type of microflexible boundary condition for gradient-plasticity, which couples the slip transmission resistance with the phase-field damage such that the resistance locally changes during the fracturing process. The formulation allows maintaining the effect of grain boundaries as obstacles for plastic slip during plastification, while also accounting for weakening of their resistance during the softening phase. In numerical experiments, the new damage-dependent boundary condition is compared with classical microfree and microhard boundary conditions in polycrystals, and it is demonstrated that it indeed produces a response that transitions from microhard to microfree as the material fails. We show that the formulation maintains resistance to slip transmission during hardening, but can generate microcracks across grain boundaries during the fracture process. We further show examples of how the model can be used to simulate void coalescence and three-dimensional crack fronts in polycrystals.