Void growth yield criteria for intergranular ductile fracture

Ductile fracture through growth and coalescence of intergranular cavities is a failure mode observed experimentally in many metallic alloys used in industrial applications. Simulation of this fracture process in polycrystalline aggregates requires modeling of the plastic yielding of porous boundaries. However, classical yield criteria for porous materials such as the Gurson–Tvergaard–Needleman model and its current extensions cannot account for the complex coupling between loading state, crystallographic orientations, void shape and material behavior at grain boundaries. In order to bridge this modeling gap, two yield criteria for intergranular ductile void growth are proposed. The first one is a GTN-like model derived from limit-analysis which, once calibrated, accounts for spherical voids at the interface of rate-independent crystals. The second one is developed from a variational approach and predicts yielding in viscoplastic crystals containing intergranular ellipsoidal cavities. Both models are validated against a wide database of numerical limit-analysis of porous bi-crystals using a FFT solver. Satisfying agreements are obtained, paving the way to microstructure-informed intergranular ductile fracture simulation. The interplay between plastic yielding inside grains and along grain boundaries is finally studied based on the proposed yield criteria.