A Multi-Scale Design Framework for Tailoring Fracture Toughness of Polycrystalline Metals

Microstructure determines fracture toughness of materials through the activation of different fracture mechanisms. To tailor fracture resistance through microstructure design, it is important to establish relations between microstructure and fracture toughness. A three-dimensional multi-scale computational framework based on the CFEM (Cohesive Finite Element Method) is developed to achieve this goal. This framework provides a means for evaluating fracture toughness through explicit simulation of fracture processes involving arbitrary crack paths, including crack tip microcracking and branching. The calculation of fracture toughness for heterogeneous microstructures uses the J-integral, accounting for the effects of grain size, texture, and competing fracture mechanisms. A rate-dependent, finite strain, crystal plasticity constitutive model is used to represent the behavior of the bulk material. Cohesive elements are embedded both within the grains and along the grain boundaries to resolve intragranular and intergranular fracture. Initial anisotropy due to crystallographic texture (as quantified by the initial grain orientation distribution function (ODF) has a strong influence on crack tip deformation and therefore fracture toughness. Parametric studies are performed to quantify the effect of grain boundary cohesion and grain cleavage strength on the competition between transgranular and intergranular fracture. The fracture toughness is evaluated as a function of microstructure characteristics and constituent properties. The framework and the relations are useful for the selection of materials and the design of new materials with tailored fracture toughness.