Effect of competing mechanisms on fracture toughness of metals with ductile grain structures

The fracture toughness of ductile materials depends on the combined effect of plastic dissipation in the constituents and energy spent on creating new crack surfaces. The design of polycrystalline metals with improved fracture toughness requires in-depth understanding of two levels of competing mechanisms: the competition between plastic deformation and crack formation as well as the competition between transgranular and intergranular fracture. Currently, no systematic approach exists to analyze the effects of the two competitions. The fundamental challenges lie in the difficulty in separating the two forms of energy dissipation and inadequate knowledge about the correlation between fracture mechanisms and material fracture toughness. In this paper, a multiscale framework based on the Cohesive Finite Element Method (CFEM) is developed to quantify the two levels of competitions and to predict the fracture toughness of ductile materials by calculating the J-integral at the macroscale. The fracture surface energy for the crack paths associated with different types of failure mechanisms is evaluated through explicit simulation of crack propagation at the microstructure level. The calculations carried out here concern the AZ31 Mg alloy, but the overall approach applies to other materials as well. Results indicate that a proper balance between transgranular and intergranular failure can lead to optimized fracture toughness. Microstructures with refined grain sizes and balanced bonding strength in grains and grain boundaries can best promote the manifestation of favorable failure mechanisms, and as a result, enhance fracture toughness.