Effect of Competing Mechanisms on Fracture Toughness of Polycrystalline Metals Available to Purchase

Fracture toughness in ductile materials is the combined effect of the plastic dissipation and the energy spent in creating new surfaces. The design of polycrystalline metals with improved fracture toughness requires in-depth understanding of two levels of competitions: the competition between plastic deformation and crack formation as well as the competition between transgranular and intergranular fracture. Currently, no systematic approach exists to address the two competitions. The fundamental challenges lie in the difficulty in separating the two energy dissipations and inadequate knowledge about the correlation between fracture mechanisms and material fracture toughness. In this paper, a Cohesive Finite Element Method (CFEM) based multiscale framework is introduced to quantify the two levels of competitions. The fracture toughness of ductile materials is predicted by calculating the J-integral at the macroscale. The fracture surface energy for different type of failure mechanisms is evaluated through explicit simulation of crack propagation at the microstructure level. The calculations carried out here concern AZ31 Mg alloy. Results indicate that the mixed transgranular and intergranular failure can lead to optimized fracture toughness. Microstructures with refined grain size and grain boundary bonding strength can best promote the favorable failure mechanisms.