Modelling the Transition from Fracture to Granular Flow

As a brittle solid is loaded, crack growth leads to fracture and fragmentation. The system then transitions to granular flow where these fragments continue to break down into smaller grains in a process known as comminution. To explore this transition, we created a discrete element model of an isotropic, brittle material. The solid is modelled as a disordered packing of locally interacting spheres. Interactions include an attractive and a repulsive pairwise force as well as a three-body angular stiffness. If stretched a critical distance, the attractive and angular interactions will break. The relative strength of interactions can be tuned to control the material’s elastic response (Poisson’s ratio) as well as the ratio of mode 1 to mode 2 fracture toughness. We use this model to explore how material properties, initial defect density, and strain rate affect both the transition to granular flow and the resulting rheology and comminution. We track the stress response of the system, the spatial and temporal locations of crack growth, and the evolution of the grain size distribution. The grain size distribution shows power-law behavior and with increasing strain rates one can identify a decreasing maximum grain size.