Microscale Characterization of Fracture Growth and Associated Energy in Granite and Sandstone Analogs: Insights Using the Discrete Element Method

We employ the Discrete Element Method to analyze the micromechanical response of numerical analogs of sandstone and granite to unstable failure. Calibrated particle-based models of sandstone and granite are subjected to biaxial experiments under confining pressures of 0-50 MPa, leading to the development of shear fractures through interactions of microcracks occurring in shear and tensile modes. We document the mode and energy associated with emergent microcracks to analyze fracture growth patterns and quantify fracture energy. Shear fracture growth in our sandstone analog occurs through cooperative interaction between shear and tensile microcracks, with shear microcracks accounting 4-44% of total microcracks and 31-92% of fracture energy. Shear microcracking increases with confining pressure resulting in an increase in fracture energy, and a transition from dilatant to compactant fracture zones in our sandstone models. Shear fracture growth in our granite analog occurs through coalescence of tensile microcracks, which account for 96-98% of total microcracks and 87-93% of fracture energy. Tensile microcracking increases with confining pressure, resulting in an increase in fracture energy and formation of dilatant fracture zones in our granite models. Our simulations show that fracture energy increases with confining pressure, accounting for 10-15% of the total input mechanical energy in sandstone versus 16-47% in granite. We estimate that the work done against friction from intergranular and fracture sliding accounts for 69-86% of total input energy in our sandstone analogs and 46-81% in our granite analogs. Our results indicate that frictional deformation during fracture is a significant component of the energy budget.