Mode I fracture at mineral grain interfaces in granite under static loading: Insights from a microscopic perspective

The static fracture behavior of granite is governed by mesoscopic heterogeneity, particularly the interactions at mineral grain interfaces (MGIs). To investigate crack propagation mechanisms, meso -scale four-point bending tests were conducted on granite from Zheduo Mountain, Kangding, China, using a single-MGI mechanical testing system. The study focused on the influence of MGIs on crack deflection and competing energy dissipation modes. Mode I fracture toughness was measured at prefabricated MGIs in 36 specimens. Based on the toughness data and energy release rate theory, a crack path deflection model was developed using maximum tensile stress and energy competition principles. Results show: (1) Crack paths are controlled by energy dissipation competition. Cracks tend to follow MGIs (e.g., quartz–feldspar interfaces), where energy is dissipated through friction and inelastic deformation. Intragranular propagation through quartz requires more energy, resulting in large-angle deflection. (2) Fracture toughness at MGIs is more scattered than in carbonaceous slate, driven by heterogeneity in grain size, shape, hardness, and arrangement. Coarse quartz grains act as stress barriers, increasing variability. (3) The model achieves deflection prediction errors below 10% and captures continuous crack paths. Compared with slate, granite exhibits stronger deflection due to greater mineralogical heterogeneity. This study offers theoretical and experimental insights into predicting crack trajectories and energy release behavior in polymineralic rocks.