Fracture Evolution and Mesoscopic Damage in Granite: A Macro–Mesoscopic Constitutive Model Under True Triaxial Stress

This study develops a macro–mesoscopic constitutive model to investigate fracture evolution in granite under true triaxial stress. Granite is modeled as a binary composite of bonded and frictional media, with microcrack propagation driving transitions between phases. Mesoscopic flaw evolution is tracked using CT scanning, enabling quantification of flaw volume and distribution during loading. A homogenization-based framework links mesoscale damage to nonlinear macroscopic behavior, explicitly capturing the influence of intermediate principal stress. A novel parameter, the flaw closure degree, is introduced to characterize compaction, providing a direct indicator of internal damage. The model accurately reproduces stress–strain responses and failure modes observed in tests, overcoming the limitations of phenomenological approaches. By incorporating fracture mechanics and internal flaw dynamics, this work offers a predictive tool for assessing structural integrity in brittle rock systems subjected to complex stress paths.