Role of Flaw Symmetry in Microscopic Damage Evolution and Asymmetric Crack Propagation in Fractured Rocks: A Grain-Based Modeling Study

Internal fractures in engineering rock masses generate intense local stress concentrations that trigger catastrophic instability. Understanding how structural symmetry governs stress redistribution and damage evolution is crucial for geotechnical stability prediction. To elucidate these mechanisms, uniaxial compression tests were performed on rock specimens containing prefabricated symmetrical V-shaped flaws with included angles of 30°, 70° and 110°. Acoustic emission (AE) monitoring and digital image correlation (DIC) were simultaneously employed to track transient microcracking and surface strain localization. Furthermore, a grain-based model (GBM) within a discrete-element framework was developed to capture polycrystalline heterogeneity and mesoscopic fracturing. The macro- and mesoscopic analyses demonstrate that although the flaw geometry is initially symmetrical, local stress concentration at the flaw tips induces stress symmetry breaking, ultimately triggering asymmetric microcrack initiation and progressive strain localization. Smaller included angles amplify this stress asymmetry, leading to crack initiation at lower stress thresholds and earlier AE activation. By capturing grain-scale failure modes, the GBM uncovers the mechanical transition from initial geometrical symmetry to asymmetric failure evolution. These findings elucidate the nonlinear mesoscopic damage mechanisms of rocks containing complex flaws and provide a solid theoretical basis for instability evaluation in geotechnical engineering.