Role of Pre-existing Microcracks and Spatial Characteristics in Controlling Rock Fracture under Indentation: A Grain-Based DEM Study

Achieving efficient and controlled rock fracture remains a core challenge in deep energy development, critical mineral extraction, and next-generation underground infrastructure. In brittle rocks, macroscopic failure emerges from the multiscale interaction between structural defects and externally applied loading. This study investigates how statistically defined pre-existing microcracks (PEMs) govern the initiation and propagation of tool-induced cracks (TICs), which collectively determine the mechanical and energy response of rocks under rock indentation. A grain-based discrete element method (DEM), combined with a micro-discrete fracture network (μDFN) framework, is developed to explicitly incorporate realistic PEM networks based on experimental microstructural measurements. By systematically varying PEM density, orientation, and distribution depth, we uncover critical thresholds and transition regimes in TIC evolution and failure behavior. A critical PEM density (P 21 ≈ 0.3 mm/mm 2 ) marks a shift from abrupt, stress-driven rupture to a structure-controlled, cooperative fracture regime. Optimal PEM orientations (e.g., 45°) promote shear-driven coalescence and energy-efficient damage growth, while spatial depth variation leads to a functional division: shallow PEMs trigger early failure, and deeper PEMs enhance lateral connectivity and stress homogenization. This study demonstrates that the spatial architecture of PEM networks exerts first-order control on failure mode, fracture morphology, and energy dissipation. The findings offer new micromechanical insights into defect-governed rock failure and establish a predictive basis for engineering high-efficiency, low-energy rock-breaking technologies in deep and complex geological environments. • A grain-based DEM-μDFN model is developed to explicitly simulate pre-existing microcrack networks in rock. • Critical PEM density thresholds control the transition from stress-driven to structure-controlled fracture under indentation. • PEM orientation influences crack coalescence and energy-efficient fracture propagation. • Spatial PEM distribution depth causes functional partitioning of early failure initiation and lateral connectivity enhancement.