Failure characteristics of fractured sandstone with varying rock bridge angles subjected to combined dynamic and static loading

Deep underground rock masses with structural defects often experience combined dynamic and static stress conditions. This study systematically examines the multi-scale failure mechanisms of fractured red sandstone subjected to combined loading by varying the rock bridge angle ( α ). A custom dynamic-static superposition system featuring an anti-rebound mechanism was employed to provide pure, uninterrupted dynamic pulses. Synchronized multi-field characterization suites, comprising acoustic emission (AE) and digital image correlation (DIC), were utilized to monitor the real-time progression of damage evolution. A dimensionless competition factor χ and a friction-corrected effective Mode-II stress intensity factor ( K II eff ) are formulated to define the mechanical transition from crack-tip tensile-shear regimes to rock-bridge pure shear regimes. Results indicate that as α increases from 0° to 90°, the peak strength demonstrates a variable decline, while the primary damage zone consistently shifts from the tips of flaws to the connecting ligament. Furthermore, an analytical model that incorporates a dynamic participation coefficient λ ( α ) is proposed. This model effectively forecasts a non-linear "Dynamic Strengthening Effect" specifically at α = 90°, where transient kinematic adjustments reformulate the primary load-transfer paths. Complementary PFC2D discrete element modeling accurately replicates the macro-mechanical profiles, confirming the localized stress-transfer pathways. These findings establish a strong theoretical and experimental foundation for evaluating stability in deep underground engineering.