Dynamic tensile behavior and failure mechanism of coal samples under water rock coupling

The dynamic mechanical behavior of water-bearing coal seams is critical for ensuring structural stability in deep underground engineering, yet the influence of moisture content on tensile strength and fracture mechanisms under dynamic loading remains poorly understood. This study investigates the dynamic tensile properties and crack propagation mechanisms of coal specimens with varying water contents (0%, 1.7%, 3.4%, and 5.1%) using the Split Hopkinson Pressure Bar system combined with digital image correlation (DIC) analysis. Unlike previous studies focusing solely on quasi-static conditions, this work reveals a strain-rate-dependent transition in tensile strength: while increasing moisture content generally reduces dynamic strength at low strain rates, the Stefan effect enhances the strength of saturated coal (w = 5.1%) compared to partially saturated samples (3.4%) at higher strain rates. Furthermore, moisture reduces coal brittleness, increasing flexibility, and decreasing fragmentation. DIC analysis demonstrates that dry coal exhibits larger crack openings post-failure, whereas saturated coal shows smaller displacement gradients, suggesting water-induced crack-tip blunting. Microstructural observations confirm that pore water not only weakens coal but also influences crack propagation in a strain-rate-dependent manner. Importantly, energy dissipation analysis reveals that higher moisture content reduces energy consumption for crack formation and propagation, providing critical insights for predicting coal seam stability under dynamic loading. These findings advance the understanding of water-weakening effects in deep mining and offer practical implications for mitigating dynamic hazards in water-rich coal seams.