Fracture mechanical properties and influence mechanism of coal samples with different moisture contents under dynamic load

To investigate the fracture mechanical response of water-bearing coal under coupled dynamic loading and water pressure, coal samples from the No 22 coal seam of the Hanjiawa Coal Industry Company were selected as the research objects. An improved SHPB system, together with a crack propagation meter, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR), was used to conduct dynamic fracture tests on coal samples with different water contents. The microstructure and internal pore–fracture characteristics of coal samples were analyzed, and the mechanism of water-induced weakening was examined from a microscopic perspective. The results show that water has a significant influence on the microstructure of the coal samples. As water content increases, both the number and size of internal cracks increase, and water promotes the formation of secondary cracks. When the coal becomes saturated, its structural integrity deteriorates significantly. Higher water content leads to lower fracture toughness. Increasing water content causes most water-bearing coal to generate more secondary cracks. The development of numerous cracks inhibits further crack propagation, resulting in a significant reduction in propagation velocity and potentially causing crack arrest. Therefore, under dynamic loading, water significantly reduces crack propagation speed, and greater water content leads to smaller fragment sizes after failure. The fracture toughness of water-bearing coal exhibits strong rate dependence. Under dynamic loading, enhanced inertia and viscous stresses in water-bearing coal dominate the fracture process, and the fracture toughness of saturated coal may exceed that of dry coal.