Dynamic fracture properties, fragmentation characteristics and failure mechanisms of coal samples subjected to water-ice phase transitions

• An orthogonal temperature–moisture matrix (20 to −20°C; 0–9% moisture) was designed to capture water–ice phase-transition effects in coal under SHPB loading. • Dynamic compressive strength and elastic modulus increase overall with decreasing temperature, showing pronounced strengthening in water-bearing coal at sub-zero conditions. • Across 0°C, energy partitioning and fragmentation reverse: water softening elevates dissipated energy and fragment fractal dimension, whereas pore-ice filling increases transmitted energy and suppresses fragmentation. • A normalized damage variable based on the maximum dynamic elastic modulus quantifies phase-transition-driven damage under coupled temperature–moisture conditions. • A normalized damage variable based on the maximum dynamic elastic modulus, together with high-speed imaging, XRD and SEM evidence and a particle–water/ice adhesion-force framework, explains the competing water-weakening and ice-cementation mechanisms. To investigate the dynamic mechanical properties and failure mechanisms of coal subjected to the water–ice phase transition, coal samples collected from the Xiwan open-pit coal mine in Yulin, Shaanxi Province, China, were tested using an orthogonal experimental design combining five temperature levels (20, 10, 0, −10, and −20°C) and four moisture contents (0%, 3%, 6%, and 9%). Dynamic uniaxial compression tests were conducted on a Split Hopkinson Pressure Bar (SHPB) apparatus under a constant impact pressure of 0.5 MPa, and the dynamic stress–strain responses and associated energy-evolution laws were obtained under different temperature–moisture combinations. Meanwhile, high-speed camera observations, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were integrated to interpret the failure mechanisms driven by the water–ice phase transition from macro- to microscale perspectives. The results indicate that both the dynamic compressive strength and the dynamic elastic modulus exhibit an overall increasing trend as the temperature decreases from 20°C to −20°C. The water–ice phase transition is the dominant factor governing the differences in the dynamic mechanical properties of coal: at room temperature and 0°C, the water-induced softening effect and water–rock interactions deteriorate the dynamic mechanical properties and aggravate fragmentation, manifested by a decreased brittleness index ( B I ), a prolonged pre-peak yielding stage, an increased fragment fractal dimension, reduced transmitted energy, and elevated dissipated energy; conversely, under sub-zero conditions, pore water freezes into ice, and pore-ice infilling provides pronounced supporting and cementation effects within the coal skeleton, shifting the failure mode toward brittle failure, as evidenced by an increased B I , a shortened pre-peak yielding stage, a reduced fractal dimension, increased transmitted energy, and reduced dissipated energy. In addition, a damage variable ( D ) was defined based on the maximum dynamic elastic modulus to quantitatively characterize the damage degree induced by the water–ice phase transition. Finally, a mathematical model incorporating interparticle forces and water/ice–particle adhesion forces was established, and SEM fracture morphologies were used in conjunction to further elucidate the micromechanical damage-evolution mechanism of coal under the water–ice phase transition.