Study on dynamics characteristics and energy dissipation laws of coal fracture under low−speed impact load

To explore the dynamic characteristics and energy dissipation laws of coal fracture instability under varied impact loads, using an improved Hopkinson bar (SHPB) test system was used to conduct one-dimensional impact load, impact load and axial static load coupling dynamic tests. The dynamic characteristics of coal samples under different impact speeds and axial static loads were studied, analyzing the macroscopic fracture morphology and pore evolution of coal samples were analyzed, the mechanism of coal rock fracture instability from the perspective of energy dissipation. The research results indicate that the stress-strain curves of coal samples under different impact load disturbances all include three stages: linear elasticity, plasticity, and plastic softening. Under one-dimensional impact load, the peak strength and peak strain of coal samples exhibit significant strain rate effects. As the strain rate increases, the peak strength and peak strain of coal samples gradually increase; Screening statistics show the mass distribution of broken fragments with different particle sizes. As the strain rate increases, the mass of larger particles decreases while the mass of smaller particles increases; The incident energy, reflected energy, and dissipated energy of coal samples gradually increase with the increase of impact load, and the energy dissipation density increases exponentially; Under the coupling effect of impact load and axial static load, a low-speed impact load of 5.54 m/s is set. As the axial static load increases, the peak strength continues to increase and the peak strain linearly weakens; Based on the characterization of pore structure using nuclear magnetic resonance (NMR) experiments, the internal micropores of coal samples continuously develop with the increase of axial static load, and the expansion trend of cracks along the axial direction increases; The incident energy of coal samples remains stable with the continuous increase of axial static load, while the reflection energy decreases and the dissipation energy gradually increases. The energy dissipation density increases linearly. According to the energy dissipation mechanism of coal rock, the initiation, expansion, and penetration of pores induce the occurrence of fracture and instability in coal; In the initial stage of axial static load, more dissipated energy was used for the development of internal micropores and crack expansion. Under the instantaneous disturbance of impact loads, the formation of macroscopic fracture within the coal is induced, which ultimately results in large-scale fracture instability. Under the instantaneous disturbance of impact load, it will induce the formation of macroscopic fracture surfaces in the coal body, resulting in large-scale fracture instability.