Relationship between micro-pores fractal characteristics about NMR T2 spectra and macro cracks fractal laws based on box dimension method of coal under impact load from energy dissipation theory

The development and utilization of deep formation resources are easily disrupted by impact loads. To investigate what effect of impact on the pore structure and energy evolution of coal, the dynamic compression tests were performed by using the Split Hopkinson Pressure Bar (SHPB) test system. The fractal characteristics of macro cracks were analyzed by box dimension, the micro-pores structure and fractal features of coal samples were studied about nuclear magnetic resonance (NMR), which clarified the intrinsic relationship between fracture structure characteristics and energy dissipation. The results showed that with increasing impact velocity from 1.27 m/s to 4.90 m/s, the dynamic strength and peak strain increased by 85.11 % and 53.76 %, respectively. The fractal dimension of the cracks grew by 26.87 %, and the fractal dimension of pore network and full aperture decreases gradually. With increasing impact velocity, the fracture dissipation energy and energy dissipation rate of coal samples increase exponentially. As the energy dissipation rate increases, the cracks fractal increases in a quadratic function relationship and the pores fractal decreases continuously. Low-velocity impacts induce dislocation plugging between coal matrix crystals, while impact effect causes more dislocations to form stress concentrations at pore tips. When the energy accumulation reaches its maximum value, the content of mesopores and macropores together with the pore connectivity increases. Instantaneous disturbance creates more macroscopic fracture surfaces in the coal, resulting in large-scale fracture instability. This research findings will provide some theoretical foundations to understand the formation mechanism of dynamic disasters in deep mines.