Abstract: In order to investigate the dynamic behaviors of coal and its deterioration law under different initial gas pressures, this study conducted impact compression experiments on gas-bearing coal by using the self-developed visualized gas-bearing coal rock dynamic and static combined loading test system, analyzed the expansion and evolution law of internal fracture in gas-bearing coal by combining with CT scanning system. Moreover, this study also quantitatively characterized the mesoscopic damage degree based on the increment of internal fracture rate of coal samples impacted under different initial gas pressures, and explored the deterioration law of macroscopic mechanical parameters of gas-bearing coal under the impact load. Some conclusions are drawn. (1) The dynamic stress-strain curves of gas-bearing coal under impact loading, which can be divided into linear elastic phase, plastic hardening phase and damage phase, have no obvious compaction phase. And it is found that the peak strength, peak strain and elastic modulus of impacted coal samples deteriorate with the increase of initial gas pressure. (2) Gas aggravates the expansion and coalescence of fractures inside the coal. CT scanning results indicate that the impact damage mode of gas-bearing coal is mainly splitting and stratified splitting damages, and the more significant the two damage modes are with the increase of initial gas pressure, so does the number of fractures and their damage degree inside the coal, which makes the spatial fracture network more complex. (3) The damage variables are defined on a mesoscopic level, and the values of damage variables show a rise of quadratic function with the increase of initial gas pressure. The comparison of the dynamic strength of coal under impact load with the theoretical strength obtained by defining the degree of damage by fracture rate increment verified the rationality of the damage variables defined by fracture rate increment of coal at the mesoscopic level. The intrinsic connection was constructed between the mesoscopic deterioration of gas-bearing coal and the loss of macroscopic parameters. The research results enrich the basic theory of gas-bearing coal dynamics and provide a theoretical reference for the prevention and control of coal-rock-gas composite dynamical hazards in mines.

Key words: gas-bearing coal, dynamic properties, industrial CT scanning, fractures, damage variables