Experimental research on dynamic characteristics of coalrock combined specimen

To explore the dynamic mechanical characteristics of the bearing structure of the coalrock combined body, a separated Hopkinson pressure bar (SHPB) test device combined with an ultrahighspeed camera system was used to carry out the impact compression test on the coalrock combined body formed by the layered combination of four different rock samples and the same coal sample. The stress wave propagation, dynamic stressstrain relationship, dynamic evolution of cracks, energy dissipation law, and failure characteristics of the coalrock combined body in the case of stress waves entering coal from the rock were compared and analyzed. The results show that the influence of the difference in the wave impedance matching effect between the rock and the incident bar on the propagation of the stress wave gradually weakens with the increase of the impact velocity. The maximum dynamic peak stress and strain of the coalrock combined body have obvious strainrate effects. Besides, with increased impact velocity, the incident energy increases linearly, the reflected energy proportion decreases linearly, and the absorbed energy proportion and the growth rate of the maximum dynamic peak stress gradually drop, changing approximately as a power function. Most of the destruction of macroscopic cracks in the coalrock combined body occur at the coal or rock ends with relatively low strength and far away from the coalrock interface. When the crack tip stress at the coalrock interface is greater than the strength of “weakened” coal or rock, the cracks will cross the coalrock interface and continue to develop. When the coalrock combined body is damaged by impact loading, the impact velocity influence on the degree of breakage of the sample is greater than rock strength influence and the rock strength effect is greater than the wave impedance matching effect on the degree of breakage of the sample. Under the same stress wave, as the strength of the rock increases, the failure degree of coal gradually increases, the broken particles gradually transition from massive to powder and the rock mode changes from splitting failure to shear failure. As a result, the average particle size of broken coal blocks decreases, and the fractal dimension of coalrock combined body increases gradually. The research results provide basic research for the control of surrounding rock of roadway under dynamic pressure.