Dynamic failure characteristics of primary coal-rock combination under hydrostatic pressure

The surrounding rock of deep roadway is weak and the fissures are developed. Under the influence of mining disturbance, it is easy to cause dynamic disasters and the overall instability failure of coal rock combination is one of key factors inducing disasters. In order to explore the dynamic damage characteristics of the primary combined coal rock body under impact load, with the help of the split Hopkinson pressure bar and high-speed camera, raw coal, artificial combination and primary combination of coal rock are studied. Based on different hydrostatic pressure and strain rate conditions, this study explores the dynamic stress-strain, crack evolution and fracture fractal characteristics of coal rock and explores the deformation and failure characteristics of coal rock using macroscopic fracture fractal and microscopic electron microscope scanning. The study shows that ① the dynamic stress-strain of raw coal, artificial combination and primary combination of coal rock is significantly nonlinear and the plastic stage of primary coal rock is similar to “plastic platform”. ② Under impact load, the strain rate effect of dynamic compressive strength of coal rock is significant and the dynamic compressive strength and hydrostatic pressure of raw coal, primary combined coal rock show a trend of first increasing and then decreasing. The critical values of hydrostatic pressure (low pressure strengthening and high pressure weakening) are 8 MPa and 10 MPa and the artificial combined coal rock and hydrostatic pressure show a positive correlation trend. ③ Under the impact load, the crack initiations of primary and artificial combined coal rock occur in the coal composition area far from the interface of coal rock. The combined coal-rock interface significantly affects the deformation and failure behavior of the sample and when the impact velocity≥10 m/s, the overall instability failure of coal rock components can occur in the original combined coal rock, while only the coal component failure occurs in the artificial combined coal rock under the same disturbance. Besides, only when the impact velocity≥14 m/s, the tip stress of the crack is greater than the strength of the coal rock, which can penetrate the artificial interface and lead to the overall crushing. The original interface has a “guiding effect” on the crack penetration of the sample, which is easy to induce crack propagation to form a macroscopic fracture surface. ④ The crushing degrees of raw coal, original coal and artificial coal rock increase with the increase of impact velocity and the crushing particle size tends to be granular and powder. Under the same disturbance, the degrees of fragmentation of coal components in three types of coal rocks show a relationship: primary-coal components > artificial-coal components > raw coal.