Experimental investigation on dynamic mechanical characteristics and fracture mechanism of coal under repeated impact loads

In underground coal mining, frequent dynamic disturbances adversely affect the stability of coal pillars. Thus, it is essential to understand the dynamic mechanical behaviour of coal under repeated impact loads. In this paper, a cone-shaped striker bar with a length of 200 mm was designed for a split Hopkinson pressure bar (SHPB) system, and repeated impact experiments of coal were carried out with this system. Based on three-wave theory, original voltage signals were recorded during the tests and then processed to study the changes in the dynamic stress-strain curves with the number of impacts. Relationships between the dynamic compressive strength, peak strain, and the number of impacts were analysed. Meanwhile, the failure mode and fracture mechanism of coal under repeated impact loads were discussed. It was found that as the number of impacts increases, the peak strain increases but the dynamic strength decreases. Furthermore, the studied coal specimens present two typical failure modes under these conditions, including axial splitting failure and hoop tensile failure. Based on the ultimate fracture modes of the specimens, two crack group failure models are established to reveal the fracture mechanism of coal under repeated impact loads. This study can make a deep understanding about the failure mechanism of coal pillar’s instability induced by impact load.