Water effect on energy evolution and fractal characteristics of coal samples under impact loading

Abstract

The dynamic tensile strength of the coal is critical to the initiation and evolution of dynamic disasters such as coal bump and coal and gas outburst. To investigate mechanical and geometrical characteristics of the coal bump subjected to the variation of the shock pressure and water content, a split Hopkinson pressure bar (SHPB) with ultra-high-speed camera and fractal dimension were employed. The results show that the stress wave is the main controlling factor of a large number of micro damage structures and damage evolution of primary pores and voids in coal samples under impact load, and the coal rock fragmentation is a process of energy absorption and dissipation. With the increase of impact load, the dissipated energy density of coal samples increases linearly, but when the incident energy is small, the dissipated energy density of coal samples has little difference; The fractal dimension of samples increases with the increase of loading pressure, and the increasing rate has a decreasing trend. Under the same loading pressure, the fractal dimension of saturated coal sample is the largest, and that of dry coal sample is the smallest; The deformation and failure of coal samples are mainly tensile splitting, and the failure cracks develop along the loading direction, first in the middle of the disc, and then multiple secondary micro cracks are initiated. It is found that there are several main strain concentration regions in the middle of saturated coal sample under impact load, and the range gradually expands, and finally develops along the radial direction.