Experimental research on evolution of surface cracks in coal mass under impact load

ZHAO Hongbao; ZHANG Huan; WANG Zhongwei; JU Nansong; WEI Ziqiang

Abstract：In this experiment,briquette samples and raw coal sample were taken as research objects,a self-developed drop hammer impact loading experimental device,three-dimensional automatic measurement and control experimental device for micro structure evolution based on meso analysis and three-dimensional micro monitoring software of coal rock surface microstructure dynamic evolution were used as main devices to investigate the crack propagation and evolution of coal samples under impact load.The results show that the cracks with a sharp angle distribution from top to bottom to the impact direction appear on the surface of two kinds coal samples under under impact load.The shape of surface cracks show serrated border,and manifests transcrystalline and bypassing the crystal mode at the same time when the crack growth meets the hard particles.The evolution of surface cracks may be bifurcated and shown as S type on the whole.Compared with briquette samples,the frequency of change of surface cracks,roughness of shape,frequency of bifurcation and through the crystal mode of raw coal is lower.As the increase of impact times,the evolution law of surface cracks width of coal samples in the same position shows progressive mode and abrupt mode,and the direction of crack propagation representes as three paragraph style that "high speed change-gentle development-rapid change".Hard particles impede the evolution of surface cracks,and original cracks plays two roles in the evolution of surface cracks,in addition,strong randomness of initiation position and propagation path of surface crack is caused by heterogeneity of coal.A function containing five parameters determining the crack growth path of surface cracks is proposed.The fractal dimension of surface cracks for raw coal samples is larger than briquette samples under the same impact effect.The fractal dimension of surface cracks decreases as the increase of impact load,and the difference of fractal dimension between each crack is gradually reduced.