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Abstract Using self-developed microscopic shear testing device for coal rock，the evolution process of shear microcracking，expansion and macroscopic failure of gas-containing coal was studied at normal stresses of 0，2 and 4 MPa under compression-shear load condition. The results show that cracks appear apparently before the peak shear stress，and the shear stress has a apparent sharp decline process after the peak shear stress. This process corresponds to the main crack coalescence，which is observed by charge-coupled device(CCD) camera. However，shear failure does not happen immediately and the coal still has a certain bearing capacity. Subsequently，the shear stress decreases slowly with the time. Accompanied by the shedding of coal，the fractured zone appears in the upper part of the coal in the process of shearing. Simultaneously，the smaller the normal stress is，the more serious the damage of the coal is. In that case，more obvious cracks appear. The cracking tips appear in the upper and lower parts of raw coal. The cracking tips appear in the upper and lower parts of raw coal. Because of the effect of original cracks in the raw coal，the main crack direction moves tortuously forward along the shear direction，but dose not coincide with shear spreading direction.
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Received: 05 January 2012
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2012, Vol. 31 
