(1. State Key Laboratory of Coal Mine Disaster Dynamics and Control，Chongqing University，Chongqing 400044，China； 2. State and Local Joint Engineering Laboratory of Methane Drainage in Complex Coal Gas Seam，Chongqing University，Chongqing 400044，China)
Using independent developmental coal-rock microscopic shear testing device，the shear tests under different loading rates were carried out. An in-depth research on micro-crack propagation process and micro-crack extension space distribution was conducted by the acoustic emission technique as well as image processing technique. The result shows that the shear strength of sandstone increases with the increase of the loading rate，and presenting a linear relation. For double sheared sandstone，crack propagation and penetration on both sides are not synchronized，and the growth of micro fissure on the one side became more rapid because of the stress concentration caused by the crack damage on the other side. The formation time of penetrating main crack shortened greatly. The micro-crack initiation positions which were generally located at a point or points near the pre-shear plane extended in the vertical direction substantially. Meanwhile，the closer micro-cracks interacted with each other until penetrating to a whole crack. During the process of micro-crack propagation of sandstone，the vertical micro-cracks turned into lateral bifurcation cracks，and the micro-cracks which are perpendicular to the vertical direction stopped expansion. The failure modes are detour，pinning，holding bifurcation，induced bifurcation，hack，stop detour，hook back etc.，mainly detour and pinning.
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2015, Vol. 34 
