Mesoscopic cracking and expansion of double sheared sandstone under compressive-shear stress

The mesoscopic test device developed studying the crack process of coal and rock under double shear was used in the shear test at different stress conditions. The acoustic emission and image processing techniques were employed to measure the mesoscopic cracking propagation process and the mesoscopic cracking extension space distribution. For the double shear test under compression and shearing, the damage of the surface of the specimen is not visible in most of the period of loading process and the surface cracks occur just before the shear stresses reach the peak value. When the shear stresses are nearly reaching the peak, the rate of Hit has a sharp increase indicating that there are already cracks inside the sandstone, while on the surface there are no cracks, and the tiny cracks can only be seen after a period of time. Presumably, the cracks developed from inside to outside. After the macroscopic damage, the left and right sides of cracks show a splayed or trapezoidal distribution. Because the internal shear failure of rock mass is mainly caused by the vertical shear force and the sand grains possess the different geometric and physical properties, the main crack along the shear plane extended irregularly. Cracks always occur in the edges of mineral particles like quartz, feldspar etc which are the weakest links of sandstone. As the shear stress level increases, the cracks will cross over the mineral particles and form transgranular fractures. This phenomenon is often seen in quartz grains because of its specific relevant microstructure features.