Test study of sandstone cracking and propagation process under compressive-shear stress

Using self-developed meso-shear test equipment for coal and rock containing gas and PCI-2 acoustic emission testing and analytical system, the compressive-shear tests of sandstone at normal stresses with 0.0, 1.5, 3.0, 4.5, 6.0 MPa under the shear load condition were carried out. The deformation and AE characteristics were studied, and the mesoscopic feature of the cracks' evolution was analyzed. The data shows that as normal stress increases, the shear strength increases, and the peak shear force and corresponding peak displacement increase linearly. Under the condition of different normal stresses, the rock has some distinctions on shapes or appearances. However, they basically extend along the expected shearing surface. As normal stress increases, the crack initiation and propagation will get later, the instability and failure of the rock become more difficult. By the impacts of normal stresses, the AE event rate doesn't reach the maximum value along with the shearing stress increases to the peak value. The fact is that the AE event rate increases rapidly after the shear stress reaches the peak value, also with the tiny cracks spread suddenly. With these two conditions, the AE event rate finally reaches the maximum value, then the rock unstable failure had been destroyed after those cracks went through it. As normal stress increases, the friction of particles between two sides of shear surface also increases. Meanwhile, the major crack is wider and the shapes of cracks become more complicated due to effects of the unbalanced stress distributions and also the internal structures heterogeneity of materials.