Cracking process and acoustic emission characteristics of sandstone with two parallel filled-flaws under biaxial compression

Discontinuities widely exist in natural rocks. To investigate the progressive micro-cracking process and failure mechanism of fissured rocks, a series of biaxial compression tests were conducted on sandstone specimens containing two parallel filled flaws using acoustic emission (AE) analysis synchronized with digital image correlation (DIC) monitoring. Experimental results show that the peak strength and elastic modulus of sandstone decrease first and then increase with the change in the ligament angle from 0° to 150°, achieving a minimum at 60°. The flaws remarkably facilitate crack coalescence under low lateral stress, such as at 2.5 MPa and 5 MPa. However, with an increase in lateral stress to 10 MPa, the crack coalescence is less influenced by the presence of pre-existing flaws. The AE events produced by flawed sandstone during the loading process conform to the Hurst statistical law. Fractal analysis shows that the lateral confinement reduces the irregularity of ultimate fracture geometry. Based on the AE dominant frequency features, the micro-tensile cracks, micro-shear cracks and micro-tensile-shear cracks are distinguished. The results show that with an increase in lateral stress, the percentage of micro-tensile cracks are constrained, but the number of micro-shear and mixed tensile-shear cracks increases. In addition, the micro-shear cracks preferentially appear in flawed sandstone specimens under high lateral stress as compared with specimens subjected to low stress.