Experimental investigation of damage evolution and fracture mechanism in rocks with a single flaw under stepwise cyclic compression

This study comprehensively investigates the damage and fracturing behaviors of sandstone specimens containing a single flaw under stepwise cyclic loading using digital image correlation (DIC) and acoustic emission (AE) techniques. The degradation of rocks is characterized by the evolution of residual strain, energy density, and cracking behaviors of flawed specimens while considering the effect of flaw inclination angle on the mechanical properties and fracturing behaviors of rocks. Experimental results reveal that residual strain gradually increases with an increasing number of cycles, and the increase in stress level induces a sudden rise in both elastic and dissipated energy density. The dissipation factor decreases initially and then reaches a constant value as the upper-stress limit increases. Moreover, the energy dissipation behavior becomes more consistent among the five cycles as the stress levels increase. Tensile wing cracks propagate stably during the stepwise cyclic loading process, accompanied by scattered low-amplitude AE events and a linear increase in cumulative AE counts. The analysis of normal and shear displacements indicates that wing cracks are primarily tensile, with significant normal opening displacements and negligible shear displacements. Horsetail cracks and anti-wing cracks initiate within fan-shaped strain zones of great size, driven by high compressive-shear stress, and rapidly propagate in the last one or two stress levels, leading to the detection of abundant high-amplitude AE events. Horsetail cracks and anti-wing cracks exhibit comparable displacement jumps in both normal and tangential directions, suggesting a mixed tensile-shear mode of crack propagation.