Mechanical response and microscopic damage mechanism of pre-flawed sandstone subjected to monotonic and multilevel cyclic loading: A laboratory-scale investigation

This study aims to investigate the mechanical response and acoustic emission (AE) characteristic of pre-flawed sandstone under both monotonic and multilevel constant-amplitude cyclic loads. Specifically, we explored how coplanar flaw angle and load type impact the strength and deformation behavior and microscopic damage mechanism. Results indicated that being fluctuated before rising with increasing fissure angle under monotonic loading, the peak strength of the specimen first increased slowly and then steeply under cyclic loading. The effect of multilevel cyclic loading on the mechanical parameters was more significant. For a single fatigue stage, the specimen underwent greater deformation in early cycles, which subsequently stabilized. Similar variation pattern was also reflected by AE count/energy/b-value. Crack behaviors were dominated by the fissure angle and load type and medium-scale crack accounted for 74.83%–86.44% of total crack. Compared with monotonic loading, crack distribution of specimen under cyclic loading was more complicated. Meanwhile, a simple model was proposed to describe the damage evolution of sandstone under cyclic loading. Finally, SEM images revealed that the microstructures at the fracture were mainly composed of intergranular fracture, and percentage of transgranular fracture jumped under cyclic loading due to the rapid release of elastic energy caused by high loading rate.