Relation between crack propagation and internal damage in sandstone during shear failure

Rock fracture mechanics have been widely applied to earthquake mechanics, hot dry rock geothermal energy extraction, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, and many other practical problems. It is important for geological disaster identification and prevention to clarify the crack propagation mechanism in rock failure process. To further investigate the evolution of crack propagation and internal damage in brittle rock, and based on the way that rock mainly occurs shear failure, we conducted direct shear tests on sandstone. Our tests combined macroscopic/mesoscopic observation and acoustic emissions monitoring. We found that crack propagation under compressive-shear stress occurs in stages, including the initiation and propagation of tension cracks, which are then connected by shear cracks. As normal stress increases, the length of a single tension crack becomes shorter, and the cracks themselves increase in number. We found that tension crack propagation is accompanied by fewer microcracks; in contrast, shear crack propagation is accompanied by considerable microcracks. This process may induce a drop off in mineral particles through slipping, and it may ultimately produce a rock bridge. The main damage occurs after shear cracks appear during shear failure, especially when the normal stress increases.