An experimental investigation on strength, deformation and crack evolution behavior of sandstone containing two oval flaws under uniaxial compression

To increase the understanding of strength and failure mechanism of fractured rock, a series of uniaxial compression tests on red sandstone specimens containing two pre-existing oval flaws was carried out using a rock mechanics servo-controlled testing system. On the basis of the experimental results, the influences of the coplanar flaw angle and ligament angle on the mechanical parameters and fracture process of sandstone specimens containing two oval flaws were analyzed in detail. The mechanical parameters including peak strength, peak strain and elastic modulus of pre-flawed specimens are all much lower than those of the intact specimens. For the specimens containing two coplanar oval flaws, the peak strength is distinctly related to the coplanar flaw angle, while the peak strain and elastic modulus are not obviously dependent on the coplanar flaw angle. For the specimens containing two non-coplanar oval flaws, the peak strength, peak strain and elastic modulus all have a nonlinear relationship with the ligament angle. By adopting photographic and AE monitoring techniques, the crack initiation, propagation and coalescence process and the AE evolution behavior of sandstone specimens were all observed and characterized. The stress drops in the stress-time curve correspond to a sudden increase in accumulated AE counts in the accumulated AE-time curve, resulting from the initiation and coalescence of cracks. The ultimate failure modes of sandstone specimens containing two pre-existing oval flaws are a mixture of several cracks (wing crack, anti-wing crack, secondary crack, shear crack and far-field crack and surface spalling), which depend on the flaw geometry. The failure patterns of pre-flawed specimens can be identified as the following four distinct modes: no crack coalescence failure, indirect crack coalescence outside the bridge area failure, single crack coalescence inside the bridge area failure and tensile crack coalescence outside the bridge area failure. The last failure mode was only observed in the specimens containing two non-coplanar oval flaws for β = 120°and 150° due to the two pre-existing overlapping flaws.