Discrete element modeling on crack evolution behavior of sandstone containing two oval flaws under uniaxial compression

To understand the fracture coalescence in specimens containing oval flaws, PFC2D was used to investigate the sandstone containing two oval flaws with different ligament angles under uniaxial compression. The micro-parameters were first confirmed by intact sandstone, and then, these were adopted to simulate the flawed specimen under uniaxial compression. Numerical results show that the peak strength, the peak strain, elastic modulus, and the ultimate failure modes were approximated well with these obtained by experiment. The failure modes obtained by numerical simulation can be divided into the following four typical categories based on the crack coalescence between the pre-existing flaws: no crack coalescence; indirect crack coalescence outside the bridge area; single crack coalescence inside the bridge area; tensile crack coalescence outside the bridge area. Based on parallel bond force fields and stress variation of measurement circles along the crack trajectory, the crack evolution processes were analyzed in detail and crack types were identified. The mode III can be divided into mode IIIa and mode IIIb. The mode IIIa is the single shear crack coalesced inside the bridge area; the mode IIIb is the single crack that initiated with tensile crack and coalesced with shear crack, and it only occurred as β = 90°. The mode IV can be divided into mode IVa and mode IVb. The mode IVa is that wing crack initiated at the middle of oval and coalesced at another oval tip, and it occurred as β = 120°. The mode IVb is that coalesced with anti-wing crack outside the bridge area, and it occurred as β = 150°. The stress distribution along the ligament has an obvious effect on ligament angle and flaw angle, and crack initiation only affects the stress concentration factor value.