Investigation on the shear fracture behavior of sandstone with two double-edged cracked Brazilian disk specimens

To investigate the compressive-shear fracture behavior in rocks, the stress intensity factors (SIFs) for double-edged cracked Brazilian disk (DCBD) and double-edged cracked flattened Brazilian disk (DCFBD) samples were calculated utilizing the weight function method (WFM) and finite element method (FEM), respectively. Subsequently, a comprehensive comparison of the SIFs between these two types of samples was conducted. Moreover, numerous fracture experiments were performed to validate their applicability in achieving shear fracture of sandstone. The results indicate that the analytic formulae of SIFs for the DCBD samples can be directly applied to the calculation of corresponding values for the DCFBD samples when the crack length ratio α ≥ 0.60 and load contact angle γ ≤ 20°. Shear fracture can be achieved in both DCBD and DCFBD samples under loading angles of θ = 10° and 15° for crack length ratios α = 0.70 and 0.80. Furthermore, the true mode II fracture toughness KIIC at α = 0.80 is consistently lower than that at α = 0.70 due to the smaller inhibition of the crack tip. Additionally, the KIIC values obtained from DCFBD samples are about 1.07 times higher than those determined from DCBD samples attributed to the larger load contact angles. Moreover, the measured KIIC/KIC ratios of this sandstone range from 2.06 to 3.45 determined from these two samples, with an average ratio of approximately 2.75. The results are crucial for facilitating the development of the shear fracture testing technology of rocks under compressive-shear loading.