Fracture toughness analysis of HCCD specimens of Longmaxi shale subjected to mixed mode I-II loading

Longmaxi shale exhibits strong anisotropy due to its bedding planes induced by diagenesis. The anisotropic properties of shale have a significant effect on the stability control of geotechnical engineering. To investigate the influence of anisotropy of shale on its mixed mode fracture toughness, we performed Brazilian disc splitting tests on hollow center cracked disc shale specimens with different bedding and crack inclination angles (alpha and beta, respectively). In this study, the fracture toughness of Longmaxi shale was calculated by the finite element method combined with either the J-integral or crack tip-opening displacement method. The results show that the dimensionless stress intensity factors and beta values for pure mode-I and mode-II fractures both show anisotmpic behavior by varying alpha. For all combinations of mode-I and mode-II loading, the fracture toughness also exhibits anisotropy by varying alpha; the mode-I fracture toughness decreases with increasing alpha when beta > 30 degrees, and the anisotropy is negligible for beta = 0 degrees. The anisotropy for mode-II fracture toughness is weak when beta is small or large, whereas the anisotropy is significant when beta = 45 degrees and 60 degrees. The effect of alpha on mode-I fracture toughness is greater than that on mode-II fracture toughness when beta is small or large. Further, a comparison of four mixed-mode fracture criteria with the experimental results shows that the maximum tangential stress criterion for a transversely isotropic body can predict the mixed-mode fracture behavior of Longmaxi shale well. At the same time, it is suggested that the role of the T-stress should be considered to obtain more accurate prediction results.