Modeling structure-dependent fracture behaviors of anisotropic cracked chevron notched Brazilian disc shale samples: Laboratory scale testing

The anisotropic fracture behaviors of layered shales affect the exploration and development of shale gas resources. Extensive investigations have been devoted to revealing the anisotropic geomechanically behaviors of intact or flawed shale under various loading paths. However, the structure-dependent fracture behavior and fracture toughness of layered shale were not well understood. The presented studies aim to conduct static fracture mechanical testing on cracked chevron notched Brazilian disc shale samples with different bedding inclinations (θ) and different crack angles (β), to reveal the influence of layer angle and crack angle on its fracture behaviors. The mechanical properties, energy conversion, fracture toughness and macro-meson fracture characteristics are studied, as well as the applicability of the conventional and generalized fracture criterion in layered rock mass is discussed. The results show that (i) The peak loads and fracture energy both increase with the increase of θ, but they increase first and then decrease with the increase of β. (ii) All samples exhibit six different failure modes corresponding to different morphological characteristics at crack surfaces. (iii) The fracture toughness of the samples increases with the increase of θ, and the mode Ⅱ fracture toughness (K_ⅡC) is about twice that of the mode Ⅰ fracture toughness (K_ⅠC) under the same θ. The traditional maximum tangential stress (MTS) criterion predicts pure mode II fracture initiation angle (φ_0Ⅱ) and K_ⅡC of the samples with a large error, but the generalized MTS criterion, which considers the T-stresses, has an error in predicting φ_0Ⅱ, but it can accurately predict K_ⅡC.