Theoretical and experimental exploration on the combined mode I + III fracture toughness of shale using the edge-notched disk bending method with acoustic emission monitoring

Abstract

The combined mode I + III fracture toughness of shale plays a significant role in the exploitation of shale gas. To investigate the combined mode I + III fracture characteristics of shale, the ENDB (edge-notched disk bending) fracture experiments with AE (acoustic emission) monitoring are implemented to accomplish the full range of fractures from pure mode I (tension) to pure mode III (tearing). By considering the contribution of the mean strain energy to the onset of fracturing, this work develops the 3D-MSED (mean strain energy density) criterion. The measured results indicate that the fracturing energy of the ENDB shale increases linearly with the move from pure tension to pure tearing, while the effective fracture toughness for the ENDB shale decreases linearly. In comparison with the published 3D fracture criteria, the developed 3D-MSED criterion can predict an excellent combined mode I + III fracture toughness envelope for the ENDB shale. Interestingly, the Gaussian function can adequately describe the distribution of AE amplitudes, and the distribution discrepancy in AE amplitudes is insignificant for the ENDB shale specimens under different loading types. However, the percentage of the AE high dominant frequency for the tearing-dominated loading case is relatively greater than that of the tension-dominated loading case, and the tearing-dominated fracturing response is characterized by the nonplanar shear deformation.