Evaluation of the fracture mechanisms and criteria of bedding shale based on three-point bending experiment

Abstract

Shale has remarkable bedding characteristics, and the key technology of shale gas extraction engineering is garnering great interest in the influence of bedding on the fracture mechanisms and fracture criteria of shale. Three-point bending tests combined with digital image correlation (DIC) were carried out on semicircular bend (SCB) specimens of shale with different bedding angles to investigate the influence of bedding on crack evolution. The results showed that the strain field of each bedding shale exhibited three stages during the three-point bending : uniform strain distribution, damage accumulation, and strain localization. In the stage of strain localization, each specimen first showed considerable tensile strain localization along a straight line extending from the tip of the prefabricated crack. With increasing loading, except for in the localized bands of the tensile strain, the maximum shear strain became aligned with the bedding orientation in the shale specimens, indicating that the original mode I cracks of the specimens were transformed into mixed-mode I/II cracks due to the influence of the bedding. A new fracture criterion (the generalized maximum circumferential stress criterion based on shale bedding, GMTS-SB) was deduced for predicting the propagation of a prefabricated mode I crack in bedding shale. The results calculated from the GMTS-SB were compared with the experimental results to evaluate the accuracy of the criterion. In the established GMTS-SB, the stress intensity factor is related to the specimen size, the load magnitude, and the bedding angle. This criterion also considers the influence of T-stress on the calculated result. For bedding shale, the GMTS-SB can effectively describe the crack angle and fracture resistance of a crack initiating along the bedding orientation.