Effects of different bedding plane strength on crack propagation process under three points bending

The issue of layered rock mechanics is critical for many geological engineering projects,such as coal min- ing,development of shale gas by hydraulic fracturing and so on. The layered rock failure model directly determines the stability of rock mass and the efficiency of energy development. In order to reveal the influence of bedding plane strength on the fracture propagation process,a bedding plane perpendicular to the direction of the maximum principal stress was created based on the method of globally embedding a cohesive unit with 0 thickness using ABAQUS simula- tion software. Combined with the results of X-ray diffraction mineral composition analysis,the software was redeveloped in Python language to realize the bedding anisotropy and mineral particle heterogeneity of shale. The effect of the strength properties of the bedding plane on the fracture growth process under the three-point bending condition was studied by adjusting the strength parameters of the bedding plane. In addition,the acoustic emission (AE) simulation during the loading process was implemented by MATLAB programming. The crack type and dynamic propagation process are further analyzed based on AE location map and AE energy data. The results show that the bedding plane inhibits the propagation of cracks along the direction of maximum principal stress,and the inhibiting effect will be en- hanced if the strength of the bedding plane is weak. Therefore,the crack initiation will occur at the bedding interface when the bedding plane strength is weak,which generates transverse cracks along the bedding. It prevents the trans- mission of stress wave,resulting in a smaller range of the stress field and the enhancement of stress concentration at the crack tip. The crack propagation path becomes complicated due to the existence of the bedding plane,which is mani- fested by the alternating occurrence of the penetrating fracture and the bedding fracture. Meanwhile,the crack presents the characteristics of discontinuous dislocation type. Therefore,the crack propagation path is determined by the bed- ding plane strength and the maximum principal stress. Moreover,the penetrating fracture belongs to the tensile type and mainly occurs along the direction of maximum principal stress,while the bedding fracture belongs to the shear type. If the bedding plane strength is weak,more energy will be required to penetrate the bedding fracture,and the peak load will also become higher. The weaker the bedding plane strength,the more the number of AE events,so there is a negative correlation between them. Similarly,the more tension and shear rupture events occur,the longer the crack length along the layer is.