Study on the Effect of Bedding Plane and Loading Style on Fracture Process Zone in Coal

In this study, we conduct the single-edge notched beam (SENB) test to study the influence of bedding planes and loading style on the fracture properties and acoustic emission (AE) characteristic parameters in coal. The DIC and AE methods are jointly used to analyze the elastic and critical opening displacements. Further, the evolutionary rule of fracture process zone (FPZ) length and width is systematically studied. The result shows that the peak load, fracture energy, and AE characteristic parameters slowly increase as the bedding-plane angles increase. The maximum crack initiation angle and surface roughness are at the bedding-plane angle of 45 degrees under pure mode I. However, the surface roughness gradually decreases as the loading mode transits from pure mode I to mixed mode I/II. Then, the loads corresponding to the elastic opening displacements of coal specimens are 70-80% of the peak loads in the pre-peak regime, and the load corresponding to the critical opening displacements are 96-57% of the peak loads in the post-peak regime. The FPZ length and width are determined according to the horizontal displacement contours from the DIC method. Furthermore, as the bedding-plane angle increases, the FPZ length gradually increases from 10.26 to 16.27 mm while the FPZ width first increases and then decreases. As the loading mode transits from pure mode I to mixed mode I/II, the FPZ length first decreases and then increases while the FPZ width slowly increases from 4.46 to 5.81 mm. Finally, we construct phase-field cohesive zone modeling (PF-CZM) to track the crack propagation path and FPZ evolution of coal under pure mode I and mixed mode I/II, respectively. The FPZ and load-CMOD curves obtained from numerical simulation match well with experimental results. These findings are beneficial for the formation of fracture networks in coal seams and the joint exploitation of coalbed methane, shale gas, and sandstone gas. The elastic and critical opening displacements of coal are jointly determined using AE and DIC methods.The evolutionary rule of fracture process zone (FPZ) length and width for coal under different bedding-plane angles and loading styles is characterized.The phase-field cohesive zone modeling is proposed for coal fracture simulation.