The Influence of Dip Angle of Rock Bridge on Mechanical Properties and Fracture Characteristics of Fractured Coal Body at Three-Dimensional Scale

To investigate the influence of geometric position distribution and interaction mechanisms of internal defects of the loaded coal body on its mechanical behavior, this study conducted a uniaxial loading test on double-fractured coal samples with different inclination angles of rock bridges. The compression failure process was scanned in stages using computed tomography equipment, and the dynamic evolution law and deformation damage characteristics of the internal fractures of the loaded coal samples were qualitatively analyzed by image processing technology, three-dimensional reconstruction technology, and digital volume correlation methods. Meanwhile, the PFC3D simulation program was used to realistically restore the stress field distribution characteristics of different stages of the loading process. Based on the experimental method and linear elastic fracture mechanics theory, the fracture characteristics of fractured coal in a three-dimensional state were discussed. The results show that: (1) With the increase of the inclination angle of the rock bridge, the macroscopic intensity of the coal sample gradually decreased, and the law was obvious. (2) Under uniaxial compression conditions, the internal microcrack development of coal samples with double fractures was mainly tensile cracks, supplemented by shear cracks. The number of tensile crack growth angles parallel to the loading direction is relatively large, and the development direction of shear microcracks is mainly concentrated in the direction parallel to the prefabricated fracture plane and the rock bridge dip angle. (3) In the early stage of loading coal samples containing fractures, the force form from the edge of the sample to the central area is first reduced and then increased. The force at the edge of the sample was the largest, and the stress distribution was more uneven, with a greater difference. When the sample reached the failure stage, the force form transformed into the middle area with the smallest force and the transition zone with the largest force. (4) The X, Z-direction average strain ratio of coal samples with different rock bridge inclination angles gradually increased with the increase of the square area, whereas the Y-direction strain ratio of coal samples with rock bridge inclination angles of 0° and 90° increased first and then stabilized, and those with rock bridge inclination angles of 30° and 60° showed a trend of first decreasing and then stabilizing. (5) The type I stress intensity factor of each crack tip was always greater than the type II and type III stress intensity factors, with a significant difference. With the increase of the inclination angle of the rock bridge, the type I stress intensity factor first decreased and then increased, while the type II stress intensity factor showed a trend of first increasing and then decreasing. Compared with the inner crack tip, the outer crack tip of coal samples with the same rock bridge inclination angle was more likely to crack.