Abstract: To comprehensively investigate the influence of fracture characteristics on rock mass strength and deformation failure, this study employs the discrete element method software PFC^3D to conduct deformation and failure tests on fractured sandstone under varying confining pressures and fracture inclinations. By integrating the macroscopic mechanical properties throughout the entire deformation and failure process and analyzing the progressive failure characteristics based on the evolution of the number of microcracks, a damage variable D is defined according to the number of microcracks, and a damage constitutive relationship is established that can reflect the asymptotic failure of fractured rock masses. The results indicate that: The fractured sandstone has obvious confining pressure effect, the larger the confining pressure, the larger the compressive strength; Under the same confining pressure, the compressive strength of fractured sandstone shows the trend of decreasing and then increasing with the increase of fracture inclination angle. The deformation and damage process of fractured sandstone corresponds to the evolution curve of the number of microscopic cracks with a stage, based on the curve of the number of cracks, a smooth section, a linear growth section, an upward concave growth section and the last tends to be flat section of the stage division, to determine the corresponding characteristics of the stress point. During the progressive damage process, the microcracks of 0° and 90° fractured rock samples were almost spread throughout the whole sample, and themicrocracks of 30°, 45° and 60° fractured rock samples tended to be generated along the pre-fractured and the upper and lower loading surfaces. At each characteristic stress point, the total number of cracks increases with the increase of confining pressure, and the larger the confining pressure, the larger the proportion of shear cracks, and the total number of cracks decreases and then increases with the increase of the inclination angle of the pre-fracture. The constitutive relationship established based on the number of microcracks is reasonable and can well reflect the basic mechanical characteristics of fractured sandstone.

Key words: fractured sandstone, crack number, progressive failure, damage constitutive model, PFC3D；confining pressures, fracture inclinations