Natural rock contains discontinuities, including pores, fractures, and inclusions or other defects. The original defects have significant influences on the physical properties and mechanical properties of rock materials, most of the rock failures are due to the initiation, propagation, coalescence of the original cracks. In the classical linear fracture mechanics theory, the stress-strain field at crack tip of rock materials is singular. In order to solve this problem, we introduce the weight function which considering the internal characteristic length of material to modify the stress field. Based on the connection between micro-structures with damage evolution of material, we established stress function which considering the damage evolution of material. Then apply the modified stress function to the existing failure criterion and obtain the strength criterion that considers the micro-structure damage evolution of material. SEM method is also applied to fracture section of pre-existing crack specimens which are fractured under uniaxial compression test. The damage degree at initiation stage of specimens are obtained based on the SEM image. Then apply the damage degree to stress function, we obtained the initiation stress at crack tip. The results shows that the initiation stress at crack tip varies with crack inclination angles, and the maximum initiation stress at the crack inclination angle of 45°.
Natural rock contains discontinuities, including pores, fractures, and inclusions or other defects, which govern the mechanical behavior of the rock mass (Goodman, 1989). The existence of internal defects in the rock mass leads to the stress concentration in the defect area. The stress field function at crack tip of rock originated from the fracture mechanics theory of the metal material. Inglis (1913) studied the stress of an elliptical hole in an infinite plate and pointed out that the stress at the crack tip was infinite and could not bear the load. Griffith (1921) introduced the surface energy to explain the phenomenon that "the tensile strength of crystals is far less than the theoretical strength", and established the linear elastic fracture mechanics. Irwin (1957) proposed the stress intensity factor (K) in considering the singularity of stress at crack tip, and classified the cracks into three types according to the mechanical characteristics of crack: type I, II, III. However, According to the classical linear fracture mechanics theory, the stress-strain field at crack tip is singular.
