Rock fracture mechanics approach becomes more applicable in different field of rock engineering works. Therefore study about crack initiation and propagation in fractured rocks under different loading conditions are necessary. Different numerical simulation methods have been proposed in the literature. Although Finite Element Method (FEM) is mostly used for crack modeling in different materials, the necessity of progressive mesh refinements around the crack tip and also the requirement of re-meshing after each crack growth step, demand large computational sources. In order to overcome the abovementioned shortcomings, the Extended Finite Element Method (XFEM) was developed. The main objective of this research work is the extensive application of XFEM for problem solving discontinuous rock media under different stress loading conditions. An object oriented code based on XFEM formulation has been developed using C ++ programming language and validated with different analytical and numerical bench mark tests in the literature. Also crack propagation in Semi-Circular Bend (SCB) specimen, Cracked Brazilian Disc (CBD) and Hollow Centre Crack Disc (HCCD) have been simulated with different crack lengths and angles. The obtained results with XFEM are in good agreement with the reported results in the literature. Since most failures in rock occur under compression condition and in such condition and even after the angle of occurrence of pure mode II, the stress intensity factor of mode I becomes negative, therefore the Maximum Tangential Stress (MTS) criterion is not applicable. The Improved MTS (IMTS) criterion has been developed, validated and employed in a cracked specimen under uniaxial compression, SCB and CBD specimens. Furthermore, in order to assess the IMTS criterion for determining the direction of crack propagation after the angle of occurrence of pure mode II, the laboratory experimental study on the CBD at different angles has been conducted. The numerical results with IMTS criterion show that the crack propagation paths are in good agreement with the experimental results and the developed improved stress-based criterion in this study could be effectively used in modeling crack initiation and propagation under both tension and compression loading conditions.
