Investigation on fracture toughness and micro-deformation field of SCB sandstone including different inclination angles cracks

The increasing use of hydraulic fracturing in mining, petroleum engineering is boosting great interest in the fracture mechanism and fracture toughness of rock. A static loading device, Geotechnical Consulting & Testing Systems, was used to conduct three-point bending tests on semicircular bend (SCB) sandstone specimens, the results showed that the concentrated force plays a guiding role in crack propagation. With the increase of crack inclination angle beta, the peak load of the specimen exponentially increases. The mode I stress intensity factor K-I decreases gradually, and the mode II stress intensity factor K-II increases first and then decreases. RFPA2D software can effectively simulate the failure process of rock, the obtained crack propagation path and peak load are basically consistent with the experimental results. The deformation field was analyzed based on the experimental and numerical results. It was found that the deformation field at crack tip varied continuously during the loading process, where the rate and range of variation show huge diversity for different beta values. As shown in the analysis of displacement field evolution, a mode I crack specimen will exhibit a uniform symmetrical trend eventually. However, the transformations of I/II mixed mode cracks are more complex, exhibiting different degrees of sliding cross phenomenon at crack tip and asymmetrical distribution in general. The comprehensive method combined with GCTS, RFPA and DIC can be effectively applied to evaluate the fracture toughness of rock material.