In this research, the effect of confining stress and loading rate on the fracture toughness has been investigated. The short beam compression test has been used to estimate the fracture toughness for Flagstaff sandstone. A brief theoretical analysis of the effects of confining stress and loading rate the fracture toughness is presented. This analysis considers changes to the critical fracture process zone size due to confining stress and loading rate. It is found that experimental results agree with the fracture mechanics theory. The mode II fracture toughness is found to increase with increasing confining stress and increasing loading rate.
In linear elastic fracture mechanics (LEFM), the stress intensity factor K describes the magnitude of stresses in the crack tip region, the size of the crack tip plastic zone or fracture process zone and its shape, and the strain energy for the crack propagation. Since it is able to correlate the crack propagation and fracture behavior, the stress intensity factor is the most important parameter in LEFM. Cracks will propagate when the stress intensity factor reaches or exceeds a critical stress intensity factor, Kc. The quantity Kc is termed the fracture toughness and is considered to be a material property. Fracture toughness is a quantitative expression of a material resistance to failure when a crack is present. Some particular applications for the fracture toughness of rock are given as follows (ISRM 1988): (i) a parameter for classification of rock materials. (ii) an index of the fragmentation process (iii) a material property in the modeling of rock fragmentation. According to the loading configurations, there are three basic fracture modes of crack tip deformation, i.e. Mode I (or opening mode), Mode II (or in-plane shearing mode) and Mode III (or out-of-plane shearing mode).