Understanding failure mechanisms is important to the fields of mining, civil, and geology. Especially, a shear fracture propagating ahead of a working face and slippage along pre-existing discontinuities in mining are possible candidates for causing rock bursts. To understand such geological phenomena, an experimental method for creating a shear fracture in brittle materials under pure shear loading conditions was developed. Based on the results of the experiments, expressions for estimating shear fracture toughness were derived by using the geometry of the specimen. The test method was used to estimate the shear fracture toughness of sandstone and Sierra White granite.
Gaining an understanding of the fracture mechanisms of brittle rock in shear rupture is essential to the understanding of the mechanisms of rock fracturing including rock bursts, rock cutting, hydro-fracturing, and explosive fracturing. Fracture toughness tests measure the amount of energy required to propagate a single crack. In order to analyze these types of failure mechanisms, the stress intensity factor of the fracture has to be defined. In linear elasticity, the ultimate strength is defined as the maximum load divided by the initial cross-sectional area, while the true fracture stress is the load at fracture divided by the final cross sectional area with corrections made for any localized deformation in the final fracture region.
Traditionally, three possible loading modes have been considered with regard to crack propagation, traditionally referred to as Modes I, II and III. As Mode I opening is encountered far more often than the other two modes, the majority of research has been devoted to this type of loading. However, research involving shear fracture has played an important role in the area of mining, civil engineering, geology and geological engineering.
Fracture toughness is a measure of the critical stress intensity KIC at the crack tip required to initiate and propagate the fracture (Ingraffea et al., 1982). It is a function of applied load and stress concentration, which is related to Poisson’s ratio and geometry. Fracture toughness can also be expressed as a critical energy release rate GIC, which is the energy required to create new surface area. It is a function of KIC, the elastic modulus, and Poisson’s ratio and is also known as the crack driving force, which has been shown to be corelatable with cutting performance of disks on a tunnel boring machine (Nelson and Fong, 1986). Fracture toughness can be calculated with an optimum combination of strength and stiffness of material.
Shear fractures in brittle materials have been the subject of both considerable study and considerable confusion. Most shear fractures created in the laboratory result from compression tests of cylinders. Study of these fractures is complicated by frictional resistance which is mobilized by normal stress across the fracture plane. A number of studies have been conducted seeking to minimize frictional effects by studying shear fracture of brittle materials under shear conditions. The results of these studies have varied widely, with some researchers reporting that they could not generate a shear fracture.