A hollow centre cracked disc specimen is proposed for determining shear fracture resistance of rocks. By setting the crack line in an appropriate direction relative to the applied load, pure shear is provided along the crack line. Depending on the crack length and the centre-hole radius, the angle of pure shear deformation varies between 20o and 35o. The shear fracture test data obtained for a rock material using the proposed sample is in good agreement with the classical fracture criteria. The ratio of shear fracture resistance to tensile fracture resistance in the tested specimen is about 0.84. This figure is in the range of 0.63 to 1 predicted by the three basic fracture criteria namely the maximum tangential stress criterion, the minimum strain energy density criterion and the maximum
The study of shear strength and deformability of rocks and rock like materials is among the favourite subjects for rock mechanics engineers and the earth science researchers. Shear deformation leads to mode II fracture that sometimes takes place in rock masses at great depth of earth which are usually under confined pressures and compressive forces. When other possible modes of deformation like opening or tensile rupture are suppressed, the in-plane sliding of rock mass layers may lead to large scale deformation and consequently large amounts of energy release. For example, in phenomena like earthquake and rock-burst the shear mechanism is the predominant cause of rupture in rocks. Meanwhile, in other engineering applications like the construction of dams, tunnels, abutments, mines and underground structures, the strength evaluation of rock structure against shear stresses becomes important. Therefore, it is necessary to study the shear failure behavior of rock masses using suitable theoretical methods and laboratory experiments. In rock mechanics, the shear characteristics of rocks are traditionally evaluated by means of Coulomb theory. In this criterion it is assumed that the loaded rock mass is primarily intact and the shear failure occurs along weak surfaces. However, rock masses very often contain a large number of joints, cracks, natural fractures, faults and other discontinuities. These cracks act as stress concentrators and hence can govern the fracture process. Therefore, for evaluating the failure behaviour of real rock structures, the use of more mechanistic methods like fracture mechanics approaches is expected to give more reliable results than traditional techniques. In fracture mechanics based methods, it is assumed that the overall failure of a cracked body initiates from the tip of pre-existing cracks. Accordingly, fracture resistance (or fracture toughness) of a cracked rock is the most important parameter for evaluating the critical load bearing capacity of cracked rocks. Based on the concepts of fracture mechanics, shear fracture resistance is the critical sustainable strength of a cracked body that is subjected to pure shear stresses along the crack line. Fig. 1 shows the pure shear crack deformation, schematically. Some laboratory test methods have been proposed in the past for determining shear fracture resistance of cracked rocks.
