Mechanism of pure shear (Mode II) fracture of brittle rock was studied by strain measurement and stress analysis in shear-box tests.The study shows that compressive loading can restrain tensile stress and create compressive stress around crack tip. The compressive stress increases as the load increases. When the load increases to 70–80 percent of peak value, the compressive stress decreases with the load and the slope of the stress-load curve changes in sign from negative to positive. That is because a great number of microcracks are initiated, propagated and coalesced, which can lead to the relaxation of the compressive stress. At the peak load maximum tensile stress is smaller than tensile strength of rock, while the maximum shear stress is larger than the shear strength of rock, which results in Model 11fracture.
Classical fracture mechanics defines three basic modes of fracture, i.e. Mode I (opening), Mode 11 (sliding) and Mode III (tearing), from the loading point of view (Irwin, 1957). It is assumed that fracture extends in pre-existing crack plane. That is true for most metal materials. For rock materials, however, pure shear loading applied on a pre-existing crack is not certain to result in a Mode II fracture. Numerical calculations show under pure shear loading, both tensile and shear stresses exist at the crack tip and they have the same order of magnitude (Rao, 1999). Since tensile strength of rock is known to be much smaller than its shear strength, Mode I fracture easily occurs before Mode Il frcture. Suggested method for determining Mode I fracture toughness of rock, K1c, have been proposed (IRSM, 1988), but there is no standard testing method available for Mode11 fracture toughness K11C up to now. Traditional testing methods for measuring K11C, e.g. anti-symmetric four-point bending test (lngraffea, 1981; Huang, 1985), edge cracked Arcan test (Banks- Sills and Arcan, 1984) and compact tension-shear test (Richard,. 1984) are based on the application of a pure shear load to the specimen regardless of fracture mode and thus mislead to the measured K11c being smaller than or close to K1c. Shear-box loading test is a potential method for achieving Mode II fracture of rock because a compressive loading applied to the original crack plane can completely suppress tensile stresses at the crack tip and facilitate Mode 11 fracture. In this study, the shear-box test was used to create Mode II fracture and determine K11c. Normal strains around the crack tip was measured throughout the tests and stresses around the crack tip were analyzed in order to better understand the mechanism of Mode II fracture of rock.
Rock type used in this study is granite. Its mechanical properties, i.e. tensile strength 0, uniaxial compressive strength 0 internal friction angle Ф, cohesion C, elastic modulus E, Poisson's ratio µ and Mode 1 fracture toughness K1c are listed in Table I. The square specimen is between two beveled dies, which can be set at anyinclination angle a (0<a<900) with respect the horizontal.
