Combined opening-sliding fracture or mixed mode I/II crack growth in rock materials is one of the main origins of collapse and catastrophic failure in rock structures. Some experimental and theoretical methods have been proposed in the past by researchers for investigating mixed mode fracture behaviour of rocks. Among these techniques, the cracked Brazilian disc (BD) specimen and the maximum tangential stress (MTS) criterion are the most commonly used experimental and theoretical methods, respectively for mixed mode fracture study of rock materials. However, the reported test data for mixed mode fracture toughness of rocks tested with the BD specimen do not comply with the theoretical curve of MTS criterion. The experimental fracture toughness data are always above the MTS curve. Therefore, a modified form of MTS criterion is employed in this paper for predicting the BD test results. The application of modified criterion is then examined for two rock materials namely: Italian light marble and Saudi Arabian limestone. It is shown that the modified criterion which uses the effects of non singular stress term along the boundary of fracture process zone can provide much better predictions than the MTS criterion.


Investigation of strength and stability in cracked rock structures is an important task for rock mechanics designers and researchers. For example, rock burst or rock fragmentation is often considered as a process of crack formation and propagation in rock masses. Fracture toughness is the most important parameter for describing the material resistance against the fracture initiation and propagation. Therefore, evaluation of fracture toughness is essential for understanding the mechanical behavior of bodies containing cracks or flaws. Most of the previous investigations on fracture toughness of rocks have focused on mode I (or opening mode) cracks. Hence, the international society for rock mechanics (ISRM) has suggested some standard procedures and test methods for determining the fracture toughness (KIc) of rock materials under pure tensile loading [1,2]. However, in practice the pre-existing cracks and discontinuities in rock structures like dam foundations, tunnel walls and rock slopes are often subjected to complex loading. For such cases, due to arbitrary orientation of flaws relative to the overall applied loads, cracks experience a combined mode I and II deformation. Fig. 1, shows the mode I and mode II crack deformations schematically. Several studies have been conducted in the past to determine the mixed mode fracture toughness of rocks. Some experimental methods and different test configurations have been used for determining mode I, mode II and various mode I/II mixtures. The cracked beam specimen subjected to asymmetric four point bend loading [3–5], the compact tension- shear specimen [6–8], 1340 the edge cracked semi circular specimen subjected to three point bending [9–11] and the Brazilian disc specimen under diametral compression [12–15] are to name a few. There are also some theoretical fracture criteria for assessing the load bearing capacity of brittle cracked materials under mixed mode loading.

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