Fracture process of inherently anisotropic rock discs under Brazilian test conditions are investigated with the use of two-dimensional Discrete Element Methods (DEM). In the DEM model, the rock matrix is represented as an assembly of rigid particles bonded at their contacts and the presence of intrinsic anisotropy is explicitly modeled by imposing individual smooth joint contacts into the bonded-particle model. A series of anisotropic models with different angles between weak layers and loading direction are tested (?=0°, 15°, 30°, 45°, 60°, 75°, 90°). The anisotropic numerical model is firstly calibrated to match the variation of Brazilian tensile strength with anisotropy angles of anisotropic rocks from published experimental data [1]. Good agreement can be found between the failure patterns of numerical model and those observed in laboratory. After that, the fracture process of anisotropic rock under diametrical compression is investigated in detail by exploring the occurrence, development and coalescence of micro cracks with different anisotropy angles. Micromechanical studies are also conducted by examining the modes, increment and distribution of micro cracks at different stages in order to gain insights on the failure mechanisms of anisotropic rocks under indirect tensile test conditions with different anisotropy angles


Among the many mechanical parameters, the tensile strength of rock material is a key one because rocks are in nature much weaker in tension than in compression. Many rock mechanics applications like the propagation of hydraulic fractures, rate of rock blasting, and drilling of boreholes in sedimentary rocks are highly dependent on the tensile strength of surrounding rocks [2, 3]. The Brazilian tensile test [4] (diametrical compression of circular discs) has been widely adopted in laboratory to determine the tensile strength of rock materials.

In laboratory, a series of experimental studies have been performed on different rocks with anisotropic properties in order to understand the responses of them under indirect tensile stress [5-8]. In fact, there is no consistent trend for the variation of Brazilian tensile strength (BTS) with the anisotropy angle due to the complex stress distribution between the weak layers and rock matrix. Based on results from nine types of rocks, Vervoort et al. [1] proposed four different trends for the BTS and failure patterns of anisotropic rocks under the Brazilain test conditions: trend1, the BTS stays constant over the entire anisotropy angles; trend 2, the BTS stays constant between 0° and 45°, followed by a linear decrease; trend 3, the BTS systematically decrease over the entire interval; trend 4: the BTS decreases from very low anisotropy angles (between 0° and 30°) and followed by a leveling off. They provided some explanations based on the relative lengths of fractures observed along the weak planes and in the matrix. However, the micromechanics for these different trends are still not fully understood. In addition, the failure patterns of anisotropic rocks are found more complex than those of isotropic rocks.

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