Reconsidering the Determination of the Tensile Strength of Orthotropic Rocks by Means of the Brazilian Disc Test

The determination of the tensile strength of rocks is a critical issue, closely related to the safety aspect of sustainable mining. In this direction, the present study deals with the problem of estimating the tensile strength of orthotropic rocks by means of the familiar Brazilian disc test. Taking into account that for orthotropic materials purely uniaxial tensile stress fields can be realized in case the material is stretched uniaxially along any one of its three anisotropy directions (weak, moderate and strong direction), it is reasonable to accept that an orthotropic material is characterized by three parameters that represent its tensile strength. To achieve the determination of these parameters, three circular discs are cut from an orthotropic body so that the axis of each disc to be parallel to one of the characteristic directions of the material. Each disc is submitted to diametral compression (according to the standardized procedure suggested by the International Society for Rock Mechanics for the Brazilian disc test) along its remaining two (in-plane) characteristic directions resulting, thus, in six loading schemes. The stress field developed at the center of the discs for each loading scheme and, also, at the disc-loading plate contact region is determined by means of proper formulation of recently introduced analytic, closed form solution based on Lekhnitskii’s pioneering studies. The procedure is applied for two natural rocks the mechanical constants of which are obtained from the literature. In the light of the results of this study, the efficiency of the indirect tensile test in predicting the three tensile strengths of the orthotropic material is critically discussed. It is pointed out that, under certain restrictions, the Brazilian disc test provides data that correspond to the tensile strengths of an orthotropic material. However, it is clearly highlighted, also, that the basic principle on which the Brazilian disc test is based, namely the direct analogy between tensile loading along a given direction and compressive loading normal to it (which is intuitively accepted for isotropic materials), must be critically reconsidered when dealing with anisotropic materials.