Estimating the direct tensile strength of rocks from indirect tests

Abstract

Within the scope of this study, a series of experimental, numerical and statistical analyses were performed in order to find the coefficients that would allow the accurate determination of the tensile strength values of the rocks. In this context, direct and indirect tensile strength values of 9 different rock types including sedimentary, metamorphic and igneous rocks were determined according to related standards and the suggested methods. Dumbbell-shaped specimens with a new geometric form were used in direct tensile tests. Brazilian disc tests having different t/D ratios (0.5, 0.75 and 1.0) and bending tests having 6 different sample sizes were used as indirect methods. In addition, numerical analyses were performed by using finite element method (FEM). According to the results of experimental and numerical analyses, new dumbbell-shaped specimen geometry which has continuously variable section has been proposed to determine the uniaxial tensile strength of the rocks. It has also been found that no indirect method actually represents the uniaxial tensile strength value. In order to estimate the direct tensile strength values of the rocks from indirect methods, new correction coefficients were found to be as 0.50 for Brazilian test and 0.33 for 3-point and 0.40 for 4-point bending tests and have been proposed. Moreover, new equations have been suggested in order to predict the tensile strength values of the rocks by using Brazilian and 3- and 4-point bending test results. When all the results are evaluated, it was found that the method that best predicts the direct tensile strength is 4-point bending test.