The flattened Brazilian disc specimen used for testing elastic modulus, tensile strength and fracture toughness of brittle rocks: analytical and numerical results

The flattened Brazilian disc specimen is proposed for determination of the elastic modulus E, tensile strength sigma(t) and opening mode fracture toughness K-1C for brittle rocks in just one test. This paper is concerned with the theoretical analysis as well as analytical and numerical results for the formulas. According to the results of stress analysis and Griffith's strength criteria, in order to guarantee crack initiation at the centre of the specimen, which is considered to be crucial for the test validity, the loading angle corresponding to the flat end width must be greater than a critical value (2alpha greater than or equal to 20degrees). The analysis shows that, based on the recorded complete load-displacement curve of the specimen (the curve should include the 'fluctuation' section after the maximum load), E can be determined by the slope of the section before the maximum load, sigma(t) by the maximum load, and K-1C by the local minimum load immediately subsequent to the maximum load. The relevant formulas for the calculation of E, sigma(t), K-1C are obtained, and the key coefficients in these formulas are calibrated by finite-element analysis. In addition, some approximate closed-form formulas based on elasticity are provided, and their accuracy is shown to be adequate by comparison with the finite-element results. (C) 2003 Published by Elsevier Ltd.