Failure, crack initiation and the tensile strength of transversely isotropic rock using the Brazilian test

Brazilian tests are used to determine tensile strengths of rocks. Failure mechanisms, and the strengths determined, are influenced by the configuration of the load contacts. Most research on how to interpret Brazilian tests is only valid for isotropic rocks. This study addresses Brazilian tests on transversely isotropic rocks, experimentally and analytically, to identify failure conditions for a range of load contact types and anisotropy angles (representing orientations of the transversely isotropic planes with respect to direction normal to the loading). Stress fields are determined analytically and coupled with a failure criterion to infer locations where failure and crack initiations occur. The analytical results are compared with experimental observations using digital image correlation (DIC) for transversely isotropic Hawkesbury sandstone. It is found that anisotropy angles affect the locations of the crack initiations, with agreement between the experimental results and analytical solutions being observed. The study goes on to show that, for a wide range of transversely isotropic rocks and for planes parallel and perpendicular to the loading directions, cracks will always initiate at disc centres as long as the contact area is large (corresponding to a subtended angle at the disc centre of at least 25 degrees). Failure and cracking at a disc centre enables reliable determination of the indirect tensile strength. The equation for strength, including a stress concentration factor for a range of transversely isotropic elastic properties, is presented for use by practitioners and laboratory technicians.