Tensile strength of rock and similar materials is a critical material property in rock engineering design. Anisotropic materials do not necessarily follow the behaviour as originally assumed when the Brazilian Tensile Strength test was created; therefore, additional care is required when analyzing tensile strength measurements of these materials. This study presents guidelines and recommendations for the analysis of strain accumulation in disc specimens with anisotropic geological fabrics using 2-dimensional digital image correlation. In addition, strain maps of specimen faces generated with digital image correlation are used to corroborate the mechanistic behaviour of a proposed three part anisotropic tensile strength criterion.
The importance of tensile strength as a critical geomechanical property is well reflected as multiple agencies including the International Society of Rock Mechanics (ISRM) and the American Society for Testing and Materials (ASTM) have published a suggested method (ISRM, 1978) and a testing standard (ASTM, 2016). The most popular method to determine tensile strength of rock is the splitting tensile test (also known as the Brazilian Indirect Tensile Strength (BTS) test), which is a well-known and well-studied laboratory test that is relatively fast and easy to perform when compared to direct tension methods. However, very few guidelines have been created to aid in the processing and understanding of BTS test results for anisotropic materials.
For homogenous, isotropic rock and similar materials, the stress distribution internal to the BTS disc specimen should develop according to the theory as originally developed by Hertz (1883) and further described by Hondros (1959). Their description of a tensile fracture that propagates from the center outward to the rock-platen contact may not always hold true for anisotropic materials. Tensile strength testing on anisotropic materials has shown the anisotropy does have an effect on the measured tensile strength (e.g. Barla and Innaurato 1972; Chen et al. 1998). Foliation or other geological fabrics in rock are known to act as planes of weakness during compressive loading; however, as tensile testing can be complicated, the development of anisotropic tensile strength is not as well understood. Recent studies have shown that fractures may develop either in tension or a combination of tension and shear which may be guided by the foliation (Dan and Konietzky 2014; Packulak et al. 2023a).
