Laboratory measurements of the rate dependence of the fracture toughness anisotropy of Barre granite

Abstract

Barre granite exhibits strong anisotropy due to its pre-existing microcracks induced by long-term tectonic loading. The quantification of rock anisotropy in fracture properties such as mode-I fracture toughness under a wide range of loading rates is critical to a variety of rock engineering applications. To quantify fracture toughness of Barre granite, notched semi-circular bend (NSCB) fracture tests are conducted statically with an MTS hydraulic servo-control testing machine and dynamically with a split Hopkinson pressure bar (SHPB) system. Barre granite samples are prepared based on the three principal directions, resulting in six orientation sample groups. For dynamic tests, pulse shaping technique is used to achieve dynamic force balance. The finite element method is then implemented to formulate equations relating the failure load to the mode-I fracture toughness using an orthotropic elastic material model. For samples in the same orientation group, the fracture toughness shows clear loading rate dependence, with the fracture toughness increasing with the loading rate. The fracture toughness anisotropy is characterized by the ratio of the largest fracture toughness over the smallest one at a given loading rate. The mode-I fracture toughness anisotropy exhibits a pronounced rate dependence, being strong under static loading while diminishing as the loading rate increases. The mode-I fracture toughness anisotropy may be understood by considering the preferentially oriented microcracks, which will be fully explored in the future.