Fracture energy variations of rocks: a mechanical investigation

Crack propagation is critical for the assessment of the strength of rocks. Linear Elastic Fracture Mechanics (LEFM) theory is commonly used to describe its propagation. However, the variation of the fracture energy, its key parameter, is generally poorly understood as its experimental measurements are influenced by temperature, stress biaxiality, and rupture velocity. This indicates other dissipative processes may occur in the vicinity of the crack. We conduct Modified Ring Tests (MRT) on Carrara marble to investigate these mechanisms. MRT provides stable mode I crack propagation under controlled velocity and stress biaxiality conditions. Coupled with a compliance method calibrated through Finite Element Method (FEM), we obtain multiple local measurements of the fracture energy within a single test. FEM also provides estimation of stress biaxiality levels as well as higher order terms of the Williams’ expansion of the stress field. The method is validated on PMMA through Digital Image Correlation (DIC) techniques. Experiments on Carrara marble show that the stress biaxiality can directly influence the fracture energy measurements. A microscopic investigation on marble is performed to look for micro-mechanisms which may cause observed variations of fracture energy.