An Insight into the Mechanical and Fracture Characterization of Minerals and Mineral Interfaces in Granite Using Nanoindentation and Micro X-Ray Computed Tomography

It is extremely important to investigate the mechanical properties and failure characteristics of granite at the mesoscale to understand the mesoscopic evolution mechanism of the time-dependent fracture of deep-buried hard rocks. This study explores the mesoscopic mechanical properties (i.e., hardness, Young's modulus, and fracture toughness) of the primary granite minerals and their interfaces using nanoindentation. Micro X-ray computed tomography was used to analyze the fracture characteristics of the failed Brazilian disc of granite. Moreover, two homogenization upscaling methods were used to calculate granite's Young's modulus and compared with that from uniaxial compression test results. The results demonstrate the following: (1) The mechanical properties of granite minerals are related to the peak load of nanoindentation. Young's modulus and hardness of quartz, K-feldspar, and plagioclase decrease with an increase in the peak load and tend to be stable when the peak load reaches 5000 mu N, whereas Young's modulus and hardness of biotite at multiple peak loads are in variability and irregularity. (2) Young's modulus and hardness of quartz-plagioclase and quartz-biotite interfaces are between quartz and plagioclase and quartz and biotite, respectively. Furthermore, a linear formula is proposed to estimate fracture toughness based on Young's modulus of granite minerals. (3) The generalized means method that predicts granite's Young's modulus is more accurate compared to the Mori-Tanaka method. (4) Under Brazilian splitting conditions, similar to 84% of tensile cracks along intragranular cracks primarily occur in feldspar minerals, while similar to 16% along grain boundaries occur at feldspar-biotite interfaces.