Tensile Microcracking Behavior of Granites After High Temperature Treatment by Considering the Effect of Grain Size and Mineralogical Composition

The tensile microcracking behavior of granites, significantly influenced by temperature, is crucial in high temperature geological and engineering applications such as geothermal energy system and nuclear waste disposal. This study investigates the impact of grain size and mineralogical composition on the mechanical properties and microcracking behavior of pre-heated granites and underlying mechanisms through a comprehensive comparative analysis. Utilizing three-point semi-circular bending tests, three distinct granite types differing in grain size and mineralogical composition were subjected to varying temperature treatments ranging from 25 °C to 1000 °C. Detailed thin-section analyses were conducted to examine the morphologies of post-failure specimens, elucidating the underlying mechanisms governing microcracking behavior. Results indicate that granites with larger average grain sizes and heterogeneous grain size distributions exhibit heightened sensitivity to thermal treatment, manifesting in increased thermal crack density. Fracture surface topographies vary with mineralogical composition and treatment temperature, with distinct patterns such as conchoidal fracture and lamellar tearing crack observed at different temperature ranges. Grain size and mineralogical composition significantly influence load–displacement curves and mode I fracture toughness, with a notable brittle-ductile transition observed in coarse-grained granites between 400 °C and 600 °C. Fracture toughness decreases with increasing temperature and grain size. Furthermore, grain size impacts the tortuosity of fracture paths under mode I loading, particularly pronounced in coarse- and medium-grained granites. In medium-grained granites, the tortuosity and width of fracture paths increase with treatment temperature, whereas the effect of temperature on the tortuosity of fracture paths in fine-grained granites is negligible.