Biotite's non-monotonic effect on fracture toughness: competing mechanisms of elastic mismatch and microcracking

Assessment of fault activity risk is crucial for earthquake disaster prevention and mitigation. Earthquakes originate from rock rupture along faults, which is a process in which the mechanical fracture behavior is significantly influenced by the constituent minerals, particularly weak minerals such as biotite, of the rock. However, the specific effects of biotite content on the fracture mechanics of rocks remain underevaluated. Therefore, in this study, the influence of biotite on the fracture mechanics of gneiss was systematically investigated, and the underlying mechanisms were elucidated. We examined the fracture behavior of gneiss through microscopic three-point bending tests. To analyze the rupture process in depth, crack propagation was captured in real time using an electron microscope coupled with a high-speed camera. Subsequently, digital image correlation (DIC) was used to retrieve strain fields, and postfailure thin sections were analyzed to investigate the fracture mechanisms. The experimental results indicate that (a) the Mode I fracture toughness of gneiss initially decreases but then increases with increasing biotite content and eventually tends to stabilize, and (b) the crack propagation mode is governed by the distribution of biotite. The mechanisms that underlie the role of biotite are as follows: (a) the elastic mismatch between biotite and brittle minerals induces internal stress concentration, which leads to a reduction in fracture toughness, and (b) the presence of biotite promotes the nucleation of intragranular microcracks within brittle minerals, thereby weakening their resistance to fracture. These findings provide significant insights into the macroscopic processes of fault rupture and seismic activity.