How Stress Biaxiality Controls Crack Morphology and Apparent Fracture Energy of Dikes and Sills

The emplacement of dikes and sills plays a crucial role in crustal mechanics. The parameter used to describe their resistance to propagation, fracture energy, remains controversial. Here, we show how different stress biaxiality levels experienced by dikes can directly affect the micromechanisms of crack propagation in rocks, consequently impacting fracture energy. We performed controlled tensile crack propagation experiments under opposite stress biaxialities. We connect fracture energy variations monitored through a compliance-based method to crack microstructures observed on post-mortem specimens. Microscopy techniques showed that biaxial tension generates intricate microstructures driven by topological instabilities, such as deflections and branches, to circumnavigate tougher grains. This yields a higher fracture energy, that we attribute to front roughening and bridging mechanisms due to front fragmentation. Bridging toughening is gradual and increase with crack size. This hints at the existence of a scale dependency of fracture energy of dikes also experiencing biaxial tension.