Deformation and toughening of graphite at the micron scale

Graphite is widely used across a range of high-performance applications, yet its fracture behaviour remains complex and insufficiently understood across lengthscales. In particular, how crystal-level mechanisms contribute to the macroscopic fracture resistance and how it can be used to design tougher materials remains unclear. Here, we investigate the fracture resistance of graphite single crystals using a microscale wedge-driven double cantilever beam (DCB) test under in situ scanning electron microscopy (SEM). The study focuses on Mode I crack propagation along the basal plane, where weak van der Waals interlayer forces dominate the crack propagation process. To accurately interpret the data, we first develop a microscale bending model based on single cantilever beam (SCB) tests, accounting for anisotropic elastic behaviour, interlayer slip and shear cracking during beam deflection. The model enables quantitative evaluation of the energy release rate in a DCB setup. Our results show that Mode I toughness in graphite is significantly higher – by factors of 3 to 30 – than Mode II measurements and theoretical predictions based solely on van der Waals bonding. These findings highlight the presence of additional toughening mechanisms at the crystal scale and provide a framework to evaluate fracture resistance in other van der Waals materials, supporting the design of microstructurally engineered materials and composites.