Probing Fracture Mechanics of Graphene through Heterocrack Propagation in a Moiré Superlattice

Abstract

Understanding the fracture properties of two-dimensional (2D) materials is essential for enhancing their mechanical performance and extending the service life of 2D-based devices. A major challenge lies in examining stress singularities near crack tips at the nanoscale. In this study, we show that we can obtain fracture toughness of monolayer graphene by investigating the propagation of heterocrack in twisted graphene layers. We developed an in situ mechanical measurement to monitor the heterocrack propagation under electron microscopy. The cracks propagated and deflected along the twisted graphene-graphene interfaces, accompanied by periodic stress fluctuations and distorted moiré superlattices. By further leveraging molecular dynamics simulations, we developed a moiré strain analysis method to track strain distributions during heterocrack propagation in the moiré superlattice. The fracture toughness can be measured through the strain fields at the crack tip. Moreover, we examined the effect of the moiré potential on the heterocrack propagation behaviors and proposed an equivalent stress intensity factor to evaluate the fracture properties of graphene under varying twist angles. This work provides key insights into the fracture mechanics of 2D materials, and also offers a foundation for assessing the reliability and mechanical stability of 2D-material-based nanodevices.