On the size-dependent critical stress intensity factor of confined brittle nanofilms

Fracture of solid materials is a complex multiscale process that essentially occurs due to macroscopic propagation of cracks formed by hierarchical evolution of atomic scale breaking of interatomic bonds. Because it depends only on the initial crack size and the applied stress, the critical stress intensity factor (Kc) is commonly used as the single parameter to predict whether a mechanically stressed brittle material will fracture by crack propagation or not. In this study, we have shown that when a crack is highly constrained in a nanoscale brittle solid, Kc becomes gage-length dependent, hence, it may not be used as the single parameter for fracture prediction. Classical molecular dynamics simulations have been used to conduct fracture simulations of center-cracked brittle single crystal sodium chloride nanosolids. Infinitely rigid end tabs were modeled to generate boundary constraints on the specimen. The degree of constraints was varied by varying the gage length between the tabs. Our study reveals that for a given crack length, the fracture strain increases as the gage length decreases. Such gage-length dependency of failure strains suggests that the critical stress intensity factor, a measure of material fracture toughness, can be affected by boundary constraints. We have also measured the K-dominance zone size for different degree of constraints and found the K-dominance zone decreases as gage length decreases. It is thus inferred that Kc alone cannot fully capture the fracture force.