The multiscale process of material failure from the interatomic bond breaking to the visible crack propagation leads us to explore the validity of linear elastic fracture mechanics (LEFM), particularly for fracture toughness as a constant from nanoscale to macroscale. In the current study, by considering the local virial stress averaged within one lattice, we overcome the barrier of ambiguous crack-tip stress field in molecular dynamics (MD) and perform direct estimation of fracture toughness not through remote stresses. By changing the specimen geometry, i.e., either the crack length or the specimen height (the dimension perpendicular to the crack line), the MD and corresponding finite element method (FEM) solutions show that fracture toughness decreases as the crack length or specimen height decreases. Consequently, fracture toughness cannot be treated as a material constant for nanostructures. The size of the singular stress zone (K-dominance zone) is used to explain the size-dependent behavior of fracture toughness. While the crack length or specimen height decreases, the decreasing K-dominance zone makes the singular part around the crack tip stress not capable of accounting for the full fracture driving force.

This content is only available via PDF.
You do not currently have access to this content.