Before fracture occurs, the local composition, structure, and mechanical properties are heterogeneous in space and show a strong dependence on the applied strain. We find that during the fracture of Na-rich glasses very irregular cavities as large as 3–4 nm form ahead of the crack front, indicating the presence of nanoductility in these glasses. A strong composition dependence is also seen in the crack velocity which, for the strain rate considered, is on the order of 1800 m/s for glasses with low Na concentration and decreases to 650 m/s if the concentration is high. The stress-strain curves demonstrate that silica glass is brittle whereas the glasses rich in Na show pronounced ductility. We use large-scale simulations to investigate the dynamic fracture of silica and sodium-silicate glasses under uniaxial tension.
