How glass breaks – Damage explains the difference between surface and fracture energies in amorphous silica

The relationship between free surface energy and fracture energy in amorphous silica is investigated using multiscale simulations. We combine the homogenization of an atomic scale fracture model with a phase-field approach to track and quantify the different energy contributions. Free surface energy, localized as potential energy at the surface, is clearly separated from damage diffusion extending over a 16-23 A range around the crack path while the plastic contribution is found negligible. Analysis of the local structure reveals that diffuse damage is primarily associated with changes in the ring structure, whereas variations in free surface energy correlate with changes in the coordination number of silicon atoms. These findings provide new insight into brittle fracture mechanisms in amorphous silicates and are consistent with experimental measurements of silica toughness.