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Densification is a major feature of silica glass that has received widespread attention. This work investigates the fracture behavior of densified silica glass upon uniaxial tension based on atomistic simulations. It is shown that the tensile strength of the silica glass approximately experiences a parabolic reduction with the initial density, while the densified samples show a faster power growth with the increase of strain rate. Meanwhile, the fracture strain and strain energy increase significantly when the densification exceeds a certain threshold, but fracture strain tends to the same value and strain energy becomes closer for different densified samples at extreme high strain rate. Microscopic views indicate that all the cracks are formed by the aggregation of nanoscale voids. The transition from brittleness fracture to ductility fracture can be found with the increase of strain rate, as a few fracture cracks change into a network distribution of many small cracks. Strikingly, for the high densified sample, there appears an evident plastic flow before fracture, which leads to the crack number less than the normal silica glass at the high strain rate. Furthermore, the coordinated silicon analysis suggests that high strain rate tension will especially lead to the transition from 4- to 3-fold Si when the high densified sample is in plastic flow.

We investigate the mechanical and microstructural changes of the densified silica glass under uniaxial loading-unloading via atomistic simulations with a modified BKS potential. The stress–strain relationship is found to include three respective stages: elastic, plastic and hardening regions. The bulk modulus increases with the initial densification and will undergo a rapid increase after complete densification. The yield pressure varies from 5 to 12 GPa for different densified samples. In addition, the Si–O–Si bond angle reduces during elastic deformation under compression, and 5-fold Si will increase linearly in the plastic deformation. In the hardening region, the peak splitting and the new peak are both found on the Si–Si and O–O pair radial distribution functions, where the 6-fold Si is increased. Instead, the lateral displacement of the atoms always varies linearly with strain, without evident periodic characteristic. As is expected, the samples are permanently densified after release from the plastic region, and the maximum density of recovered samples is about 2.64 g/cm3, which contains 15 % 5-fold Si, and the Si–O–Si bond angle is less than the ordinary silica glass. All these findings are of great significance for understanding the deformation process of densified silica glass.

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