MOLECULAR-DYNAMICS STUDY OF BRITTLE-FRACTURE IN SILICA GLASS AND CRISTOBALITE

A non-equilibrium molecular dynamics simulation approach is used to mo del the structure of silica glass and cristobalite using a two-body an d a two-body/three-body potential. A comparison of the total correlati on function is made between the two simulations and neutron scattering data. Differences in structure obtained by each simulation method sho w the influence of a lack of directional bond components in the two-bo dy potential simulation and the result of strong directional component s in the three-body potential simulation. The effect of an applied uni axial strain on the structure of the simulated glass and crystalline p hases is studied by following the resulting longitudinal stress. Data analysis shows that the glass exhibits a large strain-rate dependence in the maximum stress sustained, while the crystal does to a lesser ex tent or none at all depending on the potential function. The large str ain rate dependence in the glass is interpreted as arising from a rear rangement of the free volume structure, whereby, at low strain rates, strain added uniformly to the structure is allowed to flow from higher density regions of the glass to lower density regions. This effect le ads to coalescence of voids eventually causing fracture propagation th rough the simulated structure. The crystal, having no ability to assim ilate the applied strain, can only stretch its bonds until fracture.