Effect of initial temperature on compaction and strength of porous silica under shock compression

We use molecular dynamics simulation to study the effect of initial temperature on the shock compression of porous silica with densities of 50%, 75%, and full density. We find that the response is strongly influenced by temperature for shocks in a relatively narrow pressure range. Within this range, near the Hugoniot elastic limit, initial preheating from 300 to 1000 K can increase the final shock density by as much as 30%. However, this enhanced densification effect with preheating temperature is negligible at lower pressures (elastic compression), and is equally negligible at higher pressures (strong shock regime). For vitreous silica, the effect of initial temperature is greatest in the compaction regime (pressures between 1 and 3 GPa) where material strength plays a significant role in the mechanical response. Here, preheating can dramatically increase the final density in silica for a given pressure and porosity. Microstructure was found to influence the behavior, with aerogel structures more strongly impacted than nanopowder packings---likely due to the fact that their strengths are more susceptible to thermal softening. For similar reasons, the effect of temperature is greatest in porosities between 50% and 75% density. In some particular cases, the effect of preheating on the Hugoniot can be comparable to increasing porosity by 25%.