Shock-induced hotspot formation in amorphous and crystalline 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX): A molecular dynamics comparative study

Shock initiation of heterogeneous high-energy density materials is mediated by the formation of hotspots, and the collapse of porosity is considered the dominant mechanism behind energy localization. This is particularly important in emerging amorphous energetics yet little is known about how the intrinsic properties of glasses affect the size, shape, and temperature of hotspots in these materials. Therefore, we use large-scale molecular dynamics simulations to characterize hotspot formation in amorphous 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane originating from the collapse of cylindrical voids over a range of shock strengths. We find a transition from a viscoplastic to a hydrodynamic regime with increasing shock strength, similar to what is observed in the crystalline case. Interestingly for weak shocks, viscoplastic collapse in the amorphous system results in higher hotspot temperatures than in the crystal; this difference originates from the lower strength in the amorphous samples which results in faster collapse. On the other hand, in the hydrodynamic regime, where strength does not dominate the process of collapse, the hotspot temperature in the amorphous case is lower. The simulations reveal the molecular origin for these observations.