Hotspot formation due to shock-induced pore collapse in 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX): Role of pore shape and shock strength in collapse mechanism and temperature

Abstract

The shock to detonation transition in heterogeneous high energy density solids starts with the spatial localization of mechanical energy into so-called hotspots that form due to the interaction between the leading wave and microstructural features and defects. We used large-scale molecular dynamics to characterize the hotspots resulting from the shock-induced collapse of cylindrical voids and elongated cracks focusing on the effect of shock strength, defect shape, and size. The temperature fields resulting from the collapse of cracks elongated along the shock direction show significantly higher sensitivity to both shock strength and size than cylindrical voids. Cracks 80 nm in length result in temperatures almost three times higher than voids 80 nm in diameter, reaching values corresponding to the ideal case of isentropic recompression of a gas. The molecular dynamics trajectories reveal the atomic origin of this contrasting behavior. While circular voids undergo a transition from viscoelastic pore collapse to a hydrodynamic regime with increasing shock strength, shock focusing in elongated cracks results in jetting and vaporization which, upon recompression, leads to increased heating.

Fourier-like Thermal Relaxation of Nanoscale Explosive Hot Spots

“…We employ a combination of nonreactive all-atom MD modeling and HEq modeling to explore the thermal relaxation characteristics of realistic hot spots in TATB explosives. Nonreactive MD simulations provide direct “full physics” predictions for the formation ,− and relaxation of hot spots that implicitly include all mechanical heat generation mechanisms and ballistic transport effects within the classical approximation for phonon populations and the heat capacity. The application of a nonreactive MD model allows for isolating thermal transport without the added complication of chemical reactions leading to heat generation. − Initial hot spot structures are sampled through simulations of pore collapse in TATB single crystals with two carefully chosen orientations that provide upper and lower bounds for thermal conductivity anisotropy relative to normal conditions.…”

Unsupervised Learning-Based Multiscale Model of Thermochemistry in 1,3,5-Trinitro-1,3,5-triazinane (RDX)

“…The development and validation of such models is hindered by the lack of experimental tools with the resolution required to capture the formation of hotspots and their role in detonation initiation. There has been significant recent progress in the measurement of temperature associated with hotspots via emission spectroscopy, but these measurements lack spatial resolution or the ability to separate the initial temperature increase during the formation of the hotspot and that associated with sustaining exothermic chemical reactions . Thus, these measurements cannot be directly used to assess which hotspots become critical and sustain a developing detonation .…”

“…Advances in hardware and algorithms have enabled, in recent years, condensed matter simulations at the DFT level and using tight binding, , though the small sample sizes present problems for proper extrapolation. Large-scale reactive and nonreactive simulations enable the direct coupling of mechanical deformation and heat generation processes by means of shock-induced defects, hotspots , and the shock-to-deflagration transition , following the collapse of porosity. Reactive MD simulations provide insights into the molecular-level processes of decomposition, , and in combination with ultra-fast spectroscopy experiments on laser-driven shocks − are enabling, for the first time, a direct (if not perfect) comparison of chemical reactions under the appropriate extreme conditions.…”

A Hotspot’s Better Half: Non-Equilibrium Intra-Molecular Strain in Shock Physics

“…Hotspots are routinely characterized and analyzed by their temperature fields, across atomistic and continuum modeling ,,, and in experiments . It is well understood that applying external forces to a molecule can accelerate and change chemical reactions through mechanochemistry .…”