Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials

Abstract

The fundamental processes in shock-induced instabilities of materials remain obscure, particularly for detonation of energetic materials. We simulated these processes at the atomic scale on a realistic model of a polymer-bonded explosive (3,695,375 atoms/cell) and observed that a hot spot forms at the nonuniform interface, arising from shear relaxation that results in shear along the interface that leads to a large temperature increase that persists long after the shock front has passed the interface. For energetic materials this temperature increase is coupled to chemical reactions that lead to detonation. We show that decreasing the density of the binder eliminates the hot spot. The interaction of shock waves with nonuniform interfaces plays an essential role in the interfacial instabilities in inertial confinement fusion (ICF), in shock-induced RichtmyerMeshkov instabilities (RMIs), and in detonation in heterogeneous polymer-bonded explosives (PBXs). For detonation, it is generally accepted that hot spots form during the development of instabilities as shock waves pass through the interface or other defects.1-4 Despite numerous experimental and theoretical studies, the fundamental processes involved remain controversial. This is due to the complex environment and coupling of thermal, chemical, and mechanical degree of freedom, which is extremely difficult to unravel experimentally. It has also been very difficult theoretically to include the reactive processes involved and yet cover the enormous size and time scales intrinsic to the phenomena.To discover the origin of shock-induced hot-spot formation, we carried out reactive dynamics (RD) using the ReaxFF reactive force field on a realistic model of a real polymerbonded explosive PBX N-106. Energetic materials (EMs) are essential for applications ranging from rocket engines, to building and dam construction, and to armaments. Generally the EM is bound together in a matrix of polymer elastomers to form the PBX that can be molded into various shapes, while providing some control in resisting unintentional detonation due to shocks or friction. Unfortunately, current generations of PBXs are sensitive to accidental detonation, despite many attempts to control the safety by improvements in materials and manufacturing practices. Here we use the ReaxFF reactive force field to examine the effect of shocks on realistic models of polymer-bonded explosives, where we use these simulations to extract the mechanism of hot-spot formation. Then, based on this model, we predict how to change the system to reduce the hot spot, and carry out simulations to validate this prediction.ReaxFF has now been established to provide nearly the accuracy of quantum calculation in the various reaction barriers and rates while providing a computational efficiency nearly that of ordinary molecular dynamics (MD) with ordinary force fields (FFs), enabling us to study the complex processes involved in interfacial instabilities at the atomic scale, providing insights on such phenomenon. Thus ReaxF...

Ultrasonic hammer produces hot spots in solids

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ReaxFF reactive molecular dynamics on silicon pentaerythritol tetranitrate crystal validates the mechanism for the colossal sensitivity

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The co-crystal of TNT/CL-20 leads to decreased sensitivity toward thermal decomposition from first principles based reactive molecular dynamics

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