Session L1: Detonation and Shock-induced Chemistry IV: Propagation Modeling

Observation of sub-detonative responses in confined high density HMX-based PBXs

This paper describes experiments and modelling aimed at understanding the behaviour of highly loaded (90{\%}-95{\%}) pressed HMX-based PBXs, when subjected to shock compression and ignition by means of distinct mechanical and thermal insults under confinement. In order to separate the role of the stimuli, a test has been designed where a metal impactor is propelled at test samples using a well characterised propellant over a range of velocities to produce various levels of mechanical damage. The impactor is then heated using a characterised pyrotechnic composition which ignites the mechanically damaged explosive. Tubes have been designed to examine the effect of confinement at burst pressures of 218.5MPa and 120MPa. The high confinement tubes employ polycarbonate windows and the low confinement tubes are manufactured from polycarbonate blocks to allow the reaction of the energetic material to be captured using high-speed video. Tests carried out using these tubes have given a good insight into the processes occurring. Modelling runs have predicted an oscillating compressive wave in the explosive and considerable damage at either end of the explosive column. The latter leads to potential deconsolidation once the donor charge has burnt out allowing increased burning and violence.   

Modeling Normal Shock Velocity Curvature Relation for Heterogeneous Explosives

The normal shock velocity and curvature, $D_{\mathrm{n}}(\kappa )$, relation on a detonation shock surface has been an important functional quantity to measure to understand the shock strength exerted against the material interface between a main explosive charge and the case of an explosive munition. The $D_{\mathrm{n}}(\kappa )$ relation is considered an intrinsic property of an explosive, and can be experimentally deduced by rate stick tests at various charge diameters. However, experimental measurements of the $D_{\mathrm{n}}(\kappa )$ relation for heterogeneous explosives such as PBXN-111 [D. K. Kennedy, 2000] are challenging due to the non-smoothness and asymmetry usually observed in the experimental streak records of explosion fronts. Out of the many possibilities, the asymmetric character may be attributed to the heterogeneity of the explosives, a hypothesis which begs two questions: (1) is there any simple hydrodynamic model that can explain such an asymmetric shock evolution, and (2) what statistics can be derived for the asymmetry using simulations with defined structural heterogeneity in the unreacted explosive? Saenz, Taylor and Stewart [JFM, 2012] studied constitutive models for derivation of the $D_{\mathrm{n}}(\kappa )$ relation on porous `homogeneous' explosives and carried out simulations in a spherical coordinate frame. In this paper, we extend their model to account for `heterogeneity' and present shock evolutions in heterogeneous explosives using 2-D hydrodynamic simulations with some statistical examination. (96TW-2015-0004)   

Shock Simulations of Single-Site Coarse-Grain RDX using the Dissipative Particle Dynamics Method with Reactivity

In discrete particle simulations, when an atomistic model is coarse-grained, a trade-off is made: a boost in computational speed for a reduction in accuracy. Dissipative Particle Dynamics (DPD) methods help to recover accuracy in viscous and thermal properties, while giving back a small amount of computational speed. One of the most notable extensions of DPD has been the introduction of chemical reactivity, called DPD-RX. Today, pairing the current evolution of DPD-RX with a coarse-grained potential and its chemical decomposition reactions allows for the simulation of the shock behavior of energetic materials at a timescale faster than an atomistic counterpart. In 2007, Maillet et al. introduced implicit chemical reactivity in DPD through the concept of particle reactors and simulated the decomposition of liquid nitromethane. We have recently extended the DPD-RX method and have applied it to solid hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) under shock conditions using a recently developed single-site coarse-grain model and a reduced RDX decomposition mechanism. A description of the methods used to simulate RDX and its tranition to hot product gases within DPD-RX will be presented. Additionally, examples of the effect of microstructure on shock behavior will be shown.   

Optimum Performance of Explosives in a Quasistatic Detonation Cycle

Analyses were conducted on the behavior of explosives in a quasistatic detonation cycle. This type of cycle has been proposed for the determination of the maximum work that can be performed by the explosive. The Jaguar thermochemical equilibrium program enabled the direct analyses of explosive performance at the various steps in the detonation cycle. In all cases the explosive is initially detonated to a point on the Hugoniot curve for the reaction products. The maximum work that can be obtained from the explosive is equal to the P-V work on the isentrope for expansion after detonation to atmosperic pressure, minus one-half the square of the particle velocity at the detonation point. This quantity is calculated form the internal energy of the explosive at the initial and final atmospheric temperatures. Cycle efficiencies (net work/ heat added) are also calculated with these procedures. For several explosives including TNT RDX, and aluminized compositions, maximum work effects. were established through the Jaguar calculations for Hugoniot points corresponding to C-J, overdriven, underdriven and constant volume detonations. As expected, detonation to the C-J point is found to result in the maximum net work in all cases.   

An Arrhenius Shock-Temperature State Sensitive WSD (AWSD) model for PBX 9502

A modification to the Wescott-Stewart-Davis reactive flow model is presented that addresses several short-comings of previous formulations. Specifically, issues arising during isentropic and multi-shock compression are addressed. Furthermore, unwarranted stiffness in the ZND structure is removed, allowing for less taxing numerical computations. The calibration procedure, including many shock initiation and rate stick experiments, is presented. A relatively simple rate form, based roughly on the shock temperature, seems to adequately model a wide range of experimental configurations from shock-to-detonation transition and detonation propagation. Several validation tests, confirming the efficacy of the new model, are also given.   

Detonation properties of the nitromethane/diethylenetriamine solution

The results of the experimental determination of detonation parameters for the mixture of nitromethane (NM) with diethylenetriamine (DETA) are presented in this work. By the using of a laser interferometer VISAR the stability of detonation waves, detonation velocity and the reaction time with the change of the DETA concentration from 0 to 60 weight percentages were investigated. It is shown that detonation waves are stable up to 25{\%} DETA, and the character reaction time is reduced from 50 ns up to 30 ns with the addition of a few percentages of the sensitizer and then remains almost the constant. With further increase of the DETA concentration the detonation front becomes unstable, and it results in an arising of pulsations with amplitude of 10 microns. The limit concentration of DETA, above which the detonation of the mixture was impossible, was determined. This concentration was equal to 60{\%}. It is shown that the dependence of the detonation velocity on the DETA concentration is non-monotonic. In particular, the increase of detonation velocity in the vicinity of small concentrations of the sensitizer, about 0.1{\%}, was recorded.   

High Resolution Numerical Simulation of Detonation Diffraction of Condensed Explosives

In this paper, A high resolution large scale parallel computation software is developed based on positivity preserving for finite difference WENO method, high order boundary treatment method, multi-medium interface treatment. A new method for deriving the partial derivative of pressure in respect of every conserved quantity is proposed. The software can simulate detonation diffraction problems for two-dimensional condensed explosives. The numerical simulation results revealed the forming reasons of the low-pressure region, the low-density region, the ``vortex'' region and the ``dead zone'' in the vicinity of the corner. Furthermore, it demonstrated that the retonation will generate along the inner wall, and it plays an important role in the process of detonation diffraction. Finally, we obtain that the propagating state of detonation wave around the corner is generally determined by two factors: the transverse shock wave along the inner wall downwards and the extending curved detonation wave.