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Des etudes experimentales montrent souvent qu'une reaction de choc instantanee a un impact de projectile peut meme se produire avec des explosifs a confinement relativement epais. Le present document se propose de discuter des criteres de prevision qui peuvent etre appliques a de telles conditions et d'explorer en particulier les differents mecanismes qui interviennent lors d'impact de barreaux ou de spheres. Il s'avere que les seuils d'initiation prevues offrent de bonnes possibilites de comparaison avec les resultats experimentaux en utilisant tout au plus deux constantes empiriques pour la sensibilite au choc obtenue par des impacts sur des explosifs non confines.

In this paper we review our recent work concerned with assessing the significance of many-body forces for short-range interactions of closed shell atoms and molecules. Ab initio molecular orbital calculations (of the supermolecule type) have been carried out to determine three-body potentials for the following species: He, Ne, H 2 , N 2 , CO and CO 2 . For He, Ne, H 2 and N 2 we have also carried out calculations of the four-body potential for a limited number of orientations. These studies show that, for all these species, there are significant deviations from pair-wise additivity at short separations. The effect of these many-body interactions on the equation of state for dense fluids (such as detonation products) is being investigated by Monte Carlo simulation, and recent results for high-pressure helium are described.

Fragment and bullet impact pose a serious threat for many weapon systems because of their potential to induce violent reactions in the energetic materials (explosives and propellants) contained within them. In some scenarios detonations have been observed under conditions insufficient to cause SDT or DDT. These events have been labelled XDT (X for unknown, Detonation Transition). It has generally been assumed that XDT arises as the result of some combination of damage to the energetic material and re‐shock or re‐compression of this damaged material. In this paper we review the results of our previous experimental studies aimed at understanding the conditions under which XDT may occur as the result of fragment impact. In addition, we describe some new experiments with improved instrumentation to help elucidate the key processes. Based on the experimental evidence and some simple modelling we then propose a phenomenological model for the XDT process.

There is a clear difference in cook‐off vulnerability between highly‐loaded pressed compositions such as LX‐14 (pressed 95.5% HMX/4.5% binder), which yield violent responses, and cast compositions with low loadings, such as CPX 301 (85% RDX/15% HTPB), which yield relatively mild responses. These two classes of composition differ primarily in the quantity of binder, and in the manufacturing method used in production. An experimental study was conducted in an attempt to determine the filling proportion beyond which violent responses are observed. Here we describe a series of small‐scale cook‐off experiments which studied pressed compositions of 88%, 91%, 95% and 96% HMX, mixed with cured, cross‐linked HTPB. The experiments used a novel glass‐windowed test vehicle, instrumented internally with thermocouples. A trend of increasing event violence with increasing proportion of HMX was found, although in none of the experiments was mass reaction recorded. The results from these experiments are discussed.

In this paper we report new experimental results on the detonation characteristics of nitromethane containing high volume percentages of essentially inert additives. In particular, we have studied the detonation of packed beds of small spherical glass and aluminium particles saturated with pure nitromethane. These mixes are found to have reduced detonation velocities and critical diameters compared to the liquid explosive alone. We conclude with a general discussion of the propagation mechanism in such materials.

Initiation thresholds generated by oblique impacts of rods and spheres are examined, and their relationship to normal impacts of cones are explored. Experimental data from rod impacts show two distinct response regions for bare or lightly covered charges, the division between them depending on the point at which a steady-state oblique shock can no longer be sustained. This steady-state region is eliminated by thicker barriers, but the variation of threshold with impact angle is virtually the same as for the non-steady regime in bare HE. Both rod datasets show a dependency on impact angle rather than a function of its cosine. Cones follow the rod relationship, but spheres are unique amongst these projectile geometries in showing a dependency on the cosine of the angle.

Ultra high-speed digital photography has been used to record the onset and build-up of reaction in nitromethane charges that have been impacted by steel fragments. The nitromethane charges were housed in PMMA cylinders and back-lit using conventional flash bulbs. Flat plates of aluminum were glued to one end of the cylinder and PMMA plates to the other. The completed charge was positioned to allow normal impact of the projectiles through the aluminum barrier plate. The events were filmed using an Imacon 468, ultra high-speed digital image system, capable of framing at up to 100 million pictures per second. Using this system it was possible to record detailed photographic information concerning the onset and growth of reaction due to shock initiation of the nitromethane charges. The results obtained to date are consistent with the established concepts for initiation of homogeneous and heterogeneous materials.

In this paper we describe the development of a coupled molecular dynamics and quantum chemistry approach to the study of shock-induced chemical reactions. The method has been applied to study the interaction between small numbers of energetic molecules undergoing collisions at velocities typical of detonations. Preliminary results for nitromethane and nitric oxide are presented, and discussed in the context of mechanisms operating at, or just behind, detonation fronts. The paper concludes by describing our plans to extend the approach to consider larger systems.

This paper describes the methodology behind the development of a new explosive burn model based on Arrhenius kinetics for both homogeneous and heterogeneous explosives types. The model has been developed over the last four to five years and is incorporated into the DYNA2D hydrocode as a new equation of state. The basic homogeneous model has been extended to consider heterogeneous materials by specifying a size and number density of hotspots. The hotspots are gas filled bubbles which are assumed to collapse adiabatically. Using the model it has been possible to match threshold values for shock initiation of a secondary explosive subjected to fragment impact.

In this paper, we report results of projectile impact experiments into both pure, and amine-sensitised nitromethane. A variety of projectile geometries were used to impact charges covered by aluminium barriers of varying thickness. From these experiments, threshold conditions for detonation were determined. These results have been interpreted in terms of both a classical thermal explosion model, and by use of a 2D hydrocode incorporating an Arrhenius burn routine. The values of the Arrhenius parameters used to fit the results are discussed in the light of possible decomposition schemes for nitromethane.

In this paper we report the results of 3D molecular dynamics simulations of shock and thermally induced chemical reactions in a model energetic material. A heteronuclear diatomic lattice interacting via an Abell-Tersoff potential has been used as the model system. The energies and time-scales required to induce reaction by these stimuli are analysed, and the implications for reaction processes in detonation discussed.

Modem quantum chemistry and molecular dynamics computer codes are powerful tools with which to study the physics and chemistry of energetic materials at the molecular level. Quantum chemistry calculations, on one or two energetic molecules, can give valuable information about the initial steps in their decomposition. Molecular dynamics calculations, even with empirical potentials, can yield important information about the physical processes involved in the initiation and growth of reaction of energetic materials. The combination of Molecular dynamics and quantum chemistry techniques offers the potential to probe energetic material reaction chemistry in real systems, in some detail, in the near future. Such an approach is vital if we are to be able to create new realistic macroscopic models within hydrocodes that can describe the initiation and growth of reaction in explosives. This paper gives an overview of the approach being adopted at DRA Fort Halstead to understanding energetic materials at the molecular level. In particular, the use of quantum chemistry and Molecular dynamics to help construct new macroscopic models will be discussed.

In this paper we report the results of molecular dynamics simulations of shock initiation in an energetic material. An Abell‐Tersoff potential has been used to represent a lattice of diatomic molecules capable of undergoing an exothermic reaction. This model system has been employed to investigate the critical conditions for, initiation of a chemically sustained shock wave. The effect of shock pulses of differing amplitude and duration have been studied, and some variations in the potential parameters have been considered to examine the influence of potential barriers on the initiation thresholds. Finally the results of these simulations are compared with the classical picture of initiation in homogeneous explosives.

In this paper we report the results of recent bullet impact studies on the secondary explosive PE4 using projectiles with various tip geometries. Both experimental and theoretical results indicate that the shock pressure drops as the cone angle of the projectile decreases, and at angles less than ca, 120° the shock is insufficient to cause initiation. Our results also show that for aluminium barriers, detonation, when it occurs, is always prompt.

The trial function suggested recently by Power and Somorjai (1972) has been tested by the calculation of upper and lower energy bounds for the ground states of He and Li+. Upper bounds have also been computed for the next seven singlet, and the first seven triplet, states of both species.

The authors examine the relative accuracies of the Rayleigh upper bound and the Temple (1928), Weinstein (1934) and Stevenson (1938) lower bounds in the limit of an accurate trial function. This analysis leads to an understanding of why lower bounds have usually been observed to have larger errors than upper bounds using the same trial function. In the second part a simple relationship is given between the errors in classical lower bounds, and intermediate- operator lower bounds. The relative errors of Rayleigh and intermediate-operator methods are examined in the limit of an accurate trial function.

Ab-initio molecular orbital calculations are being carried out to study many-body effects for diatomic molecules in the close-in repulsive region. Preliminary results are reported here for H2, N2 and CO. These results indicate that at least 4-body terms must be considered to obtain a good description of the potential for H2 at separations below 1.8 A°. In N2 and CO 3-body effects appear to become significant at separations below 2.8 A°.