In the framework of low velocity impact studies, dedicated to safety analyses of plastic bonded explosives (PBX), we propose a new numerical tool, designed for restituting the ignition of a HMX (high melting point explosive) based composition. Major results are the use of a concrete-like constitutive law for the PBX and an efficient implementation of an ignition criterion. We also put forward two variants of classical Steven tests, which enable us to visualize either a dot ignition or an unusual ring-shaped ignition. It is shown that the calculation tool is able to restitute accurately both results. © 2008 Elsevier Science. All rights reserved Keywords: PBX safety, Steven test, ignition criterion ---* Corresponding author. Tel.: +33-2-47-34-41-73 ; e-mail: cyril.gruau@cea.fr, didier.picart@cea.fr IntroductionHigh explosive parts confined in pyrotechnic devices can unintentionally ignite, when submitted to impacts, even at low velocities. Ignition can then evolve to more violent and undesired events, such as deflagration or detonation. Prediction of such transitions being hardly tractable, we are focusing our attention on the ignition stage, which is the very first event that must be correctly restituted, in the framework of safety analyses.The considered high explosive is composed of a poly-dispersed distribution of HMX grains, with a mean diameter of 200 µm, which are mixed with a few percentage of a polymeric binder. An isostatic compaction process is used to reduce the composition porosity at few percentages. Its mechanical response is similar to the behavior exhibited by PBX-9501, another explosive composition.The low velocity impacts of interest may occur during transport, storage, assembling or disassembling of devices, leading to a great number of accident scenarios. Thus, safety analyses of pyrotechnic structures involve many parameters, like target configuration (shape, materials and boundary conditions) and mechanical loadings (characteristics of the projectile and its movement law), which prevents us from dealing only with experimental techniques. The aim of this paper is to describe a numerical tool designed for the ignition, applied to this kind of accidental configurations.In order to validate such a numerical tool, an experimental database is needed. The so-called "Steven test", introduced by Chidester and co-workers [1] was modified to simplify the boundary conditions and to limit strain localization [2]. This test is devoted to the determination of a safety threshold (in terms of impact velocity), under which no violent reaction is observed. Figure 1 shows three pictures taken during one of these tests.International Journal of Impact Engineering 2 Fig. 1. High speed recording of an impact test (the PBX sample is confined inside the steel target): picture before impact, during impact and at the beginning of reaction (the configuration of the target is given on Figure 15, target 3, projectile velocity = 77 m/s).Deflagrations and detonations induced by low velocity impacts are not the resul...

We analyze the evolution of the sensitivity of a TATB composition after thermal cycles at elevated temperatures. Sensitization due to thermal cycles is of variable magnitude depending on the kind of the second stimulus (mechanical or thermal). In order to investigate the possible mechanisms which govern these phenomena, we perform an extensive study of the evolution of the chemistry and microstructure of our composition and we determine the sensitivity of our explosive to various stimuli after various temperature/duration cycles. This first paper is devoted to the study of TATB chemical evolutions. We present the results obtained for explosive decomposition, furazan generation, gas analyses and solid residue characterization.

This paper is devoted to a review of microstructure observations performed on a TATB composition submitted to thermal and combined thermal/mechanical stimuli. Effects are identified and quantified in terms of decomposition extension and microstructure evolution (porosity, shear bands, grains shape and orientation, anisotropy …︁). Several complex mechanisms and important transformations are exhibited as a function of temperature, heating duration and shock wave intensity. Conclusions on decomposition mechanisms are directly derived from these observations.

We present a complete experimental and theoretical study of hot spots generation in pressed explosive compositions. First, the results of experiments leading to the identification of the hot spots origin are detailed. Then, using these results, a physical model is developed. Three applications are presented. The firts one is devoted to the study of the sensitivity of mixed HMXfTATB compositions. The second one deals with double shock desensitization. The third one is the development of an efficient SDT kinetics using the hot spot model previously described.