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...

International audienceLow velocity impacts on energetic materials induce plastic deformations and sliding friction which can lead to ignition. If some ignition criteria have been proposed, the remaining difficulty is to characterize the mechanical behavior of the material when submitted to the corresponding solicitations (high pressure and high strain rate). Thus, a technique based on the Split Hopkinson Pressure Bars system is proposed to carry out a triaxial compression test. A cylindrical specimen is placed into a confining ring and is compressed by the system of bars. The ring prevents the radial extension of the specimen and creates a lateral confining pressure. The material and dimensions chosen for the ring maintain a constant radial pressure during the test. Some tests were carried out on an inert aggregate material and proved the validity of this experimental device. The experimental data processing shows the influence of both the pressure and the strain rate. The shear stresses, which contribute to thermal dissipation and then to the ignition threshold, increase according to the pressure

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