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...

Two typos in the last two equations in the appendix have been corrected (changes highlighted in red). This preprint is consistent with the published version. Results unchanged.International audienceThe thermoelastic response of a TATB-based pressed explosive is studied using morphological modeling and a numerical Fourier scheme. First, we characterize the polycrystalline-like microstructure in terms of the (2D) granulometry and covariance functions, measured on SEM micrograph images. The granulometry is found to be close to a Rayleigh distribution. Second, we represent the polycrystal by a modified Johnson-Mehl tessellation with time-varying germination-rate, in order to approach the wide size distribution observed on the SEM images. We find excellent agreement between the numerically optimized model and the real material in terms of granulometry. Third, we compute the thermoelastic response of the microstructure model by means of full-field Fourier-based computations. Each crystal is assigned uncorrelated random orientations. The thermomechanical response of single crystals is provided by the molecular dynamic simulations of Bedrov et al. (2009) and the X-ray diffraction results of Kolb et al. (1979). Macroscopic (uniform) temperature or strain loadings are applied along various directions (tension, shear or hydrostatic). We observe strong internal stresses upon heating, owing to the highly anisotropic thermoelastic response of TATB and random crystallographic orientations in the polycrystal. The largest stress and strain gradients are observed at grain boundaries, enforcing the idea that grain boundary fracture is indeed the irreversible mechanism underlying ratchet growth. As a first attempt to account for the plastic binder, a 4-voxels soft interphase is inserted at grain boundaries. This results in a strong softening effect on elastic macroscopic properties

The text in the present document is the same as in the published article, page layout is different. v2: The bibliographical data for one reference (previously marked as submitted and published since then) has been updated.