Ignition of a confined high explosive under low velocity impact

Abstract

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...

Dynamic behavior of an aggregate material at simultaneous high pressure and strain rate: SHPB triaxial tests

“…Lastly, the end of the signal indicated gauge failure with the pyrotechnic reaction of the high explosive. Gruau et al [7] performed some numerical simulations of these experiments. To this end, they developed a numerical tool based on the ABAQUS/Explicit finite element code and a concretelike constitutive property for the high explosive [8][9].…”

“…To this end, they developed a numerical tool based on the ABAQUS/Explicit finite element code and a concretelike constitutive property for the high explosive [8][9]. Gruau and Picart [10] determined the material parameters. The simulation of the configuration reported in Figure 1 exhibited a mean plastic strain rate of 10 5 s -1 , a maximal pressure close to 500 MPa and a finite strain approaching 0.5.…”

“…This model has been improved and modified to get a better description of the behaviour in the range of relative pressure 0<P*<10 [37]. This model takes into account the third invariant (R depends on the third invariant) and introduces a non linear rate dependence (10).…”

Experimental investigation of an aggregate material behavior under confinement at high strain rate

“…However, it was shown that some explosive compositions reacted violently when submitted to a low velocity impact (the pressure is far from the shock to detonation transition threshold) [1]. The mechanical behavior of the explosive is one of the keys needed to predict such an event [2][3].…”

“…Experimental impact tests and numerical studies [3] have clarified the loading conditions during low velocity impact. The pressure can reach 300 to 500 MPa when a strain rate equal to 10 4 s -1 is expected, as well as finite strains approaching 1.…”