Deformation, strengths and strains to failure of polymer bonded explosives

Abstract

This paper describes a study of the strength and failure properties of a range of polymer bonded explosives (PBXS). These are composite systems in which small (typically micrometre up to millimetre-sized) explosive crystals are bonded by a polymer (typically 2–10% (by mass)). In PBXS it is important to optimise the mechanical properties, while maintaining a low sensitiveness (i. e. the material is safe to manufacture, store and handle) and high explosiveness (i. e. reacts powerfully to a prescribed stimulus). The Brazilian test, in which a disc-shaped specimen is loaded diametrically, was chosen for the study. The advantages are that relatively small specimens of typically 10 mm diameter and 4 mm thickness can be used, and that the tensile stresses on the central axis are achieved by applying compressive stresses at the anvil so that complicated gripping arrangements are not required. The technique of double-exposure laser speckle photography was chosen to measure the in-plane displacement field. The technique can measure displacements to sub-micrometre accuracy and provide information over the whole specimen surface. These are distinct advantages over strain gauge methods that involve attaching gauges to the specimen and which only give pointwise information. The double-exposure speckle pattern records were interpreted using an automated Young’s fringes method. The PBXS were of three explosive types and those based on HMX were studied systematically for two crystal sizes and three different binder materials, of two different weight percents. In general, compositions based on micronized crystals were the strongest. Polishing techniques were developed to study the deformation of the individual crystals, the points of nucleation of failure and the fracture paths through the PBXS. The failure modes are discussed in terms of various theoretical models. The mechanical twinning which was shown in earlier work to be an important failure mode in β -HMX also takes place in PBXS based on HMX. The general applicability of the techniques developed in this research for other composite systems is emphasized.

Macro-Micro Mechanical Behavior of a Highly-Particle-Filled Composite Using Digital Image Correlation Method

“…Careful examination can see that deformed twinning occurred on the single particle surface. It's known that twinning allows particles to accommodate large strains without fracture, but fractures can develop through the particles with increasing of the external force, it is considered that the twin is forming a step at the surface that acts as a stress concentrator [30]. Real time microscopic examination reveals that different forms of failure including interfacial debonding and particle fracture can be observed.…”

“…The mechanical properties of PBX subjected to a range of conditions are important criteria to determine a safe working life. The study on the mechanical properties and the failure mechanisms of PBX has drawn much attention in recent years [1][2][3][4][5][6][7][8][9]. Low strengths and safety concerns bring additional difficulties in preparing samples and conducting mechanical tests of PBX.…”

Influence of temperature on the high-strain-rate mechanical behavior of PBX 9501

“…A significant number of previous studies have probed the constitutive response of a wide variety of plastic-bonded explosives (1)(2)(3)(4)(5)(6)(7)(8). Beginning with the high-rate Hopkinson split-bar studies of Hoge( 1) on a range of PBX's and continuing with the highstrain rate work of Field (3,5), Palmer(6), and…”

Data Analysis, Pre-Ignition Assessment, and Post-Ignition Modeling of the Large-Scale Annular Cookoff Tests

“…BDNPA/F (a plasticizer made up of equal parts of bis(2,2-dinitropropyl)acetal (BDNPA) and bis(2,2-dinitropropyl)formal (BDNPF)). There are many different kinds of Estane (a polyurethane thermoplastic) produced by B. F. Goodrich, and different authors report two kinds that have been used in the production of PBX-9501 (Palmer et al 1993;Cady et al 2000;Cooper 1996). For example, in Los Alamos Explosive Property Data (1980), it is reported that Estane 5703 is used in PBX-9501 and that "Above 50°C, the polyester polyurethane Estane depolymerizes, so its strength decreases with time."…”