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.