No abstract

This paper describes research into the low strain rate mechanical properties of polymer bonded explosives (PBXs) and follows on from that presented by Palmer and co-workers in 1993. PBXs are highly filled composite materials comprised of crystals of a secondary explosive supported in ca. 5-10% (by mass) polymeric binder. In general, the modulus of the binder is 10 5 times lower than that of the crystalline explosive, resulting in the behaviour of the composite being heavily influenced by the properties of the binder despite the low concentration present. The Brazilian test, in which a disc of material is loaded diametrically in compression, has been used to generate tensile failure in the materials studied. Three methods of microscopy have been used to examine the nature of failure in two UK compositions and one US material (PBX 9501). Pre-and post-failure optical and electron microscopy examinations of the materials have been undertaken to gain a greater understanding of the role of the microstructure and this has been aided by the use of an environmental scanning electron microscope to follow real-time failure in the PBXs. Failure in all three compositions has been observed to start around the edges of larger filler particles perpendicular to the direction of tensile strain. Compositions with rubbery binders have been observed forming binder filaments which bridge the crack walls, while clean crystal faces are observed on the larger particles. In a composition with a nitrocellulose-based binder, hair-like features of nitrocellulose have been seen sticking out from the failure surfaces and the rougher crack walls.

An algorithm for unwrapping noisy phase maps has recently been proposed, based on the identification of discontinuity sources that mark the start or end of a 2π phase discontinuity. Branch cuts between sources act as barriers to unwrapping, resulting in a unique phase map that is independent of the unwrapping route. We investigate four methods for optimizing the placement of the cuts. A modified nearest neighbor approach is found to be the most successful and can reliably unwrap unfiltered speckle-interferometry phase maps with discontinuity source densities of 0.05 sources pixel(-1).

This paper examines the influence of microstructure on the quasi-static failure of PBX 9501, a polymer-bonded explosive (PBX) manufactured for the Los Alamos National Laboratory in America. Optical microscopy has been used to examine qualitatively cracked and pristine material. Consequent on the manufacturing process, the explosive crystals display angular features and natural facets. In addition, considerable growth twinning, internal defects and voidage has been observed. These defects are found significantly to alter the failure path. In common with other PBXs, failure paths tend to run around the long straight edges of the explosive filler and avoid regions of fine filler and binder. Explosive crystals were found to fracture due either to cracks propagating from another region or internal defects. These observations are confirmed by the use of high-resolution moiré interferometry. This sensitive optical technique allows the deformation of the sample to be measured up to and including the point of failure. By taking white-light micrographs that are in exact registration with the measured displacement maps, the influence of the underlying microstructure can be seen.

No abstract