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Quasistatic studies of the deformation, strength and failure of polymer-bonded explosives

PhD Thesis, University of Cambridge, 2000
P J Rae

Abstract

The research presented here provides some new insights into the deformation mechanisms, strength and failure criteria of polymer bonded explosives (PBXs). These materials are typically composed of 90-95% explosive crystals in a 5-10% polymer matrix. A number of experimental methods have been used to generate quantitative data including moiré interferometry and digital image cross-correlation (DICC). A particular innovation has been the development of methods that allow accurate measurement of strains as well as high magnification observation of microstructure. Optical and electron microscopy (using both a standard SEM and an environmental scanning electron microscope, ESEM) have been used to gain more qualitative information about the nature of failure. Using these techniques, information about the microscopic deformation of these materials and the way in which failure sites develop and propagate is obtained. It is important to understand the mechanical properties of these materials and how and why they fail, both to optimize their composition and more importantly, to improve safety during handling and in accident situations.

Previous work on the mechanical properties of PBXs has tended to focus on the macroscopic scale (Palmer & Field 1982, Wiegand 1995; Gray III et al. 1997, Hoge 1967). In general, mechanical data have been generated with little regard to the eventual mechanisms of failure. For this reason there is considerable interest in investigating the fundamental microscopic mechanisms at work within the material under load so that issues of composition development and ageing may be better understood.

Because of the obvious safety issues surrounding work on powerful explosives, tests for use in this laboratory have had to be developed using small amounts of the materials (<0.5 g). The Brazilian test (or diametral compression test) is used on cylindrical samples 10 mm in diameter by 4 mm thick. This geometry has the advantage that although compressive stress is applied, a tensile failure is generated in the centre of the sample without the difficulty of sample mounting present in more traditional geometries.

Three UK compositions EDC29, EDC35 and EDC37 are studied alongside one from the US, PBX9501. EDC35 is markedly more brittle than the other compositions. Therefore, only limited information could be obtained relating to strain and stress to failure. Post failure micrographs were also obtained. The other compositions are easier to study.

As well as numerical information, a study of the onset and propagation of failure has been made. In these materials, individual failures start around the edges of larger filler particles. These sites can then link up to form continuous failure routes. Sometimes secondary failure routes are created which may or may not continue to extend and open. The failure is predominately trans-granular although crystals presenting a flat crystal edge perpendicular to the advancing crack may be fractured. In rubbery compositions binder filaments are generated which span the cracks in fine-grained regions and impede the rate of crack growth. In all compositions, clean crystal faces are exposed suggesting an adhesive failure between the crystal and binder.

Some experiments relating to the behaviour of thin binder layers are included together with some efforts at modelling the bulk PBX properties from consideration of the component material properties.

References

Gray III, G. T., Blumenthal, W. R., Idar, D. J. & Cady, C. M. 1998 "Influence of temperature on the high-strain-rate mechanical behaviour of PBX9501" In "Shock compression of condensed matter - 1997", ed. S. C. Schmidt, D. P. Dandekar, J. W. Forbes, pp. 583-586, publ. American Institute of Physics, Amherst, Massachusetts

Hoge, K. G. 1967 "The behaviour of plastic-bonded explosives under dynamic compressive loads" Appl. Polym. Symp. 5 19-40.

Palmer, S. J. P. & Field, J. E. 1982 "The deformation and fracture of beta-HMX" Proc. R. Soc. Lond.A383 399-407.

Weigand, D. A. 1995 "Constant critical strain for failure of highly filled polymer composites" In "Proc. 3rd Int. Conf. Deformation and Fracture of Composites", Guildford, UK.


Thesis available upon request.

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