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Damage in energetic materials

PhD Thesis, University of Cambridge, 2014
R L Boddy

Abstract

The research described in this Thesis is concerned with mechanical damage in energetic materials, a topic which is of significant importance but into which there has been relatively little previous research. Both the nature of the damage process during mechanical loading and the subsequent on the macroscopic properties of these materials have been investigated. Studies were performed on simple prototypical Polymer Bonded eXplosives. Two different damage processes were found to occur which dominate under different conditions depending on both particle size and damage loading rate. Total particle debonding and progressive partial debonding were proposed as the two processes. The post damage mechanical response of these materials was shown to be anisotropic. The effect of varying the time a sample was held at both a fixed strain and a fixed load was studied. Mechanical fatiguing was also investigated and was found to involve additional damage processes not seen in single stage loading. Group Interaction Modelling, an approach which uses the fundamental structure of the material to predict bulk properties of the material, was applied to Elastomer Modified Double Base Composite propellant materials. Various thermal and mechanical properties of the materials were measured in order to validate the model. A comprehensive investigation into the effects of mechanical damage on the propellant materials was carried out. X-ray Computed Tomography was used to both qualify and quantify the damage. The dominant damage mechanism in these more complex materials was found to be material fracture which began once a critical input energy had been exceeded. Significant changes in the post damage mechanical response occurred before macroscopic fracture began attributed to re-orderings of polymer chains. The effect of damage on both burn response and impact initiation of the propellants was investigated. Closed vessel experiments were used to study the effect of damage on the burn response. The burn rate increased with increasing levels of damage and could be accounted for by the increased surface area of the damaged samples. The effect of damage on impact initiation was found to differ depending on the particular impact scenario. For low velocity impacts (of the order of 5 m s^-1) initiation was found to occur for both damaged and undamaged sample, the initiation mechanism in this case being adiabatic heating of trapped gas. Initiation occurred under high velocity impact (of the order of 103 m s^-1) for both damaged and undamaged samples but a critical velocity was also identified, above which damaged materials underwent a transition to a more violent reaction. Significant progress has been made into understanding both the processes by which damage occurs during damage and the effect that this has on the properties of energetic materials.

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