Microstructural Characterization of Simulated Plastic-Bonded Explosives
Yeager, John David
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Plastic-bonded explosives (PBX) are highly complex molecular composites. Recent mechanical investigations of PBX properties, in particular deformation and failure under uniaxial and cyclic loading, have revealed microstructure-dependent fracture behavior. A methodology of characterizing the relationship between microstructure and mechanical properties of PBX materials has been developed and tested on simulated materials with particular focus on the interface. Synchrotron X-ray studies of molecular crystals, explosive binders and formulated simulant composites revealed some intriguing possibilities in real-time observation of cracks, bubbles, delamination, and void collapse during high-speed loading events. A surface energy and thermomechanical study of several molecular crystals, including explosives, and potential binder candidates revealed thermodynamic interactions were not likely to be more important than mechanical properties for insensitive explosives. Ellipsometry and neutron reflectometry were used to identify the interfacial structure of polymer-acetaminophen (an explosive simulant) composites. The crystal-polymer interfacial structure was altered by inclusion of a plasticizing agent - an important result considering the commonality of plasticizing polymers in PBX formulation. The difference in interfacial properties was also observed mechanically with nanoindentation. Specifically, the plasticizer inhibited formation of a large, diffuse interface / interphase and resulted in a composite which was more likely to experience film delamination than the non-plasticized composite. The difference in mechanical behavior caused by the difference in interfacial structure has important implications for crack initiation and explosive sensitivity. Additionally, certain crystal-binder composites were investigated with a new delamination test, which, while preliminary, resulted in additional insights into fracture behavior. The methodology presented herein provides a pathway for studying PBXs, or similar composites, from the nano-scale to the macro-scale, both in terms of structure and processing, and shows important relationships between interfacial properties and mechanical behavior.