Dissertation
Defect mediated plasticity and mechanical properties of energetic and inert molecular single crystals
Washington State University
Doctor of Philosophy (PhD), Washington State University
05/2009
DOI:
https://doi.org/10.7273/000006175
Abstract
Although deformation has been extensively studied in a wide range of materials, deformation in molecular crystals has received relatively little attention. Current research efforts in the explosives and pharmaceutical industries demonstrate that while attempts have been made to understand complex compaction processes, a basic understanding of elementary defects and deformation mechanisms in molecular materials is missing. Elementary deformation mechanisms have been inconclusively implicated in industrially important processes. These processes determine the safety and performance of energetic crystalline constituents in plastic bonded explosives and the overall bioavailability of therapeutic crystalline constituents in solid dosage pharmaceutical tablets. The significance of defects and deformation mechanisms in compaction processes has not been demonstrated because of the difficulties associated with studying deformation in molecular crystals that inherently possess low symmetry, inter- and intra-molecular degrees of freedom, and susceptibility to electron beam degradation. Dynamic and quasi-static deformation investigations of molecular single crystals were conducted to produce more easily interpretable results. Plate impact experiments on single crystals of the explosive cyclotrimethylene trinitramine (RDX) demonstrated the affect of defects on Hugoniot elastic limits. This illustrated that further understanding of defects and elementary deformation in molecular materials was necessary before efforts could be undertaken to experimentally assess proposed dislocation based mechanisms for shock initiation of detonation. As a consequence, quasi-static deformation investigations were pursued to characterize deformation mechanisms. Nanoindentation and scanning probe microscopy were selected as the most applicable techniques to study incipient plasticity and the brittle nature of molecular crystal deformation. A model material for both pharmaceuticals and explosives, i.e. sucrose, was used in initial studies to develop sample preparation and testing procedures. These efforts enabled investigation of deformation, beginning with elasticity and continuing through the elastic-plastic transition to incipient cracking, for {210}, {021}, and (001) oriented single crystals of RDX. The onset of plasticity occurred near the theoretical shear strength indicating homogeneous nucleation of dislocations while evidence of slip on six planes was observed. New slip modes seemed to appear as a function of imposed stress state. Compatibility conditions for crystalline materials were used to interpret the observations of orientation dependent plasticity and cracking in RDX.
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Details
- Title
- Defect mediated plasticity and mechanical properties of energetic and inert molecular single crystals
- Creators
- Kyle James Michael Ramos
- Contributors
- David F. Bahr (Chair)David P Field (Committee Member) - Washington State University, School of Mechanical and Materials EngineeringMatthew D. McCluskey (Committee Member) - Washington State University, Department of Physics and AstronomyDaniel E Hooks (Committee Member)Stephen Sheffield (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Materials Science and Engineering Program
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 213
- Identifiers
- 99901055025901842
- Language
- English
- Resource Type
- Dissertation