High-Precision Measurements And Modeling Of How Brittle Granular Materials Behave Under Shock Compression
Voorhees, Travis John
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The objective of this research is to develop a model or modeling approach that can accurately predict the dynamic compaction behavior of brittle powders as a function of measureable initial state parameters. To generate the framework for a predictive model, the dynamic compaction behavior of a model brittle powder, CeO2, is investigated using a combined experimental and computational approach. The Hugoniot states of CeO2 powder at four initial pressed densities are measured via plate impact experiments and used to calibrate continuum compaction models. Empirical fitting parameters for the continuum compaction models are investigated for correlations with the CeO2 powder initial density. The single fitting parameter of the P-α Menikoff-Kober compaction model, PC, is shown to exponentially increase with increasing initial density, ρ00, and is replaced in the model with a functional form, PC(ρ00), producing a semi-empirical predictive compaction model. To investigate the applicability of compaction models to nonplanar compression scenarios, three commonly used continuum compaction models are calibrated to the experimentally measured planar impact Hugoniot data and used to computationally design validation experiments. Two validation experiments are executed on 3.95 and 4.03 g/cm3 CeO2 powder targets using the pulsed power driver PHELIX. In situ measurements of the CeO2 densification response are performed with proton radiography and analyzed against the model predictions. Compaction behavior is found to be best captured with a P-α model, which calculates CeO2 powder bulk densities within 80-99% of experimental values but overpredicts densification at the cylindrical target’s outer radius and center by up to 20%. The contribution of CeO2 powder strength to its nonplanar compaction response is investigated using an elementary porous strength model. This porous strength model is employed by calibrating a strength model for solid CeO2 then applying a linear transformation with respect to porosity to define porous CeO2 strength. Preliminary simulations of the PHELIX experiment using decoupled compaction and strength models show improved accuracy in late-time density calculations. Additional research into which strength models and solid-to-porous transformation methods produce the most accurate results are necessary to further improve this modeling approach. The overall outcomes of the work described in this dissertation include (1) an experimental and computational approach that can be used to generate semi-empirical predictive compaction models for brittle and granular materials, (2) a greater understanding of how brittle granular materials compact and deform under both planar and nonplanar shock compression, and (3) discernment between compaction modeling approach accuracies in extrapolated regions of phase space. Extensions of this research may allow the development of a physics-based predictive model for the dynamic compaction of brittle powders.