Publications

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The use of uniaxial strain ramp loading experiments to measure strength at extremely high strain rates is discussed. The technique is outlined and issues associated with it are examined. Results for 6061-T6 aluminum are presented that differ from the conventional view of strain rate sensitivity in aluminum alloys. ©2010 Society for Experimental Mechanics Inc.

The use of uniaxial strain ramp loading experiments to measure strength at extremely high strain rates is discussed. The technique is outlined and issues associated with it are examined. Results for 6061-T6 aluminum are presented that differ from the conventional view of strain rate sensitivity in aluminum alloys. © 2010 Society for Experimental Mechanics Inc.

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With component sizes approaching the mesoscale, conventional size microstructures offer insufficient homogeneity in mechanical properties, forcing microstructures to be reduced to the nanoscale. This work examines the effect of a nanocrystalline surface layer on the dynamic consolidation response of two different morphology Al 6061-T6 powders. Shock-propagation through equiaxed and needle morphology Al 6061-T6 powder beds initially at 73.5 and 75.0% theoretical density, respectively, is simulated at constant particle velocities ranging between 150 and 850 m/s. Shock velocity-particle velocity relationships are determined for powders both with and without the presence of a 2 μm high strength surface layer, which is representative of a nanocrystalline surface layer. Significant deviations in dynamic response are observed with the presence of the surface layer, especially at lower particle velocities. The equation of state (EOS) for both the homogeneous particles and those with a high strength surface layer are found to be best represented by a piecewise EOS. © 2009 American Institute of Physics.

Under shock loading, metals typically increase in strength with shock pressure initially but at higher stresses will eventually soften due to thermal effects. Under isentropic loading, thermal effects are minimized, so strength should rise to much higher levels. To date, though, study of strength under isentropic loading has been minimal. Here, we report new experimental results for magnetic ramp loading and impact by layered impactors in which the strength of 6061-T6 aluminum is measured under quasi-isentropic loading to stresses as high as 55 GPa. Strength is inferred from measured velocity histories using Lagrangian analysis of the loading and unloading responses; strength is related to the difference of these two responses. A simplified method to infer strength directly from a single velocity history is also presented. Measured strengths are consistent with shock loading and instability growth results to about 30 GPa but are somewhat higher than shock data for higher stresses. The current results also agree reasonably well with the Steinberg-Guinan strength model. Significant relaxation is observed as the peak stress is reached due to rate dependence and perhaps other mechanisms; accounting for this rate dependence is necessary for a valid comparison with other results. © 2009 Elsevier Ltd.

The behavior of a shocked tungsten carbide / epoxy mixture as it expands into a vacuum has been studied through a combination of experiments and simulations. X-ray radiography of the expanding material as well as the velocity measured for a stood-off witness late are used to understand the physics of the problem. The initial shock causes vaporization of the epoxy matrix, leading to a multi-phase flow situation as the epoxy expands rapidly at around 8 km/s followed by the WC particles moving around 3 km/s. There are also small amounts of WC moving at higher velocities, apparently due to jetting in the sample. These experiments provide important data about the multi-phase flow characteristics of this material.

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Nanocrystalline and nanostructured materials offer unique microstructure-dependent properties that are superior to coarse-grained materials. These materials have been shown to have very high hardness, strength, and wear resistance. However, most current methods of producing nanostructured materials in weapons-relevant materials create powdered metal that must be consolidated into bulk form to be useful. Conventional consolidation methods are not appropriate due to the need to maintain the nanocrystalline structure. This research investigated new ways of creating nanocrystalline material, new methods of consolidating nanocrystalline material, and an analysis of these different methods of creation and consolidation to evaluate their applicability to mesoscale weapons applications where part features are often under 100 {micro}m wide and the material's microstructure must be very small to give homogeneous properties across the feature.