Static and dynamic compaction of ceramic powders

Abstract

The static and dynamic compaction of ceramic powders was investigated experimentally using a high-pressure frictioncompensated press to achieve static stresses of 1.6 GPa and with a novel gas gun setup to stresses of 5.9 GPa for a tungsten carbide powder. Experiments were performed in the partial compaction region to nearly full compaction. The effects of variables including initial density, particle size distribution, particle morphology, and loading path were investigated in the static experiments. Only particle morphology was found to significantly affect the compaction response. Post-test examination of the powder reveals fracture of the grains as well as breaking at particle edges. In dynamic experiments, steady structured compaction waves traveling at very low velocities were observed. The strain rate within the compaction waves was found to scale nearly linearly with the shock stress, in contrast with many fully dense materials where strain rate scales with stress to the fourth power. Similar scaling is found for data from the literature on TiO 2 powder. The dynamic response of WC powder is found to be significantly stiffer than the static response, probably because deformation in the dynamic case is confined to the relatively narrow compaction wave front. Comparison of new static powder compaction results with shock data from the literature for SiO 2 also reveals a stiffer dynamic response.

Shock response of dry sand.

“…A plot of the calculated reshock states is given in Figure 13. In most cases, the measured reshock states lie above the Hugoniot, as was the case for WC powder [4]. This suggests that the material is able to behave in a stiffer manner after being shocked.…”

“…The current fixture is a simplified version of one used previously for tungsten carbide powder experiments [4]. The fixture was designed to be used on a 102 mm bore light gas gun, which operates by the release of compressed gas and can achieve projectile velocities as high as 900 m/s.…”

“…This behavior may be attributed to the low density of sand. Previous work [4] focused on experiments with similar shock velocities, but higher densities, producing much steeper Rayleigh lines. All of the experiments in this work, however, result in Rayleigh lines lying below both the PMMA and the Z-cut quartz Hugoniots, making a reshock the only observed off-Hugoniot state for this configuration.…”

Pressure-shear experiments on granular materials.

“…This is due to two effects. First, waves in granular materials such as sand and WC have finite rise times if the stress level is low or moderate [Brown et al, 2007;Vogler et al, 2007]. Second, because the WC powder has a lower impedance than the Ti-6Al-4V driver and anvil, the longitudinal wave reverberates within the sample until the equilibrium stress level is reached.…”

“…15 the same strength values are plotted against the relative density of the material from the previously measured Hugoniot for these powders [Brown et al, 2007;Vogler et al, 2007]. To do this, a fit to the Hugoniot data is used to determine the compressed density based on the final equilibrium stress given in Table 2.…”