Shock compression of a heterogeneous, porous polymer composite

The objective of this study was to investigate the shock compression response of several composite materials composed of 25% by volume of a ceramic powder in a THV polymer matrix. There were four different ceramic powders used to make the composites: 1, 10, and 100 mum alumina (Al2O 3) and 10 mum zirconium carbide (ZrC). Characterization of the four composite materials revealed that the ceramic particles were not homogenously distributed in the matrix. Instead, there were large (∼0.5 mm) regions of pure THV, surrounded by mixed-phase regions consisting primarily of packed ceramic powder, partially infiltrated with THV but still containing some void space. In addition, it was found that the morphology of the 1 mum Al 2O3 powder was distinctly different from that of the other Al2O3 powders as well as the ZrC powder, in that it was composed of small (∼250 nm) particles agglomerated and partially fused to make larger ∼1-10 mum porous aggregates. Furthermore, the ZrC/THV composite was found to contain large amounts of contaminants.;In order to understand the behavior of the constituent materials for the composites, the shock compression response of THV was also investigated. The US-UP Hugoniot of THV was similar to that reported for other fluoropolymers in the literature. For the composites, the Hugoniots displayed unexpected trends and, in general, did not match the predictions of a number of analytical models. The trends observed in the stress wave profiles (obtained in the course of the Hugoniot experiments) were compared with those reported in the literature for similar materials. Two of the alumina powders employed in the composites were investigated using static and dynamic methods to understand the compaction/compression response. Shock compression of a homogeneous ceramic powder/polymer composite (alumina in an epoxy matrix) was also investigated. Models that agreed with experimental results for this composite were applied to the more complex THV composites. Correlating the model predictions and the experimental data lead to questions concerning the inertness of the composites and the validity of the assumed form of the Gruneisen coefficient. Possible reactions between the ceramic particles and the THV matrix were investigated. Although no reaction was discovered between Al 2O3 and THV, evidence of a reaction was found between ZrC and THV. It was demonstrated that consideration of the reaction improves the correlation between the predictive models and the observed response.;For the 10 mum "ZrC/THV" composite, a major challenge in predicting the shock compression results is identifying the starting components. It was shown that the composite is actually composed of ZrC, monoclinic ZrO 2, THV, and some other unknown contaminants. Even with this compositional uncertainty, it was shown that predictions for the composite Hugoniot agree quite well with experimental results, provided that the higher value gamma was used and the crush-up behavior of the particles is accounted for. This work also presents evidence of a thermally-induced reaction in the ZrC/THV composite, and presents an alternative explanation for the observed Hugoniot based on an exothermic, shock-induced reaction model. Furthermore, this work shows that the waveforms of these porous, heterogeneous polymer composites follow previously noted trends of decreasing risetime with increasing pressure.;As part of the investigation into the shock-wave risetimes of the composites, this work also investigated the inherent risetime of the PVDF stress gauges used to record the pressure wave profile, using comparisons with experiments done with VISAR particle velocity gauges. A clear discrepancy in the gauge records after the initial shock wave front has passed is observed. It has been proposed in the past that this discrepancy is due to a pressure-induced phase transformation in the PVDF gauge material, although no evidence of a phase transition is observed at room temperature. However, a significant question is raised concerning the appropriate form of the Gruneisen coefficient gamma for polymeric materials. Although the exact form of gamma(V) is unclear, it is reasonable to assume an exponentially-decaying form of gamma( V), where at low temperatures gamma is an order of magnitude higher than the traditional values, and at high pressures, the traditional (lower) values are approached as the bonding becomes less anharmonic. When such an exponentially decaying form of gamma(V) is applied to the PVDF stress gauge material, it was shown that a pressure-induced phase transformation could be prevented in static high pressure experiments at room temperature, while still occurring in the shock compressed material. (Abstract shortened by UMI.)...