Shock Wave Behavior of Particulate Composites

Abstract

Material heterogeneity at some scale is common in present engineering and structural materials as a means of strength improvement, weight reduction, and performance enhancement in a great many applications such as impact and blast protection, construction, and aerospace. While the benefits of transitioning toward composites in practical applications is obvious, the methods of measurement and optimization required to handle spatial heterogeneity and bridge length scale differences across multiple orders of magnitude are not. This is especially true as loading rates transition into the shock regime. Composite materials, such as concrete, have advantages afforded to them by their microstructure that allow them to dissipate and scatter impact energy. The mechanical mismatch between constituent phases in composites (mortar and cement paste in concrete, crystals and binder in polymer bonded explosives, ceramic powder and epoxy in potting materials, etc.) provides the interfaces required for shock wave reflection. The degree to which a shock is disrupted from its accepted form as a propagating discontinuity in stress and particle velocity is highly dependent upon the size, shape, and density of the interfaces present. The experimental and computer aided simulations in this thesis seek to establish a scaling relationship between composite microstructure and shock front disruption in terms of particulate size and density through the use of multi-point heterodyne velocity interferometry. A model particulate composite has been developed to mimic the wave reflection properties of materials such as Ultra High Performace Composite (UHPC) concrete and polymer bonded explosives, while also being simple to source and manufacture repeatably. Polymethyl Methacrylate (PMMA), a thermoplastic polymer, and silica glass spheres satisfy the manufacturing constraints with a shock impedance mismatch of 4.1, when placed in-between the shock impedance of UHPC concretes (~ 10) and polymer bondedexplosives (~ 2). The flexibility afforded by the model composite allows for the use of mono-disperse bead particle diameter distributions centered at 5 discrete diameters centered in the range associated with high scattering effectiveness (5-50 times the shock thickness in the pure matrix material). Shock front disruption is measured at multiple points on the rear surface of a plate impact target to observe shock spreading and spatial heterogeneity in material response due to random particle placement. Shock rise times are reported for composites of 30% and 40% glass spheres by volume, with glass spheres of 100, 300, 500, 700, and 1000 micron diameter. Composites with single mode as well as bi-modal bead diameter distributions are subjected to plate impact loading at an average pressure of 5 GPa. In single mode composites, a linear dependence of shock wave rise time on particle diameter is observed, with a constant of proportionality equal to the bulk shock speed in the material. Bi-modal bead diameter composites were fabricated in order to achieve higher volume fractions without composite degradation. The addition of a second phase to a base 30% glass by volume composite mix results in significant increases in shock wave rise time for base mixes of 500 micron beads, while a point of maximum scattering effectiveness is observed for base mixes of 1000 micron diameter beads. A comprehensive two dimensional series of CTH hydrocode simulations has been completed in tandem with experiments. An evaluation of the discrepancies in simulation and experimental results is presented. Shock disruption mechanisms and matrix/interface damage effects are discussed as possible sources of error and potential avenues for model improvement. The scaling arguments and model deficiency corrections made in this thesis have the potential to drive the development of new approaches of modeling shock waves in heterogeneous materials as well as optimization of microstructure for maximum shock front disruption.