Session K6: Particulate, Porous and Composite Materials III

Compaction and High-Pressure Response of Granular Tantalum Oxide

The dynamic behavior of nearly fully-dense and porous tantalum oxide (Ta$_{2}$O$_{5}$) is studied. Two particle morphologies are used to obtain two distinct initial tap densities, which correspond to approximately 40\% and 15\% of crystalline density. The response is characterized from low pressures, which result in incomplete compaction, to very high pressures where the thermal component of the EOS dominates. Issues related to a possible phase transformation along the Hugoniot and to establishing reasonable error bars on the experimental data will be discussed. The suitability of continuum and mesoscale models to capture the experimental results will be examined. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Meso-scale simulation of shocked particle laden flows and construction of metamodels

In a typical multi-scale modeling problem, such as shock interaction with a dusty gas, information needs to be communicated between disparate length scales, for example between the system scale (order of meters) and the particle scale (order of microns). For the passage of a shock through a cloud of particles, the particle-gas interphase transfer terms in the macro-scale equations are typically based on empirical models of the drag force around a single particle embedded in a shocked flow. Often physical experiments to construct empirical models are restricted in parameter space and difficult or even impossible to perform for a wide range of parameters (Mach number, solid fraction, Reynolds numbers etc.). The goal of the current work is to use high-resolution meso-scale computational experiments as surrogates to physical experiments; a metamodeling approach is developed to ``lift'' information from the particle scale to the macro-scale. The research compares different metamodeling techniques and demonstrates the efficient use of metamodels to close the macro-scale equations; the meso-scale simulations provide a numerical drag law which can be readily used as a source term in macro-scale governing equations.   

Dynamic deformation of heterogeneous media: A materials scientist's perspective

Traditionally, materials design assumes full density during the usage of materials, and rather explicitly excludes open spaces. However, with increasing usage in structural applications of cellular solids and the advent of additive manufacturing to make intricate shapes this assumption is flying out the window. But this raises the question of how we deal with the underlying physics associated with the void space, particularly when such materials architectures are dynamically loaded. This builds upon decades of work on granular systems, particularly powder composites and sand. Using as examples polymeric structured lattices and particle composite mixtures we will examine the influence of the void space on the overall response of the material mesostructure.   

On mesoscale methods to enhance full-stress continuum modeling of porous compaction

The dynamic compaction of initially porous material is typically treated in continuum dynamics simulations via adjustments to the scalar equation of state (EOS) of the bulk, porous material relative to that of the solid. However, the behavior during compaction is governed by inelastic processes, as the solid material deforms, largely by shearing, to fill the voids. The resulting response depends on the strain path, e.g. isotropic versus uniaxial loading. Adjustments to the EOS are therefore fundamentally unsuited to describing porous compaction, and it is desirable to consider porous effects through the stress and strain tensors. We have investigated porous modifications to continuum strength models, designed to reproduce elastic wave speeds in porous materials and the crush response observed experimentally during compaction. We have performed mesoscale simulations, resolving the microstructure explicitly, to guide the construction of continuum models. These simulations allow us to study the interplay between strength and EOS in the solid, the extent of dissipative flow versus non-dissipative displacement, and the evolution of porosity and micro-morphological features can be captured.   

Shock Wave Structure in Particulate Composites

Shock wave experiments are conducted on a particulate composite consisting of a polymethyl methacrylate (PMMA) matrix reinforced by glass beads. Such a composite with an impedance mismatch of 4.3 closely mimics heterogeneous solids of interest such as concrete and energetic materials. The composite samples are prepared using a compression molding process. The structure and particle velocity rise times of the shocks are examined using forward ballistic experiments. Reverse ballistic experiments are used to track how the interface density influences velocity overshoot above the steady state particle velocity. The effects of particle size (0.1 to 1 mm) and volume fraction of glass beads (30-40{\%}) on the structure of the leading shock wave are investigated. It is observed that the rise time increases with increasing particle size and scales linearly for the range of particle sizes considered here. Results from numerical simulations using CTH are compared with experimental results to gain insights into wave propagation in heterogeneous particulate composites.