Session E6: ME.1 Particulate/Porous Materials IV

Percolation of Blast Waves though Sand

Previous research has concentrated on the physical processes occurring when samples of sand, of varying moisture content, were shock compressed. In this study quartz sand samples are subjected to blast waves over a range of pressure and duration. Aspects of particle movement are discussed; the global movement of a bed hundreds of particles thick is a fraction of particle width. The main diagnostics used are pressure sensors and high-speed photography. Results are presented for a range of particle sizes, aspect ratio, density and moisture content. While the velocity of the percolation through the bed is primarily controlled by density and porosity the effect of moisture reveals a more complex dependence.   

Shock and Release Behaviour of Sand

A considerable body of knowledge exists on the shock properties of dry sand. However, capturing the release properties have proven experimentally complex, and currently little information exists on the topic. The measured Hugoniot and release behaviour from a number of experiments is presented, carried out with the aim of furthering or understanding of the fundamental physics behind the unloading of dry sand from a shocked state. Additionally, progress on extending the investigation to sand with varying water content will also be presented.   

Experiments and Simulation of Split Hopkinson Bar tests on Sand

Split Hopkinson Bar data has been generated for well controlled dry and wet sand under confined and unconfined conditions. Simulations have been performed with the Lagrangian hydrocode DYNA using a Porter-Gould Equation of State (EOS) and Johnson-Holmquist type constitutive model. Comparison with the raw strain gauge data is qualitatively reasonable, although some of the details of the trace are not reproduced. This has given some insights into how the constitutive model should be improved.   

Meso-scopic Densification in Brittle Granular Materials

Particulate materials are ideally suited to shock absorbing applications due to the large amounts of energy required to deform their inherently complex meso-structure. Significant effort is being made to improve macro-scale material models to represent these atypical materials. On the long road towards achieving this capability, an important milestone would be to understand how particle densification mechanisms are affected by loading rate. In brittle particulate materials, the majority of densification is caused by particle fracture. Macro-scale quasi-static and dynamic compaction curves have been measured that show good qualitative agreement. There are, however, some differences that appear to be dependent on the loading rate that require further investigation. This study aims to investigate the difference in grain-fracture behavior between the quasi-static and shock loading response of brittle glass microsphere beds using a combination of quasi-static and dynamic loading techniques. Results from pressure-density measurements, sample recovery, and meso-scale hydrocode models (iSALE, an in-house simulation package) are discussed to explain the differences in particle densification mechanisms between the two loading rate regimes.   

Time Scales in Particulate Systems

While there are many interests of studying interactions of individual particles, macroscopic collective behavior of particles are our main interest in many practical applications. In this talk, I will give a brief overview of the multiscale methods connecting the physics at individual particles to macroscopic quantities and averaged equations. The emphasis will be on dense dissipative particulate systems, such as powders. Unlike conservative particle systems, such as molecular systems, in a dissipative particle system the concept of thermodynamic equilibrium is not very useful unless in very special cases, because the only true thermodynamically equilibrium state in these systems is the state in which nothing moves. Other than idealized simple systems, mesoscale structures are common and important in many practical systems, especially in dissipative systems. Spatial correlations of these mesoscale structures, such as force chains in dense granular system, particle clusters and streamers in fluidized beds have received some recent attentions, partly because they can be visualized. This talk will emphasize the effects of time correlations related to the mesoscale structures. To consider time correlations and history information of the system, I will introduce the mathematical foundation of the Liouville equation, its applicability and limitations. I will derive the generalized Liouville equations for particulate systems with and without interstitial fluids, and then use them to study averaged transport equations and related closures. Interactions among the time scale of particle interactions, the time scale of the mesocale structures, and the time scale of the physical problem as represented by strain rate will be discussed. The effect of these interactions on the closure relations will be illustrated. I will also discuss possible numerical methods of solving the averaged equations, and multiscale numerical algorithms bridging the particle level calculations to continuum level calculations.   

Shock-Induced Deformation in Dry and Wetted Particle Beds

The high strain rate response of granular media has received considerable attention due to increasing interest in granular penetration. It has been shown under high-rate dynamic loading dry sand particles undergo a transition in the dominant mechanism of global deformation of the particle bed from a response governed by particle slippage to one governed by particle deformation. In the present study, we investigate the response of packed particle beds, both wetted and dry, under varying flyer plate induced shock loadings. We investigate the critical conditions for the onset of particle deformation in systems of spherical macroscopic particles of various materials. Resulting particle deformations from the shock compression are characterized using scanning electron microscopy with the recovered samples, and the effects of shock strength, particle size, and particle material properties are compared.   

The Role of Intrinsic Material Properties on Shock-Induced Sliding

The high strain-rate behaviour of multi-component systems is often dominated by mediation at material interfaces. The extent to which a material's microstructure influences dynamic friction and relative sliding response remains an area of active study. We present results of a recent study into the behaviour of dry metallic interfaces under the passage of a controlled loading wave. The role of material strength linked to grain size and precipitates have been investigated through experiments on stainless steel and aluminium components of varying alloy composition and microstructure. Held in close contact along a single planar interface, oblique shock waves were generated along this boundary by direct copper flyer impact at velocities in the range 250ms$^{\mathrm{-1}}$ - 600ms$^{\mathrm{-1}}$. Both the 100mm and 13mm bore gas guns located at Imperial College London were utilised for this purpose. Multiple channels of frequency shifted PDV were employed to measure the individual far field responses of the specific materials, while a line-imaging VISAR system was used to directly record the velocity profile across the contact interface, providing a measure of any spatially dependent response. Comparisons of these results against current generation hydrocode models are presented, with good agreement attained with PDV measurements in the far field.