Session S5: PPCM: New Techniques for Porous Materials

Perturbation Decay Experiments on Granular Materials

The perturbation decay experiment has been utilized to evaluate the strength of granular materials under shock loading. Previous work has shown that material strength slows the decay of a perturbation superimposed on a propagating shock front. A novel configuration has been developed that relies on the change of optical reflectivity of a metal-coated surface during shock arrival to diagnose the shock evolution. The technique has been applied to tungsten carbide powder, tantalum powder, and mixtures of the two. Results from continuum and mesoscale calculations will also be shown. Finally, we will discuss the use of radiography for diagnosing perturbation decay experiments.   

The effect of constitutive behavior in non-planar compaction response of distended media

Shock compaction of distended media is commonly performed within a typical 1-D framework with great emphasis placed on the development of compaction models used to describe a distended equation of state.~ Historically,~minimal~effort has been placed on exploring the effects of deviatoric stresses both across and behind a propagating wave within a~porous medium, and as a result, non-planar wave evolution in such media is an often-neglected field.~ The focus of this talk is newly developed experimental techniques aimed at elucidating granular material strength.~ A dynamic loading platform coupled with X-ray (Dynamic Compression Sector at the Advanced Photon Source) has been utilized to diagnose non-planar wave evolution and material deformation within shocked granular bodies. Two different~experimental configurations of distended media compaction were explored: (i) cylindrical drive Mach stem evolution and (ii) Richtmyer-Meshkov instability growth. CTH hydrocode simulations of the aforementioned experiments~were~performed to calibrate a pressure-dependent, geologically-based constitutive model.~Resulting model applicability and future experimental designs will be~discussed.   

Fast X-ray radiography to study the dynamic compaction mechanisms in a rigid polyurethane foam under plate impact

Polymeric foams are widely used in many industrial fields as structural materials or shock wave mitigators. They would be valuable candidates to protect structures against intense mechanical stress wave loadings generated by laser irradiation or high velocity impact of very small debris. This article presents the results of plate impact experiments coupled to in situ X-ray radiography, performed on a polyurethane foam, to visualize its deformations during the propagation of a stress wave. A two-wave structure associated with the propagation of an elastic precursor and the compaction of the pores has been observed. A phenomenological compaction model, implemented in a dynamic explicit one-dimensional hydrocode, was used to simulate the dynamic macroscopic response of the foam. By using this model, which has previously been calibrated and validated by performing dedicated dynamic experiments, it is possible to compare calculated and experimental waves velocities and improve interpretations. Quasi-static tests coupled to in situ X-ray tomography have also been performed to study the mechanical behavior under low strain rates. Contrary to dynamic experiments, where the cells are crushed by brittle failure of the matrix, the quasi-static compaction of the foam is governed by elastic bending and buckling of the cell edges.   

New approaches to high-precision measurements of heterogenous materials and the path to predictive models: Controlling variation, measuring in situ brittle failure, and understanding their complex dynamic response

Heterogenous materials such as powders or minerals are far more common and varied than their homogeneous counterparts. To measure all of the varieties one might wish to understand is impractical: for example, pure quartz powder comes in many grain size distributions and bulk densities, which historically were each treated as its own ‘material.’ As the community is increasingly focused on heterogeneous materials, this historic approach is too inefficient. Instead, we must have robust models for the micromechanical response and equations of state that can account for the inherent variety of these materials. High precision dynamic and static measurements are essential to develop these models, yet most measurements of heterogeneous materials suffer from (often much) larger uncertainties than their homogeneous counterparts. As a result, the very models we need most lag behind. Our group’s strategy to close this gap has focused on three major areas: identifying and controlling the sources of uncertainty; developing models to understand the dynamic response of these materials; and designing new classes of experiments to inform these models. The variation that defines these materials makes it much harder to create well-controlled samples and to measure their dynamic response to compressive loading. We estimate the contribution to uncertainty by sample variation such as cracking, intra-sample density variation, and density/packing variation between samples. I will discuss how we control these issues during target fabrication and metrology, and how much variation one should expect if advanced metrology tools such as CT are not available. I will also discuss the results of our recent quasi-static and dynamic experiments examining compaction, failure, and the development of force networks, and the limitations and successes of our mesoscale models.