Minta Akin (16-ERD-010)
Grains are everywhere, from low-tech sands to high-tech materials with properties not found in natural materials, and their response is crucial in mining, construction, and engineering applications. The scientific community's long-term goal is to build accurate physical models of grains under compaction that will benefit fields such as geophysics, astrophysics, materials sciences, chemistry, manufacturing, and defense. To do so requires significant improvements in the physical understanding of granular compaction, new data to inform new models, and advances in computational simulations. Existing models are inadequate for these tasks, requiring a series of dynamic measurements for each of a very large set of materials and pressure ranges, while leaving major gaps in agreement. Despite known flaws and clear indications of missing physics, these models are seen as the best available and are heavily used because there is no better alternative. A lack of accurate data about the internal processes of compaction in grains underlies this knowledge gap, hindering both fundamental science and model development. Data used to inform existing compacting models are sparse, limited to integrated measurements such as velocimetry, causing these models to be inadequate. Using x-ray diffraction and imaging, we are addressing the challenge of granular compaction by obtaining new data on the internal microphysics of grain compaction and force transfer under static and dynamic compression, and relating the results to current models (see video below). We will use ideal mineral systems to study in situ force chains and the roles of packing, material, and particle shape upon compaction processes, and use the results to identify and improve deficiencies in physics models and simulations. The resulting improvement in modeling capabilities will benefit diverse efforts such as seismic and hydraulic fracturing research, monitoring nonproliferation, determining risks of underground structures and tunnels, conducting earthquake research, and defeating buried targets.
Force propagation in grains has been widely simulated, but very few experimental observations have been made—internal processes are essentially unknown. We expect to deliver improved physical understanding and models of granular media applicable to any field where grains, pores, heterogenous structures, or mixed phases are present. We will execute a series of targeted static, quasi-static, and dynamic experiments to examine force chain formation and propagation in ideal and nonideal grains. The research will begin with the simplest, most ideal case of uniform spheres, and gradually vary the system to study the effect of increased complexity and kinetics. Images will be collected and analyzed to determine variables such as particle velocity and strain in the ensemble’s interior. This will allow force chains to be mapped and equations of state to be measured. Size distributions, physical changes such as cracking, and the fraction of particles belonging to force networks can be determined. Discrepancies between simulations and observed results will be used to identify current deficiencies in physical models and codes. A successful project will result in information on compaction at varying strain rates in idealized systems. This information will provide an essential, intuitive understanding of compaction and data for numerical models, building confidence in modeling results. This work will ultimately lead to numerical models that can compound the effects of characteristics such as initial packing, particle shape, and parent material, at much greater cost efficiency.
Studies in granular media and building accurate physical models of grains under compaction benefit the LLNL core competency of earth and atmospheric science. This work also supports the strategic focus area in energy and climate security, improving force transfer models of aggregates relevant to underground energy exploration.
FY16 Accomplishments and Results
In FY16 we (1) incorporated fracture models for glass, (2) simulated existing mono- and tri-layer data of glass compaction, (3) demonstrated the presence of pressure fronts far ahead of damage fronts in simulations, (4) collected new data at the Dynamic Compression Sector at Argonne National Laboratory's Advanced Photon Source for compaction in glass and yttria-stabilized zirconium beads, (5) collected strain imaging and tomography data of single-crystal sapphire beads at the CHESS Cornell High Energy Synchrotron Source in New York, and (6) begun analysis and simulations of these results, which show that force chains lead to long-range effects on stress and strain in grains. We also determined that pores shrink, are preserved, and grow during compaction and that parent material strength will have a strong effect and must be considered in models.
Publications and Presentations
- Akin, M. C., et al., In situ studies of compaction in granular media. (2016). LLNL-POST-696439.
- Lind, J., D. C. Pagan, and M. C. Akin, Finding the weakest link in granular systems before catastrophic failure. (2016). LLNL-ABS-703019.
- Pagan, D. C., et al., Measuring the evolution of granular stress distributions using high energy x-ray diffraction and micro-tomography. Granular Matter Gordon Research Conf., Easton, MA, July 24–29, 2016. LLNL-POST-693030.