In Situ Probes of Granular Media Under Compression

Minta Akin (16-ERD-010)

Executive Summary

We are identifying and improving deficiencies in physics models and simulations of grain compaction by using ideal mineral systems to study force propagation in grain material and the roles of packing, material, and particle shape upon compaction processes. The resulting improvement in modeling capabilities will benefit diverse efforts including seismic and hydraulic fracturing research, nuclear proliferation monitoring, underground structures and tunnels risk assessment, earthquake research, and buried target defeat.

Project Description

Grains are everywhere, from low-tech sands to high-tech materials with properties not found in natural materials, and their response to compaction 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, however, requires significant improvements in the 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 about the internal microphysics of grain compaction and force transfer under static and dynamic compression, and relating the results to current models. We will use ideal mineral systems to study in situ force chains and the roles of packing, material, and particle shape in 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 including seismic and hydraulic fracturing research, nuclear proliferation monitoring, underground structures and tunnels risk assessment, earthquake research, and buried target defeat.

Force propagation in grains has been extensively 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, heterogeneous structures, or mixed phases are present. We are conducting a series of targeted static, quasi-static, and dynamic experiments to examine force-chain formation and propagation in ideal and non-ideal grains. We have begun with the simplest, most ideal case of uniformly spherical grain samples, and are gradually varying system parameters to study the effects of increased complexity and kinetics. Images are being collected and analyzed to determine variables such as particle velocity and strain in the sample ensemble’s interior. This allows 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 are being 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.

Mission Relevance

Studies in granular media and building accurate physical models of grains under compaction benefit the Laboratory's core competency of earth and atmospheric science. This work is also relevant to the strategic focus area of high-energy-density science, improving force transfer models of aggregates relevant to underground energy exploration. This research further supports DOE’s goal of a more economically competitive, environmentally responsible, secure, and resilient U.S. energy infrastructure.

FY17 Accomplishments and Results

We are researching fracture and compaction at different compression rates to understand wave propagation (including shock fronts) in common granular, heterogeneous materials and to build accurate models of these processes. We have performed detailed quasi-static experiments that compress idealized ruby spheres. Specifically, in FY17 we (1) observed phase changes, a wet-versus-dry dependence in fracture response, multiple wave fronts (predicted by our simulations), and ramp evolution for shocked powders; (2) measured shocked density directly and inferred local density of the shocked state from x-ray diffraction; (3) developed three analysis methods to measure grain compaction using image processing; (4) mapped individual grains’ strain states, observed force chain evolution and the role of broken grains, found initial packing states affecting fracture, and created mesoscale-to-continuum fields for modeling input for granular ensembles; and (5) improved fracture, strain distribution, compaction, and contact models, and used these models for experimental design and to predict outcomes.

Idealized ruby spheres used in quasi-static experiments (A). The experimental results are modeled to understand stress propagation and fracture evolution at a local level (B). These local, discontinuous models can be interpreted in a quasi-continuum model to understand stress throughout the ensemble (C). Complementary dynamic experiments are performed using a gas gun with a synchrotron x-ray source (D) to accelerate a projectile into the sample (E). Projectile velocities range up to 5.7 km/s. Upon impact, a compression front is launched into the granular sample, triggering fracture, compaction, and phase changes. X-ray (F) and optical imaging (G) are used to study dynamic fracture caused by this compression front, and are compared to detailed mesoscale simulations (H). Dynamic x-ray diffraction reveals phase changes to help extract porosity-related differences in material state. The results of these dynamic and quasi-static studies test and improve the mesoscale simulations, and in turn, the mesoscale simulations reveal details that cannot be observed in experiments and suggest new areas of study. The results lead to an improved understanding and modeling capability of these processes in common materials.

Publications and Presentations

Crum, R.S., et al. 2017a. "Probing Dynamics of 2-D Granular Media via X-Ray Imaging." 20th American Physical Society Topical Conference on Shock Compression of Condensed Matter, St. Louis, MO, 9-14 July 2017. LLNL-ABS-718864.

——— 2017b. "Probing Dynamics in Granular Media of Contrasting Geometries via X-ray Phase Contrast Imaging and PDV." American Physical Society March Meeting 2017, New Orleans, LA, 13-17 March 2017. LLNL-ABS-708917.

——— 2017c. "In Situ Probes of Granular Media via X-Ray Analysis to Advance Predictive Models." EUROMAT 2017, Thessaloniki, Greece, September 2017. LLNL-ABS-720718.

Herbold, E.B., et al. 2017. "Microscope Investigation of Dynamic Impact of Dry and Saturated Glass Powder." SCCM 2017. St. Louis, MO, 9-14 July 2017. LLNL-PRES-734666.

Homel, M.A., et al. 2017a. "Understanding Grain-Scale Mechanisms in Dynamic Compaction of Granular Materials." American Physical Society March Meeting 2017, New Orleans, LA, 13-17 Mar 2017. LLNL-ABS-716420.

——— 2017b. "Numerical Simulation of Grain-scale Mechanics in the Dynamic Compaction of Granular Materials." 14th US National Congress on Computational Mechanics, Montreal, Canada, 17-20 July 2017. LLNL-ABS-725443.

——— 2017c. "Simulations and Experiments of Dynamic Granular Compaction in Non-Ideal Geometries." 20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter, St. Louis, MO, 9-14 July 2017. LLNL-ABS-725113.

Hurley, R.C., et al. 2017a. "Microstructure and Failure Analysis During Granular Compaction Using X-ray CT and 3D X-ray Diffraction." International Conference on Tomography of Materials and Structures, Lund, Sweden, 26-30 June 2017. LLNL-ABS-719811.

——— 2017b. "Understanding the Mechanics of Granular Solids by Combining 3D X-ray Diffraction and X-ray Computed Tomography." CHESS Users' Meeting, Cornell, 6-7 June 2017. LLNL-ABS-731720.

——— 2017c. "Investigating In Situ Failure in Granular Materials." SCCM 2017, St. Louis, MO, 9-14 July 2017. LLNL-PRES-735664.

——— 2017d. "Understanding Mechanics and Stress Transmission in Granular Solids by Combining 3DXRD and XRCT." SCCM 2017, St. Louis, MO, 9-14 July 2017. LLNL-PRES-732616.

——— 2017e. "Microstructure and Failure Analysis During Granular Compaction Using X-ray Computed Tomography and 3D X-ray Diffraction." LLNL-PRES-733897.

——— 2017f. "Linking Initial Microstructure and Local Response During Quasi-Static Granular Compaction." Physical Review E. 96. LLNL-JRNL-725572.