This award concerns fundamental aspects of stress distribution and fracture behavior in sands. Granular materials such as sand, gravel, aggregates, agricultural grains, and pharmaceutical products are comprised of discrete particles in contact with each other. At the micro-scale, each particle is in contact with neighboring particles where individual particle properties and contact mechanics govern particle interaction. At the macro-scale, multiple-particle assemblies interact through networks of force chains at contact points. As the applied stresses increase, some particles may fracture, which causes a change in the distribution of force chains within the mass of the granular material. Characterization of force chains has been the subject of extensive research primarily using photoelastic materials and the discrete element method (DEM). Experimental characterization, however, of the development and evolution of force chains in three dimensions (3D) is not available. Measuring properties of force chains and contact forces between particles experimentally - while they may be fracturing - is an important step toward enhancing knowledge of particle-scale behavior in granular materials. The award supports fundamental research to provide critical experimental measurements at a range of length-scales, from the particle-scale to the size of a typical soil mechanics laboratory specimen. The research will have major impact on the development of more accurate computational models that can be applied to better understand a variety of engineering problems involving flow and deformation of granular materials, insertion of piles in sandy soils, in-situ measurement of shear strength in sands using penetrometer devices, tires rolling or skidding through granular soils on the Earth or other planets, high velocity impact of sands, explosive loading of soils, grain silo design, and more efficient manufacturing, handling, and processing of pharmaceutical, agricultural, and food products that are granular in nature.
The objective of the research is to use 3D x-ray diffraction (3DXRD) and synchrotron micro-computed tomography (SMT) to answer fundamental questions about fracture behavior, contact stresses and strains, and onset and evolution of force chains in silica sand. The research will (i) investigate the influence of crystallographic orientation on fracture behavior of silica sand; (ii) measure the distribution of strain and stress within compressed sand particles and investigate their influence on particle fracture in 3D; (iii) quantify the contact stresses between sand particles and assess their influence on fracture behavior of silica sand and on the evolution of force chains; (iv) investigate the factors that affect the onset and collapse of force chains in sand in 3D; and (v) model fracture behavior and force transmission behavior of silica sand in 3D using crystal elasticity and the finite element method (FEM). SMT and 3DXRD are powerful non-destructive 3D techniques that offer complementary experimental measurements and have the potential to yield breakthroughs in the measurement of stress and strain in granular materials at multiple length-scales.
