Collaborative Research: Three-Dimensional Assessment of Stresses and Fracture Behavior in Sand

该奖项涉及砂土中应力分布和破裂行为的基本方面。颗粒状材料，如沙子、砾石、集料、农产品和药品，是由相互接触的离散颗粒组成的。在微观尺度上，每个粒子都与邻近的粒子接触，其中单个粒子的性质和接触力学决定了粒子的相互作用。在宏观尺度上，多粒子组合通过接触点的力链网络相互作用。随着外加应力的增加，一些颗粒可能破裂，这导致颗粒材料质量内力链分布的变化。力链的表征一直是广泛研究的主题，主要使用光弹性材料和离散元方法（DEM）。然而，力链在三维（3D）中的发展和演变的实验表征是不可用的。实验测量力链的性质和颗粒之间的接触力——当它们可能断裂时——是提高颗粒材料颗粒尺度行为知识的重要一步。该奖项支持基础研究，以提供从颗粒尺度到典型土壤力学实验室标本大小的一系列长度尺度的关键实验测量。该研究将对更精确的计算模型的发展产生重大影响，这些计算模型可用于更好地理解各种工程问题，包括颗粒材料的流动和变形、砂土中桩的插入、使用贯深仪设备对砂土的抗剪强度进行现场测量、轮胎在地球或其他行星上的颗粒土中滚动或滑动、砂土的高速撞击、土壤的爆炸载荷、谷物筒仓设计、更有效地制造、处理和加工颗粒状的药品、农产品和食品。研究的目的是利用三维x射线衍射（3DXRD）和同步加速器微计算机断层扫描（SMT）来回答有关硅砂断裂行为、接触应力和应变以及力链的发生和演化的基本问题。本研究将：(1)研究晶体取向对硅砂断裂行为的影响；（ii）测量压缩砂颗粒内部的应变和应力分布，并在3D中研究它们对颗粒破裂的影响；（三）量化砂粒之间的接触应力，并评估其对硅砂断裂行为和力链演变的影响；（iv）三维研究影响砂土中力链发生和崩塌的因素；(v)利用晶体弹性和有限元方法对硅砂的断裂行为和传力行为进行三维建模。SMT和3DXRD是强大的非破坏性3D技术，它们提供了互补的实验测量，并有可能在多长度尺度下颗粒材料的应力和应变测量方面取得突破。

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.