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断裂行为的基本方面。沙、砾石、骨料、农业谷物和药品等颗粒材料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，其中单个粒子的属性和接触机制控制着粒子的相互作用。在宏观尺度上，多粒子集合通过接触点的力链网络相互作用。随着外加应力的增加，一些颗粒可能会破裂，这会导致颗粒材料质量内的力链分布发生变化。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，三维(3D)力链的发展和演化的实验表征是不可用的。当颗粒可能破裂时，通过实验测量颗粒间的力链和接触力的特性是增强颗粒材料中颗粒尺度行为的知识的重要一步。该奖项支持基础研究，以提供一系列长度尺度的关键实验测量，从颗粒尺度到典型土力学实验室样本的大小。这项研究将对开发更精确的计算模型产生重大影响，这些模型可以应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、在沙质土壤中插入桩、使用渗透仪现场测量沙子的剪切强度、轮胎在地球或其他星球上的颗粒土壤中滚动或打滑、沙子的高速撞击、土壤的爆炸装载、谷物筒仓设计以及更高效地制造、处理和加工药品、农业、这项研究的目的是利用三维X射线衍射(3DXRD)和同步辐射微电子计算机断层扫描(SMT)来回答有关石英砂中的断裂行为、接触应力和应变以及力链的开始和演化的基本问题。本研究将(I)研究结晶取向对石英砂断裂行为的影响；(Ii)测量压缩砂颗粒内部的应变和应力分布，并研究其对颗粒断裂的影响；(Iii)量化砂粒之间的接触应力，评估其对石英砂断裂行为和力链演化的影响；(Iv)研究影响砂中力链开始和崩溃的因素；(V)利用晶体弹性和有限元方法(FEM)建立石英砂三维断裂行为和力传递行为的模型。SMT和3DXRD是强大的非破坏性3D技术，提供了互补的实验测量，并有可能在测量多个长度尺度的颗粒材料中的应力和应变方面取得突破。

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.