The Origin of Geometric Friction and Cohesion

Brown， EricDry颗粒材料是通过摩擦接触相互作用的小颗粒的集合。来自自然界的颗粒状物质包括沙子、土壤和雪。颗粒材料也存在于工业生产过程中，特别是在制药、能源和农业工业中。这些材料对外力表现出复杂的空间和时间响应，因为它们表现为离散颗粒的集合，而不是纯流体，而且它们不处于热平衡状态。这些材料的机械强度以前已经被研究过，但大多数基础研究都是考虑球形或接近球形的颗粒。该项目的重点是了解由高度非球形颗粒组成的颗粒材料的机械性能，如长杆、链和可以互锁的物体，如订书钉和弯曲杆。我们将使用x射线对填充物进行成像，以识别接触和缠结，并将有关微观结构的信息与填充物在压缩下的强度联系起来。实验数据将用于计算机模拟，以验证高度非球形颗粒填料的物理模型。单轴压缩实验将对具有不同颗粒长径比和拓扑结构的非球形颗粒填料进行。实验结果将与理论模型进行比较，理论模型预测哪些颗粒形状表现出几何内聚，哪些颗粒形状产生的填料表现出显著的应变硬化应力-应变响应。对几种颗粒形状（例如棒材和订书钉）的比较将使我们能够验证模型，以便它可以用于预测颗粒形状的广泛参数空间的颗粒填料的机械响应。此外，数值模拟结合x射线微计算机断层扫描研究将使我们能够研究颗粒填料的微观结构特性(例如：产生几何凝聚和应变硬化的颗粒间接触和纠缠，以及它们如何受到摩擦和弯曲刚度的影响。

CBET - 1605178PI: Brown, EricDry granular materials are collections of small discrete particles that interact with each other through frictional contacts. Examples of granular materials from nature include sand, soil, and snow. Granular materials are also found in industrial processes, especially in the pharmaceutical, energy, and agricultural industries. These materials display complex spatial and temporal responses to external forces because they behave as collections of discrete grains instead of a pure fluid and because they are not in thermal equilibrium. The mechanical strength of these materials has been studied previously, but most fundamental research has considered spherical or nearly spherical particles. This project focuses on understanding the mechanical properties of packings of granular materials composed of highly nonspherical particles such as long rods, chains, and objects that can interlock, such as staples and bent rods. We will image the packings using x-rays to identify contacts and entanglements and connect this information about the microstructure to the strength of the packings in response to compression. The experimental data will be used in computer simulations to validate a physical model of packings of highly nonspherical grains.Uniaxial compression experiments will be performed on packings of nonspherical particles with different particle aspect ratios andtopologies. The experimental results will be compared with a theoretical model that predicts which particle shapes exhibitgeometric cohesion and which particle shapes produce packings that show significant strain-hardening stress-strain response. Comparisons for several particle shapes (e.g. rods and staples) will allow us to validate the model so that it can be employed to predict the mechanical response of granular packings for a wide parameter space of particles shapes. In addition, numerical simulations combined with x-ray micro computed tomography studies will allow us to investigate the microstructural properties of granular packings(e.g. interparticle contacts and entanglements) that give rise to geometric cohesion and strain hardening and how they are affected byfriction and bending stiffness.