Development and Characterisation of the structure of cohesive bulk solids

当剪切时，粘性粉末形成不均匀和各向异性的结构。从微观上看，这是由于颗粒团簇在体积内的破碎和团聚。粒子本身基本上保持完整，但它们之间的接触可以打开或关闭。即时粘接触点（粘附性为主）和需要时间老化的触点（胶结性为主）之间有一个重要的区别。相应的模型材料分别为kcl -粉（胶结为主）、SiO2-粉或石灰石粉（粘附为主）。kcl -颗粒可以用弹塑性接触模型很好地描述，而sio2 -颗粒表现为粘弹性。本项目的目的是了解这些接触特性对剪切诱导的非均质性和各向异性的影响。实验进行了相关的性质在个别颗粒的水平上与剪切诱导结构形成的整体。通过计算机模拟来建立这些现象的物理模型。为了验证模拟模型，将颗粒动力学与宏观现象联系起来，一个至关重要的实验工具是微观剪切测试仪，它可以与x射线显微层析成像（XMT）结合使用，以获得颗粒网络三维结构的详细图像。在第一批三维快照实验和理论分析的基础上，第三期资助将重点研究颗粒尺寸分布、颗粒形状和内聚强度对准动态过程的影响。为了提取单个粒子的轨迹和旋转，将改进高空间分辨率的时间分辨图像的数字处理，特别是对于非球形粒子。这些实验和相应的基于模拟的数据将进行统计分析，以确定粘性大块材料流变行为的相关结构特征。这项研究的结果将是基于结构信息加强对接触力学行为的理解。

When sheared, cohesive powders develop an inhomogeneous and anisotropic structure. Microscopically this is due to fragmentation and agglomeration of particle clusters within the bulk. The particles themselves remain largely intact, but contacts among them open or close. An important distinction can be made between instantly sticking contacts (adhesion dominated) and those, which need time to age (cementation dominated). Corresponding model materials are KCl-powders (cementation dominated), respectively SiO2- or limestone powders (adhesion dominated). The KCl-particles can be well described by an elasto-plastic contact model, while the SiO2-particles behave viscoelastically. The aim of this project is to understand the effect which these contact properties have on the shear induced inhomogeneity and anisotropy. Experiments are performed which correlate properties on the level of individual particles with the shear induced structure formation in the bulk. They are complemented by computer simulations to establish physical models for the phenomena. A crucial experimental tool to validate the simulation models in order to relate the particle dynamics to macroscopic phenomena is a micro shear-tester, which can be used in combination with X-ray microtomograpy (XMT) to get a detailed picture of the three dimensional structure of the particle network. Based on the experimental and theoretical analysis of the first series of three dimensional snapshots the third funding period will focus on the influence of particle size distribution, particle shape and cohesion strength on quasi-dynamic processes. In order to extract individual particle trajectories and rotations, the digital processing of time resolved images with high spatial resolution will be improved, especially also for non-spherical particles. These experimental and corresponding simulation-based data will be analyzed statistically in order to identify relevant structural signatures for the rheological behavior of cohesive bulk materials. The outcome of this research will be an enhanced understanding of contact mechanical behavior based on structural information.