Development and Characterisation of the structure of cohesive bulk solids

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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.