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Item type:Publication, Access status: with files , Membrane Particle Interaction in Triaxial Testing: FEM-DEM Coupling and CT Imaging Insights(2024-09-27); This conference presentation supplements the abstract submitted to the 5th Aspherix CFDEMcoupling conference. That has taken place from 25.09.24 - 27.09.24 in Vienna, Austria. This conference contribution presents recent advances in implementing a FEM-DEM solver into Aspherix. Implementing this FEM-DEM coupling aims to account for membrane particle interaction in numerical simulations, i.e., encountered in triaxial testing, a standard test in soil mechanics, where the DEM accounts for particle motion and the FEM for the membrane deformation. Furthermore, we showcase specific image processing techniques based on 4D (3D+time) in situ X-ray computed tomography (CT) experiments to set up and validate such numerical simulations. Miniaturized triaxial experiments have been performed on a cylindrical (height 20 mm; diameter 11 mm) dry granular particle assembly. Here, a specimen made of 1 mm glass beads is examined. Axial deformation is exerted onto the specimen, leading to a consecutive shear deformation. At each 1 % of axial strain (here: engineering strain with ε=Δl/l_0) the experiment is halted to acquire a tomography of the current state. A total of 15 % of strain is applied. On the one hand, the acquired image data of the initial particle configuration is processed to act as input for the numerical simulation. On the other hand, sophisticated image analysis based on an image correlation procedure tracks individual particles throughout the experiment solely based on the acquired image data, giving displacement vectors for each particle (see figure below). This data is complemented by externally measured axial force, which describes the examined specimen's shear resistance. Both force measurements and displacement vectors act as a validation for the numerical simulation. Finally, current limitations of the implementation are discussed, such as available element types for modelling the membrane, and an outlook for further improvements is given.Publicationtype: Conference Presentation30 21 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of oscillating water content on the structure of biomass pelletsWhen handling wood pellets, the main challenge arises during storage, where the physical properties of the pellets are affected by fluctuating relative humidity (RH). With increasing RH, the water content of the pellets increases, which leads to swelling and higher porosity as well as crack formation on the surface. However, these changes are reversible to a certain extent and the pellets can recover fully or to a certain extent when the RH subsequently decreases again. The permanent damage caused by fluctuating RH is shown in the form of larger gaps between the wood chips within the pellets. As a result, the pellets have a larger volume and higher porosity as well as higher surface roughness than in their initial state before storage. In particular, when the saturation vapor pressure was exceeded to reach a water content of over 12 wt.-%, the reversibility of the structural changes appeared to be lower.Publicationtype: Journal Article25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Pore scale investigation of unsaturated granular soil behaviour by means of in situ CT experimentsWith continuing evolution of imaging techniques from medical applications and materials science, non-destructive imaging experiments have also become an important method to investigate soil specimens. Amongst other methods, computed tomography (CT) has developed to a tool to visualise and better understand the microstructure of different soils based on 3D image data. Furthermore, the acquisition of a temporal series of CT images allows to study processes in soils on the microscale, e. g., during mechanical loading. In order to study the hydraulic and mechanical behaviour of unsaturated granular soils, we combine different custom-built miniaturised experimental set-ups with geomechanics background with computed tomography, yielding so-called in situ CT experiments. By means of image reconstruction and further image analysis based on segmented CT images acquired during different hydraulic and mechanical experiments, we study the drainage and imbibition process as well as the shear process of unsaturated sand and glass bead specimens on the pore or grain scale. The analysis of data on the microscopic level, including the phase distribution, interfacial areas, contact lines as well as radii of curvature of capillary menisci, allows to obtain insights into the macroscopic water retention behaviour and shear behaviour of granular soils.Publicationtype: Conference Paper35 74 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Capillary cohesion of different granular materials determined from uniaxial compression and water retention testsIn this conference contribution, we present and discuss the results for capillary cohesion of different granular materials determined through different approaches. First, we consider macroscopic uniaxial compression tests with varying degrees of saturation. Second, we derive theoretical capillary cohesion from the water retention curve (WRC) measured in macroscopic water retention experiments. Finally, capillary cohesion is calculated from air-water interfacial areas and solid-water interfacial areas extracted from CT images acquired during a cyclic water retention experiment. An attempt is made to link the macroscopic results of capillary cohesion based on microscopic insights from CT imaging experiments.Publicationtype: Conference PaperScopus© Citations 1 55 161