Experimental and computational analysis of random cylinder packings with applications

Random cylinder packings are prevalent in chemical engineering applications and they can serve as prototype models of fibrous materials and/or other particulate materials. In this research, comprehensive studies on cylinder packings were carried out by computer simulations and by experiments. The computational studies made use of a collective rearrangement algorithm (based on a Monte Carlo technique) to generate different packing structures. 3D random packing limits were explored, and the packing structures were quantified by their positional ordering, orientational ordering, and the particle-particle contacts. Furthermore, the void space in the packings was expressed as a pore network, which retains topological and geometrical information. The significance of this approach is that any irregular continuous porous space can be approximated as a mathematically tractable pore network, thus allowing for efficient microscale flow simulation. Single-phase flow simulations were conducted, and the results were validated by calculating permeabilities. In the experimental part of the research, a series of densification experiments were conducted on equilateral cylinders. X-ray microtomography was used to image the cylinder packs, and the particle-scale packings were reconstructed from the digital data. This numerical approach makes it possible to study detailed packing structure, packing density, the onset of ordering, and wall effects. Orthogonal ordering and layered structures were found to exist at least two characteristic diameters from the wall in cylinder packings. Important applications for cylinder packings include multiphase flow in catalytic beds, heat transfer, bulk storage and transportation, and manufacturing of fibrous composites

Lattice Element Method and its application to Multiphysics

In this thesis, a Lattice element modelling method is developed and is applied to model the loose and cemented, natural and artificial, granular matters subject to thermo-hydro-mechanical coupled loading conditions. In lattice element method, the lattice nodes which can be considered as the centres of the unit cells, are connected by cohesive links, such as spring beams that can carry normal and shear forces, bending and torsion moment. For the heat transfer due to conduction, the cohesive links are also used to carry heat as 1D pipes, and the physical properties of these rods are computed based on the Hertz contact model. The hydro part is included with the pore network modelling scheme. The voids are inscribed with the pore nodes and connected with throats, and then the meso level flow equation is solved. The Euler-Bernoulli and Timoshenko beams are chosen as the cohesive links or the lattice elements, while the latter should be used when beam elements are short and deep. This property becomes interesting in modelling auxetic materials. The model is applied to study benchmarks in geotechnical engineering. For heat transfer in the dry and full range of saturation, and fractures in the cemented granular media.How through porous media failure behaviours of rocks at high temperature and pressure and granular composites subjected to coupled Thermo hydro Mechanical loads. The model is further extended to capture the wave motion in the heterogeneous granular matter, and a few case studies for the wavefield modification with existing cracks are presented. The developed method is capable of capturing the complex interaction of crack wave interaction with relative ease and at a substantially less computational cost

Colloidal Crystals on Conical Surfaces

The curvature of surfaces affects the organisation of condensed matter upon them. In biology, the curvature of cell membranes affects how proteins are arranged within them and the growth of virus capsids is affected by their curvature, for example. Curvature is also used to guide colloidal self-assembly in industrial nanoscale manufacturing processes, to produce, among other things, drug delivery systems, biosensors and photonic crystals. An open question in the field concerns how two-dimensional crystalline systems deal with the closure constraint imposed by finite curved surfaces. A crystal on a conical surface is a good model for studying this as the curvature strain associated with surfaces of non-zero Gaussian curvature is eliminated and the closure constraint is variable, unlike on, for example, cylindrical surfaces. In this work, putative global minimum energy structures of two-dimensional, model colloidal crystals confined on conical surfaces of a range of angles are generated using basin-hopping and visualised with Voronoi tessellation. The colloidal particles interact via an isotropic Morse potential. The defect patterns observed in these model, minimum energy crystals are discussed and analysed. The interparticle potential range and the cone angle affect whether any defects are seen in the minimum energy structures, and, if defects are produced, what they are. Both wedge-shaped defects reported in experiment and novel bulk-terminating helical defects are observed. At small cone angles, the type of defect is very sensitive to changes in angle, as different defects can alleviate different amounts of strain. A phase diagram of preferred defect type against cone angle and interparticle potential range is produced for near-cylindrical conical surfaces. Another interesting feature of the crystal is the vertical position on the cone that it prefers to occupy; a preliminary line energy model is constructed which explains this behaviour qualitatively

Stress generation during the processing of epoxy-carbon composites.

The stresses generated during the processing of carbon fibre/epoxy resin composite materials are predicted using the finite element method and classical lamination theory. Elastic material behaviour is assumed. Emphasis is placed on the residual microstresses which have been less-extensively studied than other aspects of the stress generation process.Temperature and stress distributions are modelled through the thickness of laminates assuming cooling from the cure temperature of 190°C. At the microlevel the effect of varying fibre volume fraction, interfibre distance, packing geometry and fibre diameter are studied. Random and regular fibre arrays are considered.It is found that the residual stresses are generated almost entirely due to the differing properties of the fibre and the matrix and the anisotropy of the fibres, rather than any temperature gradients within the materials. At the macrolevel maximum stresses (10-100 MPa) are calculated in the transverse layers of multidirectional laminates. At the microlevel maximum stresses (10-100 MPa) are predicted at the fibre/matrix interface. The exact values depend on the assumed laminate stacking sequence and distribution of fibres, respectively. The maximum values of the microstresses are found to be approximately inversely proportional to the minimum interfibre distance and proportional to the fibre diameter. This implies that, at the shorter minimum interfibre distances typical of more realistic random arrays, the maximum stress values are greater. When the macrostress and microstress fields are superimposed it is predicted that cracks will form at some of the fibre/matrix interfaces and propagate outwards into the matrix.Observations of laminate samples under the electron microscope show no such cracking to occur, rather in a few localised regions, cracking around the fibre/matrix interface is apparent. It is suggested that in these regions the interface is weak and fails due to the weaker radial stress. Otherwise it is suggested that cracking is not observed due to a visco-elastic/visco-plastic behaviour of the matrix, the presence of an interlayer at the fibre/matrix interface with properties different to that in the matrix away from the interface and a crack suppressing mechanism resulting from the interaction of adjacent plies. The latter effect is most significant for thin plies.It is proposed that regular packing of the fibres, which precludes low interfibre distances, will prevent microcracking. Hexagonal packing is preferred since this achieves the highest volume fraction and thus the highest strengths. Sizings applied to the fibres which improve the fibre/matrix adhesion, and react/diffuse into the matrix to produce a flexible interlayer, will improve the strength and impact resistance of these composites. In multidirectional laminates thin transverse layers, less than 0.5 mm are advised

On the Characterization and Modeling of Interfaces in Fiber Reinforced Polymer Structures

Faserverstärkte Kunststoffe bieten insbesondere mit kontinuierlicher Faserverstärkung ein enormes Potential, um leichte und gleichzeitig steife und feste Strukturen zu erzeugen, weswegen sie in der Fahrzeugbranche in hochbelasteten Bauteilen Verwendung finden. Darüber hinaus sind im semistrukturellen Bereich auch häufig diskontinuierlich faserverstärkte Kunststoffe vorzufinden, da sie hier im Vergleich zu kontinuierlich faserverstärkten Kunststoffen, aber auch im Vergleich zu Metallen kostengünstiger herzustellen sind. Eine neue, hybride Werkstoffklasse zielt nun darauf ab, die Vorteile diskontinuierlicher mit den Vorteilen kontinuierlicher Faserverstärkung zu verbinden. Durch die Funktionalisierung kontinuierlicher Faserverbunde als hauptlasttragende Verstärkung einer diskontinuierlich langfaserverstärkten Grundstruktur lassen sich kostengünstige, aber leistungsfähige Faserverbundstrukturen herstellen. Derartige kontinuierlich-diskontinuierlich langfaserverstärkte Kunststoffe weisen verschiedenartige innere Grenzflächen auf, welche sich insbesondere auf das Versagensverhalten der Struktur auswirken. Um sichere Auslegungsverfahren für diese Werkstoffklasse entwickeln zu können, ist es daher notwendig, die inneren Grenzflächen untersuchen und beschreiben zu können. Die vorliegende Arbeit befasst sich mit Methoden, innere Grenzflächen von Faserverbundkunststoffen zu charakterisieren und zu modellieren und wendet diese auf werkstoffklassenspezifische Materialien und Herstellungsprozesse an. Hierzu werden experimentelle Untersuchungen der Faser-Matrix-Grenzflächen sowie der interlaminaren Grenzflächen von kontinuierlich verstärkten Schichtverbunden durchgeführt und auf die weitere Verwendbarkeit im Produktentwicklungsprozess hin untersucht. Es wird ein kombinierter experimentell-numerischer Ansatz verfolgt, um einerseits die experimentellen Ergebnisse zu validieren und andererseits eine vorteilhafte Modellierung des Materialverhaltens zu untersuchen