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L’objectif de cette thèse est l’élaboration de matériaux fibreux destinés à être utilisés en matériau d’âme dans des structures sandwiches. Les principales applications visées sont l’absorption acoustique et la résistance à l’impact. Nous avons ainsi développé des nontissés épais (environ 10 mm) en para-aramide, ayant une porosité supérieure à 90% et des densités inférieures à 150 kg/m3. Nous avons fabriqué des composites poreux à base de ces nontissés en utilisant diverses résines, dont l’époxy. La technique utilisée permet de conserver une porosité autour de 80%. Nous nous sommes intéressés à l’architecture interne poreuse des nontissés en étudiant les tailles de pores équivalents, l’isotropie, la distribution/orientation des fibres et leur degré d’enchevêtrement à l’aide de tests expérimentaux (perméabilité ; traction/compression ; capillarité ; porométrie) et de modèles théoriques (perméabilité et van Wyk). Nous avons relié les paramètres structurels aux paramètres de procédé de fabrication. Les résultats en acoustique ont démontré le bon pouvoir absorbant des nontissés au-delà de 4000 Hz. Nous avons lié avec succès l’épaisseur, la résistivité au passage de l’air et les tailles de pore obtenus par capillarité, aux propriétés acoustiques via le modèle de Delany et Bazley et le modèle de Johnson et Allard. Nous avons évalué à travers des tests de compression la capacité des matériaux fibreux à absorber et à dissiper de l’énergie : les mécanismes mis en jeu sont les frottement/rupture de fibres et la déformation de la structure. Enfin, les tests à l’impact de nontissés et de composites ont montré un réel potentiel des nontissés au cœur de sandwichs pour amortir l’impact. / The aim of this thesis is to develop fibrous structures in view of using them as core material in sandwich structures, for impact resistance and acoustic applications. Thick para-aramid nonwovens have hence been manufactured, with porosities above 90% and densities below 150 kg/m3. Their thickness was around 10 mm. Porous composites have been obtained from these nonwovens, using various resins (namely epoxy), and with a technique that allows to keep a high porosity level of 80% in the final structure.The internal porous architecture has been studied through different methods and parameters: the equivalent pore sizes, the isotropy, the fibre distribution/orientation and the degree of fibre entanglements have been evaluated with different experimental techniques (air permeability; tensile/compression tests; capillarity; porometry) and using theoretical models (permeability, van Wyk). The structural parameters have been related to the process parameters. The acoustic measurements have shown that our nonwovens were good acoustic absorbers at frequencies above 4000 Hz. The use of two models, Delany and Bazley, and Jonhson and Allard, has shown that the acoustic absorption behaviour can be modelled with the material characteristics (thickness, resistivity and an equivalent pore diameter). The capacity of our materials to absorb and dissipate energy has been evaluated with compression testing. The main mechanisms involved are fibre friction, fibre breakage and structure deformation. Finally, the impact tests performed on nonwovens and their composites have demonstrated the real potential of nonwovens to be used at the core of sandwiches to absorb impact.

The replacement of asbestos fibres with cellulose fibres in producing corrugated fibre reinforced cement sheets by the Hatschek process resulted in edge cracking for stacked sheets. This was due to the hydrophilic nature of cellulose, which increases its tendency for exchanging water with the surroundings. The drying process of corrugated sheets, in a stack, resulted in shrinkage hence edge cracking along the sheet. To reduce the magnitude of drying shrinkage and edge cracking potential, several mitigation strategies were proposed including the surface treatment of cellulose fibres, incorporation of wollastonite microfibres, addition of admixtures and superplasticizers, kaolin inclusion as partial replacement of cement and different exposure conditions. A fundamental understanding in mechanisms behind volume changes and how cracks form was crucial for optimization of the mitigation strategies. This thesis initially used a review approach to understand the mechanisms involved in different types of shrinkage and the role of different mitigation techniques. The ultimate goal was to achieve lower drying shrinkage and cracking risks in corrugated sheets along with reducing its economic impact. As a result, surface treatment of cellulose fibres, based on transforming the hydrophilic nature of cellulose to hydrophobic state, was investigated. Furthermore, inclusion of wollastonite/ kaolin as partial replacement of cement, were evaluated. Also, the potential of adding admixtures/ superplasticizers was explored. Finally, investigation on development of edge cracks in stacked corrugated fibrecement sheets was conducted under different exposure conditions. The results and findings of this research showed no significant improvement in permeability with cellulose surface treatment. Wollastonite microfibres promoted pore discontinuity hence significant reduction in permeability thus lower drying shrinkage. However, the resultant sheets were brittle. By reducing water content with addition of superplasticizers, density was enhanced thus reducing volume change from drying and wetting. Kaolin acted as internal restraint for shrinkage, refining the microstructure at the interfacial transition zone thus increasing density and its pozzolanic reaction enhanced mechanical properties. The inclusion of kaolin in the fibrecement mix in conjunction with controlling exposure conditions managed to eliminate edge cracking.

Thesis (Ph.D.)--University of the Witwatersrand, Science Faculty (Physics), 2000. / Percolation phenomena are studied in a series of composites, each with a cellular structure (small conductor particles embedded on the surfaces of large insulator particles). The DC and AC conductivities, l/f noise and magnetic properties (in some series) are measured in the systems consisting of Graphite, Graphite-Boron Nitride, Carbon Black, Niobium Carbide, Nickel and Magnetite (Fe304) as the conducting components with Talc-wax (Talc powder coated with 4% wax by volume) being the common insulating component. Compressed discs of 26mm diameter and about 3mm thickness (with various conductor volume fractions covering both the insulating and conducting region) were made from the respective powders at a pressure of 380MPa and all measurements were taken in the axial (pressure) direction. The conductivity (σm) and dielectric constant (εm) of percolation systems obey the equations: σm = σc( ɸ - ɸc)t for ɸ >ɸc; σm = σi( ɸc - ɸ-s and εm = εi( ɸc - ɸ-s' for ɸ < ɸc; outside of the crossover region given by ɸc± (δdc ~=(σi/σc)1/(t+s). Here ɸc is the critical volume fraction of the conductor (with conductivity σ = σc) and cri is the conductivity of the insulator, t and s are the conductivity exponents in the conducting and insulating regions respectively and S’ is the dielectric exponent. The values of s and t are obtained by fitting the DC conductivity results to the combined Percolation or the two exponent phenomenological equations. Both universal and non-universal values of the sand t exponents were obtained. The dielectric exponent S’, obtained from the low frequency AC measurements, is found to be frequency-dependent. The real part of the dielectric constant of the systems, has been studied as a function of the volume fraction (ɸ) of the conducting component. In systems where it is measurable beyond the DC percolation threshold, the dielectric constant has a peak at ɸ > ɸ, which differs from key predictions of the original Percolation Theory. This behaviour of the dielectric constant can be qualitatively modeled by the phenomenological two exponent equation given in Chapter two of this thesis. Even better fits to the data are obtained when the same equation is used in conjunction with ideas from Balberg's extensions to the Random Void model (Balberg 1998a and 1998b). At high frequency and closer to the percolation threshold, the AC conductivity and dielectric constant follow the power laws: σm( ɸ,שּׂ) ~ שּׂX and εm( ɸ,שּׂ) ~ שּׂ-Y respectively. In some of the systems studied, the x and y exponents do not sum up to unity as expected from the relation x + y = 1. Furthermore, the exponent q obtained from שּׂ x σm( ɸ,O)q in all but the Graphite-containing systems is greater than 1, which agrees with the inter-cluster model prediction (q = (s + t)/t). The Niobium Carbide system is the first to give an experimental q exponent greater than the value calculated from the measured DC s and t exponents. l/f or flicker noise (Sv) on the conducting side (ɸ > ɸc) of some of the systems has been measured, which gives the exponents k and w from the well-established relationships Sv/V2 = D(ɸ - ɸc)-k and Sv/V2 = KRw. V is the DC voltage across the sample with resistance R while D and K are constants. A change in the value of the exponent k and w has been observed with k taking the values kl ~ 0.92 - 5.30 close to ɸc and k2 ~ 2.55 - 3.65 further into the conducting region. Values of WI range from 0.36 -1.1 and W2 ~ 1.2 - 1.4. These values of ware generally well within the limits of the noise exponents proposed by Balberg (1998a and 1998b) for the Random Void model. The t exponents calculated from k2 and W2 (using t = k/w) are self-consistent with the t values from DC conductivity measurements. Magnetic measurements in two of the systems (Fe304 and Nickel) show unexpected behaviour of the coercive field and remnant magnetisation plotted as a function of magnetic volume fraction. Fitting the permeability results to the two exponent phenomenological equation gives t values much smaller than the corresponding DC conductivity exponents. A substantial amount of data was obtained and analysed as part of this thesis. Experimental results, mostly in the form of exponents obtained from the various scaling laws of Percolation Theory, are presented in tabular form throughout the relevant chapters. The results have been tested against various models and compare with previous studies. While there is some agreement with previous work, there are some serious discrepancies between the present work and some aspects of the standard or original Percolation Theory, for example the dielectric constant behaviour with conductor volume fraction close to but above ɸc. New results have also emerged from the present work. This includes the change in the noise exponent k with (ɸ - ɸc), the variation of the dielectric exponent s' with frequency and some DC scaling results from the Fe304 system. The present work has dealt with some intriguing aspects of Percolation Theory in real continuum composites and hopefully opened avenues for further theoretical and experimental research. / AC 2016

A Research Report submitted to the Faculty of Architecture, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Building. / Environmental and health considerations have led to the substitution of cellulosic fibres for asbestos fibres in many cement-based products. Inevitably, the substitution has required modifications to the manufacturing process. Certain production techniques associated with the switch to cellulosic fibres and other additives needed elucidation. It is possible that to ensure the required concentration of certalr additives in the final product, an excess is being used in the process because of uncertainties about the quantitative balance of the process. My intention was to examine the potential tole for Fourier Transform Infrared (FTtR) Spectroscopy in the investigation of the materials constituting fibre-reinforced cement boards. I was able to show that qualitative and quantitative determination of certain of the components in the final product are possible with adequate precision and reproducibility to be of value to the manufacturer, I also showed that care taken in the preparation of standard and analytical samples was essential for the success of such analyses. / Andrew Chakane 2018

A Research Report submitted to the Faculty of Architecture, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Building. / Environmental and health considerations have led to the substitution of cellulosic fibres for asbestos fibres in many cement-based products. Inevitably, the substitution has required modifications to the manufacturing process. Certain production techniques associated with the switch to cellulosic fibres and other additives needed elucidation. It is possible that to ensure the required concentration of certaln additives in the final product, an excess is being used in the process because of uncertainties about the quantitative balance of the process. / Andrew Chakane 2019

MSc, School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment / A study has been conducted into the synthesis of 10wt% diamond/ mullite composites through two methods: First through the hot press sintering of alumina and silica in stoichiometric composition for 3:2 mullites (mullite formed in situ) at 1400, 1450 and 1500oC. Second through the sol/ gel process. The sol/ gel method only provided the basis for future development with no further discussion of the results while the hot press sintering method yielded composites with residual cristobalite and corundum phases. Achieved densities of the composites were 93.7, 94.6 and 95.8% of the theoretical density with respect to sintering temperatures of 1400, 1450 and 1500oC for compact samples by the first method. Hardness – measured by Vickers indentation – of the composites decreased with increase in temperature with 15.5 ± 0.33GPa achieved at the lowest sintering temperature investigated. The decrease in hardness was attributed to the structural degradation of diamond to non-diamond carbon forms with increase in temperature as observed from Raman spectra of each of the composites. X-ray traces showed an increase in the mullite content with increase in temperature. The fracture toughness of compacts initially hot press sintered from alumina and silica in stoichiometric composition for 3:2 mullite with no diamond added decreased with increase in sintering temperature with 4.75 ± 0.10MPa·m1/2 achieved at the lowest sintering temperature investigated. Further discussion to the structure and physical properties is presented.

This thesis explores methods by which carbon fibre reinforced polymers may be fficiently designed with the inclusion of damage tolerance criteria. An efficient method of modelling the compression after impact (CAI) strength of composite materials is selected, and this forms the basis of analysis performed. The CAI model is initially used as the objective in an optimisation routine using a simple genetic algorithm. This indicates features of a damage tolerant composite laminate, namely that plies near the surface are less axially sti® in the loading direction than those nearer the laminate midplane, with a lower Poisson's ratio than the full laminate. This delays sublaminate buckling under laminate uniaxial compression, thus restricting delamination propagation. The designs produced by the optimisation are verified experimentally. In order to improve the computational efficiency of the CAI model a simple surrogate modelling technique for sublaminate buckling is presented. This allows a complete database of results to be produced for a given set of ply angles, in this case standard 0/90/§45± plies. This is used in the full analysis of a collection of layups produced elsewhere to be fully uncoupled, but without the stipulation of midplane symmetry. The surrogate method is shown to reduce computation time by over 99%, and produce results with an average error of less than 0.1% compared to exhaustive analysis. The analysis of the damage tolerance of fully uncoupled laminates shows that the relaxation of midplane symmetry as a design rule gives the designer far more flexibility in layup, and may allow for more damage tolerant laminates to be selected. Finally, the CAI model is incorporated into a stiffened panel design optimisation problem as a constraint. Firstly the panel is optimised using the in¯nite strip analysis tool VICONOPT, with three stiffener geometries. The objective function is minimum mass for a panel subject to compressive and out-of-plane loading, with buckling and strain allowable constraints applied. Damage tolerance constraints are then applied in place of a strain allowable, using a bi-level optimisation approach. This method is shown to allow efficient inclusion of damage tolerance as a constraint in stiffened panel design, although it does not account for interactions in global buckling and local sublaminate buckling which may reduce the strength of the panel. Results indicate that the inclusion of damage tolerance analysis in stiffened panel design shows little benefit for low load panels, but can give significant reductions in mass (up to 30%) for higher load panels.