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Magnesium alloys, as the lightest of all structural metallic materials, and aluminium-based particulate metal matrix composites (PMMCs), offering unified combination of metallic and ceramic properties, have attracted increased interest from the automotive, aerospace, electronic and recreation industries. Current processing technologies for PMMCs do not achieve a uniform distribution of fine-sized reinforcements and produce agglomerated particles in the ductile matrix, which are detrimental to the ductility. At the same time, molten magnesium alloys contain impurities and oxides and when cast conventionally, the final components usually exhibit a coarse and non-uniform microstructure with various casting defects. The key idea in this thesis has been to adopt a novel intensive melt conditioning process, allowing the application of sufficient shear stress that would disperse solid particles present in the melt and offer unique solidification behaviour, improved fluidity and die-filling during casting. The Melt Conditioned High Pressure Die Casting (MC-HPDC) process, where intensive shearing is directly imposed on the alloy melt, which is then cast by the conventional HPDC process, has been used to produce PMMC and magnesium alloy castings. The MC-HPDC process for PMMCs leads to a uniform dispersion of the reinforcement in the matrix, confirmed by quantitative statistical analysis, and increased mechanical performance as indicated by an increase in the hardness and the tensile properties of the composites. We describe a solidification path for aluminium containing magnesium alloys, where intensive shearing prior to casting leads to effective dispersion of solid oxide particles, which then effectively act as nucleation sites for magnesium grains, resulting in significant grain refinement. The MC-HPDC processed magnesium castings have a significantly refined microstructure, with reduced porosity levels and casting defects. Evaluation of the mechanical properties of the castings reveals the beneficial effect of intensive shearing. After careful optimization, the MC-HPDC process shows promising potential for the direct recycling of high purity magnesium die casting scrap, producing casting with mechanical properties comparable to those of primary magnesium alloys.

This thesis reports the systematic approaches to overcome the key drawbacks of the pure PHBV, namely low crystallisation rate, tensile strength, ductility, melt viscosity, thermal stability and high materials cost. The physical, mechanical, thermal, and rheological properties of the pure PHBV were studied systematically first to lay a solid foundation for formulation development. The influence of blending with other biopolymers, inclusion of filler, and chain extender additives in terms of mechanical properties, rheology, thermal decomposition and crystallization kinetics were then followed. Creating lightweight structures by foaming is considered to be one of the effective ways to reduce material consumption, hence the reduction of density and morphology of PHBV-based foams using extrusion foaming technique were studied comprehensively in terms of extrusion conditions (temperature profiles, screw speed and material feeding rate) and the blowing agent content. The material cost reduction was achieved by adding low-cost filler (e.g. CaCO3) and reduction of density by foaming. The thermal instability was enhanced by incorporation of chain extender (e.g. Joncryl) and blending with a high thermal stability biopolymer (e.g. PBAT). The polymer blend also improved the ductility. Adding nucleation agent enhanced the crystallization rate to reduce stickiness of extruded sheet. The final formulation (PHBV/PBAT/CaCO3 composite) was successfully extruded into high quality sheet and thermoformed to produce prototype trays in an industrial scale trial. The effect of the extrusion conditions (temperature profiles, screw speed and material feeding rate) and the blowing agent content are correlated to the density reduction of the foams. 61 and 47 % density reduction were achieved for the commercial PHBV and the PHBV/PBAT/CaCO3 composite respectively and there exists further scope for more expansion if multiple variable optimisation of the conditions are carried out.

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Nanoporous gold is an exciting topic that has been highly researched due to its potential in applications including sensing, catalysts, gas storage, and heat exchangers, made possible by its high surface area to volume ratio and high porosity. However, these applications tend to require a specific morphology, which is often difficult to control. In this work, significant strides have been made in tuning the morphology of nanoporous gold by studying the effect of different fabrication parameters on the ligament diameter, pore diameter, and ligament length, three characteristics which are most discussed in previous studies concerning nanoporous gold. This material also, generally shows a brittle behavior despite it consisting of a normally ductile constituent element, limiting many commercial applications. There have been multiple simulated studies on the tensile mechanical properties and the fracture mode of this material, but limited experimental tensile testing research exists due to technical difficulty of conducting such experiments with small fragile samples. We examine the tensile mechanical behavior of nanoporous gold with ligament sizes ranging from 10 to 30 nm using in situ tensile testing under an environmental scanning electron microscope (ESEM). A specially designed tensile stage and sample holders are used to deform the sample inside the ESEM, allowing us to observing both the macro and microscopic structure changes. Our experimental results advance our understandings of how porous structure influence the mechanical properties of nanoporous gold, and they also serve to increase the accuracy of future simulation studies that will take this material a step towards commercial use by providing a thorough understanding of its structural mechanical limitations. / MS

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The early age cracking of concrete, which includes plastic shrinkage cracking (PShC) and plastic settlement cracking (PSeC), commonly occurs in flat concrete elements such as bridge decks and slabs or at the change of a concrete section depth. These cracks typically occur once the concrete has been cast and consolidated up to the final setting time, and initiate when the tensile stresses developed in the concrete exceeds its ultimate tensile strength or, alternatively phrased, when the restrained shrinkage induced strain in the concrete exceeds its tensile strain capacity. These cracks have a premature detrimental effect on the durability and strength of concrete structures as they allow deleterious materials to penetrate the concrete, which could cause the corrosion of steel reinforcing. With this in mind, the objective of this study is to gain a fundamental understanding of the tensile properties of early age concrete, up to the point of final setting, as well as the variables that affect these properties. This is done to better understand, and ultimately reduce the risk of early age cracking. To achieve this, experimental assemblies found in literature were evaluated and built upon to create a multi-component uniaxial tensile testing setup that is able to capture the complete stress-strain behaviour of early age concrete, while still in a plastic state. The following significant findings were attained from this study: • Reducing the coarse aggregate size in a concrete mix increases both the tensile strength and Young’s modulus of early age concrete, while reducing both its fracture energy and fracture process zone (FPZ) characteristic length. • The low volume addition of microfibres to a conventional concrete mix increases both the fracture energy and the FPZ characteristic length of early age concrete. • The low volume addition of microfibres to a conventional concrete mix increases the strain capacity of early age concrete shortly before and after the initial setting time. This increased strain capacity is believed to be of great significance for the prevention of PShC. • The addition of an accelerator to a conventional concrete mix accelerates the development of the tensile properties of early age concrete, while a retarder reduces it. • The addition of a retarder to a conventional concrete mix increases the strain capacity of early age concrete shortly before and after the initial setting time. This provides a reason for the reduced PShC severity observed in retarded mixes in certain instances. From this study it is concluded that the results from the tensile tests provide a greater understanding of the tensile properties of early age concrete as well as the variables that affect them. When interpreting these results in combination with those obtained from PShC experiments, it is suggested that it is possible to determine when and if PShC will occur. / AFRIKAANSE OPSOMMING: Die vroëe-ouderdom kraking van beton, wat plastiese krimp krake (PKK) en plastiese versakkings krake (PVK) insluit, kom algemeen voor in plat betonelemente soos brug-dekke en blaaie, of by die die verandering in die deursnit diepte van betonelemente. Die krake kom tiepies voor vandat beton gegiet en gekompakteer is totdat dit die finale settyd bereik, en vind plaas sodra die trekspanning wat in die beton ontstaan sy treksterkte oorskry of, anders bewoord, wanneer die verhinderde krimp geinduseerde vervorming van die beton, die vervormings-kapasiteit van die beton oorskry. Hierdie krake het ʼn voortydige nagelige uitwerking op die duursaamheid en sterkte van betonstrukture aangesien hulle toelaat dat skadelike stowwe die beton binnedring, wat die korrosie van staalbewapening veroorsaak. Met dit ingedagte is die doel van die studie om fundamentele kennis rakende die vroëe-ouderdom trekeienskappe van beton, tot by die punt van finale set, asook die veranderlikes wat die eienskappe beinvloed, te verwerf. Om vroëe-ouederdom krake beter te verstaan en uiteindelik, te voorkom, is hierdie kennis nodig. Eksperimentele opstellings in literatuur is ge-evalueer en op voortgebou vir die bou van ʼn multi-komponet eenassige terktoetsopstelling om die volledige spanning-vervorming gedrag van vroëe-ouderdom beton vas te vang. Die volgende bevindings het uit die studie aan die lig gekom: • ʼn Kleiner aggregaat grootte in n betonmeng verhoog beide die trekstrekte en Young se modulus van vroëe-ouderdom beton, terwyl dit beide die fraktuur-energie en die fraktuur proses sone (FPS) se karakteristieke lengte verminder. • Die lae volume byvoeging van mikrovesels tot ʼn betonmeng verhoog beide die fraktuur-energie en die FPS se karakteristieke lengte van vroëe-ouderdom beton. • Die lae volume byvoeging van mikrovesels tot ʼn betonmeng verhoog die vervormings kapasiteit van vroëe-ouderdom beton kort voor en na die aanvanklike settyd. Daar word geglo dat hierdie verhoogde vervormings-kapasiteit van groot belang is vir die voorkoming van PKK. • Die byvoeging van ʼn versneller tot ʼn betonmeng versnel die ontwikkelingstempo van die trekeienskappe van vroëe-ouderdom beton, terwyl ʼn vertrager dit verlaag. • Die byvoeging van ʼn vertrager tot ʼn betonmeng verhoog die vervormings-kapasiteit van vroëe-ouderdom beton kort voor en na die aanvanklike settyd. Dit verskaf die rede vir die bevinding dat die byvoeging van ʼn vertrager PKK in sekere gevalle verminder. Hierdie studie het bevind dat die die trektoetse ʼn groter begrip rakende die trek-eienskappe van vroëe-ouderdom beton, en die veranderlikes wat die eienskappe beinvloed, gelewer het. Wanneer die resultate van die studie tesame met PShC toetse geinterpreteer word, will dit voorkom dat dit moontlik is om te bepaal wanneer, en of PKK sal plaasvind.

Mechanical characterization of four polymer nanocomposite systems and two pure polymer reference systems was performed. Alumina (Al2O3) and magnetite (Fe3O4) nanoparticles were embedded in poly(methyl methacrylate) (PMMA) and polystyrene (PS) matrices. Mechanical testing techniques utilized include tensile testing, dynamic mechanical analysis (DMA), and nanoindentation. Consistent results from the three techniques proved that these nanocomposite systems exhibit worse mechanical properties than their respective pure polymer systems. The interphase, an interfacial area between the nanoparticle filler and the polymer matrix, was investigated using two approaches to explain the mechanical testing results. The first approach utilized data from thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) to predict the structure and density of the interphase for the four nanocomposite systems. The second approach analyzed the bonding between the polymer and the nanoparticle surfaces using Fourier Transform Infrared Spectroscopy (FT-IR) to calculate the density of the interphase for the two PMMA-based nanocomposite systems. Results from the two approaches were compared to previous studies. The results indicate that Al2O3 nanoparticles are more reactive with the polymer matrix than are Fe3O4 nanoparticles, but neither have strong interaction with the polymer matrix. The poor interaction leads to low density interphase which results in the poor mechanical properties.

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