Alkali compounds were added to a South African coal with a high swelling propensity and the behaviour of the blends were investigated. A vitrinite-rich bituminous coal from the Tshikondeni coal mine in the Limpopo province of South Africa was used. To reduce the influence of the minerals in the coal, the coal was partially demineralized by leaching with HCl and HF. The ash content of the coal sample was successfully reduced from 17.7% to 0.6%. KOH, KCl, K2CO3 and KCH3CO2 were then added to the demineralized coal in mass percentages of 1%, 4%, 5% and 10%. The free swelling indices (FSI) of the blends were determined and the samples were subjected to acquisition of TMA and TG-MS data. Addition of these potassium compounds to the demineralized coal reduced the swelling of the vitrinite-rich coal. From the free swelling indices of the various mixtures, it was concluded that the potassium compounds reduce the swelling of the coal in the following order of decreasing impact: KCH3CO2 > KOH > K2CO3 > KCl. From dilatometry experiments done on the blends with the 10% addition of potassium compounds, it was seen that with the addition of potassium compounds to the demineralized coal, a reduction in dilatation volume was obtained. The influence of the potassium compound in decreasing order: K2CO3> KOH> KCH3CO2> KCl. An increase in the softening temperature was observed for the demineralized coal-alkali blends. Thermogravimetric analyses were performed on the demineralized coal-potassium blended samples (<75 μm). These samples were pyrolyzed under a nitrogen atmosphere to a maximum temperature of 1200 °C using a heating rate of 10 °C/min. The relative reactivity for each of the blends with the different wt% addition was determined. From these results it was seen that KCH3CO2 increased the relative reactivity, whereas the KOH, KCl and K2CO3 showed an inhibiting influence. The attached mass spectrometer provided information on the low molecular mass gaseous products formed in the various temperature ranges as the thermal treatment proceeded. From the mass spectroscopy results, it was found that the potassium compounds decreased the temperature at which maximum evolution of H2 takes place. Thermomechanical analyses were performed on the 10 wt% addition of the potassium compounds to the demineralized coal. During TMA analyses, the sample was heated to 1000 °C using a heating rate of 10 °C/min. From the TMA result obtained it was clear that the addition of KCl did not have an influence on the swelling of the demineralized coal. All results are discussed. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

In contrast to composites and woven fabrics, nonwoven materials have a unique web structure, which is composed of randomly oriented fibres bonded in a pattern by mechanical, thermal or chemical techniques. The type of nonwovens studied in this research is a thermally bonded one with polymer-based bicomponent fibres. Such fibres have a core/sheath structure with outer layer (sheath) having a lower melting temperature than that of the core. In thermal bonding of such fibres, as the hot calender with an engraved pattern contacts the fibrous web, bond points are formed thanks to melting of the sheath material. Molten sheath material acts as an adhesive while core parts of the fibres remain fully intact in the bond points. On the other hand, web regions, which are not in contact with the hot engraved pattern, remain unaffected and form the fibre matrix that acts as a link between bond points. With two distinct regions, namely, bond points and fibre matrix, with different structures, nonwovens exhibit a unique deformation behaviour. This research aims to analyse the complex mechanical behaviour of thermally bonded bicomponent fibre nonwoven materials using a combination of experimental and numerical methods. A novel approach is introduced in the thesis to predict the complex mechanical behaviour of thermally bonded bicomponent fibre nonwovens under various threedimensional time-dependent loading conditions. Development of the approach starts with experimental studies on thermally bonded bicomponent fibre nonwovens to achieve a better understating of their complex deformation characteristics. Mechanical performance of single bicomponent fibres is investigated with tensile and relaxation tests since they are the basic constituents of nonwoven fabrics. The fabric microstructure, which is one of the most important factors affecting its mechanical behaviour, is examined with scanning electron microscopy and X-ray micro computed tomography techniques. At the final part of experimental studies, mechanical response of thermally bonded bicomponent fibre nonwovens is characterised with several mechanical tests. (Continues...).

No description available.

Self-consolidating concrete (SCC) is a special type of concrete that flows under its own weight and spreads readily into place while remaining stable. Although SCC technology has been rapidly progressing over the last 20 years and continues to develop, the relationships between the fresh, hardened and durability properties of SCC are not well documented. The focus of this investigation is twofold. Firstly, the use of SCC necessitates reliable and accurate characterization of material properties. A variety of laboratory test methods are used to evaluate SCC’s plastic properties. Recognizing that various test methods evaluate the same plastic properties, there is a need to critically investigate the adequacy and sensitivity of each test. Secondly, outcomes from this project are expected to advance the fundamental understanding of the interplay between the fresh properties of SCC and their implications on hardened properties and durability performance.

Self-consolidating concrete (SCC) is a special type of concrete that flows under its own weight and spreads readily into place while remaining stable. Although SCC technology has been rapidly progressing over the last 20 years and continues to develop, the relationships between the fresh, hardened and durability properties of SCC are not well documented. The focus of this investigation is twofold. Firstly, the use of SCC necessitates reliable and accurate characterization of material properties. A variety of laboratory test methods are used to evaluate SCC’s plastic properties. Recognizing that various test methods evaluate the same plastic properties, there is a need to critically investigate the adequacy and sensitivity of each test. Secondly, outcomes from this project are expected to advance the fundamental understanding of the interplay between the fresh properties of SCC and their implications on hardened properties and durability performance.

The increasing use of alpha/beta Ti-Al alloys in critical aircraft gas turbine engine and airframe applications necessitates the further development of physically-based constitutive models that account for their complex microdeformation mechanisms. Alpha/beta Ti-Al alloys are dual-phase in nature consisting of a mixture of hcp (alpha) and bcc (beta) crystal structures, which through variation in alloying elements and/or processing techniques can be produced in a wide range of microstructural compositions and morphologies. A constitutive model for these materials should address the various sources of material anisotropy and heterogeneity at both the micro and macroscales. The main sources of anisotropy in these materials are the low symmetry of the hcp phase, the texture, the relative strengths of different slip systems, non-planar dislocation core structures, phase distributions, and dislocation substructure evolution. The focus of this work is the development of a 3-D crystal plasticity model for duplex Ti-6Al-4V (Ti-64), an (alpha+beta) alloy. The model is used to study the process of attachment fatigue. Attachment fatigue is a boundary layer phenomenon in which most of the plastic deformation and damage accumulation occurs at depths on the order of tens of microns and encompasses regions of only a few grains into the depth of the material. The use of computational micromechanics-based crystal plasticity models to study attachment fatigue is a relatively new approach. This approach has the potential to offer additional insight to classical homogeneous plasticity models, since the length scales over which relative slip and crack initiation occur during this process is on the order of microstructural dimensions. Emphasis is placed on understanding the effects that texture, slip strength anisotropy, and phase distribution have on the surface and subsurface deformation fields during attachment fatigue. The deformation fields are quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and plastic strain-based critical plane multiaxial fatigue parameters.

Alkali compounds were added to a South African coal with a high swelling propensity and the behaviour of the blends were investigated. A vitrinite-rich bituminous coal from the Tshikondeni coal mine in the Limpopo province of South Africa was used. To reduce the influence of the minerals in the coal, the coal was partially demineralized by leaching with HCl and HF. The ash content of the coal sample was successfully reduced from 17.7% to 0.6%. KOH, KCl, K2CO3 and KCH3CO2 were then added to the demineralized coal in mass percentages of 1%, 4%, 5% and 10%. The free swelling indices (FSI) of the blends were determined and the samples were subjected to acquisition of TMA and TG-MS data. Addition of these potassium compounds to the demineralized coal reduced the swelling of the vitrinite-rich coal. From the free swelling indices of the various mixtures, it was concluded that the potassium compounds reduce the swelling of the coal in the following order of decreasing impact: KCH3CO2 > KOH > K2CO3 > KCl. From dilatometry experiments done on the blends with the 10% addition of potassium compounds, it was seen that with the addition of potassium compounds to the demineralized coal, a reduction in dilatation volume was obtained. The influence of the potassium compound in decreasing order: K2CO3> KOH> KCH3CO2> KCl. An increase in the softening temperature was observed for the demineralized coal-alkali blends. Thermogravimetric analyses were performed on the demineralized coal-potassium blended samples (<75 μm). These samples were pyrolyzed under a nitrogen atmosphere to a maximum temperature of 1200 °C using a heating rate of 10 °C/min. The relative reactivity for each of the blends with the different wt% addition was determined. From these results it was seen that KCH3CO2 increased the relative reactivity, whereas the KOH, KCl and K2CO3 showed an inhibiting influence. The attached mass spectrometer provided information on the low molecular mass gaseous products formed in the various temperature ranges as the thermal treatment proceeded. From the mass spectroscopy results, it was found that the potassium compounds decreased the temperature at which maximum evolution of H2 takes place. Thermomechanical analyses were performed on the 10 wt% addition of the potassium compounds to the demineralized coal. During TMA analyses, the sample was heated to 1000 °C using a heating rate of 10 °C/min. From the TMA result obtained it was clear that the addition of KCl did not have an influence on the swelling of the demineralized coal. All results are discussed. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The general problem of plastic buckling of flat metal plates is a fundamental area of investigation in mechanics not only because of its intrinsic importance in the design of engineering structures, but also because it still has not been settled in a satisfying manner. Which theory of plasticity is the correct one to predict the buckling loads in the plastic range is a long-argued problem. / This thesis presents finite element analyses of plastic buckling and postbuckling behaviour of columns and plates, taking into account the presence of initial out-of-plane imperfections. The FE programs constructed by the author for this purpose are used to analyze the imperfection growth of such columns and plates under axial loading and simply supported edge conditions. The material behaviour is modeled according to both the incremental and the deformation theories of strain-hardening plasticity. The programs combine both the geometric and material nonlinearities to trace the load-deflection behaviours of these structures in prebuckling (up to the maximum load) as well as postbuckling ranges. The results of the analyses for plates show the extreme sensitivity of the incremental theory, and the relative insensitivity of the deformation theory, to the initial imperfections. / The programs are used to simulate the plastic buckling experiments on Aluminum tubes, taking into account their measured imperfections. The imperfection growth analyses demonstrate that the maximum load predictions of the incremental theory are quite close to those recorded in the experiments.

Ti-6Al-4V, known for high strength-to-weight ratio and good resistance to corrosion, has been widely used in aerospace, biomedical, and high-performance sports applications. A wide range of physical and mechanical properties of Ti-6Al-4V can be achieved by varying the microstructures via deformation and recrystallization processes. The aim of this thesis is to establish a microstructure-sensitive fatigue analysis approach that can be applied in engineering applications such as fretting fatigue to permit explicit assessment of the influence of microstructure. In this thesis, crystal plasticity constitutive relations are developed to model the cyclic deformation -TiAl has beenabehavior of Ti-6Al-4V. The development of the slip bands within widely reported and has been found to play an important role in deformation and fatigue behaviors of Ti-6Al-4V. The shear enhanced model is used to simulate the formation and evolution of slip bands triggered by planar slip under static or quasi-static loading at room temperature. Fatigue Indicator Parameters (FIPs) are introduced to reflect driving force for the different crack formation mechanisms in Ti-6Al-4V. The cyclic stress-strain behavior and fretting fatigue sensitivity to microstructure and loading parameters in dual phase Ti-6Al-4V are investigated.

No description available.