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Theses and dissertations (Engineering and Built Environment)

Permanent URI for this collectionhttp://ir-dev.dut.ac.za/handle/10321/10

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End date: 2009Subject: search.filters.subject.Mechanical engineering--Dissertations, Academic

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    Methodologies for the optimization of fibre-reinforced composite structures with manufacturing uncertainties
    (2006) Hamilton, Ryan Jason
    Fibre Reinforced Plastics (FRPs) have been used in many practical structural applications due to their excellent strength and weight characteristics as well as the ability for their properties to be tailored to the requirements of a given application. Thus, designing with FRPs can be extremely challenging, particularly when the number of design variables contained in the design space is large. For example, to determine the ply orientations and the material properties optimally is typically difficult without a considered approach. Optimization of composite structures with respect to the ply angles is necessary to realize the full potential of fibre-reinforced materials. Evaluating the fitness of each candidate in the design space, and selecting the most efficient can be very time consuming and costly. Structures composed of composite materials often contain components which may be modelled as rectangular plates or cylindrical shells, for example. Modelling of components such as plates can be useful as it is a means of simplifying elements of structures, and this can save time and thus cost. Variations in manufacturing processes and user environment may affect the quality and performance of a product. It is usually beneficial to account for such variances or tolerances in the design process, and in fact, sometimes it may be crucial, particularly when the effect is of consequence. The work conducted within this project focused on methodologies for optimally designing fibre-reinforced laminated composite structures with the effects of manufacturing tolerances included. For this study it is assumed that the probability of any tolerance value occurring within the tolerance band, compared with any other, is equal, and thus the techniques are aimed at designing for the worst-case scenario. This thesis thus discusses four new procedures for the optimization of composite structures with the effects of manufacturing uncertainties included.
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    Increasing the use of fibre-reinforced composites in the Sasol group of companies : a case study
    (2007) Mouton, Jacques
    A composite material comprises two or more materials with properties that are superior to those of the individual constituents. Composites have become important engineering materials, especially in the fields of chemical plant, automotive, aerospace and marine engineering. The development of more advanced materials and manufacturing techniques in composites has grown from humble beginnings in the 1930s to a recognized and well-respected engineering discipline, providing solutions to conventional and challenging applications. At present, fibre-reinforced composites (FRCs) are amongst the most common types of composites used. They are produced in various forms with different structural properties, and designers, specifiers and end-users can choose from an almost endless list of these materials, providing design flexibility as well as low manufacturing and maintenance cost. Many suggest that composites have revolutionised the chemical and petro-chemical industries. Examples of applications include tanks and chemical reactor vessels that contains many hundreds of litres of hazardous chemicals, reinforced pipes measuring up to several meters in diameter conveying dangerous gases and so on. The South Africa Coal, Oil and Gas Corporation Limited (SASOL) was established in September 1950. From a small start-up, the company has grown to be a world leader in the commercial production of liquid fuels and chemicals from coal and crude oil. Sasol manufactures more than 200 fuel and chemical products at its main plants in Sasolburg and Secunda in South Africa as well as at several other plants abroad. Its products are exported to more than 90 countries around the world. The use of composites in general, and fibre reinforced composites in particular has received little support in Sasol through the years. Some sporadic use of these materials in the construction of process equipment, e.g. tanks, vessels and piping has taken place with varying degrees of success. While the use of equipment fabricated with fibre-reinforced composites has proven extremely successful in the chlorine producing facility in Sasolburg, catastrophic failures have taken place in Secunda in critical fire water systems made of these materials. The history of correct use and application of fibre-reinforced equipment has shown that the cost of ownership of such equipment is significantly lower than similar metallic equipment, therefore reducing costs and safety risks. However, even though this technology brings a company like Sasol closer to the realisation of the vast number of advantages and solutions offered by these materials, the reality is that most engineering personnel are still applying traditional (viz. steel and wood) technology as used by our predecessors. The work presented here attempts to indicate the relevance of fibre-reinforced composites for Sasol, and to detail efforts aimed at the raising of awareness amongst appropriate personnel at Sasol to increase the use of these materials in major capital projects and day-to-day maintenance contracts, therefore taking advantage of the superior performance of fibre-reinforced composites in demanding applications. In support of this drive, part of the work presented indicates the status as well as progress of the composites industry in the last few years. This project was therefore aimed at identifying the level of utilization of fibre-reinforced composites at Sasol, and the possible improvement in benefits of using these technologies. A methodology was developed, using engineering as well as marketing principles, to reach the engineering personnel in various divisions and seniority levels of Sasol to increase the awareness of the capabilities of composites materials, specifically regarding fibre-reinforced composites. Questionnaires were used to gauge the level of awareness while various methods, e.g. one-on-one meetings, seminars, conferences, electronic media, etc were used to upgrade the target groups’ knowledge. The results of the initial survey to determine the status of various dimensions in the company are indicated as well as the outcomes at the end of the research period. In support of the process in Sasol, the development, interaction and cross-pollination of international and national role-players in the fibre-reinforcement industry with respect to chemical containment and Sasol are indicated. The importance of this two-legged process is demonstrated: it ensures a professional national support framework for companies like Sasol. Results are indicated, compared and discussed to give future direction in this ongoing process. As important to this process was the development of appropriate technical resources (like design standards and codes) to enable their use within the group. It was recognised early on that raising the level of awareness of the target groups was not enough and that these resources had to be in-place down the line so that those who chose to could start to implement these material technologies with the aid of the resources. The development of the necessary resources is also discussed. Finally, it will be shown that significant growth has taken place regarding the awareness within the group over the course of implementation of this project. Specifically, about 20% of the target groups have moved from a stage of no knowledge to higher levels of confidence. In terms of use of these materials, significant growth has also taken place judging by the number of plant requests, activity on major capital projects and so on. In fact, from almost nothing in 1999, over the last 5 years in excess of R137 Million has been spent on capital equipment manufactured from composite materials, with the majority in the last 2 years.
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    The synthesis, structure and properties of polypropylene nanocomposites
    (2007) Moodley, Vishnu Kribagaran
    Polymer nanocomposites may be defined as structures that are formed by infusing layered-silicate clay into a thermosetting orthermoplastic polymer matrix. The nanocomposites are normally particle-filled polymers for which at least one dimension of the dispersed particles is in nanoscale. These clay-polymer nanocomposites have thus attracted great interest in industry and academia due to their exhibition of remarkable enhancements in material properties when compared to the virgin polymer or conventional micro and macro-composites. The present work describes the synthesis, mechanical properties and morphology of nano-phased polypropylene structures. The structures were manufactured by melt- blending low weight percentages of montmorillonite (MMT) nanoclays (0.5, 1, 2, 3, 5 wt. %) and polypropylene (PP) thermoplastic. Both virgin and infused polypropylene structures were then subjected to quasi-static tensile tests, flexural tests, micro-hardness tests, impact testing, compression testing, fracture toughness analysis, dynamic mechanical analysis, tribological testing. Scanning electron microscopy studies were then conducted to analyse the fracture surfaces of pristine PP and PP nanocomposite. X-ray diffraction studies were performed on closite 15A clay and polypropylene composites containing 0.5, 1, 2, 3 and 5 wt. % closite 15A nanoclay to confirm the formation of nanocomposites on the addition of organo clays. Transmission electron miscopy studies were then performed on the PP nanocomposites to determine the formation of intercalated, exfoliated or agglomerated nanoclay structures. Analysis of test data show that the mechanical properties increase with an increase in nanoclay loading up to a threshold of 2 wt. %, thereafter the material properties degrade. At low weight nanoclay loadings the enhancement of properties is attributed to the lower percolation points created by the high aspect ratio nanoclays. The increase in properties may also be attributed to the formation of intercalated and exfoliated nanocomposite structures formed at these loadings of clay. At higher weight loading, degradation in mechanical properties may be attributed to the formation of agglomerated clay tactoids. Results of XRD, transmission electron microscopy studies and scanning electron microscopy studies of the fractured surface of tensile specimens verify these hypotheses.
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    Fracture properties of fibre and nano reinforced composite structures
    (2007) Ramsaroop, Avinash
    Interlaminar cracking or delamination is an inherent disadvantage of composite materials. In this study the fracture properties of nano and fibre-reinforced polypropylene and epoxy composite structures are examined. These structures were subjected to various tests including Single Edge Notched Bend (SENB) and Mixed Mode Bending (MMB) tests. Polypropylene nanocomposites infused with 0.5, 1, 2, 3 and 5 weight % nanoclays showed correspondingly increasing fracture properties. The 5 weight % specimen exhibited 161 % improvement in critical stress intensity factor (KIC) over virgin polypropylene. XRD and TEM studies show an increase in the intercalated morphology and the presence of agglomerated clay sites with an increase in clay loading. The improvement in KIC values may be attributed to the change in structure. Tests on the fibre-reinforced polypropylene composites reveal that the woven fibre structure carries 100 % greater load and exhibits 275 % lower crack propagation rate than the chopped fibre specimen. Under MMB conditions, the woven fibre structure exhibited a delamination propagation rate of 1.5 mm/min which suggests delamination growth propagates slower under Mode I dominant conditions. The woven fibre / epoxy structure shows 147 % greater tensile modulus, 63 % greater critical stress intensity factor (KIC), and 184 % lower crack propagation rate than the chopped fibre-reinforced epoxy composite. MMB tests reveal that the load carrying capability of the specimens increased as the mode-mix ratio decreased, corresponding to an increase in the Mode II component. Delamination was through fibre–matrix interface with no penetration of fibre layers. A failure envelope was developed and tested and may be used to determine the critical applied load for any mode-mix ratio. The 5 weight % nanocomposite specimen exhibited a greater load carrying capability and attained a critical stress intensity factor that was 10 % less than that of the fibre-reinforced polypropylene structure, which had three times the reinforcement weight. Further, the nanocomposite exhibited superior strain energy release rates to a material with ten times the reinforcement weight. The hybrid structure exhibited 27 % increase in tensile modulus over the conventional fibre-reinforced structure. Under MMB conditions, no significant increase in load carrying capability or strain energy release rate over the conventional composite was observed. However, the hybrid structure was able to resist delamination initiation for a longer period, and it also exhibited lower delamination propagation rates.