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Doctoral Degrees (Mechanical Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6861

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Browsing Doctoral Degrees (Mechanical Engineering) by Author "Bemont, Clinton Pierre."

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    Optimisation of welding parameters to mitigate the effect of residual stress on the fatigue life of nozzle–shell welded joints in cylindrical pressure vessels.
    (2017) Zondi, Mthobisi Clyde.; Adali, Sarp.; Bemont, Clinton Pierre.
    The process of welding steel structures inadvertently causes residual stress as a result of thermal cycles that the material is subjected to. These welding-induced residual stresses have been shown to be responsible for a number of catastrophic failures in critical infrastructure installations such as pressure vessels, ship’s hulls, steel roof structures, and others. The present study examines the relationship between welding input parameters and the resultant residual stress, fatigue properties, weld bead geometry and mechanical properties of welded carbon steel pressure vessels. The study focuses on circumferential nozzle-to-shell welds, which have not been studied to this extent until now. A hybrid methodology including experimentation, numerical analysis, and mathematical modelling is employed to map out the relationship between welding input parameters and the output weld characteristics in order to further optimize the input parameters to produce an optimal welded joint whose stress and fatigue characteristics enhance service life of the welded structure. The results of a series of experiments performed show that the mechanical properties such as hardness are significantly affected by the welding process parameters and thereby affect the service life of a welded pressure vessel. The weld geometry is also affected by the input parameters of the welding process such that bead width and bead depth will vary depending on the parametric combination of input variables. The fatigue properties of a welded pressure vessel structure are affected by the residual stress conditions of the structure. The fractional factorial design technique shows that the welding current (I) and voltage (V) are statistically significant controlling parameters in the welding process. The results of the neutron diffraction (ND) tests reveal that there is a high concentration of residual stresses close to the weld centre-line. These stresses subside with increasing distance from the centre-line. The resultant hoop residual stress distribution shows that the hoop stresses are highly tensile close to the weld centre-line, decrease in magnitude as the distance from the weld centre-line increases, then decrease back to zero before changing direction to compressive further away from the weld centre-line. The hoop stress distribution profile on the flange side is similar to that of the pipe side around the circumferential weld, and the residual stress peak values are equal to or higher than the yield strength of the filler material. The weld specimens failed at the weld toe where the hoop stress was generally highly tensile in most of the welded specimens. The multiobjective genetic algorithm is successfully used to produce a set of optimal solutions that are in agreement with values obtained during experiments. The 3D finite element model produced using MSC Marc software is generally comparable to physical experimentation. The results obtained in the present study are in agreement with similar studies reported in the literature.
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    Structural characterisation and response modelling of paraffin-based hybrid rocket motor fuel grains.
    (2020) Veale, Kirsty Lynn.; Adali, Sarp.; Pitot de la Beaujardiere, Jean-Francois Philippe.; Bemont, Clinton Pierre.
    Abstract available in PDF.
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    Trip steels as smart sensor alloys.
    (2013) Bemont, Clinton Pierre.; Bright, Glen.; Cornish, Lesley.
    Upon deformation, TRIP steels undergo progressive irreversible transformation from paramagnetic austenite to more thermodynamically stable, ferromagnetic αʹ-martensite. The change in magnetic permeability is readily detectable, and since TRIP steels also have excellent mechanical properties, this presents the opportunity for implementing cheap, robust structural health monitoring systems. However, the extent of martensitic transformation in TRIP steels is affected not only by the degree of deformation, but by environmental temperature at the time of deformation and strain rate. This creates inherent inaccuracy when implementing TRIP steels as sensor materials. In this thesis it has been demonstrated that it is possible to design TRIP steels that are less susceptible to these factors, show good deformation induced transformation, and can function simultaneously as sensors and structural elements. As-cast alloys were tested in compression, while annealed, hot-rolled and warm-rolled alloys were tested primarily in tension. There was considerable variation between alloys in rate of transformation with deformation. Martensitic transformation was evaluated magnetically and correlated with optical and scanning electron microscopy and X-ray diffraction results. Changes in magnetisation and magnetic permeability curves with deformation were characterised to ensure optimal electronic monitoring. Equations from literature for determining characteristic transformation temperatures, Ms and Md30 were evaluated experimentally for the alloy range of interest, and the best equations were selected to aid in the design of high alloy TRIP steels exhibiting strong transformation and low temperature sensitivity. Temperature sensitivity between alloys was found to vary as predicted. Temperature sensitivity was also compared in annealed, hot rolled and warm rolled conditions; the annealed condition showed the lowest sensitivity, and this is thought to be related to lower dislocation densities. Mining was targeted as a primary industry for application of these sensor systems because of the pressing need for greater safety and more efficient structural support at low cost. Two distinct devices for monitoring the structural health of mines were designed, built and tested, and a third was developed for the aerospace industry. Better understanding and control of the temperature sensitivity of martensitic transformation in TRIP steels is expected to aid not only structural health monitoring, but also the application of such materials to other areas of technology, such as sheet forming and high impact resistance applications. Although there are limitations on the extent to which TRIP steel transformation characteristics can be controlled, it was shown that they can be manipulated to enable successful implementation of new alloys for smart load or damage sensors. Practical, robust, low cost structural health monitoring sensors based on the smart properties of TRIP steels were shown to be feasible.