http://bura.brunel.ac.uk/handle/2438/236 2013-05-20T06:44:54Z 2013-05-20T06:44:54Z Iida, Yusuke http://bura.brunel.ac.uk/handle/2438/7405 2013-04-29T11:14:39Z 2007-01-01T00:00:00Z

Title: The effects of magnetic fields on rolling contact fatigue wear Authors: Iida, Yusuke Abstract: The thesis describes the effects of the magnetic field in rolling contact tests of steel by using a two-disc machine and the investigation of its mechanism from contact stress analysis by using FEM and the agnetisation of a ferromagnetic substance. In the tests, two contact kinematic conditions, that is pure rolling and 10% rolling with sliding together with 1.1 and OAT [Tesla] of horizontal static magnetic fields created by permanent magnets were pplied. The results of optical and scanning electron microscopy observations show that finer wear particles and smoother worn surfaces are produced in the presence of the magnetic field. For the generation of the finer wear particles, it is considered necessary that the subsurface crack initiation point is moved towards the surface due to the magnetic field. Wear amounts of the discs are lowered III the magnetic fields under the pure rolling conditions. However, at 100/0 rolling with sliding, the wear amounts are increased in the magnetic fields even though finer particles and smoother surfaces are observed. Both tendencies are unified by calculating the number of cycles required to generate wear particles, which are reduced due to the magnetic field's presence. For these mechanisms, it is considered that domain walls near the contact region are caught by dislocations when the specimen is agnetised and part of the energy for magnetisation activates the dislocation movement resulting in crack initiation. Description: This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.

2007-01-01T00:00:00Z Wang, Xin http://bura.brunel.ac.uk/handle/2438/7403 2013-04-29T11:01:15Z 2001-01-01T00:00:00Z

Title: Non-destructive characterisation of structural ceramics using impedance spectroscopy Authors: Wang, Xin Abstract: The aim of this project was to explore the potential applications of impedance spectroscopy (IS) in the non-destructive characterisation of structural ceramics. A major advantage in the use of the IS technique is its capability in distinguishing the properties of different microstructural origins in materials, e.g., grains and grain boundaries, etc. In this thesis, a review of the theoretical aspects of IS is presented. An analytical approach and numerical analyses are conducted to illustrate how impedance spectra become resolved and why the spectral resolution is dependent on the representation formalism as well as on the difference between the electrical properties of different microstructural features. Three categories of structural ceramics, i. e. A1203/SiC nanocomposites, thermal barrier coatings and clay-based ceramics, were used as the model materials for this IS study. Both sintering and degradation phenomenon have been examined. Conventional analytical techniques, such as SEM, XRD, EDS, TGA and dilatometry, were used to aid in the understanding of IS and to prove the reliability of impedance measurements. The research results indicate: * For A1203/SiC nanocomposites, IS can be used to examine the conducting mechanisms for the materials containing different quantities of SiC particles. The content of SiC can be correlated to the impedance spectral features and dielectric constant of the material. The oxidation scale formed at the surface gives a separate relaxation process. Based on the capacitive effect of this relaxation process, the thickness can be determined non-destructively using IS. * For thermal barrier coatings, three relaxation processes can be found in the impedance spectra, which correspond to the top coating, oxidation layer and microstructural defects in the top coating, respectively. The thickness of the oxidation layer can be quantitatively related to the diameter of the corresponding semicircle in electrical modulus spectrum. In the meantime, the top coating degradation could also be monitored using IS. * For clay-based ceramics, the impedance spectra consist of a high frequency semicircle and a low frequency tail, which correspond to bulk effect and electrode effect, respectively. The variation of the bulk conductivity with sintering time can be quantitatively correlated to the densification of the material during sintering. The electrode effect tail is directly related to the capacitive effect of the electrode/specimen interface, which could be an effective indicator of the electrode temperature. Therefore IS is a useful technique for non-destructive characterisation of structural ceramics. Description: This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.

2001-01-01T00:00:00Z Bornoff, Robin B http://bura.brunel.ac.uk/handle/2438/7400 2013-04-29T10:05:59Z 1997-01-01T00:00:00Z

Title: A numerical investigation of the interactions between adjacent cooling tower plumes Authors: Bornoff, Robin B Abstract: Cooling tower plume rise, dilution and dispersion is investigated using a numerical model. Both single and double sources are considered. The main aim of the investigation is concerned with comparison of the computational results to existing wind tunnel experimental data as well as simple empirical rise height formula. Analysis of the interaction of adjacent sources, and subsequent rise augmentation compared to that of a single source, is a central theme of the work. A full-scale hybrid mechanical cooling tower is modelled as a surface mounted cuboid block 20 m high with an internal development duct of 10 m diameter. Both jet and moderately buoyant plume type sources are studied. Two exit velocity ratios are also considered. An oncoming atmospheric boundary is modelled with an associated logarithmic velocity profile and profiles of turbulence kinetic energy and length scale. Two double source orientations, tandem and side-by-side with respect to the oncoming cross wind, are studied. Physical symmetry is utilised and so only half of the domain is modelled. Both the small-scale (wind tunnel) and full-scale were modelled. The small-scale work used combinations of a low Reynolds number k-e turbulence model and both hybrid and QUICK discretisation schemes. The high Reynolds numbers encountered in the fullscale allowed the use of a number of different turbulence models, namely the standard k-e model, the RNG k-e model and a differential flux model, combined again with the hybrid and QUICK discretisation schemes. The results of a number of sensitivity tests showed that plume rise in this case was not sensitive to the turbulence model constant C3 or to source turbulence levels. A decrease in the turbulent Prandtl number led to a marked increase in the turbulent diffusion of the thermal plume. Horizontal plume spreading was underpredicted in both small and full-scales compared to the experimental data. Plume rise and dilution was, in the majority of cases, predicted accurately compared to both the experimental data and also to rise heights given by simple empirical relationships. Generally, the choice of discretisation scheme was a more important factor than choice of turbulence model. Interaction of side-by-side plumes was dominated by the interaction of the rotating vortex pairs within the plumes. A tandem source arrangement led to early merging and efficient rise enhancement. Merging into a single type plume occurred sooner with an decrease in exit velocity ratio, R. Description: This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.

1997-01-01T00:00:00Z Knight, Matthew G http://bura.brunel.ac.uk/handle/2438/7387 2013-04-26T13:51:06Z 2002-01-01T00:00:00Z

Title: Numerical modelling of particulate and fibre reinforced composites Authors: Knight, Matthew G Abstract: This thesis presents research into the micromechanical modelling of composite materials using numerical techniques. Composite materials are generally examined from two points of view: macromechanics and micromechanics, owing to their inherent heterogeneous nature. In this research, the material behaviour is examined on a microscopic scale, as the properties of interest, i.e. strength and toughness, are dependent on local phenomena. In general, the strength and toughness of composite materials are not as well understood as the simpler elastic properties, because in many cases the modes of failure under a given system of external load are not predictable in advance. Previous research in this field has typically involved specially designed experiments, theoretical/statistical studies, or the use of numerical models. In this study, advanced implementations of numerical methods in continuum mechanics, i.e. the boundary element and the finite element methods are employed to gain a greater understanding of composite behaviour. The advantage of using numerical methods, as opposed to experimental studies, is that the geometric and material characteristics can be investigated parametrically, in addition to the reduced time and expense involved. However, to model the complete behaviour of real composites is still not possible, due to the degree of complexity and uncertainty involved in modelling the various mechanisms of damage and failure, etc. and also due to the immense computational cost. Therefore, simplified models must be employed which are limited by their assumptions. For the preliminary studies within this thesis, geometrically simplified models are presented to provide an understanding of the influence of embedding second phase inclusions on the local stress fields, and also to validate the numerical techniques with readily available analytical solutions. These models are then extended to accommodate additional phenomena, such as inclusion interaction, spatial inclusion arrangement, material formulation, i.e. consisting of two- and three-phases of various material properties. The influence of such factors on the local stress concentrations, which play an important role in determining the strength of the composite, is analysed through a series of parametric studies. The localised toughening of composites is also considered through novel investigations into the interaction between a propagating crack with inclusions and microcracks. Through the development of the numerical models a more realistic representation of composite behaviour is achieved, which in tum, provides an improved knowledge of the factors that control strength and toughness. Such information is invaluable to composite material designers, who presently rely heavily on experimental studies to develop composite materials. Description: This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.

2002-01-01T00:00:00Z