Mechanical and Materials Engineering Theses

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This collection is made up of doctoral and master theses by research, which have been received in accordance with university regulations.

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Now showing 1 - 5 of 14
  • Publication
    Optical Eye Modelling for Myopia Control
    (University College Dublin. School of Mechanical and Materials Engineering, 2022)
    Myopia is posing a big threat to the global eye health and putting the young generation in danger of blindness. To prevent the development of high myopia, optical lenses have been developed based on the findings that the peripheral optical properties of the eye can affect myopia progression. As the core for many applications in myopia control, a realistic eye model should be established that reproduces the optical and structural features of the human eye. Based on the ocular data obtained from the recent measurement technologies, this thesis investigates the peripheral optical features of the ocular components and develops new models to achieve a more realistic description of the human eye for the wide visual field. As the most complex component in the eye model, a new crystalline lens model is proposed in Chapter 2 to represent the structural and functional features of the lens of children. The model has the capability of involving most parameters measurable on the in vivo human lens, while maintaining realistic values of optical power and spherical aberration. Starting from the lens model proposed in Chapter 2, Chapter 3 develops a method for evaluating the peripheral refracting properties of the lens. The impacts of the lens structural parameters to the peripheral lens power are systematically evaluated. Specifically, the contribution of the gradient refractive index structure to the peripheral lens power has been revealed. In Chapter 4, a fast computation method based on generalized ray tracing is developed to analyze the peripheral optical power of the cornea defined in a similarly way as proposed in Chapter 3. The method is tested on the realistic corneal model constructed from measurement data. The contribution of the cornea to the ocular refractions over the entire visual field can be formulated based on the proposed procedure. Chapter 5 and 6 investigates the role of the retinal contour to the peripheral optical properties of the eye. Specifically, a highly efficient method is proposed that can reproduce the peripheral spherical equivalent refraction over the entire visual field by retinal contour modeling. Overall, this work contributes a theoretical framework and knowledge base on the peripheral optics of the human eye, which are instrumental for developing potential approaches aimed for higher efficacy in myopia control.
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  • Publication
    Investigations of Tribological Performance of Structured Surfaces on Bioimplants
    (University College Dublin. School of Mechanical and Materials Engineering, 2022)
    Bioimplants are man-made medical devices to replace the malfunctional natural joints. In the past six decades, total joint replacement is credited as one of the most successful surgical operations. Metal alloy matching with polymer is the most common material combination used in the bearing parts of artificial joints. However, most commercial products have a relatively short in-vivo lifespan due to the unsatisfactory tribological performance of bearing parts. This study aims at increasing the longevity of bioimplants by modifying the surface topographies of bearing parts, including surface roughness and surface texturing. This thesis starts with the motivation of carrying out this project, followed by a comprehensive literature review. In Chapter 3, the validity of molecular-mechanical frictional theory is tested for the bioimplant application using the pin-on-disk tribometer. By a long-term sliding test (10 km) and the dynamic analysis, the study firstly highlights the importance for the metallic bearing surface to keep its original surface finish after implantation other than only getting a superfinished surface before implantation. As a result, surface texturing approach is proposed to achieve this goal. In Chapter 4, the impact of four potential working mechanisms related to surface texturing in bioimplants are investigated. An important finding on the role of hydrodynamic pressure on the tribological performance of textured implants is presented: the extra lifting force provided by hydrodynamic pressure is negligible. This unique property distinguishes the bioimplants application from other conventional bearing systems. Further numerical simulation and experimental experiments attribute this novel finding to the working conditions of implanted joints: slow sliding speed and low viscosity lubricating solution. Alternatively, a new understanding, namely squeezing effect, is established to explain the increased thickness of lubricant film which helps to improve the tribological performance of textured bioimplants. Meanwhile, a novel failure mechanism, interlocking effect (stress concentration and two-body abrasive wear), is put forward to explain why some pattern designs, such as sharp-corner structure, are detrimental to the bioimplants. Afterwards, a technical solution, namely round-corner structure, is developed to resolve the interlocking effect. In Chapter 5, sliding orientation is found to play a minor role on the tribological performance of textured bioimplants and this phenomenon is explained by the proposed working mechanism of squeezing effect. Furthermore, the regularly arranged texturing patterns are proved to be more suitable for bioimplants than the irregularly arranged micro patterns. Finally, an orthogonal experiment is designed to reveal the influential level of five main pattern parameters: area density > size > shape > depth > distribution mode. The conclusion is that the optimal pattern design with specific parameters: triangle structure with 200 µm side length, 8-10 µm depth, 10% area density and square distribution mode, can provide the optimized tribological performance. In Chapter 6, a long-term wear experiment with 1 million cycles is carried out to compare the tribological performance between the only polished bioimplants and the ones with optimal structured bearing part. The study confirms that, by applying the optimal structured surface, the in-vivo longevity of polymer-based bioimplants can be effectively increased.
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  • Publication
    XR in Health and Wellbeing: Establishing a New, Inclusivley Designed Participatory Practice using Augmented Reality Helpers to Support Teens and Young Adults with Autism Spectrum Disorder to Address Life Challenges
    (University College Dublin. School of Mechanical and Materials Engineering, 2022) ;
    0000-0002-5667-8246
    The 2017 Health Evidence Awareness Report from the Irish Health Repository identified that 1- 2% of the global population has Autism Spectrum Disorder (ASD). This percentage was confirmed in 2021 by the US Disease Control and Prevention (CDC). Attempting to ‘fit into’ societal infrastructures and norms can present complex challenges for individuals with ASD, in a world that has not catered for their needs or tailored opportunities for their development. For instance, over 35% of young adults with ASD in the USA have not held a job, nor received formal education after leaving high school. Global comparators also show disparity and deprivation for this age group of young adults with ASD. This thesis offers novel design insights achieved through the design and testing of a novel Augmented Reality (AR) Digital Helper, the aim of which is to bridge the needs of individuals with ASD with possible societal supports. Augmented Reality is applied in this thesis projects as a technology tool that offers a layer of information and interpretation for young people with ASD engaged in real-world activities. AR as an Assistive Technology was applied in the project through the process of co-designing an AR helper with the young people in need of its affordances. In this way, an Inclusive Co-Design methodology was applied, establishing participants with ASD as co-designers of their own AR Digital Helpers and testing their responses to the use of their own Helpers in real-world settings. The research also gathered insights from the participants’ care ecosystems of family members and care workers. This research makes an original contribution to scholarship by introducing this novel assistive technology in the context of specific domains of knowledge, reaching across disciplines and practices to provide new insights focused on the characteristics of, and activities with which, AR Digital Helpers can assist. The thesis details the establishment of a new participatory design method, and includes a literature review covering the areas of Critical Disability Studies, XR capabilities in well-being practices, and the area of social challenges for young adults with ASD, alongside the technical areas and applicable research in Virtual Reality (VR), AR and 3D non player character guides or helpers. The PhD thesis, including the body of data, data analysis, and details of the Digital Helper prototype, are all offered together as a practice-based PhD making an original and substantial contribution to the Field of Knowledge of XR in Health and Wellbeing for individuals with ASD, and to the cognate fields of Extended Reality for ASD, and Inclusive Design. It is hoped that future scholars, design practitioners, clinicians, and people with ASD, will be able to apply this research to their studies and their lives.
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  • Publication
    Ab-initio Simulations and Structure Fabrication at Atomic and Close-to-atomic Scale using Atomic Force Microscopy
    (University College Dublin. School of Mechanical and Materials Engineering, 2022) ;
    0000-0002-8056-2198
    To increase the number of electronic components in a single integrated circuit chip, the functional feature size should be reduced to the atomic and close-to-atomic scale (ACS). For this, the application of molecules could be utilised as a channel for current conduction. This thesis focuses on the fundamental aspects of this theme to help us achieve atomic scale device fabrication in the future. A literature review on advances in moletronics and atomic and close-to-atomic scale manufacturing (ACSM) research with the application of atomic force microscopy (AFM) is given in chapter 1. ACS device manufacturing using molecules as the building block requires to overcome mainly three fundamental problems. Firstly the orientation of the molecule when placed between the electrodes plays a critical role in electronic transport. This is explained in chapter 2, which gives a detailed ab-initio simulation studies of current flow in inorganic molecule, such as polyoxometalates (POMs) and organic molecules such as phthalocyanines (Pc) and porphyrins (Pr), by incorporating them between gold electrodes. For the POM molecule, longitudinal orientation showed better conduction than lateral orientation, whereas for Pc and Pr molecules, the geometrically optimised orientation displayed better electronic transport properties than the tautomerized structure. Secondly, the bonding interaction between the electrode and the molecular terminal atoms helps us to determine the rate of electronic transport at the junction. Chapter 3 inspects this interaction through a periodic energy decomposition analysis on Pc and Pr derivatives. The attractive and repulsive energy terms of the bonding interactions proved that Pr molecules are better interactive over the gold substrate in comparison to Pc molecules. Electronic transport studies performed on their derivatives with and without thiol linkers further supported this result. Thus, a link between these two studies were established. This paves path for future work to select appropriate molecules and electrodes to demonstrate transistor actions for atomic scale device fabrication. Finally, the possibility of the fabrication of ACS electrodes with a single atomic protrusion for the attachment of molecules needs to be experimentally validated. As a first step towards this, fundamental studies using AFM to achieve atomic layer removal were carried out taking into account different machining parameters. This is given in chapter 4 and chapter 5. In chapter 4, mechanical AFM-based scratching techniques over gold and silicon using diamond tips were performed. In silicon substrate, material removal having a minimum depth of 3.2Å which is close to about 3 silicon atom thickness, has been achieved. On gold, a minimum depth of 9.7Å, close to 7 atom thickness has been achieved. In chapter 5, electrochemical AFM-based lithography over HOPG and silicon using platinum coated tips were carried out. Results showed that in bare silicon local anodic oxidation took place instead of material removal. Even in hydrofluoric (HF) treated silicon, oxidation occurred but in a controlled and well defined manner. From this, it can be deduced that HF treated silicon is better suited for structure fabrication than bare silicon. In the case of HOPG, different patterns such as nano-holes, nanolines and intrinsic patterns were machined and material removal close-to-a single atomic layer, ~3.35Å was achieved. Results from chapter 4 and 5 reveal that controlled AFM-based scratching techniques can ensure the fabrication of well-defined atomic structures for the application of molecular devices. Since ACSM represents the next phase of manufacturing, this thesis proposes some of the primary works required to realise ACSM using the currently available techniques and simulation methodologies to bring us one step closer in achieving considerable advancements in this field in the near future.
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  • Publication
    Comparison of Ti-6Al-4V and 316L Stainless Steel Diamond Lattice Structures Fabricated by Additive Manufacturing
    (University College Dublin. School of Mechanical and Materials Engineering, 2022) ;
    0000-0001-7986-5151
    Laser Based Powder Bed Fusion (PBF-LB) is an additive manufacturing process, which can be used for the manufacture of geometrically complex metallic structures. The focus of this thesis is to evaluate the properties of PBF-LB fabricated Ti-6Al-4V and 316L stainless steel lattice structures, obtained using multiple laser scanning strategies. Amongst the investigations carried out was to determine the effect of both strut diameter and overhang angle, two key lattice design parameters, on the lattice morphology and internal porosity. Ti-6Al-4V structures were manufactured using a point-based approach, in which laser parameters were varied in order to control strut diameter, 316L stainless steel structures were manufactured with a hatch and contour approach, in which strut diameter is varied via. CAD. Evaluation of the structures showed limitations of the minimum achievable overhang angle for both structures of approximately 20°. Porosity formation within the titanium alloy structures was found to be largely associated with keyholing defects, while stainless steel porosity was attributed to lack of fusion defects, both of which were attributed to the selected laser process parameters. In-process monitoring used in the manufacture of the titanium alloy struts, demonstrated that reduced optical emission intensities were obtained from melt pools, from which higher levels of strut failures occurred. A further study evaluated the morphological and compressive mechanical properties of diamond lattice structures fabricated from the titanium and stainless steel alloys. The diamond lattice structure was selected for its noted ease of manufacture and its application within the biomedical sector. Samples fabricated in titanium alloy were found to be of higher quality, with lower amounts of external particle adhesion and more cylindrical struts in comparison to the stainless-steel samples. Both titanium and stainless steel were found to have similar relative compressive strengths, while stainless steel samples were found to have higher relative elastic modulus. Despite the same lattice structures differences in failure modes between the two alloys were obtained. Titanium samples were found to deform in a brittle manner, showing failure through sudden rupture of struts along parallel planes, while stainless steel samples underwent ductile failure where struts and nodes were found to deform consistently without rupture.
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