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(2022)ThesisThe mesoscale failure behaviour of textile Carbon Fibre Reinforced Polymer (CFRP) is investigated using in-situ CT scan. The research focused on the potential of using CT acquired geometries, deformation, and failure information to validate traditional modelling techniques and improve the accuracy of future modelling. Carbon Fibre Reinforced Polymer (CFRP) has been widely used in aerospace, automobile, and sporting industry due to its high strength to weight ratio, high stiffness and resistance to fatigue or corrosion. Textile CFRP is a fabric weaved from fibre bundles. Compared to traditional unidirectional CFRP, textile CFRP is usually easier to handle and to form into complex shapes during manufacturing. Due to the yarn interlacing, textile CFRP is also more stable and damage tolerant. However, the interlacing fibre bundles (yarn) introduced additional layer of complexity when predicting the strength and failure of textile composite parts. The main approaches used in current modelling techniques, assume the textile fabric has a regular and repeating structure, failed to capture the irregularity in mesoscale structure introduced during the manufacturing process. As experimental observation shown, the irregularity in mesoscale structure initiates micro crack and failure and has a considerable influence on the properties and failure behaviour of textile CFRPs. The ability of capturing a textile CFRP meso-structure and reconstructing it for FE modelling improves the accuracy of numerical analyses and result in a more reliable and efficient CFRP structure. This research demonstrated the potential of using computer tomography (CT) to improve the understanding of CFRP mesoscale failure behaviour both numerically and experimentally. A more realistic numerical model was constructed using the geometry extracted from the CT image. The CT image volume was classified based on the tow direction and the material property was adjusted based on the fibre orientation. Irregularities in the specimen could be fully reflected in the finite element model. This improves the ability of predicating the onset and progression of failure of textile CFRP. In-situ CT scan was used to investigate the failure mode and crack propagation in several textile CFRP tensile specimens. An in-situ tensile testing rig was designed and manufactured to allow a reliable CT scan of textile CFRP specimens while under tension. One major challenge identified in mesoscale CFRP study is to have a specimen large enough to encompass complete meso-structures while have sufficient resolution. Tensile specimens with a gauge width of 10mm was found to be large enough while providing a good CT image result when scanned using the lowest voltage of 60 kV with 6 accumulations. This resulted in a CT image resolution of 3.2μm. The CT results showed cracks that were usually not visible under conventional scanning technique. Digital volume correlation (DVC) was used to calculate the 3D displacement field by comparing the pre and peri load CT images, providing a quantitative understanding on the failure process. Strain fields were calculated using the forward difference of the displacement field and were used to validate the result of numerical models. The original contribution of this project is the use of CT scan to improve the procedure of obtaining the numerical and experimental results, thus improve the understanding of textile CFRP failure behaviour. A novel in-situ testing rig and the corresponding specimens were manufactured to allow the observation of textile CFRP failure under load. The novel algorithm developed during the project provided a more detailed and accurate geometry of the specimen and allowed for a more accurate model. These contributes to the significant improvement in the understanding of textile composite failure behaviour, both experimentally and numerically.
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(2018)ThesisThe characteristic length of the thin film systems used nowadays in nanoscale thermoelectric and microelectric devices are comparable to the mean free path and wavelength of energy carriers. As a result, the application of classical theorise to characterise thermal transport at the nanoscale is questionable. However, it is essential to understand the underlying physics of heat propagation in thin film systems to control, manipulate, and manage thermal properties in micro and nanodevices. Understanding thermal properties by experiment are challenging, especially for materials with low thermal conductivity and small mean free path (MFP). On the other hand, molecular dynamics (MD) allows investigation of sophisticated crystalline, bulk, interface and surface effects of thermal conduction problem with accuracy, fidelity, and reliability. Nevertheless, the computational results of MD can suffer from the wrong choice of critical parameters, unfit empirical potentials for thermal application and provide unreliable thermal conductivity. Moreover, the dependence of the thermal boundary resistance (TBR) on temperature, thin film's dimension, and defects are not systematically assessed for the important thin films in the thermal application. To solve this problem, a systematic equilibrium molecular dynamics (EMD), addressing the critical issues in thermal conduction characterisation is proposed at the classical temperature range, where thermal conduction is dominated by phonons. The model has been validated by investigating the thermal conduction of Si and dielectrics used in thin film systems. The issue with empirical potential is addressed by critically assessing the performance of a potential on the basis of thermal conductivity, atomic energy and phonon density of state prediction. Later, thin film systems are studied to understand relative phonon propagation at the interface and quantify TBR's dependence on interface area, interfacial distance as well as temperature. Finally, the thermal resistance of thin film systems with defects is characterised to understand realistic phonon propagation scenario in the thermal application of thin film systems.
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(2023)ThesisHigh-entropy alloys (HEAs) are a new class of metallic materials that contain five or more elements in near-equiatomic compositions and that have received significant research attention in the past decade. The reason for this is that some alloy systems have been reported to crystallise as single-phase materials with face-centred cubic (FCC) crystal structure despite their individual elements often having very different crystal structures. One such example, the CrMnFeCoNi alloy, one of the first HEAs reported by Cantor et al. in 2004, has, furthermore, excellent combinations of mechanical properties such as high strength, excellent ductility, and outstanding fracture toughness at room temperature; interestingly, in contrast to many other materials, these properties improve with decreasing temperature down to liquid nitrogen. The reason for this is the staggered activation of deformation mechanisms such as dislocation glide and nano-scale deformation twinning. While numerous studies have reported the CrMnFeCoNi alloy as well as many other multi-component alloy systems to be chemically homogeneous, little attention has been drawn to the impact of processing history on chemical complexity and as such clustering and ordering phenomena that may impact mechanical performance. In this work, CrMnFeCoNi alloys have been fabricated using different processing routes resulting in a chemically homogeneous and a chemically heterogeneous versions of the material. Using various characterisation methods such as a wavelength-dispersive x-ray spectroscopy-based electron micro-probe analyser (WDXS-EPMA) in combination with energy-dispersive x-ray spectroscopy (EDXS) and atom probe tomography (APT), methods that can be utilised across multiple length scales, element composition as well as spatial distribution of the materials have been investigated. The two materials have furthermore been utilised for an APT parameter study to tailor data acquisition parameter for multi-component alloy systems to obtain high quality atom probe data. Based on the obtained results, a generalised multi-component short-range order (GM-SRO) parameter study was conducted to analyse ordering phenomena in the APT data of the HEAs and compared with medium-entropy alloy subsystems containing 3-4 elements and that can be associated with the HEAs in terms of their mechanical and/or elastic properties. Finally, two non-equiatomic alloys from the chemically distinct regions of the heterogeneous material were fabricated and compared in terms of microstructure development and associated mechanical performance to each other and the equiatomic HEA.