MAX phase ceramics are promising candidates as high temperature materials. The self-healing capabilities of these materials may further improve their reliability and reduce maintenance costs during service. However, previous two dimensional research on their healing behaviour is either inadequate or sometimes even biased, because crack healing is essentially a three dimensional process. In this study, the self-healing behaviour of two MAX phase ceramics: Ti2AlC and Cr2AlC were investigated in three dimensions using high resolution synchrotron and laboratory X-ray tomography. The Ti2AlC showed remarkable healing ability of repeatedly repairing cracks at 1150 °C for three healing cycles with each healing cycle lasting less than 66 minutes. The healing kinetics of Ti2AlC was revealed to have a strong dependence on crack location, which deviates from a previously proposed uniform healing model. The Ti2AlC maintains its healing kinetics when the post-healing crack propagation follows a different growth path to the original crack. While a decreased healing kinetics was observed if the crack grows through a previous healed zone. The strength recovery is closely related to the healing percentage, where a full strength recovery was achieved with a healing percentage of over 90 %. In comparison, the Cr2AlC showed a much slower healing kinetics, with a parabolic constant of 4.3×10-3 µm2•s-1. The healing process of Cr2AlC was revealed to be more or less independent of crack location, where crack tip is always healed first. The crack gap is filled by purely Al2O3 after healing without the formation of Cr2O3. The composition of the healed area varies along the crack. A Cr7C3 sublayer beneath the Al2O3 layer was found at the healed zone of crack root, while missing in the healed zone at crack tip. The influence of impurity Cr particles on healing kinetics was investigated through a correlative microscopy study, combining X-ray tomographic slices with SEM images. The existence of Cr particles was found to accelerate the healing rate. However, this was always accompanied by the formation of large pores, which may be detrimental to the strength recovery after healing.

No description available.

This thesis details the work conducted over three years on the development of strain-based defect assessment techniques for carbon-fibre reinforced composites. This material, whilst exhibiting a high specific strength, is sensitive to defects and thus there is an industrial need for assessment techniques that are capable of characterising defects and obtaining predictions of residual strength or life. The most commonly applied techniques are currently ultrasonic and thermographic non-destructive evaluation. A strain-based defect assessment could lead to more accurate predictions of residual strength, resulting in a reduction of the costs associated with operating composite aerospace structures. The aim of this project is to increase the quality and confidence in residual strength information gained from the non-destructive evaluation of composite defects using strain-based assessments, in addition to currently applied ultrasonic practices for composite structures. A literature review on composite defects and existing techniques for assessing defects was conducted. Knowledge gaps were then identified that if filled, could improve residual strength predictions. Initially, a statistical framework was developed that used Bayesian regression to predict the residual strength of impacted composites, based on ultrasonic non-destructive measurements, that is robust to data outliers. As part of this framework a performance metric for quantifying the accuracy of residual strength predictions was introduced, allowing currently applied assessment techniques to be compared with the novel strain-based assessment. Then, a novel technique for performing strain-based defect assessments was developed that utilised image decomposition and the statistical framework to make residual strength predictions. Digital image correlation was used to measure strain fields which were then dimensionally reduced to feature vectors using image decomposition. The difference between feature vectors representing virgin and defective laminates were quantified, resulting in a strain-based defect severity measure. Bayesian regression was used to fit an empirical model capable of predicting the residual strength of an impacted laminate based on the strain-based defect severity. The accuracy of the strain-based predictions were compared to the accuracy of ultrasound-based predictions and found to outperform the currently applied ultrasonic technique. Strain-based assessment of in-plane fibre-waviness was also explored, as minimal research had been conducted studying waviness defects with full-field techniques. This required the development of a procedure for creating controlled levels of local waviness in laminates. The same strain-based assessment used for assessing impact damage was applied to the fibre-waviness specimens, but for this defect the accuracy of predictions were found to be comparable to the ultrasound-based predictions. However, residual strain measurements were found to be effective for predicting the strength of laminates, indicating that knowledge of the residual strains around a waviness defect may be important when predicting a laminates residual strength.

Warm mix asphalt (WMA) is a relatively new technology that allows asphalt mixtures to be mixed and compacted at lower temperatures than the conventional hot mix asphalt (HMA). Previous WMA investigations have predominantly been limited to basic performance indicators and properties, especially in the Middle East region. Implementing the WMA technology in the asphalt pavement industry in the State of Qatar requires investigation of material properties and performance under the prevailing climatic conditions in the country. This study provides a fundamental material evaluation of WMA material damage using advanced testing and analysis approaches. The results led to a conceptual understanding of the impact of WMA additives on asphalt materials’ performance. It was found that Sasobit is the most effective WMA additive, as it increases the material’s (bitumen and mastic) stiffness, enhances resistance to rutting, and causes a reduction in the ageing effect on fatigue. The study also evaluated the impact of ageing on WMA material damage. Fine Aggregate Mix (FAM) samples were placed in an accelerated weathering machine that was controlled to simulate the climatic condition in Qatar (UV light and heat). The samples showed significant changes after ageing in the weathering instrument. The aged FAM samples were then subjected to repeated creep and recovery testing and the performance of the material at different ageing levels was studied. The results showed that the Sasobit mix had the highest ability to recover damage induced during the loading period. The results showed a high correlation between the recovery modulus of the material and the reduced damage captured during the unloading. In order to further characterise damage, air void sizes in the asphalt mixture before and after adopting the repeated creep and recovery test were examined by scanning them using the X-Ray CT system. The air voids of the control mix (without any WMA additives) iii showed a remarkable change after the repeated creep and recovery test; however, all WMA samples had lower air void sizes and minor changes after testing. The CT imaging also confirmed that ageing using the UV light has an insignificant impact on air void changing before and after the repeated loading test. The results confirmed the benefits of implementing the WMA technology in the asphalt industry in the State of Qatar. This is particularly the case for Sasobit, which showed significant improvement in the performance testing results in comparison with the control mix and other WMA additives.

The present thesis is the result of a four year experimental research, which aims at studying the impact of non-Newtonian droplets (i.e., droplets of complex fluids such as polymer solutions) on heated surfaces (i.e., surfaces with a temperature above the Leidenfrost point) and soft surfaces (i.e., surfaces that undergo temporary or permanent deformations upon drop impact) through high-speed imaging. In the first year we focused on the Leidenfrost drop impact of different model fluids with matching flow curves. We demonstrate that the total kinetic energy carried by low-viscosity Newtonian drops during retraction is partly transformed into rotational kinetic energy rather than dissipated (published on Physical Review E, 2016). In the second year we extended the Leidenfrost drop impact experiment to viscoplastic fluids. The results show that the main contribution to drop rebound is due to surface forces rather than the elasticity of vapour cushion (published on Soft Matter, 2016). A systematic investigation on the impact of viscoplastic drops onto viscoplastic substrates was carried out in the third year. It is shown that the yield stress magnitude of drop/substrate strongly affects the final shape of the impacting drop (published on Soft Matter, 2017). The fourth year was devoted to the drop impact on spherical elastic surfaces. The dynamic contact angle measured using a novel digital image processing scheme is found to be significantly affected by the impact parameters and a quantitative estimation of the deformation energy is proposed (published on Physics of Fluids, 2017).

In this project, I have modelled the adsorption and diffusion of hydrogen and oxygen on and through the three low-index planes of two phases of iron, namely the body-centred cubic ferromagnetic alpha iron, alpha-Fe, and face-centered cubic gamma iron phase, gamma-Fe. This was done using spin-polarised Density Functional Theory, and the minimum energy path for the diffusion calculation was derived from potential energy surfaces created from a tight 3D mesh through the crystal. It was found that oxygen and hydrogen atoms strongly chemisorb on the (110) phase. Oxygen strongly chemisorbs on alpha-Fe(110) at the quasi-threefold site, with a surface stretch ~500 cm-1 for higher coverage. The structural changes at the highest coverage (>0.5 ML) indicated the incipient formation of FeO(111) from the O-Fe(110) overlay. Studying the electronic properties of the formation of FeO(111) yields an understanding of the earliest stage of oxide formation. Hydrogen was found to strongly chemisorb on the (110) surface of alpha-Fe. The hydrogen adsorbs at the quasi-threefold site with an adsorption energy of ~3 eV/H atom and surface stretches at ~1100 cm-1 for higher coverages. The (111) surface of gamma-Fe has been found to have the highest barrier for bulk-like diffusion. The bulk-diffusion barrier for hydrogen through gamma-Fe is ~0.7 eV for the (111) surface, which is ~0.2 eV higher than the (110) surface. The presence of magnetism in the (001) surface of gamma-Fe resulted in a lowering in the bulk-like diffusion barrier, with an ~0.2 eV barrier in the ferromagnetic surface as opposed to the ~0.6 eV in the non-magnetic surface. The high barrier for the (111) surface of gamma-Fe demonstrates that producing textured austenitic steel components with this surface exposed to the hydrogen source may work to lower the hydrogen damage in these samples. The strong effect of magnetism in lowering the barrier for diffusion demonstrates the importance of avoiding ferromagnetic austenitic steel alloys in environments where hydrogen is in abundance. These results may be applied in the process of development of Gen IV fission and fusion reactors. Ferritic and austenitic steels are ideal candidates for a number of components in these reactors, such as the first wall/breeding blanket. There is an abundance of presence of hydrogen in nuclear reactors. Hydrogen may enter the metallic matrix through diffusion processes, leading to the embrittlement of these components. Additionally, oxygen is readily present in the environment, which may oxidise components. In this project, I have modelled the adsorption and diffusion of hydrogen and oxygen on and through the three low-index planes of two phases of iron, namely the body-centred cubic ferromagnetic alpha iron, alpha-Fe, and face-centered cubic gamma iron phase, gamma-Fe. This was done using spin-polarised Density Functional Theory, and the minimum energy path for the diffusion calculation was derived from potential energy surfaces created from a tight 3D mesh through the crystal. It was found that oxygen and hydrogen atoms strongly chemisorb on the (110) phase. Oxygen strongly chemisorbs on alpha-Fe(110) at the quasi-threefold site, with a surface stretch ~500 cm-1 for higher coverage. The structural changes at the highest coverage (>0.5 ML) indicated the incipient formation of FeO(111) from the O-Fe(110) overlay. Studying the electronic properties of the formation of FeO(111) yields an understanding of the earliest stage of oxide formation. Hydrogen was found to strongly chemisorb on the (110) surface of alpha-Fe. The hydrogen adsorbs at the quasi-threefold site with an adsorption energy of ~3 eV/H atom and surface stretches at ~1100 cm-1 for higher coverages. The (111) surface of gamma-Fe has been found to have the highest barrier for bulk-like diffusion. The bulk-diffusion barrier for hydrogen through gamma-Fe is ~0.7 eV for the (111) surface, which is ~0.2 eV higher than the (110) surface. The presence of magnetism in the (001) surface of gamma-Fe resulted in a lowering in the bulk-like diffusion barrier, with an ~0.2 eV barrier in the ferromagnetic surface as opposed to the ~0.6 eV in the non-magnetic surface. The high barrier for the (111) surface of gamma-Fe demonstrates that producing textured austenitic steel components with this surface exposed to the hydrogen source may work to lower the hydrogen damage in these samples. The strong effect of magnetism in lowering the barrier for diffusion demonstrates the importance of avoiding ferromagnetic austenitic steel alloys in environments where hydrogen is in abundance. These results may be applied in the process of development of Gen IV fission and fusion reactors. Ferritic and austenitic steels are ideal candidates for a number of components in these reactors, such as the first wall/breeding blanket. There is an abundance of presence of hydrogen in nuclear reactors. Hydrogen may enter the metallic matrix through diffusion processes, leading to the embrittlement of these components. Additionally, oxygen is readily present in the environment, which may oxidise components.

The aim of this study is to investigate consumer responses to emerging digital fit and sizing tools in online fashion retailing. Digital fit and sizing tools are assumed to reduce returns, increase sales conversions (Dacko 2016) and bridge physical and digital assessment in online retailing contexts (Pantano and Timmermans 2016, Roy et al. 2016). However little is undestood regarding their impact on consumer behaviour. Research is required to gain a holistic understanding of how the tools aid consumer decisions and to deduce whether they can assist with key retailer metrics. This research is split into three stages: stage 1 which investigates the responses of 400 females aged 18-24 to pre-purchase virtual fit website Metail.com. Stage 2 explores the garment fit evaluation of 20 females, aged 18-34 to physical garments purchased using size and style recommender tools on Very.co.uk or Topshop.com. Stage 3 explores the opinions of 6 industry experts from top UK online fashion retailers through semi-structured interviews. Existing research in digital fit and sizing technology has been approached by garment technologists (Kim and LaBat 2012, Song and Ashdown 2015) or experiential marketers (Pantano and Servidio 2012, Beck and Crie 2016). This research aims to provide cohesion between the distinct areas of approach. The study reviews existing literature on experiential fashion technology, consumer behaviour and garment fit and sizing, and makes links between the approaches of each discipline. The research follows the consumer shopping journey framework proposed by Lemon and Verhoef (2016). Existing studies have often not captured consumer responses beyond online purchase intention (Huang and Liao 2015, Kim et al. 2016), and the shopping journey model recognises the importance of consumer experience pre and post-purchase. In addition, consumer fit evaluation is under-explored in extant research (Kasambala et al. 2016, deKlerk and Tselepis 2007). The study adopts the Technology Acceptance Model (TAM) (Davis 1989) to theoretically underpin the conclusive research on pre-purchase consumer evaluations using Metail. The TAM has been used in extant virtual fit or augmented reality fashion research (Perry 2016, Huang and Liao 2015). However, this study builds from existing research and extends the TAM to include the user trait: technology confidence and the experiential state: playfulness. The constructs measured in the post-purchase fit evaluation were derived through existing consumer behaviour research and the evaluation of garment fit and sizing (Kasambala et al. 2016, Eckman et al. 1990, McKinney and Shin 2016). Results of the study provide a guide to understanding the end-to-end processes behind consumer use of fit and sizing tools. The results of the research are conclusive (stage 1) and exploratory (stage 2 and 3) and were triangulated in the discussion to validate each approach.

This Ph.D. thesis focusses upon computational fluid dynamics simulations of circulation control in transonic freestream speeds for applications to unmanned combat air vehicles. The work addresses Coanda shape designs and their effectiveness for transonic circulation control using supersonic jets, with comparisons against traditional control surfaces. Previous works have thus far only investigated transonic circulation control on elliptical sectioned wings with unrepresentatively thick trailing edges and improvements in performance made by considering elliptical Coanda devices or increasing radii of curvature. In this work, a supercritical aerofoil was first modified to accommodate a small Coanda surface with minimal effects on the base drag and a comparison made between the performance of using several Coanda designs and a hinged control surface. The use of a step was demonstrated to make a circulation control device with a simple converging nozzle as effective as ailerons and flaps up to moderate deflection angles and that the limitations are due to breakdowns in the mean flow in a similar fashion to traditional devices. In addition an optimisation study was performed using modern numerical methods on the contouring of the Coanda surface, which identified a shape that performed well for both transonic and subsonic freestream conditions. Circulation control was then applied to a three-dimensional unmanned combat air vehicle planform and assessed at transonic conditions for use in roll, pitch and yaw control. From the range of conditions investigated the findings suggest that, for a three-dimensional representative geometry, circulation control can match the performance of conventional controls for roll and pitch. The results also suggest that for benign transonic conditions, circulation control can also provide control similar effectiveness to split flaps for yaw control. The findings open up insights into transonic circulation control and hopefully will promote further research in both academia and industry, where a lack of CFD validation quality experimental data for a transonic test case with supersonic blowing prohibits the technology from advancing.

In this thesis, a framework that quantifies the uncertainties introduced when using surrogate models for Uncertainty Quantification is proposed. The proposed framework have been adopted for a variety of Nuclear Engineering problems.

No description available.