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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Heterogeneous nucleation on convex and concave spherical surfaces

Ma, Jie January 2008 (has links)
Nucleation is a phenomenon of broad scientific interest and technological importance. It refers to the very early stages of the formation of a new phase, which can be solid, gaseous, and liquid, in a metastable parent phase. Most nucleation occurs heterogeneously unless the metastable parent phase from which the nuclei form is perfectly homogeneous and isolated from any catalyzing medium. This thesis project deals with heterogeneous nucleation on convex and concave spherical surfaces. In brief, the main achievements of the project are • An innovative analytical thermodynamic approach has been invented, which enabled rigorous thermodynamic formation of the energy barrier to nucleation, the critical radius and the shape factor, for nucleation on both convex and on concave surfaces. The rigorous thermodynamic analyses conducted have revealed a number of features for heterogeneous nucleation on convex and concave spherical surfaces as opposed to heterogeneous nucleation on a flat substrate surface. These are described in detail in Chapters 2 and 4. • Nucleation is the easiest on a concave spherical surface while it is the most difficult on a convex spherical surface assuming the contact angle and the critical embryo radius are the same. Nucleation on a flat substrate surface falls in between. This is determined by their shape factors. • The ratio R = 20r*, where R is the radius of the spherical substrate and r* is the critical embryo radius, (always define the symbols when they are first used. No one knows what they mean. R could be gas constant) can be regarded as a sufficiently accurate boundary that distinguishes between spherical and flat substrates for heterogeneous nucleation. • The investigation of the growth of crystal nuclei on a convex spherical surface has revealed that no growth barrier exists to the growth of a nucleus on a convex spherical substrate surface regardless of R < r* or R > r*. All nuclei formed on a convex spherical substrate surface are thus transformation nuclei. Turnbull’s transformation nucleus model or the recently developed free growth model does not apply to the growth of a spherical-cap nucleus on a convex spherical surface. • For heterogeneous nucleation in undercooled liquid metals, the cap thickness varies in a very narrow range by just a few angstroms and is typically about a few atomic layers thick according to Turnbull’s nucleation rate equation. The variations in the cap thickness are generally limited to less than 1Å when the contact angle Ɵ is varied in the range from 0° to 45°. It is anticipated that these findings will help to better understand the classical models for heterogeneous nucleation and provide new insights into the control of heterogeneous nucleation where convex or concave spherical substrate surfaces are involved.

Effort reduction and collision avoidance for powered wheelchairs : SCAD assistive mobility system

Langner, Martin January 2012 (has links)
The new research described in this dissertation created systems and methods to assist wheelchair users and provide them with new realistic and interesting driving opportunities. The work also created and applied novel effort reduction and collision avoidance systems and some new electronic interactive devices. A Scanning Collision Avoidance Device (SCAD) was created that attached to standard powered wheelchairs to help prevent children from driving into things. Initially, mechanical bumpers were used but they made many wheelchairs unwieldy, so a novel system that rotated a single ultra-sonic transducer was created. The SCAD provided wheelchair guidance and assisted with steering. Optical side object detectors were included to cover blind spots and also assist with doorway navigation. A steering lockout mode was also included for training, which stopped the wheelchair from driving towards a detected object. Some drivers did not have sufficient manual dexterity to operate a reverse control. A reverse turn manoeuvring mode was added that applied a sequential reverse and turn function, enabling a driver to escape from a confined situation by operating a single turn control. A new generation of Proportional SCAD was created that operated with proportional control inputs rather than switches and new systems were created to reduce veer, including effort reduction systems. New variable switches were created that provided variable speed control in place of standard digital switches and all that research reduced the number of control actions required by a driver. Finally, some new systems were created to motivate individuals to try new activities. These included a track guided train and an adventure playground that including new interactive systems. The research was initially inspired by the needs of young people at Chailey Heritage, the novel systems provided new and more autonomous driving opportunities for many powered wheelchair users in less structured environments.

An experimental and computational study of damage and crack growth for a nickel-based superalloy under fatigue-oxidation conditions

Karabela, Alkistis January 2011 (has links)
Oxidation damage, in conjunction with fatigue, is a concern for nickel-based superalloys utilised as disc rotors in high pressure compressor and turbine of aero-engines. A combined experimental and numerical study has been carried out for alloy RR1000, developed at Rolls-Royce plc through a powder metallurgy route to meet the demands for higher overall pressure ratios, compressor discharge temperatures and rotational speeds for the latest aero-engines. Cyclic experiments have been carried out for waisted specimens at selected temperatures (700°C-800°C), followed by microscopy examination using Focused Ion Beam (FIB). The results suggest that the major mechanism of oxidation damage consists of the formation of surface oxide scales and internal micro-voids and oxide particles beneath the oxide scales, which becomes more severe with the increase of temperature. Applying a cyclic stress does not change the nature of oxidation damage but tends to enhance the extent of oxidation damage for temperatures at 750°C and 800°C. Further energy dispersive X-ray (EDX) analyses show the enrichment of Cr and Ti, together with lower Ni and Co levels, in the surface oxide scales, suggesting the formation of brittle Cr2O3, TiO2, NiO and Co3O4 oxides on the specimen surface. Penetration of oxygen into the material and associated internal oxidation, which leads to further material embrittlement and associated failure, are evidenced from both secondary ion imaging and EDX analyses. Scanning electron microscopy (SEM) studies of fracture surfaces have been performed for dwell crack growth, and the results confirmed the transition from transgranular to predominant intergranular cracking in alloy RR1000 for increased dwell time. This change in fracture mode supports the oxidation-assisted crack growth mechanism via grain boundary embrittlement. Oxidation embrittlement has also been supported by the FIB analyses of fracture surfaces which confirmed the oxidation reaction for alloy RR1000 at high temperatures. A stress-assisted diffusion approach has been used to model oxygen penetration in the waisted specimen based on Fick’s first and second laws. Grain microstructures were considered explicitly in the model using a finite element submodelling technique, and the grain boundary was taken as the primary path for oxygen diffusion. The material constitutive behaviour was described by a crystal plasticity model to consider the effects of heterogeneous deformation at grain level on oxygen diffusion. Two essential diffusion parameters, i.e., oxygen diffusivity and pressure factor, have been obtained from the simulated oxygen penetration as well as the FIB measurements of internal oxidation depth. Using the obtained diffusion parameters, finite element analyses of a compact tension specimen have been carried out to model oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure curve was then utilized to predict crack growth rates under fatigue-oxidation conditions for selected loading frequencies and dwell periods, with comparison against the experimental results and those predicted from the viscoplastic model alone.

Investigating the factors affecting readiness for lean system adoption within Kuwaiti small and medium-sized manufacturing industries

Al-Najem, Mohamad January 2014 (has links)
The central belief in lean systems (LS) is that the implementation of lean practices will reduce different type of wastes. However, LS implemented without an evaluation of organisational readiness may lead to failure. In this study, a measurement framework to evaluate the lean readiness (LR) and LS within Kuwaiti small and medium-sized manufacturing industries (K-SMMIs) has been developed. This measurement framework encompasses the quality and management practices related to LS (processes; planning and control; human resources (HR); top management and leadership; customer relations; and supplier relations) to assess the practices in K-SMMIs and determine whether they have the foundation to implement LS. Mixed methods are adopted in this study, including quantitative approaches (questionnaire administered to 50 K SMMIs and structured observation conducted in 27 K-SMMIs), and qualitative approaches (two case studies (observation and semi-structured interviews with staff of various levels), and semi-structured interviews with 27 managers of K-SMMIs and 26 experts). In addition, a comprehensive literature review has been carried out. The findings indicate that current quality and management practices within K-SMMIs are not very supportive towards LS. Many factors are revealed, both external and internal, that affect K-SMMIs with respect to LS readiness, including language barriers, and deficiencies in aspects including quality workers in terms of education and skills; technology; government attention; know-how regarding LS; market competitiveness; and urgency for adopting LS. This LS and LR measurement framework relating to K-SMMIs provides a unique effort in the area of lean system, and the study’s findings can be used as an internal checklist prior to and during LS implementation. However, this research study contains some obvious limitations, such as very limited information being available on LS and quality initiatives in Kuwait, and small sample size. Further, the LR framework should be tested in small and medium-sized manufacturing industries that have successfully used LS, in order to provide a benchmark.

Characterisations and properties of nanocomposites based upon vinyl ester matrix and layered silicate

Alateyah, Abdulrahman Ibrahim S. January 2014 (has links)
In this research, various concentrations of layered silicate based on vinyl ester nanocomposites were prepared and the effect of the incorporation of layered silicate into the polymer matrix on the different properties was investigated. The characterisations of interlaminar structure of the nanocomposites by X-ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectrometry and Transmission Electron Microscopy are undertaken. This study revealed that the incorporation of layered silicate into the polymer matrix formed uniformly distributed nanocomposites structure at low clay content (i.e. 1, 2, and 3 wt.%). At 4 wt.% clay loading, the partially intercalated / exfoliated nanocomposites system was observed as proved by the different characterisations' results. However, the addition of more clay such as 5 wt.% resulted in decreasing the overall intercalation level due to the existence of aggregation layers. The addition of layered silicate into the vinyl ester matrix increased the environmental, mechanical and thermal properties, and the enhancements were correlated to the results of the characterisations' outputs. The mechanical properties such as flexural, tensile, nanoindentation, impact, and creep properties of neat samples were improved by the incorporation of layered silicate. The presence of layered silicate into the polymer matrix increased the tensile strength and modulus and flexural strength and modulus up to 4 wt.% clay content. The level of intercalation of nanocomposites played an important role in the improvements of the mechanical properties. So, the tensile and flexural properties were correlated to the characterisations' results. At 5 wt.% clay content, the modulus and strength of both tensile and flexural were reduced due to the effect of aggregation layers where the interfacial interaction between the layered silicate and the polymer is reduced. The nanoindentation test showed that the addition of layered silicate increased the reduced modulus and hardness of the nanocomposites compared to the neat vinyl ester. The presence of only 1 wt.% clay loading increased the hardness and reduced modulus at up to 13% and 11% respectively compared to the pristine polymer. The improvement percentage of hardness and modulus at 2 wt.% were 31% and 19% respectively. The ultimate improvements were observed at 4 wt.% clay loading, where the enhancements in hardness and modulus were 56 and 50% respectively compared to the neat vinyl ester. Further addition of clay resulted in marginal reductions in these properties. The impact properties of the neat vinyl ester and the nanocomposites were investigated using a low velocity impact testing. The addition of layered silicate into the polymer matrix showed that an optimum range of nanoclay reinforcement in the vinyl ester matrix can produce enhanced load bearing and energy absorption capability compared to the neat matrix. Likewise, the influence of the clay addition into the neat polymer on the creep relaxation behaviour at 25°C and 60°C was studied. In both cases, the presence of the layered silicate remarkably improved the creep behaviour. The strain reduction is related to the clay concentration level. The neat polymer illustrated higher strain compared to the nanocomposites samples. Moreover, the addition of layered silicate into the polymer matrix improved the thermal properties. Thermal Gravimetric Analysis (TGA) showed that the nanocomposites represent better stability compared to the neat polymer. The onset temperature of the nanocomposites was higher than the neat polymer. At 1, 2, 3, and 4, wt.% clay content, the improvements in onset temperature were 7 %, 4.2 %, 4 %, 2.5 % respectively compared to the virgin polymer. In addition, the incorporation of layered silicate into the polymer matrix increased the thermal conductivity. At 4 wt.% clay, the thermal conductivity was increased by 12% compared to the neat polymer. Differential Scanning Calorimetry (DSC) is also performed in order to study the effect of the addition of layered silicate into the polymer on the glass transition temperature. The level of intercalation is critical to the Tg values. The nanocomposites represented a marginal reduction in Tg, however at 4 wt.% clay loading the Tg was as same as the neat polymer which was traced to the well-dispersed structure. Furthermore, the study of environmental measurements, which included the water absorption behaviour and its effect on the nanoindentation test, was investigated. The improvement of the water repellence behaviour was observed for the nanocomposites. The enhancements in barrier properties were related to the clay content. At 5 wt.% clay loading, the reduction of water uptake was about 1266% compared to the neat polymer. The hardness and elastic moduli after water absorption was reduced due to the effect of water molecules entering into the polymer chains. However, the higher amount of clay reinforcement led to less reduction in hardness due to the formation of the barrier properties by the layered silicate. The hardness of neat polymer after immersing in water was reduced by 30% whereas the hardness of 5 wt.% nanocomposites showed only a reduction by 10.3% compared to the dry sample.

Investigation of surface defects for extruded aluminium profiles using pattern recognition techniques

Chondronasios, Apostolos January 2015 (has links)
This research investigates detection and classification of surface defects in extruded aluminium profiles in order to replace the traditional eye inspection which is still the method widely used today. Through an extensive literature review it is evident that extruded aluminium surface is not investigated properly, while similar industrial products such as copper strips or rolled steel have attracted more interest. An experimental machine vision system is used to capture images from surfaces of extruded aluminium profiles. Extensive feature selection is investigated and appropriate statistical features from a novel technique based on Gradient-Only Co-occurrence Matrices are used to detect and classify defects. The methodology created in this research, makes use of the Sobel edge detector to obtain the gradient magnitude of the image and is followed by the extraction of statistical texture measures from the gradient, after a transformation of the gradient values. Comparisons are made between the statistical features extracted from the original image (Gray-Level Co-occurrence Matrix) and those extracted from the gradient magnitude using a novel approach (Gradient-Only Co-occurrence Matrix). The features extracted from the image processing are classified by feed-forward artificial neural networks. Experiments were conducted for a three class and a four class case study, with the first consisting of Good Surface, Blisters and Scratches, and the second introducing Die Lines to the classes of the first case study. The artificial neural network training is tested using different combinations of statistical features with different topologies. Features are compared individually and grouped, showing better classification accuracy for the novel technique (98.9%) compared to research standard methodology of gray-level co-occurrence matrices (55.9%).

Fatigue crack growth in laser shock peened aerofoils subjected to foreign object damage

Spanrad, Sven Klaus January 2011 (has links)
Foreign Object Damage (FOD) is one of the main life limiting factors for aeroengine fan blades. The FOD impacts during takeoff and landing cause severe damage to aerofoils, resulting in reduced air safety and life time with an estimated annual cost of $4 billion for the aeroengine industry. Advanced surface treatments, such as Laser Shock Peening (LSP) have significantly improved the fatigue strength and crack growth resistance of critical components under FOD. However, it is not yet possible to predict the protective residual stresses and utilise their full potential for enhancing fatigue resistance and damage tolerance capacity in service. This research programme aims to utilise some of the established methods for fatigue tolerance assessment of critical components, based on fracture mechanics principles, to address the effects of complex residual stresses due to LSP and FOD on fatigue crack growth in aerofoils under simulated service loading conditions. The experimental study involved fatigue testing of LSPed and FODed specimens with a geometry representative of fan blades made from Ti-6Al-4V alloy. A four point bend fatigue test setup was designed and calibrated. A real-time computer-controlled crack growth monitoring system and optical crack monitoring techniques were developed. Scanning Electron Microscopy (SEM) and Back-Scatter Electron (BSE) were used to conduct metallographic and fractographic studies, including crack initiation, early fatigue crack growth and FOD damage characterisation. The fracture mechanics analyses used the weight function method and the finite element method to obtain a modified stress intensity factor considering residual stresses due to LSP and FOD. Fatigue crack growth data under low cycle fatigue(LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions were correlated using a standard and the modified stress intensity factors. The influence of impact angles and loading conditions on fatigue crack growth behaviour was assessed, and the results were compared with those from untreated FODed specimens.

A hybrid intelligent technique for induction motor condition monitoring

Wen, Xin January 2011 (has links)
The objective of this research is to advance the field of condition monitoring and fault diagnosis for induction motors. This involves processing the signals produced by induction motors, classifying the types and estimating the severity of induction motors faults. A typical process of condition monitoring and fault diagnosis for induction motors consists of four steps: data acquisition, signal analysis, fault detection and post-processing. A description of various kinds of faults that can occur in induction motors is presented. The features reflecting faults are usually embedded in transient motor signals. The signal analysis is a very important step in the motor fault diagnosis process, which is to extract features which are related to specific fault modes. The signal analysis methods available in feature extraction for motor signals are discussed. The wavelet packet decomposition results consist of the time-frequency representation of a signal in the same time, which is inherently suited to the transient events in the motor fault signals. The wavelet packet transform-based analysis method is proposed to extract the features of motor signals. Fault detection has to establish a relationship between the motor symptoms and the condition. Classifying motor condition and estimating the severity of faults from the motor signals have never been easy tasks and they are affected by many factors. AI techniques, such as expert system (ES), fuzzy logic system (FLS), artificial neural network (ANN) and support vector machine (SVM), have been applied in fault diagnosis of very complex system, where accurate mathematical models are difficult to be built. These techniques use association, reasoning and decision making processes as would the human brain in solving diagnostic problems. ANN is a computation and information processing method that mimics the process found in biological neurons. But when ANN-based methods are used for fault diagnosis, local minimums caused by the traditional training algorithms often result in large approximation error that may destroy their reliability. In this research, a novel method of condition monitoring and fault diagnosis for induction motor is proposed using hybrid intelligent techniques based on WPT. ANN is trained by improved genetic algorithm (IGA). WPT is used to decompose motor signals to extract the feature parameters. The extracted features with different frequency resolutions are used as the input of ANN for the fault diagnosis. Finally, the proposed method is tested in 1.5 kW and 3.7 kW induction motor rigs. The experimental results demonstrate that the proposed method improves the sensitivity and accuracy of the ANN-based methods of condition monitoring and fault diagnosis for induction motors.

Statistical-based optimization of process parameters of fused deposition modelling for improved quality

Alhubail, Mohammad A. M. J. January 2012 (has links)
Fused Deposition Modelling (FDM) is a rapid prototyping system that produces physical models directly from the computer aided design (CAD) drawings. These models can be used to evaluate the assembly and the functionality of the design, also producing a manufacturing tools, and end-use parts. Parts built with production-grade thermoplastics that match the traditional machined parts, and according to the realworld conditions. FDM can produce instantly functional parts that used mainly in medical and automotive applications, with the use of reverse engineering techniques such as engineering scanning or digitizing systems. Knowledge of the quality characteristics of FDM fabricated parts is crucial. Quality significantly depends on process variable parameters. Optimizing the process parameters of FDM can make the system more precise and repeatable and such advancement can lead to use of FDM in rapid manufacturing applications rather than only producing prototypes. The part building is influenced by variant processing conditions. Thus, FDM process variable parameters are required to be collectively optimized rather than individually. In order to understand this issue, this study presents results of the experimental work on the effect of the main FDM process variable parameters of layer thickness (A), air gap (B), raster width (C), contour width (D), and raster orientation (E) on the quality characteristics of surface roughness (Ra), dimensional accuracy (DA), and tensile strength (TS). Previous studies have investigated the quality characteristics but limited knowledge is available on FDM newly improved materials. Thus, the new ABS- M30i biomedical material was used in this experimental work to build parts. To conduct this study, a full factorial experiment was used to obtain the test runs. A number of analytical methods such as regression analysis, Analysis of Variance (ANOVA), and Pareto analysis were used to determine the influence of the variable FDM process parameter settings. Results show that these process parameters have significant effect on the quality of finished products. For example, it has been found that the surface roughness and tensile strength of processed parts are greatly influenced by the air gap parameter as it affects the part’s beads structure, because it overlapping the material beads and consequently strengthen the beads bonding, and reduce the voids between the beads. Scanning Electron Microscope (SEM) work has been undertaken to characterise the experimental results. The results will be important for FDM produced parts in different functional applications as rapid manufacturing becomes increasingly accepted.

In vitro studies of bone-cement interface and related work on cemented acetabular replacement

Tozzi, Gianluca January 2012 (has links)
The lasting integrity of the bond between bone cement and bone defines the long-term stability of cemented acetabular replacements. Although several studies have been carried out on bone-cement interface at continuum level, micromechanics of the interface has been studied only recently for tensile and shear loading cases. Furthermore, the mechanical and microstructural behaviour of this interface is complex due to the variation in morphology and properties that can arise from a range of factors. In this work in vitro studies of the bone-cement interfacial behaviour under selected loading conditions were carried out using a range of experimental techniques. Damage development in cemented acetabular reconstructs was studied under a combined physiological loading block representative of routine activities in a saline environment. A custom-made environmental chamber was developed to allow testing of acetabular reconstructs in a wet condition for the first time and damage was monitored and detected by scanning at selected loading intervals using micro-focus computed tomography (μCT). Preliminary results showed that, as in dry cases, debonding at the bone-cement interface defined the failure of the cement fixation. However, the combination of mechanical loading and saline environment seems to affect the damage initiation site, drastically reducing the survival lives of the reconstructs. Interfacial behaviour of the bone-cement interface was studied under tensile, shear and mixed-mode loading conditions. Bone-cement coupons were first mechanically tested and then μCT imaged. The influence of the loading angle, the extent of the cement penetration and the failure mechanisms with regard to the loading mode on the interfacial behaviour were examined. Both mechanical testing and post failure morphologies seem to suggest an effect of the loading angle on the failure mechanism of the interface. The micromechanical performance of bone-cement interface under compression was also examined. The samples were tested in step-wise compression using a custom-made micromechanical loading stage within the μCT chamber, and the damage evolution with load was monitored. Results showed that load transfer in bone-cement interface occurred mainly in the bone-cement contact region, resulting in progressively developed deformation due to trabeculae bending and buckling. Compressive and fatigue behaviour of bovine cancellous bone and selected open-cell metallic foams were studied also, and their suitability as bone analogous materials for cemented biomechanical testing was investigated. Whilst the morphological parameters of the foams and the bone appear to be closer, the mechanical properties vary significantly between the foams and the bone. However, despite the apparent differences in their respective properties, the general deformation behaviour is similar across the bone and the foams. Multi-step fatigue tests were carried out to study the deformation behaviour under increasing compressive cyclic stresses. Optical and scanning electron microscopy (SEM) were used to characterise the microstructure of foams and bone prior to and post mechanical testing. The results showed that residual strain accumulation is the predominant driving force leading to failure of foams and bones. Although foams and bone fail by the same mechanism of cyclic creep, the deformation behaviour at the transient region of each step was different for both materials. Preliminary results of foam-cement interface performance under mixed-mode loading conditions are also presented.

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