Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. TBC is a duplex material system consisting of an insulating ceramic topcoat layer and an intermetallic bondcoat layer. TBC microstructures are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating's final thermal and mechanical properties, and the service lives of the coatings. Failure in APS TBCs is mainly associated with the thermo-mechanical stresses developing due to the thermally grown oxide (TGO) layer growth at the topcoat-bondcoat interface and thermal expansion mismatch during thermal cycling. The interface roughness has been shown to play a major role in the development of these induced stresses and lifetime of TBCs.The objective of this thesis work was two-fold for one purpose: to design an optimised TBC to be used for next generation gas turbines. The first objective was to investigate the relationships between coating microstructure and thermal-mechanical properties of topcoats, and to utilise these relationships to design an optimised morphology of the topcoat microstructure. The second objective was to investigate the relationships between topcoat-bondcoat interface roughness, TGO growth and lifetime of TBCs, and to utilise these relationships to design an optimal interface. Simulation technique was used to achieve these objectives. Important microstructural parameters influencing the performance of topcoats were identified and coatings with the feasible identified microstructural parameters were designed, modelled and experimentally verified. It was shown that large globular pores with connected cracks inherited within the topcoat microstructure significantly enhanced TBC performance. Real topcoat-bondcoat interface topographies were used to calculate the induced stresses and a diffusion based TGO growth model was developed to assess the lifetime. The modelling results were compared with existing theories published in previous works and experiments. It was shown that the modelling approach developed in this work could be used as a powerful tool to design new coatings and interfaces as well as to achieve high performance optimised morphologies.

This thesis is dedicated to the study of structural and petroleum geology problems. To this purpose, a frictional elastoplastic law based on the Van Eekelen criterion is formulated, which avoids the classical drawbacks of the Drücker Prager criterion. Also, a 2D automatic adaptive re-meshing algorithm is developed for complex multidomains configurations, in order to overcome the limitation of the Lagrangian mesh. Details of the hydromechanical formulation implemented in the LAGAMINE FE code in a large strain context are presented. Application cases (reproduction of sandbox simulation, study of a hydrocarbon trap) concentrate on the study of the strain localisation and potential fracturation using different criteria. In the first case, re-meshing technique allowed to reproduce successfully analogue experiment of thrusting propagation. In the second case, a detailed study based on different initial conditions has brought new insight to the reactivated origin of some faults and has allowed to obtain information on the potential fracturing of the hydrocarbon reservoir unit. As an academic case, the study of anorthosite diapirism is carried out, which confirms the validity of the petrological model of diapirism. Eventually, the hydromechanical coupling effects between a layered porous medium and a fault are illustrated on a simple case.

In recent years, vibration-based energy harvesting technologies have gained great importance because of reduced power requirement of small electronic components. External power source and maintenance requirement can be minimized by employment of mechanical vibration energy harvesters. Power sources that harvest energy from the environment have the main advantages of high safety, long shell life and low cost compared to chemical batteries. Electromagnetic, electrostatic and piezoelectric transduction mechanisms are the three main energy harvesting methods. In this thesis, it is aimed to apply the piezoelectric elements technology to develop means for energy storage in munitions launch. The practical problems encountered in the design of piezoelectric energy harvesters are investigated. The applicability of energy harvesting to high power needs are studied. The experience compiled in the study is to be exploited in designing piezoelectric energy harvesters for munitions applications. Piezoelectric energy harvesters for harmonic and mechanical shock loading conditions with different types of piezoelectric materials are designed and tested. The test results are compared with both responses from analytical models generated in MATLAB&reg / and ORCAD PSPICE&reg / , and finite element method models generated in ATILA&reg / . Optimum energy storage methods are considered.

This thesis presents structural analysis of a cockpit of a jet trainer type aircraft and the correlation studies performed by using ground pressurisation test results. For this purpose, first the response of the complex integrated fuselage structure is investigated under the complex type cabin pressure load. Then, cockpit part of the fuselage structure is modelled using commercial finite element software MSC/PATRAN&reg / and MSC/NASTRAN&reg / . The finite element model (FEM) of the cockpit structure is improved by the examination of the ground pressurisation test data and is finalised after achieving a good correlation between the finite element analysis (FEA) and the test results. This final form of the FEM of the cockpit structure serving as a benchmark is proved to be reliable for any future modifications.

Aluminium sheet production is a multi-stage process in which altering processing conditions can drastically alter the size and type of second phase particles found in the final product. The properties of these second phase particles also affects deformation and annealing processes, meaning that any attempt to create a through process model would require the ability to predict both how the particles would develop in the material, and how these particles then affect the alloy moving forward. This project first focuses on gaining insight into how the particles in a model aluminium alloy change during homogenisation heat treatment and hot rolling. This has been accomplished by utilising serial block face scanning electron microscopy (SBF-SEM), a technique which allows the capture of 3D data sets at sub micron resolutions. This has allowed the populations of primary (constituent) and secondary (dispersoid) particles to be analysed at different stages of sheet production, and thus allowing the effects of homogenisation and hot rolling on particle populations to be quantified. To discover how the particles would go on to affect further processing, digital image correlation has been used to examine the localised strain in the alloy near to a selection of particle configurations. This highlighted the heterogeneity in slip behaviour within the alloy and illustrated that plumes of rotation develop near to non deformable regions. Rotation plumes have previously been modelled using a crystal plasticity model, and so further work is also presented expanding upon this model to simulate a variety of particle configurations. This has shown that in the case of single particles, local deformation is dependent on both the aspect ratio of the particle and how it is aligned to the active slip system. With the incorporation of a second particle, the interparticle spacing must also be considered.

La prostate est un organe pelvien qui joue un rôle important dans son environnement anatomique, notamment en assurant la sécrétion d’un liquide essentiel dans la composition du liquide séminal. Le cancer de la prostate représente la première cause mortalité chez l’homme à un âge avancé. Ce travail concerne le développement par la méthode des éléments finis d'un modèle numérique du mouvement des organes pelviens (prostate, vessie, rectum) et de leurs interactions. L’objectif est la réduction des marges d'irradiation du volume cible au cours d'une séance de radiothérapie, afin de ne pas altérer les organes sains avoisinants. Les marges choisies sont importantes compte tenu du fait que la prostate subi des déplacements importants lors des interactions permanentes avec les organes avoisinants. Un premier modèle est construit à partir de la géométrie des organes pelviens générée à partir d'images scanner acquises sur patients. Des lois hyper-élastiques sont adoptées pour modéliser le comportement mécanique du rectum et de la vessie et un comportement Hookéen est considéré pour la prostate. Les paramètres physiques du modèle sont déterminés à partir de la littérature, des données expérimentales et de nos propres mesures. Les conditions aux limites cinématiques et statiques (pressions de distension intra-vésicale et intra-rectale) sont définies à partir des observations anatomiques et reflètent la présence de l’entourage anatomique et les conditions de chargement. Des comparaisons entre les variations de forme et de position d'organes obtenues par simulation et les mesures obtenues par imagerie scanner (Keros et al. ; 2006) montrent des amplitudes de déplacements proches, avec des écarts variant entre 8% et 11%. Un modèle prenant en compte la variabilité des paramètres physiques inter et intra patients est envisagé en perspectives / The prostate plays an important biological role in the human body, such as secretion of some prostatic liquid essential in the semen composition. Prostate cancer is the first cause of mortality for men at an advanced age. The prostate motion due to the interactions with the surrounding anatomic entities is difficult to predict, hence important margins are usually adopted during X-ray irradiation, in order not to damage the surrounding healthy organs (bladder and rectum). The principal objective of this work is to set up a FE model of the motion and deformation of the human pelvic organs in order to reduce the margins. A first model is constructed from CT-scans of the human pelvic organs, allowing the generation of the organ geometrics. Hyperelastic modeling of the bladder and rectum behaviors were considered whereas a Hookean model was retained for the prostate. The model parameters are fixed by adopting literature data, experimental data (from CHU-Nancy) and experimental measurements achieved on pig. Boundary conditions are defined according either surrounding anatomy kinematic constraints or internal pressures that correspond to the bladder and rectum repletion’s. Simulated displacements show order of magnitudes of the prostate motion very close to measurements carried out by Keros et al. (2006) on a deceased person, with a relative error ranging from 8% to 11%. Those differences are essentially due to the variability in the physical parameters, pointing out the need for a statistical approach in order to take into account the material, geometrical and loading variability related to a panel of patients

Most tyre models developed to date require a fair amount of data before an accurate representation of the tyre can be obtained. This study entails the development of a simplified, yet accurate, non-linear Finite Element (FE) model of an “off-road” tyre to study the behaviour of the tyre due to radial loading conditions. The study aims to develop a FE tyre model that can solve fast and be accurate enough to be used in multibody dynamic vehicle simulations. A model that is less complex than conventional detailed FE models is developed. The work explores the use of superimposed finite elements to model the varying stiffness in the respective orthogonal directions of the sidewall and tread of the tyre. Non-linear elements defined by Neo-Hookean or Ogden models and elements with different linear orthogonal stiffnesses are superimposed onto each other to simulate the global material properties of the tread and the sidewall of the tyre investigated. The geometry of the tyre studied was measured experimentally using laser displacement transducers and digital image correlation techniques. Material properties of segments of the tyre were obtained by performing tensile tests on samples. Since the rubber slipped against the clamps during the experiment, deformation of the segments was also measured using digital image correlation. These geometrical and material properties were used as input to develop a finite element model of an “off-road” tyre. Measurements were conducted using laser displacement transducers, load cells mounted to actuators, etc. to obtain accurate sidewall deformation profiles and global radial load vs. displacement curves for different radial loading conditions. The data obtained from the results was used to validate the tyre model developed. Numerous analyses are performed with different combinations of moduli of elasticity in the respective orthogonal directions of the sidewall stiffness and the tread to investigate its influence on the global behaviour of the tyre model. The main focus of the project was to develop a tyre model from data obtained from laser and photogrammetry measurements in a laboratory that accurately represents tyre behaviour due to radial forces. A finite element model that can simulate the effect of radial forced and obstacles on a tyre was developed. The use of two subsets of elements, superimposed onto each other to simulate global material properties of the rubbers, steel wires, polyester and nylon threads, was investigated. The combination of material properties that gave the best fit for all the load cases investigated were determined. The finite element model correlated well with the load vs. displacement graphs and sidewall displacement profiles determined experimentally. The solving time is still fairly high and is still not quite suitable for real-time dynamic simulation. However, it solves faster than more complex tyre models where details of steel wires, etc. are included in the model. For future studies it is recommended that different element types be investigated in the tyre model. The study proves that equivalent material properties can be used to simulate the composite properties of the materials in tyres. Most tyres can be divided into a few regions that each has its own material structure right through the region. These regions can be characterized by simple tests and the input can be used as a first estimation of the tyre’s material properties for the model. Accurate validation criteria should be used to validate the tyre model if time does not allow for excessive testing of the material properties of all the rubber, steel wires, polyester threads, etc. Geometric displacement data at various loading conditions can be used for validation of the tyre model. The model developed can be used to investigate the effect of different stiffnesses and other material changes in the sidewall or tread of a tyre. Useful insight can be obtained from the finite element model developed for dynamic simulation where the force vs. global displacement data is important. / Dissertation (MEng)--University of Pretoria, 2014. / tm2015 / Mechanical and Aeronautical Engineering / MEng / Unrestricted

During neural activity, action potentials travel down axons, generating effective charge current pulses, which are central in neuron-to-neuron communication. Consequently, said current pulses generate associated magnetic fields with amplitudes on the order of picotesla (pT) and femtotesla (fT) and durations of 10’s of ms. Magnetoencephalography (MEG) is a technique used to measure the cortical magnetic fields associated with neural activity. MEG limitations include the inability to detect signals from deeper regions of the brain, the need to house the equipment in special magnetically shielded rooms to cancel out environmental noise, and the use of superconducting magnets, requiring cryogenic temperatures, bringing opportunities for new magnetic sensors to overcome these limitations and to further advance neuroscience. An extraordinary magnetoresistance (EMR) tunable graphene magnetometer could potentially achieve this goal. Its advantages are linear response at room temperature (RT), sensitivity enhancement owing to combination of geometric and Hall effects, microscale size to place the sensor closer to the source or macroscale size for large source area, and noise and sensitivity tailoring. The magnetic sensitivity of EMR sensors is, among others, strongly dependent on the charge mobility of the sensing graphene layer. Mechanisms affecting the carrier mobility in graphene monolayers include interactions between the substrate and graphene, such as electron-phonon scattering, charge impurities, and surface roughness. The present work reviews and proposes a material set for increasing graphene mobility, thus providing a pathway towards pT and fT detection. The successful fabrication of large-size magnetic sensors employing CVD graphene is described, as well as the fabrication of trilayer magnetic sensors employing mechanical exfoliation of h-BN and graphene. The magneto-transport response of CVD graphene Hall bar and EMR magnetic sensors is compared to that obtained in equivalent trilayer devices. The sensor response characteristics are reported, and a determination is provided for key performance parameters such as current and voltage sensitivity and magnetic resolution. These parameters crucially depend on the material's intrinsic properties. The Hall cross magnetic sensor here reported has a magnetic sensitivity of ~ 600 nanotesla (nT). We find that the attained sensitivity of the devices here reported is limited by contaminants on the graphene surface, which negatively impact carrier mobility and carrier density, and by high contact resistance of ~2.7 kΩ µm at the metallic contacts. Reducing the contact resistance to < 150 Ω µm and eliminating surface contamination, as discussed in this work, paves the way towards pT and ultimately fT sensitivity using these novel magnetic sensors. Finite element modeling (FEM) is used to simulate the sensor response, which agrees with experimental data with an error of less than 3%. This enables the prediction and optimization of the magnetic sensor performance as a function of material parameters and fabrication changes. Predictive studies indicate that an EMR magnetic sensor could attain a sensitivity of 1.9 nT/√Hz employing graphene with carrier mobilities of 180,000 cm²/Vs, carrier densities of 1.3×10¹¹ cm^-2 and a device contact resistance of 150 Ω µm. This sensitivity increments to 443 pT/√Hz if the mobility is 245,000 cm²/Vs, carrier density is 1.6×10¹⁰ cm^-2, and a lower contact resistance of 30 Ω µm. Such devices could readily be deployed in wearable devices to detect biomagnetic signals originating from the human heart and skeletal muscles and for developing advanced human-machine interfaces.

Railway sleepers have important roles in the complex railway system. Due to different loading condition, poor maintenance of sleeper or bad quality of ballast, a random load distribution along the sleeper-ballast interface may occur. A sleeper design, and also the track system design, which do not consider the random load distribution, could influence the performance of the sleeper and even damage the whole railway system. Thus, a numerical static and dynamic analysis for a pre-stressed concrete mono-block railway sleeper is carried out using finite element method. The structural behaviour of a single sleeper subjected to a random sleeper-ballast interaction is studied in three steps. First, four typical scenarios of support condition for sleeper are discussed in numerical analysis. Second, large enough numerical results under different random support conditions are conducted. Finally, Neural Network methodology is used to study the performance of sleeper under a stochastic support condition. Results of vertical displacement of rail seat, tensile stress at midpoint and underneath rail seat are presented. Moreover, the worst support condition is also identified.

<p>Aluminum alloys have broad applications in aerospace, automotive, and defense industries as structural material due to the low density, high-specific strength, good castability and formability. However, aluminum alloys commonly suffer from problems such as low yield strength, low stiffness, and poor wear and tear resistance, and therefore are restricted to certain advanced industrial applications. To overcome the problems, one promising method is the fabrication of aluminum matrix composites (AMCs) by introducing ceramic reinforcements (fibers, whiskers or particles) in the metal matrix. AMCs typically possess advanced properties than the matrix alloys such as high specific modulus, strength, wear resistance, thermal stability, while remain the low density. Among the AMCs, particulate reinforced aluminum matrix composites (PRAMCs) are advantageous for their isotropic properties, ease of fabrication, and low costs. Particularly, the PRAMCs with in-situ particulate reinforcements have received great interest recent years. The in-situ fabricated particles are synthesized in an aluminum matrix via chemical reactions. They are more stable and finer in size, and have a more uniform distribution in the aluminum matrix and stronger interface bonding with aluminum matrix, compared to the ex-situ particulate reinforcements. As a consequence, the in-situ PRAMCs have superior strength and mechanical properties as advanced engineering materials for a broad range of industrial applications.</p> <p>This dissertation focuses on the investigation of high strength aluminum matrix composites reinforced with in-situ particulates. The first chapter provides a brief introduction for the studied materials in the dissertation, including the background, the scope, the significance and the research questions of the study. The second chapter presents the literature review on the basic knowledge, the fabrication methods, the mechanical properties of in-situ PRAMCs. The strengthening mechanisms and strategies of in-situ PRAMCs are summarized. Besides, the micromechanical simulation is introduced as a complementary methodology for the investigation of the microstructure-properties relationship of the in-situ PRAMCs. The third chapter shows the framework and methodology of this dissertation, including material preparation and material characterization methods, phase diagram method and finite element modelling. </p> <p>In Chapter 4, the microstructures and mechanical properties of in-situ Al₃Ti particulate reinforced A356 composites are investigated. The microstructure and mechanical properties of in-situ 5 vol. % Al₃Ti/A356 composites are studied either taking account of the effects of T6 heat treatment and strontium (Sr) addition or not. Chapter 5 studies the evolution of intermetallic phases in the Al-Si-Ti alloy during solution treatment, based on the work of Chapter 4. The as-cast Al-Si-Ti alloy is solution treated at 540 °C for different periods between 0 to 72 h to understand the evolution of intermetallic phases. In Chapter 6, a three-dimensional (3D) micromechanical simulation is conducted to study the effects of particle size, fraction and distribution on the mechanical behavior of the in-situ Al₃Ti/A356 composite. The mechanical behavior of the in-situ Al₃Ti/A356 composite is studied by three-dimensional (3D) micromechanical simulation with microstructure-based Representative Volume Element (RVE) models. The effects of hot rolling and heat treatment on the microstructure and mechanical properties of an in-situ TiB₂/Al2618 composite with minor Sc addition are investigated in Chapter 7. TiB₂/Al2618 composites ingots were fabricated <i>in-situ</i> via salt-melt reactions and subjected to hot rolling. The microstructure and mechanical properties of the TiB₂/Al2618 composite are investigated by considering the effects of particle volume fraction, hot rolling thickness reduction, and heat treatment. </p>