19 research outputs found

    Strength Assessment of a Precession Driven Dynamo

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    A pressure vessel, which shall be filled with eight tons of liquid sodium, has to be designed for a large-scale experimental setup to investigate flow-induced magnetic fields. In addition to the centrifugal forces and gyroscopic loads induced by the rotation about two non-parallel axes, the complex internal pressure distribution, the imbalance of the container, as well as the thermal loads resulting from the elevated temperatures, which are required for the experiments, must be taken into account. This leads to several millions of load cases. That is why a calculation procedure is developed using the finite element method, which strongly reduces the computational complexity by utilizing sector symmetry, load case decomposition and superposition. Here, the focus is to determine the most critical load cases, which will be used for the strength assessment, regarding both the static and the fatigue strength.Besides the structural strength, the welded joints and the bolted joints are analyzed. Therefore, nonlinear effects are considered, for example the contact status of the bolted joints. The submodelling technique is used to investigate structural details

    Development of methodologies for the solution of the forward problem in magnetic-field tomography (MFT) based on magnetoencephalography (MEG)

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    The prime topic of research presented in this report is the development and validation of methodologies for the solution of the forward problem in Magnetic field Tomography based on Magnetoencephalography. Throughout the report full aspects of the accurate solution are discussed, including the development of algorithms and methods for realistic brain model, development of realistic neuronal source, computational approaches, and validation techniques. Every delivered methodology is tested and analyzed in terms of mathematical and computational errors. Optimizations required for error minimization are performed and discussed. Presented techniques are successfully integrated together for different test problems. Results were compared to experimental data where possible for the most of calculated cases. Designed human brain model reconstruction algorithms and techniques, which are based on MRI (Magnetic Resonance Imaging) modality, are proved to be the most accurate among existing in terms of geometrical and material properties. Error estimations and algorithm structure delivers the resolution of the model to be the same as practical imaging resolution of the MRI equipment (for presented case was less than 1mm). Novel neuronal source modelling approach was also presented with partial experimental validation showing improved results in comparison to all existing methods. At the same time developed mathematical basis for practical realization of discussed approach allows computer simulations of any known neuronal formation. Also it is the most suitable method for Finite Element Method (FEM) which was proved to be the best computer solver for complex bio-electrical problems. The mathematical structure for Inverse problem solution which is based on integrated human brain modelling technique and neuronal source modelling approach is delivered and briefly discussed. In the concluding part of the report the practical application case of developed techniques is performed and discussed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Development of a numerical and experimental framework to understand and predict the burning dynamics of porous fuel beds

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    Understanding the burning behaviour of litter fuels is essential before developing a complete understanding of wildfire spread. The challenge of predicting the fire behaviour of such fuels arises from their porous nature and from the strong coupling of the physico-chemical complexities of the fuel with the surrounding environment, which controls the burning dynamics. In this work, a method is presented to accurately understand the processes which control the burning behaviour of a wildland fuel layer using numerical simulations coupled with laboratory experiments. Simulations are undertaken with ForestFireFOAM, a modification of FireFOAM that uses a Large Eddy Simulation solver to represent porous fuel by implementing a multiphase formulation to conservation equations (mass, momentum, and energy). This approach allows the fire- induced behaviour of a porous, reactive and radiative medium to be simulated. Conservation equations are solved in an averaged control volume at a scale su cient to contain both coexisting gas and solid phases, considering strong coupling between the phases. Processes such as drying, pyrolysis, and char combustion are described through temperature-dependent interaction between the solid and gas phases. Di↵erent sub-models for heat transfer, pyrolysis, gas combustion, and smouldering have been implemented and tested to allow better representation of these combustion processes. Numerical simulations are compared with experiments undertaken in a controlled environment using the FM Global Fire Propagation Apparatus. Pine needle beds of varying densities and surface to volume ratios were subject to radiative heat fluxes and flows to interrogate the ignition and combustion behaviour. After including modified descriptions of the heat transfer, degradation, and combustion models, it is shown that key flammability parameters of mass loss rates, heat release rates, gas emissions and temperature fields agree well with experimental observations. Using this approach, we are able to provide the appropriate modifications to represent the burning behaviour of complex wildland fuels in a range of conditions representative of real fires. It is anticipated that this framework will support larger-scale model development and optimisation of fire simulations of wildland fuels

    Investigation on Fatigue Failure in Tyres

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    PhDTyres are highly engineered complex rubber composite products. They are constructed from a wide range of different materials in addition to the rubber. In different parts of the tyre’s construction, the rubber elements are expected to perform different functions and as a consequence many different types of rubber are used, each of which will have its own specific detailed compound formulation. These different regions of a tyre’s construction are joined together by different types of molecular bonding. This variety of materials introduces potential sources of failure both in the homogenous regions within the tyre’s construction but also at the interfaces between them. This thesis investigates the crack growth resistance of the rubber materials used in different regions of a tyre’s construction as well as the interfaces that are found between the different parts of the tyre. A fracture mechanics framework was used to investigate the fatigue behaviour of bulk rubber and some of the interfaces. The loading of a tyre is periodic in nature as a consequence of the wheel’s rotation therefore the materials were characterised over a range of loading conditions. The effect of cyclical loading frequency on the fatigue behavior of the bulk rubber was also investigated. This work discovered that the amount of crack growth per cycle was comprised from two different crack growth contributions. The first is related to the steady tear which is related to the length of time the load is applied. The second resulted from additional damage caused by the repeated loading and unloading of the material. Potential reasons for this additional crack growth contribution are discussed. The interfacial fatigue properties between adjoining and potentially dissimilar rubber compounds were examined using a fatigue peeling experiment. A novel test piece geometry was developed to evaluate the fatigue properties of interfaces in tyres and it was also used to investigate how different processing parameters such as the pressure at the interface during vulcanisation alter the interfacial strength. A significant effect was observed and this was related to the different phenomena occurring when two rubbery polymers are brought into contact. Finally, a fracture mechanics approach was also used to derive the value of the tearing energy, the variable governing crack growth propagation in the rubber materials found in tyres, using submodelling technique in finite element analysis. The tearing energy values at different locations within a tyre were calculated and are shown not to exceed the minimum energy criteria for crack propagation under normal service conditions.EPSRC and Dunlop Aircraft Tyres Lt

    Thermo-mechanical reliability studies of lead-free solder interconnects

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    N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/

    Wear fatigue in nickel superalloys.

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    To date, work to assess the progression of wear and the effects of wear damage on low cycle fatigue has tended to be focused on specific components and their operating conditions. Although effective in the short term to solve today’s problems, these efforts often deliver insufficient understanding of the overall design space limits to have much influence of future component design.Therefore, the following research attempts to understand how wear damage progresses and how it impacts on fatigue performance in order to develop more accurate lifing models to predict the behaviour and life of real engine components.In order to do this, a survey of the internal Rolls-Royce database and public literature on wear damage on components from ex-service and current service engines was performed. Information relating to the wear scar morphologies in the reports was extracted as well as physically measuring and analysing wear damage on worn components within the Rolls- Royce failure investigation department. The wear damage was then replicated onto Udimet720Li laboratory fatigue specimens by a means of altering the pad pressure and pad sliding distance to produce a range of wear damage in order to carry out fatigue testing. Fatigue testing of the damaged specimens allowed fatigue knockdown factors to be calculated to determine the impact of wear on the fatigue life.A fretting fatigue rig was also designed and built for this research to focus on in-situ fretting fatigue at high temperatures of 600°C

    SIRM 2017

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    This volume contains selected papers presented at the 12th International Conference on vibrations in rotating machines, SIRM, which took place February 15-17, 2017 at the campus of the Graz University of Technology. By all meaningful measures, SIRM was a great success, attracting about 120 participants (ranging from senior colleagues to graduate students) from 14 countries. Latest trends in theoretical research, development, design and machine maintenance have been discussed between machine manufacturers, machine operators and scientific representatives in the field of rotor dynamics. SIRM 2017 included thematic sessions on the following topics: Rotordynamics, Stability, Friction, Monitoring, Electrical Machines, Torsional Vibrations, Blade Vibrations, Balancing, Parametric Excitation, and Bearings. The papers struck an admirable balance between theory, analysis, computation and experiment, thus contributing a richly diverse set of perspectives and methods to the audience of the conference

    Application of LES-PDF methods on turbulent reacting flows

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    This research concerns the application of the Probability Density Function (PDF) on Large Eddy Simulations (LES) of turbulent reacting flows in a wide range of open flame configurations spanning between the premixed and non-premixed regime. The aim is to validate the applicability of the PDF model on a wide range of flames without any special treatment. Additionally, the \textit{a-posteriori} Chemical Exposive Mode Analysis (CEMA) has been applied to the results in order to examine the flame structure and identify locations of extinction, re-ignition, etc. Four different series of flames are studied, each one of them belonging to a completely different combustion regime. The F1-F3 premixed turbulent flames is the first family of flames where the PDF method is applied. The LES-PDF model is shown to accurately predict the flow field and the scalar field even on a very coarse grid. The simulations were performed on a personal computer, so the computational power was severely restricted. Nevertheless, the PDF model was able to give accurate predictions, so one of the flames was chosen for a further sensitivity analysis. A large number of modelling parameters were studied and the results show little sensitivity to them in contrast to RANS-PDF approaches in premixed flames. Finally, the model is able to capture large scale quenching at qualitatively the correct extinction speed. The Cambridge-Sandia series of swirling stratified flames was also examined. It encompasses a wide range of flames with various combinations of swirl and stratification ratio levels. Four distinct cases were selected and tested. For the most simple flames (SwB1 and SwB5), the model gives excellent prediction for both the flow field and the scalar distribution. The introduction of the additional fields improves slightly the results, especially at locations further away from the nozzle exit. For the flames which exhibit more complex flow fields and complex characteristics (SwB6 and SwB11), the model gives reasonable results, given the complexity of the flow field. The introduction of differential diffusion and heat losses towards the ceramic cap was studied independently on the SwB11 flame and was found to have counteracting effects. Therefore, their combination was tested and was found to give a significant improvement. The next series of flames is the Sydney Swirl flames. The SM1 and SM2 flames are two complex swirling flames with a difficult flow field to capture. The field is composed of recirculating zones and vortex break-down bubble areas. The SM2 has not been tested in the literature and this work is the first modelling approach. The flow field simulation results are reasonable, given the complexity of the flame. The biggest discrepancies are observed close to the nozzle exit. The Chemical Explosive Mode Analysis is also performed to give information about the flame structure. The flame is divided into three distinct zones with the second one being a very large quenching region. The CEMA analysis explains why the flame does not quench, but re-ignites further down. Finally, the Delft III premixed flame is studied, a difficult flame to model as it shows quenching with large extinction pockets despite the moderately low Reynolds number. The major flow characteristics were accurately captured by the simulation and the introduction of the additional stochastic fields improves the results close to the nozzle exit. Contrary to most researchers that model the pilot flow as a single heat source close to the nozzle exit, in this work the pilot flow is modelled as a separate flow stream, something that increased the complexity of the simulations due to the extremely thin pilot rim which was comparable to the cell size. Nevertheless, the model was able to accurately capture the localized extinction throughout the flame and the application of the Chemical Explosive Mode Analysis gave further insight into the structure of the flame.Open Acces
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