This thesis reports the investigation into the modelling and simulation of an aircraft tyre and wheel assembly in finite element environment. The finite element simulations basing on aircraft tyre test and operational scenarios could predict the loads transferred from tyre and the stresses distributed to the wheel rim. The virtual analysis could assess the safety criteria of different tyre structures, which would lead to the cost and time circle reduction in tyre R&D process. An H41x16.0R20 radial ply aircraft test tyre and its corresponding test wheel, provided by Dunlop Aircraft Tyres Limited, are adopted as the subject of this research. The material properties, especially the rubber and fabric materials, have been investigated. The finite element hyperelastic models have been utilized to represent rubbers and been correlated to experimental data. The 2D and 3D finite element tyre models, along with the finite element wheel models are created in the commercial finite element code, LS-Dyna. The finite element models have been validated with either industrial standardised simulation results or experimental data. Basing on the validated models, simulations that duplicating static test and dynamic operational scenarios have been developed. The researches have provided knowledge in comparing single and double bead tyre designs with respect to wheel loading mechanisms. The computational model also allowed manufacturers to assess the performance and safety criteria of a particular tyre at its design stage. The development of such models would add to the general drive towards the use of more virtual prototypes in an area traditionally reliant on experimental testing.

The principal objective of this investigation is to develop a constitutive model to simulate the hysteresis behaviour of unbonded flexible risers. A new constitutive model for flexible risers is proposed and a procedure for the identification of the related input parameters is developed using a multi-scale approach. The constitutive model is formulated in the framework of an Euler-Bernoulli beam model, with the addition of suitable pressure terms to the generalised stresses to account for the internal and external pressures, and therefore can be efficiently used for large-scale analyses. The developed non-linear relationship between generalised stresses and strains in the beam is based on the analogy between frictional slipping between different layers of a flexible riser and frictional slipping between micro-planes of a continuum medium in nonassociative elasto-plasticity. Hence, a linear elastic relationship is used for the initial response in which no-slip occurs; an onset-slip function is introduced to define the ‘noslip’ domain, i.e. the set of generalised stresses for which no slip occurs; a nonassociative rule with linear kinematic hardening is used to model the full-slip phase. The results of several numerical simulations for a riser of small-length, obtained with a very detailed (small-scale) non-linear finite-element model, are used to identify the parameters of the constitutive law, bridging in this way the small scale of the detailed finite-element simulations with the large scale of the beam model. The effectiveness of the proposed method is validated by the satisfactory agreement between the results of various detailed finite-element simulations for a short riser, subject to internal and external uniform pressures and cyclic bending and tensile loadings, with those given by the proposed constitutive law. The merit of the present constitutive law lies in the capturing of many important aspects of risers structural response, including the energy dissipation due to frictional slip between layers and the hysteretic response. This privilege allows one to accurately study the cyclic behavior of unbonded flexible risers subject to axial tension, bending moment, internal and external pressures.

In this thesis two kinds of phase change i.e., solid state phase transformation in steels and solid-to-liquid phase transformation in paraffin, have been modeled and numerically simulated. The solid state phase transformation is modeled using the phase field theory while the solid-to-liquid phase transformation is modeled using the Stokes equation and exploiting the viscous nature of the paraffin, by treating it as a liquid in both states.The theoretical base of the solid state, diffusionless phase transformation or the martensitic transformation comes from the Khachaturyan's phase field microelasticity theory. The time evolution of the variable describing the phase transformation is computed using the time dependent Ginzburg-Landau equation. Plasticity is also incorporated into the model by solving another time dependent equation. Simulations are performed both in 2D and 3D, for a single crystal and a polycrystal. Although the model is valid for most iron-carbon alloys, in this research an Fe-0.3\%C alloy is chosen.In order to simulate martensitic transformation in a polycrystal, it is necessary to include the effect of the grain boundary to correctly capture the morphology of the microstructure. One of the important achievements of this research is the incorporation of the grain boundary effect in the Khachaturyan's phase field model. The developed model is also employed to analyze the effect of external stresses on the martensitic transformation, both in 2D and 3D. Results obtained from the numerical simulations show good qualitative agreement with the empirical observations found in the literature.The microactuators are generally used as a micropump or microvalve in various miniaturized industrial and engineering applications. The phase transformation in a paraffin based thermohydraulic membrane microactuator is modeled by treating paraffin as a highly viscous liquid, instead of a solid, below its melting point.  The fluid-solid interaction between paraffin and the enclosing membrane is governed by the ALE technique. The thing which sets apart the presented model from the previous models, is the use of geometry independent and realistic thermal and mechanical properties. Numerical results obtained by treating paraffin as a liquid in both states show better conformity with the experiments, performed on a similar microactuator. The developed model is further employed to analyze the time response of the system, for different input powers and geometries of the microactuator. / <p>QC 20130219</p>

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Master of Science / Department of Mechanical and Nuclear Engineering / Kevin B. Lease / Xiao J. Xin / The study of buckling behavior of tubular and cellular structures has been an intriguing area of research in the field of solid mechanics. Unlike the global Euler buckling of slender structures under compressive loads, tubular and cellular structures deform with their walls buckling as individual supported plates. The aspect ratio and the dimensional characteristics of the tube define the buckling behavior of any tube structure. In this thesis, a thorough study on the buckling of polygon tubular structures with different cross sections is discussed. In the first study, the theoretical buckling formulation of a square tube using the energy method is reviewed from existing solutions in literature. The elastic critical load of a square tube derived from the theoretical solution is then compared with results of finite element elastic buckling simulations. The formation of lobes along the height of the walls at different aspect ratios of the tube is investigated and compared to theory. Also, the buckling behavior of multi-wall structures is studied and the relationship between these structures and a rectangular simply supported plate is established. A brief study on the buckling behavior of rhombic tubes is also performed. The results of the simulation match closely with the theoretical predictions. The study is then extended to quadrilateral tubes with cross-sections in the shape of square, rectangle, rhombus and parallelogram. The theory of buckling of these tubes is explicitly defined using classical plate mechanics based on the previous works presented in literature. Also, the possibility of global Euler buckling in the tubular structures after a certain critical height is discussed. The prediction from the theory is validated using extensive finite element elastic buckling simulations and experimental tests on square and rhombic tube specimens. The results of the simulations and experiments are observed to be consistent with the theory. Using the formulation of plate buckling under different boundary conditions, the buckling behavior of triangular tubes is also determined. A theoretical formulation for calculating the critical load of triangular tubes is derived. The theoretical critical loads for a range of aspect ratios are compared with corresponding finite element simulation results. The comparisons reveal high degree of similarity of the theoretical predictions with the simulations.

Die simulative Absicherung von Produkten in den frühen Phasen der Produktentwicklung wird immer wichtiger, um den Anforderungen nach steigender Effizienz gerecht zu werden. Da das Angebot an erfahrenen Berechnungsingenieuren mit langjähriger Berufserfahrung begrenzt ist gilt es weniger erfahrene Simulationsanwender bei der Durchführung von aussagekräftigen Finite-Elemente-Simulationen zu unterstützen. Die Autoren stellen im Rahmen des Beitrags das Konzept des Analyseteils FEdelM eines FEA-Assistenzsystems vor, welches strukturmechanische Finite-Elemente (FE) Simulationen auf Plausibilität überprüft und auftretende Fehler möglichst automatisiert zu erkennt und behebt. Hierbei werden die einzelnen Module und deren Verknüpfungen untereinander und zu anderen Anwendungen vorgestellt.

Mechanical surface treatments using an elastic-plastic cold working process can develop residual stresses on the surface of a workpiece. Compressive residual stresses on the surface increase resistance against surface crack propagation, so the overall mechanical performance can be improved by this technique. Compressive residual stresses can be created by different methods such as hammering, rolling, and shot peening. Shot peening is a well-established method to induce compressive residual stresses in the metallic components using cold working, and often ascribed to being beneficial to fatigue life in the aerospace and automobile industries. In this method, the surface is bombarded by high-velocity spherical balls which cause plastic deformation of the substrate, leading to a residual compressive stress after shot peening on the surface of the part. Computational modeling is an appropriate and effective way which can predict the amount of produced residual stresses and plastic deformation to obtain surface roughness after shot peening simulation. Finally, an experimental method to measure the magnitude of the residual stress using a nanoindentation technique was developed. The experimental indentation method was compared to both computational predictions (in aluminum) and with x-ray diffraction measurements of stress (in an alloy steel). The current study validates the relation between the nanoindentation method and numerical simulation for assessing the surface residual stresses resulting from single or multiple shot peening processes.

La thèse s'inscrit dans un axe de recherche visant à améliorer les connaissances et prédictions des chargements hydrodynamiques subis par les réservoirs de carburant lors d'impacts balistiques (coup de bélier hydrodynamique) pour améliorer la survivabilité de la structure. Les modèles numériques les plus avancés ne permettent toujours pas de simuler le phénomène complet. De plus les modèles développés sont trop coûteux pour être utilisés lors d'optimisations de réservoirs durant la phase de conception. L'étude proposée consiste à développer un modèle analytique capable de simuler la séquence d'expansion et d'effondrement de la bulle de cavitation et d'utiliser ce modèle pour déterminer les paramètres influant sur les chargements hydrodynamiques lors de coups de bélier hydrodynamiques. La problématique du phénomène de coup de bélier hydrodynamique et l'état de l'art sont présentés dans le premier chapitre du mémoire. Dans le deuxième chapitre, le modèle de Rayleigh-Plesset est modifié pour prendre en compte l'effet de confinement d'un réservoir sphérique sur la dynamique d'expansion de la bulle en négligeant la présence du gaz dans la cavité dans les prédictions des chargements hydrodynamiques. Ce modèle a été appliqué à des cas expérimentaux de coup de bélier, en calibrant un paramètre de confinement lié à la rigidité de la structure. L'étape suivante, présentée dans le troisième chapitre, consiste à évaluer la capacité d'un modèle analytique linéaire élastique (de type plaque) à estimer la valeur du paramètre de confinement. Un bon accord est trouvé entre cette valeur et celle calibrée précédemment, ce qui valide la méthode d'estimation de ce paramètre et supprime la nécessite de la calibration expérimentale. Dans le quatrième chapitre le modèle incompressible de Rayleigh-Plesset est comparé à des simulations explicites éléments finis pour déterminer les effets de l'inertie de la structure et de la compressibilité du liquide sur la dynamique des bulles confinées. Enfin dans le cinquième chapitre un modèle analytique de dynamique de bulles confinées, dans un liquide compressible, est développé et validé. Pour cela une formulation basée sur l'équation de Keller-Miksis est proposée. La pertinence de ce modèle est vérifiée puis il est validé par rapport à des simulations éléments finis, ce qui permet d'estimer l'amélioration apportée par le modèle de Keller-Miksis dans la prédiction des chargements hydrodynamiques par comparaison au modèle de Rayleigh-Plesset. Finalement une analyse critique du travail de thèse et des perspectives sont données. / The context of the thesis consists of improving the knowledge and the predictions of hydrodynamic loads applied on fuel tanks during ballistic impacts (Hydrodynamic Ram) to improve the survivability of aircraft structures. The most advanced numerical models still cannot simulate the entire phenomenon. Moreover, these models are too expensive to be used for optimisation or for design purposes during the tank development stage. The proposed study consists of developing an analytical model capable of simulating the expansion and collapse of the cavitation bubble and to use it to determine the parameters that influence the consecutive hydrodynamic loads. The state of the art hydrodynamic ram problem is first presented. In the second chapter, the Rayleigh-Plesset model is modified to include the stiffness effect of a spherical container on the described bubble dynamics, when neglecting the presence of the bubble gas on the hydrodynamic loads. This model is applied to hydrodynamic ram test cases, by calibrating a confinement parameter which is related to the structure rigidity. The next step, presented in the third chapter evaluates the capacity of an elastic linear analytical model (plate formulae) to provide the value of the confinement parameter. A good agreement is found with the calibrated value in the previous chapter, which validates the method to get this parameter and suppress the need for experimental calibration. In the fourth chapter, the incompressible Rayleigh-Plesset model is compared to explicit finite element simulations to determine the influence of the liquid compressibility and structural inertia on the dynamics of confined bubbles. Then, in the fifth chapter an analytical model for confined bubbles in a compressible liquid is developed. A formulation based on the Keller-Miksis model is proposed. The relevance of this model is verified and it is validated again with respect to finite element simulations. Then the improvement of hydrodynamic loads predictions using this model is estimated by comparison with the Rayleigh-Plesset model. Finality a critical analysis of the thesis work and some outlooks are given.

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