As coberturas constituem um elemento essencial para a sobrevivÃncia humana. As transformaÃÃes mais portantes das cobertas relacionaram-se, por um lado, com a prÃ-fabricaÃÃo do material a ser utilizado, que veio permitir maiores garantias de Ãxito em qualquer aplicaÃÃo, e por outro lado, com o aparecimento de novos materiais na construÃÃo, como o vidro laminado, peÃas de madeira, os materiais plÃsticos e principalmente o alumÃnio e o aÃo. Neste contexto os telhados de metal sÃo bem avaliados por vÃrios motivos, sÃo resistentes, durÃveis, vencem grandes vÃos, resistem à corrosÃo e sÃo mais leves em relaÃÃo Ãs cerÃmicas, gerando economia nos custos da estrutura e facilidade no manuseio, transporte e montagem. A problemÃtica dos ventos de alta velocidade, como furacÃes e tempestades, muitas vezes causam danos graves Ãs telhas metÃlicas. Os danos causados pelo vento mostraram que a fadiga promove uma trinca no telhado em torno dos furos do prendedor ocasionando um despendimento das telhas em sequÃncia. Nesse trabalho desenvolve-se uma mÃquina para estudos de simulaÃÃo de carregamento cÃclico em telhas metÃlicas simulando a forÃa do vento. Utilizando a tÃcnica da CorrelaÃÃo Digital de Imagem (CDI) desenvolveu-se uma metodologia que auxiliou em termos quantitativos e qualitativos a avaliaÃÃo da integridade da telha. / Shelter is an essential element for human survival. The most important changes in shelter have been in regard, on the one hand, of pre-fabrication of the material to be used, which better guarantees success at any application, and, on the other hand, of the rise of new building materials, such as laminated glass, wooden parts, plastic materials, and mainly aluminum and steel. In this context, metal roofs are praised for several reasons. They are sturdy and durable, span across wide areas, stand corrosion, and are lighter compared to ceramic tiles, which leads to savings in structure costs and ease of handling, transport, and assembly. The issue of high- speed winds, such as hurricanes and storms, often causes great damage to metal tiling. Wind damages show that the low-cycle wear fissures the roofing around the fastening holes, causing the tiles to become loose. This research developed a machine to study the simulation of cyclic loads in metal tiles by simulating wind force. By using the Digital Image Correlation (DIC) technique, a methodology was developed that will aid in assessing tile integrity both quantitatively and qualitatively.

The main aim of this research was to investigate the behaviour of adhesive joints exposed to repeated low-velocity impact i.e. impact fatigue (IF), and to compare this loading regime with standard fatigue (SF), i.e. non-impacting, constant amplitude, sinusoidal loading conditions. Two types of lap joint configuration using rubber toughened modified epoxy adhesives were used and exposed to various loading conditions in order to determine the fatigue behaviour of the joints for each load conditions. The fatigue life was investigated using bonded aluminium alloy (7075-T6) single lap joint (SLJ) specimens, where it was seen that IF is an extremely damaging load regime compared to SF. Different trends were visible in force-life plots for these two types of loading. In SF a gradual decrease in the fatigue life with increasing load was observed, whereas, in IF a significant decrease in life was seen at relatively modest levels of maximum force after relatively few cycles. Comparisons of the fatigue life show a considerably earlier failure in IF than in SF for comparable levels of force and energy. Additionally, it was demonstrated that the maximum force per cycle, loading time, stiffness and strength decreased as a result of damage generated in the sample during IF.

This dissertation investigates the development of numerical algorithms for coupling computational fluid dynamics (CFD) and computational solid dynamics (CSD) solvers, and the use of these solvers for simulating fluid-solid interaction (FSI) problems involving large deformation, shock waves, and multiphase flow. The dissertation consists of two parts. The first part investigates the use of Robin interface conditions to resolve the well-known numerical added-mass instability, which affects partitioned coupling procedures for solving problems with incompressible flow and strong added-mass effect. First, a one-parameter Robin interface condition is developed by linearly combining the conventional Dirichlet and Neumann interface conditions. Next, a numerical algorithm is developed to implement the Robin interface condition in an embedded boundary method for coupling a parallel, projection-based incompressible viscous flow solver with a nonlinear finite element solid solver. Both an analytical study and a numerical study reveal that the new algorithm can clearly outperform conventional Dirichlet-Neumann procedures in terms of both stability and accuracy, when the parameter value is carefully selected. Moreover, the studies also indicate that the optimal parameter value depends on the materials and geometry of the problem. Therefore, to efficiently solve FSI problems involving non-uniform structures, a generalized Robin interface condition is presented, in which the constant parameter is replaced by a spatially varying function that depends on the local material and geometric properties of the structure. Numerical experiments using two benchmark problems show that the spatially varying Robin interface condition can clearly improve numerical accuracy compared to the constant- parameter version with the same computational cost. The second part of this dissertation focuses on simulating complex FSI problems featuring shock waves, multiphase flow (e.g., bubbles), and shock-induced material damage and fracture. A recently developed three-dimensional computational framework is employed, which couples a multiphase, compressible CFD solver and a nonlinear finite element CSD solver using an embedded boundary method and a partitioned procedure. In particular, the CFD solver applies a level-set method to capture the evolution of the bubble surface, and the CSD solver utilizes a continuum damage mechanics model and an element erosion method to simulate the dynamic fracture of the material. Two computational studies are presented. The first one investigates the dynamic response and failure of a brittle material exposed to a prescribed shock wave. The predictive capability of the computational framework is first demonstrated by simulating a series of laboratory experiments in the context of shock wave lithotripsy. Then, a parametric study is conducted to elucidate the significant effects of the shock wave's profile on material damage. In the second study, the computational framework is applied to simulate shock-induced bubble collapse near various solid and soft materials. The reciprocal effect of the material's properties (e.g., acoustic impedance, Young's modulus) on bubble dynamics is discussed in detail. / Doctor of Philosophy / Numerical simulations that couple computational fluid dynamics (CFD) solvers and computational solid dynamics (CSD) solvers have been widely used in the solution of nonlinear fluid-solid interaction (FSI) problems underlying many engineering applications. This is primarily because they are based on partitioned solutions of fluid and solid subsystems, which facilitates the use of existing numerical methods and computational codes developed for each subsystem. The first part of this dissertation focuses on developing advanced numerical algorithms for coupling the two subsystems. The aim is to resolve a major numerical instability issue that occurs when solving problems involving incompressible, heavy fluids and thin, lightweight structures. Specifically, this work first presents a new coupling algorithm based on a one-parameter Robin interface condition. An embedded boundary method is developed to enforce the Robin interface condition, which can be advantageous in solving problems involving complex geometry and large deformation. The new coupling algorithm has been shown to significantly improve numerical stability when the constant parameter is carefully selected. Next, the constant parameter is generalized into a spatially varying function whose local value is determined by the local material and geometric properties of the structure. Numerical studies show that when solving FSI problems involving non-uniform structures, using this spatially varying Robin interface condition can outperform the constant-parameter version in both stability and accuracy under the same computational cost. In the second part of this dissertation, a recently developed three-dimensional multiphase CFD - CSD coupled solver is extended to simulate complex FSI problems featuring shock wave, bubbles, and material damage and fracture. The aim is to understand the material’s response to loading induced by a shock wave and the collapse of nearby bubbles, which is important for advancing the beneficial use of shock wave and bubble collapse for material modification. Two computational studies are presented. The first one investigates the dynamic response and failure of a brittle material exposed to a prescribed shock wave. The causal relationship between shock loading and material failure, and the effects of the shock wave’s profile on material damage are discussed. The second study investigates the shock-induced bubble collapse near various solid and soft materials. The two-way interaction between bubble dynamics and materials response, and the reciprocal effects of the material’s properties are discussed in detail.

L’objectif de ce travail est de proposer une méthodologie simple pour évaluer l’intégrité et la durée de vie d’échangeurs thermiques soudés. Une approche à deux échelles est proposée. Une description macroscopique avec la prise en compte de la structure de l’échangeur est menée pour permettre des calculs thermoélastiques par éléments finis. La réponse mécanique de l’échangeur pour deschargements thermiques, cycliques, simples est évaluée. Notamment, les zones de concentration decontraintes sont repérées. A partir cette étude, une étude micromécanique du comportement dumatériau composant l’échangeur est menée. Le matériau considéré est un acier 316L. Soncomportement élastoplastique est identifié avec un écrouissage isotrope et cinématique. La tenuemécanique pour des chargements en fatigue oligocyclique est évaluée à l’aide d’un dispositif deflexion 4 points alternée et un critère de Manson-Coffin est identifié. Ce critère est utilisé pour évaluerle nombre de cycle admissible par l’échangeur pour une amplitude de chargement donnée. Pour cela,la déformation plastique attendue dans l’échangeur est évaluée à partir d’une équivalence en énergieaux endroits où la contrainte se concentre. Les prédictions du modèle ont été comparées de manièresatisfaisante avec les résultats expérimentaux menés sur un échangeur test, pour la réponsethermoélastique que pour l’évaluation du nombre de cycles à rupture. / This study proposes a simple methodology to estimate the mechanical response and lifetime of weldedheat exchangers subjected to thermal loadings. The structure of the heat exchanger is modeled toestimate its mechanical response for thermal loads. Thermoelastic calculations are carried out with thefinite elements method. From these simulations, the regions where the stress concentrates areidentified. Then, a micromechanics approach is adopted to identify the material’s elastic plasticresponse with isotropic and kinematic hardening. Its durability under oligocyclic fatigue isinvestigated with an original 4 points alternate bending.d vice. From these experiments, a Manson-Coffin criterion is identified. This criterion is used to estimate the heat exchangers lifetime in terms ofmaximum cycles for thermal loadings with different magnitude. To this end, the plastic deformation isestimated from the macroscopic calculation with an energy equivalence between the thermoelasticcalculation and the non linear one. The predictions are found in agreement with experimental datacarried out on test-heat exchangers, for both the thermoelastic response and the number at cycles atrupture.

Zyklische Kohäsivzonenmodelle beschreiben irreversibles Separationsverhalten und Schädigungsakkumulation unter zyklischer Belastung. In der vorliegenden Arbeit wird die Formulierung zyklischer Kohäsivzonenmodelle systematisiert und ihr Potenzial zur Simulation von Ermüdungsvorgängen analysiert. Die Kohäsivspannungs-Separations-Beziehungen werden auf Basis etablierter thermodynamischer Konzepte der Schädigungsmechanik aufgestellt. Zyklische Schädigungsakkumulation wird über die Entwicklungsgleichung der Schädigungsvariablen unter Berücksichtigung einer zustandsabhängigen Dauerfestigkeit beschrieben. Das Kohäsivzonenmodell wird erfolgreich für die Simulation von Werkstoffermüdung angewandt. Numerisch mithilfe der Methode der finiten Elemente erzeugte Rissfortschrittskurven bilden das experimentell beobachtete Ermüdungsrisswachstumsverhalten in allen Bereichen ab. Über Parameterstudien wird der Einfluss der einzelnen Modellparameter ermittelt. Darüber hinaus wird die Anwendung des zyklischen Kohäsivzonenmodells auf die Simulation von Wöhler-Versuchen vorgestellt und der Probengrößeneffekt auf das Ermüdungsverhalten untersucht. Der Zusammenhang zwischen den lokalen Beanspruchungszuständen in der Kohäsivzone und dem vorhergesagten globalen Versagensverhalten wird aufgeklärt. Die gewonnenen Erkenntnisse bilden die Grundlage für ein Konzept zur Identifikation der Kohäsivparameter, das auf der Auswertung von Wöhler- und Rissfortschrittskurven beruht. / Cyclic cohesive zone models describe irreversible separation behaviour and damage accumulation under cyclic loading. In the present thesis, the formulation of cyclic cohesive zone models is systemised and their potential to simulate fatigue processes is analysed. The relation between traction and separation is described based on established thermodynamical concepts of damage mechanics. Cyclic damage accumulation is controlled by a damage evolution equation taking into account a state-dependent endurance limit. The cohesive zone model is applied successfully to the simulation of material fatigue. Fatigue crack growth rate curves, which were obtained numerically by means of the finite element method, reproduce the experimentally observed behaviour in all stages. The influences of the particular parameters of the model are determined by parametric studies. In addition, simulations of uniaxial fatigue tests using the cyclic cohesive zone model are presented. Furthermore, the size effect on the fatigue behaviour is investigated. The relation between the local states within the cohesive zone and the predicted global failure modes is explained. These findings form the foundation for a concept of parameter identification which bases on the evaluation of Wöhler-curves and fatigue crack growth rate curves.