Reliable flutter analysis of aircraft structures is a major requirement to determine safe flight envelops. Dynamically equivalent finite element model of an aircraft structure correlating well with experimental modal is a major requirement for a reliable flutter analysis. Currently available model updating techniques require enormous time and engineering work to achieve appropriate finite element models of aircraft structures. The method developed within the scope of this thesis work aims to remove important disadvantages of common model updating procedures. In doing this, the method starts with a simple finite element mesh obtained by connecting measurement points, used in the Ground Vibration Test of an aircraft structure, with 3 D Euler-Bernoulli beam elements. Initial estimates of the geometric and material properties are determined by solving structural identification equations derived from the mass and stiffness orthogonality of experimental modes. By using those initial estimates, an initial finite element model is constructed. Starting from this initial finite element model, structural identification equations are updated and solved iteratively by using experimental natural frequencies and eigenvectors of the v updated finite element model representing the same mode shapes with measured normal modes. Iterations are continued until eigen solution of the updated finite element model closely correlates with experimental modal data. The applicability of the method is illustrated on a scaled aircraft model and a real aircraft structure. The results are quite satisfactory but the method requires further improvements to achieve a much better correlation level in case of real aircraft structures.

This thesis presents a new modeling framework and application methodology for the study of aircraft structures. The framework provides a ‘cradle-to-grave’ approach to structural analysis of a component, where structural integrity encompasses all phases of its lifespan. The methodology examines the holistic structural design of aircraft components by integrating fatigue and damage tolerance methodologies. It accomplishes this by marrying the load inputs from a fatigue analysis for new design, into a risk analysis for an existing design. The risk analysis incorporates the variability found from literature, including recorded defects, loadings, and material strength properties. The methodology is verified via formal conceptualization of the structures, which are demonstrated on an actual hydraulic accumulator and an engine nacelle inlet. The hydraulic accumulator is examined for structural integrity utilizing different base materials undergoing variable amplitude loading. Integrity is accomplished through a risk analysis by means of fault tree analysis. The engine nacelle inlet uses the damage tolerance philosophy for a sonic fatigue condition undergoing both constant amplitude loading and a theoretical flight design case. Residual strength changes are examined throughout crack growth, where structural integrity is accomplished through a risk analysis of component strength versus probability of failure. Both methodologies can be applied to nearly any structural application, not necessarily limited to aerospace.

The analysis and optimum design of stiffened, shear webs in aircraft structures is addressed. The post-buckling behaviour of the webs is assessed using the interactive algorithm developed by Grisham. This method requires only linear finite element analyses, while convergence is typically achieved in as few as five iterations. The Grisham algorithm is extensively compared with empirical analysis methods previously used for aircraft structures and also with a refined, non-linear quasi-static finite element analysis. The Grisham algorithm provides for both compressive buckling in two directions as well as shear buckling, and overcomes some of the conservatism inherent in conventional methods of analysis. In addition, the method is notably less expensive than a complete non-linear finite element analysis, even though global collapse cannot be predicted. While verification of the analysis methodology is the main focus of the stud, an initial investigation into optimization is also made. In optimizing stiffened thin walled structures, the Grisham algorithm is combined with a genetic algorithm. Allowable stress constraints are accommodated using a simple penalty formulation. / Dissertation (MEng (Mechanical and Aeronautical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

Estruturas aeronáuticas são em sua grande parte fabricadas em material compósito para que sejam atendidas as especificações de projeto. Entre essas estruturas destaca-se a estrutura sanduíche. A utilização desse tipo de estrutura requer estudos extensos em novos materiais, bem como na aplicação dos mesmos. Uma atenção especial para o núcleo dessas estruturas é necessária, pois este material é na verdade uma estrutura celular, como as espumas poliméricas. Esta dissertação busca concatenar a literatura com a prática ao estudar a calibração de modelos de material para descrever o comportamento mecânico de espumas poliméricas, bem como avaliar suas potencialidades e limitações. Estas espumas são estruturas celulares cujos mecanismos de falha consistem em respostas micro e macroscópicas. A identificação e quantificação desses comportamentos podem ser feitas através da investigação de modelos de material micro-mêcanicos ou fenomenológicos (macro-mecânicos) associados a ensaios e análises experimentais tanto do material celular quanto da estrutura na qual este material é utilizado. Cada abordagem, micro ou macro-mecânica, possui vantagens e desvantagens que no presente trabalho são discutidas para o material estudado (espuma polimérica rígida de PVC, poli-cloreto de vinila, com estrutura de células fechada e densidade de 60kg/m³). Uma série de ensaios experimentais com bases em normas é realizada e os dados coletados são comparados com dados obtidos simultaneamente através de uma técnica de correlação de imagens. Todas as informações experimentais são confrontadas e associadas aos mecanismos de falha da espuma polimérica. Finalmente, os dados experimentais são utilizados nas identificações de parâmetros de modelos de material disponíveis em um programa comercial de elementos finitos - ABAQUS. Com os modelos de material calibrados, o presente trabalho investiga a representatividade e as limitações dos mesmos quando aplicados a estruturas aeronáuticas submetidas a cargas localizadas, monotônicas ou não. Observou-se que há uma forte dependência da resposta macroscópica da espuma com sua estrutura celular quando submetida a cargas localizadas e/ou não-monotônicas. Ademais, o uso de modelos de material simplificados, e/ou com hipóteses de implementação, gera resultados duvidosos quando estes modelos são aplicados a materiais celulares com respostas complexas (mecanismos micro-mecânicos, anisotropia, viscosidade, etc.). Todavia, o presente trabalho mostra que uma calibração estratégica relevando as hipóteses de implementação e as limitações do modelo de material, fornece bons resultados macroscópicos que são fortemente influenciados pelos mecanismos de falha micro-mecânicos. / Aircraft structures are mostly made of composite material in order to achieve the specifications of a project. Among these structures one highlights the sandwich structure. The usage of this structure requires extensive studies on new materials as well as on the application of these very materials. A special attention for the cores material of these structures is needed because it is in fact a cellular structure, as the polymeric foams. This dissertation seeks to concatenate the literature and practice, studying the calibration of material models to describe the mechanical behavior of polymeric foams, as well as to analyse their potentials and limitations. These foams are cellular structures whose failure mechanisms comprise micro and macro responses. The identification and quantification of these behaviors can be done through micro-mechanical or phenomenological (macro-mechanical) material models along with experimental tests and analyses of both the cellular material and the structure in which this material is used. Each approach, micro or macro, has advantages and disadvantages that in the present work are discussed for the studied material (PVC, poly-vinyl-chloride, rigid closed-cell polymeric foam with a density of 60kg/m³). A series of experimental tests based on standard procedures are carried out and the data collected are compared with data obtained simultaneously through an image correlation technique. All the experimental information are confronted and associated to the failure mechanisms of the polymeric foam. Finally, the experimental data are used for the identification of material models parameters, currently available in the commercial finite elements software - ABAQUS. With the material models calibrated, the present work investigates the representativeness and the limitations of these very models when applied to aircraft structures submitted to monotonic or not localized loads. One has observed that there is a strong dependence of the foams macroscopic response with its cellular structure when it is submitted to localized and/or non-monotonic loads. Moreover, the usage of simplified material models, and/or with some implementation hypotheses, renders doubtful results when these models are applied to cellular materials with complex responses (micro-mechanical mechanisms, anisotropy, viscosity, etc.). Nevertheless, the present work shows that a strategic calibration taking into account the implementation hypotheses and the limitations of the material model, yields good macroscopic results that are strongly influenced by the micro-mechanical failure mechanisms.

Solid surfaces featuring a spatial variation of surface wettability along particular directions on their surface, referred to as wettability gradient surfaces, are becoming increasingly important in practical applications such as enhancement of boiling and condensation heat transfer and separation of immiscible liquids in smart micro-fluidic devices. With the aid of an external energy input, such as mechanical vibration or impact kinetic energy, a liquid drop on such surfaces gets propelled towards more wettable region on the surface. A fundamental study of impact dynamics of liquid drops on such solid surfaces is relevant in understanding their effectiveness. The present thesis reports a combined experimental and theoretical study on the impact dynamics of liquid drops on solid surfaces comprising a smooth portion and a groove-textured portion separated by a junction line (dual-textured surfaces). Three different dual-textured surfaces – two made of intrinsically hydrophilic stainless steel and one of intrinsically hydrophobic poly-di-methyl-siloxane (PDMS) – are considered. Liquid drops, with Weber number (We) in the range 1–100, are impacted on the junction of the dual-textured surfaces and the entire impact dynamics across the junction is captured using a high speed video camera. Experiments of drop impact on the homogeneous surface portions of dual-textured surfaces (far away from the junction) are also conducted. The temporal variation of drop contact radius measured from the junction line on smooth and groove-textured portions of the dual-textured surfaces exhibits four distinct stages – primary spreading, primary receding, secondary spreading on more wettable surface portion, and final equilibrium – with the final outcome being the bulk movement and deposition of liquid drop away from the junction towards the more hydrophilic surface portion. Secondary parameters characterizing each of these different stages are extracted from these measurements and a one-to-one comparison between dual-textured and homogenous surfaces is presented. A significant effect of dual-texture nature is seen on the receding process of impacting drops. On the dual-textured surfaces, the receding velocity of impacting drop on the groove-textured portion is always greater than that on the smooth portion. The asymmetry in drop receding results in a drop drift velocity towards the more wettable surface portion leading to an enhanced secondary drop spreading on the more wettable smooth portion. The drop drift velocity shows a decrease with We at low We and remains almost constant at higher We after a particular value of We. Correspondingly, the ratio of the maximum drop spread factor achieved during the secondary spreading (βm2) to that during the primary spreading (βm) is seen to decrease with We at low We and remains constant at higher We. Owing to the differences in the static equilibrium wetting difference, βm2/βm is more on the stainless steel dual-textured surfaces than on the PDMS dual-textured surface. The presence of dual-texture results in a higher final spread on more wettable smooth portion and smaller final spread on less wettable textured portion of the dual-textured surfaces and this difference decreases with We. The difference in final spread factors between the smooth and textured portions is more on the stainless steel dual-textured surfaces than the PDMS dual-textured surface. The bulk drop movement (ξ), characterized in terms of distance measured from the junction to the final drop center, decreases with We at low We and remains constant at higher We on the stainless steel dual-textured surfaces whereas it remains constant at low We and decreases at higher We on the PDMS dual-textured surface. ξ on the PDMS dual-textured surface is always less than that on the stainless dual-textured surface due to the lower wetting difference across the junction of the former. Comparison of the trends of secondary parameters with the predictions from theoretical models reported in literature showed a lack of agreement. This is due to various physical processes encountered by impacting drop on the groove-textured surface, identified through experiments of drop impact on homogeneous groove-textured surfaces, such as (i) convex shape of liquid-vapor interface near contact line at maximum spreading, (ii) impregnation of drop liquid into the grooves during impact, and (iii) contact line pinning of spreading drop at the asperity edges of surface texture, as well as the wetting difference in dual-textured surfaces. The inclusion of these physical processes under conventional energy conservation approach is seen to predict the experimentally observed trends of maximum drop spread factor on the groove-textured portions. A force balance model, applied to the liquid drop configuration at the beginning of drop receding on the dual-textured surfaces, predicts the qualitative trend of ξ with We on all surfaces. Drop liquid impregnation into the grooves of textured portion at We > Wecr (critical We corresponding to transition from Cassie to impaled state) is proposed as a possible physical mechanism to account for the explanation of the specific trends of ξ with We. A theoretical model formulated using force balance at the three phase contact line beneath impacting drop on groove-textured surface is presented for the prediction of Wecr.

Estruturas aeronáuticas são em sua grande parte fabricadas em material compósito para que sejam atendidas as especificações de projeto. Entre essas estruturas destaca-se a estrutura sanduíche. A utilização desse tipo de estrutura requer estudos extensos em novos materiais, bem como na aplicação dos mesmos. Uma atenção especial para o núcleo dessas estruturas é necessária, pois este material é na verdade uma estrutura celular, como as espumas poliméricas. Esta dissertação busca concatenar a literatura com a prática ao estudar a calibração de modelos de material para descrever o comportamento mecânico de espumas poliméricas, bem como avaliar suas potencialidades e limitações. Estas espumas são estruturas celulares cujos mecanismos de falha consistem em respostas micro e macroscópicas. A identificação e quantificação desses comportamentos podem ser feitas através da investigação de modelos de material micro-mêcanicos ou fenomenológicos (macro-mecânicos) associados a ensaios e análises experimentais tanto do material celular quanto da estrutura na qual este material é utilizado. Cada abordagem, micro ou macro-mecânica, possui vantagens e desvantagens que no presente trabalho são discutidas para o material estudado (espuma polimérica rígida de PVC, poli-cloreto de vinila, com estrutura de células fechada e densidade de 60kg/m³). Uma série de ensaios experimentais com bases em normas é realizada e os dados coletados são comparados com dados obtidos simultaneamente através de uma técnica de correlação de imagens. Todas as informações experimentais são confrontadas e associadas aos mecanismos de falha da espuma polimérica. Finalmente, os dados experimentais são utilizados nas identificações de parâmetros de modelos de material disponíveis em um programa comercial de elementos finitos - ABAQUS. Com os modelos de material calibrados, o presente trabalho investiga a representatividade e as limitações dos mesmos quando aplicados a estruturas aeronáuticas submetidas a cargas localizadas, monotônicas ou não. Observou-se que há uma forte dependência da resposta macroscópica da espuma com sua estrutura celular quando submetida a cargas localizadas e/ou não-monotônicas. Ademais, o uso de modelos de material simplificados, e/ou com hipóteses de implementação, gera resultados duvidosos quando estes modelos são aplicados a materiais celulares com respostas complexas (mecanismos micro-mecânicos, anisotropia, viscosidade, etc.). Todavia, o presente trabalho mostra que uma calibração estratégica relevando as hipóteses de implementação e as limitações do modelo de material, fornece bons resultados macroscópicos que são fortemente influenciados pelos mecanismos de falha micro-mecânicos. / Aircraft structures are mostly made of composite material in order to achieve the specifications of a project. Among these structures one highlights the sandwich structure. The usage of this structure requires extensive studies on new materials as well as on the application of these very materials. A special attention for the cores material of these structures is needed because it is in fact a cellular structure, as the polymeric foams. This dissertation seeks to concatenate the literature and practice, studying the calibration of material models to describe the mechanical behavior of polymeric foams, as well as to analyse their potentials and limitations. These foams are cellular structures whose failure mechanisms comprise micro and macro responses. The identification and quantification of these behaviors can be done through micro-mechanical or phenomenological (macro-mechanical) material models along with experimental tests and analyses of both the cellular material and the structure in which this material is used. Each approach, micro or macro, has advantages and disadvantages that in the present work are discussed for the studied material (PVC, poly-vinyl-chloride, rigid closed-cell polymeric foam with a density of 60kg/m³). A series of experimental tests based on standard procedures are carried out and the data collected are compared with data obtained simultaneously through an image correlation technique. All the experimental information are confronted and associated to the failure mechanisms of the polymeric foam. Finally, the experimental data are used for the identification of material models parameters, currently available in the commercial finite elements software - ABAQUS. With the material models calibrated, the present work investigates the representativeness and the limitations of these very models when applied to aircraft structures submitted to monotonic or not localized loads. One has observed that there is a strong dependence of the foams macroscopic response with its cellular structure when it is submitted to localized and/or non-monotonic loads. Moreover, the usage of simplified material models, and/or with some implementation hypotheses, renders doubtful results when these models are applied to cellular materials with complex responses (micro-mechanical mechanisms, anisotropy, viscosity, etc.). Nevertheless, the present work shows that a strategic calibration taking into account the implementation hypotheses and the limitations of the material model, yields good macroscopic results that are strongly influenced by the micro-mechanical failure mechanisms.

En raison de la gravité des accidents liés au phénomène de fatigue-propagation de fissure, les préoccupations de l’industrie aéronautique à assurer l’intégrité des structures soumises à ce mode de sollicitation revêtent un caractère tout à fait essentiel. Les travaux de thèse présentés dans ce mémoire visent à appréhender le problème de sûreté des structures aéronautiques dimensionnées en tolérance aux dommages sous l’angle probabiliste. La formulation et l’application d’une approche fiabiliste menant à des processus de conception et de maintenance fiables des structures aéronautiques en contexte industriel nécessitent cependant de lever un nombre important de verrous scientifiques. Les efforts ont été concentrés au niveau de trois domaines dans ce travail. Une méthodologie a tout d’abord été développée afin de capturer et de retranscrire fidèlement l’aléa du chargement de fatigue à partir de séquences de chargement observées sur des structures en service et monitorées, ce qui constitue une réelle avancée scientifique. Un deuxième axe de recherche a porté sur la sélection d’un modèle mécanique apte à prédire l’évolution de fissure sous chargement d’amplitude variable à coût de calcul modéré. Les travaux se sont ainsi appuyés sur le modèle PREFFAS pour lequel des évolutions ont également été proposées afin de lever l’hypothèse restrictive de périodicité de chargement. Enfin, les analyses probabilistes, produits du couplage entre le modèle mécanique et les modélisations stochastiques préalablement établies, ont entre autre permis de conclure que le chargement est un paramètre qui influe notablement sur la dispersion du phénomène de propagation de fissure. Le dernier objectif de ces travaux a ainsi porté sur la formulation et la résolution du problème de fiabilité en tolérance aux dommages à partir des modèles stochastiques retenus pour le chargement, constituant un réel enjeu scientifique. Une méthode de résolution spécifique du problème de fiabilité a été mise en place afin de répondre aux objectifs fixés et appliquée à des structures jugées représentatives de problèmes réels. / Ensuring the integrity of structural components subjected to fatigue loads remains an increasing concern in the aerospace industry due to the detrimental accidents that might result from fatigue and fracture processes. The research works presented here aim at addressing the question of aircraft safety in the framework of probabilistic fracture mechanics. It should be noticed that a large number of scientific challenges requires to be solved before performing comprehensive probabilistic analyses and assessing the mechanical reliability of components or structures in an industrial context. The contributions made during the PhD are reported here. Efforts are provided on each step of the global probabilistic methodology. The modeling of random fatigue load sequences based on real measured loads, which represents a key and original step in stochastic damage tolerance, is first addressed. The second task consists in choosing a model able to predict the crack growth under variable amplitude loads, i.e. which accounts for load interactions and retardation/acceleration effects, at a moderate computational cost. The PREFFAS crack closure model is selected for this purpose. Modifications are brought in order to circumvent the restrictive assumption of stationary load sequences. Finally, probabilistic analyses resulting from the coupling between the PREFFAS model and the stochastic modeling are carried out. The following conclusion can especially be drawn. Scatter in fatigue loads considerably affects the dispersion of the crack growth phenomenon. Then, it must be taken into account in reliability analyses. The last part of this work focuses on phrasing and solving the reliability problem in damage tolerance according to the selected stochastic loading models, which is a scientific challenge. A dedicated method is established to meet the required objectives and applied to structures representative of real problems.