Models (both physical and numerical) of the friction stir (FS) welding process are used to develop a greater understanding of the influence of independent process parameters on dependent process output variables, such as torque, power, specific weld energy, peak temperature, cooling rates and various metallurgical factors (e.g., grain size and precipitates). An understanding of how the independent process parameters influence output variables and ultimately their effect on resultant properties (e.g., strength, hardness, etc..) is desirable. Most models developed have been validated primarily for aluminum alloys with relatively small amounts of experimental data. Fewer models have been validated for steels or stainless steels, particularly since steels and stainless steels have proven more challenging to friction stir than aluminum alloys. The Gleeble system is also a powerful tool with the capability to perform thermomechanical simulations in a known and controlled environment and provide physical representation of resultant microstructure and hardness values. The coupling of experimental data and physical simulated data can be extremely useful in assessing the capabilities of friction stir numerical process models. The overall approach is to evaluate Isaiah an existing three-dimensional finite element code developed at Cornell University by comparing against experimental and physically-simulated data to determine how well the code output relates to real FS data over a range of nine processing conditions. Physical simulations replicating select thermomechanical streamline histories were conducted to provide a physical representation of resultant metallurgy and hardness. Isaiah shows promise in predicting qualitative trends over a limited range of parameters and is not recommended for use as a predictive tool but rather a complimentary tool, Once properly calibrated, the Isaiah code can be a powerful tool to gain insight into the process, strength evolution during the process and coupled with a texture evolution model may also provide insight into microstructural and texture evolution over a range for which it is calibrated.

This dissertation deals with the solution of three inverse problems in structural mechanics. The first one is load updating for finite element models (FEMs). A least squares fitting is used to identify the load parameters. The basic studies are made for geometrically linear and nonlinear FEMs of beams or frames by using a four-noded curved beam element, which, for a given precision, may significantly solve the ill-posed problem by reducing the overall number of degrees of freedom (DOF) of the system, especially the number of the unknown variables to obtain an overdetermined system. For the basic studies, the unknown applied load within an element is represented by a linear combination of integrated Legendre polynomials, the coefficients of which are the parameters to be extracted using measured displacements or strains. The optimizer L-BFGS-B is used to solve the least squares problem. The second problem is the placement optimization of a distributed sensing fiber optic sensor for a smart bed using Genetic Algorithms (GA), where the sensor performance is maximized. The sensing fiber optic cable is represented by a Non-uniform Rational B-Splines (NURBS) curve, which changes the placement of a set of infinite number of the infinitesimal sensors to the placement of a set of finite number of the control points. The sensor performance is simplified as the integration of the absolute curvature change of the fiber optic cable with respect to a perturbation due to the body movement of a patient. The smart bed is modeled as an elastic mattress core, which supports a fiber optic sensor cable. The initial and deformed geometries of the bed due to the body weight of the patient are calculated using MSC/NASTRAN for a given body pressure. The deformation of the fiber optic cable can be extracted from the deformation of the mattress. The performance of the fiber optic sensor for any given placement is further calculated for any given perturbation. The third application is stiffened panel optimization, including the size and placement optimization for the blade stiffeners, subject to buckling and stress constraints. The present work uses NURBS for the panel and stiffener representation. The mesh for the panel is generated using DistMesh, a triangulation algorithm in MATLAB. A NASTRAN/MATLAB interface is developed to automatically transfer the data between the analysis and optimization processes respectively. The optimization consists of minimizing the weight of the stiffened panel with design variables being the thickness of the plate and height and width of the stiffener as well as the placement of the stiffeners subjected to buckling and stress constraints under in-plane normal/shear and out-plane pressure loading conditions. / Ph. D.

The laterally-loaded pile has long been a topic of research interest. Several models of the soil surrounding a pile have been developed for simulation of lateral pile behavior, ranging from simple spring and dashpot models to sophisticated three-dimensional finite-element models. However, results from the available pile-soil models are not accurate due to inherent approximations or constraints. For the springs and dashpots representation, the real and imaginary stiffness are calculated by idealizing the soil domain as a series of plane-strain slices leading to unrealistic pile behavior at low frequencies while the three-dimensional finite-element analysis is very computationally demanding. Therefore, this dissertation research seeks to contribute toward procedures that are computationally cost-effective while accuracy of the computed response is maintained identical or close to that of the three-dimensional finite-element solution. Based on the fact that purely-elastic soil displacement variations in azimuthal direction are known, the surrounding soil can be formulated in terms of an equivalent one-dimensional model leading to a significant reduction of computational cost. The pile with conventional soil-slice model will be explored first. Next, models with shear stresses between soil slices, including and neglecting the soil vertical displacement, are investigated. Excellent agreement of results from the proposed models with three-dimensional finite-element solutions can be achieved with only small additional computational cost. / text

This document presents a methodology that (using tunable finite element models) allow validating during the development phase of the project, the behavior that equipment and mechanical systems will have once they are installed in microgravity. To achieve this purpose, a general methodology is developed initially. It can be applied to any space system or equipment necessary to know the behavior of any engineering parameter in case this parameter is affected by the particular space conditions. Later, the general methodology in applied over the VIF System. Thus, a concrete methodology for a mechanical system (VIF System) is developed in order to analyze a set of mechanical parameters that allow predicting on ground the mechanical behavior of this system once it is in microgravity. Once defined the mechanical parameters that are required to be analyzed and characterized form the VIF System (forces, moments, velocities, accelerations, etc), then the test phase started. In this phase, test equipment is developed and the VIF System is installed on. The VIF System is externally excited and it is acquired the VIF System behavior, for different levels of excitement, using the appropriate sensors (accelerometers, strain gauges, position sensors, etc). In this way, it is obtained the characterization of VIF System on ground. Once the VIF system has been already characterized on ground, a finite element model (FEM) of the VIF System is performed. This model is properly characterized to achieve the same behavior that the VIF System showed during previous tests. Finally, the FEM model is modified applying a zero gravity condition. This allows getting the VIF System behavior under these particular microgravity conditions. / El present treball planteja una metodologia que, basant-se fonamentalment en models d’elements finits caracteritzables, permet analitzar, a la fase de desenvolupament del projecte, el comportament que equips i sistemes mecànics tindran un cop es trobin instal•lats en condicions de microgravetat. Per aconseguir aquest propòsit, inicialment es desenvolupa una metodologia general que permet ser aplicada a qualsevol equip o sistema espacial on sigui necessari conèixer el comportament de qualsevol paràmetre d’enginyeria que es trobi afectat per les particulars condicions de l’espai. Posteriorment, la teoria general es particularitza pel cas concret d’un sistema mecànic, el Sistema VIF, plantejant així una metodologia particular que permet predir a terra el comportament que tindran un seguit de paràmetres mecànics quan el Sistema VIF es trobi en condicions de microgravetat. Un cop definits els paràmetres mecànics que es volen analitzar i caracteritzar del Sistema VIF (forces, moments, velocitats, acceleracions, etc), s’inicia la fase de test. En aquesta fase es desenvolupa un equip de test on s’hi instal•la el Sistema VIF. L’excitació externa que s’efectua sobre el Sistema VIF permet recollir, usant els sensors adients (acceleròmetres, galgues extensomètriques, sensors de posició, etc), el comportament del Sistema VIF per diferents nivells d’excitació. D’aquesta manera, s’obté la caracterització del Sistema VIF a terra. Una vegada es disposa del Sistema VIF ja caracteritzat a terra, cal realitzat un model d’elements finits (FEM) del Sistema VIF. Aquest model es caracteritza adequadament per tal que assoleixi el mateix comportament que mostrava el Sistema VIF durant la fase de tests. Finalment, s’aplica sobre el model FEM la condició d’ingravidesa imposant un valor de gravetat nul•la. D’aquesta manera, s’obté el comportament del Sistema VIF en aquestes particulars condicions de microgravetat que proporciona l’espai.

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.

Made available in DSpace on 2015-05-08T14:59:32Z (GMT). No. of bitstreams: 1 parte1.pdf: 3354442 bytes, checksum: 532efa08cfa28b912005d769b2265b38 (MD5) Previous issue date: 2011-03-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Temporary grandstands are structures subjected to dynamic loading. This is mainly due to the type of use they are subjected. Vibration problems were detected in these structures and occurrences of accidents have been reported. In this sense, it is necessary to study and identify dynamic characteristics of grandstand components, in order to develop numerical models that can be used for a better understanding of issues linked to comfort and safety of these structures. The numerical model of this type of structure for vibration analysis enables the design to be analyzed with respect to natural frequencies and mode shapes of the structure, avoiding possible effects of resonance with the excitation due to the public. The objective is to develop and adjust numerical models of a temporary grandstand for the characterization of numerical-experimental behaviour in relation to natural frequencies. A temporary grandstand with a capacity of about 96 people was assembled in the laboratory. Previous studies of structural components are used and supplemented with tests and numerical modeling. Modal tests were performed to characterize the structure and calibrate the numerical models. Such models were developed with different degrees of refinement in order to obtain a comparison of the effect of simplifications while modeling the structure. The study provides knowledge about the development of more accurate numerical models of this type of structure, enabling diagnostic of dynamic behavior in use, with guidelines on the level of refinement that should be adopted to model this type of structure. / Estruturas de arquibancadas temporárias estão sujeitas a carregamentos dinâmicos. Isto se deve principalmente ao tipo de uso a que são submetidas. Problemas de vibração foram detectados nestas estruturas e ocorrência de acidentes vêm sendo registradas. Neste sentido, faz-se necessário estudo e identificação das características dinâmicas dos componentes que constituem as estruturas de arquibancadas temporárias, de forma a elaborar modelos numéricos que possam ser utilizados para melhor entendimento de aspectos associados ao conforto e segurança destas estruturas. O modelo numérico deste tipo de estrutura, para análise de vibração, possibilita a análise quanto às freqüências naturais e as formas modais da estrutura, evitando possíveis efeitos de ressonância com a excitação do público. O objetivo deste trabalho é desenvolver e ajustar modelos numéricos de arquibancada temporária com vista à caracterização do comportamento numérico-experimental no que se refere às freqüências naturais. Para tanto, uma arquibancada temporária em estrutura metálica com capacidade de aproximadamente 96 pessoas foi montada em laboratório. Estudos prévios com componentes da estrutura são aproveitados e complementados com a realização de novos testes e modelagens numéricas. Testes modais foram realizados para caracterização da estrutura e calibração de modelos numéricos. Tais modelos foram desenvolvidos com graus de refinamentos diferentes, de forma a se obter uma comparação do efeito de simplificações na modelagem da estrutura. O estudo possibilita aprofundar o conhecimento quanto ao desenvolvimento de modelos numéricos mais precisos deste tipo de estrutura, viabilizando diagnósticos do comportamento dinâmico em uso, com diretrizes quanto ao nível de refinamento que se deve adotar ao modelar este tipo de estrutura.

Les fractures de fragilité représentent un problème de santé publique pour les personnes âgées. L'évaluation de la résistance osseuse et du risque de fracture par la méthode de référence (absorption bi-photonique à rayons X, DXA) est limitée. Les micro-modèles en éléments finis (µFEM) ont montré de meilleures prédictions de la résistance osseuse, mais on ne peut confirmer qu’ils améliorent l’estimation du risque de fracture par rapport à la DXA. L'objectif de cette thèse était donc d'évaluer si la prédiction par simulation numérique pouvait être améliorée en prenant en compte des conditions réalistes de chargement. Tout d’abord, les conditions de chargement correspondant à une chute vers l’avant ont été reproduites sur 32 radius humain dans une expérimentation ex-vivo. Les résultats expérimentaux ont conduits à deux groupes : un fracturé et un non fracturé. Puis, la capacité de prédiction d’un modèle « ségment » (9 mm de radius distal) créé en utilisant un scanner à très haute résolution (High Resolution peripheral Quantitative Computed Tomography) a été évaluée. . Différentes configurations (axiale (configuration standard) et 5 non-axiales) ont été simulées. L’implémentation de chargement non-axial n’a pas amélioré la capacité de prédiction du modèle « segment ». Finalement, un modèle hétérogène du radius distal entier a été créé à partir d’un scanner clinique (Cone Beam Computed Tomography). Ce modèle a pris en compte les conditions d’une chute en termes d’orientation et de vitesse. Le modèle de radius distal entier a montré une meilleure prédiction de la charge à la rupture expérimentale que le modèle « segment ». Cette étude propose des données originales pour la validation de modèles numériques pour l’amélioration de la prédiction du risque de fracture / Fragility fractures represent a public health problem for elderly. The assessment of the bone strength and of the risk of fracture by the gold standard method (Dual X-ray Absorptiometry - DXA) is limited. Micro-finite element models (µFEM) have shown to better predict the bone strength, but it is not possible to confirm that they do better than the density measured by DXA to estimate the risk of fracture. Thus, the aim of this thesis was to evaluate whether including realistic loading conditions could improve the level of prediction of the FEM. First, we reproduced the loading conditions of a forward fall on 32 radii in an ex-vivo experiment. This experiment leaded to two groups: one fractured and one non - fractured. Then, we evaluated the prediction capability of a segment FEM (9 mm of the distal radius) created using the High Resolution peripheral Quantitative Computed Tomography. This segment FEM was tested under the axial loading (standard analysis), and under five additional non-axial configurations. It was found that the prediction capability of the segment FEM was not improved by the implementation of non-axial loadings. Finally, a heterogeneous FEM of the whole distal radius was created using data from a Cone Beam Computed Tomography. This model considers the fall loading configurations in orientation and speed of the ex-vivo experiment. The FEM of the whole distal radius has a better accuracy to predict the experimental failure load than the segment FEM. This study proposes original data for model validation dedicated to further improvements of fracture risk prediction

A hybrid method is presented for the analysis of layers, plates, and multi-layered systems consisting of isotropic and linear elastic materials. The problem is formulated for the general case of a multi-layered system using a total potential energy formulation and employing the layerwise laminate theory of Reddy. A one-dimensional finite element model is used for the analysis of the multi-layered system through its thickness, and integral Fourier transforms are used to obtain the exact solution for the in-plane problem. Explicit expressions are obtained for the fundamental solution of the typical infinite layer, which are applied in the two-dimensional boundary integral equation model to produce the integral representation of the solution. The boundary integral equation model is two-dimensional, displacement-based and assumes piecewise continuous distribution of the displacement components through the system's thickness. The developed model describes the three-dimensional displacement field, the stress field, the strains and the interlaminar stresses over the entire domain of the problem as continuous functions of the position. This detailed three-dimensional analysis is achieved by incorporating only contour integrals. The boundary integral equations are discretized using the boundary element method and a numerical model is developed for the single numerical layer (element). This model is extended to the case of a multilayered system by introducing appropriate continuity conditions at the interfaces between the layers (firmly bonded layers, or separation, slip and friction between the layers). Assembly of the element matrices yields the global system of equations, which can be solved via iterative techniques. In addition, numerical techniques are developed for the evaluation of the boundary and domain integrals involved in the construction of the element matrices. The singular boundary integrals are computed using a special coordinate transformation, along with a subdivision of the boundary element and a transformation of the Gauss points. The domain integrals (regular, singular or near-singular) are transformed to regular definite integrals along the boundary through a semi-analytical approach. The proposed method provides a simple, efficient, and versatile model for a three-dimensional analysis of thick plates or multilayered systems. It can also be used to study plates resting on elastic foundations or plates with internal supports. The proposed method can be applied in an obvious manner to anisotropic materials and vibration problems. / Ph. D.

Neste trabalho estuda-se o comportamento de laminados fibra-metal em regime de impacto, a partir de uma abordagem teórica, numérica e experimental. Os materiais estudados consistem em camadas finas intercaladas de alumínio 2024-T4 e de um novo material termoplástico de polipropileno (PP) de alta resistência mecânica. Eventos de impacto de baixa e alta velocidade contra placas destes laminados foram realizados a partir de um martelo de impacto e de um canhão pneumático, respectivamente. Nestes experimentos buscou-se identificar as condições limite de ruptura e perfuração das amostras, assim como parâmetros de comportamento do material. O laminado fibra metal de PP (ou TFML) e seus constituintes foram caracterizados a taxas de deformação entre 10-4 / s e 102 / s, utilizando-se máquinas de ensaio universal comerciais e um dispositivo desenvolvido especificamente para este estudo, capaz de caracterizar materiais em taxas intermediárias de deformação. Os modelos teóricos de Jones e Reid-Wen foram adaptados para utilização com TFMLs, sendo capazes de identificar o comportamento do material em regime de baixa e alta velocidade de impacto, respectivamente. Um modelo numérico do TFML em regime de impacto foi desenvolvido utilizando o programa comercial LSDyna. Resultados experimentais e teóricos foram confrontados com esse, apresentado boa correlação na predição do limiar de falha e limite balístico do material. Uma vez que o comportamento do TFML ao impacto foi modelado, buscou-se identificar o efeito da distribuição de camadas e composição de constituintes no comportamento do material ao impacto. Estudos também foram conduzidos com o intuito de identificar a influência da taxa de deformação, geometria do indentador e localização do impacto no comportamento dos laminados. Por fim, uma configuração de TFML foi proposta visando melhoria de seu desempenho ao impacto. / In this work, the behaviour of fibre-metal laminates under impact loading is studied by using theoretical, numerical and experimental approaches. The material is a combination of thin aluminium 2024-T3 layers and an innovative high strength thermoplastic polypropylene material. Low and high velocity impact events were performed using a falling weight machine and a gas-gun projectile launcher, respectively. The thermoplastic fibre-metal laminates (or TFML) and its constituents were mechanically characterized in the range of strain rates between 10-4 / s and 102 / s , using commercial universal testing machines and a specifically designed rig for tensile tests at intermediate strain rates. The Jones and Reid-Wen theoretical models were adapted to be used with TFML plates. A finite element model of the TFML under impact events was developed using LS-Dyna software. The numerical model revalled results that were compared with the theoretical models and the experimental data, providing reasonably similar results. Once the TFML impact behaviour was identified and modelled, the effect of the layers distribution and constituent composition on the TFML impact response was studied. Studies of the strain rate effect, identor geometry and the impact location were also performed. Finally, a TFML configuration was suggested in order to improve the TFML impact performance.

Neste trabalho estuda-se o comportamento de laminados fibra-metal em regime de impacto, a partir de uma abordagem teórica, numérica e experimental. Os materiais estudados consistem em camadas finas intercaladas de alumínio 2024-T4 e de um novo material termoplástico de polipropileno (PP) de alta resistência mecânica. Eventos de impacto de baixa e alta velocidade contra placas destes laminados foram realizados a partir de um martelo de impacto e de um canhão pneumático, respectivamente. Nestes experimentos buscou-se identificar as condições limite de ruptura e perfuração das amostras, assim como parâmetros de comportamento do material. O laminado fibra metal de PP (ou TFML) e seus constituintes foram caracterizados a taxas de deformação entre 10-4 / s e 102 / s, utilizando-se máquinas de ensaio universal comerciais e um dispositivo desenvolvido especificamente para este estudo, capaz de caracterizar materiais em taxas intermediárias de deformação. Os modelos teóricos de Jones e Reid-Wen foram adaptados para utilização com TFMLs, sendo capazes de identificar o comportamento do material em regime de baixa e alta velocidade de impacto, respectivamente. Um modelo numérico do TFML em regime de impacto foi desenvolvido utilizando o programa comercial LSDyna. Resultados experimentais e teóricos foram confrontados com esse, apresentado boa correlação na predição do limiar de falha e limite balístico do material. Uma vez que o comportamento do TFML ao impacto foi modelado, buscou-se identificar o efeito da distribuição de camadas e composição de constituintes no comportamento do material ao impacto. Estudos também foram conduzidos com o intuito de identificar a influência da taxa de deformação, geometria do indentador e localização do impacto no comportamento dos laminados. Por fim, uma configuração de TFML foi proposta visando melhoria de seu desempenho ao impacto. / In this work, the behaviour of fibre-metal laminates under impact loading is studied by using theoretical, numerical and experimental approaches. The material is a combination of thin aluminium 2024-T3 layers and an innovative high strength thermoplastic polypropylene material. Low and high velocity impact events were performed using a falling weight machine and a gas-gun projectile launcher, respectively. The thermoplastic fibre-metal laminates (or TFML) and its constituents were mechanically characterized in the range of strain rates between 10-4 / s and 102 / s , using commercial universal testing machines and a specifically designed rig for tensile tests at intermediate strain rates. The Jones and Reid-Wen theoretical models were adapted to be used with TFML plates. A finite element model of the TFML under impact events was developed using LS-Dyna software. The numerical model revalled results that were compared with the theoretical models and the experimental data, providing reasonably similar results. Once the TFML impact behaviour was identified and modelled, the effect of the layers distribution and constituent composition on the TFML impact response was studied. Studies of the strain rate effect, identor geometry and the impact location were also performed. Finally, a TFML configuration was suggested in order to improve the TFML impact performance.