The objective of this study is to develop crack growth analysis methods for functionally graded materials under mode I cyclic loading by using finite element technique. The study starts with the analysis of test specimens which are given in ASTM standard E399. The material properties of specimens are assumed to be changing along the thickness direction according to a presumed variation function used for the modeling of functionally graded materials. The results of the study reveal the influence of different material variation functions on the crack growth behavior. In the second part, the growth of an elliptical crack which is a common case in engineering applications is analyzed. First, mode I cycling loading is applied perpendicular to the crack plane and crack growth profiles for a certain number of cycles are obtained for homogeneous materials. Then, the code is extended for the analysis functionally graded materials. The material properties are assumed to vary as an exponential function along the major or minor axis direction of the crack. The results can be used to examine the crack profile and material constants&rsquo / influence for a certain number of cyclic loading.

Submitted by DAYANA CAMPANELLI DE MORAIS null (dayanacampanelli@gmail.com) on 2017-12-22T03:50:36Z No. of bitstreams: 1 para impressão final.pdf: 3931660 bytes, checksum: d52b37974d258d1d9c1647f7cca67f54 (MD5) / Approved for entry into archive by Silvana Alvarez null (silvana@ict.unesp.br) on 2018-01-04T18:53:29Z (GMT) No. of bitstreams: 1 morais_dc_me_sjc.pdf: 3931660 bytes, checksum: d52b37974d258d1d9c1647f7cca67f54 (MD5) / Made available in DSpace on 2018-01-04T18:53:29Z (GMT). No. of bitstreams: 1 morais_dc_me_sjc.pdf: 3931660 bytes, checksum: d52b37974d258d1d9c1647f7cca67f54 (MD5) Previous issue date: 2017-11-24 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O presente estudo teve como objetivo desenvolver vitrocerâmicas de dissilicato de lítio com gradiente funcional de densidade, inspirado no gradiente natural que existe entre o esmalte e a dentina. Primeiramente o vidro de composição: 33,33% mol de Li2O e 66,67% mol de SiO2 foi obtido pelo método fusão/resfriamento. Em seguida foram preparadas amostras com estruturas homogêneas do vidro a base de dissilicato de lítio para determinação dos melhores parâmetros de sinterização. Três diferentes tratamentos térmicos, determinados com base no resultado da análise diferencial de calorimetria foram utilizados: 850 °C/3h; 900 °C/3h e 950 °C/3h. A caracterização desses materiais foi realizada através da difração de raios X, microscopia eletrônica de varredura, método de Arquimedes e ensaio de flexão biaxial (n=10). O tratamento térmico de 950 oC obteve os melhores resultados, sendo o escolhido para a realização das próximas etapas do estudo. Com a finalidade de otimizar a estética, foi adicionada cerâmica feldspática (VITAVM®9) ao vidro SiO2-Li2O na proporção de 10%, 15% e 20% (n=30). Foi observado que a adição de 10% de VM9 não alterou a resistência do material, e quanto maior a quantidade de VM9, maior foi a translucidez e menor foram o módulo elástico e a densidade. Com isso, foram preparadas vitrocerâmicas bioinspiradas com gradiente funcional de densidade nas seguintes sequências de camadas: uma com 10% de VM9, outra com 15% e a última com 20%. Não houve diferença na resistência à flexão biaxial do grupo com gradiente, quando a camada mais densa estava voltada para o lado de tração, com o grupo com 15% de VM9 com estrutura homogênea. A translucidez do grupo com gradiente foi equivalente ao grupo homogêneo mais translúcido, com 20% de VM9. Concluiu-se que foi possível sintetizar uma vitrocerâmica de dissilicato de lítio funcionalmente graduada, onde uma boa resistência mecânica e uma boa translucidez foram unidas.

Made available in DSpace on 2016-06-02T19:11:48Z (GMT). No. of bitstreams: 1 dissABML.pdf: 4339605 bytes, checksum: b2fcb2bbac41c9e9cc7c77720f7b0b4c (MD5) Previous issue date: 2007-03-01 / Universidade Federal de Sao Carlos / The search for alternatives sources to fossil fuel has stimulated the technological development of solid oxide fuel cell. Hydrogen is the ideal fuel for this application. However, due to technical reasons, alternatives fuels to hydrogen such as methanol and natural gas has been tested. Therefore, the use of these fuels makes necessary the substitution of the anode materials that are normally used with hydrogen. The anode materials substitution must be based on the knowledge about the relationship between anode microstructure and its performance during cell operation. This knowledge makes possible introduce new materials besides new fabrication process for solid oxide fuel cell anodes. The main characteristics of anode are: high chemical activity and durability. The durability of the anode can be attained using FGM material that makes possible to match the thermal expansion of the interfaces. Ni-8YSZ cermet is normally used as anode material. In this work it was studied the cermets Ni-8YSZ as electrodes on the YSZ electrolyte. Two types of electrodes were prepared: with and without compositional gradient along the electrode thickness. This work was developed in two steps. The first one was the development of the powder 8YSZ-NiO suspension with the appropriated rheological characteristics. The second one was the electrodes characterization by impedance spectroscopy and scanning electron microscopy. It was found that electrodes containing layers with ≤30 vol% Ni are inefficient to promote the reactions because de Ni particles are not percolated. / A busca por fontes alternativas aos combustíveis fósseis para geração de energia tem estimulado a tecnologia para a fabricação de células a combustível (CaC). Gás hidrogênio é o combustível ideal para CaC porém outros combustíveis, tais como metanol e gás natural, são testados. O material de anodo utilizado, quando se emprega hidrogênio como combustível, é o compósito Ni-8YSZ. O anodo deve atender dois requisitos: alta atividade química das reações e durabilidade. O conceito de material FGM (gradação funcional) é aplicado a eletrodo com o objetivo de aumentar a durabilidade pela compatibilização da expansão térmica do eletrólito e coletor de corrente. Nesta dissertação foram estudados eletrodos compósitos Ni-8YSZ, em eletrólitos de 8YSZ, com e sem gradiente composicional ao longo da espessura. Este trabalho foi realizado em duas etapas: desenvolvimento do processamento cerâmico e caracterização elétrica dos eletrodos. A primeira etapa consistiu no desenvolvimento de suspensões da mistura 8YSZ-NiO com características adequadas ao processo de deposição por jato de suspensão e cujo filme granular resultante fosse livre de defeitos. A segunda etapa consistiu na caracterização dos eletrodos, com e sem gradação de composição, por espectroscopia de impedância e microscopia eletrônica. Eletrodos contendo camadas ≤30%vol Ni são ineficientes para promover as reações de anodo pois as partículas de níquel não estão percoladas.

Made available in DSpace on 2016-12-08T17:19:39Z (GMT). No. of bitstreams: 1 pre-textuais.pdf: 99837 bytes, checksum: 26af6e9b202afa9fa60be43b442df83a (MD5) Previous issue date: 2011-07-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The finite element method has been traditionally applied in solving problems of elasticity and heat transfer, being widely used in cases with homogeneous isotropic materials. With the advent of new technologies, it has been more frequent to use components with more than one material, aiming to get the most out of each phase. A common example is the application of coating films, with optimal properties, on a substrate. These coating films may be homogeneous or functionally graded (FGM). The latter presents a continuous variation of properties through the geometry, allowing to reduce differences at the interface with the substrate. It is important to analyze the behavior of stresses along the interface between film and substrate, a region prone for the occurrence of failure. As current commercial finite element programs have limitations to handle FGM's, especially at the interfaces, one of the motivations of this dissertation is to develop a method for stress and flux nodal recovery, adequate to work with this type of material. The work presents the analysis of a component made of steel or aluminum coated with a titanium nitride film (homogeneous or FGM) under thermo-mechanical loading. The thermal part of the problem is set in an Eulerian description, leading to heat transfer by conduction and convection in the solid. The mechanical (elasticity) part is set in the usual Lagrangean description. A staggered thermo-mechanical approach is implemented and promising results are obtained. / O método de elementos finitos tem sido tradicionalmente aplicado na solução de problemas de elasticidade e transferência de calor, sendo amplamente utilizado em casos com materiais homogêneos e isotrópicos. Com o advento de novas tecnologias tem sido mais freqüente a utilização de componentes com mais de um material, onde se procura obter o máximo proveito de cada fase. Um exemplo comum é a aplicação de filmes de revestimento com propriedades ótimas sobre um substrato. Estes filmes de revestimento podem ser homogêneos ou funcionalmente gradados (FGM), sendo que o último tem variação continua de propriedades ao longo da geometria, permitindo atenuar diferenças na interface com o substrato. A análise do comportamento das tensões ao longo da interface entre filme e substrato é de suma importância, pois esta é uma região potencial para ocorrência de falhas. Dado que programas comerciais de elementos finitos atuais apresentam limitações para lidar com FGM s, principalmente na região de interface, uma das motivações desta dissertação é desenvolver um método de pós-processamento de tensões e fluxos capaz de trabalhar adequadamente com este tipo de material. O trabalho apresenta a análise de um componente de aço ou alumínio revestido com filme de nitreto de titânio (homogêneo ou funcionalmente gradado), sob carregamento termomecânico. A parte térmica do problema é descrita usando uma abordagem Euleriana, que resulta em condução de calor por convecção e condução no sólido. A parte mecânica (elasticidade) é descrita pela tradicional abordagem Lagrangeana. Um procedimento de solução termomecânica aninhada (staggered) é implementado e resultados promissores são obtidos.

No description available.

3D printing technology, which has its origins in rapid prototyping, is increasingly used to build functional devices. Although 3D printing technology has been well developed for thermoplastic polymers and metals, it is still in the research phase for soft polymeric materials such as silicones. Silicones are an industrially vital polymer characterized by a broad spectrum of chemical and physical properties for several smart applications, including on skin printing, smart sensors, multigradient material, and soft actuators. Extrusion-based multimaterial printing is one of the 3D printing techniques that have been adapted due to its compatibility to process silicone-based materials for constructing various functional devices. However, there are several challenges such as achieving on the fly mixing at low Reynolds numbers regime, achieving fast switching while using Newtonian/non-Newtonian inks, and achieving multimaterial printing on nonplanar surfaces. The development of suitable and robust printheads that are able to tackle those challenges can expand the application of this technology to a wide range of fields. In this thesis, several deposition-controlled printhead designs have been created for 3D printing multifunctional devices using an understanding of microfluidics. The established printhead can be controlled to formulate different multigradient structures through on the fly mixing during the material printing. Moreover, the developed printhead can be adapted to print multi viscous inks with high switching rates up to 50 Hz. Through the developed system, the printhead was able to track topologies in real-time, allowing objects to be printed over complex substrates. These new capabilities were applied to fabricate functional structures in order to demonstrate the potential of the developed printhead approaches that can be used in various applications, including smart sensors, soft robotics and multigradient objects. / Thesis / Doctor of Philosophy (PhD) / 3D printing techniques, such as extrusion-based multimaterial printing, have recently been utilized to process silicones due to their versatility in different smart applications, including multigradient material and soft actuators. Although it represents significant progress, there are still several challenges, including the proper mixing during printing with a laminar flow regime, the fast switching between different inks, and the printing over complex topographies. Therefore, various printhead designs have been developed in this thesis to tackle these challenges. In particular, a mixer printhead has been designed to allow mixing during printing for building multigradient objects. Also, a scalable printhead has been developed to allow fast switching for creating pixelated structures. Finally, a simple mechanical system has achieved multimaterial printing over various nonplanar surfaces. To the best of the author's knowledge, the developed printheads can be used in many fields, such as soft robotics and smart devices.

O objetivo precípuo deste estudo é a implementação computacional de modelos constitutivos elásticos e elastoplásticos para materiais com gradação funcional em regime de grandes deslocamentos e elevadas deformações. Para simular numericamente um problema estrutural, são empregados aqui elementos finitos sólidos (tetraédrico e hexaédrico) com ordem de aproximação polinomial qualquer. Grandezas da Mecânica Não Linear do Contínuo, como deformação e tensão, são utilizadas na formulação deste estudo. Para reproduzir os grandes deslocamentos, é empregada a análise não linear geométrica. A descrição adotada aqui é a Lagrangiana total, e o equilíbrio da estrutura é expresso pelo Princípio da Mínima Energia Potencial Total. Com relação à resposta elástica do material, são usadas leis constitutivas hiperelásticas, nas quais a relação tensão-deformação é obtida a partir de um potencial escalar. O comportamento elastoplástico do material é definido pela decomposição da deformação nas parcelas elástica e plástica, pelo critério de plastificação de von-Mises, pela lei de fluxo associativa, pelas condições de consistência e de complementaridade, pelo parâmetro de encruamento isotrópico e pelo tensor das tensões inversas, relacionado ao encruamento cinemático. Duas formulações elastoplásticas são utilizadas aqui: a de Green-Naghdi, na qual a deformação é decomposta de forma aditiva; e a hiperelastoplástica, em que o gradiente é decomposto de forma multiplicativa. É empregado também o conceito de material com gradação funcional (GF), a qual é definida como a variação gradual (contínua e suave) das propriedades constitutivas do material. A solução numérica do equilíbrio de forças é feita via método iterativo de Newton-Raphson. Para satisfazer o critério de plastificação, são utilizadas as estratégias de previsão elástica, e de correção plástica via algoritmos de retorno. Basicamente foram desenvolvidos cinco programas computacionais: o gerador automático das funções de forma; o gerador de malhas de elementos finitos sólidos; o código para análise de materiais em regime elástico; o código para análise de materiais em regime elastoplástico; e o programa de pós-processamento. Além desses, o aluno teve contato com os programas EPIM3D e DD3IMP ao longo de seu estágio de doutorado na Universidade de Coimbra (Portugal). Os programas EPIM3D e DD3IMP são empregados para analisar, respectivamente, materiais em regime elastoplástico, e processos de conformação de metais. Para o problema da barra sob tração uniaxial uniforme, são descritas equações e soluções analíticas para materiais homogêneos e com GF em regime elastoplástico. Para reduzir o tempo de simulação, foi empregada a programação em paralelo. De acordo com os resultados das simulações numéricas, as principais conclusões são: o refinamento da malha de elementos finitos melhora a precisão dos resultados para materiais em regimes elástico e elastoplástico; as formulações elastoplásticas de Green-Naghdi e hiperelastoplástica parecem ser equivalentes para pequenas deformações; a formulação hiperelastoplástica é equivalente ao modelo mecânico dos programas EPIM3D e DD3IMP para materiais em regime de pequenas deformações elásticas; foram constatados ganhos significativos, em termos de tempo de simulação, com a paralelização dos códigos computacionais de análise estrutural; e os programas desenvolvidos são capazes de simular - com precisão - problemas complexos, como a membrana de Cook e o cilindro fino transversalmente tracionado. / The main objective of this study is the computational implementation of elastic and elastoplastic constitutive models for functionally graded materials in large deformation regime. In order to numerically simulate a structural problem, the finite elements used are solids (tetrahedric and hexahedric) of any order of approximation. Entities from Nonlinear Continnum Mechanics, as strain and stress, are used in the present formulation. To reproduce the finite displacements, the geometrically nonlinear analysis is employed. The description adopted here is the total Lagrangian, and the structural equilibrium is expressed by means of the Principal of Minimum Total Potential Energy. Regarding the elastic material response, hyperelastic constitutive laws are used, in which the stress-strain relation is obtained from a scalar potential. The elastoplastic material behavior is defined by the strain decomposition in the elastic and plastic parts, by the von-Mises yield criterion, by the associative flow law, by the consistency and complementarity conditions, by the isotropic hardening parameter, and by the backstress tensor, related to the kinematic hardening. Two elastoplastic formulations are used here: the Green-Naghdi one, in which the strain is additively decomposed; and the hyperelastoplasticiy, in which the gradient is multiplicatively decomposed. The concept of functionally graded (FG) material, in which the constitutive properties vary gradually (continuous and smoothly), is also used. The numerical solution of the forces equilibrium is obtained via Newton-Raphson iterative procedure. In order to satisfy the yield criterion, the strategies of elastic prediction and plastic correction (via return algorithms) are used. Basically, five computer codes have been developed: the automatic shape functions generator; the solid mesh generator; the code for analysis of materials in the elastic regime; the code for analysis of materials in the elastoplastic regime; and the post-processor. Besides these, the student had contact with the programs EPIM3D and DD3IMP during his doctoral stage in the University of Coimbra (Portugal). The programs EPIM3D and DD3IMP are employed to analyze, respectively, materials in the elastoplastic regime, and sheet-metal forming processes. For the problem of the bar under uniform uniaxial tension, equations and analytical solutions are described for homogeneous and FG materials. To reduce the simulation time, the parallel programming has been employed. According to the numerical simulation results, the main conclusions are: the results accuracy is improved with mesh refinement for materials in the elastic and elastoplastic regimes; the Green-Naghdi elastoplastic formulation and the hyperelastoplasticity appear to be equivalent for small strains; the hyperelastoplastic formulation is equivalent to the mechanical model of the programs EPIM3D and DD3IMP for materials the small elastic strains regime; simulation time reduction has been obtained with the parallelization of the computer codes for structural analysis; the developed programs are capable of simulating, precisely, complex problems, such as the Cook\'s membrane and the pulled thin cylinder.

Depuis 1984, les matériaux à gradients de fonction (FGM) permettent de former une barrière thermique et réduire les fortes discontinuités des propriétés entre deux matériaux de nature différente. Ces multi-matériaux, qui consistent en une variation intentionnelle de la composition chimique entrainant par conséquent une modification des propriétés microstructurales, chimiques, mécaniques et thermiques, permettent de lisser la distribution des contraintes thermiques. L’élaboration in situ de ces alliages sur mesure est rendu possible grâce à l’utilisation de procédés de fabrication additive tel que le procédé par dépôt de poudres DED-CLAD®. Ces procédés connaissent un essor considérable depuis les années 1980 et sont idéaux dans la fabrication de FGM. Dans le cadre de cette thèse CIFRE, des développements techniques ont été effectués pour adapter le procédé DED-CLAD® et permettre la réalisation de FGM. Grâce à plusieurs collaborations industrielles, une étude complète a été réalisée sur les alliages titane-molybdène et titane-niobium. Ces alliages permettent dans le premier cas de réaliser des pièces résistantes à de fortes sollicitations thermiques (secteur spatial), et dans le second cas d’associer les propriétés mécaniques et la biocompatibilité (secteur biomédical). L’originalité de cette thèse repose sur l’étude d’un gradient complet, c’est-à-dire que l’ajout en élément d’alliage varie de 0% à 100%. En effet, les études reportées dans la littérature ne font pas mentions des alliages titane-matériaux réfractaire pour des taux élevés en élément réfractaire. Les analyses microstructurale (DRX, structure cristallographique par EBSD, microstructure), chimique (EDS) et mécanique (microdureté, tests de traction et essais d’indentation instrumentée) ont mis en évidence une évolution des propriétés le long du gradients de composition. La caractérisation mécanique des échantillons par indentation instrumentée s’est par ailleurs révélée particulièrement pertinente dans les cas de ces multi-matériaux / Since 1984, the Functionally Graded Material (FGM) allow to create a thermal barrier and to reduce the strong discontinuities of properties between two materials of different composition. These multimaterials,whose consist of an intentional variation in the chemical composition and, consequently, modify the microstructural, chemical, mechanical and thermal properties, lead to a smooth distribution of the thermal stress. The in-situ development of these custom-made alloys is made possible by the use of additive manufacturing processes such as the DED-CLAD® powder deposition process. These processes have grown substantially since the 1980s and are optimal for the manufacture of FGM. During this industrial thesis, technical developments have been carried out to adapt the DED-CLAD® process and to allow the manufacturing of FGM. Thanks to two industrial collaborations, a full study was carried out on titanium-molybdenum and titanium-niobium alloys. These alloys make it possible, in the first case, to produce parts resistant to strong thermal stress (space sector), and in the second case to combine mechanical properties and biocompatibility (biomedical sector). The originality of this thesis rests on the study of a complete gradient, that is the addition in alloy element varied from 0% to 100%. In fact, studies reported in the literature do not mention titanium-refractory material for high levels of refractory element. Microstructural (XRD, crystallographic analysis by EBSD technique), chemical (EDS) and mechanical (microhardness, tensile test and instrumented indentation) analyses revealed an evolution of the properties along the chemical gradient. The mechanical characterization of the sample by instrumented indentation has also proved particularly relevant in the case of these multi-materials

Cette thèse est un travail prospectif sur la caractérisation multi-échelle de matériaux à gradient de propriétés générés par des traitements de surface de type mécanique (grenaillage à air comprimé ou par ultrasons) ou thermochimique (nitruration, implantation ionique, cémentation basse température). Les apports de plusieurs techniques de caractérisation (microscopie électronique à balayage, spectrométrie, indentation instrumentée, microscopie interférométrique), à différentes échelles, et l’existence possible d’une signature des traitements de surface étudiés sur le matériau ont été examinés. Une analyse multi-échelle des échantillons grenaillés par ultrasons a permis d’établir un lien entre les paramètres procédé et la rugosité du matériau. Une approche originale statistique a été proposée pour déterminer la dureté d’un matériau modifié par un traitement de surface donné sans altérer la surface par une rectification. Elle a permis d’établir un lien entre la rugosité des échantillons grenaillés par ultrasons et leur dureté. Une recherche bibliographique détaillée a été réalisée sur la simulation de l’essai d’indentation instrumentée par éléments finis en étudiant une centaine d’articles afin d’évaluer l’influence des hypothèses des modèles sur leurs résultats. A l’aide d’un modèle éléments finis, la sensibilité des courbes d’indentation à une variation des paramètres matériau a été examinée. Cela a permis de mettre en place une réflexion sur l’identification des propriétés d’un matériau à gradient à l’aide de l’essai d’indentation. / This thesis is a prospective work on the multiscale characterization of plastically graded materials obtained with mechanical (shot peening using air pressure or ultrasounds) or thermochemical (nitriding, ion implantation, low temperature carburizing) surface treatments. The benefits of several characterization techniques (scanning electron microscopy, spectrometry, instrumented indentation, interferometric microscopy), at different scales, and the possible existence of a signature of the studied surface treatments on the material were examined. A multiscale analysis of the ultrasonically shot-peened specimens linked the process conditions with the material roughness. A novel statistical approach was proposed to determine the treated material roughness without deteriorating the surface through resurfacing. The latter enabled to establish a link between the shot-peened specimen roughness and their hardness. A detailed literature review of a hundred articles examined the IV simulation of the instrumented indentation test with finite elements in order to assess the effect of different hypotheses on the simulation results. Using a finite element model, the sensitivity of indentation curves to a variation of material parameters was examined. The latter enabled to reflect on the identification of the properties of plastically grade materials using the instrumented indentation test.

The main aim of this study is to utilize a Jk-integral based computational method in order to calculate crack tip parameters for orthotropic functionally graded materials (FGMs). The crack is subjected to mixed mode thermal loading. Mixed mode thermal fracture analysis requires the calculation of mode-I and mode-II stress intensity factors (KI ,KII ). In addition to stress intensity factors, energy release rate and T-stress are calculated by means of Jk-integral. Jk-integral is defined as a line integral over a vanishingly small curve. Since it is difficult to deal with a line integral on a vanishing curve , Jk-integral is converted to a domain independent form containing area and line integrals by the help of plane thermoelasticity constitutive relations. Steady-state temperature distribution profiles in FGMs and the components of the Jk-integral are computed by means of the finite element method. In both thermal and structural analyses, finite element models that possess graded isoparametric elements are created in the general purpose finite element analysis software ANSYS. In the formulation of Jk-integral, all required engineering material properties are assumed to possess continuous spatial variations through the functionally graded medium. The numerical results are compared to the results obtained from Displacement Correlation Technique (DCT). The domain independence of Jk-integral is also demonstrated. The results obtained in this study show the effects of crack location and material property gradation profiles on stress intensity factors, energy release rate and T-stress.