Ce travail vise à améliorer la compréhension et la modélisation de la rupture des matériaux métalliques à gradient de propriétés. L'application se fait sur des pignons en acier 20MnB5 carbonitrurés insérés dans un "recliner", mécanisme de sécurité des siègesautomobiles. Le traitement de carbonitruration consisteà enrichir en carbone et azote une couche externe despièces en les chauffant dans le domaine austénitiquedans une atmosphère riche en ces deux éléments. Puisles pièces sont trempées afin de déclencher unetransformation martensitique. On obtient ainsi unmatériau à gradient de propriétés, intéressant pour despièces de transmission de puissance comme lesengrenages. Ce projet a commencé par l’analyse de larupture de mécanismes de sièges en test industriel. Celaa confirmé le double comportement à rupture binaire dumatériau : fragile sur une couche externe, ductile àl’intérieur. Un banc d’essai, spécialement conçu pour ceprojet, soumet une dent à un effort latéral jusqu’àrupture complète. Des observations in situ sonteffectuées et la courbe force-déplacement estenregistrée, montrant la diversité de comportement enfonction de la profondeur d'engagement des dents et dela présence ou non de la couche carbonitrurée. Desessais de traction, de flexion 4 points et de cisaillementsur éprouvettes papillons sont utilisés pour mesurer lespropriétés plastiques et calibrer les critères de rupturede la couche carbonitrurée comme de l'acier de base. Laplasticité de Von Mises avec une loi d'écrouissagesimple rend très bien compte de tous ces essaismécaniques. Différents critères de rupture ductile issusde la littérature sont calibrés; ils ne parviennent pas àreprésenter correctement tous les essais réalisés.Un critère plus adapté est donc proposé enconclusion de cette campagne expérimentale. Lasimulation de la rupture dans LS-Dyna est réaliséeavec une technique d'érosion d'éléments dont leslimitations sont discutées. La comparaison avec larupture de dent expérimentale permet d'évaluer lescritères numériques identifiés et d'analyser leslimites actuelles de la simulation, en particulier lanécessité de prendre en compte plus finement àl'avenir le gradient de propriétés mécaniques ainsique les contraintes résiduelles de compression dela couche carbonitrurée. / This work aims at a better understanding and modeling of the failure of gradient metallic materials. It is applied to carbonitrided pinions made out of 20MnB5 steel, inserted in a "recliner", a safety mechanism of automotive seats. Carbonitriding induces high surface hardness while preserving significant core ductility. The experimental analysis of the fracture behavior of seat recliners in an industrial test confirmed the dual failure behavior of the component : brittle external layer, ductile core material. A test bench has been specifically designed for the project: one tooth is submitted to a lateral force until complete failure. In situ observations are performed and the load-displacement curve recorded, showing a variety of behaviors as a function of the teeth engagement depth and of the presence or not of the carbonitrided layer. Experimental tests with various tress states were conducted to measure plastic properties as well as to calibrate fracture criteria, for the carbonitrided layer and for the core steel. Von Mises plasticity and a simple strain hardening curve fit very well all these experiments. As fracture criteria from the literature were unable to predict failure correctly for all the mechanical tests, an adapted criterion has therefore been proposed as an outcome of this extensive mechanical testing campaign. Fracture simulation in LS Dyna has been performed using the element erosion technique, the limitations of which are discussed. Comparison with the experimental tooth fracture measurements allows evaluation of the proposed failure criteria, and enables to stress out and discuss the present limits of the simulation, concluding that it will be necessary in future work to account more finely for the mechanical property gradient together with the compressive residual stresses in the carbonitrided layer.

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2017-12-12T13:03:18Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Eduardo Pagano.pdf: 5564139 bytes, checksum: 41a00843092c58a3dcd62662b42000f1 (MD5) / Made available in DSpace on 2017-12-12T13:03:18Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Eduardo Pagano.pdf: 5564139 bytes, checksum: 41a00843092c58a3dcd62662b42000f1 (MD5) Previous issue date: 2017-08-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste estudo foi realizado a obtenção e caracterização de materiais cerâmicos de alumina com gradiente funcional de porosidade. Foram utilizados como os métodos de processamento a rota de freeze casting em água e a fase de sacrifício utilizando o amido. Os materiais utilizados para a produção das amostras foram a alumina e o amido de milho de pureza analítica. Foram investigadas as características de porosidade quanto a morfologia de poros, distribuição dos poros pela matriz e interferência do processamento na formação destes. Para o estudo da porosidade, os métodos de porosimetria via método de Arquimedes em água e penetração de mercúrio foram aplicados na analise desta porosidade obtida. As propriedades mecânicas, bem como o modo de fratura obtido, também foram investigadas em ensaio de compressão uniaxial em uma máquina de ensaios universal. As imagens obtidas por microscopia eletrônica de varredura auxiliaram a correlacionar os dados obtidos nos ensaios de porosimetrias com a resposta mecânica obtida do material. Os resultados obtidos entre os métodos, quanto à porosidade, revelam diferenças consideráveis na distribuição e quantidade de poros ao se comparar o freeze casting com a fase de sacrifício com amido. Porém quanto a tamanho, os valores se mantiveram bem próximos. A resistência mecânica em compressão é maior nas amostras com amido, porém estas amostras falham de forma catastrófica. As amostras de freeze casting, por sua vez, possuem menor resistência mecânica e tiveram suas fraturas influenciadas por vários fatores durante o processamento. A morfologia de poros e microestrutura apresentada pelos dois métodos aplicados são consequência direta da rota de processamento utilizada. Nas amostras por freeze casting, foram observados canais de poros. Já as amostras com amido como fase de sacrifício, os poros são arredondados e não conectados / In this study, preparation and characterization of alumina ceramic materials with functional porosity gradients were performed. Freeze casting of alumina slurries, using water as the chosen solvent, and sacrificial phase method using corn starch as sacrificial phase, were implemented. The materials used to produce the samples were alumina and analytical purity corn starch. Porosity characteristics were investigated concerning pore morphology, pore distribution in the ceramic matrix and processing route interference in pore formation. The evaluation of porosity was made utilizing porosity determination methods as mercury intrusion and Archimedes method with water as immersion liquid. The mechanical properties, as well as the fracture mode, were investigated during uniaxial compressive test in a universal testing machine. Using the images obtained by scanning electron microscopy (SEM), it was possible to correlate the data obtained in the porosimetry analysis with the mechanical response obtained from the material. In terms of porosity, the results between the processing routes reveal considerable differences in pore distribution and quantity of pores. However, the pore sizes obtained for both methods were pretty close to each other. The mechanical strength during compressive stress was higher in the samples produced via sacrificial phase method, but those samples failed catastrophically. Freeze casting samples, on the other hand, had lower mechanical resistance and their fractures were directly influenced by several factors during their assembly processing. The pore and microstructure morphology presented by both methods were direct consequence of the processing route used. In the samples produced by freeze casting, pore channels were observed. The samples produced via sacrificial phased method using corn starch presented unconnected rounded pores.

Cerâmicas piezelétricas possibilitam posicionamento e sensoriamento de precisão ou captação de energia mecânica valendo-se do efeito piezelétrico, capaz de converter energia mecânica em elétrica ou o contrário. Para aprimorar ou estender as aplicações dessas cerâmicas, mecanismos flexíveis podem ser acoplados a elas, formando um Dispositivo Piezelétrico Flextensional (DPF). No projeto desse tipo de estrutura, o conceito de Material com Gradação Funcional (MGF) é interessante, já que esses materiais apresentam variações graduais de suas propriedades efetivas, permitindo a alternância entre um material mais flexível e um mais rígido de acordo com a intensidade de deslocamento desejada em cada região da estrutura. Assim, neste trabalho, implementa-se o Método de Otimização Topológica (MOT) no projeto de estruturas gradadas com o intuito de identificar as vantagens e desvantagens da utilização do conceito de MGF em DPF. Esse método combina algoritmos de otimização e o Métodos dos Elementos Finitos (MEF) para distribuir material dentro de um domínio fixo através de um modelo de material, que no presente caso é o de Material Isotrópico Sólido com Penalização (MISP) adaptado a MGF. Na fabricação desses dispositivos otimizados, utiliza-se a Sinterização por Jato de Plasma (SJP) para a obtenção de tarugos gradados que são submetidos a processos de eletro-erosão e de corte a laser. Por fim, para a verificação dos resultados numéricos, utiliza-se um vibrômetro para aferir os deslocamentos dos protótipos de atuadores fabricados. / Piezoelectric devices enable precision positioning and sensing or mechanical energy harvesting based on the piezoelectric effect. In flextensional piezoelectric devices, flexible coupling structures are attached to ceramics to improve or extend the application possibilities. On the design of this kind of structure, the concept of Functionally Graded Materials (FGM) can be interesting, since it allows gradual variations of its effective properties along some direction by mixing two or more materials. Thus, in order to identify the advantages and disadvantages of using FGM, graded flexible coupling structures that maximize the performance of piezoelectric devices are obtained by implementing the Topology Optimization Method (TOM). This method combines optimization algorithms and the Finite Element Method (FEM) to distribute material inside a fixed domain. In this work, the formulation is based on the Solid Isotropic Material with Penalization (SIMP) material model adapted for the FGM concept, which can represent continuous change in material properties along the domain. Resulting optimal graded topologies of coupling structures are presented and compared with homogeneous structures. Finally, graded devices are manufactured through Spark Plasma Sintering (SPS) technique in order to be characterized, validating numerical results. The numerical results demonstrate the TOM efficacy in designing functionally graded piezoelectric devices and show, by its implementation, significant gains in graded mechanisms performance when compared with analogous homogeneous. Furthermore, the feasibility of proposed manufacturing process is confirmed, allowing the fabrication of prototypes with expected behavior.

Functionally graded materials (FGMs) are composites with continuous material property variations. The distinct interfaces between the reinforcement and the matrix in classical composites are potential damage initiation sites. The concept of FGM aims at avoiding the material mismatch at the interfaces. Functionally graded materials originated from the need for a material that has high-toughness at very high operating temperatures that occur in rocket nozzles and aeroplane engines. One of the early applications of graded materials can be thus found in thermal barrier coatings of gas turbine blades. Recent applications of FGMs include optoelectronics, ballistic impact resistance structures, wear resistant coatings and others. Although the manufacturing and applications of FGMs are well developed the basic mechanics of failure is not well understood, which is important in developing engineering design methodologies. Modern day design practice uses the concepts of fracture mechanics and the fracture properties of graded materials is not well understood. Most studies in the literature have assumed that the material response of the bulk functionally graded material to be elastic even though the constituents are nominally ductile. Some asymptotic analysis available in the literature have described the effect of ductility on the fracture parameters. However, these analysis are not complete in the sense that they have some undetermined constants. The present thesis aims at performing whole-field finite element (FE) simulations of a crack embedded in a ductile functionally graded material subjected to an anti-plane shear (mode-3) loading. A J2-deformation theory based power-law hardening nonlinear material response is assumed. The material property variation is assumed to be in the radial-direction (r-FGM), tangential to the crack (x-FGM), normal to the crack plane (y-FGM) and also at an arbitrary angle to the crack-plane (xy-FGM). Yet another power law described the material property variation. The competition between the indices of the hardening and material property variation is understood by performing a parametric analysis by varying both systematically. Our results indicate that the first most singular term of the asymptotic series remains unaffected. For some values of the material property variation index, the second asymptotic term is affected. The semi-closed form solutions available in the literature were unable to decipher the relative range of dominance of the first and second terms. From the present whole-field FEM analysis were able to extract this relative range of dominance. Our results indicate the range of dominance of the first term is least for FGMs when the material property variation is in the direction to the crack (x-FGM), and it is more for y-FGM.

[pt] Cascas cilíndricas são usadas em muitas aplicações de engenharia e, devido a sua forma e capacidade de transporte de carga, são bastante usadas na indústria aeroespacial e em estruturas civis. Elas minimizam a quantidade de material do qual são fabricadas, tornando-se assim estruturas muito leves e esbeltas. Em décadas recentes tem se procurado criar novos materiais que conjuguem múltiplas propriedades como maior resistência, melhor proteção térmica, proteção contra corrosão e adequado nível de amortecimento, dentre outras. Uma classe de materiais que podem atender simultaneamente várias destas exigências é o chamado material com gradação funcional, onde as propriedades do material variam de forma contínua em uma ou mais direções. Materiais com gradação funcional são particularmente indicados para a construção de cascas. Como a maioria destas estruturas estão sujeitas a cargas dinâmicas, torna-se importante o estudo do comportamento dinâmico de cascas fabricadas com materiais com gradação funcional. O objetivo deste trabalho é estudar as vibrações não lineares de cascas cilíndricas esbeltas com gradação funcional. Para isto utiliza-se a teoria não linear de cascas de Sanders, considerada uma das teorias mais precisas para a análise de cascas esbeltas. Inicialmente, derivam-se as equações de movimento considerando um estado de tensões iniciais. Usando as equações linearizadas, obtêm-se às frequências naturais e as cargas críticas, sendo estes resultados comparados favoravelmente com resultados encontrados na literatura para materiais homogêneos e com gradação funcional. A seguir, usando uma expansão modal que atende as condições de contorno e continuidade, além de expressar os acoplamentos modais característicos de cascas cilíndricas no regime não linear, as equações de movimento são discretizadas usando-se o método de Galerkin. As equações algébricas resultantes são resolvidas pelo método de Newton-Raphson, sendo assim obtida a relação não linear frequência-amplitude. Finalmente, realiza-se uma análise paramétrica para estudar a influência da geometria da casca, da gradação do material funcional e dos modos de vibração no grau e tipo de não linearidade da casca cilíndrica, sendo esta a principal contribuição deste trabalho de pesquisa. / [en] Cylindrical shells are used in many engineering applications and, due to its shape and load carrying capacity, are frequently used in aerospace and civil structures. They minimize the amount of material from which they are manufactured, thus making it a very lightweight and slender structure. In recent decades, there has been a search for new materials that combine multiple properties such as increased strength, better thermal protection, corrosion protection and appropriate damping level, among others. A material that can meet several of these requirements simultaneously is the so called functionally graded material, where the material properties vary continuously in one or more directions. Functionally graded materials are particularly suitable for the construction of shells. As most of these structures are subjected to dynamic loads, it is important to study the dynamic behavior of shells made of functionally graded materials. The objective of this work is to study the nonlinear vibrations of slender functionally graded cylindrical shells. For this, the Sanders non-linear shell theory, which is considered one of the most precise theories for the analysis of slender shells, is adopted. Initially, the equations of motion are derived considering an initial stress state. Using the linearized equations of motion, the natural frequencies and critical loads are obtained. These results compare favorably with results reported in the literature for homogeneous and functionally graded shells. Then, using a modal expansion that satisfies the boundary and continuity conditions and expresses the modal couplings characteristic of cylindrical shells in the nonlinear regime, the equations of motion are discretized using the Galerkin method. The resulting algebraic equations are solved by the Newton-Raphson method, thus obtaining the nonlinear frequency-amplitude relation. Finally, a parametric analysis is conducted to study the influence of the geometry of the shell, the gradient of the functional material and vibration modes on the degree and type of nonlinearity of the cylindrical shell, which is the main contribution of this research work.

Cerâmicas piezelétricas possibilitam posicionamento e sensoriamento de precisão ou captação de energia mecânica valendo-se do efeito piezelétrico, capaz de converter energia mecânica em elétrica ou o contrário. Para aprimorar ou estender as aplicações dessas cerâmicas, mecanismos flexíveis podem ser acoplados a elas, formando um Dispositivo Piezelétrico Flextensional (DPF). No projeto desse tipo de estrutura, o conceito de Material com Gradação Funcional (MGF) é interessante, já que esses materiais apresentam variações graduais de suas propriedades efetivas, permitindo a alternância entre um material mais flexível e um mais rígido de acordo com a intensidade de deslocamento desejada em cada região da estrutura. Assim, neste trabalho, implementa-se o Método de Otimização Topológica (MOT) no projeto de estruturas gradadas com o intuito de identificar as vantagens e desvantagens da utilização do conceito de MGF em DPF. Esse método combina algoritmos de otimização e o Métodos dos Elementos Finitos (MEF) para distribuir material dentro de um domínio fixo através de um modelo de material, que no presente caso é o de Material Isotrópico Sólido com Penalização (MISP) adaptado a MGF. Na fabricação desses dispositivos otimizados, utiliza-se a Sinterização por Jato de Plasma (SJP) para a obtenção de tarugos gradados que são submetidos a processos de eletro-erosão e de corte a laser. Por fim, para a verificação dos resultados numéricos, utiliza-se um vibrômetro para aferir os deslocamentos dos protótipos de atuadores fabricados. / Piezoelectric devices enable precision positioning and sensing or mechanical energy harvesting based on the piezoelectric effect. In flextensional piezoelectric devices, flexible coupling structures are attached to ceramics to improve or extend the application possibilities. On the design of this kind of structure, the concept of Functionally Graded Materials (FGM) can be interesting, since it allows gradual variations of its effective properties along some direction by mixing two or more materials. Thus, in order to identify the advantages and disadvantages of using FGM, graded flexible coupling structures that maximize the performance of piezoelectric devices are obtained by implementing the Topology Optimization Method (TOM). This method combines optimization algorithms and the Finite Element Method (FEM) to distribute material inside a fixed domain. In this work, the formulation is based on the Solid Isotropic Material with Penalization (SIMP) material model adapted for the FGM concept, which can represent continuous change in material properties along the domain. Resulting optimal graded topologies of coupling structures are presented and compared with homogeneous structures. Finally, graded devices are manufactured through Spark Plasma Sintering (SPS) technique in order to be characterized, validating numerical results. The numerical results demonstrate the TOM efficacy in designing functionally graded piezoelectric devices and show, by its implementation, significant gains in graded mechanisms performance when compared with analogous homogeneous. Furthermore, the feasibility of proposed manufacturing process is confirmed, allowing the fabrication of prototypes with expected behavior.

Dans l'industrie du forage pétrolier, les conditions de travail de plus en plus sévères requièrent sans cesse de nouveaux outils plus résistants à l'usure abrasive et à l'impact. Afin de répondre à ce défi, les travaux présentés ici, ont pour but l'élaboration de matériaux en carbure cémenté à gradient de composition par le procédé d'imbibition réactive. Ce procédé peut être décomposé en deux procédés de gradation, à savoir : l'imbibition et le revêtement réactif. L'imbibition a pour rôle d'enrichir graduellement, en phase liante, le coeur d'une pièce en carbure cémenté dense et repose sur le principe de migration de phase liquide dans un corps solide-liquide. Le revêtement réactif est un procédé qui s'applique également sur un carbure cémenté dense sur lequel est déposé un revêtement de nitrure de bore. Après avoir atteint le liquidus de la phase liante du carbure cémenté, une précipitation de borures ternaires prend place à la surface du matériau et s'étend graduellement sur des distances millimétriques. Dans le cas de pièces industrielles (inserts tricône et supports de taillants PDC (Polycrystalline Diamond Compact)), le traitement d'imbibition réactive permet de générer des gradients de dureté pouvant atteindre 450 HV sur 25 mm. L'élaboration de tels gradients a nécessité une meilleure compréhension des cinétiques ainsi que des phénomènes mis en jeu durant l'imbibition et le revêtement réactif. Dans le cas des taillants PDC, l'influence des paramètres du procédé HPHT de synthèse de la plaquette diamantée, sur le gradient de composition présent dans le support WC-Co après imbibition réactive, a été étudiée. Ces travaux se sont également intéressés à l'effet de ces gradients de composition sur les propriétés des plaquettes diamantées ainsi obtenues. A la suite d'essais mécaniques, les inserts WC-Co et taillants PDC gradués montrent une augmentation significative de leur résistance à l'abrasion (de 30 à 100%) et de leur tenue à l'impact (de 20 à 40%). / In oil drilling industry, harsher working conditions require ever new and more abrasive wear and impact resistant tools. To meet this challenge, the work presented here, is to develop graded cemented carbide materials by reactive imbibition process. This method can be divided into two gradation processes, namely, reactive coating and imbibition. Imbibition gradually enriches the core of dense cemented carbide with binder phase and is based on the principle of liquid phase migration in a solid-liquid body. Reactive coating also takes place in dense cemented carbide on which a boron nitride coating is deposited. After reaction with the WC-Co liquid binder, some ternary boride precipitations take place from surface to several millimeters deep. For industrial parts (inserts for roller cone bits and PDC (Polycrystalline Diamond Compact) cutters substrates), hardness gradients obtained can reach 450 HV on 25 mm. Such gradients development is passed through a better understanding of kinetics and phenomena occurring during imbibition and reactive coating. In case of PDC cutters, influence of HPHT process parameters, allowing diamond table synthesis, on the WC-Co substrate gradient, generated by reactive imbibition, was studied. This work was also interested in gradient effect on the obtained diamond tables properties. After mechanical tests, graded WC-Co inserts and graded PDC cutters show a significant increase of both wear resistance (from 30 to 100%) and impact resistance (from 20 to 40%).

This thesis was written under the supervision of Assoc. Prof. Luboš Náhlík, Ph.D. and Assoc. Prof. Pavel Hutař, Ph.D. The topic of this thesis is the study of a free surface effect and gradient change of material properties on a crack behavior. The common theme of the work is a fracture mechanics description of a crack behavior near a material nonhomogeneity. Here, the material nonhomogeneity can be understood either as a boundary of a body (interface between body and surrounding) or as a continuous change of material properties. The thesis is introduced by a review part where the state-of-the-art of the concerned topic is described. This part presents several stress state descriptions in the vicinity of some general singular stress concentrators, particularly a crack and a V-notch. The influence of the free surface on a fatigue crack front shape is discussed here. The review part follows with the fracture mechanics description of the graded materials. The problem formulation and the main aims of the thesis are stated in the following chapter. The core of this work is the fracture mechanical description of a crack propagating near the material nonhomogeneity. The thesis focuses on a stress field description near the free surface of the body where a change in a type of the singular stress field occurs. Methods used in generalized fracture mechanics are applied here to describe the stress field near the free surface. The difference between crack behavior in thin-walled and thick-walled structures is shown and supplied by relevant examples. Methods and procedures used are later utilized for estimation of a crack behavior in graded structures. The thesis is concluded by the discussion of obtained results in appropriate context.

No description available.

With a great need for new and better cancer treatments, microwaves and gold nanoparticles (GNPs) are increasingly being suggested to be used in radiotherapy. In thisreport, the authors present an investigation of how human tissue,both malignant and healthy, behaves under microwave radiation.Specifically, the focus has been on gradual transitions between healthy and GNP-treated cancerous tissue inside a waveguide.Here, the electromagnetic material property of concern is therelative permittivity. The permittivities of human tissues andGNPs have been modelled using well known analytical models.Furthermore, numerical simulations have been performed usingCOMSOL Multiphysics and the results have been compared to analytical equations suggested to describe the energy absorption in the material. The study shows that the analytical equations are in agreement with the numerical simulations for lower propagating electromagnetic (EM) modes. Some possible electromagnetic resonance has been seen within the GNP treated cancer tissue. Furthermore, the extensive analytical models and numerical software tools created will be of importance for future research of the feasibility of this cancer treatment method. / Med ett stort behov av nya, bättre cancer-behandlingar föreslås mikrovågsstrålning och guldnanopartiklar (GNPs) att användas i strålterapi. I denna rapport presenterar författarna en undersökning av hur mänsklig vävnad beter sig under mikrovågsstrålning. Specifikt har fokus varit på gradvisa övergångar mellan frisk och GNP fylld cancervävnad i en vågledare. Här är den elektromagnetiska egenskapen av intresse den relativa permittiviteten. Permittiviteterna av mänsklig vävnad och GNPs har modellerats med välkända analytiska modeller. Vidare har numeriska simuleringar utförts i COMSOL Multiphysics och resultaten av dessa har jämförts med de analytiska ekvationerna som sägs beskriva absorbtion av energi inuti materialet. Undersökningen visar att de analytiska ekvationerna stämmer överens med de numeriska simuleringarna för lägre propagerande elektromagnetiska (EM) moder. Möjlig elektromagnetisk resonans har setts inom den GNP fyllda cancervävnaden. De omfattande analytiska modellerna och numeriska mjukvaruverktygen som tagits fram kommer vara viktiga för framtida forskning på denna cancerbehandling. / Kandidatexjobb i elektroteknik 2022, KTH, Stockholm