Por vezes o comportamento vinculado a fadiga é a principal fonte de dúvidas em projetos de engenharia. Aqui, o crescimento de trincas por fadiga é analisado numericamente utilizando uma plataforma de elementos finitos, onde o problema da fratura é modelado de forma discreta através do uso de elementos coesivos de interface. Os casos estudados abordam estruturas que apresentam uma trinca inicial de tamanho conhecido que propaga por um caminho condicionado. Os problemas são restritos ao estado plano de deformações e sujeitos em sua grande maioria a abertura normal entre as superfícies da trinca. O processo de separação das interfaces da trinca é descrito pelo uso de dois modelos de zona coesiva irreversível. Os modelos possuem relações de tensão-separação que não seguem um caminho pré-definido, sendo dependentes da evolução do dano ligado as propriedades da zona coesiva. Inicialmente são mostradas respostas básicas uniaxiais para os elementos coesivos, que provam a existência de curvas S-N e que o acumulo de dano ocorre de forma não linear. Em seguida, analisa-se crescimento de trincas por fadiga em uma viga do tipo double cantilever beam, onde a estrutura observada é formada pela união de duas chapas metálicas através de um adesivo coesivo que direciona o crescimento da trinca Casos com carga de amplitude única são computados, resultando na representatividade da propagação por uma lei de Paris. Além disso, uma análise de fadiga transiente é realizada através de respostas geradas pela aplicação de sequências de carregamentos em blocos, que mostram a dependência entre o formato do bloco e a maneira a qual ocorre a propagação da trinca. Na sequência, observa-se o efeito escala no crescimento de trincas por fadiga em uma placa submetida a tração com comportamento quase-frágil. Obtém-se uma conexão entre a altura da placa e o formato da ruptura. Por fim, o efeito da plastificação sobre a propagação de trincas, incluindo a captura do efeito crack closure, é comprovado em uma estrutura multicamada que emprega uma malha refinada na ponta da trinca. Os resultados mostram que o modelo computacional implementado reproduz comportamentos semelhantes atingidos nos trabalhos de referência para os problemas analisados. / Sometimes fatigue-related behavior is the main source of doubt in engineering projects. Here, the fatigue crack growth is analyzed numerically using a finite element platform, where the fracture problem is discretely modeled through the use of cohesive interface elements. The cases studied deal with structures that present an initial crack of known size that propagates through a conditioned path. The problems are restricted to plane strain and are mostly subject to the normal opening between the surfaces of the crack. The process of separating the interfaces of the crack is described by the use of two models of irreversible cohesive zone. The models have stress-separation relationships that do not follow a predefined path, being dependent on the evolution of the damage linked to the properties of the cohesive zone. Initially basic uniaxial responses are shown for the cohesive elements, which prove the existence of S-N curves and that the accumulation of damage occurs non-linearly. Then, fatigue crack growth is analyzed in a double cantilever beam, where the observed structure is formed by the union of two metal plates through a cohesive adhesive that directs the growth of the crack Cases with single amplitude loading are computed, resulting in the representativeness of propagation by a Paris law. In addition, a transient fatigue analysis is performed through responses generated by the application of block loading sequences, which show the dependence between the block shape and the manner in which crack propagation occurs. In the sequence, the scale effect on the growth of fatigue cracks in a plate subjected to traction with quasi-fragile behavior is observed. A connection is obtained between the height of the plate and the rupture format. Finally, the plastification effect on crack propagation, including crack closure capture, is demonstrated in a multilayer structure that employs a refined mesh at the crack tip. The results show that the computational model implemented reproduces similar behaviors reached in the reference works for the analyzed problems.

Por vezes o comportamento vinculado a fadiga é a principal fonte de dúvidas em projetos de engenharia. Aqui, o crescimento de trincas por fadiga é analisado numericamente utilizando uma plataforma de elementos finitos, onde o problema da fratura é modelado de forma discreta através do uso de elementos coesivos de interface. Os casos estudados abordam estruturas que apresentam uma trinca inicial de tamanho conhecido que propaga por um caminho condicionado. Os problemas são restritos ao estado plano de deformações e sujeitos em sua grande maioria a abertura normal entre as superfícies da trinca. O processo de separação das interfaces da trinca é descrito pelo uso de dois modelos de zona coesiva irreversível. Os modelos possuem relações de tensão-separação que não seguem um caminho pré-definido, sendo dependentes da evolução do dano ligado as propriedades da zona coesiva. Inicialmente são mostradas respostas básicas uniaxiais para os elementos coesivos, que provam a existência de curvas S-N e que o acumulo de dano ocorre de forma não linear. Em seguida, analisa-se crescimento de trincas por fadiga em uma viga do tipo double cantilever beam, onde a estrutura observada é formada pela união de duas chapas metálicas através de um adesivo coesivo que direciona o crescimento da trinca Casos com carga de amplitude única são computados, resultando na representatividade da propagação por uma lei de Paris. Além disso, uma análise de fadiga transiente é realizada através de respostas geradas pela aplicação de sequências de carregamentos em blocos, que mostram a dependência entre o formato do bloco e a maneira a qual ocorre a propagação da trinca. Na sequência, observa-se o efeito escala no crescimento de trincas por fadiga em uma placa submetida a tração com comportamento quase-frágil. Obtém-se uma conexão entre a altura da placa e o formato da ruptura. Por fim, o efeito da plastificação sobre a propagação de trincas, incluindo a captura do efeito crack closure, é comprovado em uma estrutura multicamada que emprega uma malha refinada na ponta da trinca. Os resultados mostram que o modelo computacional implementado reproduz comportamentos semelhantes atingidos nos trabalhos de referência para os problemas analisados. / Sometimes fatigue-related behavior is the main source of doubt in engineering projects. Here, the fatigue crack growth is analyzed numerically using a finite element platform, where the fracture problem is discretely modeled through the use of cohesive interface elements. The cases studied deal with structures that present an initial crack of known size that propagates through a conditioned path. The problems are restricted to plane strain and are mostly subject to the normal opening between the surfaces of the crack. The process of separating the interfaces of the crack is described by the use of two models of irreversible cohesive zone. The models have stress-separation relationships that do not follow a predefined path, being dependent on the evolution of the damage linked to the properties of the cohesive zone. Initially basic uniaxial responses are shown for the cohesive elements, which prove the existence of S-N curves and that the accumulation of damage occurs non-linearly. Then, fatigue crack growth is analyzed in a double cantilever beam, where the observed structure is formed by the union of two metal plates through a cohesive adhesive that directs the growth of the crack Cases with single amplitude loading are computed, resulting in the representativeness of propagation by a Paris law. In addition, a transient fatigue analysis is performed through responses generated by the application of block loading sequences, which show the dependence between the block shape and the manner in which crack propagation occurs. In the sequence, the scale effect on the growth of fatigue cracks in a plate subjected to traction with quasi-fragile behavior is observed. A connection is obtained between the height of the plate and the rupture format. Finally, the plastification effect on crack propagation, including crack closure capture, is demonstrated in a multilayer structure that employs a refined mesh at the crack tip. The results show that the computational model implemented reproduces similar behaviors reached in the reference works for the analyzed problems.

No trabalho estuda-se o crescimento de trincas em carga monotônica e cíclica nos casos de materiais quase-frágeis, introduzindo uma lei de dano cíclico. Revisam-se conceitos sobre modelos coesivos, leis de carga-descarga, leis de evolução de dano e efeito de escala. É seguido o modelo coesivo irreversível proposto por Wang e Siegmund (2006). Em particular se dá ênfase aos efeitos de escala não estatísticos. O modelo de zona coesiva irreversível apresenta uma formulação de dano e considera carregamento em fadiga. Quando o tamanho estrutural é reduzido (ou as trinca se extendem), a fratura por fadiga não mais ocorre por propagação de trinca, mas sim por uma decoesão uniforme. O objetivo desde trabalho é implementar este modelo e verificar sua potencialidade na captura de efeitos de escala, comparando com experimentos e dados disponíveis na literatura. / At present work is intended to study crack growth in cyclic and monotonic loading in the case of quasi-brittle materials, introducing a damage mechanism, is reviewed concepts of cohesive models, loading-unloading laws, damage evolution laws and effect of scale. The irreversible cohesive zone model proposed by Wang e Siegmund (2006) is followed. In particular emphasizes in the not statistical size effects. The irreversible cohesive zone model, presents a damage formulation and considers fatigue loading. It is demonstrated in this study that, when the structure size is reduced (or extend cracks), the fatigue fracture no longer occurs by crack propagation, then occurs by uniform decohesion . The objetive of this work is implementing this model and verify its capability to capture the scale effect compared with experiments and data available in literature.

Por vezes o comportamento vinculado a fadiga é a principal fonte de dúvidas em projetos de engenharia. Aqui, o crescimento de trincas por fadiga é analisado numericamente utilizando uma plataforma de elementos finitos, onde o problema da fratura é modelado de forma discreta através do uso de elementos coesivos de interface. Os casos estudados abordam estruturas que apresentam uma trinca inicial de tamanho conhecido que propaga por um caminho condicionado. Os problemas são restritos ao estado plano de deformações e sujeitos em sua grande maioria a abertura normal entre as superfícies da trinca. O processo de separação das interfaces da trinca é descrito pelo uso de dois modelos de zona coesiva irreversível. Os modelos possuem relações de tensão-separação que não seguem um caminho pré-definido, sendo dependentes da evolução do dano ligado as propriedades da zona coesiva. Inicialmente são mostradas respostas básicas uniaxiais para os elementos coesivos, que provam a existência de curvas S-N e que o acumulo de dano ocorre de forma não linear. Em seguida, analisa-se crescimento de trincas por fadiga em uma viga do tipo double cantilever beam, onde a estrutura observada é formada pela união de duas chapas metálicas através de um adesivo coesivo que direciona o crescimento da trinca Casos com carga de amplitude única são computados, resultando na representatividade da propagação por uma lei de Paris. Além disso, uma análise de fadiga transiente é realizada através de respostas geradas pela aplicação de sequências de carregamentos em blocos, que mostram a dependência entre o formato do bloco e a maneira a qual ocorre a propagação da trinca. Na sequência, observa-se o efeito escala no crescimento de trincas por fadiga em uma placa submetida a tração com comportamento quase-frágil. Obtém-se uma conexão entre a altura da placa e o formato da ruptura. Por fim, o efeito da plastificação sobre a propagação de trincas, incluindo a captura do efeito crack closure, é comprovado em uma estrutura multicamada que emprega uma malha refinada na ponta da trinca. Os resultados mostram que o modelo computacional implementado reproduz comportamentos semelhantes atingidos nos trabalhos de referência para os problemas analisados. / Sometimes fatigue-related behavior is the main source of doubt in engineering projects. Here, the fatigue crack growth is analyzed numerically using a finite element platform, where the fracture problem is discretely modeled through the use of cohesive interface elements. The cases studied deal with structures that present an initial crack of known size that propagates through a conditioned path. The problems are restricted to plane strain and are mostly subject to the normal opening between the surfaces of the crack. The process of separating the interfaces of the crack is described by the use of two models of irreversible cohesive zone. The models have stress-separation relationships that do not follow a predefined path, being dependent on the evolution of the damage linked to the properties of the cohesive zone. Initially basic uniaxial responses are shown for the cohesive elements, which prove the existence of S-N curves and that the accumulation of damage occurs non-linearly. Then, fatigue crack growth is analyzed in a double cantilever beam, where the observed structure is formed by the union of two metal plates through a cohesive adhesive that directs the growth of the crack Cases with single amplitude loading are computed, resulting in the representativeness of propagation by a Paris law. In addition, a transient fatigue analysis is performed through responses generated by the application of block loading sequences, which show the dependence between the block shape and the manner in which crack propagation occurs. In the sequence, the scale effect on the growth of fatigue cracks in a plate subjected to traction with quasi-fragile behavior is observed. A connection is obtained between the height of the plate and the rupture format. Finally, the plastification effect on crack propagation, including crack closure capture, is demonstrated in a multilayer structure that employs a refined mesh at the crack tip. The results show that the computational model implemented reproduces similar behaviors reached in the reference works for the analyzed problems.

The use of composite materials brought a tremendous breakthrough in the scientific world of aerospace engineering. The lack of understanding of the failure of composite materials can be disastrous. Composite laminated structures need to be thoroughly studied and investigated in the design stage. In this thesis, formed-hat skin stringer made of composite laminates is investigated. Delamination is the most common failure of laminated composites, which has two stages delamination onset and delamination propagation. In the preliminary design phase, firstly the structures need to be investigated for low-velocity impact to check the formation of damage onset due to the impact that may arise during manufacturing. In the detailed design phase, the structure is investigated to study the evolution of delamination growth under loading conditions. The structure is modeled using 3 D elements because of the presence of Interlaminar stresses in the width and thickness direction and anisotropic nature. In this thesis, more emphasis is given on the interface between the skin and the stringer. The debonding effect of the interface is studied using cohesive zone model(CZM).

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The Two Element in Series (TEIS) and Two Zone models stand out as powerful tools that enable deeper understanding of compressor stage designs after they have been tested. The insights gained from these investigations have aided in improving new stage designs. Up to now, it has only been possible to use the TEIS and Two Zone models for analysis of test data due to the inability to predict the four required input parameters for untested machines. Empirical models for the TEIS and Two Zone model input parameters, ETAa5, ETAb5, Chi5, and DELTA5p, for two different types of vaned diffusers, channel and cascade, are proposed. These models were developed with frozen impeller modeling. This is the first time that modeling the TEIS and Two Zone input parameters has been attempted for vaned diffusers and impeller-diffuser coupling was not considered in this initial investigation. The centrifugal compressor experimental data used in the model building was obtained from Concepts NREC, an industry sponsor. Each dataset provided was evaluated for quality and reliability and only the data deemed reliable were used in the model building databases. The empirical models presented are built solely on this higher quality data. Seven models are proposed for use in predicting the TEIS and Two Zone model input parameters ETAa5, ETAb5, Chi5, and DELTA5p. Models for ETAa5, ETAb5, and DELTA5p are specific to the type of vane present in the diffuser, while the model for Chi5 is common to both diffuser types. These are the first models ever built for the TEIS and Two Zone model inputs applied to channel and cascade diffusers and become a benchmark for future studies. The work with these models is not complete, however. The databases are not of a size that data could be withheld from empirical model building for the express purpose of validation. Instead the model performance is evaluated by applying all of the models, simultaneously, to the database from which they were built. The determination of the effectiveness of the combined modeling is based on the average error across the entire speedline. The models proved to be effective and a contributing step to employing such models for use in future compressor design.

The polymer laminated sheet metal (PLSM) is a layered material which involves a sheet metal substrate, a thin polymer film and an adhesive layer between the film and the substrate. The adhesion properties between the bonded materials are among the most important issues in PLSM forming operations. In this thesis, the main focus has been devoted to characterizing and improving the adhesion properties of the PLSM system for forming applications. Metallic surface roughness evolution and residual stress development in polymer adherends are two consequences of the plastic deformation of the PLSMs. In chapter 2, the effect of these factors on interfacial adhesion strength between metallic substrate and polymer adherend (polymer film with a thin uniform pressure-sensitive adhesive layer on one side) is investigated by devising a new experimental methodology. This methodology is based on two different protocols for preparation of peel sample, one involving pre-straining in uniaxial tension of the metallic substrate prior to lamination and the other involving post-lamination pre-straining of the PLSM. In chapter 3, the peel test results of two different types of PLSMs at different peel speeds are analyzed with two different approaches common in cohesive zone modeling in the literature, namely linear elastic stiffness approach and critical maximum stress approach. The modeling results revealed the significance of the peel speed in determining the interface strength between the adhesive and metallic substrate. In chapter 4, two mechanical treatment techniques of grinding and knurling are implemented to alter the metallic substrate surface roughness before lamination. Peel strength of these samples are investigated at different peel speeds and at different peel loading directions with respect to the grinding and knurling directions. / Thesis / Doctor of Philosophy (PhD) / The polymer laminated sheet metal (PLSM) is a layered material which involves a sheet metal substrate, a thin polymer film and an adhesive layer between the film and the substrate. In this thesis, the main focus has been devoted to characterizing and improving the adhesion properties of the PLSM system for forming applications. A new experimental methodology has been devised for analyzing the effects of deformation-induced surface roughness of metallic substrate and deformation-induced residual stress in polymer adherends on interfacial peel properties of PLSMs. A novel interpretation of the results obtained from rate-independent cohesive zone modeling of peel test has revealed the significance of peel speed in determining the interface strength between the adhesive and the metallic substrate. In another part of this thesis, the effects of two substrate surface alteration techniques, grinding and knurling, on peel properties of PLSMs have been studied.

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