In order to characterize mechanical components used in high responsibility applications, the knowledge of chemical composition and results from regular experiments such as traction, impact and hardness tests is important, but not sufficient. They cannot supply the necessary information that permit anticipating, in a reliable way, the components behavior in actual working conditions. As an example, there are engines cylinder head submitted to mechanical and thermal tensions that are relatively high during the in service use, and very high in same very demanding condition. During long run times and any failure in the cooling and/or lubrication the temperature may reach 300ºC. This temperature variation causes thermal shocks which may generate cracks and/or a wide ranging of plastic deformation in regions close to the pistons. Even not considering the thermal shock effects caused by failure, even so, a short number of start-up and shutdown cycles of engine, are considered the main cause of small cracks. This indicates that the generation of cracks in cylinder head may be considered as low cycle thermomechanical fatigue problem. Another problem is the microstructure heterogeneity in the component due to the casting process, leading to different physical and mechanical properties in the same piece. Besides the presence of porosity generated by gas bubbles and voids of solidification, which may be as great as short crack, reducing the nucleation life and changing the problems focus for the fatigue crack growth. The purpose of this study was to determine the isothermal and thermomechanical fatigue property through low cycle fatigue, as well as the fatigue crack growth, relaxation, microestrutural characterization and modeling of mechanical behavior by finite element for the aluminum alloy employed in the manufacture of engine cylinder head by the national automotive industry. Isothermal fatigue experiments were carried out at temperature of 120ºC and 280ºC and the thermomechanical performed in phase between the temperatures of 120ºC and 280ºC. The relaxation experiments were performed at some temperatures with trapezoidal wave loading, whereas the experiments of fatigue crack growth carried out at temperatures of 120ºC, 200ºC and 280ºC for sine and trapezoidal wave loading in displacement and load control. The microstructure analysis was also made in the specimens after the fatigue test by optical microscopy and scanning electron microscopy SEM. The results of these experiments showed that the casting defects and materials inhomogeneities, coupled long run times in high temperatures and loads, are a critical factor in the component performance. These results help us to establish accurate models for life prediction of the engine cylinder head. / Para caracterizar componentes usados em aplicações de alta responsabilidade não basta apenas conhecer a composição química e os resultados de ensaios de tração, impacto e dureza, pois estes podem não fornecer os subsídios necessários que permitam prever, de maneira confiável, o comportamento dos componentes nas condições reais de trabalho. Exemplo disto são os cabeçotes de motor automotivos, submetidos a tensões térmicas e mecânicas relativamente altas durante seu uso normal e altíssimo em condições extremas. Durante longos tempos de funcionamento e eventuais falhas na refrigeração e ou lubrificação a temperatura pode chegar a valores próximos de 300ºC. Esta variação de temperaturas provoca choques térmicos que podem gerar trincas e/ou uma grande quantidade de deformação plástica em regiões próximas aos pistões. Desconsiderando a presença de choques térmicos provocados por falhas, ainda assim, uma pequena quantidade de ciclos de acionamento e parada do motor, é considerada como os principais causadores de pequenas trincas. Isso indica que o surgimento de trincas em cabeçotes de motor deve ser considerado um problema de fadiga termomecânica de baixo ciclo. Outro problema é a heterogeneidade microestrutural no componente devido ao processo de fundição, levando a propriedades mecânicas e físicas diferentes em uma mesma peça. Além da presença de porosidade gerada por bolhas de gás e vazios de solidificação, que podem adquirir tamanho tal que se aproximem de pequenas trincas, diminuindo a vida para a nucleação e assim mudando o foco do problema para o de propagação de trinca por fadiga. A proposta deste trabalho foi a de determinar as propriedades de fadiga isotérmica e termomecânica através de ensaios de fadiga de baixo ciclo, bem como as propriedades de propagação de trinca por fadiga, relaxação, caracterização microestrutural e modelagem do comportamento mecânico por elementos finitos para a liga de alumínio utilizada na fabricação de cabeçotes de motores automotivos pela indústria nacional. Os ensaios de fadiga isotérmica foram realizados nas temperaturas de 120ºC e 280ºC e os ensaios termomecânicos foram realizados em fase entre as temperaturas de 120ºC e 280ºC. Os ensaios de relaxação foram realizados em várias temperaturas com carregamento de onda trapezoidal, enquanto que os ensaios de propagação de trinca por fadiga foram realizados nas temperaturas de 120ºC, 200ºC e 280ºC para carregamentos de onda senoidal e trapezoidal em controle de carga e de deslocamento. Foi feita também análise microestrutural nos corpos de prova, após os ensaios de fadiga, por microscopia óptica e por microscopia eletrônica de varredura MEV. Os resultados destes ensaios mostraram que os defeitos de fundição e a falta de homogeneidade no material, aliados a longos tempos de exposição a carregamentos e em altas temperaturas, constituem um fator crítico no desempenho do componente. Estes resultados ajudarão a estabelecer modelos precisos de previsão de vida para os cabeçotes de motor.

The main subject of this master’s thesis was to evalute inluence of loading cycle asymmetry and long-term thermal exposure on fatigue crack growth rate in martensitic P91 and P92 steels. Experiments were carried out in Material and metallurgical research Ostrava Ltd. and their main aim was to study the influences of different loading cycle asymmetries (R = 0,1 and R = 0,6) as well as 5000 hours/600 °C (P91) respectively 5000 hours/650 °C (P92) of thermal exposure on fatigue crack growth rate.

Master’s thesis deals with study of fatigue crack growth retardation models. This document presents description, evaluation, verification and finally application of retardation models demonstrated on the damage tolerant structure. Analysis was provided on the L 410 NG aircraft in the location of the rear spar lower flange joint. Output of this thesis is comparison of the damage tolerance analysis with the current method using in the Aircraft Industry a. s., i.e. linear model without using load interaction models. The comparisons are fatigue crack growth curves, threshold and intervals of inspection program. AFGROW software was used for verification and application of retardation models.

L’influence des renforts nanoparticules de TiB2 (6 wt.%) sur la précipitation interfaciale de la phase (Zn1.5Cu0.5)Mg, la résistance à la traction et la fissuration sous chargement de fatigue (fatigue crack growth-FCG) des composites à matrice de Al-Zn-Mg-Cu ont été étudiées. Des échantillons de composites ont été obtenus par réaction in-situ pendant le moulage suivi d’un FSP (friction stir processing) et une extrusion à chaud. Seuls les échantillons moulés et extrudés ont été utilisés pour étude de FCG à cause de la limitation de la taille après FSP. Des observations au microscope électronique à balayage (SEM), avec la diffraction des électrons rétrodiffusés (SEM/EBSD) et au microscope électronique en transmission à haute résolution (HRSTEM) ont été réalisées pour caractériser la microstructure.Des échantillons présentent une structure des grains équi-axiaux et des nanoparticules de TiB2 sont distribuées de façon homogène dans la matrice. En état de solution solide, l’interface TiB2/Al est de nature semi-cohérente et très propre. En état de vieillissementou ou sur vieillissement, la précipitation interfacaile hétérogène de la phase (Zn1.5Cu0.5)Mg a été observée. La cinétique de la précipitation interfaciale a été discutée. Les interfaces entre Al/(Zn1.5Cu0.5)Mg/TiB2 sont quasi cohérentes et l’interface TiB2/Al a été renforcée grâce à la réduction de l’énergie de l’interface. Ce mécanisme de précipitation interfaciale peut expliquer l’effet de renforcement de l’interface contribuant simultanement l’augmentation de la résistance et de l’élongation des échatillons de composite.La majorité de nanoparticules TiB2 tentent de s’agglomérer le long des joints de grains dans des échantillons sans FSP. La vitesse de croissance de fissure a été augmentée à l’intérieur des grains avec un facteur d’intensité (ΔK) intermédiaire ou important à cause de l’affinement de grains. Cependant, la vitesse de croissance de fissure a été diminuée aux joints de grains avec (ΔK) faible ou intermédiaire à cause de la présence des clusters de TiB2 tandis que cette vitesse augmente avec (ΔK) important à cause de la coalescence des micropores. / The influences of TiB2 reinforcement nanoparticles (6 wt.%) on the interfacial precipitation of (Zn1.5Cu0.5)Mg phase, the associated tensile and fatigue crack growth (FCG) properties of the Al-Zn-Mg-Cu matrix composites have been studied. The composite samples were produced by in-situ reaction during casting followed by friction stir processing (FSP) and hot extrusion, while only casted and extruded samples were used for evaluating FCG due to size limit of the nugget zone after FSP. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and high-resolution scanning transmission electron microscopy (HRSTEM) were employed for the microstructure characterization.The as-processed composite samples contain the fine equiaxed-grain structure, where TiB2 nanoparticles are homogenously distributed. At solid-solution state, the TiB2/Al interfaces are featured by the clean and semi-coherent nature. At the peak-aged and overaged states, the interface precipitate determined as (Zn1.5Cu0.5)Mg phase was formed, and the underlying heterogeneous interfacial precipitation kinetics was discussed. The Al/(Zn1.5Cu0.5)Mg/TiB2 multi-interfaces were revealed to be almost coherent, and the TiB2/Al interfaces were thus strengthened due to the greatly reduced coherency strains. This mechanism was proposed as precipitation assisted interface strengthening, which has contributed to the simultaneously enhanced tensile strength and uniform elongation of the as-processed composite.The majority of TiB2 nanoparticles tend to aggregate along grain boundaries (GBs) in the composite samples without FSP. The FCG rate is increased inside grains at intermediate and high stress intensity factor (ΔK) ranges due to the refined grain size. However, the FCG rate at the GBs is decreased at the low and intermediate ΔK ranges by fatigue crack deflection and trapping due to the presence of TiB2 clusters, while it increases at the high ΔK range due to microvoid coalescence.

Nickel superalloys are used for high-temperature application in energetic and aerospace industry. They are exposed to aggressive environment at high temperatures with the interactions between fatigue and creep processes, high-temperature oxidation, corrosion and erosion. Lifetime extension of such strained parts while increasing the performance of particular machine is possible by applying protective surface coatings. The subject of this work is to investigate the fatigue failure mechanisms of superalloy Inconel 713LC at 800 °C and to compare these mechanisms between material with a protective coating based on Al-Si and material without coating. The location of initiation fatigue cracks, their propagation and the fatigue crack propagation rate in some areas were analyzed by optical microscopy, scanning electron microscopy and confocal laser scanning microscopy. Based on previous research it was found that the application of the coating AlSi has a positive effect on lifetime of alloy Inconel 713LC. These results were confirmed and estimated in the diploma thesis.

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<div>As the use of fiber-reinforced polymer composites grows in aerospace structures, there is an emerging need to implement damage tolerant approaches. The use of <i>in-situ</i> synchrotron X-ray tomography enables direct observations of progressive damage relative to the microstructural features, which is studied in a T650/5320 laminate composite with varying layup orientations (using 45^o and -45^o plies) in a compact tension specimen geometry. Specifically, the interactions of micromechanical damage mechanisms at the notch tip were analyzed through 3D image processing as the crack grew. First, monotonic tests were conducted where X-ray tomography was acquired incrementally between the unloaded state and maximum load. The analysis of the monotonic tension specimens showed intralaminar cracking was dominant during crack initiation, delamination became prevalent during the later stages of crack progression, and fiber breakage was, in general, largely related to intralaminar cracking. After the monotonic tension analysis, modifications were made to the specimen geometry and the loading assembly, and fatigue tests were conducted, also using <i>in-situ</i> synchrotron X-ray tomography. Specifically, tomography images were acquired after select intervals of cyclic loading to examine the crack growth behavior up to 5802 cycles. The analysis of the fatigue tests showed that intralaminar cracking was also dominant, while localized delamination allowed ply cross-over. A finite element analysis was conducted by comparing the crack profile at varying intervals of loading, and the change in stored energy per cycle, dU/dN, was calculated. The combined experimental and simulation analysis showed that when the per ply values of dU/dN were examined, the intralaminar cracking rate collapsed to one curve regardless of the ply orientation, where direct observations of fiber bridging were characterized and associated with a reduction in crack growth rate for the influenced ply. Overall, this work provides a physical understanding of the micromechanics facilitating intralaminar crack growth in composites, providing engineers the necessary assessments for slow crack growth approaches in structural composite materials.<br></div>

[pt] Na análise da predição de vida de estruturas a presença de defeitos por trincas é de fundamental importância. Estudos iniciais utilizando a Mecânica da Fratura Linear Elástica mostram que o Fator de Intensidade de Tensão controla o crescimento de trinca por fadiga. Contudo, sobrecargas podem induzir efeitos de memória no material ao reduzir, parar ou acelerar taxas de propagação, comportamento não previsto considerando-se um único parâmetro elástico. Devido às hipóteses consideradas, as soluções fornecidas pelos métodos analíticos não verificam condições importantes de equilíbrio e compatibilidade, especialmente quando são usados campos singulares idealizados de tensão e de deformação na região da ponta da trinca. Neste trabalho uma extensa revisão dos resultados analíticos recentemente publicados para materiais nos regimes elástico e elastoplástico são apresentadas: Williams e HRR com campo de distribuição singular das tensões e Creager-Paris com campo não-singular das tensões com o material elástico. Neste estudo, estas soluções são comparadas com resultados da análise numérica utilizando a discretização por elementos finitos tridimensionais elastoplásticos com o critério de escoamento do material de von Mises na representação do cegamento da ponta da trinca e importantes conclusões são apresentadas com referência aos limites de aplicação dos modelos teóricos bem como a extensão dos requerimentos para o modelo numérico. Além de carregamentos monotonicamente crescentes, o descarregamento foi também considerado na análise numérica, apesar desta condição não estar presente nas referências consideradas para as soluções analíticas. Para as simulações numéricas consideradas algoritmos de elastoplasticidade foram implementados em um framework com arquitetura baseada em plugins. / [en] In structure life prediction analysis the occurrence of crack defects are of paramount importance to be considered. Basic studies using Linear Elastic Fracture Mechanics approach shows that the Stress Intensity Factor (SIF) parameter controls Fatigue crack growth (FCG). However, overloads may induce material memory effects that delay, arrest or accelerate the FCG rate, a behavior that is not described by considering a single elastic parameter. To account for service variable amplitude loadings, the use of a prescribed stress-strain distribution has been proposed in recent research studies, as the driving force of FCG, using the critical damage approach. Due to the assumptions considered in the analytical derivations, important equilibrium and compatibility conditions are violated in the obtained solutions, since an idealized singular stress-strain field at the crack front region is used. In this work a comprehensive review of the recently published analytical results for solutions under the elastic and elastoplastic material behavior regimens are presented: Williams and HRR with singular stress distribution field and Creager-Paris with a non singular stress field, but with the material elastic approach. These solutions are compared, throughout the study, to results obtained from the numerical analysis using a 3D finite element discretization with elastoplastic von Mises yielding criteria employed for the modeling of the crack tip blunt and some comprehensive conclusions are derived regarding application limits of the theoretical models as well as the required extent of the numerical model representation. In addition to monotonic increasing applied loads, unloading conditions were also considered in the numerical analysis, although no reference to this condition is available in the literature, considering the analytical approach. For the numerical simulations presented elastoplastic algorithms were implemented in a plugin based framework.

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios.