Les pales d’hélicoptère sont des structures composites soumises à un chargement cyclique multiaxial, et leur criticité impose de porter une attention particulière à la tolérance aux dommages. Leur revêtement peut potentiellement présenter des criques suite à certains évènements (impact, défaut, foudre). Ces travaux se focalisent sur un matériau de revêtement donné (tissu de verre) et concernent l’étude de la propagation de coupure (crique) sous chargement cyclique. Les sollicitations de service ont amené à considérer la traction et le cisaillement plan. Une étude expérimentale a été menée afin d’étudier les modes d’endommagement du matériau et sa résistance à la propagation de coupure pour différentes sollicitations (en traction et en cisaillement) et pour les drapages les plus courants. Elle a permis de dégager les mécanismes d’endommagement mis en jeu, et a fourni un ensemble important de propriétés matériau et de données quantitatives de vitesse de propagation. Elle a par ailleurs guidé vers une modélisation par éléments finis adaptée à l’architecture du matériau, et la manière dont il se dégrade en fatigue. Ce modèle repose sur un maillage à l’échelle de la mèche, et la prédiction de la propagation est obtenue par l’utilisation d’une courbe de fatigue S-N. La simulation a été évaluée par comparaison des faciès de rupture, des vitesses de propagation et de l’étendue des zones d’endommagement avec les essais réalisés sur éprouvettes. / Helicopter blades consist of composite structures which have to sustain multi-axial cyclic loading. Because of their criticality, damage tolerance has to be considered carefully. Their skin is subjected to environmental events like impact, flaw, lightning which can cause through-thethickness cracks. The present work focuses on one given skin material (woven glass fabric) and concerns the study of the through-the-thickness crack growth under cyclic loading. In-flight loading lead to consider tension and shear. An experimental study has been carried out to study damage in the material and its crackgrowth resistance under different loadings (tension and shear) and for usual stacking sequences. It highlighted damage mechanisms and provided an important set of material data and crack growth speeds. Besides, this led to a finite element approach adapted to the woven fabric architecture, anddamage feature under fatigue loading. This modeling is based on a bundle scale mesh, a semidiscrete damage modeling and an S-N curve to predict fiber failure. Numerical simulations of crack growth tests were carried out, and results were compared with experiments in terms of crack direction, crack growth speed, and size of damaged area.

No description available.

A austenita retida está sempre presente na microestrutura de camada cementada de aços, em maior ou menor quantidade. Como é uma fase dúctil comparada à martensita, sua presença tem sido alvo de muita controvérsia. Este trabalho apresenta um estudo sobre a influência da austenita retida na propagação de trincas curtas por fadiga em camada cementada de aço SAE 8620. Foram feitos ensaios de fadiga por flexão em quatro pontos, a temperatura ambiente, em corpos de prova sem entalhe com três níveis de amplitude de tensão e razão de tensões de 0,1. Através de diferentes ciclos de cementação e tratamentos térmicos, foram obtidas camadas cementadas com quatro níveis de austenita retida na microestrutura. O teor de austenita retida foi medido através da técnica de difração de Raios-X. Trincas superficiais foram monitoradas por meio da técnica de réplicas de acetato. Como resultados foram obtidos tamanho de trinca em função do número de ciclos e taxa de crescimento de trincas curtas. Corpos de prova com maiores níveis de austenita retida apresentaram maior vida em fadiga. / The retained austenite is always present in case carburized steel microstructure in small or high percentages. Since it is a ductile phase, its presence has long been a controversial subject. The influence of retained austenite on short fatigue crack propagation in case carburized SAE 8620 steel was studied in this work. Four-point-bend fatigue tests were carried out at room temperature in specimens without notch using three levels of stress range and a stress ratio of 0.1. Four different amount of retained austenite in the case carburized microstructure were obtained through different cycles of carburizing and heat treating. The retained austenite content was measured by X-ray technique, and the surface short crack growth was monitored by means of acetate replication technique. Crack length versus number of cycles and crack growth rate versus mean crack length were obtained as results. Specimens with higher levels of retained austenite in the carburized case showed longer fatigue life.

<p>QC 20110523</p>

No description available.

[pt] Medindo a rigidez de corpos de prova em testes de propagação de trinca por fadiga, Elber identificou que a ponta da trinca só está totalmente aberta durante uma parte do ciclo de carregamento, e nomeou o carregamento onde a trinca abre totalmente de carga de abertura Pab. Baseado nisso, Elber assumiu que o dano à frente da trinca era induzido apenas pela fração do carregamento acima da Pab, propondo que a força motriz na propagação de trincas por fadiga é a gama do fator de intensidade de tensões efetivo (delta)Kef. Para verificar esta hipótese, neste trabalho foram examinados diferentes cenários em testes de propagação de trinca por fadiga. Primeiramente, em corpos DC(T) de aço AISI 1020 e em corpos DC(T) e C(T) de alumínio 6351-T6 foi propagada uma trinca com (delta)K e Kmax quase constantes, medindo Pab em campo próximo e em campo distante, com extensômetros e com um sistema 3D de correlação digital de imagens (DIC). Depois, usando novos corpos DC(T) de aço e de alumínio, foi propagada uma trinca com (delta)K e Kmax quase constantes, antes e depois de um evento de sobrecarga, medindo a Pab ao longo do teste, em campo próximo e campo distante. Cabe salientar que nesses testes as espessuras dos corpos de prova foram projetadas para que a trinca propagasse em tensão plana e em deformação plana. Finalmente, em testes de propagação de trinca com (delta)K e Kmax quase constantes, em corpos DC(T) de aço e de alumínio foi medido o campo de deformação à frente da ponta da trinca com um sistema de microscópio estéreo DIC, para analisar o comportamento dentro da zona plástica, medindo também a Pab com os métodos mencionados anteriormente. Dos resultados dos testes três comportamentos foram particularmente relevantes. Nos primeiros testes de propagação com deltaK e Kmax quase constantes, a razão Kab/Kmax diminuía enquanto a trinca propagava com uma taxa constante. Nos testes de sobrecarga o valor mínimo de (delta)Kef estava defasado em relação ao valor mínimo de taxa de propagação (da/dN). Já nos testes onde foi medido o campo deformação com o sistema de microscópio estéreo DIC, a deformação 0.1mm à frente da ponta da trinca mostrou que existia dano para cargas abaixo da Pab. Portanto, estes resultados não podem ser explicados pela hipótese de Elber, e contestam o (delta)Kef como a força motriz na propagação de trincas por fadiga. / [en] Measuring the stiffness of a fatigue-cracked plate during its loading cycle, Elber discovered that this crack only completely opened after reaching the crack opening load Pop. Based on this, Elber assumed that the damage ahead of the crack is induced by the loading part above the Pop. In this way, he proposed that the range of the effective stress intensity factor (delta)Keff is the driving force in fatigue crack growth. In order to verify this hypothesis, this study investigated different scenarios in fatigue crack growth tests. For this, DC(T) specimens of AISI 1020 steel, and DC(T) and C(T) specimens of 6351-T6 aluminum were tested. The fatigue crack growth tests were performed under quasi-constant K and Kmax conditions. The Pop measurements in the near and far field were obtained from strain gage readings and 3D Digital Image Correlation (DIC) analysis. In addition, simple variable amplitude tests were performed in steel and aluminum DC(T) specimens. A single tensile overload was introduced in the fatigue crack growth experiments under quasi-constant (delta)K and Kmax conditions, also measuring the Pop throughout the test in the near and the far field. It is important to note that the thickness of the specimens was designed to perform a crack propagation in plane stress and plane strain conditions. Moreover, near tip strain measurements in steel and aluminum DC(T) specimens were obtained with a stereo microscope DIC system, to analyze the behavior within the plastic zone, also measuring the Pop with the methods previously mentioned. From the experimental results, three behaviors were particularly relevant. In the first fatigue crack growth tests with quasi-constant {(delta)K, Kmax} conditions, the ratio Kop/Kmax decreased as the crack propagates under a constant rate. In the fatigue crack growth tests with overload application, the minimum value of (delta)Keff was lagged in relation to the minimum value of the propagation rate (da/dN). Finally, in the strain measurements performed with the stereo microscope DIC system, the de-formation at 0.1mm ahead of the crack tip showed the existence of damage in load values below Pop. Therefore, these results cannot be explained by Elber s hypothesis, and question the (delta)Keff as the driving force in fatigue crack growth.

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

Die Arbeit ruht auf zwei Säulen: Die eine besteht in der Aufbereitung, Erprobung und konsequenten Anwendung von Methoden der Skaleninvarianzanalyse, die andere in einem breiten Fundus an experimentellen Daten für aushärtbare Aluminiumknetlegierungen in der Form dünner Bleche, die unter gleichartigen, streng kontrollierten Bedingungen gewonnen worden sind. Als methodische Weiterentwicklungen sind die Fundierung des Umgangs mit der algebraischen Korrelation zwischen Vorfaktor und Exponent einer beliebigen Potenzgleichung, die Übertragung des Ansatzes der finiten Skaleninvarianz auf die Ermüdungsrißausbreitung sowie die Kombination der Idee eines geschwindigkeitsbestimmenden Schrittes mit der Dimensionsanalyse der umgebungsabhängigen Ermüdungsrißausbreitung bis hin zur Kartierung der geschwindigkeitsbestimmenden Schritte zu nennen. Auf experimenteller Seite wurde eine Datensammlung mit gemessenen Streubändern für die Ermüdungsrißausbreitung und das Verfestigungsverhalten von 39 Orientierungen bzw. Auslagerungszuständen von Aluminiumlegierungen aufgebaut. Diese Sammlung wird durch ausgewählte Messungen der Ermüdungsrißausbreitung im schwellenwertnahen Bereich, Restfestigkeitsversuche, Rißschließmessungen, Rauheitsmessungen an Bruchflächen, frequenzabhängige Messungen zum Umgebungseinfluß sowie Untersuchungen an drei Stählen und einer Magnesiumlegierung sinnvoll ergänzt. Auf der Basis der Meßdaten und der Analysemethoden wurde der Werkstoffeinfluß auf die Ermüdungsrißausbreitung in dünnen Blechen aus Aluminiumknetlegierungen bei Belastung mit konstanter Amplitude im Gültigkeitsbereich der linear-elastischen Bruchmechanik untersucht. Dabei wurden folgende Größen als wesentliche Einflußfaktoren identifiziert: - für die Gruppenzugehörigkeit: der Kohärenz- und Ordnungsgrad der festigkeitsbestimmenden Ausscheidungen und die resultierende Gleitverteilung, - für den gemeinsamen Vorfaktor der Legierungen der Gruppe 1: die elastischen Eigenschaften und das Spannungsverhältnis (Translation der Paris-Geraden), - für die Exponenten der Legierungen der Gruppe 1: 0,2%-Dehngrenze, athermischer Verfestigungsparameter, Probendicke und Kc-Wert als dimensionsloses Potenzprodukt (Rotation der Paris-Geraden), - für die Legierungen der Gruppe 2: das Ausmaß der Rißablenkung und eine bleibende Mode-II-Komponente der Rißöffnungsverschiebung, - für den Umgebungseinfluß der Legierung 6013 T6: Frequenz und Schwingbreite des Spannungsintensitätsfaktors. Die Diskussion umfaßt den wertenden Vergleich der experimentellen Ergebnisse mit Befunden und Modellen aus der Literatur, Erklärungsansätze für die Ursachen der Einflußnahme der wesentlichen Parameter sowie einen Modellansatz für die Legierungen der Gruppe 1 auf der Basis einer Mischungsregel. Dabei hatte sich erwiesen, daß keines der aus der Literatur bekannten Modelle alle Befunde richtig wiedergibt. Einige der ausgearbeiteten Erklärungsansätze bedürfen der zukünftigen Vertiefung. / The work is based upon two essentials: the first one is the preparation and application of techniques of scale invariance analysis, the second one consists in a database of experimental results for heat-treatable thin-sheet wrought aluminium alloys obtained under uniform conditions. Progress with respect to methodology was achieved regarding, first, the algebraic correlation between sets of coefficients and exponents of any power law, second, the transfer of the concept of finite scale invariance to the phenomenon of fatigue crack growth (FCG), and third, the combination of the ideas of a rate-controlling step and dimensional analysis of environmental-assisted FCG including the mapping of rate-controlling steps. In the experimental part, a database containing both measured scatterbands of FCG and strengthening characteristics for several orientations and aging conditions of aluminium alloys amounting to a total of 39 different material conditions was established. This database was supplemented with results of selected measurements of near-threshold FCG rates, residual strength, crack closure, roughness of fatigue cracks, and frequency-dependent environmental-assisted FCG as well as investigations of three plain-carbon steels and a magnesium alloy. Based on these prerequisites, the influence of the material on the FCG behaviour of thin-sheet wrought aluminium alloys under constant-amplitude loading was investigated within the limits of validity of linear-elastic fracture mechanics. The following influence factors were identified to be essential: The assignment of alloys to one out of two groups is mainly determined by the degrees of coherency and order of the strength-controlling precipitates and the resulting type of slip distribution. The normalized-Paris-law coefficient for the first group is mainly dependent on the modulus of elasticity and the stress ratio. The Paris-law exponents for the first group are dominated by a dimensionless power monomial of the 0.2% proof stress, the athermal strengthening coefficient, sheet thickeness and the critical stress intensity factor. The retardation of the FCG rates of alloys of the second group relative to the first group is mainly determined by the amount of crack deflection and by a residual mode-II component of crack opening displacement. Finally, the environment-assisted FCG for aluminium alloy 6013 T6 reveals a coupled dependence on loading frequency and cyclic stress intensity factor. The discussion covers the evaluation of the results in relation to observations and models from the literature, the explanation of the modes of operation of the major influence factors and a model based on a mixing rule for the alloys of the first group. It turned out that there is not any model that reflects all of the observations simultaneously. Some of the ideas presented require to be worked out in more detail.

[pt] A resistência a fadiga dos elementos estruturais pode ser limitada pela presença de trincas curtas, as quais podem passar despercebidas em inspeções não-destrutivas tradicionais. Para modelar o comportamento dessas trincas pode-se utilizar o tamanho característico das trincas curtas ao, proposto por El Haddad, Topper e Smith (ETS), que permite ajustar adequadamente os dados experimentais do diagrama de Kitagawa-Takahashi. Partindo do modelo ETS, neste trabalho se apresenta o modelo do Gradiente de Tensão (GT), desenvolvido para prever o efeito das trincas curtas que nascem em pontas de entalhes. Este modelo reconhece que trincas podem ser facilmente geradas por fadiga em entalhes afiados, que introduzem efeitos de concentração de tensão elevados nas suas pontas. Mas devido ao alto gradiente de tensão que atua em torno das pontas desses entalhes, as trincas curtas que deles emanam também podem parar de crescer por fadiga após terem se propagado por uma pequena distância, tornando-se assim trincas não-propagantes que podem ser toleradas em serviço. Corpos de prova tipo C(T) modificados foram projetados especificamente para estudar a geração e o crescimento inicial dessas trincas em ensaios de fadiga, e usados para verificar experimentalmente as previsões do modelo proposto. / [en] The fatigue strength of structural elements may be limited by the presence of short cracks, which can not be detected with traditional nondestructive inspections. The behavior of these cracks can be modeled using the characteristic size of short cracks a0, proposed by El Haddad, Topper and Smith (ETS), which allows one to properly adjust experimental data from the Kitagawa-Takahashi diagram. Based on the ETS model, this work presents the Stress Gradient (SG) model, developed to predict the effect of short cracks that grow from the notch tips. This model considers that cracks are easily generated due to fatigue in sharp notches, which introduce high stress concentration effects at their tips. Because of the high stress gradient acting at these notch tips, the short cracks can also stop to grow by fatigue after having propagated through a small distance, thereby becoming non-propagating cracks that can be tolerated in service. C(T) modified specimens were specifically designed to study the generation and initial growth of these cracks in fatigue tests, and used to experimentally verify the predictions of the proposed model.

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.