[pt] A grande maioria das estruturas que trabalham sob cargas alternadas precisa ser dimensionada para evitar a iniciação de trincas por fadiga, o principal mecanismo de dano mecânico nesses casos. Os vários parâmetros dos modelos de previsão de dano à fadiga usados nesses projetos devem ser preferencialmente medidos a partir do ajuste otimizado de suas equações a dados experimentais medidos de forma adequada. Na realidade, a precisão das previsões baseadas nesses modelos depende diretamente da qualidade dos ajustes utilizados para obtenção desses parâmetros. Sendo assim, o objetivo principal deste trabalho é estudar a melhor maneira de se obter os parâmetros dos principais modelos de previsão da iniciação de trincas por fadiga através de ajustes de dados experimentais baseados no algoritmo de LevenbergMarquardt. Primeiro, foram realizados diversos ensaios εN em uma liga de alumínio 6351-T6 para averiguar o desempenho do ajuste proposto para asequações de Coffin-Manson e de Ramberg-Osgood. Em seguida, foram usados dados da literatura de outros oito materiais para ajustar modelos deformaçãovida clássicos, assim como com o expoente de Walker, para assim avaliar o efeito de cargas médias não-nulas em testes εN. Por fim, foi estudado o ajuste de um modelo SN com expoente de Walker que considera limites de fadiga e efeitos de carga média. Esse estudo também inclui considerações estatísticas para quantificar o fator de confiabilidade a partir de diferentes hipóteses de funções densidade de probabilidade, baseadas em dez conjuntos de dados da literatura. / [en] Most structures working under alternate loadings must be dimensioned to prevent fatigue crack initiation, the main mechanism of mechanical damage in these cases. The various parameters from the fatigue damage prediction models used in these projects should preferably be measured by optimally fitting their equations to well-measured experimental data. In fact, the accuracy of the predictions based on these models depends directly on the quality of the adjustments used to obtain these parameters. As a result, the main purpose of this work is to study the best way to obtain the parameters of the leading prediction models of fatigue crack initiation through experimental data fittings based on the Levenberg-Marquardt algorithm. First, several εN tests were performed on a 6351-T6 aluminum alloy to verify the performance of the proposed fit for the Coffin-Manson and Ramberg-Osgood equations. Then, data from the literature of eight other materials were used to fit classic strainlife models, as well as models based on the Walker exponent, to evaluate the effect of non-zero mean loads in εN tests. Finally, the fitting of an SN model including the Walker exponent was studied, which considers fatigue limits and mean load effects. This study includes as well statistical considerations to quantify the reliability factor from different probability density function assumptions, based on ten data sets from the literature.

No description available.

This study is part of an original approach based on quantitative imaging techniques, such as Infrared Thermography, to study the damage under fretting loadings. The main objective was the establishment of new experimental method for a rapid determination of crack initiation conditions under fretting loadings, based on the analysis of the thermal signal. One other objective was the determination of the different heat sources in order to assess a local analysis of the fretting damage. This work underlined the great potential of Infrared Thermography to study this damage. / Cette étude s'inscrit dans une approche originale basée sur des techniques d'imagerie quantitatives comme la thermographie infrarouge pour étudier l'endommagement sous chargement de fretting. Le premier objectif était de développer une méthode de construction rapide de carte de fretting basée sur l'analyse du signale thermique au cours d'essais de fretting. Un deuxième objectif était de déterminer les sources de chaleurs accompagnant le fretting pour une meilleure compréhension des mécanismes d'endommagement. Ce travail a souligné le grand potentiel de la thermographie infrarouge pour l'étude de c'est phénomènes.

Les alliages d'aluminium utilisés dans les structures aéronautiques (fuselage, voilure) sont soumis à des chargements cycliques, faisant de la fatigue l'un des facteurs dimensionnant. Dans cette thèse, les mécanismes d'amorçage de ces fissures de fatigue – au niveau des particules intermétalliques – et de micropropagation sont étudiés expérimentalement et numériquement sur les alliages 2050-T8 et 7050-T7451. Les analyses des premiers chapitres portent sur la description des particules intermétalliques qui sont les plus susceptibles de donner lieu à une amorce de fissure dans la matrice d'aluminium. Les effets de la nature des particules et de leur taille sont quantifiés. La proximité entre les particules intermétalliques et les pores y est décrite. Cette analyse expérimentale fait intervenir des observations de surface en microscopie électronique à balayage (MEB), ainsi que des caractérisations tridimensionnelles (3D) réalisées à l'aide de la tomographie par rayonnement synchrotron. Dans les chapitres suivants, les analyses traitent de la prévision des chemins de fissuration à l'échelle d'un grain. Elles s'appuient sur des observations expérimentales de surface et des simulations de plasticité cristalline 3D pour comprendre les chemins de fissuration. Un modèle d'endommagement tenant compte de la cristallographie est alors proposé pour simuler la propagation de fissure par éléments finis. / Aluminium alloys used for aerospace structures (wing, fuselage) are subjected to cyclic loading. Fatigue properties of such alloys are therefore taken into account for the design of such parts. In this thesis, initiation mechanisms of fatigue cracks – near intermetallic particles – and micropropagation are studied experimentally and numerically on alloys 2050-T8 and 7050-T7451. In the first chapters, the analysis focuses on intermetallic particles which are most prone to initiate a fatigue crack in the aluminium matrix. The effects of the nature of particles as well as their size are quantified. The proximity between intermetallic particles and pores is described. This experimental analysis use surface observations obtained with a scanning electron microscope (SEM), and three-dimensional (3D) characterizations using synchrotron tomography. In the last chapters, analysis are oriented towards the prediction of crack paths at the grain size. They rely on surface experimental observations and 3D crystal plasticity modelling in order to understand crack paths. A damage model taking into account crystallography is proposed to simulate crack propagation using the finite element method.

Fretting fatigue is a damage process that may arise in engineering applications where small cyclic relative displacements develop inside contacts leading to detrimental effects on the material fatigue properties. Fretting is located in regions not easily accessible, which makes it a dangerous phenomenon. It is therefore important to be able to make reliable predictions of the fretting fatigue lives. The work presented in this thesis has its focus on different aspects related to fretting fatigue in the titanium alloy Ti-17. A fretting experiment was developed which allowed for separate control of the three main fretting loads. Initially, the evolution of the coefficient of friction inside the slip region was investigated experimentally and analytically. Subsequently, 28 fretting tests were performed in which large fatigue cracks developed. The fretting tests were firstly evaluated with respect to fatigue crack initiation through five multiaxial fatigue criteria. The criteria predicted a too high fretting fatigue limit. A possible clue to the discrepancy was found in the fretting induced surface roughness with the asperity-pit interactions. The fatigue growth of the large fretting cracks was numerically modelled through a parametric crack growth procedure. The predicted lives were compared to the experimental outcome. The numerical simulations showed that linear elastic fracture mechanics was an appropriate tool for the prediction of fretting fatigue propagation lives in the long crack regime. Fatigue cracks spend most of their propagation life in the small crack regime. The possibility of modelling the small crack behaviour is therefore very important from the engineering point of view. The fatigue growth of through thickness short cracks was studied experimentally and numerically in the four-point bend configuration. It was found that linear elastic fracture mechanics and closure-free material growth data furnished conservative estimates for cracks longer than 50 μm. One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental results on fretting life with or without shot peening were simulated. The fatigue life enhancement in shot peened specimens could be explained by slower crack growth in the surface material layer with residual compressive stresses. / QC 20100827

This research establishes a physics-based life determination model for the second generation single crystal superalloy PWA 1484 experiencing out-of-phase thermomechanical fatigue (TMF). The life model was developed as a result of a combination of critical mechanical tests, dominant damage characterization and utilization of well-established literature. The resulting life model improves life prediction over currently employed methods and provides for extrapolation into yet unutilized operating regimes. Particularly, the proposed deformation model accounts for the materials' coupled fatigue-environment-microstructure response to TMF loading. Because the proposed model is be based upon the underlying deformation physics, the model is robust enough to be easily modified for other single crystal superalloys having similar microstructure. Future use of this model for turbine life estimation calculations would be based upon the actual deformation experienced by the turbine blade, thereby enabling turbine maintenance scheduling based upon on a "retirement for a cause" life management scheme rather than the currently employed "safe-life" calculations. This advancement has the ability to greatly reduce maintenance costs to the turbine end-user since turbine blades would be removed from service for practical and justifiable reasons. Additionally this work will enable a rethinking of the warranty period, thereby decreasing warranty related replacements. Finally, this research provides a more thorough understanding of the deformation mechanisms present in loading situations that combine fatigue-environment-microstructure effects.

In der vorliegenden Arbeit wurden Untersuchungen zum Initiierungs- und Ausbreitungsverhalten kurzer Ermüdungsrisse in einem austenitischen sowie einem austenitisch-ferritischen Edelstahl durchgeführt. Dazu erfolgten zyklische Verformungsexperimente sowohl ex situ als auch in situ im Rasterelektronenmikroskop. Die Auswertung der Experimente erfolgte im Rasterelektronenmikroskop sowohl abbildend in verschiedenen Modi als auch über Rückstreuelektronenbeugungsmessungen. Bezüglich der Rissinitiierung wurde eine Häufigkeitsverteilung der Rissinitiierungsorte für beide Stähle erstellt. Die dabei ermittelte stark unterschiedliche Häufigkeit für die transkristalline Rissinitiierung in der austenitischen Phase konnte mit der unterschiedlichen Textur und Mikrostruktur der beiden Stähle in Zusammenwirken mit den elastischen Eigenschaften der beiden Phasen erklärt werden. Für die Rissausbreitung wurde gezeigt, dass eine Korrelation zwischen der Risslänge und der Rissausbreitungsrate besteht, aus der hervorgeht, dass die Übergangsrisslänge zwischen den Bereichen der mikrostrukturell und mechanisch kurzen Risse etwa einen Korndurchmesser beträgt. Anhand der in situ Messung der (plastischen) Rissöffnung und -scherung wurden die Unterschiede im Rissausbreitungsverhalten in den verschiedenen Phasen herausgearbeitet. Für die austenitische Phase ergibt sich dabei ein öffnungsdominierter und für die ferritische Phase ein scherungsdominierter Mechanismus. Die im Ferrit auftretenden zwei unterschiedlichen Ausprägungen der Oberflächenrisspfade („rau“ und „glatt“) konnten mit der Orientierung der jeweils risstragenden Körner korreliert werden. In Zusammenwirken mit der beobachteten Systematik der Anordnung der Gleitspuren um kurze Risse im Ferrit sowie einer Analyse der Spannungsverteilung um die Rissspitze wurde ein Modell des Rissausbreitungsmechanismus\' erstellt sowie die bisher in der Literatur vorherrschende These des Einfachgleitens widerlegt. Schließlich konnte die Barrierenwirkung von Korn- und Phasengrenzen sowohl anhand der Messung der Rissausbreitungsrate als auch der plastischen Rissöffnung bzw. -scherung gezeigt und daraus Rückschlüsse auf die Ausdehnung der plastischen Zone vor der Rissspitze gezogen werden. Gleichzeitig wurde dabei die Korrelation zwischen Rissausbreitungsrate und plastischer Rissöffnung bzw. -scherung nachgewiesen.

[pt] Estudos recentes têm mostrado que para muitos materiais de Engenharia não existe um limite de fadiga. Numa análise convencional, se admite uma vida infinita em fadiga de alto ciclo quando o material atinge 10(6) - 10(7) ciclos sem a incidência de falhas. Entretanto, em função do desenvolvimento tecnológico atual, a vida-fadiga de inúmeros componentes mecânicos e estruturais pode ultrapassar a fronteira da fadiga de alto ciclo, fazendo com que a avaliação do comportamento em fadiga de altíssimo ciclo (10(7) – 10(12)), tenha se tornado extremamente importante para projetos, por ter estabelecido que uma tensão limite de fadiga não existe em muitos casos. Pesquisas recentes demonstram que a maioria dos materiais, incluindo ligas ferrosas, apresentam falhas em até 10(10) ciclos, com um decréscimo contínuo do limite de fadiga após 10(6) ciclos, o que torna a resistência à fadiga associada com um número de ciclos mais importante do que o próprio limite de fadiga. No regime de altíssimo ciclo de fadiga as trincas se iniciam a partir de defeitos internos do material, como inclusões, gerando mecanismos de iniciação de trincas caraterísticos das superfícies de fratura, tais como olho de peixe (fish-eye), ODA (Optically Dark Area) e FGA (Fine Granular Area). Neste estudo foram usinados corpos de prova do aço DIN 34CrNiMo6, que foram ensaiados entre 10(6) e 10(9) ciclos, sob fadiga ultrassônica do tipo tração-compressão, com frequência de 20 kHz e razão de carregamento -1. Os resultados mostraram que o material tem uma tendência maior a vida-fadiga sob valores baixos de tensão e na superfície de fratura de alguns corpos de prova formação de fish-eye. / [en] Recent studies have shown that for many engineering materials there is no fatigue limit. In a conventional analysis, infinite life in high cycle fatigue is allowed when the material reaches 10(6) - 10(7) cycles without the occurrence of failures. However, due to the current technological development, the fatigue life of several mechanical and structural components can exceed the boundary of high cycle fatigue, making the evaluation of ultra high cyle fatigue behavior (10(7)-10(12)), or fatigue of very high cycle, has become extremely important for projects, because it has established that a stress of fatigue limit does not exist in many cases. Current research has shown that most materials, including ferrous alloys, exhibit failures in up to 10(9) cycles, with a continuous decrease in the fatigue limit after 10(6) cycles, which makes strength fatigue associated with a number of cycles most important than own fatigue limit. In the very high fatigue cycles regime, cracks start from internal defects of the material, such as inclusions, generating a feature mechanisms of crack initiation on the fracture surfaces, such as fish-eye, ODA (Optically Dark Area) and Fine Granular Area (FGA). This study, specimens of DIN 34CrNiMo6 steel were machined and were tested between 10(6) and 10(9) cycles, under ultrasonic fatigue, with a frequency of 20 kHz and a loading ratio of -1. The results showed that the material has a tendency to fatigue life under low stress values and in some fracture sufaces of the specimens the fish-eye formation.

Les méthodes de calcul en fatigue à grande durée de vie sont en cours de développement depuis des décennies et sont utilisées par les ingénieurs pour dimensionner les structures. Généralement, ces méthodes se basent sur la mise en équations de quantités mécaniques calculées à l'échelle macroscopique ou mésoscopique. Les critères de fatigue multiaxiale reposent généralement sur des hypothèses de changement d'échelle dont l'objectif est d'accéder à l'état de contraintes ou de déformations à l'échelle du grain. Dans les approches de type plan critique (Dang Van, Papadopoulos, Morel), l'amorçage d'une fissure de fatigue est considéré comme piloté par une quantité mécanique liée à une orientation matérielle particulière (plan critique). Si ces phénomènes sont bien établis dans le cas des chargements uniaxiaux, la nature des mécanismes liés à l'activation des systèmes de glissement, à la multiplicité du glissement et aux différents sites préférentiels d'amorçage de fissures sous chargements complexes reste peu connue.Afin de mieux comprendre les mécanismes d'endommagement en fatigue multiaxiale, les techniques d'analyse et de caractérisation de l'activité plastique (activation des systèmes de glissements, bandes de glissement persistantes) et d'observation de l'endommagement par fatigue ont été mises en place en se basant principalement sur des observations MEB et analyses EBSD. Ces investigations ont permis de mettre en lumière les effets des chargements non proportionnels sur la multiplicité du glissement sur du cuivre pur OFHC. L'étude statistique des sites préférentiels d'amorçage de fissures montre que les grains à glissement multiple présentent une forte probabilité d'amorçage de fissures, surtout sous les chargements non proportionnels. Nous avons également mis en évidence le rôle des joints de grains et des joints de macle sur le développement de la plasticité à l'échelle de la microstructure. Les résultats expérimentaux sont confrontés à ceux du calcul éléments finis (EF) en plasticité polycristalline sur des microstructures synthétiques 3D semi-périodiques. L'application du critère de Dang Van à l'échelle mésoscopique (le grain) montre une forte variabilité de la contrainte hydrostatique et du cisaillement. Cette variabilité est plus importante pour un modèle de comportement cristallin élastique anisotrope. Le rôle de la plasticité cristalline se révèle secondaire. Ces analyses permettent de remettre en perspective les hypothèses usuelles de changement d'échelle utilisées en fatigue multiaxiale. Enfin, une méthode basée sur la statistique des valeurs extrêmes est proposée pour le dépouillement des calculs EF sur agrégats. Cette analyse a été appliquée sur la contrainte équivalente associée au critère de fatigue de Dang Van pour les calculs d'agrégats polycristallins avec différentes morphologies et orientations des grains. Les effets de la surface libre, du type de chargement et du modèle de comportement mécanique des grains ont été analysés. Les résultats offrent des perspectives intéressantes sur la modélisation de l'amorçage des fissures en fatigue multiaxiale des matériaux et des structures avec une prise en compte de la microstructure. / The development of high cycle fatigue (HCF) strength assessment methods has now been running for more than a century, leading to relatively efficient methods for engineers. Generally, these methods are based on mechanical quantities calculated at macroscopic or mesoscopic scales and validated by the model's ability to accurately reproduce experimental results. Multiaxial fatigue strength criteria are usually based on scaling transition assumptions aiming at capturing the stress or strain state in the grain. In the case of critical plane based criteria (Dang Van, Papadopoulos, Morel), fatigue crack initiation is supposed to be controlled by a mechanical quantity linked to a particular orientation (critical plane). If fatigue crack initiation phenomena are well established in the case of uniaxial loadings, the nature of the mechanisms involved in the activation of slip systems, multiple slip and preferential sites of rack initiation under complex loadings remains little known.To better understand the mechanisms of multiaxial fatigue crack initiation, analysis and characterization of the plastic activity (e.g. activation of slip systems, persistent slip bands) and observations of fatigue damage have been carried out on pure OFHC copper, using SEM and EBSD analyses. These investigations enabled to highlight the effects of non-proportional multiaxial loadings through the induced multiplicity of slip. The statistical study of preferential crack initiation sites shows that grains with multiple slip have a high probability of crack initiation, especially under non-proportional loading. We also highlighted the role of grain boundaries and twin boundaries on the development of plasticity across the microstructure. The experimental results were compared with those of finite element crystal plasticity computations on synthetic 3D semi-periodic microstructures. The application of the Dang Van criterion at the mesoscopic (grain) scale showed a strong variability of the hydrostatic stress and the shear stress. This variability was greater for anisotropic elastic behavior, while the role of crystal plasticity seemed to be secondary. These analyses allowed putting into perspective the usual assumptions of scaling transition rules used in multiaxial fatigue. Finally, a method based on the extreme values statistics ​​was proposed and applied to the equivalent stress associated to the Dang Van fatigue criterion for polycrystalline aggregate computations with different morphologies and grains orientations. The effects of the microstructure, free surface, loading types and mechanical behavior were analyzed. The results offered interesting insights into the multiaxial fatigue modeling of metals and structures taking into account the microstructure.

L’amélioration des performances spécifiques des alliages métalliques de l’aéronautique est une démarche constante. Les alliages de titane sont des matériaux privilégiés par les constructeurs aéronautiques car ils allient hautes propriétés mécaniques et faible densité.Parmi ces matériaux, les alliages β-métastables qui ont pour particularité de retenir jusqu’à 40% de phase β connaissent un fort regain d’intérêt pour les motoristes (Ti-17) comme pour des applications de structure type trains d’atterrissage (Ti-5553 et Ti-10-2-3). Ce travail a pour but d’analyser le comportement mécanique et la durabilité de ces alliages soumis à des sollicitations monotones et cycliques en lien avec les microstructures.Des essais mécaniques ont pour cela été développés à partir de différentes microstructures métallurgiques qu’elles soient issues d’un traitement industriel ou spécifique visant à simplifier ces dernières. Les mécanismes de déformation (systèmes de glissement) et d’endommagement (amorçage de fissures) ont été identifiés et analysés à différentes échelles par microcopie optique et électronique à balayage en intégrant les notions d’orientation cristallographique (EBSD). Le recours à des essais réalisés in situ sous microscope (optique et MEB) et à une métrologie adaptée aux échelles pertinentes a permis d’identifier les éléments micro-structuraux clés et les cinétiques de développement de ces processus. Un des faits marquants est le rôle majeur de l’anisotropie de propriétés mécaniques de la phase β qui a également fait l’objet de simulations numériques. / The improvement of specific performances of metallic materials used for aerospaceapplications needs continuous researches and developments. Titanium alloys are materials ofchoice for aerospace companies thanks to their high mechanical properties and low density.Among them, the β-metastable alloys that retain up to 40% of β phase are more and moreintroduced in aircraft engines (Ti-17) and for structural parts (e.g. landing gears in Ti-5553and Ti-10-2-3).This work aims to analyse the mechanical behaviour and durability of these alloyssubmitted to monotonic or cyclic loadings. Mechanical tests have been developed on differentindustrial microstructures as on academic simplified ones produced by specific thermaltreatments. Deformation mechanisms (slip systems) and damage processes (cracks initiation)were identified and analyzed at different scales using microscopes (optical and SEM) andcrystallographic features were studied by EBSD. Specific in situ tests performed undermicroscopes (optical and SEM) and digital images correlation techniques at scales of interesthave permitted to identify and to quantify the key microstructural parameters and the kineticsof these processes. One major result concerns the influence of the anisotropy of mechanicalproperties associated to the β phase.