Le matériau étudié est un alliage Al A356, utilisés pour produire des têtes de cylindres pour l'industrie automobile par fonderie. Le matériau présente une microstructure dendritique assez grossière dans une matrice eutectique, avec une taille moyenne de grains de 25 microns, des précipités intermetalliques et des porosités. Les propriétés de traction sont fortement affectées par la température d'essai, avec une baisse assez sensible du module de Young, de la limite d'élasticité lorsque la température augmente. La durée de vie de fatigue isotherme chute de façon marquée (approximativement d'un facteur 10) lorsque la température d'essai est portée de 120 à 280 °C, en déformation imposée. Du essais thermomecaniques cycliques en phase, avec une température variant de 120 à 280 °C, on montré que la durée de vie en anisotherme est assez similaire à celle obtenue en conditions isothermes à 280 oC. Dans ce cas, les dommages causés par le chargement thermomécanique cyclique se produisent à la température la plus élevée du cycle. Les essais de relaxation ont montré l'existence de deux comportements distincts. À basse température, le matériau présente de l'écrouissage cyclique tandis qu'il s'adoucit cycliquement à des températures plus élevées. A partir des résultats de croissance des fissures de fatigue, on a observé que la température et la forme du cycle de fatigue a une forte influence sur la vitesse de fissuration par fatigue ainsi que sur le facteur d'intensité des contraintes seuil. Une loi de comportement élastique visco-plastique non-isotherme a été identifiée pour le matériau. Les paramètres de comportement mécanique sont statistiquement distribués du fait de la fabrication du matériau par fonderie. Toutefois, il a été démontré que le modèle était capable de se reproduire, avec une approximation raisonnable, les essais contrainte-déformation à des températures différentes, pour le cas isotherme et anisotherme. / The studied material is an A356 Al alloy, used to produce engine cylinder heads for the automotive industry by die casting process. The material displays a quite coarse dendritic microstructure in a eutectic matrix, with a mean grains size of 25 microns, intemetallic precipitates and porosities. The tensile properties are strongly affected by testing temperature, with a quite sensitive drop of the Young's modulus, the Yield stress as the temperature was raised. The isothermal fatigue life dropped of markedly (approximately 10 times) when the testing temperature is raised from 120 to 280 °C, under strain control. From the themomechanical in-phase cyclic tests, with temperature varying from (120 to 280 oC), it was possible to observe that life is quite similar to the isothermal fatigue test at 280 oC. In this case, the more sensitive damage caused the in-phase mechanical and thermal cycle take place at the highest temperature. Relaxation tests indicated two distinct behaviors, with the temperature of 240°C being a threshold. At lower temperatures, the material hardens cyclically whereas it softens cyclically at higher temperatures. From the fatigue crack growth results, it was observed that temperature and wave shape has a strong influence on the crack growth rate as well as on the stress intensity threshold. Considering sinusoidal wave shape (10 Hz), as the temperature increased the DKth decreased and the crack propagation rate increased. However, the rate as da/dN change with temperature is quite similar, as an indicative that the micromechanism of crack growth has not changed due to the high frequency used, and it was due only to loss of mechanical strength. An elastic-visco-plastic non-isothermal constitutive law was identified for the material. For the cast material studied in this work, the mechanical behavior parameters are statistically distributed. However, it was shown that the model was able to reproduce, with a reasonable approximation, the stress – strain relationship at different temperatures, for the isothermal and anisothermal cases. / 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 mecânicasrelativamente 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 provocadospor 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. Todos os ensaios e análises computacionais foram realizados nos laboratórios da Universidade de São Paulo (EESC-USP) e da École Normale Supérieure de Cachan (ENS-LMT). 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 arregamentos 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 aim of this diploma thesis is to estimate the residual fatigue lifetime of the model train axle. Primarily, the thesis is focused on determination of the stress intesity factor, the main variable for estimation of the residual fatigue lifetime. At first, the theoretical approaches of estimation of the residual fatigue lifetime are listed and then the residual fatigue lifetime of the model train axle is estimated.

Zyklische Kohäsivzonenmodelle beschreiben irreversibles Separationsverhalten und Schädigungsakkumulation unter zyklischer Belastung. In der vorliegenden Arbeit wird die Formulierung zyklischer Kohäsivzonenmodelle systematisiert und ihr Potenzial zur Simulation von Ermüdungsvorgängen analysiert. Die Kohäsivspannungs-Separations-Beziehungen werden auf Basis etablierter thermodynamischer Konzepte der Schädigungsmechanik aufgestellt. Zyklische Schädigungsakkumulation wird über die Entwicklungsgleichung der Schädigungsvariablen unter Berücksichtigung einer zustandsabhängigen Dauerfestigkeit beschrieben. Das Kohäsivzonenmodell wird erfolgreich für die Simulation von Werkstoffermüdung angewandt. Numerisch mithilfe der Methode der finiten Elemente erzeugte Rissfortschrittskurven bilden das experimentell beobachtete Ermüdungsrisswachstumsverhalten in allen Bereichen ab. Über Parameterstudien wird der Einfluss der einzelnen Modellparameter ermittelt. Darüber hinaus wird die Anwendung des zyklischen Kohäsivzonenmodells auf die Simulation von Wöhler-Versuchen vorgestellt und der Probengrößeneffekt auf das Ermüdungsverhalten untersucht. Der Zusammenhang zwischen den lokalen Beanspruchungszuständen in der Kohäsivzone und dem vorhergesagten globalen Versagensverhalten wird aufgeklärt. Die gewonnenen Erkenntnisse bilden die Grundlage für ein Konzept zur Identifikation der Kohäsivparameter, das auf der Auswertung von Wöhler- und Rissfortschrittskurven beruht. / Cyclic cohesive zone models describe irreversible separation behaviour and damage accumulation under cyclic loading. In the present thesis, the formulation of cyclic cohesive zone models is systemised and their potential to simulate fatigue processes is analysed. The relation between traction and separation is described based on established thermodynamical concepts of damage mechanics. Cyclic damage accumulation is controlled by a damage evolution equation taking into account a state-dependent endurance limit. The cohesive zone model is applied successfully to the simulation of material fatigue. Fatigue crack growth rate curves, which were obtained numerically by means of the finite element method, reproduce the experimentally observed behaviour in all stages. The influences of the particular parameters of the model are determined by parametric studies. In addition, simulations of uniaxial fatigue tests using the cyclic cohesive zone model are presented. Furthermore, the size effect on the fatigue behaviour is investigated. The relation between the local states within the cohesive zone and the predicted global failure modes is explained. These findings form the foundation for a concept of parameter identification which bases on the evaluation of Wöhler-curves and fatigue crack growth rate curves.

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.

Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility vi programvaran NASGRO där livsförutsägelserna visade god överensstämmelse med experimentella livscykler i de flesta fall. vii Abstract Title: Microstructure and mechanical properties of low-temperature hot isostatic pressed Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility viii was observed at 150 ˚C compared to that at room temperature, but the ductility decreased between 150 - 350 ˚C because of activation of different slip systems. The low cycle fatigue (LCF) behavior of such a modified HIP (low-temperature HIP) material is assessed at two different strain levels and compared with the corresponding LCF properties for the standard HIP material. Even though the modified HIP material had lowest minimum life cycles to failure, the overall fatigue performance is comparable with that of the standard HIP material. Also, fatigue life predictions were made from the measured defect size at the crack initiation site using NASGRO. The calculated life predictions showed good agreement with the experimental values in most cases. In-situ neutron diffraction measurements on tensile test specimens were conducted, at both room temperature and at 350˚ C, for the standard and modified HIP-treated materials. The objective was to gain essential insights on how the crystal lattice strains relate to the macroscopic strengths in these specific microstructures. This investigation helped to understand the load partitioning between different slip planes and constituent phases in the microstructure at different temperatures. / Ti-6Al-4V är den mest använda α+β titanlegeringen i flygmotortillämpningar på grund av sin höga specifika hållfasthet. Vanligtvis tillverkas legeringen som gjutgods eller smide och bearbetas sedan till slutlig geometri. Dessa konventionella tillverkningsprocesser genererar dock en hel del avfallsmaterial, medan additiv tillverkning (AM) potentiellt kan producera en nästan slutgiltlig geometri direkt från råvaran. Under det senaste decenniet har elektronstrålesmältning (EBM), en av pulverbäddsfusionsteknikerna, undersökts mycket för att bygga Ti-6Al-4V-komponenter. Ändå kan det byggda materialet innehålla defekter såsom gasporer som kräver efterbearbetning, såsom varm isostatisk pressning (HIP) för att producera nästan helt täta komponenter. HIP[1]behandling av konventionellt gjutet Ti-6Al-4V utförs normalt vid 920 ˚C, 100 MPa under 2 timmar. Samma HIP-behandling har sedan anpassats även för EBM[1]tillverkat Ti-6Al-4V, vilket dock resulterar i förgrovning av α-lameller och minskning av sträckgränsen. Att hitta en mer lämplig HIP-behandling för denna nya typ av Ti-6Al-4V-material, dvs EBM-tillverkat, skulle därför vara till stor fördel för industrin. Att sänka HIP-behandlingstemperaturen till 800 ˚C och öka trycket till 200 MPa har nyligen visat sig stänga porositeten i hög grad samtidigt som den höga sträckgränsen bibehålls. Ti-6Al-4V används huvudsakligen i applikationer för flygmotorer upp till en maximal driftstemperatur på 300 ˚C. Därför studerades högtemperaturdragegenskaperna hos de olika HIP-behandlade EBM[1]byggmaterialen i detta forskningsarbete. Denna studie visade att duktiliteten påverkas av aktiveringen av olika glidsystem baserat på temperatur. Ytterligare neutrondiffraktionsexperiment utfördes tillsammans med in-situ dragprovning för att bestämma det aktiva glidsystemet vid en specifik temperatur. Utmattningsbeteendet hos det lågtemperaturbehandlade HIP-materialet utvärderas också genom lågcykelutmattningstestning och utmattningsspricktillväxttest. Utmattningsprestandan för det modifierade HIP[1]materialet utvärderades mot standard HIP- material och visade sig ha jämförbara utmattningsegenskaper. Förutsägelser om utmattningsliv utfördes med hjälp av vi programvaran NASGRO där livsförutsägelserna visade god överensstämmelse med experimentella livscykler i de flesta fall. / <p>Submitted papers or manuscripts have been excluded from the fulltext file.</p>

Les travaux présentés dans ce mémoire concernent le développement de méthodes de pronostic à base de modèles. Le pronostic à base de modèles a pour but d'estimer le temps qu'il reste avant qu'un système ne soit défaillant, à partir d'un modèle physique de la dégradation du système. Ce temps de vie restant est appelé durée de résiduelle (RUL) du système.Le pronostic à base de modèle est composé de deux étapes principales : (i) estimation de l'état actuel de la dégradation et (ii) prédiction de l'état futur de la dégradation. La première étape, qui est une étape de filtrage, est réalisée à partir du modèle et des mesures disponibles. La seconde étape consiste à faire de la propagation d'incertitudes. Le principal enjeu du pronostic concerne la prise en compte des différentes sources d'incertitude pour obtenir une mesure de l'incertitude associée à la RUL prédite. Les principales sources d'incertitude sont les incertitudes de modèle, les incertitudes de mesures et les incertitudes liées aux futures conditions d'opération du système. Afin de gérer ces incertitudes et les intégrer au pronostic, des méthodes probabilistes ainsi que des méthodes ensemblistes ont été développées dans cette thèse.Dans un premier temps, un filtre de Kalman étendu ainsi qu'un filtre particulaire sont appliqués au pronostic de propagation de fissure, en utilisant la loi de Paris et des données synthétiques. Puis, une méthode combinant un filtre particulaire et un algorithme de détection (algorithme des sommes cumulatives) a été développée puis appliquée au pronostic de propagation de fissure dans un matériau composite soumis à un chargement variable. Cette fois, en plus des incertitudes de modèle et de mesures, les incertitudes liées aux futures conditions d'opération du système ont aussi été considérées. De plus, des données réelles ont été utilisées. Ensuite, deux méthodes de pronostic sont développées dans un cadre ensembliste où les erreurs sont considérées comme étant bornées. Elles utilisent notamment des méthodes d'inversion ensembliste et un observateur par intervalles pour des systèmes linéaires à temps discret. Enfin, l'application d'une méthode issue du domaine de l'analyse de fiabilité des systèmes au pronostic à base de modèles est présentée. Il s'agit de la méthode Inverse First-Order Reliability Method (Inverse FORM).Pour chaque méthode développée, des métriques d'évaluation de performance sont calculées dans le but de comparer leur efficacité. Il s'agit de l'exactitude, la précision et l'opportunité. / In this manuscript, contributions to the development of methods for on-line model-based prognosis are presented. Model-based prognosis aims at predicting the time before the monitored system reaches a failure state, using a physics-based model of the degradation. This time before failure is called the remaining useful life (RUL) of the system.Model-based prognosis is divided in two main steps: (i) current degradation state estimation and (ii) future degradation state prediction to predict the RUL. The first step, which consists in estimating the current degradation state using the measurements, is performed with filtering techniques. The second step is realized with uncertainty propagation methods. The main challenge in prognosis is to take the different uncertainty sources into account in order to obtain a measure of the RUL uncertainty. There are mainly model uncertainty, measurement uncertainty and future uncertainty (loading, operating conditions, etc.). Thus, probabilistic and set-membership methods for model-based prognosis are investigated in this thesis to tackle these uncertainties.The ability of an extended Kalman filter and a particle filter to perform RUL prognosis in presence of model and measurement uncertainty is first studied using a nonlinear fatigue crack growth model based on the Paris' law and synthetic data. Then, the particle filter combined to a detection algorithm (cumulative sum algorithm) is applied to a more realistic case study, which is fatigue crack growth prognosis in composite materials under variable amplitude loading. This time, model uncertainty, measurement uncertainty and future loading uncertainty are taken into account, and real data are used. Then, two set-membership model-based prognosis methods based on constraint satisfaction and unknown input interval observer for linear discete-time systems are presented. Finally, an extension of a reliability analysis method to model-based prognosis, namely the inverse first-order reliability method (Inverse FORM), is presented.In each case study, performance evaluation metrics (accuracy, precision and timeliness) are calculated in order to make a comparison between the proposed methods.

No description available.

Finite element analyses are frequently used to model growing fatigue cracks and the associated plasticity-induced crack closure. Two-dimensional, elastic-perfectly plastic finite element analyses of middle-crack tension (M(T)), bend (SEB), and compact tension (C(T)) geometries were conducted to study fatigue crack closure and to calculate the crack opening values under plane-strain and plane-stress conditions. The loading was selected to give the same maximum stress intensity factor in both geometries, and thus similar initial forward plastic zone sizes. Mesh refinement studies were performed on all geometries with various element types. For the C(T) geometry, negligible crack opening loads under plane-strain conditions were observed. In contrast, for the M(T) specimen, the plane-strain crack opening stresses were found to be significantly larger. This difference was shown to be a consequence of in-plane constraint. Under plane-stress conditions, it was found that the in-plane constraint has negligible effect, such that the opening values are approximately the same for the C(T), SEB, and M(T) specimens. Next, the crack opening values of the C(T), SEB and M(T) specimens were compared under various stress levels and load ratios. The effect of a highly refined mesh on crack opening values was noted and significantly lower crack opening values than those reported in literature were found. A new methodology is presented to calculate crack opening values in planar geometries using the crack surface nodal force distribution under minimum loading as determined from finite element analyses. The calculated crack opening values are compared with values obtained using finite element analysis and more conventional crack opening assessment methodologies. It is shown that the new method is independent of loading increment, integration method (normal and reduced integration), and crack opening assessment location. The compared opening values were in good agreement with strip-yield models.

No description available.

No description available.