L’objectif de ces travaux de thèse est d’étudier l’influence du rapport entre taille de défaut et longueur caractéristique de la microstructure, sur la limite de fatigue pour une durée de vie fixée d’un matériaux métallique,dans le cadre de la fatigue à grand nombre de cycle (FGNC). Le fer pur Armco est choisi comme matériau d’étude, car sa microstructure simple présente une seule longueur caractéristique à l’échelle mésoscopique (échelle des grains) : la taille de grain. Le but de l’étude revient ainsi à étudier la compétition entre un effet de structure (défaut surfacique)et un effet matériau (taille de grain) dans le cadre de sollicitations mécaniques en FGNC.Afin d’obtenir une taille de défaut comparable mais aussi inférieure à la taille de grain du matériau, un protocole thermomécanique a été élaboré pour augmenter la taille de grain. Des essais de FGNC, utilisant des éprouvettes issues des deux tailles de microstructures (matériau initial et celui écroui traité) dans lesquelles sont introduits des défauts hémisphériques de tailles différentes, ont été effectués pour estimer les limites de fatigue pour différents rapports taille de défaut / taille de grain. Lorsque les diagrammes de Kitagawa sont présentés en valeurs relatives(limite de fatigue / celle du matériau sans défaut en fonction de la taille de défaut / taille de grain), il existe une seule courbe qui combine les deux microstructures. Ce diagramme de Kitagawa sans dimension permet ainsi d’analyser la réduction de la limite de fatigue causée par un défaut. L’utilisation de la taille relative du défaut par rapport à la dimension microstructurale caractéristique apparaît comme plus pertinente que l’emploi de la taille physique réelle du défaut.Ces résultats expérimentaux sont utilisés pour reproduire les essais en FGNC avec des simulations Éléments Finis sur des microstructures 3D représentatives du fer Armco. La compétition existant entre la concentration de contrainte induite par le défaut géométrique et les régions fortement sollicitées de la microstructure engendrées parl’anisotropie du comportement mécanique des grains est étudiée. Un critère mésoscopique (à partir des grandeurs mécaniques moyennées par grain) basé sur une approche statistique permet de retrouver l’allure du diagramme de Kitagawa adimensionné, c’est-à-dire la taille relative du défaut critique à partir de laquelle ce dernier prend le passur l’hétérogénéité de la réponse de la microstructure et conditionne ainsi la tenue en fatigue du polycristal. La modification du critère mésoscopique par la prise en compte des hétérogénéités intragranulaires (via l’écart-type par grain des grandeurs mécaniques) est discutée. / The objective of this thesis is to study the influence of the ratio between the defect size and themicrostructure characteristic length on the fatigue limit (for a fixed fatigue life) of a metallic material, under highcycle fatigue (HCF). Pure Armco iron is chosen because its simple microstructure has a single characteristic lengthat the mesoscopic scale (grain scale) : the grain size. The aim of the study is thus to study the competition betweena structural effect (surface defect) and a material effect (grain size) in the context of mechanical stresses in HCF.In order to obtain a comparable different grain size, a thermomechanical protocol has been developed. HCF tests,using specimens from both microstructure sizes (initial material and processed hardened material) in which hemisphericaldefects of different sizes were introduced, were performed to estimate the fatigue limits for different defectsize / grain size ratios. When Kitagawa diagrams are presented in relative values (fatigue limit / fatigue limit ofdefect free material versus defect size / grain size), there is a single curve that combines the two microstructures.This dimensionless Kitagawa diagram thus makes it possible to analyze the reduction of the fatigue limit inducedby a defect. The use of the relative size of the defect with respect to the characteristic microstructural dimensionappears to be more relevant than the use of the physical size of the defect.These experimental results are used to reproduce the HCF tests with Finite Element simulations on 3D microstructuresrepresentative of Armco iron. The competition existing between the stress concentration induced by thegeometrical defect and the highly stressed regions of the microstructure generated by the anisotropy of the mechanicalbehavior of the grains is studied. A mesoscopic criterion (involving mechanical quantities averaged by grain)based on a statistical approach allows to find the evolution of the dimensionless Kitagawa diagram, ie the relativesize of the critical defect from which it predominates over the response heterogeneity of the microstructure and thusgoverns the fatigue behavior of the polycrystal. The modification of the mesoscopic criterion by taking into accountintragranular heterogeneities (with the standard deviation per grain of mechanical quantities) is discussed.

Ligas de titânio do tipo ??compostas de elementos não tóxicos são materiais com potencial para aplicações biomédicas por apresentarem baixo módulo de elasticidade, efeito de memória de forma, biocompatibilidade satisfatória e boa conformabilidade. Para os biomateriais, o conhecimento das propriedades de fadiga é essencial para garantir uma alta confiabilidade para implantes ortopédicos e odontológicos. As propriedades mecânicas dependem diretamente do processamento termomecânico, da taxa de resfriamento imposta à liga e da composição química, que são fatores responsáveis pela determinação de sua microestrutura. Neste contexto, neste trabalho foi avaliado o comportamento em fadiga da liga Ti-35Nb-7Zr (% em p.) para aplicações biomédicas. Trata-se de uma liga de titânio do tipo ??e a motivação desse estudo baseou-se em complementar resultados de suas propriedades microestruturais e mecânicas obtidos em estudos anteriores realizados no DEMAR - EEL/USP. A liga foi produzida por fusão a arco a partir de materiais (Ti, Nb, Zr) de pureza comercial. A rota de processamento termomecânico envolveu as etapas de tratamento térmico de solubilização, forjamento rotativo a frio, tratamento térmico de recristalização. Um tratamento térmico ultrarrápido (Flash) foi adicionado para promover o refino de grãos e aumento da resistência mecânica. A caracterização microestrutural foi realizada por técnicas de microscopia óptica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão, difratometria de raios X e medidas de dureza Vickers. As propriedades mecânicas foram investigadas por ensaios de tração uniaxial e ensaios de fadiga de alto ciclo por flexão rotativa de corpos de prova lisos e entalhados. Com relação ao conjunto de propriedades, os melhores resultados podem ser considerados para a condição recristalizada à 1000ºC/2h + flash. Os resultados obtidos neste trabalho confirmam a possibilidade de uso da liga Ti-35Nb-7Zr para aplicações biomédicas. / ?-type titanium alloys composed of non-toxic elements are materials with potential for biomedical applications because they have low elastic modulus, shape memory effect, satisfactory biocompatibility and good workability. For biomaterials, knowledge of fatigue properties is essential to ensure high reliability for orthopedic and dental implants. The mechanical properties depend directly on thermomechanical processing, alloy cooling rate and chemical composition, which are responsible for the determination of its microstructure. In this context, this work was evaluated the fatigue behavior of the Ti- 35Nb-7Zr alloy (wt. %) for biomedical applications. It is a ?-type alloy and the motivation of this study was based on complementing the results of its microstructural and mechanical properties previously obtained at DEMAR-EEL/USP. The alloy was produced from materials of commercial purity (Ti, Nb and Zr) by arc meling. The thermomechanical processing route consisted the steps of solubilization heat treatment, cold rotary forging, recrystallization heat treatment. A ultrafast thermal treatment (Flash) was additioned to promote the grain refinement and to increase mechanical strength. The microstructural characterization was done by optical microscopy, scanning electron microscopy, transmission electron microscopy, X ray diffraction techniques and Vickers microhardness tests. The mechanical properties was investigated by uniaxial tensile tests and rotary bending high cycle fatigue tests of smooth and notched specimens. With respect to the set of properties, the best results can be considered for recrystalized condition at 1000ºC/2h + flash. The results obtained in this work confirm the possibility of using the Ti-35Nb-7Zr alloy for biomedical applications.

The design parameters axial gap and stator count of highpressure turbine stages are evaluated numerically towards theirinfluence on the unsteady aerodynamic excitation of rotorblades. Of particular interest is if and how unsteadyaerodynamic considerations in the design could reduce the riskofhigh cycle fatigue (HCF) failures of the turbine rotor. A well-documented 2D/Q3D non-linear unsteady code (UNSFLO)is chosen to perform the stage flow analyses. The evaluatedresults are interpreted as aerodynamic excitation mechanisms onstream sheets neglecting 3D effects. Mesh studies andvalidations against measurements and 3D computations provideconfidence in the unsteady results. Three test cases areanalysed. First, a typical aero-engine high pressure turbinestage is studied at subsonic and transonic flow conditions,with four axial gaps (37% - 52% of cax,rotor) and two statorconfigurations (43 and 70 NGV). Operating conditions areaccording to the resonant conditions of the blades used inaccompanied experiments. Second, a subsonic high pressureturbine intended to drive the turbopump of a rocket engine isinvestigated. Four axial gap variations (10% - 29% ofcax,rotor) and three stator geometry variations are analysed toextend and generalise the findings made on the first study.Third, a transonic low pressure turbine rotor, known as theInternational Standard Configuration 11, has been modelled tocompute the unsteady flow due to blade vibration and comparedto available experimental data. Excitation mechanisms due to shock, potential waves andwakes are described and related to the work found in the openliterature. The strength of shock excitation leads to increasedpressure excitation levels by a factor 2 to 3 compared tosubsonic cases. Potential excitations are of a typical wavetype in all cases, differences in the propagation direction ofthe waves and the wave reflection pattern in the rotor passagelead to modifications in the time and space resolved unsteadypressures on the blade surface. The significant influence ofoperating conditions, axial gap and stator size on the wavepropagation is discussed on chosen cases. The wake influence onthe rotorblade unsteady pressure is small in the presentevaluations, which is explicitly demonstrated on the turbopumpturbine by a parametric study of wake and potentialexcitations. A reduction in stator size (towards R≈1)reduces the potential excitation part so that wake andpotential excitation approach in their magnitude. Potentials to reduce the risk of HCF excitation in transonicflow are the decrease of stator exit Mach number and themodification of temporal relations between shock and potentialexcitation events. A similar temporal tuning of wake excitationto shock excitation appears not efficient because of the smallwake excitation contribution. The increase of axial gap doesnot necessarily decrease the shock excitation strength neitherdoes the decrease of vane size because the shock excitation mayremain strong even behind a smaller stator. The evaluation ofthe aerodynamic excitation towards a HCF risk reduction shouldonly be done with regard to the excited mode shape, asdemonstrated with parametric studies of the mode shapeinfluence on excitability. <b>Keywords:</b>Aeroelasticity, Aerodynamics, Stator-RotorInteraction, Excitation Mechanism, Unsteady Flow Computation,Forced Response, High Cycle Fatigue, Turbomachinery,Gas-Turbine, High-Pressure Turbine, Turbopump, CFD, Design

Microelectronic systems are subjected to thermal cycling, power cycling, and vibration environments in various applications. These environments, whether applied sequentially or simultaneously, affect the solder joint reliability. Literature is scarce on predicting solder joint fatigue failure under such multiple loading environments. This thesis aims to develop a unified modeling methodology to study the reliability of electronic packages subjected to thermal cycling, power cycling, and vibration loading conditions. Such a modeling methodology is comprised of an enriched material model to accommodate time-, temperature-, and direction-dependent behavior of various materials in the assembly, and at the same time, will have a geometry model that can accommodate thermal- and power-cycling induced low-cycle fatigue damage mechanism as well as vibration-induced high-cycle fatigue damage mechanism. The developed modeling methodology is applied to study the reliability characteristics of ceramic area array electronic packages with lead-based solder interconnections. In particular, this thesis aims to study the reliability of such solder interconnections under thermal, power, and vibration conditions individually, and validate the model against these conditions using appropriate experimental data either from in-house experiments or existing literature. Once validated, this thesis also aims to perform a design of simulations study to understand the effect of various materials, geometry, and thermal parameters on solder joint reliability of ceramic ball grid array and ceramic column grid array packages, and use such a study to develop universal polynomial predictive equations for solder joint reliability. The thesis also aims to employ the unified modeling methodology to develop new understanding of the acceleration factor relationship between power cycling and thermal cycling. Finally, this thesis plans to use the unified modeling methodology to study solder joint reliability under the sequential application of thermal cycling and vibration loading conditions, and to validate the modeling results with first-of-its-kind experimental data. A nonlinear cumulative damage law is developed to account for the nonlinearity and effect of sequence loading under thermal cycling, power cycling, and vibration loading.

A novel microstructure-sensitive extreme value probabilistic framework is introduced to evaluate material performance/variability for damage evolution processes (e.g., fatigue, fracture, creep). This framework employs newly developed extreme value marked correlation functions (EVMCF) to identify the coupled microstructure attributes (e.g., phase/grain size, grain orientation, grain misorientation) that have the greatest statistical relevance to the extreme value response variables (e.g., stress, elastic/plastic strain) that describe the damage evolution processes of interest. This is an improvement on previous approaches that account for distributed extreme value response variables that describe the damage evolution process of interest based only on the extreme value distributions of a single microstructure attribute; previous approaches have given no consideration of how coupled microstructure attributes affect the distributions of extreme value response. This framework also utilizes computational modeling techniques to identify correlations between microstructure attributes that significantly raise or lower the magnitudes of the damage response variables of interest through the simulation of multiple statistical volume elements (SVE). Each SVE for a given response is constructed to be a statistical sample of the entire microstructure ensemble (i.e., bulk material); therefore, the response of interest in each SVE is not expected to be the same. This is in contrast to computational simulation of a single representative volume element (RVE), which often is untenably large for response variables dependent on the extreme value microstructure attributes. This framework has been demonstrated in the context of characterizing microstructure-sensitive high cycle fatigue (HCF) variability due to the processes of fatigue crack formation (nucleation and microstructurally small crack growth) in polycrystalline metallic alloys. Specifically, the framework is exercised to estimate the local driving forces for fatigue crack formation, to validate these with limited existing experiments, and to explore how the extreme value probabilities of certain fatigue indicator parameters (FIPs) affect overall variability in fatigue life in the HCF regime. Various FIPs have been introduced and used previously as a means to quantify the potential for fatigue crack formation based on experimentally observed mechanisms. Distributions of the extreme value FIPs are calculated for multiple SVEs simulated via the FEM with crystal plasticity constitutive relations. By using crystal plasticity relations, the FIPs can be computed based on the cyclic plastic strain on the scale of the individual grains. These simulated SVEs are instantiated such that they are statistically similar to real microstructures in terms of the crystallographic microstructure attributes that are hypothesized to have the most influence on the extreme value HCF response. The polycrystalline alloys considered here include the Ni-base superalloy IN100 and the Ti alloy Ti-6Al-4V. In applying this framework to study the microstructure dependent variability of HCF in these alloys, the extreme value distributions of the FIPs and associated extreme value marked correlations of crystallographic microstructure attributes are characterized. This information can then be used to rank order multiple variants of the microstructure for a specific material system for relative HCF performance or to design new microstructures hypothesized to exhibit improved performance. This framework enables limiting the (presently) large number of experiments required to characterize scatter in HCF and lends quantitative support to designing improved, fatigue-resistant materials and accelerating insertion of modified and new materials into service.

<p>The design parameters axial gap and stator count of highpressure turbine stages are evaluated numerically towards theirinfluence on the unsteady aerodynamic excitation of rotorblades. Of particular interest is if and how unsteadyaerodynamic considerations in the design could reduce the riskofhigh cycle fatigue (HCF) failures of the turbine rotor.</p><p>A well-documented 2D/Q3D non-linear unsteady code (UNSFLO)is chosen to perform the stage flow analyses. The evaluatedresults are interpreted as aerodynamic excitation mechanisms onstream sheets neglecting 3D effects. Mesh studies andvalidations against measurements and 3D computations provideconfidence in the unsteady results. Three test cases areanalysed. First, a typical aero-engine high pressure turbinestage is studied at subsonic and transonic flow conditions,with four axial gaps (37% - 52% of cax,rotor) and two statorconfigurations (43 and 70 NGV). Operating conditions areaccording to the resonant conditions of the blades used inaccompanied experiments. Second, a subsonic high pressureturbine intended to drive the turbopump of a rocket engine isinvestigated. Four axial gap variations (10% - 29% ofcax,rotor) and three stator geometry variations are analysed toextend and generalise the findings made on the first study.Third, a transonic low pressure turbine rotor, known as theInternational Standard Configuration 11, has been modelled tocompute the unsteady flow due to blade vibration and comparedto available experimental data.</p><p>Excitation mechanisms due to shock, potential waves andwakes are described and related to the work found in the openliterature. The strength of shock excitation leads to increasedpressure excitation levels by a factor 2 to 3 compared tosubsonic cases. Potential excitations are of a typical wavetype in all cases, differences in the propagation direction ofthe waves and the wave reflection pattern in the rotor passagelead to modifications in the time and space resolved unsteadypressures on the blade surface. The significant influence ofoperating conditions, axial gap and stator size on the wavepropagation is discussed on chosen cases. The wake influence onthe rotorblade unsteady pressure is small in the presentevaluations, which is explicitly demonstrated on the turbopumpturbine by a parametric study of wake and potentialexcitations. A reduction in stator size (towards R≈1)reduces the potential excitation part so that wake andpotential excitation approach in their magnitude.</p><p>Potentials to reduce the risk of HCF excitation in transonicflow are the decrease of stator exit Mach number and themodification of temporal relations between shock and potentialexcitation events. A similar temporal tuning of wake excitationto shock excitation appears not efficient because of the smallwake excitation contribution. The increase of axial gap doesnot necessarily decrease the shock excitation strength neitherdoes the decrease of vane size because the shock excitation mayremain strong even behind a smaller stator. The evaluation ofthe aerodynamic excitation towards a HCF risk reduction shouldonly be done with regard to the excited mode shape, asdemonstrated with parametric studies of the mode shapeinfluence on excitability.</p><p><b>Keywords:</b>Aeroelasticity, Aerodynamics, Stator-RotorInteraction, Excitation Mechanism, Unsteady Flow Computation,Forced Response, High Cycle Fatigue, Turbomachinery,Gas-Turbine, High-Pressure Turbine, Turbopump, CFD, Design</p>

The purpose of this thesis is to investigate the fatigue life of two components in different hydraulic quick couplings. The crack which initiates the fatigue failure usually occurs in the bottom of one of the threads. Thus, the threads in the coupling are carefully investigated. A few different approaches for determining the fatigue life are studied for each component. CAD (Computer Aided Design) models for both components are created and analyzed with FEM (Finite Element Method). By analyzing these components with FEM the stresses and the strains are calculated. These values for stress and strain are used to determine the fatigue life for the different components. The first component is suffering from high cycle fatigue. Four different approaches for high cycle fatigue are investigated. These four are Wöhler-curve, Haigh-diagram, von Mises fatigue criterion and Mises-Sines fatigue criterion. It was found that all of them but the von Mises fatigue criterion give a good approximation of the fatigue life. von Mises fatigue criterion does not consider the midstress, which can be a reason for this. The three other approaches indicate that the component will be suffering from fatigue, but will be relatively close to infinite life. The Wöhler-curve indicates that the component will withstand about 470 000 loading cycles. Compared to the experimental tests where the component withstands about 700 000 loading cycles, it can be said that the approach is a good estimation of the fatigue life. Approximated material data are used. For further work it is essential to obtain experimental material data for better approximation of the fatigue life. The second component is suffering from low cycle fatigue, which means that different approaches are needed. The approaches are originally from Coffin-Manson and Morrow. A total of four low cycle fatigue criterion have been studied. The first one is made by CoffinManson and takes the plastic strain in to account, the second is made by Morrow and takes both the elastic and the plastic strain in to account.  The third is a modification of Morrow’s criterion, and will consider the midstresses as well. The fourth is a simplification of Morrow’s criterion where more common material parameters are used. These four have been compared and it can be seen that the CoffinManson criterion gives the best approximation of the reality. The Coffin-Manson criterion gives an approximation of 50 000 loading cycles to fatigue failure which, compared to that the experimental tests suffered from fatigue failure after about 40 000 loading cycles, is a good estimation of the fatigue life.

No description available.

[Truncated abstract. Please see pdf format for complete text.] Over the years much work has been done on studying sequence effects under multilevel loading. Yet, the underlying fatigue mechanisms responsible for such interactions are not fully understood. The study of fatigue under complex loading histories begins by investigating strain interaction effects arising from simple 2-step loading sequences. Fatigue for all investigations were conducted under uniaxial push-pull mode in strain-control. Fatigue is traditionally classified as either low or high cycle fatigue (LCF and HCF respectively). The boundary for LCF and HCF is not well-defined even though the fatigue life of LCF is typically dominated by crack “initiation”, while for HCF, fatigue life is usually dominated by stable crack growth. The terms LCF and HCF, apart from referring to the low and high number of fatigue cycles required for failure, also bear little physical meaning in terms of describing the state of fatigue imposed. As a result, conventional definitions of the two distinct regimes of fatigue are challenged and a new method of classifying the boundary between the two regimes of fatigue is proposed. New definitions are proposed and the terms plastically dominant fatigue (PDF) and elastically dominant fatigue (EDF) are introduced as suitable replacements for LCF and HCF respectively. PDF refers to the condition of a material undergoing significant reverse plasticity during cyclic loading, while for EDF, minimal reverse plasticity is experienced. Systematic testing of three materials, 316 L stainless steel, 6061-T6 aluminium alloy and 4340 high strength steel, was performed to fully investigate the cycle ratio trends and “damage” accumulation behaviour which resulted from a variety of loading conditions. Results from this study were carried over to investigate more complex multilevel loading sequences and possible mechanisms for interaction effects observed both under 2-step and multi-step sequences were proposed. Results showed that atypical cycle ratio trends could result from loading sequences which involve combinations of strain amplitudes from different fatigue regimes (i.e. PDF or EDF). Mean strain effects on fatigue life were also studied. The objective of this study was to identify regimes of fatigue which are significantly influenced by mean strains. Results indicated that mean strains affected EDF but not PDF. 2-step tests, similar to those performed in earlier studies were conducted to investigate the effects of mean strain on variable amplitude loading. Again, atypical cycle ratio trends were observed for loading sequences involving combinations of PDF and EDF. It is understood that fatigue crack growth interaction behaviour and mean stress effects are two dominant mechanisms which can be used to explain cycle ratio trends observed. The significance and importance of proper PDF/EDF definition and specification are also stressed. The study of fracture mechanics is an important component of any fatigue research. Fatigue crack growth in 4140 high strength steel CT specimens, under conditions of plane stress and plane strain were studied. In this investigation, the effects of R and overload ratios were also studied for both plane stress and plane strain conditions. Results indicate that differences in the point of crack “initiation” under both plane stress and plane strain conditions decrease with increasing load range, while the extent of crack retardation as a result of overloading, is greater under plane stress than plane strain conditions. The extent of crack growth retardation increases with decreasing R ratios and increasing overload ratios. The final phase of this project involves the proposal of two practical models used to predict cumulative “damage” and fatigue crack propagation in metals. The cumulative “damage” model proposed takes the form of a power law and the exponent which governs “damage” accumulation can easily be calculated by knowing the failure life, Nf, for a given strain or load level. Predictions for the “damage” model performed better when compared to other popular cumulative “damage” models. The second model proposed predicts fatigue crack growth behaviour from known monotonic and smooth specimen fatigue data. There are several benefits of having a model that can predict fatigue crack growth from monotonic and smooth specimen fatigue data: a) traditionally, engineers had to rely on expensive and time-consuming crack propagation tests to evaluate and select materials for maximum fatigue resistance, and b) monotonic and smooth specimen fatigue data are readily available. The crack propagation model is proposed to alleviate the material selection process by providing engineers a means to rapidly eliminate and narrow down selections for possible material candidates.

Dans les générateurs de vapeur des centrales nucléaires à eau pressurisée, les tuyauteries sont soumises à des chargements thermique et mécanique, qui sont variables et divisés en deux régimes différents : la fatigue oligocyclique et la fatigue gigacyclique. Les aciers au carbone – manganèse, type A42, A48 et Tu48 (normes françaises) sont souvent utilisés dans de telles applications. Les propriétés du matériau manifestent certains caractères spéciaux en mécanique et métallurgie comme le vieillissement dynamique conduisant à une augmentation de la valeur de la contrainte maximale et une diminution de la ductilité à la température 200 ℃. Le comportement en fatigue oligocyclique et gigacyclique sont étudiés à température ambiante et 200 ℃. Des essais de fatigue cumulée ont été mis en oeuvre pour étudier l’effet du cumul de dommage combinant des cycles de fatigue oligocyclique suivis de cycles en fatigue gigacyclique. Tous les résultats sont analysés en utilisant la mécanique de l’endommagement des milieux continus et l’analyse microfractographique. Les cycles d’hystérésis en fatigue oligocyclique sont dus à la déformation plastique de durcissement cinématique, ils peuvent être décrits par sous le modèle d’Armstrong – Frederick ; le durcissement isotrope est utile pour prédire l’évolution de l’amplitude de contrainte. Mais avec l’augmentation de la déformation plastique accumulée, le dommage ne peut être négligé. Le modèle de Chaboche d’endommagement par fatigue est utilisé pour décrire l’évolution des dommages oligocyclique et il est étendu au régime gigacyclique. Un modèle de fatigue cumulée des dommages a été développé à partir du modèle de Chaboche et appliqué à l’estimation des dommages de fatigue pour décrire le comportement de l’évolution de la contrainte en fonction du nombre de cycles. En fatigue oligocyclique à la température de 200 ℃, l’acier A48 est sensible au phénomène de vieillissement dynamique et il apparaît un durcissement secondaire, qui peut être prédit par la théorie des dislocations et est simulé dans la thése. L’analyse des surfaces de rupture est effectuée par fractographie au Microscope Electronique à Balayage pour les essais en oligocyclique, gigacyclique et cumul. En fatigue oligocylique, la fissure est initiée en surface. En fatigue gigacyclique, certaines fissures sont initiées sur des inclusions situées à l’intérieur d’éprouvettes. / In steam generators of nuclear power plants, typical pipes components are subjected to thermal and mechanical loading which are variable and divided into two different regimes: low cycle fatigue and gigacycle fatigue. Carbon-manganese steels A42, A48 and Tu48 steels (French standards) are often used in such applications. The material properties manifest some special characters in mechanics and metallurgy such as Dynamic Strain Aging, increasing UTS values in 200℃ temperature domain. The LCF and VHCF behaviors are investigated respectively by test method at room temperature and 200℃. The cumulative fatigue tests are implemented through referencing the load as prior LCF following gigacycle fatigue from the steam generator pipes thermal loads in order to obtain the performance of material under accumulated fatigue damage. All the test results are analyzed by using plastic mechanics, continuums damage mechanics and microscopic analysis. Hysteresis loops are due to plastic deformation in LCF which is the effect of kinematic hardening and they can be described by Armstrong – Frederick form models; the isotropy hardening is used to predict the evolution of stress amplitude in LCF. But with rising of accumulated plastic deformation, the damage can not be neglected. The Chaboche fatigue damage model is applied to describe the damage evolution of LCF and extended to VHCF regime. The cumulative fatigue damage model is extended from Chaboche model and applies to the estimation cumulative fatigue damage. The constitutive relationship and isotropy rule are coupled with fatigue damage model that can describe the whole fatigue behavior. In 200℃ for LCF, A48 is sensitive to dynamic strain aging and its secondary hardening behavior is important which can be predicted by dislocation theory and is simulated in the paper. The fractographic analysis is performed by SEM for LCF, VHCF and cumulative fatigue tests. The LCF crack is initiated in surface. Some of cracks of VHCF are given birth from the inclusions located at interior of sample.