O uso das ligas de alumínio em aplicações estruturais tem crescido consideravelmente nas últimas décadas. Nos transportes, a baixa massa específica do alumínio resulta em uma alta razão resistência/ peso, favorecendo a fabricação de aviões, trens e automóveis. Com a crescente preocupação em reduzir a emissão de gases poluentes, vem tornando-se promissora a alternativa de reduzir o peso dos veículos substituindo peças convencionalmente produzidas com outros materiais por peças de alumínio. As ligas tratáveis termicamente da série 6xxx são frequentemente escolhidas para estas aplicações. Assim, para que o emprego dessas ligas seja otimizado, é importante um estudo mais detalhado de suas propriedades mecânicas, principalmente sob solicitações cíclicas. Neste trabalho foram estudadas ligas de Al-Si-Mg que são amplamente utilizadas nas indústrias automotivas, em especial na fabricação de componentes de carroçarias para caminhões e ônibus. Foi realizado o estudo do comportamento em fadiga de baixo ciclo e fadiga multiaxial das ligas AA6005 T6, AA6063 T6 e AA6351 T6, fornecidas pela CBA (Companhia Brasileira de Alumínio) visando caracterizar e comparar essas ligas em sua microestrutura, propriedades de tração e fadiga. As propriedades básicas de fadiga foram determinadas por meio do método ε-N (fadiga de baixo ciclo) e os ensaios foram realizados com controle de deformação total, onda triangular e taxa de deformação 0,005 seg-1. As análises dos laços de histerese elasto-plástica permitiram inferir sobre aspectos microestruturais relacionados às propriedades mecânicas das ligas estudadas. O comportamento em fadiga multiaxial foi avaliado por meio de carregamentos combinados axial-torcional em fase e fora de fase. Para ajustar os dados experimentais obtidos, foram testados alguns dos modelos encontrados na literatura. Os cálculos baseados no modelo de plano crítico, proposto por Fatemi e Socie, apresentaram resultados satisfatórios. Também foram realizadas análises microestruturais e fractográficas para as três ligas. As superfícies de fratura dos ensaios de fadiga multiaxial mostraram resultados diferentes de acordo com o carregamento adotado. A avaliação comparativa das três ligas estudadas fornece subsídios para fundamentar a seleção de materiais para a fabricação de componentes estruturais para o setor automotivo. / The use of aluminum alloys in structural applications has grown considerably in recent decades. In transportation, the low density of aluminum results in a high strength-to weight ratio, proving attractive for production of aircrafts, trains and automobiles. With a growing concern for the reduction of pollutant gas emissions, aluminum alloys are becoming a promising alternative to diminish vehicle weight through the replacement of conventionally produced parts made from other heavier materials for aluminum parts. The heat treatable alloys from the 6xxx series are often chosen for these applications. Therefore, to optimize the employment of these alloys, a detailed study of their mechanical properties, primary under cyclic solicitations is necessary For the present study Al-Mg-Si alloys were chosen, which are widely used in automotive industries, particularly in the manufacturing of components for trucks and bus bodies. The low-cycle fatigue behavior and multiaxial fatigue of the three following aluminum alloys: AA6005 T6, AA6063 T6 and AA6351 T6, provided by CBA (Brazilian Aluminum Company), were assessed, with the aim of characterizing and comparing these alloys in their microstructure, tensile properties and fatigue. The basic properties of fatigue were studied by ε-N method (low cycle fatigue) and the experiments were performed with total strain control, triangular waveform and with a constant deformation rate of 5.0x10-3 s-1. The analyses of hysteresis loops elasto-plastic provided insight about microstructural aspects, related to mechanical properties of the studied alloys. Multiaxial fatigue behavior was assessed in combined axial-torsion loading in phase and out of phase. To adjust the experimental data, some models found in the literature were tested. Calculations based on critical plane model, proposed by Fatemi Socie, presented satisfactory results. Furthermore, microstructure analyses and fractography were performed for these three alloys. The fracture surface of multiaxial fatigue assays demonstrated different results according to the adopted loading. Comparative evaluation of the three studied alloys provides support for the selection of materials for manufacturing structural components of the automotive sector.

Para o correto dimensionamento da maioria dos componentes estruturais é necessário informações sobre a resposta do material quando submetido à fadiga de alto e baixo ciclo, bem como conhecer as propriedades monotônicas (não-cíclicas) e cíclicas dos materiais. Na literatura são encontradas amplas divulgações de dados sobre diversos materiais de engenharia (Ex. SAE J1099 Technical Report on Fatigue Properties). Porém, quando se trata de ferro fundido nodular estas informações são limitadas, sendo assim, visa-se neste trabalho caracterizar as propriedades monotônicas e cíclicas destes materiais em complemento aos trabalhos já publicados na literatura e propor métodos para a estimativa destas propriedades a partir da dureza. Faz-se necessário a proposição de métodos estimativos das propriedades mecânicas destes materiais baseados na dureza devido às suas grandes variações que são inerentes ao processo de fundição. Em um mesmo componente podem existir diferentes classes de ferro fundido, que apesar de possuir a mesma composição química, podem apresentar variações nas propriedades mecânicas devido à formação de diferentes estruturas metalúrgicas que são sensíveis às taxas de resfriamento do material que variam de acordo com as características geométricas da peça que está sendo fundida, principalmente a variação da espessura. Neste estudo a determinação das relações entre as propriedades monotônicas e cíclicas dos ferros fundidos nodulares foram obtidas a partir do tratamento dos dados publicados na literatura levando-se em consideração o índice de qualidade. Foi proposto um modelo contínuo com relação à dureza para a estimativa das propriedades monotônicas, do coeficiente de resistência cíclico e do expoente de encruamento cíclico e para a estimativa das propriedades cíclicas que experimentalmente demonstraram não ter correlação com a dureza foi proposto uma forma discreta, que consistiu na recomendação de valores típicos definidas por faixas de dureza. / For the correct design of the most part of structural components is necessary to have information about the material response under both high cycle and low cycle fatigue, as well as the knowledge of monotonic and cyclic materials properties. In literature a major publication of several engineering material data can be found (e.g, SAE J1099 - Technical Report on Fatigue Properties), but regarding to ductile iron this information is quite limited. Therefore, this work aims to characterize the monotonic and cyclic properties of this material in complementing to the available data in the literature and also make a proposition of methods to estimate this properties from hardness. The mechanical properties estimation model from hardness is relevant to take into account the inherent variations of casting process, which for the same chemical composition can be found different grades in a same part. This fact occurs due to the formation of different metallurgical structures that is influenced by cooling ratio which changes accordingly to geometrical characteristic of the part, especially the thickness variation. In this study the determination of the relation between monotonic and cyclic properties from hardness has been determined from literature data processing taking into account the Quality Index. For monotonic properties, the cyclic strength coefficient and the cyclic strain hardening exponent estimation has been proposed a continuous method based on hardness and for the cyclic properties that experimentally showed to remain independent of hardness has been recommended one set of properties for specific hardness ranges.

Neste trabalho, foram determinadas as propriedades mecânicas de fadiga de baixo e alto ciclo e de tenacidade à fratura JIC de um aço inoxidável austenítico do sistema Cr-Mn-N, utilizado na confecção de colares para perfurações pela indústria de exploração de petróleo. Foi também avaliada a influência da temperatura de serviço nas propriedades mecânicas do aço, que apresentou um decréscimo significativo nos valores dos limites de escoamento e de resistência e do alongamento com o aumento da temperatura. Na temperatura ambiente, os ensaios de fadiga de baixo e alto ciclo foram realizados segundo as normas ASTM E606 e ASTM E466. Em temperaturas superiores, as propriedades de fadiga foram estimadas utilizando alguns métodos atualmente disponíveis, que utilizam somente as propriedades mecânicas de tração. Os resultados obtidos mostraram uma pequena influência da temperatura na vida à fadiga na região de baixo ciclo e uma grande influência na região de alto ciclo. A curva tensão-deformação cíclica do material foi obtida pelo método convencional e pelo método do passo incrementai, que utiliza somente um corpo de prova. Os ensaios de JIC foram realizados nas temperaturas ambiente e 150°C, segundo a norma ASTM E813, utilizando a técnica de variação da flexibilidade elástica. Na temperatura ambiente, foi observada uma grande dispersão nos valores obtidos de JIC, atribuída à presença de uma fase frágil, identificada como precipitados de carbonitretos mistos de ferro, cromo, manganês e nióbio. Foi também observada uma diminuição no valor médio de JIC com o aumento da temperatura. / In the present work, the low and high cycle fatigue and fracture toughness properties of a Cr-Mn-N austenitic stainless steel used for application in drill collars were investigated. Also, the temperarure dependence of the tensile properties was determined, and it was found that the yield strength, ultimate strength and elongation values decrease significantly with the temperature. The low and high cycle fatigue testing were carried out at room temperature according to ASTM E606 and ASTM E466 standards. At high temperarure, the fatigue properties were estimated from monotonic propetties obtained in the tensile testing. The results showed small influence of temperature in the fatigue life in the short-life range and great influence in the long-life range. The cyclic stress-strain curve was obtained by companion specimen tests and incremental step tests methods. The last method showed to be advantageous because only one specimen and short testing time were required. The fracture toughness was detennined at room temperature and at 150°C applying elastic-plastic fracture mechanics concepts. The single specimen elastic compliance technique for crack length determination was employied. At room temperature a great scattering of the JIC values was observed due the presence of the brittle phase identified as a precipitates of carbonitride content Fe, Cr, Mn, Nb elements. Also a decreasing of average value o f the JIC was observed with the increasing of temperature.

Realizou-se neste trabalho uma investigação comparativa do comportamento dos aços ferramenta H13 e THYROTHERM 2999 EFS SUPRA, destinados à fabricação de matrizes para conformação a quente, quando submetidos à fadiga de baixo ciclo a altas temperaturas (FBCAT). A partir de suas curvas de revenimento, foram definidas três durezas de trabalho para cada material (durezas de 42, 52 e 58 HRC), correspondendo a três temperaturas de revenimento distintas e três condições de estudo, buscando-se a condição ótima apresentada por estes materiais para este tipo de aplicação, visando-se então analisar a influência da dureza inicial do material na vida do componente. Foi determinada também a temperatura de ensaio de fadiga isotérmica, em 400°C, correspondente à temperatura de utilização da matriz, ou seja, uma temperatura crítica típica que a matriz atinge durante a solicitação em trabalho. A seguir foram realizados para cada material os ensaios de tração a temperatura ambiente, e na seqüência, os ensaios de tração na temperatura de trabalho definida, que permitiram a determinação dos primeiros parâmetros monotônicos dos materiais, dentre eles uma previsão para os níveis de deformação a serem utilizados nos ensaios de fadiga (0.5,0.6,0.7,0.8,0.9,1.0 e 1.1%), e demais parâmetros como E, k, n, σe, σ’f, ε’f, b, c, que permitiram a elaboração de curvas ε−N, com um modelo estimativo já existente. Finalmente, foram então realizados os ensaios de fadiga isotérmica de baixo ciclo, à temperatura de 400°C, e os resultados foram utilizados para a elaboração das curvas ε−N, resultando então na proposta de um modelo de previsão de resistência à fadiga específico para os materiais pesquisados. / It was made in this work an investigative comparison of the behavior of the tool steels H13 and THYROTHERM 2999 EFS SUPRA, designed for die steels for hot forming, when exposed to high temperature low cycle fatigue (HTLCF). From their tempering curves three material working hardness were defined for each material (hardness of 42, 52 and 58 HRC), corresponding to three different tempering temperatures, and so three study cases for each material, searching for the best condition for this kind of application, and to assess the influence of the initial hardness on the part material life. The isothermal low cycle fatigue test temperature was either defined at 400°C, corresponding to the used temperature at the die steel, i.e., a critical typical temperature that the forging dies reach on hot working. After that, tensile tests were performed for both materials, at room temperature, and at the working temperature formerly defined, and these tests allowed the definition of the first monotonic parameters for these materials, among them predictions for strain levels (0.5, 0.6, 0.7, 0.8, 0.9, 1.0 and 1.1%), to be used on fatigue tests, and further parameters like E, k, n, σe, σ’f, ε’f, b, c, that allowed the elaboration of ε−N curves, based on a still existing prediction model. Finally, isothermal low cycle fatigue tests were performed, at 400°C, and the results were used for ε−N curves elaboration, resulting on a prediction model of the fatigue strength specified for the assessed materials.

High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such environments superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface. The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with a thermodynamically stable oxide layer that acts as a diffusion barrier. During service life, mechanical properties of metallic coatings can be changed due to the significant interdiffusion between substrate and coating. There are also other degradation mechanisms that affect nickel-base superalloys such as low cycle fatigue, thermo-mechanical fatigue and creep. The focus of this work is on a study of low cycle fatigue and out-of-phase thermo-mechanical fatigue behaviour of three uncoated and coated nickel-base superalloys. Polycrystalline IN792 and two single crystals CMSX-4 and SCB were coated with four different coatings; an overlay coating AMDRY997 (NiCoCrAlYTa), a platinum aluminide modified diffusion coating RT22 and two innovative coatings with a NiW interdiffusion barrier in the interface called IC1 and IC3. A low cycle fatigue and thermo-mechanical fatigue device was designed and set-up to simulate service loading of turbine blades and vanes. The low cycle fatigue tests were run at 500oC and 900oC while the thermo-mechanical fatigue tests were run between 250oC and 900oC.To simulate long service life, some coated specimens were exposed at 1050oC for 2000 h before the tests. The main conclusions are that the presence of the coatings is, in most cases, detrimental to LCF lives of the superalloys at 500oC while the coatings do improve the LCF lives of the superalloys at 900oC. Under TMF loading conditions, the coatings have negative effect on the lifetime of IN792. On single crystals, they are found to improve TMF life of the superalloys, especially at lower strains. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged specimens. The aged specimens exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing transgranularly perpendicular to the load axis.

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.

Atlikta literatūros apžvalga parodė, kad tyrimų, nagrinėjant medžiagas būdingame intervale kambario ir aukštesnėje temperatūroje nebuvo atlikta. Todėl šiame darbe, nustatyti analizuojamų duomenų sklaidos intervalai: legiruotiems konstrukciniams plienams bei legiruotų konstrukcinių plienų suvirinimo siūlių medžiagoms (kambario ir aukštesnėje temperatūroje). Atliktas deformavimo, nuovargio parametrų ir modifikuoto plastiškumo kriterijaus reikšmių pasiskirstymo pagal normalųjį dėsnį hipotezių tikrinimas, kuris parodė, kad dauguma nuovargio nuo modifikuoto plastiškumo kriterijaus duomenų yra pasiskirstę pagal normalųjį dėsnį. Pasiūlyti analitinių priklausomybių koeficientai tarp suirimo kreivės parametrų ir modifikuoto plastiškumo kriterijaus. Gauti koreliacinės analizės duomenys patvirtino medžiagų atmetimo (būdingo intervalo nustatymo) tikslingumą. Tai reiškia, kad darbe nustatytos analitinės priklausomybės gali būti taikomos įrenginių konstrukcijų preliminariam ilgalaikiškumui bei patikimumui apskaičiuoti. / Performed literature review showed that any studies of examining materials in intrinsic range at room and elevated temperature haven’t been performed. That’s why, in this paper, analyzed data setted diffusion ranges-alloyed for structural steels and alloy structural steels weld (at room and elevated temperatures). Performed deformation, fatigue parameters and modified plasticity criteria value distribution by the normal law hypothesis testing, which showed that most of the fatigue plasticity criteria from modified data is distributed according to the normal principle. Offered analytical dependency ratios between the disintegrate curve parameters and modified plasticity criteria. Counted correlation analysis appropriate rejection of materials (in typical range). This means that in paper setted analytical relationships can be used for equipment preliminary calculation of durability and reliability.

Emission regulations for heavy-duty diesel engines are becoming increasingly restrictive to limit the environmental impacts of exhaust gases and particles. Increasing the specific power output of diesel engines would improve fuel efficiency and greatly reduce emissions, but these changes could lead to increased exhaust gas temperature, increasing demands on the exhaust manifold material. This is currently the ferritic ductile cast iron alloy SiMo51, containing about 4 wt% Si and ~1 wt% Mo, which operates close to its fatigue and oxidation resistance limits at peak temperature (750C). To ensure high durability at higher temperatures, three different approaches to improving the life of exhaust manifolds were developed in this thesis. The first approach was to modify SiMo51 by adding different combinations of Cr and Ni to improve its high-temperature strength and oxidation resistance, or by applying a thermal barrier coating (TBC) to reduce the material temperature and thereby improve fatigue life. In the second approach, new materials for engine components, e.g. austenitic ductile iron and cast stainless steel, were investigated for their high-temperature fatigue and oxidation properties. In order to identify the most suitable alloys for this application, in the third the environmental effects of the corrosive diesel exhaust gas on the fatigue life of SiMo51 were investigated. The high-temperature oxidation resistance of SiMo51 at 700 and 800C in air was found to be improved by adding Cr, whereas Ni showed adverse effects. The effects of solid-solution hardening from Ni and precipitation hardening from Cr were low at 700C, with improvements only at lower temperatures. Applying a TBC system, providing thermal protection from a ceramic topcoat and oxidation protection from a metallic bond coat, resulted in only small reductions in material temperature, but according to finite element calculations still effectively improved the fatigue life of a turbo manifold. Possible alternative materials to SiMo51 identified were austenitic cast ductile iron Ni-resistant D5S and austenitic cast stainless steel HK30, which provided high durability of exhaust manifolds up to 800 and 900C, respectively. Corrosion fatigue testing of SiMo51 at 700C in diesel exhaust gas demonstrated that the corrosive gas reduced fatigue life by 30-50% compared with air and by 60-75% compared with an inert environment. The reduced fatigue life was associated with a mechanism whereby the crack tip oxidized, followed by crack growth. Thus another potential benefit of TBC systems is that the bond coat may reduce oxidation interactions and further improve fatigue life. These results can be used for selecting materials for exhaust applications. They also reveal many new research questions for future studies. Combining the different approaches of alloy modification, new material testing and improving the performance using coatings widened the scope of how component life in exhaust manifolds can be improved. Moreover, the findings on environmental interactions on SiMo51 fatigue provide a completely new understanding of these processes in ductile irons, important knowledge when designing components exposed to corrosive environments. The novel facility developed for high-temperature corrosion fatigue testing can be useful to other researchers working in this field. / <p>QC 20150507</p>

[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.