Ein zentraler Baustein zur Reduktion von CO2-Emissionen ist der Ausbau der erneuerbaren Energien, insbesondere der Windenergie. Forschungsbedarf besteht dabei bei der ressourceneffizienten Herstellung der Turmstrukturen. Bei Nabenhöhen von über 100 Metern sind Hybridtürme aus vorgespannten Stahlbetonsegmenten die geeignetste Konstruktion. Hierfür ist jedoch eine genaue Kenntnis des Ermüdungsverhaltens von Beton erforderlich. In der Literatur existieren überwiegend Untersuchungen an kleinformatigen zylindrischen Probekörpern, deren Ergebnisse nur bedingt auf die großmaßstäblichen Bauteile übertragen werden können. Im Rahmen dieses Vorhabens wurden daher zum einen Großversuche mit zyklisch biegebeanspruchten, vorgespannten Betonbalken sowie Begleitversuche an zylindrischen Probekörpern und zum anderen numerische Simulationen der Balkenversuche durchgeführt. Das numerische Materialmodell wurde aufbauend auf einem additiven Dehnungsmodell im Finite-Elemente-Programm ANSYS Mechanical in einem iterativen Berechnungsablauf implementiert. Die Betondehnungen setzen sich hierbei aus vier Anteilen zusammen, einem elastischen, einem plastischen, einem viskosen und einem Temperaturdehnungsanteil. Somit konnte der kombinierte Einfluss der Anteile auf das Ermüdungsverhalten von Beton dargestellt werden. In den Großversuchen konnte bei den Balkenprobekörpern ein Ermüdungsversagen der Betondruckzone erzeugt werden, das sich an dieser Stelle durch Risse parallel zur Drucknormalspannung sowie teilweises Abplatzen der Betondruckzone, die der größten Spannungsschwingbreite ausgesetzt war, einstellte. Es zeigte sich, dass dies erst nach deutlich mehr Lastwechseln eintrat als bei den axial beanspruchten Betonzylindern in den zyklischen Begleitversuchen mit derselben Spannungsschwingbreite. Dies ist auf die Spannungsumlagerung zurückzuführen, die im Querschnitt aufgrund der ermüdungsbedingten Materialdegradation und Steifigkeitsverringerung der stark beanspruchten Randbereiche stattfand. In den Begleitversuchen wurden die Materialparameter für das numerische Modell ermittelt, mit dem im Anschluss die Balkenversuche nachgerechnet wurden. Es konnten die in den Versuchen beobachteten Effekte der Steifigkeitsdegradation und Spannungsumlagerung und die daraus resultierende Lebensdauerverlängerung nachgebildet werden. Das Modell kann somit für weitergehende Lebensdaueruntersuchungen von ermüdungsbeanspruchten Betonbauteilen verwendet werden.:ABSCHLUSSBERICHT 1 Allgemeine Angaben 2 Zusammenfassung / Summary 3 Wissenschaftlicher Arbeits- und Ergebnisbericht 4 Veröffentlichte Projektergebnisse

[pt] Pesquisas atuais evidenciam que diversos materiais com aplicação nas indústrias automotiva, ferroviária, aeronáutica e aeroespacial não apresentam um limite de fadiga. Porém, em torno de 1860 Wohler propôs a curva S-N (tensão - número de ciclos), admitindo um conceito que há um limite de fadiga, estipulado na época como 1E5 ciclos e considerado como vida infinita em fadiga. Contudo, com os avanços tecnológicos, inúmeros componentes falham na condição de vida infinita, de maneira contrária ao proposto por Wöhler. Atualmente, se considera 3 regimes distintos de fadiga: fadiga de baixo ciclo, fadiga de alto ciclo e fadiga de altíssimo ciclo. Nas indústrias automotivas há um extensivo uso de chapas de aço para a fabricação de componentes de geometria complexa. Neste contexto, o aço interstitial free (IF – aços com baixa quantidade de átomos intersticiais), é um dos materiais de largo emprego, em função da sua excelente estampabilidade. O aço IF apresenta uma microestrutura ferrítica com uma baixíssima quantidade de elementos intersticiais. Interstícios com carbono e nitrogênio são estabilizados pelo titânio (Ti) ou nióbio (Nb) formando frações de volume extremamente pequenas na matriz ferrítica de carbetos e nitretos. O trabalho baseia-se na investigação experimental dos danos microestruturais no aço IF após carregamentos cíclicos de longa duração. Amostras do aço IF estabilizado pelo Ti foram submetidas a ensaios ultrassônicos de fadiga com carregamentos superiores a 1E7 ciclos. Se observou o aparecimento de danos microestruturais (persistent slip bands, PSB s) após determinado número de ciclos. Por meio de análise no microscópio ótico foi estimado que 97,76 por cento da vida em fadiga foi destinado para nucleação de trinca enquanto 2,23 por cento para a propagação de trinca. Houve o projeto de um novo corpo de prova para a máquina de fadiga de altíssimo, com o intuito de acompanha a superfície do material durante os ensaios. É proposto a equação da curva S-N fadiga de altíssimo ciclo com os dados extraídos dos testes. E por fim, é sugerido um fator de correção na equação de deformação plástica cisalhante nos PSB s formados / [en] Current research shows that several materials with application in the automotive, railway, aeronautical and aerospace industries do not have a fatigue limit. However, around 1860 Wohler proposed the S-N curve (stress - number of cycles), admitting a concept that there is a fatigue limit, stipulated at the time as 1E5 cycles and considered as infinite fatigue life. Although, with technological advances, numerous components fail in the condition of infinite life, contrary to what Wöhler proposed. Currently, 3 different fatigue regimes are considered: low cycle fatigue, high cycle fatigue and very high cycle fatigue. In the automotive industries there is an extensive use of sheet steel for the manufacture of components with complex geometry. In this context, interstitial free steel (IF steel) is one of the widely used materials, due to its excellent stampability. IF steel has a ferritic microstructure with a very low amount of interstitial elements. Interstices with carbon and nitrogen are stabilized by titanium (Ti) or niobium (Nb) forming extremely small volume fractions in the ferritic matrix of carbides and nitrides. The work is based on the experimental investigation of microstructural damage in IF steel after long-term cyclic loading. Samples of Ti-stabilized IF steel were subjected to ultrasonic fatigue tests with loads greater than 1E7 cycles. It was observed the arising of microstructural damage (persistent slip bands, PSB s) after a certain number of cycles. By means of image analysis in the optical microscope, it was estimated that 97.76 percent of the fatigue life was destined for crack nucleation while 2.23 percent for crack propagation in very high cycle fatigue regime. There was the project of a new specimen for the very high fatigue machine, in order to accompany the surface of the material during the tests. It is proposed the equation of the curve S-N fatigue of very high cycle os with data extracted from the tests. Finally, a correction factor is suggested in the plastic shear deformation equation in PSB s based on experimental data.

L’Inconel 718 est utilisé dans les disques de turbine des moteurs d’avion, de par sa haute résistance à la corrosion, à l’oxydation, au fluage et sa haute résistance mécanique à très haute température. Le nombre total de cycles de ces composants mécaniques s’élève à 109~1010 durant sa vie. Ils subissent des chargements de grande amplitude à faible fréquence, comme les forces centrifuges ou les contraintes thermiques mais aussi des chargements de faibles amplitudes à très haute fréquence, du aux vibrations des pales. Dans ce travail, on se propose d’étudier la fatigue à très grand nombre de cycles (VHCF) de l’Inconel 718 en utilisant des machines de fatigue ultrasonique, fonctionnant à 20KHz. Le système d’acquisition utilise des cartes NI et le logiciel LabView pour superviser la fréquence, la température, les déplacements durant toute la durée des tests. Des capteurs laser Keyence utilisant deux sondes pour les faces supérieure et inférieure de l’éprouvette permettent de capturer la fréquence et les modes de vibration. La différence entre les valeurs moyennes mesurées permet d’accéder à l’allongement de l’éprouvette, dû à l’auto-échauffement.3 types de matériaux avec différents traitements thermiques, AR, DA et DAHQ de l’ONERA et SAFRAN sont comparés. La différence au niveau de la taille de grain, de la phase, des précipités, … est analysée par micrographie métallographique en utilisant un microscope optique (MO) et un microscope électronique à balayage (MEB). Le comportement en traction quasi-statique et sous chargement cyclique contrainte-déformation est aussi proposé. La transition entre durcissement et adoucissement cyclique apparait à l’issue du traitement thermique. Finalement, les surfaces de ruptures sont observées en utilisant des caméras optiques et un MEB afin d’identifier les mécanismes de ruptures de l’Inconel 718 dans le domaine de la fatigue à très grand nombre de cycles. / Inconel 718 is widely used in turbine disk of aeronautic engines, due to its high resistance to corrosion, oxidation, thermal creep deformation and high mechanical strength at elevated temperature. The total cycle of these mechanical components is up to 109~1010 during its whole lifetime. It endures high-amplitude low-frequency loading including centrifugal force or thermal stress, and also low-amplitude high-frequency loading came from vibration of blade.In this work, the very high cycle fatigue (VHCF) behaviour of Inconel 718 with self-heating phenomenon without any cooling is studied using ultrasonic fatigue system at 20KHz. Acquisition system is improved using NI capture card with LabView for monitoring the frequency, temperature, displacement and so on during all the tests. Keyence laser sensor with two probes at the top and bottom surfaces of the specimens is used to reveal the frequency and vibration mode. The difference of mean values between these two probes is the elongation of the specimen caused by self-heating phenomenon.Three sets of materials with different heat treatment, As-Received (AR), Directly Aged (DA) and Directly Aged High Quality (DAHQ) from ONERA and SAFRAN are compared. The difference of grain size, phase, precipitate particle, etc. is investigated by metallographic micrograph using optical microscope (OM) and scanning electron microscope (SEM). Quasi-static uniaxial tensile property and cyclic stress-strain response is also proposed. The transition from cyclic hardening to cyclic softening appears after aged heat treatment. Finally, fracture surfaces are observed using optical camera and scanning electron microscope in order to identify the mechanism of fracture of Inconel 718 in the VHCF domain.

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.

Résumé : Cette étude a pour objectif de mieux comprendre les mécanismes précurseurs de l'amorçage de fissures dans le cas de métaux ductiles monophasés, comme le cuivre pur, sollicités à des amplitudes de contrainte inférieures à la limite de fatigue conventionnelle et jusqu'à des nombres de cycles atteignant le domaine de la fatigue gigacyclique (Very High Cycle Fatigue, VHCF). Les essais ont été réalisés sur une machine de fatigue ultrasonique à une fréquence de sollicitation de 20 kHz. Les mécanismes précurseurs de l'amorçage des fissures se manifestent (1) sous forme de bandes de glissement qui apparaissent sur la surface de l'éprouvette et (2) par un auto-échauffement du matériau dû à la dissipation intrinsèque. Les cartographies de température de la surface des éprouvettes nous ont permis d'estimer des dissipations d'énergie moyennes et de caractériser leur évolution avec le nombre de cycles et l'amplitude de contraintes. En parallèle, l'évolution du relief de la surface, initialement lisse et sans contrainte résiduelle, a été analysée à partir d'observations en microcopie optique, électronique à balayage et à force atomique. Nous avons établi que l'amplitude de contrainte nécessaire pour faire apparaître les premières bandes décroit en fonction du nombre de cycles. Des analyses EBSD, couplées à des calculs éléments finis intégrant l'anisotropie élastique des grains, ont révélé le rôle clé (1) des joints de macles et (2) du glissement dévié dans l'amorçage de bandes de glissement intenses.Mots clés : Fatigue gigacyclique, Bandes de glissement, Microplasticité cyclique, Dissipation, Anisotropie élastique, Simulation multicristalline, Thermographique infrarouge. / Abstract : This work aims to better understanding mechanisms leading to crack initiation in ductile single phase metals such as pure copper, loaded stress amplitudes lower than the conventional fatigue threshold and after about 109 cycles, the so-called Very High Cycle Fatigue regime. Tests were conducted using an ultrasonic technique at loafing frequency of 20 kHz. The mechanisms leading to crack initiation express (1) via slip bands at the specimen surface and (2) via self-heating due to intrinsic dissipation. Thermal maps were used to estimate the mean dissipation and its change with number of cycles and stress amplitudes. At the same time, the surface relief changes were characterized using optical, scanning electronic and atomic force microscopes. The stress amplitude required to observe the slip bands was found to decrease as a function of number of cycles. EBSD investigations combined with finites elements simulations accounting for elastic anisotropy of copper revealed the key role of (1) twin boundaries and (2) cross slip in slip band initiation.Keywords : Very High Cycle Fatigue, Slip bands, Cyclic microplasticity, Dissipation, Elastic anisotropy, Multicrystals simulation, Infrared thermography.

[pt] As tecnologias de manufatura aditiva (MA) tornaram-se alvo de grande interesse industrial para fabricação de componentes e peças finais destinadas a inúmeras aplicações em diversos setores da indústria. Estes componentes, em sua maioria, são projetados para apresentar vida em serviço superior à 107ciclos, fazendo com que a análise do comportamento em fadiga no regime de altíssimo ciclo (VHCF) se tornasse um critério de projeto imprescindível. O aço inoxidável AISI 316L é um dos materiais mais processados por MA, com inúmeras abordagens na literatura. Contudo, ainda não há um conhecimento consolidado a respeito do comportamento deste material quando submetido a longas ou super longas vidas em fadiga após processamento pelas distintas técnicas disponíveis, nem tampouco dos mecanismos de iniciação de trincas predominantes. Em geral, no regime de VHCF as trincas tendem a se iniciar internamente ou em regiões subsuperficiais a partir de defeitos internos presente no material. Esse fato resulta em uma característica na superfície de fratura conhecida como fish-eye. Outro fenômeno que ocorre na superfície de fratura, mais especificamente dentro da região do fish-eye, é a formação de uma camada fina granular (FGA) ao redor dos sítios de iniciação de trincas. O presente trabalho analisou a resistência à fadiga de longa duração do aço AISI 316L manufaturado pela técnica de deposição por energia direcionada a laser (L-DED). Corpos de prova de duas condições de pós-processamento do material, com e sem tratamento térmico, foram submetidos a ensaios de fadiga em equipamento ultrassônico (frequência de 20±0,5 kHz), com R = -1 e tendo como alvo 109ciclos. Após o levantamento das curvas S-N das duas condições microestruturais, as superfícies de fratura foram analisadas. Os resultados da pesquisa indicaram que o tratamento térmico reduziu a vida em fadiga do material em função da população de defeitos metalúrgicos encontrada, bem como influenciou a formação de fish-eye e FGA durante a iniciação e propagação das trincas. Por fim, o tamanho do FGA foi quantificado experimentalmente, para permitir a comparação com dimensões de FGA estimadas por equações empíricas presentes na literatura. / [en] Additive manufacturing (AM) technologies have become a target of great industrial interest for manufacture of components and final parts intended for several applications in many sectors of the industry. Most of these components are designed to have a service life higher than 107cycles, making the analysis of the fatigue behavior in the very high cycle regime (VHCF) an essential design criterion. AISI 316L stainless steel is one of the most processed by AM, with numerous approaches in the literature. However, there is still no consolidated knowledge about the behavior of this material when subjected to long or super long fatigue lives after processing by the different available techniques, nor about the predominant crack initiation mechanisms. In general, in the VHCF regime, cracks tend to start internally or in subsurface regions from internal defects present in the material. This fact results in a characteristic in the fracture surface known as fish-eye. Another phenomenon that occurs at the fracture surface, more specifically within the fish-eye region, is the formation of a fine granular area (FGA) nearby the crack initiation sites. The present work analyzed the long-term fatigue strength of AISI 316L steel manufactured by laser directed energy deposition (L-DED) technique. Specimens of two post-processing conditions of the material, with and without heat treatment, were subjected to fatigue tests in ultrasonic equipment (frequency 20 ± 0.5 kHz), with R = -1 aiming 109cycles. After surveying the S-N curves of the two microstructural conditions, the fracture surfaces were analyzed. The research results indicated that the heat treatment reduced the fatigue life of the material as a function of the population of metallurgical defects found, as well as influenced the formation of fish-eye and FGA during the initiation and propagation of cracks. Finally, the size of FGA was experimentally quantified to allow comparison with the dimensions of the FGA estimated by empirical equations present in the literature.

[pt] A determinação da vida à fadiga para projetar estruturas e componentes mecânicos é extremamente importante. A curva S-N pode ser afetada por diferentes condições operacionais e alguns fatores são mais pronunciados no teste ultrassônico de fadiga, dependendo do material. A influência da alta frequência nos fenômenos de autoaquecimento e o efeito da frequência são discutidos. Um aspecto relevante em VHCF é o mecanismo de iniciação e propagação de trincas. Os fenômenos como olho de peixe e área fina granular (FGA) foram encontrados nas superfícies de fratura. Esta tese está dividida em 3 tópicos: evolução da temperatura, amplitude de deformação da microplasticidade e investigação da região FGA. Os materiais em estudo são os aços DIN 34CrNiMo6 e DIN 42CrMo4. O teste de fadiga ultrassônica foi realizado em diferentes condições de carregamento no modo intermitentes e acompanhado por câmera termográfica infravermelha. Esses resultados obtidos para a câmera termográfica foram usados para desenvolver um modelo de inteligência artificial usando aprendizado de máquina para prever a curva temperatura-número de ciclos. O modelo foi capaz de prever a temperatura e os valores do coeficiente de determinação estão acima de 0,98. Para prever a vida à fadiga, foram escolhidos parâmetros baseados em tensão, curva S-N tradicional, temperatura no início do teste, (razão de Rayleigh), dissipação de calor, Qcyc e gradiente de temperatura. A temperatura em estado estacionário foi atingida em aproximadamente 5E+04 ciclos em ambos os aços. Observa-se que o número de ciclos até a falha aumenta à medida que a inclinação da temperatura (fase I) e a dissipação de calor diminuem. (razão de Rayleigh) forneceu melhor concordância com os resultados experimentais seguido por Qcyc. Além disso, grãos ultrafinos na seção transversal do FGA entre 500 -700 nm dentro da superfície da fratura foram detectados pela análise FIB e EBSD. O local grain refinament foi escolhido como o melhor modelo para explicar a formação de FGA. As inclusões não metálicas Al2O3 foram responsáveis por todas as iniciações internas da trinca de fadiga. / [en] The determination of fatigue life to design structures and mechanical components is extremely important. The S-N curve can be affected by different operational conditions, and some factors are more pronounced under ultrasonic fatigue test, depending on the material. The influence of the high frequency in self-heating phenomena and frequency effect are discussed. A relevant aspect in VHCF is the mechanism of crack initiation and propagation. The fish-eye and fine granular area (FGA) phenomena were encountered on the fracture surfaces. This thesis is divided in 3 topics: temperature evolution, microplasticity strain amplitude, and investigation of the FGA region. The materials under study are DIN 34CrNiMo6 and DIN 42CrMo4 steel. Ultrasonic fatigue test was conducted at different intermittent driving and loading ratios, accompanied by an infrared thermographic camera. These results obtained by the thermographic camera were used to developed an artificial intelligence model using machine learning to predict the temperature-number of cycle curves based on the fatigue life. The model was able to predict the temperature and the coefficient of determination values to be above 0.98. In order to predict the fatigue life, parameters were selected based on stress, the traditional S-N curve, slope temperature at the beginning of the test, (Rayleigh ratio), heat dissipation, Qcyc, and gradient temperature. A steady state temperature was reached approximately in 5E+04 cycles with both steels. It is noticeable that the number of cycles to failure increases as slope temperature and heat dissipation decreases. (Rayleigh ratio) provided better agreement with the experimental results followed by Qcyc. Moreover, ultrafine grains in the cross–section of the FGA between 500 - 700 nm within the fracture surface were detected by FIB and EBSD analysis. Local grain refinement was choose the best model to explain FGA formation. The non-metallic inclusions were ultimately responsible for all internal crack initiations of Al2O3.

Ce travail vise à améliorer notre compréhension du comportement en fatigue à grand et très grand nombre de cycles d’un acier ferrito-martensitique dual phase, notamment les effets de la température et de la vitesse de déformation résultant de chargements cycliques à haute fréquence. L'effet de la fréquence sur la réponse en fatigue de l'acier DP600 a été étudié en effectuant des essais de fatigue sur une machine ultrasonique travaillant à 20 kHz et sur une machine conventionnelle travaillant à des fréquences inférieures à 100 Hz. Des études de fractographie et des observations microscopiques à la surface des échantillons ont été effectuées pour étudier les mécanismes de déformation et de rupture. De plus, la thermographie infrarouge in situ a été utilisée pour étudier la réponse thermique et les mécanismes dissipatifs du matériau lors des essais de fatigue. Les courbes S-N ont été déterminées à partir de chargements de fatigue ultrasoniques à 20 kHz et d’essais conventionnels à 30 Hz. La durée de vie pour une amplitude de contrainte donnée est plus élevée dans le cas de la fatigue ultrasonique bien que la limite de fatigue soit identique dans les deux cas. L’augmentation inévitable de la température en fatigue ultrasonique à fortes amplitudes de contraintes, ainsi que le comportement dépendant de la vitesse de déformation de la ferrite, en tant que structure CC, ont été trouvés comme les paramètres clés expliquant le comportement observé en fatigue, et la réponse thermique sous les fréquences faibles et ultrasoniques. Les écarts observés entre l’essai de fatigue conventionnel et celui ultrasonique ont été évalués à travers les mécanismes de mobilité des dislocations vis dans la phase ferritique de structure cubique centrée (CC). La durée de vie plus élevée et l’amorçage de la fissure principale sur une inclusion observés en fatigue ultrasonique ont été attribués au vieillissement dynamique résultant du fort auto-échauffement du matériau aux fortes amplitudes de contraintes. L'existence d'une transition du régime thermiquement activé au régime athermique avec l’augmentation de l'amplitude de contrainte a été mise en évidence. Au-dessous de la limite de fatigue, la déformation a lieu dans un régime thermiquement activé alors qu'elle est dans un régime athermique au-dessus de la limite de fatigue. En fatigue conventionnelle, la déformation est athermique pour toutes les amplitudes de contrainte. Une carte de transition a été produite en utilisant les résultats expérimentaux pour l'acier DP600 ainsi que les données disponibles dans la littérature pour d'autres aciers à base de ferrite, montrant ainsi la corrélation entre le mouvement des dislocations vis thermiquement activé et l'absence de rupture en fatigue à très grand nombre de cycle. / This work is an attempt towards a better understanding of the high cycle and very high cycle fatigue behaviors of a ferritic-martensitic dual-phase steel, with a regard to temperature and strain rate effects, resulting from accelerated fatigue loading frequencies. The influence of frequency on fatigue response of DP600 steel was investigated by conducting ultrasonic and conventional low frequency fatigue tests. Fractography studies and microscopic observations on the surface of specimens were carried out to study the deformation and fracture mechanisms under low and ultrasonic frequencies. Moreover, in situ infrared thermography was carried out to investigate the thermal response and dissipative mechanisms of the material under fatigue tests. The S-N curves were determined from ultrasonic 20-kHz fatigue loadings and conventional tests at 30 Hz. Fatigue life for a given stress amplitude was found to be higher in the case of ultrasonic fatigue whereas the fatigue limit was the same for both cases. Moreover, crack initiation was always inclusion-induced under ultrasonic loading while under conventional tests it occurred at slip bands or defects on the surface. The inevitable temperature increase under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a body centered cubic (BCC) structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. The discrepancies observed between conventional and ultrasonic fatigue tests were assessed through the mechanisms of screw dislocation mobility in the ferrite phase as a BCC structure. The higher fatigue life and inclusion-induced crack initiations in the case of ultrasonic loading were attributed to the dynamic strain aging, which resulted from the high temperature increases at high stress amplitudes. The existence of a transition in deformation regime from thermally-activated to athermal regime under ultrasonic fatigue loading by increasing the stress amplitude was confirmed. Below the fatigue limit, deformation occurred in thermally-activated regime while it was in athermal regime above the fatigue limit. Under conventional loading deformation occurred in athermal regime for all stress amplitudes. From the analysis of the experimental data gathered in this work, guidelines were given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue.

[pt] O presente trabalho se baseia na teoria dos defeitos equivalentes, também conhecida como teoria de Murakami, que permite a predição da vida-fadiga de materiais estruturais adotando o parâmetro (raiz quadrada de area), responsável por uma equivalência quantitativa entre heterogeneidades microestruturais (inclusões metalúrgicas) existentes na região de análise do material e descontinuidades mecânicas (furos) usinados nos corpos de prova. Neste contexto, a tese propõe novas equações, a partir da teoria de Murakami, para prever falhas por fadiga em carregamentos ultralongos (fadiga de altíssimo ciclo, VHCF) do aço DIN42CrMo4, de larga aplicação na fabricação de eixos virabrequins para unidades geradoras de usinas termoelétricas. Corpos de prova do aço DIN42CrMo4 foram usinados com furos de diâmetros variando entre 0,18 mm e 0,70 mm e ensaiados num regime de fadiga de altíssimo ciclo, variando o valor da amplitude de tensão do ensaio, para assim determinar em quais condições o material falha e obter a curva experimental S-N do aço DIN42CrMo4 na presença de diferentes tamanhos de defeitos (furos). Os resultados experimentais permitiram o desenvolvimento de equações em função do parâmetro e curvas de Wohler do material, em conformidade com diferentes descontinuidades mecânicas nos corpos de prova e resistências à fadiga do material. Consequentemente, se estabeleceu uma metodologia que permite uma correlação entre inclusões metalúrgicas, tensão aplicada e vida superlonga em fadiga de eixos virabrequins em serviço em usinas termoelétricas. / [en] The present work is based on the theory of equivalent defects, also known as Murakami s theory, which allows the prediction of the fatigue life of structural materials by adopting the parameter (square root of area), responsible for a quantitative equivalence between microstructural heterogeneities (metallurgical inclusions) existing in the region analysis of the material and mechanical discontinuities (holes) machined in the specimens. In this context, the thesis proposes new equations, based on Murakami s theory, to predict fatigue failures in ultralong loads (very high cycle fatigue, VHCF) of DIN42CrMo4 steel, widely used in the manufacture of crankshafts for generating units of thermoelectric power plants. DIN42CrMo4 steel specimens were machined with holes with diameters varying between 018 mm and 0.70 mm and tested in a very high cycle fatigue regime, varying the value of the test stress amplitude, to determine under what conditions the material failure and obtain the experimental S-N curve of DIN42CrMo4 steel in the presence of different sizes of defects (holes). The experimental results allowed the development of equations as a function of the parameter and Wohler curves of the material, in accordance with different mechanical discontinuities in the test specimens and resistance to fatigue of the material. Consequently, a methodology was established that allows a correlation between metallurgical inclusions, applied stress and super-long fatigue life of crankshafts in service in thermoelectric power plants.