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

Very high cycle fatigue (VHCF) is generally considered as the domain of fatigue lifetime beyond 10 million (107) load cycles. Few examples of structural components which are subjected to 107-109 load cycles during their service life are engine parts, turbine disks, railway axles and load-carrying parts of automobiles. Therefore, the safe and reliable operation of these components depends on the knowledge of their fatigue strength and the prevalent damage/failure mechanisms. Moreover, the fatigue life of materials in the VHCF regime is controlled by the fatigue crack initiation and early growth stage of short cracks. This study was focussed on the evaluation of fatigue properties of duplex stainless steels in the VHCF regime using the ultrasonic fatigue testing equipment. The ultrasonic fatigue tests were conducted on the cold rolled duplex stainless strip steel and hot rolled duplex stainless steel grades. Two different geometries of ultrasonic fatigue test specimens were tested. Considerable attention was devoted to the evaluation of fatigue crack initiation and growth mechanisms using the high resolution scanning electron microscopy. The fatigue crack initiation was found to be surface initiated phenomena in all the tested grades, albeit different in each case. The second part of this thesis work was the development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane. Cracks with dimensions much smaller than the overall size of the domain were considered. The main goal of the development of this technique was the evaluation of stress intensity factor at each crack tip. The comparison of results from the stress intensity factor evaluation by the developed procedure and the well-established Finite Element Method software ABAQUS showed difference of less than 1% for Jacobi polynomial expansion of sixth order in the dipole density representation. / <p>Article III was still in manuscript form at the time of the defense.</p> / Very high cycle fatigue of stainless steels

[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.

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena. In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism. Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method. / Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Structural components that have service life in the VHCF regime include wheels and axles of high speed trains, gas turbine disks, and components of paper production machinery. Safe and reliable design, and the longevity, of these components depends on the knowledge of their VHCF properties. The overall aim of the experimental portion of this thesis was to gain in-depth knowledge of the VHCF properties of stainless steels. Fatigue test data in the VHCF regime was generated for different stainless steel grades using ultrasonic fatigue testing. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. Theoretical modelling of complex crack geometries involving kinks and branches was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The main aim of this development was to compute the stress intensity factors and to analyse the stress state around the cracks. The results showed that accuracy of 2% was attainable compared to the solutions obtained by Finite Element Method (FEM). / <p>Artikel 4 publicerad i avhandlingen som manuskript</p>

Classical fatigue models teach that there is an intrinsic fatigue limit for steels, representing a level of stress that is too low for regular crack growth where every cyclic load propagates a fatigue crack through the material. Modern application with extreme lifetimes has shown that fatigue will still take place in steels with stress levels well below the expected fatigue limit. This relatively new area of study has been named Very High Cycle Fatigue, or VHCF, and describes fatigue failures with a number of load cycles exceeding 107. Fractography of steels that has suffered VHCF tends to reveal an especially rough crack surface adjacent to where the fatigue crack originates, which is typically some form of defect in the bulk of the steel. This area is believed to be critical for VHCF and has been referred to in a number of ways by different studies, but will herein be called Fine Granular Area, or FGA. The aim of this study is to try and get a better understanding of VHCF. This was done by fractography analysis of test specimens of high strength tool steel that suffered fatigue failure at lifetimes ranging from about 106 cycles to 1,9x109 cycles. The lower lifetimes were achieved using hydraulic testing equipment, while the specimens in the VHCF range suffered fatigue failure in ultrasonic testing equipment allowing the application of a cyclic stress at a rate of 20 000 Hz. The resulting fracture surfaces were then investigated using a scanning electron microscope, or SEM, taking special note of the fatigue initiating defects and, in the case of VHCF, the rough area found adjacent to it. In combination with the SEM an elemental analysis of the fatigue initiating defects as well as the bulk of the material was done using energy-dispersive X-ray spectroscopy, or EDS. This was done to find out what the defects consisted of; confirming that they were slags and checking that the composition of the material of the bulk of the specimen matches what was expected. Using light optical microscopy in combination with acid etching of the surface of samples cut out of the test specimens the structure of the steel was investigated. Calculating the local stresses at the location of the fatigue initiating defect was done using FEM in combination with displacement amplitude gathered from the ultrasonic testing equipment. The data gathered was then measured and compared to that of previous studies, using models of prediction and seeing how they match the experimental results. The results suggest that the stress intensity factor at the internal slags is critical for VHCF and that with lower stress intensity factors one can expect longer lifetimes. Another observation is a relatively consistent stress intensity factor at the edge of the FGA combined with the original defect, likely signifying the transition from the creation of FGA to traditional crack propagation. There also seems to be a connection between the size of the FGA and the number of cycles to failure, with larger FGA with increasing lifetimes. The most glaring shortcoming of this study is the amount satisfactory tests conducted, and thus amount of data points, is very low due to the majority of specimens suffered failure at the threading used to connect them to the ultrasonic testing equipment at lifetimes far too low to be relevant. / Klassiska utmattningsmodeller lär ut att det finns en utmattningsgräns för stål, vilket representerar en spänningsnivå som är för låg för regelbunden sprickväxt där varje cyklisk belastning sprider en utmattningsspricka genom materialet. Moderna applikation med extrema livstider har visat att utmattning fortfarande äger rum i stål med spänningsnivåer långt under den förväntade utmattningsgränsen. Detta relativt nya studieområde har fått namnet Very High Cycle Fatigue, eller VHCF, och beskriver utmattningsfall med ett antal belastningscykler som överstiger 107. Fraktografi av stål som har drabbats av VHCF tenderar att ha en särskilt gropig sprickyta som ligger intill där utmattningssprickan har sitt ursprung, vilket typiskt är någon form av defekt i stålets bulk. Detta område tros vara kritiskt för VHCF och har hänvisats till på ett antal sätt av olika studier, men kommer här att kallas Fine Granular Area eller FGA. Syftet med denna studie är att försöka få en bättre förståelse för VHCF. Detta gjordes genom fraktografianalys av testprover av verktygsstål med hög hållfasthet som drabbades av utmattningsbrott vid livstider från cirka 106 cykler till 1,9x109 cykler. De lägre livslängderna uppnåddes med hjälp av hydraulisk testutrustning, medan proverna i VHCF-området drabbades av utmattningsbrott i ultraljudstestutrustning som klarar att applicera en cyklisk stress med en frekvens på 20 kHz. De resulterande sprickytorna undersöktes sedan med hjälp av ett svepelektronmikroskop, eller SEM, med särskild fokus på utmattningsinitierande defekter och, i fallet med VHCF, det grova området som hittades intill det, FGA. I kombination med SEM utfördes en elementanalys av utmattningsinitierande defekter liksom huvuddelen av materialet med energidispersiv röntgenspektroskopi, eller EDS. Detta gjordes för att ta reda på vad inneslutningarna bestod av för att bekräfta att de var slagg samt kontrollera att sammansättningen av materialet i huvuddelen av provet matchar det som förväntades. Med användning av optisk ljusmikroskopi i kombination med syraetsning av ytan på prover som skars ut ur testproverna undersöktes stålets struktur. Beräkning av de lokala spänningarna på platsen för den utmattningsinitierande defekten gjordes med hjälp av FEM i kombination med förskjutningsamplituden som samlats från ultraljudsutrustningen. De insamlade uppgifterna mättes sedan och jämfördes med tidigare studier genom att använda diverse modeller och se hur de matchar de experimentella resultaten. Resultaten antyder att stressintensitetsfaktorn vid inneslutningarna är kritisk för VHCF och att man med lägre stressintensitetsfaktorer kan förvänta sig längre livstid. En annan observation är en relativt konsekvent stressintensitetsfaktor vid kanten av FGA, vilket sannolikt markerar övergången från skapandet eller utbredning av FGA till traditionell sprickutbredning. Det verkar också finnas en koppling mellan storleken på FGA och antalet cykler till fel, med större FGA med ökande livslängd. Den mest uppenbara bristen i denna studie är mängden tillfredsställande tester som genomförts. Därmed är mängden datapunkter mycket låg, detta på grund av att majoriteten av proverna misslyckades vid gängningen som användes för att ansluta dem till ultraljudstestutrustningen vid livstider alltför låga för att vara relevanta.

[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.