Hlavním cílem této práce bylo vysvětlit únavové chování ocelí ODS Eurofer, 14Cr ODS feritické oceli vyráběné v CEA (Commissariat a l’énergie atomique, Saclay, France) a 14Cr ODS feritické oceli vyráběné v EPFL (École Polytechnique Fédérale de Laussane, Switzerland). Pokud to bylo možné byly získané výsledky porovnány s ocelí Eurofer 97. Tato práce je součástí široké spolupráce, která je zaměřena na vývoj konstrukčních materiálů pro fuzní reaktory. V průběhu práce byly provedeny následující experimenty: • Standardní únavové zkoušky za pokojové teploty, při 650 a při 750 , byly získány křivky cyklického zpevnění/změkčení, cyklické deformační křivky, Coffinovy–Mansonovy a Wöhlerovi křivky. Hysterezní smyčky byly detailně analyzovány. • Pozorování mikrostruktury výchozího stavu pomocí TEM a porovnání s mikrostrukturou po cyklickém zatížení. • Pozorování povrchového reliéfu, který vznik díky lokalizaci cyklické plastické deformace - byla odhalena místa iniciace únavových trhlin a byl analyzován mechanismus jejich vzniku. • Měření kinetiky únavových trhlin. Oxidická disperse značně zpevňuje materiály, redukuje cyklické změkčení a stabilizuje mikrostrukturu v celém rozsahu teplot. Byly nalezeny velké rozdíly v životnosti jednotlivých ocelí. Ty byly vysvětleny pomocí rozdílných mechanismů iniciace trhlin a jejich různou rychlostí.

This thesis describes selection and shows calibration of material model, capable of describing cyclic softening of material. Stress-strain FEM analysis of circumferential blade attachment for last section of rotor blades of steam turbine is performed, expected lifetime of existing attachment is evaluated. Multi-parameter optimization of new-shape attachment was done, resulting in dimensions for new-shape attachment with longer lifetime. Improvements in strain amount in comparison with existing attachment were evaluated and possible RPM increase of turbine with new attachment type was calculated.

Detta examensarbete utfördes på Scania CV AB och behandlar delar i lastbilens avgassystem som utsätts för korrosion och nötning. Huvudmålet med detta arbete är att utveckla en provmetod för livslängdsundersökning av flexibla slangar. En serie tester utfördes för att förstå grundorsakerna till brott och en FE-analys utfördes för att verifiera resultaten till dessa prov. En stor del av den flexibla slangen blir stel på grund av höga avgastemperaturer och som leder till plastisk deformation av slangen. Men de tre första lindningarna som ligger närmast motorn behåller sin flexibilitet på grund av kontinuerliga motorvibrationer. Slutsatsen är att huvudorsaken som gör att de flexibla slangarna går av, är slitage som leder till sprickinitiering. Den sprickan sprider sig i form av lågcykelutmattning d.v.s att det tar ganska kort tid tills brott sker i den flexibla slangen. Korrosion initierar mellan de icke-flexibla delar av slangen Korrosionshastigheten ökar med tiden och det orsakar små hål på ytorna. Baserat på testresultaten är den mest lämpliga stället för livslängdsundersökningar Scanias komponentprovceller. Genom att mäta slitagedjupet under repeterbara förhållanden som rekommenderas i rapporten och med hjälp av data från kundutfall, kan en livslängd uppskattas. För att kunna förbättra livslängden för de flexibla slangarna, rekommenderas att använda rostfritt stål typ 1.4828 vid tillverkning av dessa slangar bör varmformning användas, samt att avståndet mellan lagrena respektive tjockleken ökas. Flera tester med olika teststidsintervall behöver göras för att ta reda på nötningshastigheten. / This thesis was conducted at Scania CV AB with information based on corrosion and wear. The main goal of this work is to develop a test method to investigate the stripwounded hose’s lifetime. A series of tests were performed to understand the reasons of their failure and a FE analysis was performed to verify the results obtained from these tests. It was observed that a large part of stripwounded hose becomes stiff due to high temperatures of the exhaust gas. The high temperature leads to plastic deformation of the hose. In contrast, the three first windings closest to the engine keep their flexibility due to continuously engine vibrations while there is sign of wear between the layers in these three windings more than other parts of the stripwounded hose. Hence, it is concluded that the main reason for stripwounded hose’s unpredictable failures is wear which leads to crack initiation. The crack propagation is in type of low cycle fatigue which means that it takes a short time until the stripewounded hose breaks. Corrosion is initiated between the non-flexible parts of the hose. The rate of corrosion is increased by time and causes small holes on the surfaces. According to results from the experiments the most suitable location for the lifetime investigation is Scania’s component test cells . Several tests with different time spans needs to be performed. By measuring the wear depth rate under repeatable test conditions and bycomparing with stripwounded hoses from customer trucks, a lifetime can be estimated. To improve the fatigue lifetime of the stripwounded hoses, it is recommended to use stainless steel of type 1.4828, manufactured through hot-forming with larger distance between layers and thicker layers to find out the wear rate.

Dans cette thèse, nous proposons une analyse globale multi-échelles de la fatigue à faible nombre de cycles des matériaux à mémoire de forme (MMF). Dans un premier temps, une large campagne d’essais a été menée pour différents chargements thermomécaniques comprenant des tests de fatigue sous contrainte et déformation imposée et pour différentes fréquences de chargement. A partir des résultats des essais, un critère de fatigue, basé sur l’énergie de déformation, a été développé ; on montre que l’énergie de déformation est un paramètre pertinent pour prédire la fatigue des MMF en tenant compte du couplage thermomécanique et du type de chargement : contrainte ou déformation imposée. Ensuite, en prenant appui sur la répartition de l’énergie de l’hystérésis en dissipation et énergie stockée, on avance une interprétation physique du mécanisme de la fatigue des MMF. Dans la troisième partie, on propose une modélisation multi-échelles de l’initiation des fissures de fatigue dans les MMF à partir de la notion de plasticité de transformation (PlTr). Dans ce cadre, on montre que la fatigue de MMF est contrôlée par la (PlTr) et que la température maximale lors de la transformation de phase est le paramètre à retenir pour prédire la rupture par fatigue des MMF. Le modèle permet également de prédire le lieu d’initiation des premières fissures de fatigue. Enfin, un procédé – fondé sur l’«éducation» des MMF – permettant d’améliorer la résistance à la fatigue est proposé. / The thesis proposes a multi-scale comprehensive analysis of low cycle fatigue of shape memory alloys (SMAs). First, low cycle fatigue of SMAs is experimentally investigated; comprehensive tensile-tensile fatigue tests under both stress and strain controlled loadings at different frequencies are carried out and results are discussed. Second, a new strain energy-based fatigue criterion is developed; it is shown that the use of total strain energy is a relevant parameter to predict fatigue lifetime of SMAs for different thermomechanical conditions and under different types (strain-control or stress-control) loadings. A physical interpretation of the mechanism related to the low-cycle fatigue of SMAs is then provided based on the conversion of hysteresis work into dissipation and stored energy. Third, fatigue crack initiation during cyclic stress-induced phase transformation is modeled based on transformation induced plasticity (TRIP); it is shown that the maximum temperature during the cyclic loading is a relevant indicator of the fatigue of SMA. Furthermore, the effect of the macroscopic mechanical load on the the fatigue lifetime is addressed as well as the spatial location of crack initiation. Finally, a mechanical training process that allows enhancing resistance to low cycle fatigue of SMAs is proposed.

This work is focused on study of mechanical properties of NiTi alloy which shows pseudoelastic and shape memory behaviour. Functional and structural fatigue of the material is examined. The main aim of this work was to perform and to evaluate a series of fatigue tests. The material was supplied in the form of wire which was gripped in the machine using special grips. In the first part of the experiment, tensile tests are evaluated and the reproducibility of measurements is demonstrated. Then, a series of cyclic tests was performed. Results were analysed together with previous measurements. One part of discussion concerned changes of the hysteresis loops during cycling and their dependence on strain rate. The fatigue life curves were plotted. It was found that these curves have non-standard shapes. The reasons for this are explained in the work.

The present doctoral thesis is focused on the effect of dwells on the low cycle fatigue behaviour of advanced high temperature materials. 10 minutes strain holds are introduced into the cyclic straining of cast Ni-based superalloy IN792-5A and cast intermetallic alloy TiAl-7Nb which were chosen as experimental material. Fatigue experiments were conducted in strain control mode with constant total strain amplitude and strain rate. IN792-5A was subjected to continuous cyclic loading test as well as fatigue tests either with tensile peak strain holds or compressive peak strain holds at temperature 800 °C. TiAl-7Nb was experimentally examined by continuous cyclic loading tests and also by fatigue test with tensile peak strain hold times at temperature 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves, Manson-Coffin curves and Basquin curves were obtained. Stress relaxation data were measured during hold times. Mean stress evolution was observed and documented. Microstructure of both alloys was observed in as-received state and also after cyclic loading by means of SEM and TEM. Surface relief investigation revealed cyclic plastic strain localisation as well as fatigue crack initiation sites. Fatigue crack propagation was described using fracture surface and longitudinal gauge section observations. Dislocation structures developed in the course of fatigue tests were studied and documented.

Due to coming emission legislations, the temperature is expected to increase in heavy-duty diesel engines, specifically in the hot-end of the exhaust system affecting components, such as exhaust- and turbo manifolds. Since the current material in the turbo manifold, a ductile cast iron named SiMo51, is operating close to its limits there is a need for material development in order to maintain a high durability of these components. When designing for increased life, many material properties need to be considered, for example, creep-, corrosion- and fatigue resistance. Among these, the present work focuses on the latter two up to 800°C improving the current material by additions of Cr, for corrosion resistance, and Ni, for mechanical properties. The results show improved high-temperature corrosion resistance in air from 0.5 and 1wt% Cr additions resulting in improved barrier layer at the oxide/metal interface. However, during oxidation in exhaust-gases, which is a much more demanding environment compared to air, such improvement could not be observed. Addition of 1wt% Ni was found to increase the fatigue life up to 250°C, resulting from solution strengthening of the ferritic matrix. However, Ni was also found to increase the oxidation rates, as no continuous SiO2-barrier layers were formed in the presence of Ni. Since none of the tested alloys showed improved material properties in exhaust gases at high temperature, it is suggested that the way of improving performance of exhaust manifolds is to move towards austenitic ductile cast irons or cast stainless steels. One alloy showing good high-temperature oxidation properties in exhaust atmospheres is an austenitic cast stainless steel named HK30. This alloy formed adherent oxide scales during oxidation at 900°C in gas mixtures of 5%O2-10%H2O-85%N2 and 5%CO2-10%H2O-85%N2 and in air. In the two latter atmospheres, compact scales of (Cr, Mn)-spinel and Cr2O3 were formed whereas in the atmosphere containing 5%O2 and 10%H2O, the scales were more porous due to increased Fe-oxide formation. Despite the formation of a protective, i.e. compact and adherent, oxide scale on HK30, exposure to exhaust-gas condensate showed a detrimental effect in form of oxide spallation and metal release. Thus, proving the importance of taking exhaust-gas condensation, which may occur during cold-start or upon cooling of the engine, into account when selecting a new material for exhaust manifolds. / <p>QC 20130508</p>

This thesis is mainly focused on the seismic behaviour of cross laminated timber (CLT) buildings. The document can be subdivided into three main sections closely related to each other. In the first part, after a short introduction on the state of the art on timber buildings regarding the constructive and legislative issues, the behaviour of CLT buildings is presented. The research is focused on the study on single shear-walls, on the multi-storey single-walls and on the behaviour of the whole buildings. The analyses are performed in order to assess the ductility level achievable by a CLT building as a result of different choices for the ductility of the connections at the foundation level. In order to estimate the ductility level, a large number of non-linear analyses were performed. This was possible thanks to a Matlab code, specifically developed, which allowed to reduce the computational burden. The results are used to evaluate a reliable set of behaviour factors to be applied in the seismic design of CLT buildings. In the second part of thesis, the low-cyclic fatigue strengths for different typologies of dissipative timber connections are presented. The low-cyclic fatigue strength represents a key-parameter in the assessment of the seismic behaviour of timber connections. In fact, high values of ductility associated with low values of strength degradation ensure a remarkable and reliable energy dissipation without a significant loss of strength. Despite the current version of chapter 8 of Eurocode 8 requires specific values of seismic demand for timber connections in terms of low-cyclic fatigue strength, no specific provision is reported to this regard in the European Standard for the cycling testing of timber connections and assemblage in seismic design (EN 12512). In This Standard the ductility capacity and the impairment of strength are calculated as separate mechanical parameters. For this reason, a proposal of revision of European Standard EN12512 is presented and discussed. The third and last part of the thesis describes an innovative connection for CLT buildings. This innovative connection was originally developed in order to absorb both traction and shear actions. Furthermore, a good performance has been obtained in terms of low-cyclic fatigue strength and ductility, with the aim of conceiving a connection able of satisfy the requirements of the current seismic European Standard. The design of this new connection was an iterative process, starting from some simplified numerical models. After some improvements, it was possible to obtain the expected performance levels. The strength and rigidity of the designed connection were initially obtained through numerical analysis, and then compared with the results of physical tests carried out in the Materials and Structures Testing Laboratory (MSTL), that is a part of the Department of Civil, Environmental and Mechanical Engineering (DICAM) of the University of Trento.

[pt] O método EN baseia-se na quantificação do dano causado pela deformação decorrente do histórico de carregamento. O objetivo desse modelo consiste em estimar o número de ciclos para iniciar uma trinca por fadiga. Diferente do método SN o número de variáveis envolvidas nesse modelo é menor. Entretanto, requer um número maior de constantes, que nem sempre podem ser obtidas experimentalmente. Esse fato levou a formulação de diversos modelos que se propõem a estimar as constantes de Coffin-Manson, que na maioria dos casos levam a previsões de vida não conservativas, como visto em outros trabalhos. Contudo a maioria dos casos existentes na literatura não abordam o efeito de regiões de concentração de tensão e a influência de carregamentos com amplitudes variáveis. A metodologia usada nessas pesquisas consiste em avaliar a influência desses aspectos através da comparação entre a vida em fadiga experimental e a estimada segundo as constantes de Coffin-Manson medidas e estimadas segundo diferentes modelos. Esse estudo avalia o dano à fadiga segundo 2 tipos de concentradores de tensão e quatro históricos de carregamentos de amplitude variável. Considerando os modelos de Manson, Muralindhran-Manson, Bäumel-Seeger and Meggiolaro-Castro os resultados mostram que a vida em fadiga prevista usando os parâmetros de Coffin Manson obtidos segundo o método de Meggiolado-Castro e por Baumel-Segger são os que mais se aproximam das vidas estimadas baseada nas constantes experimentais. Nos demais casos nota-se que a regra de concentração de tensão exerce uma maior influência no resultado e levam a vidas à fadiga não conservativas. / [en] The strain-life designed is based on the damaged caused by the strain as a result of a loading history. The purpose of this approach is estimating the number of cycles to initiate a fatigue crack. Compared with the stress life method the EN needs a lower number of variables. However, it needs a large number of parameters that are not directly measurable properties of a material and conduct the necessary number of tests to obtain them is not always an option. This fact leads to the formulation of different models to estimate Coffin- Manson s constants, which in most of the cases can result in a non-conservative fatigue life prediction, as seen in several papers. However, in these studies, it is not considered a specimen with a stress concentration factor neither under a variable amplitude loading history. The methodology used in this research is evaluating the influence of these aspects by comparing the fatigue life experimental with the estimated by using the Coffin Manson s experimental constants and the fatigue life calculated with different models to estimate the Coffin-Manson s parameters. This study evaluated the fatigue damage based on four load histories and two types of samples with different stress risers. Considering the models of Manson, Muralidharan-Manson, Bäumel-Seeger, and Meggiolaro-Castro the results indicated that the fatigue life predicted using the Coffin Manson s parameter obtained by Meggiolaro-Castro and Baumel-Segger s model are more similar to the life calculated with the experimental constants. The other models are more influenced by the stress concentration s rule and can lead to not conservative fatigue life.

[pt] A falha mecânica conhecida como fadiga é caracterizada pela iniciação e/ou propagação de trincas, causada por forças variáveis. Suas metodologias tradicionais calculam uma tensão elástica uniaxial equivalente que atua no componente, a fim de compará-la com os dados experimentais de comportamento mecânico do material do componente sob cargas uniaxiais. Esta hipótese pode levar a resultados não conservativos, por considerar que o material é igualmente sensível a tensões normais e cisalhantes, o que é falso em várias aplicações práticas. Portanto, dados torcionais e multiaxiais são necessários para melhor prever a vida em fadiga dos componentes. Para executar estes experimentos, o presente trabalho propõe uma variedade de projetos de componentes e metodologias de montagem para que se possa usar em uma máquina de tração-torção Instron 8874 uma garra hidráulica originalmente projetada para uma máquina tração pura Instron 8501. É proposto um método simplificado para estimar, por controle de deslocamento, as propriedades de fadiga de baixo ciclo em cisalhamento (gama)N, evitando assim a necessidade de usar equipamentos caros e diferentes tipos de corpos de prova. Este método é usado para caracterização das ligas Aço SAE 1020 e Alumínio 6351-T6 e os dados levantados são comparados com as propriedades medidas de fadiga de baixo ciclo em tração (epsilon)N, identificando assim se o material é mais sensível a tensões normais ou cisalhantes. Um programa numérico é usado para ajustar as curvas (epsilon)N e (gama)N nos dados experimentais, e seus procedimentos de implementação são discutidos. Por fim, são propostos e calibrados modelos de fadiga multiaxial de plano crítico mais adequados para cada material testado, com base nos dados medidos. / [en] The mechanical failure known as fatigue is characterized by the formation and/or propagation of cracks caused by variable forces. Its traditional methodologies normally calculate an equivalent uniaxial tensile stress acting on the component, in order to compare it with the known experimental mechanical behavior data of the component s material measured under uniaxial loads. This assumption can lead to non-conservative results because it considers the material to be equally sensitive to shear and tensile stresses, which is not true in a wide range of practical applications. Therefore, torsional and multiaxial experimental data is necessary to better predict the fatigue life of components. To execute those experiments, the present work proposes a variety of component designs and assembly methodologies to use on an Instron 8874 axial-torsional testing machine a hydraulic grip originally designed for an Instron 8501 uniaxial testing machine. Furthermore, a simplified method to estimate shear (gamma)N low-cycle fatigue properties via displacement-controlled experiments is proposed to avoid the need of using expensive equipment and different specimen designs, and used for characterization of SAE 1020 Steel and 6351-T6 Aluminum alloys. This data is compared with the measured tensile (epsilon)N low-cycle fatigue properties to identify if these materials are tensile or shear sensitive under multiaxial loading conditions. A numerical computing code is used to fit (epsilon)N and (gamma)N curves to the experimental data, and its implementation procedures are discussed. Finally, the most suitable critical-plane multiaxial fatigue models are proposed and calibrated for each material tested, based on the measured data.