Pour améliorer la caractérisation et le dimensionnement des disques de turbines pour les moteurs d’avion, le motoriste Safran Aircraft Engines (SAE) développe des modèles de comportement, des lois d’endommagement et des critères de fatigue plus adaptés aux chargements réels. Pour aider à cette démarche, le but de cette étude est de développer un modèle de plasticité adapté à l'Inco718DA (un alliage à base nickel utilisé dans la fabrication des turbines haute pression), capable de représenter différents chargements (monotone, cyclique symétrique et non-symétrique). La proposition puis l'identification du modèle a été possible grâce à une campagne expérimentale favorisant des tests complexes et innovants aux essais de fatigue nombreux et coûteux. Les essais faits incluent un essai monotone avec décharges élastiques, un essai multi-niveau à Rε=-1 et deux essais multi-niveau à Rε=0 pour mieux caractériser la relaxation de la contrainte moyenne et un essai pour quantifer le rochet. Le comportement cyclique du matériau a été identifié en utilisant un écrouissage cinématique non saturant avec des éléments s'inspirant de la surface mémoire de Chaboche. Un des défis a été d'obtenir des boucles stabilisées "pointues" dans un régime de plasticité cyclique saturante, en utilisant une évolution du paramètre Γ en fonction de la déformation plastique équivalente maximale (prefacteur du terme de rappel de la loi d'écrouissage cinématique).Une deuxième difficulté apparaît dans la description de la relaxation de la contrainte moyenne, phénomène complexe avec un impact considérable sur la durée de vie en fatigue. Dans le chapitre 3, un modèle est proposé pour la caractérisation de la relaxation partielle de la contrainte moyenne. Une originalité du modèle est l'idée que la relaxation incomplète est une conséquence directe de la différence entre la charge et la décharge de la boucle de hystérésis. Le paramètre choisi pour décrire cette différence a été le préfacteur du terme de rappel Γ, pour lequel la thermodynamique donne de la liberté. Par rapport à d'autres lois d'écrouissage confirmées, notre modèle présente l'avantage d'utiliser un seul terme de rappel, mais avec une formulation plus complexe. En plus, le modèle est incrémental (écrit en taux/en vitesse), il peut donc prendre en compte des chargements complexes tels que aléatoires ou plus simplement tels que dans les essais multi-niveaux pilotés en déformation. Dans le dernier chapitre de la thèse, une campagne biaxiale vaste est présentée, avec les développements pour réaliser des essais biaxiaux pilotés en déformation. La campagne biaxiale a été réalisée sur des éprouvettes cruciformes en utilisant des capteurs LASER et des mesures de champs mono et stéréo analysées en utilisant la Corrélation d'Images Numeriques (CIN). Pour analyser la relaxation de la contrainte moyenne en biaxial un moyen de mesure et de contrôle fiable a dû être développé, adapté aux déformations plastiques élevées qui apparaissent dans la région d'intérêt de l'éprouvette. En utilisant la corrélation d'images intégrée (I-CIN) avec des fonctions de forme adaptées sur un seul élément et des calculs sur GPU, on a obtenu des fréquences de mesure de 100~Hz. En plus, avec sa précision et vitesse, I-CIN a été une technique adaptée pour contrôler une machine d'essais multiaxiale hydraulique. Un résultat important obtenu quand on a réalisé des essais equi-biaxiaux pilotés en déformation a été l'observation d'une relaxation de la contrainte moyenne très faible par rapport au cas uniaxial. Ce résultat doit être pris en compte dans les études futures avec des calculs éléments finis sur l'éprouvette complète. / To improve the characterization and design of aircraft engine turbine disks, the propulsion systems manufacturer Safran Aircraft Engines (SAE) develops constitutive equations, damage laws and fatigue criteria that are more adapted to real loadings. As part of this effort, the purpose of the current study is to develop a plasticity model for Inco718DA (a nickel-based alloy used in the manufacturing of high-pressure turbine disks), capable of representing several loading conditions (monotonic, symmetrical and non-symmetrical cyclic loading). The identification of the model was possible thanks to a uniaxial campaign, favoring a few but complex, innovative, tests to numerous costly fatigue tests. The tests we performed include a monotonic test with elastic discharges, a multi-level Rε = -1 test and two multi-level Rε = 0 tests that better quantify the mean stress relaxation and a test to identify ratcheting. The cyclic behavior was identified using a non-saturating kinematic hardening law with elements of Chaboche's memory surface. One of the challenges was to obtain sharp stabilized loops in a saturated cyclic plasticity regime, which was possible using parameter Γ evolving with respect to the maximum equivalent plastic strain, in the back-stress of kinematic hardening rule.A second difficulty appears in the description of mean stress relaxation, which has a considerable impact on fatigue lifetime. In chapter 3, a model is proposed for the description of the incomplete mean stress relaxation. One of the originalities is the idea that incomplete mean stress relaxation is a direct consequence of the difference between the loading and the unloading part of the hysteresis loop. The parameter we used to describe this difference, was the prefactor of the back-stress term Γ, for which the thermodynamics allows liberty. When compared to confirmed kinematic hardening laws that model non-zero mean stress relaxation, our model presents the advantage of using only one backstress, even if its description is more complex. Moreover, the model is incremental (written in a rate form in chapter 3 section 5) so it can take into account complex loadings such as multi-level strain-controlled tests.In the last chapter of the thesis, a vast biaxial campaign is presented, along with developments to make biaxial strain-controlled tests. The biaxial campaign was performed on cross-shaped samples using LASER sensors, mono and stereo full-field measurements using Digital Image Correlation (DIC). In order to analyze biaxial mean stress relaxation, a reliable measurement and control method had to be developed for the high plastic strains occurring in the region of interest of the sample. By using an Integrated-DIC (I-DIC) algorithm with adequate shape functions on one element and GPU computations we were able to obtain measurement frequencies of 100 Hz. Moreover, with its precision and speed, I-DIC proved to be a suitable technique for controlling a biaxial hydraulic machine. An important result obtained when performing equi-biaxial I-DIC strain-controlled tests was that there was very little biaxial mean stress relaxation, with respect to the uniaxial case. This result will have to be taken into account in future studies when performing finite element computations of the whole sample.

This study investigates the cyclic deformation, fatigue behavior, and failure mechanisms for Ti-6Al-4V ELI (extra low interstitial) with and without mean strain/stress. Mean stress effects on fatigue behavior were studied using four strain ratios. Fatigue data generated was used to assess mean stress fatigue life prediction approaches, including stress-based methods such as Goodman, Gerber, Morrow, Walker and Kwofie; as well as strain-based models, such as Morrow, Smith-Watson-Topper, Walker, Kwofie, Ince-Glinka and a modified version of the Smith-Watson-Topper. The stress-based models did not yield reasonable results and data scatter was observed. The strain-based models offered better results, specifically the Morrow approach which provided more accurate fatigue life predictions. Fractography analysis determined that the influence of material defects on fatigue life had no major differences across all the strain ratios considered. Overall observations indicate that inclusions near the surface had great influence on the fatigue behavior.

No description available.

The design of any mechanical components requires an understanding of the general statical, dynamical and environmental conditions where the components will be operating to give a satisfactory results in terms of performance and endurance. The premature failure of any components is undesirable and potentially catastrophic, therefore predictions on performances and endurances of components to proceed with repair or substitution is vital to the stability of the structure where the component is inserted. The capability of a component of withstanding fatigue loading conditions during service is called fatigue life and the designed predictions can be conservative or non conservative. Improvements to a strain based approach to fatigue were obtained in this study, studying the effects of mean stresses on fatigue life and investigating cyclic mean stress relaxation of two aluminum alloys, 7075-T6511 and 7249-T76511, used in structural aircraft applications. The two aluminum alloys were tested and their fatigue behavior characterized. The project, entirely funded by NAVAIR, Naval Air Systems Command, and jointly coordinated with TDA, Technical Data Analysis Inc., was aimed to obtain fatigue data for both aluminum alloys, with particular interest in 7249 alloy because of its enhanced corrosion resistance, and to give guidelines for improving the performances of FAMS, Fatigue Analysis of Metallic Structures, a life prediction software from the point of view of both mean stress effects and mean stress relaxation. The sensitivity of engineering materials to mean stresses is of high relevance in a strain based fatigue approach. The performance of the most common models used to calculate mean stress correction factors was studied for the two aluminum alloys 7075 and 7249 to give guidelines in the use of those for life predictions. Not only mean stresses have a high influence on fatigue life, but they are also subjected to transient cyclic behaviors. The following study considered both an empirical approach and a plasticity theory approach to simulate and include these transient effects in life calculations. Results will give valid directions to a successful modification of FAMS like any other life calculation software to include in the picture transient phenomena. / Ph. D.

Plankerbverzahnungen zählen zu den Vertretern der Kupplungsverzahnungen, mit deren Hilfe zwei Bauteile koaxial miteinander verbunden werden können. Neben einer hohen Winkelgenauigkeit zeichnen sie sich durch eine enorme Drehmomentkapazität bei gleichzeitig geringem Montageaufwand aus. Die zur Leistungsübertragung erforderliche Vorspannkraft kann über eine zentrale Spannschraube oder mehrere dezentral, auf einem gemeinsamen Teilkreis, angeordnete Spannschrauben erzeugt werden. Diese Arbeit liefert einen Beitrag zur beanspruchungsgerechten Dimensionierung von Plankerbverzahnungen mit zentraler und dezentraler Verschraubung. Es wird eine Berechnungsmethode zur Bestimmung der Tragfähigkeit des Zahnfußes, der Zahnflanke und der Spannschrauben vorgestellt und anhand von Beispielrechnungen erläutert. Neben den umfangreichen theoretischen Betrachtungen werden die Ergebnisse zahlreicher experimenteller Untersuchungen präsentiert. Diese dienen der Gewinnung von grundlegenden Aussagen zur Stützwirkung und zum Mittelspannungseinfluss an einfach und mehrfach gekerbten Proben sowie zur Verifizierung des Tragfähigkeitsnachweises.

<p>Many geotechnical engineering problems involve combining two or more independent physical processes as a coupled solution of seepage, volume change and shear strength. For any physical process being modeled, it is desirous to be able to compute any of the volume-mass soil properties. When the volume-mass soil properties are combined with the stress state of the soil, the result is a volume-mass constitutive relationship. Three volume-mass constitutive relationships (i.e., void ratio, water content and degree of saturation) are generally viewed as being the most fundamental; however, only two of the relations are independent. The unsaturated soil properties associated with seepage, volume change and shear strength problems are also related to the volume-mass soil properties. While the unsaturated soil properties are often estimated as simply being a function of the soil-water characteristic curve, it is more accurate to define the properties in a more rigorous manner in terms of the volume-mass soil properties. The advancement of computing capabilities means that it is quite easy to formulate constitutive relations for shear strength and permeability, for example, in terms of all volume-mass properties of the unsaturated soil.</p><p>The objectives of this dissertation include: i) the development of volume-mass constitutive models for unsaturated soils; ii) the further study and verification of the volume-mass constitutive behavior of unsaturated soils; and iii) the development of techniques for visualization of volume-mass constitutive surfaces for unsaturated soils. To achieve these objectives, the present research study was conducted from both theoretical and experimental bases.</p><p>The theoretical program commenced with a comprehensive literature review of the volume-mass constitutive relationships for unsaturated soils. A new, more rigorous volume-mass constitutive model was then proposed. Appropriate terminology was introduced for the development of the model, followed by an outline of the assumptions used and the mathematical derivation. The proposed model requires conventionally obtainable soil properties for its calibration. The model is capable of predicting both the void ratio and water content constitutive relationships for various unsaturated soils, taking into account elastic and plastic volume changes. Various stress paths can be simulated and hysteresis associated with the soil-water characteristic curve can be taken into account. </p><p>Two closed-form equations for the volume-mass constitutive relationships were derived. A computer software program was written based on the theory of the proposed volume-mass constitutive model. Techniques for the visualization of the volume-mass constitutive surfaces were then presented. An experimental program was conducted in the laboratory. The experimental program involved the verification of a new testing apparatus. Several soils were selected for testing purposes and appropriate testing procedures were established (i.e., soil specimens were initially slurry). The testing stress paths followed in the experimental program were different from most research programs conducted in the past and reported in the research literature. Conclusions regarding the compressibility, stress path dependency, and hysteretic nature of the soil-water characteristic curve of an unsaturated soil were presented.</p><p>A considerable number of test results (i.e., from both the experimental program and the research literature) were used in the verification of the new volume-mass constitutive model. This model has proven to be effective in predicting both collapse and expansion of a soil. The volume-mass constitutive model appears to predict behaviour in a satisfactory manner for a wide range of soils; however, the predictions appear to be superior for certain soils. In all cases the volume-mass predictions of the model appear to be satisfactory for geotechnical engineering practice.

<p>Many geotechnical engineering problems involve combining two or more independent physical processes as a coupled solution of seepage, volume change and shear strength. For any physical process being modeled, it is desirous to be able to compute any of the volume-mass soil properties. When the volume-mass soil properties are combined with the stress state of the soil, the result is a volume-mass constitutive relationship. Three volume-mass constitutive relationships (i.e., void ratio, water content and degree of saturation) are generally viewed as being the most fundamental; however, only two of the relations are independent. The unsaturated soil properties associated with seepage, volume change and shear strength problems are also related to the volume-mass soil properties. While the unsaturated soil properties are often estimated as simply being a function of the soil-water characteristic curve, it is more accurate to define the properties in a more rigorous manner in terms of the volume-mass soil properties. The advancement of computing capabilities means that it is quite easy to formulate constitutive relations for shear strength and permeability, for example, in terms of all volume-mass properties of the unsaturated soil.</p><p>The objectives of this dissertation include: i) the development of volume-mass constitutive models for unsaturated soils; ii) the further study and verification of the volume-mass constitutive behavior of unsaturated soils; and iii) the development of techniques for visualization of volume-mass constitutive surfaces for unsaturated soils. To achieve these objectives, the present research study was conducted from both theoretical and experimental bases.</p><p>The theoretical program commenced with a comprehensive literature review of the volume-mass constitutive relationships for unsaturated soils. A new, more rigorous volume-mass constitutive model was then proposed. Appropriate terminology was introduced for the development of the model, followed by an outline of the assumptions used and the mathematical derivation. The proposed model requires conventionally obtainable soil properties for its calibration. The model is capable of predicting both the void ratio and water content constitutive relationships for various unsaturated soils, taking into account elastic and plastic volume changes. Various stress paths can be simulated and hysteresis associated with the soil-water characteristic curve can be taken into account. </p><p>Two closed-form equations for the volume-mass constitutive relationships were derived. A computer software program was written based on the theory of the proposed volume-mass constitutive model. Techniques for the visualization of the volume-mass constitutive surfaces were then presented. An experimental program was conducted in the laboratory. The experimental program involved the verification of a new testing apparatus. Several soils were selected for testing purposes and appropriate testing procedures were established (i.e., soil specimens were initially slurry). The testing stress paths followed in the experimental program were different from most research programs conducted in the past and reported in the research literature. Conclusions regarding the compressibility, stress path dependency, and hysteretic nature of the soil-water characteristic curve of an unsaturated soil were presented.</p><p>A considerable number of test results (i.e., from both the experimental program and the research literature) were used in the verification of the new volume-mass constitutive model. This model has proven to be effective in predicting both collapse and expansion of a soil. The volume-mass constitutive model appears to predict behaviour in a satisfactory manner for a wide range of soils; however, the predictions appear to be superior for certain soils. In all cases the volume-mass predictions of the model appear to be satisfactory for geotechnical engineering practice.

The fatigue life and the kinetics of short crack growth in specimen of austenitic-ferritic duplex steel cycled with constant stress amplitude and different mean stresses is studied. Fatigue life was determined on smooth cylindrical specimens. Strain amplitude and mean strain were recorded. Short crack growth was studied in cylindrical specimens with a shallow notch. Cyclic straining was interrupted and crack length was measured using an optical microscope mounted on the testing machine. Crack length and crack growth rates were evaluated. The effect of the mean stress on the rate of short cracks and fatigue life were discussed.

Geometrie, Werkstoff und Belastung sind die maßgeblichen Faktoren, welche die Dauerfestigkeit beeinflussen. Allgemein bekannt ist, dass die Dauerfestigkeit bei einer Überlagerung von ruhenden und schwingenden Belastungen geringer ist als bei rein schwingenden Belastungen. Dieser Effekt wird Mittelspannungseinfluss genannt. Die vorliegende Arbeit widmet sich dem Einfluss von Mittelspannungen auf die Dauerfestigkeit. Hierfür wurden Versuche durchgeführt und ergänzt durch die Auswertung von Ver-suchsdaten aus der Literatur. Der Mittelspannungseinfluss wird ermittelt und ein Vorschlag zur Abschätzung im Nennspannungskonzept vorgestellt. Dieser Vorschlag ist einfacher und genauer als bestehende Methoden zur Abschätzung des Mittelspannungseinflusses auf die Dauerfestigkeit. Unter Berücksichtigung von plastischen Verformungen wird der lokale Spannungszustand für die im Versuch auftretenden Belastungen ermittelt. Der Mittelspannungseinfluss wird für lokale Spannungen bestimmt und eine Methode zur Berechnung der lokalen Dauerfestig-keit entwickelt, die auch mit Festigkeitswerten aus dem Nennspannungskonzept verwen-det werden kann.:1 Einleitung 1 2 Stand des Wissens 3 2.1 Grundlagen 3 2.2 Experimentelle Ermittlung der Dauerfestigkeit 17 2.3 Mittelspannungseinfluss nach dem Nennspannungskonzept 22 2.4 Mittelspannungseinfluss im Kerbgrundkonzept 38 2.5 Zusammenfassung zum Stand des Wissens 42 3 Experimentelle Untersuchungen 45 3.1 Versuchsplanung und Vorbereitung 45 3.2 Werkstoffuntersuchungen 49 3.3 Versuchsbedingungen und Durchführung 52 3.4 Auswertung und Ergebnisse 54 4 Recherche nach experimentellen Untersuchungen 61 4.1 Kriterien 61 4.2 Aufbereitung und Auswertung 62 4.3 Ergebnisse der Versuchsdaten aus der Literatur 62 4.4 Versuche aus der Literatur, die nicht den Kriterien entsprechen 63 5 Auslegungskonzept und Validierung 69 5.1 Numerische Voruntersuchungen 70 5.2 Untersuchungen mit Nennspannungen 83 5.3 Untersuchungen mit lokalen Spannungen 107 6 Zusammenfassung 123 6.1 Nennspannungskonzept 123 6.2 Lokale Spannungen 124 7 Ausblick 125 7.1 Lastwechselzahl, Lastfolge, Lasthöhe 125 7.2 Werkstoff 125 Literatur 127 A Anhang 136 / Geometry, material and load are the main factors influencing fatigue strength. Loads can be static or oscillating. If static and oscillating loads are superimposed, the fatigue strength is lower than with purely oscillating loads. This effect is called mean stress effect. This paper is dedicated to the mean stress effect on fatigue strength. Tests are carried out on shaft-typical geometries and materials. The tests are supplemented by the evaluation of test data from literature. The mean stress effect is determined for the test and literature data. A proposal for the estimation of the mean stress effect in the nominal stress concept is presented. This proposal is simpler and more accurate than existing methods for estimating the mean stress effect on fatigue strength. Taking plastic deformations into account, the local stress state for the loads occurring in the test is determined. These are used to determine the mean stress effect and to present a concept for local stresses and a method for calculating the local fatigue strength is de-veloped, which can also be used with strength values from the nominal stress concept.:1 Einleitung 1 2 Stand des Wissens 3 2.1 Grundlagen 3 2.2 Experimentelle Ermittlung der Dauerfestigkeit 17 2.3 Mittelspannungseinfluss nach dem Nennspannungskonzept 22 2.4 Mittelspannungseinfluss im Kerbgrundkonzept 38 2.5 Zusammenfassung zum Stand des Wissens 42 3 Experimentelle Untersuchungen 45 3.1 Versuchsplanung und Vorbereitung 45 3.2 Werkstoffuntersuchungen 49 3.3 Versuchsbedingungen und Durchführung 52 3.4 Auswertung und Ergebnisse 54 4 Recherche nach experimentellen Untersuchungen 61 4.1 Kriterien 61 4.2 Aufbereitung und Auswertung 62 4.3 Ergebnisse der Versuchsdaten aus der Literatur 62 4.4 Versuche aus der Literatur, die nicht den Kriterien entsprechen 63 5 Auslegungskonzept und Validierung 69 5.1 Numerische Voruntersuchungen 70 5.2 Untersuchungen mit Nennspannungen 83 5.3 Untersuchungen mit lokalen Spannungen 107 6 Zusammenfassung 123 6.1 Nennspannungskonzept 123 6.2 Lokale Spannungen 124 7 Ausblick 125 7.1 Lastwechselzahl, Lastfolge, Lasthöhe 125 7.2 Werkstoff 125 Literatur 127 A Anhang 136

No description available.