No description available.

No description available.

Les pieds d’aubes de soufflantes de turboréacteurs étant soumis à des sollicitations de type fretting, l’introduction de matériaux composites dans la nouvelle génération de moteur d’avion a rendu nécessaire le développement d’outils permettant de modéliser le contact entre des matériaux hétérogènes. En particulier, le comportement tribologique et l’endommagement de ces matériaux est encore mal compris. La mise en place de méthodes numériques capable de prédire les endommagements dans le contact permettrait de mieux prédire la durée de vie des pièces en service et de garantir la sécurité des passagers. Cette thèse porte sur le développement de méthodes semi-analytiques pour la modélisation de l’endommagement dans des conditions de fretting et de roulement. / The blade/disk interface in turbofan is subject to fretting loading. Fan blade of the new generation of aircraft engines are made of woven composite materials. The introduction of these new kind of materials create the need for a new numerical tool able to simulate the contact between heterogeneous materials. The tribological behavior and the damage mechanism associated with these kind of material are still not well understood. The developpment of new numerical tool able to model the damage in the contact area would allow to predict the life of engines parts and to guarantee the security of passengers. This PhD is about the developpment of semi-analytical methods for modeling the damage in fretting and rolling contact conditions with some applications to heterogeneous materials.

This dissertation aims to study the benefit of rail grinding on service life of railroad rails, focusing on failures due to rolling contact fatigue (RCF) at the rail head. Assuming a tangent rail with one-point contact at the running surface, a finite element analysis of a full-scale wheel-rail rolling contact with a nonlinear isotropic kinematic hardening material model is performed to simulate the accumulation of residual stresses and strains in the rail head. Using rolling stress and strain results from the sixth loading cycle, in which residual stresses and strains are at their steady-state, as input, two critical plane fatigue criteria are proposed for fatigue analyses. The first fatigue criterion is the stress-based approach—namely the Findley fatigue criterion. It suggests an important role of tensile residual stresses on subsurface crack nucleation and early growth in the rail head, but applications of the criterion to the near-running-surface region are limited because of plastic deformation from wheel-rail contact. The second fatigue criterion is the strain-based approach—namely the Fatemi-Socie fatigue criterion. Contributed mainly from shear strain amplitudes and factorized by normal stress components, the criterion also predicts fatigue crack nucleation at the subsurface as a possible failure mode as well as fatigue crack nucleation at the near-surface, while maintaining its validity in both regions. A collection of fatigue test data of various types of rail steel from literature is analyzed to determine a relationship between fatigue damages and number of cycles to failure. Considering a set of wheel loads with their corresponding number of rolling passage as a loading unit (LU), optimizations of grinding schedules with genetic algorithm (GA) show that fatigue life of rail increases by varying amount when compared against that from the no-grinding case. Results show that the proposed grinding schedules, optimized with the exploratory and local-search genetic algorithms, can increase fatigue life of rail by 240 percent. The optimization framework is designed to be able to determine a set of optimal grinding schedules for different types of rail steel and different contact configurations, i.e. two-point contact occurred when cornering.

Sous l’effet des sollicitations mécaniques répétées induites par les passages des trains, on observe l’apparition de fissures de fatigue de contact dans les rails. Une fois amorcées, celles-ci peuvent se propager et mener à la rupture du rail. Dans un contexte d’intensification du trafic et d’augmentation globale des vitesses de circulation, il devient stratégique pour SNCF d’optimiser sa politique de maintenance. Afin de définir des pas de surveillance adaptés et une planification optimisée des opérations de maintenance, une meilleure connaissance des mécanismes d’endommagement par fatigue du rail s’avère nécessaire. Tendre vers cette stratégie de maintenance prédictive passe par la mise en place d’outils de simulation numérique adaptés. Dans ce contexte, une chaîne d’outils a été développée : détermination des sollicitations transmises au rail, des champs de contraintes et de déformations résiduelles, localisation des zones critiques vis-à-vis du risque de fissuration. L’étape suivante consiste à estimer le risque lié à la présence de fissures et à étudier leurs propagations. Elle constitue une partie des objectifs de ces travaux de thèse. La résolution du problème tri-dimensionnel d’une structure fissurée, avec contact et frottement entre les lèvres, est effectuée grâce à un modèle tri-dimensionnel éléments finis étendus multi-échelles. Ce modèle fait appel à une formulation mixte stabilisée où chaque champ est écrit à l’aide d’enrichissement. La fissure est représentée grâce à une stratégie implicite-explicite. Le problème est résolu à l’aide du solveur non-linéaire LATIN. Une étude empirico-numérique a permis de proposer des formules a priori assurant à la méthode de résolution un taux de convergence proche de l’optimal. La simulation de la propagation des fissures de fatigue est réalisée à l’aide de critères spécifiques, adaptés à un chargement multi-axial et non-proportionnel, et d’une loi de propagation dédiée en mode mixte. La confrontation des résultats de simulation avec des essais réalisés sur une configuration cylindre-plan a validé la stratégie X-FEM/LATIN à deux échelles. Tous ces développements ont été implémentés dans le code de calcul éléments finis CAST3M. Des contraintes résiduelles réalistes, provenant d’un logiciel externe, ont été introduites. Cette étape a requis la mise en place d’une procédure de transfert des champs entre les deux maillages (celui utilisé pour le calcul des contraintes résiduelles et celui utilisé pour la résolution du problème de mécanique élastique linéaire de la rupture). L’étude de la flexion du rail a révelé l’influence de ce phénomène uniquement lors du passage du chargement sur la fissure.Enfin, une étude numérique a montré la très forte influence de l’orientation du chargement tangentiel, des contraintes résiduelles et de la présence de plusieurs fissures sur la direction et les vitesses de propagation des fissures de fatigue. / To optimize the rail grinding strategy, the prediction of crack growth rates has a vital role. Contact, with friction between the crack faces, notably occurs in rolling contact fatigue (RCF) problems. These time-dependent, multi-axial, non proportional loadings may lead to a crack initiation and propagation, and sometimes to the development of very complex 3D crack network. Numerical simulations of frictional fatigue crack are efficiently performed using the eXtended Finite Element Method (X-FEM). Within this method, the mesh does not need to conform to the crack geometry. Most difficulties associated to complex mesh generation around the crack and the re-meshing steps during the propagation are hence avoided. A 3D two-scale frictional contact fa-tigue crack model developed within the X-FEM framework is presented. It allows the use of a refined discretization of the crack interface independent from the underlying finite element mesh and adapted to the frictional contact crack scale. The model is used here to analyze the crack propagation, rate and direction, under rolling contact fatigue. The wheel-rail contact loading is modeled as a traveling hertzian load. The stress intensity factors are computed at the crack tips during the wheel passage. Criteria for determining crack growth direction under multiaxial non proportional conditions and mixed mode Paris’ law are used. Actual residual stresses are accounted for in the simulation. They are determined thanks to a dedicated model used at SNCF in which the asymptotic mechanical state of the rail is computed when submitted to cyclic loads. A non-uniform elastic-plastic stabilized state is calculated and introduced, by projection of the mechanical fields onto the finite element mesh, in the crack propagation simulation. All this strategy has been implemented in CAST3M and is now used to model 3D frictional crack growth under RCF.

Ce travail réalisé dans le cadre d'une collaboration industrielle avec la société ALSTOM Transport porte sur l'étude de la dégradation des voies ferrées urbaines. Les composantes de voie retenus pour cette étude sont le rail et la dalle de voie en béton. Concernant le rail, différents problèmes sont abordés : contact roue – rail, usure du rail, usure ondulatoire du rail, et fatigue de contact de roulement (RCF) du rail. Un outil numérique avec des interfaces graphiques, nommé CONUS, est développé pour le problème de contact roue – rail et le problème d'usure du rail. Des théories classiques (Hertz, Kalker, Archard, etc.) sont implantées dans cet outil. La méthode stationnaire est implantée dans un code de calcul par éléments finis pour étudier l'état asymptotique de l'acier du rail sous le chargement répété des trains. Ceci nous permet de prédire les régimes de RCF du rail. La mécanique de l'endommagement est utilisée pour prédire la fatigue du matériau béton. Le formalisme de Marigo couplé avec le modèle d'endommagement de Mazars permet de modéliser la dégradation progressive de la rigidité du matériau sous chargement cyclique. Une campagne d'essais de fatigue du béton en flexion a été réalisée. Elle a pour but de valider le modèle théorique et d'identifier les paramètres du matériau. Le dimensionnement d'une dalle de voie en béton a fait l'objet d'une application de cette méthode. Le modèle de réseau de poutres (lattice model) a été utilisé pour étudier la propagation des fissures dans les structures en béton. Ce modèle a été implanté dans le logiciel de calcul par éléments finis, CESAR-LCPC. Les résultats numériques (propagation de fissures) obtenus pour les structures simples sous chargement statique sont en tout point comparables avec les résultats d'essais expérimentaux. Ce modèle a ensuite été utilisé pour étudier la fissuration sous chargement de fatigue. Pour cela un modèle d'endommagement simple modélisant la dégradation des éléments «poutres» s'est avéré suffisant pour décrire la cinématique de propagation des fissures / This work is part of the collaboration between the laboratory Navier (UMR ENPC /IFSTTAR/ CNRS) and ALSTOM Transport company (TGS/Trackway). It focuses on the study of the degradation of urban railways. The components of track considered in this study are the rail and the concrete slab. Regarding the rail, different problems are discussed : wheel – rail contact, rail wear, rail corrugation and rolling contact fatigue (RCF). A numerical tool with graphical interfaces, called CONUS, is developed to predict the behaviour of the wheel - rail contact, the rail wear, and the rail corrugation problems. Classical theories (Hertz, Kalker, Archard, etc...) are implemented in this tool. The stationary method is implemented in a finite element software to study the asymptotic state of the rail steel under repeated loading of trains.The damage mechanics is used to predict the fatigue life of concrete. Marigo's formalism coupled with Mazars' damage model is used to predict the gradual degradation of material stiffness under cyclic loading. A campaign of fatigue tests for concrete in bending was conducted. It aims at validating the theoretical model and identifying material parameters. We applied this method in order to design the concrete slabs of urban railway. The lattice model was used to study the crack propagation in concrete structures. This model was implemented in the finite element software, CESAR-LCPC. The numerical results obtained for simple structures under static loading are consistent with the results of laboratory experiments. This model was then used to study the crack propagation under fatigue loading. For that purpose, a simple damage model of degradation of the "beams" elements describes the kinematics of crack propagation with a satisfying accuracy

This master’s thesis deals with a fatigue analysis of a roller bearing. The fatigue analysis consists of determining service-life of a roller bearing using a stress-strain analysis with finite element method and subsequent numeric calculation using software fe-safe with application of Brown-Miller multiaxial fatigue criterion. Service-life of a roller bearing is determined by number of cycles to failure in the form of pitting formation which is consequence of contact fatigue. According to calculations was found an inappropriateness of Brown-Miller criterion for high-cycle contact fatigue. Test of other available criteria was performed. Principal-Strain criterion provided the best match with ISO 281 standard.

Achieving sustainable lubrication by using environmentally friendly formulated lubricants has became an essential component of the transition process from fossil-powered vehicles to electrified transportation. Mixtures, or aqueous solutions of molecules such as polyhydroxy alcohols, and glycols usually known as green lubricants make it possible to achieve low friction coefficients under different lubrication conditions, which constitutes a potential alternative to improve the tribological performance of moving parts in automotive systems, at the same time that the environmental requirements are satisfied. There is a need to improve the protection against mild-wear and micropitting offered by green-lubricants before consider using them in the transmission gear box of battery electric vehicles BEVs, where their low shear-stress resistance can potentially help to improve the vehicle efficiency. Therefore, this research work aims to gain understanding of the tribological behaviour of rolling-sliding elements under glycerol-based lubrication, with focus on assessing the influence of different glycerol-based lubricant formulations on the wear modes associated to rolling contact fatigue such as micropitting, and macropitting.  Micropitting tests were carried out by using a micropitting rig, with a roller on ring planetary configuration. The evolution of microcracks, and micropitting was studied for several glycerol-based lubricants at different slide-to-roll ratios (SRRs) 5-30%, and different load contact conditions 1.5-2.5 GPa. A comparison against a fully formulated transmission oil has been performed. A relationship between the surface damage morphology and the operating conditions has been established. The lubrication regimes of DLC coated contacts and uncoated contacts in presence of glycerol-based lubricants were investigated through the construction of 3D friction maps, and Stribeck curves in a wide range of rolling speeds, and SRR testing conditions. The capability of two different DLC coating systems to prevent micropitting onset due to rolling contact fatigue in presence of glycerol-based lubricants was studied: Cr/a-WC:H/a-C:H and a-C:Cr.  Under mixed-lubrication regime it was found that a reduction up to 51% of friction can be achieved by using glycerol-based lubricants compared to a commercial transmission oil. The initial low friction coefficients at low contact cycles was attributed to the low pressure-viscosity coefficient of the glycerol-based lubricants. Friction was even further reduced with the increasing contact cycles since glycerol aqueous solutions were found to promote mild-wear, causing a smoothing of the surface asperities and therefore an enhancement of the hydrodynamic effect due a higher lambda ratio (Λ), making possible to trigger superlubricity in the contact. Surface micro-cracking was found to decrease at low SRR values under glycerol-based lubrication contrasted to the contacts lubricated with a fully formulated transmission oil. Once microcracks were nucleated, micropitting progressed faster in the contacts lubricated with glycerol-based lubricants, while micropitting was null or significantly delayed in presence of transmission oil. With the increasing SRR surface microcracks density and micropitting was found to became higher. The reduction of microcracks density with a glycerol-glycol based lubricant was attributed to the adsorption of long glycol molecules on the steel surface that avoid the direct interaction between asperities. The faster micropitting progression was attributed to the presence of dissolved water in the contact. Thin DLC a-C:Cr coatings were shown to significantly mitigate the penetration of surface initiated pits into the steel substrate, which was translated in a lower volume loss. Local micro-delamination was found to be the main reason behind coating failure in presence of glycerol-based lubricants, as a result, pitting took place on the steel exposed areas.

No description available.

Hypoid gears have been used extensively in automobiles, aerospace, marine and other applications for decades. The special advantages of hypoid gears come with inherent contact complexities of varying curvature and sliding in both profile and lengthwise direction. Spalling failure is catastrophic and needs to be addressed with deeper roots in gear design. Analytical methods present several limitations. Iterative development from experimentation is expensive and time consuming with different nonlinear parameters difficult to interpret. This thesis aims to develop a method to calculate contact fatigue life for initiation of spalling using finite element methods. Experiments have played a major role in understanding the causal factors for failure, determining the fatigue life and to study the major system design parameters. A failure analysis of the fractured flank is performed. It clarified the design causal factors for failure and the mechanism of failure. Pinion being the vulnerable part is the focus of this thesis, a finite element model was developed on ANSOL HFM and the residual stresses were superposed on MSC Marc. A finite element fatigue analysis is performed on FEMFAT and the component fatigue life is determined. The calculated fatigue life is compared with physical testing results using Weibull statistical analysis in combination with probabilistic bearing life models to formulate emphatical correlation methods. The goal of this thesis is to establish a method to estimate fatigue life by taking up example of computing subsurface fatigue life of a hypoid pinion. The influence factors like the method of contact analysis, different types of residual stresses due to case hardening and shot peening, fatigue criteria, friction, material properties are studied in this thesis to develop a conscience for the methodology to computing contact fatigue life. The bulk material properties based on hardness represented fatigue properties more accurately. Scaled normal stress in critical plane fatigue criteria was found more suitable for contact analysis with pre-stresses and multi-axial non-proportional contact stress state on FEMFAT. Finite element based contact analysis method was found to be more suitable for subsurface fatigue life estimation despite the inherent advantages of the hybrid surface integral method and its accurate representation of friction. It was found that inclusion of friction in model did not change the fatigue life significantly, showing that the influence of hardness, surface topographies lubrication and contact temperature on shear stresses are too large to be neglected. Contact fatigue life increased by a factor of 4.4 times due to shot peening of gears in comparison with case hardening indicating the influence of residual stresses. For the estimation of fatigue life at the initiation of failure, a complete correlation with the fatigue test results could not be achieved and reasons for deviations were clearly identified. The area of damage indicated by this computation method correlated with the damage observed during tests. The observations and calculations indicated premature failure of pinion with explanation of mechanism of failure of pinion flank using contact conditions. / Hypoidväxlar har använts i stor utsträckning i bilar, flyg-, marin- och andra applikationer under årtionden. De speciella fördelarna med hypoidväxlar kommer med inneboende kontaktkomplexitet med varierande krökning och glidning i både profil och längdriktning. Spallingfel är katastrofala och måste hanteras med grundlig redskapsdesign. Analytiska metoder har flera begränsningar. Iterativ utveckling baserad på experiment är dyrt och tidskrävande med olika icke-linjära parametrar som är svåra att tolka. Denna avhandling syftar till att utveckla en metod för att beräkna kontaktutmattningslivslängden för initiering av spalling med finitaelementmetoden. Experiment har spelat en viktig roll för att förstå orsaksfaktorerna för fel, bestämma utmattningslivslängden och för att studera de viktigaste systemdesignparametrarna. En felanalys av den skadade kuggflanken utfördes, vilket förtydligade felens orsaksfaktorer och den underliggande felmekanismerna. Driften är fokus för denna avhandling, en finitaelementmodell utvecklades med ANSOL-HFM och restspänningarna överlagrades med FEM-verktyget MSC-Marc. En slutlig elementutmattningsanalys utfördes med FEMFAT och komponentens utmattningstid bestämdes. Den beräknade utmattningslivslängden korrelerades med fysiska provningsresultat genom att tillämpa statistisk Weibullanalys i kombination med probabilistiska livslängdsmodeller. Målet med denna avhandling är att utveckla en metod för att uppskatta utmattningslivslängden och att tillämpa metoden för att beräkna bulkmaterialets utmattningslivslängd för ett hypoidrev. Påverkningsfaktorer som metoden för kontaktanalys, olika typer av restspänningar på grund av ythärdning och kulblästring, utmattningskriterier, friktion, materialegenskaper studeras i denna avhandling för att utveckla denna metod för att prediktera kontaktutmattningslivslängden. Bulkmaterialegenskaperna för hårdhet representerade utmattningsegenskaperna mera exakt. Skalad normalspänning i kritiska planutmattningskriterier befanns vara mer lämplig för kontaktanalys med förspänningar och multi-axiell icke-proportionell kontaktspänningsstatu än andra egenskaper med FEMFAT. Metodbaserad kontaktanalysmetod visade sig vara mer lämplig för uppskattning av ytutmattningslivslängd trots de inneboende fördelarna med hybridytaintegralmetoden och dess mera exakta friktionsrepresentation. Det visade sig att inkludering av friktion i modellen inte markant förändrade livslängden, vilket visade att påverkan av hårdhet, smörjning av ytorna och kontakttemperatur på skjuvspänningarna är för stor för att försummas. Kontaktutmattningslivslängden ökade med en faktor 4,4 gånger på grund av kugghjulning jämfört med ythärdning, vilket indikerar restspänningspåverkan. För att uppskatta utmattningslivslängden vid inledningen av ytfel kunde en fullständig korrelation med utmattningstestresultaten inte uppnås och orsakerna till avvikelser identifierades tydligt. Det skadade område som indikeras av denna beräkningsmetod korrelerade väl med den skada som observerades under testerna. Observationerna och beräkningarna indikerade tidigt fel i kuggen med förklaring av mekanismen för fel hos kuggflanken med hjälp av aktuellt kontaktförhållande.