Analyse tribologique du contact roue-railModélisation et expérimentations– Cas de l'usure ondulatoire –

Depuis plus d'un siècle, les rails des transports urbains ferrés (tramways, métros) sont affectés d'une usure périodique de leur surface de contact appelée "usure ondulatoire". Lors du passage d'un train sur une voie en étant affectée, il apparaît d'importantes nuisances sonores et vibratoires dont les niveaux dépassent fréquemment les normes fixées par la loi. Les réseaux urbains sont alors contraints de procéder à des opérations coûteuses et répétées d'abrasion mécanique de la surface des rails par meulage, actuellement le seul moyen efficace de contrôle du développement de l'usure ondulatoire.Depuis des décennies, cette usure a été l'objet d'études centrées principalement sur des analyses vibratoires de ses conséquences plutôt que sur des analyses temporelles des interactions locales de contact à l'origine de sa formation. Afin de pallier ce manque, une démarche centrée sur l'analyse tribologique locale du contact roue-rail est proposée et appliquée aux cas particuliers de l'usure ondulatoire des voies rectilignes en zone d'accélération et celle des voies courbes de faible rayon.Tout d'abord, les résultats des analyses tribologiques des surfaces frottantes ont permis, dans les deux cas, de caractériser le circuit tribologique de l'usure ondulatoire établie. Cependant, l'influence de sa géométrie sur les débits qui ont mené initialement à sa formation reste à déterminer.Dans le cas de l'usure ondulatoire des voies rectilignes, un modèle numérique longitudinal en 2-dimensions de contact roue-rail a été mis en œuvre pour pallier le manque d'instrumentation physique du contact. Dans des conditions reproduisant l'accélération d'un train, des régimes d'instabilités (adhérence-glissement) se développent naturellement dans le contact et ont pour conséquence d'induire des températures de contact et des déformations plastiques périodiques de la surface du rail qui sont comparables à celles de l'usure ondulatoire étudiée expérimentalement.Dans le cas de l'usure ondulatoire des voies courbes, une instrumentation sur site à l'échelle de la roue et du rail (collaboration RATP et Railtech) a été mise en œuvre. Des glissements latéraux périodiques spécifiques des zones d'usure ondulatoire ont alors été identifiés. Confirmés par des essais sur banc visant à reproduire ce type d'usure (collaboration Lucchini et Politecnico di Milano), ces glissement latéraux périodiques sont bien à l'origine de la formation de l'usure ondulatoire des voies courbes.For more than a century, the rails of mass transit (tramways, undergrounds) have been affected by periodical wear on their contact surfaces, called "corrugation". The traffic of a train on an affected track leads to noise and vibration pollution, that levels frequently exceed law-allowed limits. Then, the urban networks have to proceed to expensive and repetitive mechanical abrasive operations on the rails' surface by grinding, which is the only efficient means to control corrugation development for now.For decades, this wear has been studied, mainly through vibration analyses of its consequences rather than through temporal analyses of the local contact interactions at the origin of its birth. In order to compensate for this lack, an approach centred on the local tribological analysis of the wheel-rail contact is proposed and applied to the specific corrugation cases on straight tracks in acceleration zones and to the one on sharp curved tracks.First, the results of the tribological analyses of the friction surfaces have allowed, in both cases, to characterize the tribological circuit of corrugation. Yet, the influence of its geometry on the flows that initially led to its birth is still undetermined.In the case of corrugation on straight tracks, a 2-dimensions numerical longitudinal model of the wheel-rail contact is implemented to compensate for the lack of physical instrumentation of the contact. In conditions that reproduce train acceleration, instability states (stick-slip) naturally spread in the contact and result in contact temperatures and periodical plastic strains of the rail, comparable with the ones of the corrugation experimentally studied.In the case of the corrugation on sharp curved tracks, an on-site instrumentation at the scale of the wheel-rail group (collaboration RATP and Railtech) is implemented. Periodical lateral slidings specific of corrugated zones were identified. Confirmed by laboratory tests that reproduce this type of wear (collaboration Lucchini and Politecnico di Milano), these periodical lateral slidings are at the origin of the creation of this corrugation on sharp curved tracks

Utilisation raisonnée de contaminants pour caractériser la rhéologie des 1ers et 3ème corps solides (Application à la tribologie en ambiances spatiales)

Dès les débuts de la conquête spatiale, la tribologie spatiale est considérée comme une discipline à part entière car il s agit de lubrifier un mécanisme non seulement dans le vide spatial, mais aussi lors des opérations au sol, et ceci dans différents environnements (vide simulé, air sec et humide, azote sec). Un lubrifiant spatial, fluide ou solide, doit donc relever le défi de procurer le comportement tribologique désiré dans ces environnements successifs, mais doit aussi résister aux sollicitations dynamiques induites par le lancement. Dans l espace, le mécanisme doit y être autonome sans maintenance et sa lubrification doit être maintenue pendant au moins 15 ans (800 millions de cycles pour un roulement) tout en conservant fiabilité et précision de positionnement ( rad). Malgré ces enjeux, la maîtrise tribologique spatiale se limite à de l empirisme et du conservatisme de solutions techniques, notamment lors de l emploi des matériaux sources de lubrifiants solides qui font l objet de cette étude. Dans les faits, seules des recommandations d utilisation existent sans pouvoir prédire le comportement tribologique des solutions. Cette prédiction nécessite la reconstitution de la vie des contacts . Cette dernière nécessite l évaluation des débits de 1ers et 3ème corps. Ces débits dépendent d un couplage multi-échelles et multi-physique forcé par les sollicitations tribologiques. Pour décrypter ces débits, une démarche expérimentale qui couple des mesures mécano-chimiques en temps réel et des analyses post-essai a été développée (forces, analyse de gaz, visualisation électronique et photonique). La simultanéité des mesures permet d étudier le couplage entre les phénomènes mécaniques (créations de particules de 3ème corps, déformations plastiques des 1er corps) et physicochimiques (adsorption/désorption, modifications chimiques des 1ers et 3ème corps). La démarche est appliquée à l étude de deux triplets tribologiques ayant respectivement des 1ers corps en MoS2 et MoS2+Ti dont les comportements tribologiques sous vide et sous air humide sont antagonistes. L utilisation de leur antagonisme permet de mieux comprendre la réponse des 1er et 3ème corps aux sollicitations tribologiques. La reconstitution de la vie de ces triplets a montré : (1) le rôle bénéfique de la structure colonnaire dans l obtention d un facteur de frottement faible et d une longue durée de vie, non parce qu elle est intrinsèquement colonnaire mais parce qu elle localise la production de 3ème corps; (2) le rôle bénéfique des contaminations internes et externes aux 1ers et 3ème corps qui délocalisent l accommodation du volume du 3ème corps vers les complexes de surface, ce qui explique par exemple que l environnement N2 sec ne puisse, même pour des raisons économiques, simuler l ultravide. In fine, ces travaux permettent de spécifier une démarche de conception de triplets tribologiques qui sera généralisée ultérieurement grâce à la tribologie numériqueFrom the very beginning of space exploration, space tribology became a specific field of competences in its own right because it implies lubricating a mechanism not only in the vacuum of space but also in several environments (simulated vacuum, dry and humid air, dry N2 environment, etc.) on Earth. Consequently, a lubricant, fluid or solid, must provide the desired tribological behaviour in all those successive environments, but it must also resist to the dynamic loading induced by the launch operations. Once in space, a space mechanism must be autonomous, with no possible maintenance, and its lubrication must be sustained during 15 to 30 years (i.e. up to 800 millions cycles for a ball bearing) in space while sustaining high reliability and great precision (to within a few rad). Despite those stakes, the mastery of space tribology is limited to empiricism and conservatism of technical solutions, especially when solid lubricants, whose are studied in this study, are used. In facts, only directions for use exist without allowing predicting the tribological behaviour of the solutions. That prediction requires the reconstitution of the contacts life . The latter requires evaluating the 1st and 3rd bodies flows. Those flows depend on a multiscale and multiphysical coupling effect forced by the tribological stresses. To decrypt those flows, an experimental procedure which couples both real time and post-mortem mechano-chemical measurements (friction forces, gas analysis, photonic and electronic visualization,) has thus been developed. The simultaneity in the measure allows studying the relationship between mechanical (3rd body particles creation, plastic deformations of 1st bodies, etc.) and physicochemical (adsorption/desorption, chemical changes of 1st and 3rd bodies, etc.) phenomena. The procedure is applied to two tribological triplets respectively containing 1st bodies comprised of MoS2 and MoS2+Ti coatings whose tribological behaviours in vacuum and humid air are antagonistic. Such antagonism allows better understanding the responses of the 1st and 3rd bodies to the tribological stresses. The reconstruction of the contact life of those triplets allowed showing, among others: (1) the beneficial role of coating columnar structure in the obtaining of both a low friction coefficient and a long wear life. This is not because it is columnar but because such a structure localizes the 3rd body creation; (2) the beneficial role of contaminations, which is either internal or external to the 1st and 3rd bodies, by delocalizing the accommodation from the 3rd body volume to the surface complexes, which explains, for example, that dry N2 environment can not, even for financial reasons, mimic ultrahigh vacuum. In fine, the study allows specifying a procedure for designing tribological triplets which will be generalized at a latter stage thanks to the numerical tribology.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Experimental and numerical modelling of the ignition of solid propellant

International audienceMixing processes of solid propellants can result in friction. Solid propellant ignitions can be observed under safety tests. Analysing a solid propellant elementary friction test from a mechanical, thermal, and physiochemical point of view, it appears that specific friction conditions allow the emergence of component flows in the solid propellant volume, readying the self-ignition. Numerical simulations of the solid propellant elementary friction test involve discrete elements to model these dynamic behaviours of the components within the contact. Indeed, comparisons between experiments and simulations are performed on mechanical parameters and on the evolution of the solid propellant components within the contact. Such comparisons exhibit qualitative and quantitative results by validating local parameters (adhesion), which make understand the solid propellant ignition scenario

Adhesion measurements in MOS2 dry lubricated contacts to inform predictive tribological numerical models : comparison between laboratory-tested samples and ball bearings from the niriss mechanism

International audiencePredicting the tribological behaviour of dry lubricants remains difficult because it greatly depends on their mechanical and physicochemical environment. While it is difficult to analytically model dry lubrication, Discrete Element Method (DEM)-based modelling has been able to provide valuable insight into the tribological behaviour of dry lubricated contacts. The present study aims to experimentally define interactions between the discrete elements used for simulating different materials in contact, in order to accurately model and predict the tribological behaviour of dry lubricants. Those interactions are here defined by using the work of adhesion (W) between engineering materials: AISI440C, pristine MoS2 coating, as well as the related transfer film. A method was developed and applied on regular laboratory tribological test samples and ball bearings from the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument of the James Webb Space Telescope. Measured W values were consistent between all worn surfaces. The first DEM modelling results exhibit behaviours similar to those observed experimentally including surface plasticization and transfer

Self-lubricating polymer composites : using numerical trbology to hightlight their design criterion

International audienceAfter the cessation of RT/Duroid 5813, manufacturing tests were performed by CNES and ESA/ESTL in order to find an alternative material. Although PGM-HT was selected as the best candidate, limitations about its tribological capabilities to replace RT/Duroid 5813 were later pointed out. Today, the predictability of the tribological behaviour of those materials is not fully overcome. The motivation to this work is to complement studies of self-lubricating materials by coupling experimental analyses with numerical modelling, in order to predict their tribological behaviour. A Discrete Element Method is chosen to construct the numerical material, because it allows to represent wear and the third body generation at the scale of the ball/retainer contact. An underlying role of the adhesion between components in controlling the tribological properties of the transfer film has been observed

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

International audienceSelf-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60's in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM)

Development of Temperature-Controlled Shear Tests to Reproduce White-Etching-Layer Formation in Pearlitic Rail Steel

The formation of a white etching layer (WEL), a very hard and brittle phase on the rail surface, is associated with a progressive transformation of the pearlitic grain to very fragmented grains due to the cumulative passage of trains. Its formation is associated with a complex thermomechanical coupling. To predict the exact conditions of WEL formation, a thermomechanical model previously proposed by the authors needs to be validated. In this study, monotonic and cyclic shear tests using hat-shaped specimens were conducted in the temperature range of 20 C to 400 C to reproduce the WEL formation. The tests showed a strong sensitivity of the material to temperature, which does not necessarily favor WEL formation. For the monotonic tests, no WELs were produced; however, a localization of the plastic deformation was observed for tests performed at 200 C and 300 C. In this temperature range, the material was less ductile than at room temperature, leading to failure before WEL formation. At 400 C, the material exhibited a much more ductile behavior, and nanograins close to WEL stages were visible. For the cyclic tests, a WEL zone was successfully reproduced at room temperature only and confirmed the effect of shear in WEL formation. The same cyclic tests conducted at 200 C and 300 C yielded results consistent with those of the monotonic tests; the deformation was much more localized and did not lead to WEL formation.This work is part of the multi-disciplinary project MOPHAB, which aims to improve our knowledge and understanding of the mechanisms leading to the formation of the white etching layer in the materials used to construct railways and to develop corresponding numerical models. This project was supported by IRT Railenium and other industrial partners (RATP: Régie Autonome des Transports Parisiens, France, SNCF: Société Nationale des Chemins de Fer Francais, France, SAARSTAHL rail)

Analyse tribologique du contact roue-rail : modélisation et expérimentations - Cas de l\u27usure ondulatoire

Depuis plus d\u27un siècle, l\u27usure périodique de la surface des rails, appelée usure ondulatoire, reste un problème majeur des transports urbains ferrés. Induites en erreur par la périodicité de ce phénomène, les études précédentes ont ainsi délaissé l\u27analyse des conditions locales de contact roue-rail, véritable cur du problème, au profit d\u27analyses fréquentielles globales des interactions véhicules-voie. Compensant ce manque, la démarche proposée de cette thèse est centrée sur l\u27analyse des conditions de contact roue-rail. Ainsi en couplant des analyses tribologiques a posteriori des rails affectés d\u27usure ondulatoire avec, d\u27une part, des simulations numériques dynamiques par éléments finis de contact roue-rail et, d\u27autre part, des expérimentations sur site et en laboratoire, les circuits tribologiques menant à la formation de l\u27usure ondulatoire des voies rectilignes en condition d\u27accélération et de l\u27usure ondulatoire des voies courbes ont finalement été déterminés

Sharp curved track corrugation: From corrugation observed on-site, to corrugation reproduced on simulators

International audienceFor more than a century, rail corrugation has been a major issue for mass transit networks. Although studied mainly by global analyses of train/track dynamics, knowledge about why corrugation appears on some types of trackform in some circumstances and not in others remains inadequate. One explanation is that these studies mainly overlooked the question of local wheel-rail interactions due to the difficulty to obtain contact data by in situ instrumentation and also to model them reliably. To provide more insight into rail corrugation modelling, this paper gives more tribological details on the onset of corrugation: * * Firstly, characteristics of corrugation are highlighted from both points of views: local, through a posteriori observations, and global, through on-site measurements. Corrugation troughs are characterized by a detachment of particles and a lateral sliding. Such characteristics may nevertheless be the cause or consequence of the periodical geometry of corrugation pre-existing on the rail surface. * * The veil on such dilemma is then lifted by laboratory testing at both full-scale, under imposed global train-track conditions, and reduced-scale, under imposed local contact conditions. Lateral slip variation in the wheel-rail contact is clearly the dominant cause of the periodical detachment of particles that digs corrugation troughs