Statistical identification of geometric parameters for high speed train catenary

Pantograph/catenary interaction is known to be strongly dependent on the static geometry of the catenary, this research thus seeks to build a statistical model of this geometry. Sensitivity analyses provide a selection of relevant parameters affecting the geometry. After correction for the dynamic nature of the measurement, provide a database of measurements. One then seeks to solve the statistical inverse problem using the maximum entropy principle and the maximum likelihood method. Two methods of multivariate density estimations are presented, the Gaussian kernel density estimation method and the Gaussian parametric method. The results provide statistical information on the significant parameters and show that the messenger wire tension of the catenary hides sources of variability that are not yet taken into account in the model

Introduction of variability in pantograph-catenary dynamic simulations

Currently, pantograph-catenary dynamic simulations codes are mainly based on deterministic approaches. However, the contact force between catenary and pantograph depends on many key parameters that are not always quantified precisely. To get a better chance of addressing extreme or combinations of critical conditions, methodologies to consider variability are thus necessary. Aerodynamic forces and geometrical irregularities of catenaries are thought to be significant sources of variability in measurement and this paper proposes methods to take them into account. Results are compared with measurements to see the importance of the considered parameters with respect to global variability observed in measurements

Damping characterization of a high speed train catenary

Catenary damping has long been a tuning parameter in pantograph-catenary dynamic interaction models. As the computed contact force is highly sensitive to the choice of damping model or coefficients, it became critical to measure it independently of the pantograph. Original tests have been conducted on a real catenary and damping identification shows a very low level of damping for a large frequency range. A fitted Rayleigh model and a combined modal and Rayleigh model are proposed and compared with a reference damping model found in literature as well as with the tests. Finally, the consequences on a typical contact force simulation are analysed and the most relevant model is chosen

Local deformations and incommensurability of high quality epitaxial graphene on a weakly interacting transition metal

We investigate the fine structure of graphene on iridium, which is a model for graphene weakly interacting with a transition metal substrate. Even the highest quality epitaxial graphene displays tiny imperfections, i.e. small biaxial strains, ca. 0.3%, rotations, ca. 0.5^{\circ}, and shears over distances of ca. 100 nm, and is found incommensurate, as revealed by X-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of the lattice parameter mismatch when cooling down the sample from the graphene preparation temperature to the measurement temperature. Although graphene weakly interacts with iridium, its thermal expansion is found positive, contrary to free-standing graphene. The structure of graphene and its variations are very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pining of graphene at steps

Rail Crack Propagation Forecasting Using Multi-horizons RNNs

The prediction of rail crack length propagation plays a crucial role in the maintenance and safety assessment of materials and structures. Traditional methods rely on physical models and empirical equations such as Paris law, which often have limitations in capturing the complex nature of crack growth. In recent years, machine learning techniques, particularly Recurrent Neural Networks (RNNs), have emerged as promising methods for time series forecasting. They allow to model time series data, and to incorporate exogenous variables into the model. The proposed approach involves collecting real data on the French rail network that includes historical crack length measurements, along with relevant exogenous factors that may influence crack growth. First, a pre-processing phase was performed to prepare a consistent data set for learning. Then, a suitable Bayesian multi-horizons recurrent architecture was designed to model the crack propagation phenomenon. Obtained results show that the Multi-horizons model outperforms state-of-the-art models such as LSTM and GRU

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.Comment: To appear in Appl. Phys. Let

Magnetism of cobalt nanoclusters on graphene on iridium

The structure and magnetic properties of Co clusters, comprising from 26 to 2700 atoms, self-organized or not on the graphene/Ir(111) moir\'e, were studied in situ with the help of scanning tunneling microscopy and X-ray magnetic circular dichroism. Surprisingly the small clusters have almost no magnetic anisotropy. We find indication for a magnetic coupling between the clusters. Experiments have to be performed carefully so as to avoid cluster damage by the soft X-rays

Epitaxial graphene on metal for new magnetic manometric systems

Graphène est un candidat pour la préparation de dispositifs spintroniques de nouvelle génération tirant partie de sa grande longueur de diffusion de spin et de la grande mobilité de ses porteurs de charge. En interagissant avec matériau ferromagnétique, il pourrait en outre devenir un élément actif, comme le suggèrent des études récentes par physique des surfaces, qui mettent en évidence un moment magnétique de quelques fractions de magnéton de Bohr dans le graphène en contact avec du fer, et une séparation en spin des bandes électroniques du graphène, d'environ 10 meV, par un effet Rashba au contact d'un élément de grand numéro atomique (l'or). La façon dont le graphène peut influencer les propriétés, par exemple magnétiques, des matériaux qui y sont contactés, reste peu étudiée. Les systèmes hybrides de haute qualité, constitués de graphène en contact avec des couches minces magnétiques ou des plots de taille nanométrique, sont des terrains de jeu pour explorer les deux aspects, la manipulation des propriétés du graphène par son interaction avec d'autres espèces, et vice versa. Dans le graphène contacté à des couches magnétiques ultra-minces par exemple, de forts effets d'interface pourraient être exploités pour contrôler l'aimantation du matériau magnétique. L'auto-organisation quasi-parfaite récemment découverte pour des plots nanométriques sur graphène, pourrait permettre d'explorer les interactions magnétiques, potentiellement transmises par le graphène, entre plots. Trois systèmes hybrides de haute qualité, intégrant du graphène préparé par dépôt chimique en phase vapeur sur le surface (111) de l'iridium, ont été développés sous ultra-haut vide (UHV) : des films ultra-minces de cobalt déposés sur graphène, et intercalés à température modérée entre graphène et son substrat, ainsi que des plots nanométriques riches-Co et -Fe, organisés avec une période de 2.5 nm sur le moiré entre graphène et Ir(111). Auparavant, des films de 10 nm d'Ir(111), monocristallins, déposés sur saphir, ont été développés. Ces films ont été par la suite utilisés comme substrats en remplacement de monocristaux massifs d'Ir(111). Ces nouveaux substrats ont ouvert la voie à des caractérisations multi-techniques ex situ, peu utilisées jusqu'alors pour étudier les systèmes graphène/métaux préparés sous UHV. Au moyen d'une combinaison de techniques de surface in situ et de sondes ex situ, les propriétés structurales, vibrationnelles, électroniques et magnétiques des trois nouveaux systèmes hybrides ont été caractérisées et confrontées à des calculs ab initio. Un certain nombre de propriétés remarquables ont été mises en évidence. L'interface entre graphene et cobalt implique de fortes interactions C-Co qui conduisent à une forte anisotropie magnétique d'interface, capable de pousser l'aimantation hors de la surface d'un film ultra-mince en dépit de la forte anisotropie de forme dans ces films. Cet effet est optimum dans les systèmes obtenus par intercalation entre graphène et iridium, qui sont par ailleurs naturellement protégés des pollutions de l'air. Les plots nanométriques, au contraire, semblent peu interagit avec le graphène. Des plots comprenant environ 30 atomes restent superparamagnétiques à 10 K, n'ont pas d'anisotropie magnétique, et leur aimantation est difficile à saturer, même sous 5 T. D'autre part, la taille des domaines magnétiques semble dépasser celle d'un plot unique, ce qui pourrait être le signe d'interactions magnétiques entre plots.Graphene is a candidate for next generation spintronics devices exploiting its long spin transport length and high carrier mobility. Besides, when put in interaction with a ferromagnet, it may become an active building block, as suggested by recent surface science studies revealing few tenth of a Bohr magneton magnetic moments held by carbon atoms in graphene on iron, and a Rashba spin-orbit splitting reaching about 10 meV in graphene on a high atomic number element such as gold. The extent to which graphene may influence the properties, e.g. magnetic ones, of the materials contacted to it was barely addressed thus far. High quality hybrid systems composed of graphene in contact with magnetic thin layers or nanoclusters are playgrounds for exploring both aspects, the manipulation of the properties of graphene by interaction with other species, and vice versa. In graphene contacted to ultra-thin ferromagnetic layers for instance, strong graphene/ferromagnet interface effects could be employed in the view of manipulating the magnetization in the ferromagnet. The recently discovered close-to-perfect self-organization of nanoclusters on graphene, provides a way to probe magnetic interaction between clusters, possibly mediated by graphene. Three high quality hybrid systems relying on graphene prepared by chemical vapor deposition on the (111) surface of iridium have been developed under ultra-high vacuum (UHV): cobalt ultra-thin and flat films deposited on top of graphene, and intercalated at moderate temperature between graphene and its substrate, and self-organized cobalt- and iron-rich nanoclusters on the 2.5 nm-periodicity moiré between graphene and Ir(111). Prior to these systems, 10 nm-thick Ir(111) single-crystal thin films on sapphire were developed: they were latter employed as a substrate replacing bulk Ir(111) single-crystals usually employed. This new substrate opens the route to multi-technique characterizations, especially ex situ ones which were little employed thus far for studying graphene/metal systems prepared under UHV. Using a combination of in situ surface science techniques (scanning tunneling microscopy, x-ray magnetic circular dichroism, spin-polarized low-energy electron microscopy, auger electron spectroscopy, reflection high-energy electron diffraction) and ex situ probes (x-ray diffraction, transmission electron microscopy, Raman spectroscopy, MOKE magnetometry) the structural, vibrational, electronic, and magnetic properties of the three new graphene hybrid systems were characterized and confronted to first-principle calculations. Several striking features were unveiled. The interface between graphene and cobalt involves strong C-Co interactions which are responsible for a large interface magnetic anisotropy, capable of driving the magnetization out-of-the plane of the surface of an ultra-thin film in spite of the strong shape anisotropy in such films. The effect is maximized in the system obtained by intercalation between graphene and iridium, which comes naturally air-protected. Nanoclusters, on the contrary, seem to weakly interact with graphene. Small ones, comprising ca. 30 atoms each, remain super paramagnetic at 10 K, have no magnetic anisotropy, and it turns out difficult, even with 5 T fields to saturate their magnetization. Besides, the magnetic domains size seem to exceed the size of a single cluster, possibly pointing to magnetic interactions between clusters.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Damping characterization of a high speed train catenary

Catenary damping has long been a tuning parameter in pantograph-catenary dynamic interaction models. As the computed contact force is highly sensitive to the choice of damping model or coefficients, it became critical to measure it independently of the pantograph. Original tests have been conducted on a real catenary and damping identification shows a very low level of damping for a large frequency range. A fitted Rayleigh model and a combined modal and Rayleigh model are proposed and compared with a reference damping model found in literature as well as with the tests. Finally, the consequences on a typical contact force simulation are analysed and the most relevant model is chosen

Waves, modes and properties with a major impact on dynamic pantograph-catenary interaction

Understanding the dynamic behavior of the pantograph-catenary system is crucial for design improvement, but many factors inuence the contact force, which is the main design objective. To give a proper un-derstanding of dynamic characteristics, the paper uses a combination of mass drop tests on a catenary, analytic models and parametric _nite element model simulations allowing a ne analysis of the inuence of train speed. The _rst contributor to contact force variations is the geometry of the catenary under gravity loading. This parameter is however shown to be insu_cient to explain higher frequency e_ects. The second contributor is the propagation of waves in the contact and messenger wires. The inuence of wave dis-persion is _rst demonstrated, which emphasizes the importance of considering the bending sti_ness. Wave compensation by droppers and reections at the mast are then shown to be important. Characteristic times associated with wave group velocities are _nally used to explain the series of harmonic contributions visible in spectra in the catenary and pantograph frames. Finally, modes are shown to play a role particularly when their frequencies coincide with other contributions. The notion of mode groups, associated wave velocities and relevant design variables are discussed. Several observations pave the way for future work on catenary design