Etude et validation de la tenue en fatigue vibratoire d'élastomères basée sur de nouvelles méthodes expérimentales

De nombreux composants élastomères soumis à des chargements vibratoires sont exposés à des phénomènes de fatigue. La validation de la tenue à la fatigue nécessite des caractérisations expérimentales, avec des méthodes et moyens d'essais appropriés au comportement complexe des élastomères. Deux applications sont présentées, basées chacune sur l'utilisation d'un moyen d'essai DMA (Dynamic Mechanical Analysis). La première application propose une adaptation des méthodes existantes de spécifications d'essais d'endurance mécanique au cas particulier des élastomères. La deuxième concerne l'évolution des moyens d'essais pour la caractérisation du comportement de propagation de fissures dans les élastomères.Many elastomeric components subjected to dynamic mechanical loads are exposed to fatigue. The validation of the resistance to fatigue requires experimental characterizations with methods and test means appropriate to the complex behaviour of elastomers. Two applications are presented, each based on the use of a DMA (Dynamic Mechanical Analysis) testing mean. The first application proposes an adaptation of existing methods related to the specification of mechanical test endurance to the particular case of elastomers. The second one deals with the evolution of the test means for the characterization of crack growth behaviour in elastomers.

Centrality-Based Eventual Leader Election in Dynamic Networks

International audienceThis paper presents CEL, a new distributed eventual leader election algorithm for dynamic networks, which exploits topological information to improve the choice of a central leader and reduce message exchanges. The algorithm has a crosslayer neighbors detection, with a neighbor-aware mechanism, to improve the sharing of topological knowledge and elect a central leader faster. It uses a self-pruning mechanism based on topological knowledge, combined with probabilistic gossip, to improve the performance of broadcast propagation. Evaluations were conducted on the OMNeT++ environment, simulating realistic MANET with interference, collision, and messages loss. Using different parameters values, we have compared CEL to Gómez-Calzado et al. algorithm [1], on the Random Walk and the Truncated Lévy Walk mobility models. The results show better performances than [1], including fewer messages sent, shortest paths to the leader, and a more stable algorithm

Crystal Structure of Saccharomyces cerevisiae ECM4, a Xi-Class Glutathione Transferase that Reacts with Glutathionyl-(hydro)quinones

International audienceGlutathionyl-hydroquinone reductases (GHRs) belong to the recently characterized Xi-class of glutathione transferases (GSTXs) according to unique structural properties and are present in all but animal kingdoms. The GHR ScECM4 from the yeast Saccharomyces cerevisiae has been studied since 1997 when it was found to be potentially involved in cell-wall biosyn-thesis. Up to now and in spite of biological studies made on this enzyme, its physiological role remains challenging. The work here reports its crystallographic study. In addition to exhibiting the general GSTX structural features, ScECM4 shows extensions including a huge loop which contributes to the quaternary assembly. These structural extensions are probably specific to Saccharomycetaceae. Soaking of ScECM4 crystals with GS-menadione results in a structure where glutathione forms a mixed disulfide bond with the cysteine 46. Solution studies confirm that ScECM4 has reductase activity for GS-menadione in presence of glutathione. Moreover, the high resolution structures allowed us to propose new roles of conserved residues of the active site to assist the cysteine 46 during the catalytic act

Confirmation de l'asymétrie traction-compression du NiTi à l'aide de bulge tests circulaires sur plaques minces

La disymétrie traction-compression du comportement superélastique d'alliages NiTi polycristallin est étudiée à l'aide d'essais de traction simple et de bulge-test sur des tôles fines. Pour ce dernier test, à partir du chargement en pression et de la mesure du champ de déplacements, on déduit la courbe de traction équibiaxiale au centre de l'échantillon. Le déviateur des contraintes est alors identique à celui de la compression dans l'épaisseur. Un comportement asymétrique tension-compression est alors observé, similaire à ceux déjà relevés dans de précédentes études dans le plan de la plaque

Un algorithme d'élection de leader cross-layer pour réseaux mobiles ad hoc (résumé)

National audienceUn MANET (mobile ad hoc network) est un réseau dynamique décentralisé qui s’autocon-figure continuellement, composé de nœuds pouvant se déplacer librement, rejoindre et quitter le système à tout moment. Les nœuds ne connaissent initialement que leur identifiant. Seuls les nœuds situés dans le rayon de transmission l’un de l’autre communiquent directement. En revanche, toute paire de nœuds peut communiquer via un ensemble de nœuds relais intermé-diaires.L’élection de leader est un composant essentiel des systèmes répartis permettant par exemple, de résoudre le problème du consensus. L’élection consiste à trouver une entente sur l’identité d’un unique nœud, considéré comme leader par l’ensemble des nœuds du réseau. Lors d’un départ ou d’une défaillance du leader, les autres nœuds détectent son indisponibilité et déclenchent une nouvelle élection.De nombreux travaux portent sur l’élection de leader dans les systèmes statiques [6, 8]. Dans un système dynamique, [1] proposent un algorithme adapté aux topologies mobiles et reposant sur des TVG [3]. D’autres algorithmes spécifiques aux MANET utilisent une vision partielle du réseau, le plus souvent pour élire le nœud ayant le plus faible identifiant [7, 5, 4, 2, 9].Notre algorithme obtient une vision globale à partir de la liste des voisins que chaque nœud maintient et échange. Périodiquement, les nœuds envoient un checksum de leur vision pour détecter les incohérences. Les communications sont en broadcast uniquement. Différents critères permettent de choisir le leader comme son ancienneté dans le réseau ou sa centralité.L’algorithme est conçu pour les réseaux mobiles génériques et possède une implémen-tation cross-layer adaptée aux MANET. Notre algorithme exploite la connexion sans fil et les informations maintenues par la couche MAC pour améliorer les performances des échanges.L’algorithme a été implémenté sur une plateforme de dix Raspberry Pi mobiles et de pre-miers résultats sont prometteurs. Dans le futur, le choix du leader sera amélioré en prenant en compte la stabilité des nœuds. Des expérimentations dans le simulateur OMNeT++ permet-tront de tester le passage à l’échelle avec différents patterns de mobilité

Topology Aware Leader Election Algorithm for Dynamic Networks

International audienceThis paper proposes an algorithm that eventually elects a leader for each connected component of a dynamic network where nodes can move or fail by crash. A node only communicates with nodes in its transmission range and locally keeps a global view, denoted topological knowledge, of the communication graph of the network and its dynamic evolution. Every change in the topology or in nodes membership is detected by one or more nodes and propagated over the network, updating thus the topological knowledge of the nodes. As the choice of the leader has an impact on the performance of applications that use an eventual leader election service, our algorithm, thanks to nodes topological knowledge, exploits the closeness centrality as the criterion for electing a leader. Experiments were conducted on top of PeerSim simulator, comparing our algorithm to a representative flooding algorithm. Performance results show that our algorithm outperforms the flooding one when considering leader choice stability, number of messages, and average distance to the leader

Topology Aware Leader Election Algorithm for MANET

National audienceThis article presents an eventual leader election algorithm for mobile dynamic networks. Each node builds knowledge of the communication graph of connected nodes, by broadcasting changes in their neighborhood. This knowledge provides the current topology of the network, used to compute the closeness centrality as the choice of the leader. Experiments were realized on PeerSim simulator, comparing our algorithm with static and dynamic flooding algorithms, on different network topologies and mobility patterns. Our algorithm improves leader stability up to 24%, sends half less messages and aims to an 8% shorter leader path

Retrieval of snow properties from the Sentinel-3 Ocean and Land Colour Instrument

The Sentinel Application Platform (SNAP) architecture facilitates Earth Observation data processing. In this work, we present results from a new Snow Processor for SNAP. We also describe physical principles behind the developed snow property retrieval technique based on the analysis of Ocean and Land Colour Instrument (OLCI) onboard Sentinel-3A/B measurements over clean and polluted snow fields. Using OLCI spectral reflectance measurements in the range 400–1020 nm, we derived important snow properties such as spectral and broadband albedo, snow specific surface area, snow extent and grain size on a spatial grid of 300 m. The algorithm also incorporated cloud screening and atmospheric correction procedures over snow surfaces. We present validation results using ground measurements from Antarctica, the Greenland ice sheet and the French Alps. We find the spectral albedo retrieved with accuracy of better than 3% on average, making our retrievals sufficient for a variety of applications. Broadband albedo is retrieved with the average accuracy of about 5% over snow. Therefore, the uncertainties of satellite retrievals are close to experimental errors of ground measurements. The retrieved surface grain size shows good agreement with ground observations. Snow specific surface area observations are also consistent with our OLCI retrievals. We present snow albedo and grain size mapping over the inland ice sheet of Greenland for areas including dry snow, melted/melting snow and impurity rich bare ice. The algorithm can be applied to OLCI Sentinel-3 measurements providing an opportunity for creation of long-term snow property records essential for climate monitoring and data assimilation studies—especially in the Arctic region, where we face rapid environmental changes including reduction of snow/ice extent and, therefore, planetary albedo.publishedVersio