Observing the Forest Canopy with a New Ultra-Violet Compact Airborne Lidar

We have developed a new airborne UV lidar for the forest canopy and deployed it in the Landes forest (France). It is the first one that: (i) operates at 355 nm for emitting energetic pulses of 16 mJ at 20 Hz while fulfilling eye-safety regulations and (ii) is flown onboard an ultra-light airplane for enhanced flight flexibility. Laser footprints at ground level were 2.4 m wide for a flying altitude of 300 m. Three test areas of ∼500 × 500 m2 with Maritime pines of different ages were investigated. We used a threshold method adapted for this lidar to accurately extract from its waveforms detailed forest canopy vertical structure: canopy top, tree crown base and undergrowth heights. Good detection sensitivity enabled the observation of ground returns underneath the trees. Statistical and one-to-one comparisons with ground measurements by field foresters indicated a mean absolute accuracy of ∼1 m. Sensitivity tests on detection threshold showed the importance of signal to noise ratio and footprint size for a proper detection of the canopy vertical structure. This UV-lidar is intended for future innovative applications of simultaneous observation of forest canopy, laser-induced vegetation fluorescence and atmospheric aerosols

Friction and lifetime of a contact lubricated by a solid third body formed from an MoS1.6 coating at low temperature

International audienceA special test device has been designed to investigate the friction and wear behaviour of materials under very low temperature conditions. This paper describes an analysis of friction and wear in sliding contacts “lubricated” by a solid third body formed in situ from a non-stoichiometric MoS1.6 coating at 297K, 223K, 173K and 123K, under reciprocating kinematics in a gaseous atmosphere. It was found that under given test conditions, the friction coefficient was somewhat higher at low temperature than at room temperature under oxidising atmosphere.The lifetime was ten times lower at 123K than at room temperature. Under no oxidising atmosphere the temperature does not effect on the friction value. The consequences of decreasing temperature on friction and lifetime are interpreted on the basis of experimental evidence and from the view point of mechanics in order to evaluate third body flows and the rheology activated in the contact

Analyse tribologique du contact roue-rail (modélisation et expérimentations)

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 courbesVILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Etude du comportement tribologique de composites carbone/carbone sous sollicitations de freinage aéronautique (approches mécanique et physico-chimique)

Les composites C/C constituent actuellement la meilleure solution technologique pour assurer le freinage aéronautique. En effet, ces matériaux possèdent des caractéristiques (densité, capacité thermique, ) et des propriétés thermomécaniques très adaptées au freinage très dissipatif. Dans ce contexte, cette étude a pour objectif l identification des mécanismes d usure de composites C/C lors de freinages de type taxiage à basse et à haute températures.Pour ce faire, deux tribomètres sont utilisés (pion/disque et disque/disque) ainsi qu un spectromètre de masse couplé au tribomètre pion/disque afin d étudier les réactions chimiques dans le contact. L analyse des gaz, la caractérisation des surfaces et des particules de troisième corps formées lors du frottement (MOLP, MEB, AFM, DRX, XPS, adsorption d azote) ont permis de suivre l endommagement des matériaux étudiés.Le frottement à basse température se caractérise par une usure particulièrement élevée, des surfaces majoritairement endommagées (rayures, fissurations, décohésions fibres/matrice,..), et un détachement important de particules de grande taille qui participent activement à l abrasion des surfaces. L usure tribo-chimique particulièrement élevée se traduit par une contribution prépondérante de l usure mécanique par rapport à l usure chimique (oxydation).Quant au frottement à haute température, il conduit à une usure faible, des surfaces peu endommagées et la production de particules de petite taille plus oxydées, plus cohésives et donc nettement moins abrasives. L usure tribo-chimique relativement faible se caractérise par une plus faible contribution de l usure mécanique et une oxydation plus importante.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

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

Formation conditions of mechanically modified superficial structures for two steels

International audienceThe aim of this work was to study the formation mechanisms of tribologically transformed structures (TTS). As these transformations need not be associated with phase modifications but usually with grain refinement or nanostructuring, the term mechanically modified superficial structures (M2S2) has been proposed to name any intermediary modification of material structures between the initial and nanostructured states. Severe plastic deformation processes allow producing transformations of material in volume and reproducing conditions that are close to those found under contact. Thus, a Bridgman Anvil apparatus which allows shearing samples in torsion under pressure is used to test two carbon steels (AISI 1005, AISI 1045). This experimental study is coupled with finite element analysis used to simulate the test and to study the specificity of the stress and strain cycles of the transformed layers obtained. For both materials, M2S2 layers are produced. The results show that the two materials follow the same trends. M2S2 formation can be associated with high hydrostatic pressure, high plastic strains, high strain rates and high strain gradients. This work is a first step for proposing mechanical criteria for the M2S2/TTS formation, but also for studying the influence of the initial alloy (microstructure, composition) on these transformations

Experimental identification and characterization of the effects of contaminants in the wheel-rail contact

International audiencePrevious results have highlighted the presence of a natural third body ranging in thickness from a few micrometres to several dozen micrometres on the rail and wheel. The third body layer, initially composed of particles stemming from the wheels and rails, flows into the contact to accommodate local sliding inside it. The work presented in this paper focuses on the identification of contaminants whose influence on the wheel–rail contact is significant. This influence can be considered as significant if it enters the contact, affects surface properties, modifies the third body layer, and possibly damages or protects the rail. The third body layer can progressively absorb and assimilate solid (ballast stone, sand) and fluid (oil) contaminants existing on rails, and thus reduce their negative consequences on rail lifetime. These phenomena are the result of the exchange of third body flows between the wheel and rail. A high-speed camera was used for this experimental study performed on a real site. The analyses of the dynamic images are coupled with tribological analyses of the surfaces by scanning electron microscopy and X-ray energy dispersive analysis

Presence and role of the third body in a wheel–rail

Publié suite au congrès : Sixth International Conference on Contact Mechanics and Wear of Rail/Wheel Systems (CM2003), Gothenburg, Sweden, June 10–13, 2003International audienceNote: Persons concerned by modelling wheel–rail contacts should take into account the existence of the third body as a boundary condition of their models. Similar articles have been published in two journals: one dealing specifically with this subject in Fatigue Fract. Eng. Mater. Struct. while the other, in this special issue of Wear, describes a more general experimental approach. Some parts of this article, which describe in details the reality of the wheel–rail contact, have been used in the synthesising article: Y. Berthier, S. Descartes, M. Busquet, E. Niccolini, C. Desrayaud, L. Baillet, M.C. Baietto-Dubourg, The role and effects of the third body in the wheel–rail interaction, Fatigue Fract. Eng. Mater. Struct. 27 (5) (2004) 423–436. doi: 10.1111/j.1460-2695.2004.00764.x Published by Blackwell. This paper focuses on the presence of the third body, a solid interfacial layer, in the wheel–rail contact. A phenomenological analysis is carried out as thoroughly as possible of the real tribological behaviour of this contact. To improve the understanding of the wheel–rail contact reality and the reconstitution of contact dynamics, this paper is presented a synthesis of different studies coming from: analysis of specimens taken out periodically from rails and wheels in service, and thus under real contact conditions, test laboratories, allowing us to impose rolling–sliding conditions with very high precision. The results show the presence of natural third body ranging in thickness from a few micrometers to several dozen micrometers on the rail and wheel. Initially composed of particles stemming from wheels and rails, it flows into the contact to accommodate the sliding between wheel and rail while absorbing and digesting solid and fluid contaminants. Up to now, the third body is a means of tracing local conditions in the wheel–rail contact, in order to pass through the difficulties of in situ instrumentation. From all these studies and results, a better understanding of the role of the third body and its influence on friction is reached. It also controls the rail's lifetime, the lubrication as well as the wheel–rail adherence via its “degradation and/or reformation” mechanisms

Tribological study of oral care Silica

International audienceThis study focuses on the effect of mechanical parameters for a better understanding of oral care silica tribological behavior. Reciprocating motion tests were performed with variable parameters (tooth- brushes design, load, brushing speed, with and without silica slurries). The analysis of the tests is based on the third body concept.The flows of third body were evaluated in-situ and post-mortem related to the evolution of the friction coefficient. Clues to understand the mechanisms involved are given, showing the multi-scales and multi-physical aspects of the problem

Modulating the bioactive peptides content in Swiss cheeses by combinations of thermophilic Lactobacilli strains and species

Peptides within the sequence of milk proteins can be released during food processing such as cheese making.Depending on their amino acid sequence, these peptides may exert a number of activities in vivo, affectingcardiovascular, endocrine, immune and nervous systems. Bioactive peptides have been described in cheeses, but theirquantification and the way to produce them in situ are far from being well controlled. In this work, we demonstrate thatthe amount of antihypertensive peptides in cheese can be modulated by combinations of lactobacilli starters.Selected strains of the three starter species encountered in Swiss-type cheeses were used in various combinations:Streptococcus thermophilus (ST, 2 levels of the same strain), Lactobacillus helveticus (LH, 2 strains H1 and H2 and noneH0), Lactobacillus delbrueckii (LD, 2 strains D1 and D2 and none D0). 18 model cheeses were then manufacturedunder identical controlled conditions. Microbial and physico-chemical analyses were performed on cheeses at differenttimes of process and ripening.Cheese aqueous extracts exhibited various in vitro anti-hypertensive activities, expressed as concentration that inhibits50 % of angiotensin I converting enzyme activity, i.e. IC50. Levels and dynamics of this anti-hypertensive activity variedaccording to the combination of lactobacilli strains used, up to 6 fold at the end of the ripening time. The combinationH2D2 led to the lowest IC50, i.e. the highest antihypertensive activity.Twenty-one peptides with potential anti-hypertensive activity were identified by tandem mass spectrometry: 15from β-casein, four from αs1- and two from αs2-caseins. Two particular anti-hypertensive peptides, VPP and IPP, werespecifically quantified by mass spectrometry showing a strong correlation between their content and the IC50. Strainbiodiversity and starter combinations are powerful tools to modulate bioactive peptides content in cheese