The Distribution of Patterns in Random Trees

Let $T\_n$ denote the set of unrooted labeled trees of size $n$ and let $T\_n$ be a particular (finite, unlabeled) tree. Assuming that every tree of $T\_n$ is equally likely, it is shown that the limiting distribution as $n$ goes to infinity of the number of occurrences of $M$ as an induced subtree is asymptotically normal with mean value and variance asymptotically equivalent to $\mu n$ and $\sigma^2n$, respectively, where the constants $\mu>0$ and $\sigma\ge 0$ are computable

Ceramic matrix composites for liner system of radioactive waste disposal cells

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Comportement à l'impact des composites fibres de verre/Epoxy modifié copolymère à bloc.

Epoxy resins are widely used in the design of fibre composite materials. This increasing use finds its reason in the fact that these materials have excellent mechanical and thermal properties. Playing on its chemical composition and curing speed, it’s possible to vary the mechanical properties from the extreme flexibility to high rigidity. However, the inherent toughness of a highly crosslinked epoxy is relatively low. It therefore seems desirable for high impact resistance applications, to improve the toughness, without affecting the other usual properties of this polymer. Recent studies have shown a significant improvement in impact resistance of epoxy in the presence of block copolymers. Our work aims to investigate the effect of triblock copolymer-based acrylate (Nanostrength) on the impact resistance of glass / epoxy composite. An experimental device, called "drop tower" is used to perform impact tests on composites with or without Nanostrength. Dynamic mechanical analyses (DMA) were conducted, first to characterize the effect of addition of Nanostrength on the thermomechanical properties, but also to establish a correlation between thermomechanical properties and impact resistance. Different observation tools, such as optical observation, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to visualize the material damages. Adding Nanostrength in epoxy leads to the improvement of impact resistance of the composite material. A slight decrease in the storage modulus and glass transition temperature have been observed. Microscopic observations shown that the different failure modes of the composite are mainly delamination, fibres breakage and matrix cracking. It was also observed that the presence of Nanostrength role as cracks deflect

Vers un modele pour l'analyse multiechelle de surface 3D

L'analyse d'une surface a plusieurs echelles a pour objectif la hierarchisation des structures qui la composent. Dans le cas ou l'on desire representer une surface a echelle donnee, differente de celle a laquelle elle est decrite originellement, on est amene a eliminer les structures qui ne sont pas significatives a cette echelle. Un exemple typique de cette demarche est la cartographie. Les cartographes ont cependant des soucis qui outrepassent les considerations simples d'elimination graduelle des structures, ils doivent dans la majorite des cas, preserver la nature des structures geomorphologiques qui composent la surface et a toutes les echelles. Ils ne peuvent donc pas se contenter d'un lissage global de la surface qui entrainerait des modifications morphologiques inevitables de certaines structures importantes. Pour remedier a ce probleme, ils incluent dans le processus, une interpretation humaine qui a pour but de preserver les points importants de la surface (manuellement). Dans ce rapport, nous presentons une modelisation de surface qui permet d'effectuer une analyse multiechelle en tenant compte des phenomenes locaux inherents a la morphologie du relief. A partir d'un modele discret nous derivons un operateur de changement d'echelle adaptatif (i.e. dependant du contexte) qui permet de realiser un lissage isotrope ou anisotrope

Nanostructuration de résines polyester insaturé par des copolymères à blocs (application aux composites SMC et BMC)

On s intéresse dans cette étude au potentiel de copolymères à blocs (BCP) de types PBA-b-P(MMA-co-DMA) et PBA-b-P(BA-co-DMA) en tant qu additifs multifonctionnels nanostructurant la matrice thermodurcissable polyester insaturé de composites SMC et BMC. La nanostructuration de la résine polyester insaturé (UPR) est assurée par la ségrégation du bloc élastomère poly(acrylate de butyle), ainsi que par la miscibilité du deuxième bloc dans le réseau grâce à la bonne compatibilité des motifs diméthylacrylamide (auto-assemblage). Ces BCP sont tout d abord étudiés en tant qu agents renforçants et anti-retrait dans les composites SMC et BMC, en étant substitués et comparés aux additifs anti-retrait conventionnels PVAc et P(MMA-co-S) incorporés traditionnellement afin de compenser le retrait de la résine polyester. Ces additifs nanostructurants sont ensuite évalués en tant qu agents de mûrissement des compounds SMC. La nanostructuration des BCP dans la résine UP réactive pouvant entrainer une importante augmentation de la viscosité (formation d un gel), ces additifs pourraient peut-être constituer des agents de mûrissement plus efficaces que l oxyde de magnésium couramment utilisé à cet effet. Une étude des matrices [UPR + BCP, UPR + PVAc et UPR + P(MMA-co-S)] est réalisée dans un premier temps, les composites BMC et SMC chargés par du carbonate de calcium et renforcés par des fibres de verre étant étudiés par la suite. En absence de charges et de fibres de verre (matrice seule), la nanostructuration est moins efficace que la macroséparation de phase des additifs conventionnels PVAc et P(MMA-co-S) pour compenser le retrait du réseau polyester. Dans un composite en revanche, la nanostructuration conduit à une compensation du retrait intermédiaire entre celles du P(MMA-co-S) et du PVAc. Au niveau de la matrice seule, la nanostructuration permet d éviter une diminution conséquente de l énergie de rupture du réseau polyester et permet même dans certains cas une amélioration significative de cette dernière, alors que les additifs conventionnels engendrent un effondrement de cette propriété. Cette différence est cependant beaucoup moins visible dans le cas des composites, les charges entrainant une fissuration prématurée de la matrice. Un composite nanostructuré peut tout de même présenter un renforcement significatif par rapport aux composites conventionnels lorsqu il contient des nanovides compensateurs de retrait de tailles conséquentes qui améliorent la ténacité du matériau. En ce qui concerne le mûrissement d un compound SMC par nanostructuration au cours du refroidissement (passage de la transition ordre-désordre), si l augmentation de la viscosité d un système réactif UPR + BCP semble suffisamment importante et abrupte, elle est cependant insuffisante en présence des charges ou alors intervient à une température trop faible en raison de l influence de ces dernières sur la nanostructuration.The aim of this study is to evaluate the potential of PBA-b-P(MMA-co-DMA) and PBA-b-P(BA-co-DMA) block copolymers (BCP) as multifunctional additives nanostructuring the unsaturated polyester thermosetting matrix of SMC and BMC composites. The nanostructuration of the unsaturated polyester resin (UPR) is ensured by the segregation of the poly(butyl acrylate) elastomeric block, and by the miscibility of the second block in the thermoset thanks to the good compatibility of dimethylacrylamide units (self-assembly). These BCP are first studied as reinforcing and anti-shrinkage agents in SMC and BMC composites, as compared to conventional low profile additives PVAc and P(MMA-co-S) traditionally blended to compensate the polyester resin shrinkage. These nanostructuring additives are then studied as thickening agents in SMC compounds. As the nanostructuration of BCP in the UP reactive resin can lead to a high viscosity increase (gel formation), BCP might provide a more efficient thickening than the conventional additive magnesium oxide. Matrices are studied first [UPR + BCP, UPR + PVAc and UPR + P(MMA-co-S)], BMC and SMC composites filled with calcium carbonate and reinforced by chopped glass fibres being then considered. In the absence of fillers and fibres (neat matrix), the nanostructuration is less efficient than the macrophase separation of PVAc and P(MMA-co-S) to compensate the polyester resin shrinkage. However, in a composite, the nanostructuration leads to an intermediate shrinkage compensation between those of P(MMA-co-S) and PVAc. In the neat matrix, the nanostructuration avoids a substantial decrease of polyester network toughness and can even lead sometimes to a significant reinforcement, whereas conventional additives decrease tremendously this property. However, this difference is not observed in composites, fillers giving rise to an early cracking of the matrix. A nanostructured composite can still present a significant reinforcement compared to conventional composites when it contains large nanovoids (compensating shrinkage) which improve the fracture toughness of the material. As regards the thickening of an SMC compound by nanostructuration during cooling (crossing of the order-disorder transition), the viscosity increase of a UPR + BCP reactive system seems important and sharp enough, but in the presence of fillers the viscosity increase is not sufficient or the order-disorder transition temperature is too low because of the influence of fillers on the nanostructuration.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Trace metal concentrations in Posidonia oceanica of North Corsica (northwestern Mediterranean Sea): use as a biological monitor?

BACKGROUND: Within semi-closed areas like the Mediterranean Sea, anthropic wastes tend to concentrate in the environment. Metals, in particular, are known to persist in the environment and can affect human health due to accumulation in the food chain. The seagrass Posidonia oceanica, widely found in Mediterranean coastal waters, has been chosen as a "sentinel" to quantify the distribution of such pollutants within the marine environment. Using a technique similar to dendrochronology in trees, it can act as an indicator of pollutant levels over a timeframe of several months to years. In the present study, we measured and compared the levels of eight trace metals (Cr, Ni, Cu, Zn, As, Se, Cd, and Pb) in sheaths dated by lepidochronology and in leaves of shoots sampled from P. oceanica meadows collected from six offshore sites in northern Corsica between 1988 and 2004; in the aim to determine 1) the spatial and 2) temporal variations of these metals in these areas and 3) to compared these two types of tissues. RESULTS: We found low trace metal concentrations with no increase over the last decade, confirming the potential use of Corsican seagrass beds as reference sites for the Mediterranean Sea. Temporal trends of trace metal concentrations in sheaths were not significant for Cr, Ni, Cu, As or Se, but Zn, Cd, and Pb levels decreased, probably due to the reduced anthropic use of these metals. Similar temporal trends between Cu levels in leaves (living tissue) and in sheaths (dead tissue) demonstrated that lepidochronology linked with Cu monitoring is effective for surveying the temporal variability of this metal. CONCLUSION: Leaves of P. oceanica can give an indication of the metal concentration in the environment over a short time period (months) with good accuracy. On the contrary, sheaths, which gave an indication of changes over long time periods (decades), seem to be less sensitive to variations in the metal concentration in the environment. Changes in human consumption of metals (e.g., the reduction of Pb in fuel) are clearly reflected in both organs. These results confirm that P. oceanica is a good bioindicator of metals and a good biomonitor species for assessing Cu in the environment

Steinberg's Conjecture and near-colorings

Let F be the family of planar graphs without cycles of length 4 and 5. Steinberg's Conjecture (1976) that says every graph of F is 3-colorable remains widely open. Focusing on a relaxation proposed by Erd˝os (1991), many studies proved the conjecture for some subfamilies of F. For example, Borodin et al. proved that every planar graph without cycles of length 4 to 7 is 3- colorable. In this note we propose to relax the problem not on the family of graphs but on the coloring by considering near-colorings. A graph G = (V,E) is said to be (i, j, k)-colorable if its vertex set can be partitioned into three sets V1, V2, V3 such that the graphs G[V1],G[V2],G[V3] induced by the sets V1, V2, V3 have maximum degree at most i, j, k respectively. Under this terminology, Steinberg's Conjecture says that every graph of F is (0, 0, 0)-colorable. A result of Xu (2008) implies that every graph of F is (1, 1, 1)-colorable. Here we prove that every graph of F is (2, 1, 0)-colorable and (4, 0, 0)-colorable

Application of homogeneous potentials for the modeling of the Bauschinger effects in ultra low carbon steel

In this work, an approach is proposed for the description of the plastic behavior of materials subjected to multiple or continuous strain path changes. In particular, although it is not formulated with a kinematic hardening rule, it provides a reasonable description of the Bauschinger effect when loading is reversed. This description of anisotropic hardening is based on homogeneous yield functions/plastic potentials combining a stable, isotropic hardening-type, component and a fluctuating component. The capability of this constitutive description is illustrated with applications on an ultra low carbon steel sheet sample deformed in three-stage uniaxial loading with two load reversals [1].ope

Emploi des seniors : les leçons des pays de réussite

Du fait du vieillissement de la population, l’emploi des seniors devient un enjeu primordial de la politique du travail dans les pays européens. Retarder l’âge de fin d’activité permettrait d’augmenter le niveau de production et d’équilibrer les systèmes de retraite sans réduire le niveau des retraites. Encore faut-il que les seniors soient effectivement employés. Les réformes en cours des systèmes de retraite font courir le risque qu’une partie importante des travailleurs seniors ne trouvent pas d’emploi et soient contraints de partir à la retraite avec un faible niveau de pension. Aussi, l’article étudie-t-il la stratégie suivie par les pays qui ont réussi à maintenir un taux d’emploi des seniors élevé (Suède, Danemark, Royaume-Uni) ou qui ont connu des relèvements importants de ce taux durant ces dernières années (Finlande, Pays-Bas). Ceux-ci sont généralement des pays proches du plein emploi, même si les facteurs de plein emploi diffèrent : temps partiel, stratégie macroéconomique qui allie recherche de la compétitivité et politique expansionniste, libéralisation du marché du travail ou gestion par les partenaires sociaux, développement d’emplois sociaux. Ils ont bénéficié d’institutions favorables (faiblesse des hausses de salaire à l’ancienneté, meilleures conditions de travail). La stratégie libérale diminue les retraites, ce qui incite les seniors à travailler, quitte à accepter des salaires plus bas, ce qui est permis par la flexibilité du marché du travail. La stratégie des pays nordiques, le vieillissement actif, organise une mobilisation sociale afin d’augmenter les emplois disponibles pour les seniors et d’inciter ceux-ci à prolonger leurs carrières : amélioration des conditions de travail, formation permanente, refonte des carrières, lutte contre les discriminations liées à l’âge, campagne de sensibilisation, accords au niveau des branches et des entreprises. La France n’a guère cette tradition d’accord entre État et partenaires sociaux. Or un compromis social fructueux est un préalable nécessaire. Il devrait comporter une différentiation des conditions de départ à la retraite selon les professions et des garanties sur l’évolution du taux de remplacement.Older workers’ employment rates are high in three EU countries (Sweden, Denmark, UK) and have risen substantially in Finland and the Netherlands in recent years. These five countries are also close to full employment, even if the factors behind their employment performance differ. Labour market institutions are a factor (moderate wage increases for older workers, better working conditions). In countries with a liberal strategy, early-retirement schemes have been abolished; older workers are requested to work and accept lower wages, which is allowed by labour market flexibility. In Nordic countries, the active ageing strategy relies on a social mobilisation to increase jobs available for older workers and to give them incentives to work longer. The article concludes that France should consider the experience from Nordic countries to set up a fruitful social compromise