1,017 research outputs found
Investigation of the effect of aggregates' morphology on concrete creep properties by numerical simulations
International audiencePrestress losses due to creep of concrete is a matter of interest for long-term operations of nuclear power plants containment buildings. Experimental studies by Granger (1995) have shown that concretes with similar formulations have different creep behaviors. The aim of this paper is to numerically investigate the effect of size distribution and shape of elastic inclusions on the long-term creep of concrete. Several microstructures with prescribed size distribution and spherical or polyhedral shape of inclusions are generated. By using the 3D numerical homogenization procedure for viscoelastic microstructures proposed by Ć milauer and BaĆŸant (2010), it is shown that the size distribution and shape of inclusions have no measurable influence on the overall creep behavior. Moreover, a mean-field estimate provides close predictions. An Interfacial Transition Zone was introduced according to the model of Nadeau (2003). It is shown that this feature of concrete's microstructure can explain differences between creep behaviors
Analyse de sensibilité globale d'un modÚle spatialisé pour l'évaluation économique du risque d'inondation
L'analyse de sensibilitĂ© globale peine Ă se dĂ©velopper dans le champ de la modĂ©lisation environnementale. Dans sa formulation initiale, elle est limitĂ©e Ă l'Ă©tude de modĂšles Y = f (X1; : : : ;Xp) oĂč les variables d'entrĂ©e Xj et la sortie Y sont scalaires, alors que nombre de modĂšles environnementaux incluent une dimension spatiale marquĂ©e, soit qu'ils fassent appel Ă des cartes comme variables d'entrĂ©e, soit que leurs sorties soient distribuĂ©es spatialement. Au travers d'une Ă©tude de cas dĂ©taillĂ©e, nous prĂ©sentons dans cet article une extension de l'analyse de sensibilitĂ© globale Ă l'Ă©tude de modĂšles spatialisĂ©s. Le modĂšle Ă©tudiĂ©, nommĂ© ACB-DE, est un outil d'Ă©valuation Ă©conomique du risque d'inondation. Il est ici appliquĂ© sur la basse-vallĂ©e de l'Orb (HĂ©rault). Des spĂ©cifications spatialisĂ©es de l'incertitude sont utilisĂ©es pour gĂ©nĂ©rer un nombre fini de rĂ©alisations alĂ©atoires Ă©quiprobables des variables d'entrĂ©e qui sont des cartes : les effets de structure spatiale ou d'auto-corrĂ©lation dans ces cartes peuvent ainsi ĂȘtre pris en compte. La rĂ©alisation de cartes d'indices de sensibilitĂ© permet ensuite d'Ă©tudier les sorties spatialisĂ©es du modĂšle ACB-DE et de rendre compte de la variabilitĂ© spatiale des indices de Sobol. L'influence relative des variables d'entrĂ©e Ă diffĂ©rentes Ă©chelles d'Ă©tude est analysĂ©e par la rĂ©alisation de cartes d'indices de sensibilitĂ© de rĂ©solution croissante. L'analyse rĂ©alisĂ©e permet d'identifier les variables d'entrĂ©e incertaines qui expliquent la plus grande part de la variabilitĂ© de l'indicateur Ă©conomique fourni par le modĂšle ACB-DE ; elle apporte un Ă©clairage nouveau sur le choix de l'Ă©chelle adĂ©quate de reprĂ©sentation spatialisĂ©e de cet indicateur selon la prĂ©cision des variables d'entrĂ©e. L'approche proposĂ©e pourrait ĂȘtre aisĂ©ment appliquĂ©e Ă d'autres modĂšles spatialisĂ©s peu coĂ»teux en temps de calcul. / Variance-based Sobol' global sensitivity analysis (GSA) was initially designed for the study of models with scalar inputs and outputs, while many models in the environmental field are spatially explicit. As a result, GSA is not a common practise in environmental modelling. In this paper we describe a detailed case study where GSA is performed on a spatially dependent model for flood risk economic assessment on the Orb valley (southeast France). Spatial input factors are handled by associating randomly generated map realizations to scalar values sampled from discrete uniform distributions. The realisations of random input maps can be generated by any method including geostatistical simulation techniques, allowing for spatial structure and auto-correlation to be taken into account. The estimation of sensitivity indices on ACB-DE spatial outputs makes it possible to produce maps of sensitivity indices. These maps describe the spatial variability of Sobol' indices. Sensitivity maps of different resolutions are then compared to discuss the relative influence of uncertain input factors at different scales
Estimation of creep strain and creep failure of a glass reinforced plastic by semi-analytical methods and 3D numerical simulations
International audienceGlass reinforced plastics based on polyvinyl chloride (PVC) is a material of choice for construction applications, such as pipes. The lifetime of pipes may be limited by creep failure and polymers exhibit a viscoelastic response that depends on the time of loading. In this paper, homogenization methods are designed to upscale the viscoelastic properties of a composite material made of chopped glass fibers with random orientations and PVC. The estimates of the MoriâTanaka scheme and 3D numerical computations for creep strains and creep failure are compared, validating the MoriâTanaka model as a practical tool to predict the effect of fiber length and volume fraction of fibers on creep strain and creep failure. In particular, it appears that, for a given creep load, the lifetime of the material is increased if the volume fraction of fibers increases or if the length of fibers decreases, as long as the failure mode is fiber breakage
Spontaneous reduction of a hydroborane to generate a B-B single bond using a Lewis pair
The ansaâaminohydroborane 1âNMe2â2â(BH2)C6H4 crystallizes in an unprecedented type of dimer containing a BâH bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported
Global patterns of β-diversity along the phylogenetic time-scale : the role of climate and plate tectonics
Aim: We aimed to assess the relative influence of the historical and contemporary processes determining global patterns of current \u3b2-diversity. Specifically, we quantified the relative effects of contemporary climate and historical plate tectonics on \u3b2-diversity at different phylogenetic scales. Location: Global. Time Period: Contemporaneous. Major taxa studied: Mammals and birds. Methods: We analysed the current \u3b2-diversity patterns of birds and mammal assemblages at sequential depths in the phylogeny, that is, from the tips to deeper branches. This was done by slicing bird and mammal phylogenetic trees into 66 time slices of 1 Ma (from 0 to 65 Ma) and recording the branches within each slice. Using global distribution data, we defined the branches\u2019 geographical distribution as the union of the corresponding downstream species distributions. For each time slice, we (a) computed pairwise \u3b2-diversity across all the grid cells for the whole world and (b) estimated the correlation between this \u3b2-diversity matrix and contemporary climatic and geographical distances, and past geological distances, a proxy for plate tectonics. Results: Contemporary climate best explained the \u3b2-diversity of shallow branches (i.e., species). For mammals, the geographical isolation of landmasses generated by plate tectonics best explained the \u3b2-diversity of deeper branches, whereas the effect of past isolation was weaker for birds. Main conclusions: Our study shows that the relative influence of contemporary climate and plate tectonics on the \u3b2-diversity of bird and mammal assemblages varies along the phylogenetic time-scale. Our phylogenetic time-scale approach is general and flexible enough to be applied to a broad spectrum of study systems and spatial scales
Relations entre qualité du substrat, flux de matiÚres et activités humaines. Connaissance pour la gestion des ressources en eau de la Région Aquitaine
Les résultats de ce travail de recherche, confirment une fois de plus l'absence de déterminisme apparent entre qualité du substrat et occupation des sols. Ils montrent qu'il n'y a pas de variables prépondérantes ou explicatives de facteurs sociaux ou physiques qui soient responsables des variations affectant les milieux étudiés. Cependant, les analyses régressives des rapports entre homme et milieu montrent les liens étroits entre habitat et types de sol. L'action humaine a des effets sur la couverture végétale, sur les sols et sur l'hydraulique qui avec le climat et la topographie constituent les principaux facteurs d'érosion
Drug hypersensitivity caused by alteration of the MHC-presented self-peptide repertoire
Idiosyncratic adverse drug reactions are unpredictable, dose independent and
potentially life threatening; this makes them a major factor contributing to
the cost and uncertainty of drug development. Clinical data suggest that many
such reactions involve immune mechanisms, and genetic association studies have
identified strong linkage between drug hypersensitivity reactions to several
drugs and specific HLA alleles. One of the strongest such genetic associations
found has been for the antiviral drug abacavir, which causes severe adverse
reactions exclusively in patients expressing the HLA molecular variant B*57:01.
Abacavir adverse reactions were recently shown to be driven by drug-specific
activation of cytokine-producing, cytotoxic CD8+ T cells that required
HLA-B*57:01 molecules for their function. However, the mechanism by which
abacavir induces this pathologic T cell response remains unclear. Here we show
that abacavir can bind within the F-pocket of the peptide-binding groove of
HLA-B*57:01 thereby altering its specificity. This supports a novel explanation
for HLA-linked idiosyncratic adverse drug reactions; namely that drugs can
alter the repertoire of self-peptides presented to T cells thus causing the
equivalent of an alloreactive T cell response. Indeed, we identified specific
self-peptides that are presented only in the presence of abacavir, and that
were recognized by T cells of hypersensitive patients. The assays we have
established can be applied to test additional compounds with suspected HLA
linked hypersensitivities in vitro. Where successful, these assays could speed
up the discovery and mechanistic understanding of HLA linked hypersensitivities
as well as guide the development of safer drugs
Deep-ocean mixing driven by small-scale internal tides
Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earthâs climate. In the deep ocean, tides supply much of the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fateâthe relative importance of their local versus remote breaking into turbulenceâremains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models
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