79 research outputs found
Utilisation de la spectromĂ©trie infrarouge pour une quantification rapide du taux dâhumiditĂ© dans des fibres vĂ©gĂ©tales
International audienceIn the field of composite materials, natural fibres appear to be a viable replacement for glass fibres. However, in humid conditions, strong hydrophilic behavior of such materials can lead to high level of moisture absorption. This feature can result in a structural modification of the fibres and a modification of their own mechanical properties as well as those of the composites they are fitted in. Then, understanding moisture sorption mechanisms in these materials is an important issue for their efficient use. In this work, water absorption on three natural fibres (flax, hemp and sisal) studied qualitatively and quantitatively using non-invasive approach, i.e. Fourier transformed infrared spectroscopy. Fibres with different moisture content were prepared using climatic chambers, allowing strictly controlled hygrothermal ageing. The equilibrium water content were assessed by gravimetric measurement and determined for each relative humidity conditions (Relative humidity ranging from 30 % until 97 %). The principal chemical functions of fibres involved in the water absorption phenomenon were thereby identified and the water content of tested fibres was determined using a Partial Least Square Regression approach. The typical sorption isotherm curves described by Park model were clearly fitted, confirming the validity of our predictive model. Finally, this model has been applied for monitoring the water diffusion.Les fibres vĂ©gĂ©tales sont une alternative possible aux fibres de verre utilisĂ©es classiquement comme renforts dans certains matĂ©riaux composites. En plus dâavantages environnementaux, leur faible densitĂ© leur confĂšre des propriĂ©tĂ©s mĂ©caniques spĂ©cifiques Ă©levĂ©es. Cependant, leur comportement hydrophile marquĂ© entraĂźne une forte absorption dâeau lors dâune utilisation en environnement humide, ce qui peut modifier leurs propriĂ©tĂ©s mĂ©caniques. La comprĂ©hension de ces mĂ©canismes dâabsorption dâhumiditĂ© est donc indispensable Ă leur maĂźtrise. Dans ce travail, lâabsorption dâeau a Ă©tĂ© Ă©tudiĂ©e par spectroscopie infrarouge Ă transformĂ©e de Fourier sur des fibres de lin, chanvre et sisal. Ces fibres ont Ă©tĂ© soumises Ă des conditions hygromĂ©triques contrĂŽlĂ©es. Les spectres infrarouges des Ă©chantillons ont Ă©tĂ© enregistrĂ©s en mode rĂ©flexion totale attĂ©nuĂ©e. Les informations spectrales ont permis de mettre en Ă©vidence les fonctions chimiques impliquĂ©es dans les mĂ©canismes dâabsorption de lâhumiditĂ© et de quantifier le taux dâhumiditĂ© retenu par les fibres. Pour cela, des modĂšles de rĂ©gression partielle des moindres carrĂ©s ont Ă©tĂ© dĂ©veloppĂ©s en exploitant des donnĂ©es expĂ©rimentales issues de mesures gravimĂ©triques. Ces modĂšles se sont rĂ©vĂ©lĂ©s de bons outils pour la caractĂ©risation des cinĂ©tiques de diffusion de lâeau dans ces diffĂ©rentes fibres
Effet de lâĂ©rosion par des particules de sable sur la performance des pales dâĂ©oliennes dans les zones arides et semi-arides
Cet article prĂ©sente les rĂ©sultats dâune recherche expĂ©rimentale visant Ă Ă©tudier la dĂ©gradation de la surface des pales dâĂ©oliennes situĂ©es dans des zones arides causĂ©e par les impacts de particules de sable et lâinfluence qui en rĂ©sulte sur leur performance Ă©nergĂ©tique. Les essais sont effectuĂ©s dans une sableuse Ă Ă©rosion puis dans la soufflerie pour la pale en fibre de verre / polyester non revĂȘtue sous diffĂ©rents angles et durĂ©es dâimpact. Les forces aĂ©rodynamiques sur les profiles NACA 4412 sont dĂ©terminĂ©es expĂ©rimentalement. Tous les profiles (AR = 0,571) sont testĂ©s dans une soufflerie Ă une vitesse de lâair de 10 m/s au nombre de Reynolds 6.5Ă105. Les rĂ©sultats montrent les diffĂ©rences de comportement dĂ©tectĂ©es pour les Ă©chantillons avec T = 160s, T = 240s et T = 340s (quand Ra augmente) en les comparant avec lâĂ©chantillon lisse (T = 0s, Ra = 0,94). Les coefficients de force (CL et CD) montrent des changements significatifs qui conduisent Ă une diminution du rapport portance-traĂźnĂ©e et donc une perte de performance notable
Statistical properties of effective elastic moduli of random cubic polycrystals
The homogenized elastic properties of polycrystals depend on the grain morphology and crystallographic orientations. For simplification purposes, the orientations of the grains are usually considered three independent Euler angles. However, experimental investigations reveal spatial correlations in these angles. The KarhunenâLoĂšve expansion is used to generate random fields of Euler angles having exponential kernel functions with varying correlation lengths. The effective elastic moduli for numerically generated statistically equiaxed cubic polycrystals are estimated via the classical EshelbyâKröner Self-Consistent homogenization model. The influence of the correlation lengths of the orientationsâ random fields on the statistical properties of the effective elastic moduli has been investigated. Our results show that spatially correlated Euler angles could increase the variability of the homogenized elastic properties compared to the ones having uncorrelated Euler angles. Nevertheless, using independent random variables for Euler angles remains valid when correlation lengths are close to the average grain size
Nouvelle mĂ©thode de maintenance CBM pour amĂ©liorer la durĂ©e de vie des pales dâĂ©oliennes implantĂ©es dans les zones arides
lâamĂ©lioration de la durĂ©e de vie des pales Ă©oliennes opĂ©rant dans une rĂ©gion aride au-delĂ de sa durĂ©e de vie habituelle de 20 ou 25 ans oblige leur propriĂ©taire Ă augmenter le nombre des inspections dans le cadre de la maintenance prĂ©ventive sachant que les coĂ»ts de maintenance et dâarrĂȘt dâune dâĂ©olienne ont un impact direct sur la production totale et peuvent reprĂ©senter jusquâĂ 25% de lâĂ©nergie produite. Sachant que dans une rĂ©gion poussiĂ©reuse, les pales sont soumises Ă une usure Ă©rosive qui contribue Ă la modification de la forme aĂ©rodynamique du bord dâattaque et rĂ©duit ainsi les performances de lâĂ©olienne. Ce papier dĂ©crive et propose une nouvelle mĂ©thode de maintenance CBM pour amĂ©liorer la durĂ©e de vie dâune pale Ă©olienne. Les rĂ©sultats expĂ©rimentaux montrent que lâutilisation de cette mĂ©thode est trĂšs efficace pour dĂ©tecter toute dĂ©gradation provoquĂ©e par lâusure Ă©rosive Ă la surface dâune pale pendant son service
ModÚles de transition d'échelles pour l'étude de matériaux biphasés
International audienceLargement utilisés pour des applications industrielles, les matériaux biphasés polycristallins présentent des hétérogénéités de comportement élastique, thermique et plastique à l'échelle de la phase et des cristallites. L'identification des lois de comportement des phases est nécessaire pour dimensionner une structure composée à partir de ces matériaux. Ce livre décrit des méthodes originales pour estimer les constantes d'élasticité et de dilatation thermique d'une des phases d'un systÚme multiphasé. La démarche repose sur un couplage de résultats expérimentaux avec des modÚles de transition d'échelles. Les propriétés de la phase beta du Ti-17 ont ainsi été caractérisées depuis l'échelle du cristal. Les hétérogénéités de comportement élasto-plastique des matériaux biphasés ont été investiguées lors de l'étude d'un alliage de Ti-662 fortement texturé. Des analyses de contraintes par Diffraction des Rayons X ont été confrontées à la simulation. Les outils numériques et les procédures expérimentales décrits dans cet ouvrage assurent une meilleure compréhension des mécanismes intervenant dans la génération des états mécaniques multi-échelles de matériaux biphasés
Multi-scale analysis of fiber reinforced composite parts submitted to environmental and mechanical loads
The purpose of this work is to present various application of statistical scale transition models to the analysis of polymer-matrix composites submitted to thermo-hygro-mechanical loads. In order to achieve such a goal, two approaches, classically used in the field of modelling heterogeneous material are studied: Eshelby-Kröner self-consistent model on the one hand and Mori-Tanaka approximate, on the second hand. Both models manage to handle the question of the homogenization of the microscopic properties of the constituents (matrix and reinforcements) in order to express the effective macroscopic coefficients of moisture expansion, coefficients of thermal expansion and elastic stiffness of a uni-directionally reinforced single ply. Inversion scale transition relations are provided also, in order to identify the effective unknown behaviour of a constituent. The proposed method entails to inverse scale transition models usually employed in order to predict the homogenised macroscopic elastic/hygroscopic/thermal properties of the composite ply from those of the constituents. The identification procedure involves the coupling of the inverse scale transition models to macroscopic input data obtained through either experiments or in the already published literature. Applications of the proposed approach to practical cases are provided: in particular, a very satisfactory agreement between the fitted elastic constants and the corresponding properties expected in practice for the reinforcing fiber of typical composite plies is achieved. Another part of this work is devoted to the extensive analysis of macroscopic mechanical states concentration within the constituents of the plies of a composite structure submitted to thermo-hygro-elastic loads. Both numerical and a fully explicit version of Eshelby-Kröner model are detailed. The two approaches are applied in the viewpoint of predicting the mechanical states in both the fiber and the matrix of composites structures submitted to a transient hygro-elastic load. For this purpose, rigorous continuum mechanics formalisms are used for the determination of the required time and space dependent macroscopic stresses. The reliability of the new analytical approach is checked through a comparison between the local stress states calculated in both the resin and fiber according to the new closed form solutions and the equivalent numerical model: a very good agreement between the two models was obtained. The purpose of the final part of this work consists in the determination of microscopic (local) quadratic failure criterion (in stress space) in the matrix of a composite structure submitted to purely mechanical load. The local failure criterion of the pure matrix is deduced from the macroscopic strength of the composite ply (available from experiments), using an appropriate inverse model involving the explicit scale transition relations previously obtained for the macroscopic stress concentration at microscopic level. Convenient analytical forms are provided as often as possible, else procedures required to achieve numerical calculations are extensively explained. Applications of this model are achieved for two typical carbon-fiber reinforced epoxies: the previously unknown microscopic strength coefficients and ultimate strength of the considered epoxies are identified and compared to typical expected values published in the literature
Eshelby-Kröner Self-Consistent elastic model: the geometric mean versus the arithmetic mean - A numerical investigation
International audienceScale-transition models, such as Eshelby-Kroner self-consistent framework, which are often used for predicting the effective behavior of heterogeneous materials or estimating the distribution of local states from the knowledge of the corresponding macroscopic quantities, require the extensive use of set averages. The present paper is devoted to the comparison of the numerical results provided in pure elasticity by Eshelby-Kroner model depending on the average type chosen for achieving set average operations: either the traditional arithmetic mean or the geometric average. Various numerical applications of the model to the case of predicting either the effective stiffness or the lattice strains of single-phase polycrystals will be provided. The particular case when an extreme grain-shape occurs will also be investigated
Extension of the Vook-Witt and inverse Vook-Witt elastic grain-interaction models to general loading states
International audienceThe recently developed VookâWitt and inverse VookâWitt elastic grain-interaction models have been employed for the calculation of mechanical elastic constants and diffraction (X-ray) stress factors of, in particular, thin films. However, their applicability is limited to a planar, rotationally symmetric state of macroscopic, mechanical stress. For such a loading state (and an, at least, transversely, elastically isotropic specimen), only two mechanical elastic constants are necessary to describe mechanical elastic behaviour and only the sum of two diffraction (X-ray) stress factors is needed to relate lattice strains to the one independent component of the mechanical stress tensor. The restriction to a planar, rotationally symmetric state of mechanical stress will be removed in this work. Calculation of the full stiffness tensor and all six diffraction (X-ray) stress factors then becomes possible. It was found previously that the VookâWitt and inverse VookâWitt models become (but only approximately) equivalent to the EshelbyâKröner model for certain ideal grain-shape textures. For this reason, results of numerical calculations of mechanical elastic constants and diffraction (X-ray) stress factors, based on the VookâWitt and inverse VookâWitt models, will be presented and compared to corresponding results obtained from the Eshelby--Kröner grain-interaction model considering ideal grain-shape (âmorphologicalâ) textures
Water-mechanical property coupling
The present contribution investigates the effects related to theplasticization of the polymer matrix occurring during the water sorption process onthe internal mechanical state profiles, at the constituent and ply scales. Then, twomulti-physics models are considered to account that the moisture sorption dependson the internal mechanical states: the free volume theory and a thermodynamicapproach
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