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International audience; Core-shell mesostructure in a reinforced polymer composite could dramatically affect the damage mechanisms of the material during monotonic loading. A growing understanding of the damage scenario has been possible by the use of ex-situ and in-situ microtomography tests for non-destructive 3D investigation of the material bulk. Fibre orientation analysis has been extensively developed to describe the composite mesostructure. However, complexity of the damage mechanisms still makes it difficult to predict the mechanical response. This study is aimed to understand the effects of the local mesostructure on damage mechanisms and to assess the possibility to measure strains within the material. Flat notched specimens of a short glass fibre-reinforced polyamide 6,6 were used to perform in-situ fracture tests. Fibres local orientations and crack propagation features were studied and quantified using synchrotron radiation laminography, a technique adapted to image regions of interest in flat samples. Damage mechanisms were identified independently in the core and the shell zones. Displacement and strain fields were successfully calculated by a Digital Volume Correlation (DVC) method; then, the results were compared to the mesostructure/damage analysis. Using the DVC correlation residuals the cavitation quantification was possible. As expected, fibres in the specimen core were mainly orientated in the loading direction, whereas, in the shell, the fibres were mainly oriented perpendicularly to the loading direction, see Figure 1.a. The crack roughness and heterogeneity of the crack front showed a strong influence of the local fibre orientation. In Figure 1.b, the void volume fraction thickness-distribution seems to be linked to the local fibre orientation. Qualitative results from the DVC showed that the evolution of the void volume fraction follows the same trend as the fibre orientation gradient with faster void and crack growth in the specimen center. Thus, it may be concluded that the mesostructure gradient strongly influences the damage process.

International audience; Carbon fibres have exceptional mechanical properties and are used for critical structural applications such as composite pressure vessels and aerospace components. For such high performance applications, reliability-based designs and lifetime assessments require very accurate strength models. Accuracy of the predictions made by composite strength models depend on realistic material properties of constituents, which are used as input. In practice however, fibre strength properties reported by different sources show significant variations. The work described here aims at understanding the influence of measurement uncertainty and sampling randomness on the uncertainty in calculated tensile strength distribution parameters. Tensile strength data for T700 carbon fibres obtained from single fibre testing process has been analysed for uncertainties. A parametric bootstrap method has been used for the evaluation. It has been shown that although both the causes studied of uncertainty are critical, the sampling randomness has a larger influence on the uncertainty of fibre strength, as compared to the uncertainty due to measurement. Choosing an insufficient sample size for analysis can thus result in uncertain or even inaccurate fibre strength properties, which would limit the reliability of composite strength models. The knowledge of the causes and effects of these uncertainties can help in taking appropriate measures for improving the accuracy of results. This would thereby enhance the capability of composite strength models to estimate the behaviour of different composite structures more accurately.

International audience; Knowledge of fibre strength is crucial for understanding the failure behaviour of fibre-reinforced composite materials and structures. Measuring the properties of technical fibres has been known to be very challenging, and the different challenges associated with single fibre characterisation are illustrated in this article. An improved and automated experimental methodology for tensile testing of single fibres is described. This process has been used to generate fibre strength data for T700 carbon fibres at three different gauge lengths of 4, 20 and 30 mm. The variability in strength and modulus of short fibres was found to be much larger than that of longer fibres. Statistical analysis of this large data set has also highlighted the limitations of the standard Weibull distribution for representing fibre strength behaviour. The need for a better statistical representation of the fibre strength data in order to provide a more accurate description of the fibre strength behaviour has been emphasized.

International audience; This paper proposes to take into account the influence of some morphological fluctuations – often observed in unidirectional composite materials – on the modelling of the elastic behaviour of such materials. This work relies on a Generalized Self-Consistent Scheme coupled with a Morphologically Representative Pattern based approach. An analytical model is proposed to deal with “non-percolated” or trapped matrix regions. Closed-form analytical expressions are provided to investigate different kinds of morphological fluctuation effects on the effective transverse elastic behaviour. Finally some examples are given to illustrate the effectiveness of this approach.

International audience; Experimental studies are presented on quasi-static tensile and compressive loading of composites hybridised at two levels: intraply and interply. Consistent reinforcements in the form of novel unidirectional fabrics were developed using T700SC carbon and E-CR glass fibres. Composites were manufactured using Resin Transfer Moulding process with epoxy resin and characterised to ensure consistency and comparability, further enabling easier understanding and confirmation of hybrid effect in a reliable way. Failure strain in tension for interply hybrid revealed a positive hybrid effect of +7.4%, while interply hybrid showed a negative hybrid effect of −6.4% in compression. Intraply hybrid with three carbon and three glass tows blocked together demonstrated the best mechanical performance among all hybrids; synergistic effects of +17.8% and +39.6% in tensile and compressive strength, respectively, was observed for this hybrid configuration. The results show that different hybridisation strategies can be exploited to balance cost and performance of composites for structural and lightweight applications.

The objective of this work is to determine a low cycle fatigue criterion for a thermoplastic matrix composite reinforced with both short and continuous fibres. The proposed criterion is based on the steady state creep strain rate and unifies the results for different fibre orientations, stress ratio, frequency and water content. The criterion is applicable in low cycle fatigue and creep. By means of this criterion, the Monkman–Grant relationship is verified both for creep and low cycle fatigue and offers interesting perspectives for numerical approaches.

International audience; This study deals with the cavitation of semi-crystalline Polyamide 11 during deformation. The link between the microstructural evolution and the material macroscopic behaviour is demonstrated thanks to tensile tests on small notched round bars using in-situ X-ray synchrotron computed tomography (SRCT). 3D rendering, image processing and both qualitative and quantitative data analyses are at the heart of this work so as to provide a comprehensive description of micro-mechanisms occurring in the material throughout the tensile tests. Whitening appeared whilst only few microvoids were observed via the tomographic scans in the central part of the specimen where the stress triaxiality is the highest. Significant stages – chronologically, denotching, renotching and a final denotching phase, resulting in the elongation of the central part of the specimen – are detailed. Corresponding evolution of the morphologies (in terms of growth and prolate and oblate shapes) of the void cloud, as well as of the individualized voids within the latter, are discussed.

FD2019, 8th Edition of the international conference on fatigue design, Senlis, FRANCE, 20-/11/2019 - 21/11/2019

International audience; The current requirements for composite cylinders are still based on an arbitrary approach derived from the behaviour of metal structures, that the designed burst pressure should be at least 2.5 times the maximum in-service pressure [1]. This could lead to an over-designed composite cylinder for which the weight saving would be less than optimum. Moreover, predicting the lifetime of composite cylinders is a challenging task due to their anisotropic characteristics. A federal research institute in Germany (BAM) has proposed a minimum load-cycle requirement that mitigates this issue by using a Monte-Carlo analysis of the burst test results [2-3]. To enrich this study, more experiments are required however they are normally limited by the necessity of long duration testing times (loading rate and number of cylinders) and the design (stacking sequence of the composite layer). A multi-scale model incorporating the micromechanical behaviour of composite structures has been developed at Mines ParisTech. The model has shown similar behaviour to that of composite cylinders under different loading rates [4]. This indicates that the model could assist the Monte-Carlo analysis study. An evaluation of the multi-scale model therefore has been carried out to determine its limitations in predicting lifetimes of composite cylinders. The evaluation starts with the comparison of burst pressures with type IV composite cylinders under different loading rates. A μCT-Scan of a type IV cylinder has been carried out at the University of Southampton. The produced images were analysed using the Fast-Fourier Transform (FFT) technique to determine the configuration of the composite layers, which is required by the model. Finally, the time dependent effect studied, by using the multi scale model, has been described. In the long-term, this study can be used to conduct a parametric study for creating more efficient design of type IV cylinders.

Mechanical properties of fibre reinforced composites are primarily dependent on those of fibres. Fibre properties are used for estimating the damage and strength behaviour of composite materials and structures. Tensile strength of fibres is commonly determined by single fibre tensile tests, which is challenging and is prone to measurement errors. In this study, different possible sources of errors due to experimental limitations in the fibre testing process were identified. Their effect on fibre tensile strength was analytically modelled. This model was used to evaluate the uncertainty in experimentally determined fibre strength. A sensitivity analysis was conducted to rank the relative significance of input quantities on the calculated fibre strength. Since composite models require fibre properties determined at very small gauge lengths, the results of the sensitivity analysis were extrapolated to determine critical parameters for tests done at those small gauge lengths of a few millimetres. It was shown that, for sufficiently long fibres, their strength depends mainly on the diameter and failure force, however, for shorter gauge lengths, the effects of misalignment become very significant. The knowledge of uncertainty would be useful in estimating the reliability of the predictions made by composite strength models on the damage and failure behaviour of composite materials and structures. Minimising the influence of critical parameters on fibre strength would help in designing improved single fibre testing systems capable of determining fibre strength more accurately.

International audience; Fibre properties are used for estimating the damage and strength behaviour of composite materials and structures, as the mechanical properties of fibre reinforced composites are primarily dependent on those of fibres. Fibre reinforced composites owe their outstanding properties to their constituents, mainly the fibres which reinforce matrices in composites. The accuracy of the predictions made by composite strength models would be highly dependent on the accuracy of the fibre strength distribution which is used as input. Any error in input fibre strength properties would lead to inaccurate model predictions. It has however been highlighted by many studies that there is discrepancy in experimental data even for the same type of fibres. The fibre strength data used to determine fibre strength distributions may be inaccurate. Due to the fibre preselection effect, experimental data is usually incomplete and does not appropriately represent the true fibre strength variation. An adaptation to the Weibull analysis has been presented to predict a more accurate statistical distribution for the representation of the strength behaviour of fibres inside composites. This analysis considers the preselection effect, which is used in modelling the fibre strength behaviour. The described analysis has the potential to significantly improve the accuracy of fibre strength distribution determined using experimentally generated fibre strength data.

International audience; This paper presents an enhanced micromechanical analytical model to predict the transverse elastic properties of unidirectionnal (UD) composite materials. Unlike the elastic longitudinal constants, the transverse effective properties does depend on the spatial fibre distribution. This model allows to take into account the effect of local morphological fluctuations, mainly induced by random fibre arrangements, on the effective transverse properties. For that purpose, a “n-phase” Generalized Self consistent Scheme (GSCS) has been extended by using a Morphologically Representative Pattern (MRP) approach. As an example, a two bi-phasic pattern-based model is combined with some recent data of the literature (ultrasonic methods) to explore the broad predictive ability of the model. A full range of fibre volume fractions with fibre packing effects can be covered by the proposed method. Analytical solutions based on two morphological parameters (to which is added the overall volume fraction of fibers) are provided to predict the transverse shear modulus and the transverse bulk modulus of any UD composite material. The two moduli are shown to be linked together. Very simple analytical calculations make possible parametric studies less expensive than numerical or experimental characterizations; Cet article présente un modèle analytique micromécanique amélioré permettant de prédire les propriétés élastiques transverses des matériaux composites unidirectionnels (UD). Contrairement aux propriétés longitudinales, les propriétés effectives transverses dépendent beaucoup de la répartition spatiale des fibres. Le modèle présenté ici permet de prendre en compte l’effet des fluctuations morphologiques locales, principalement induites par les distributions aléatoires des fibres,sur les propriétés transverses effectives. Pour cela, un modèle Auto-Cohérent Généralisée (ACG) “à n phases” a été étendu avec une approche par Motifs Morphologiques Représentatifs (MMR). À titre d’exemple d’application, un modèle à deux motifs de deux phases a été combiné à des données expérimentales de la littérature (méthodes par ultrasons) pour explorer la capacité prédictive globale du modèle. La méthode proposée couvre une gamme complète de fractions volumiques de fibres avec des fibres qui peuvent se regrouper. Des solutions analytiques basées sur deux paramètres morphologiques (qui s’ajoutent à la fraction volumique en fibres du composite) sont fournies pour prédire le module de cisaillement transverse et le module de compressibilité plan de tout matériau composite UD. Ces deux modules sont liés l’un à l’autre. Des calculs analytiques très simples rendent les études paramétriques possibles moins coûteuses que les caractérisations numériques ou expérimentales.

International audience; Accuracy of strength models for composite material applications is dependent on the accuracy of the input data for its constituents, namely fibre and matrix properties[1].Any errors or variability in constituent properties would lead to inaccurate or variable model predictions. In practicethe characterization of fibre properties have been known to be very challenging, due to their small cross-sections, inhomogeneity, morphological variations, etc. which may lead to variations in the results[2]. Fibre properties reported in the literature are usually not consistent, mainly due to these challenges. The variation is even larger for very brittle materials such as carbon fibres which are one of the most popular reinforcements for composites[3]. These challenges directly affect determination of fibre properties especiallystrength which isthus strongly dependent on measurement accuracy[4]. No measurement is perfectly accurate and there is some degree of uncertainty associated with every result. Uncertainties can arise from many sources such as limited precision of measurement tool, the individual performing the measurement, etc.[5, 6]. The work described here aims at understanding the influence that measurement errors can have on the calculated strengthof carbon and glass fibres and their corresponding strength distributions. This would help in identifying the critical parameters which strongly affect strength and modulus measurements and lead to uncertainty. Minimising the effect of these parameters may significantly improve the accuracy of determined results.REFERENCES:[1] F. Islam, S. Joannes, and L. Laiarinandrasana,Evaluation of critical parameters in tensile strengthmeasurement of single fibres,Journal of Composites Science, 3 (3), 69.[2] J.L. Thomason. “On the application of weibull analysis to experimentally determined single fibre strength distributions”. Composites Science and Technology, Vol. pp 77:74 –80, 2013.[3] S.Joannès,F.Islam, L.Laiarinandrasana,. Uncertainty in fibre strength characterisationdue to uncertainty in measurement and samplingrandomness,Applied Composite Materials.[4] F. Islam, S. Joannes, and L. Laiarinandrasana,Investigation of tensilestrength anddimensional variationof T700 carbon fibresusing an improvedexperimentalsetup, Journal of Reinforced Plastics and Composites.[5] F.Islam, S.Bucknell, Y.Leray, A.Bunsell, L.Laiarinandrasana, S.Joannès. Improvements in determination of carbon fibre strength distribution using automation and statistical data analysis. Fiber Society's Spring Conference 2018, Jun 2018, Tokyo, Japan. 3 p. ⟨hal-01959309⟩.[6] F.Islam, S.Joannès, S.Bucknell, Y.Leray, A.Bunsell, L. Laiarinandrasana,Towardsaccurate and efficient single fibre characterization to better assess failure strength distribution. ECCM 18 -18th European conference on composite materials, Jun 2018, Athenes, Greece. 7 p. ⟨hal-01958211⟩

International audience

International audience; A Fibre-Break Model (FBM) developed at Mines ParisTech can predict the burst pressure of high pressure composite vessels. This model uses random values of fibre strength at each Gauss point of the considered vessels meshed with finite element (FE). However, previous studies has determined the optimum FEs to be used on real-scale structures (0.1 mm x 0.1 mm x 8 mm). A simple calculation shows that, on a real-scale pressure vessel, this induces a gigantic number of FEs, hence the extensive computation time. To overcome this problem, the integral range method is proposed to find a reduced volume zone of the vessels, on which an equivalent calculation can be made and gives the same results with the one that employs the FBM on the entire structure.

International audience; Failure in unidirectional organic composites, loaded longitudinally, usually originates within fibres, which makes their failure kinetics a key parameter for failure modelling. Correctly capturing fibre strength statistics and failure kinetics is crucial to the success of composite stochastic strength models. The characterization of fibres is challenging, as the fibre diameter can be just a few microns. Nevertheless mechanical testing of single fibres is the only unambiguous means of characterising fibres or exploring their morphologies. Despite great progress in characterisation techniques, many obstacles remain to obtain accurate fibre strengths. Most studies have acknowledged the difficulty in obtaining fibre strength data which can accurately represent the entire fibre population in a given application. In this study an automated single fibre tensile testing machine has been used which overcomes most obstacles usually faced during fibre strength determination. Advanced statistical data processing techniques have been implemented to determine the reliability of the obtained results. Such analysis permits the calculation of confidence/accuracy associated with the generated fibre strength data. This may be used to extract more useful information from the limited resources available. The inadequacy of the popular statistical distributions for representing fibre strength has been highlighted. It is shown that inclusion of a trunca-tion parameter in the Weibull distribution could help in estimating the strength behaviour of a given fibre population.

International audience

Characterizing fibers—both natural and synthetic—is challenging, especially when the fiber diameter can be as small as just a few microns. Nevertheless, mechanical testing at the single fiber scale is the only unambiguous means of characterizing fibers or exploring their morphologies. Traditionally the properties of fibers have been normalized to their linear weight because of the difficulties of measuring fiber cross sections exactly; however, precise measuring techniques are now available, which allow their properties to be expressed in engineering terms familiar to all engineers working on structural materials. Traditional and conventional engineering units will both be found in this book, usually with an explanation of how to convert from one to the other. The identification of the mechanical responses together with understanding the micromechanisms involved are invaluable in guiding the design and use of fibers with advanced properties. The fiber structure can be rather complex and involves multiple characteristic length scales that demand specific techniques, e.g., from spectroscopic techniques to probe molecular or atomic structures, X-ray diffraction to identify crystalline or amorphous domains or both optical and electron microscopy. Tomography techniques are new tools allowing the internal morphology of fibers to be imaged and recent developments in this direction open up previously unimagined prospects of exploring fiber structure.

International audience; The transverse and longitudinal mechanical properties of aramid fibers like Kevlar™ 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed.

National audience

International audience; This paper introduces an experimental procedure aiming at performing a consistent mechanical characterization of textile composites behavior, under static and dynamic loadings, and particularly their macroscopic strain rate-dependent mechanisms. The procedure includes the design and validation of a reduced specimen adapted to dynamic testing, which guarantees the consistency of the identified macroscopic properties under a wide range of strain rates. An interrupted high speed tensile apparatus was also developed to investigate the strain rate sensitivity of the nonlinear constitutive behavior from math formula = 10 − 4 s − 1 to 102 s − 1. The originality of this experimental device lies in its ability to stop high speed loadings before ultimate failure, at adjustable, accurate strain levels. Finally, an extensive characterization campaign was performed. Results reveal a high strain rate dependency of the linear and nonlinear behavior. The procedure therefore generates consistent characterization data that may trustworthily be used for modeling purposes.

International audience

International audience

International audience; The case of molecular diffusion is investigated and a method is proposed to predict the effective diffusion properties of transversely isotropic composite materials. This paper is the second one of a set of two consecutive papers published in this volume. The general modelling strategy was devised in the first paper (Hervé-Luanco and Joannès, 2016) and applied to the biphasic case in this one. Closed-form analytical expressions are given to investigate fibre packing effects on the effective transverse diffusivity of any unidirectional composite. The specific case of insulated fibres embedded in a diffusive matrix is studied and a clear procedure is then described to determine the model parameters. It is worth noting that only a very simple cross-section image analysis of the unidirectional composite medium is required to provide the two morphological parameters needed to predict the overall diffusivity tensor. Finally, an application on a real microstructure is provided to demonstrate the efficiency of the suggested turnkey method. Very simple analytical calculations make possible parametric studies less expensive than numerical or experimental characterizations.

International audience; Numerical and analytical modelling approaches are often competing to describe a desired phenomenon. However, there is no reason to look for a single method and complementarities between distinct modelling ways are always profitable. This paper, the first of a set of two consecutive papers published in this volume, provides such a modelling methods reconciliation in the case of transport phenomena acting in transversely isotropic multi-phased materials. Microstructural Finite Element Modelling (FEM) helps supplying the analytical implementation for a better description of interactions between the constituents. As an application example, the case of molecular diffusion within a unidirectional composite medium is investigated without any restriction for considering any other transport mechanism. Based on a rational microstructure description, this study proposes a model able to predict the overall diffusion tensor of the composite and investigates more particularly diffusivity effects of interphase areas and fibre packings. For this purpose, a “n-phase” Generalized Self-Consistent Scheme (GSCS) coupled with a Morphologically Representative Pattern (MRP) approach has been developed. This first part is focused on the modelling strategy and the concept of “transfer matrices” has been used. This approach is applied in Part II (Joannès and Hervé-Luanco, 2016) to study the influence of fibre packings on the effective behaviour of composite materials made of insulated fibres embedded in a diffusive matrix.

National audience

Colloque avec actes et comité de lecture. Internationale.; International audience; De nombreuses études sur les matériaux composites à matrice organique, en particulier thermoplastique, ont montré que le comportement mécanique de tels matériaux est sensible à la vitesse de sollicitation. Cependant, il n’existe pas de consensus permettant de définir les moyens de caractériser, comparer et modéliser ce genre de comportement. Cette étude se focalise sur une démarche expérimentale visant à caractériser de manière robuste le comportement mécanique d’un composite à matrice polyamide 6,6 renforcé de fibres de verre tissées. Une large gamme de vitesses de déformation est étudiée (de ε=10-4 s-1 à ε=102 s-1), à l’aide d’une machine de traction conventionnelle et d’une machine de traction à vitesses rapides. En regard des précautions qu’imposent les essais dynamiques, une investigation portant sur l’optimisation d’une géométrie d’éprouvette a été réalisée. Cette dernière assure la cohérence de l’identification des propriétés macroscopiques, ainsi que le respect de critères émanant de la littérature concernant les effets de forme et les phénomènes de propagation d’ondes de choc. Dans un second temps, afin de minimiser les oscillations dues au montage, un atténuateur d’ondes volumiques a été étudié à l’aide d’une simulation dynamique du montage d’essais. Les résultats révèlent une diminution de l’amplitude des oscillations de la force mesurée et une meilleure homogénéité des déformations dans la zone utile de l’éprouvette. Il en résulte une procédure robuste permettant d’aboutir à des données fiables et donc exploitables pour identifier un modèle.

International audience

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International audience; The phenomenon of diffusion coefficients depending on concentration has been observed and studied experimentally for many years. This is particularly the case for the inter-diffusion in metals [1] but has also been observed on various polymeric systems [2]. This study presents a concentration-dependent diffusion coefficient model for water sorption in a polymer matrix composite. It has been developed by studying the water uptake of a polyphthalamide short glass fiber reinforced material. According to experimental data, we postulate that the diffusion coefficient follows a modified Arrhenius equation with a concentration dependence of the pre-exponential factor but also of the activation energy. This choice is related to the evolution of the glass transition temperature during sorption, which has a profound influence on transport properties. The coupled partial differential equation of the model is solved for some applied examples by using an easy-to-implement finite difference scheme.

International audience; Current structural adhesives can be used to assemble a variety of similar and dissimilar materials, usually with minimal surface preparation and sometimes even without degreasing. This kind of performance is obtained by using filled structural polymers which, as a consequence, lead to complex multi-phased composite material microstructures. Nevertheless, the use of mineral fillers such as talc also affects the adhesive mechanical behavior which is of major importance for structural applications. In order to study this type of material, a multi-axial mechanical characterization of an epoxy structural adhesive has been undertaken using a modified Arcan setup. An unfilled and not marketed formulation of the epoxy adhesive has also been tested to emphasize the role of the mineral particles on its mechanical behavior. As a first step, this paper reports the mechanical differences that have been observed between the filled and the unfilled versions of the adhesive, and proposes possible explanations related to microstructural processes. Contribution of the mineral fillers leads to a deterministic failure process. This is suggested, most significantly, by microstructural observations which have revealed useful information concerning the damage evolution and its kinetics. Indeed, the applied mechanical load provokes an exfoliation of talc flakes leading to a pattern of micro-cracks. In a subsequent step in the study, based on both macro and micro experimental results of the adhesive, a damage model has been derived from the vectorial description of cracking modes and its performance has been assessed.

International audience; Comprendre le comportement mécanique de la fibre aramide à l’échelle mésoscopique est indispensable pour développer de nouveaux champs d’application pour ces matériaux. Cette démarche est illustrée ici à partir d’un exemple d’application : l’utilisation des fibres aramides pour le renforcement pneumatique. La réponse mécanique, à l’échelle de la fibre, ainsi que les mécanismes moléculaires associés seront discutés en vue d’apporter des solutions face aux défis de performance et d’économie d’énergie.

International audience; Transport problems have long been studied and appear in numerous physical situations: thermal conduction, flow through porous media, molecular diffusion, etc. In this paper, we are interested in the water sorption of a polymer matrix composite which exhibits a concentration and temperature dependent diffusivity. Since it is time consuming and difficult to measure, it is necessary to estimate the water uptake and the time required to reach saturation with an evolving hydro-thermal environment without using sophisticated tools. This study provides an easy-to-implement and practical tool based on a finite difference scheme to solve this non-linear and coupled problem. We focus on the concentration dependency and propose two ways to improve the local space accuracy and thus capture the induced high-gradient frontier. The present mono-dimensional approach can be easily extended to bi-dimensional cases. Some numerical examples are given to show the effectiveness of the scheme.Composites

International audience

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International audience; In recent years, the use of continuous fiber thermoplastic composites has been growing steadily, especially in the transportation industry. The purpose of the work reported here consists in developing an experimental and numerical approach to bring help in the design of laser welded TP reinforced composites. The current paper proposes a test procedure for a multi-axial mechanical characterization of the laser welded seam using the ARCAN-MINES device. This set up allows varying the loading direction while controlling the state of stress inside the joint. On the basis of experimental observations, a non-linear phenomenological material model based on a generalized Drucker-Prager criterion is used. An estimation of strains and stress inside the laser-welded seam could be computed and related to failure observations.

International audience; The present work aims to investigate the mechanical behaviour and the damage kinetics of a3D interlock woven fabric composite, especially used for natural gas tanks dedicated to thetransportation industry. On the one hand, we aim at predicting the macroscopic coefficients ofthe stiffness matrix by homogenization multiscale approach. For this, we identified a basicunit cell which represents well the composite microstructure. On the other hand, damagemechanisms are analyzed: optical microscopy examinations on damaged specimens revealedseveral types of defects. We used the same multi-scale approach to assess the impact of thesedefects on the decrease of stiffness.

International audience; Experimental investigations on PA6GF composite material through SEM images showed high microstructural variability. Local fibre volume fractions were quantified. The use of X-ray tomography techniques allowed examinations of matrix damage mechanisms by void growth. A constitutive model for bulk PA6 polymer was set up in order to simulate PA6 behavior submitted to complex triaxial loadings. Unit cell computations were then carried out to better understand the influence of hydrostatic pressure on composite materials.

International audience; The ability of structural adhesive to withstand combination of cyclic loading and service environment makes them suitable for a wide range of applications. It is nevertheless true that their mechanical and micromechanical behaviour is still little understood. The purpose of the work reported here is to study the fracture process and the role of talc fillers of an epoxy adhesive subjected to oligocyclic fatigue. Tests were conducted by using a set-up based on the Arcan fixture and experimental results have been reported for both tensile and shear cyclic loadings. The cohesive failure of the adhesive seems to set off at a specific amplitude level, whatever the loading value. By an incursion at the microstructural scale using scanning electron microscopy, we suggest an explanation of these phenomena. The foliated form of talc fillers are of major importance and seem to provide a deterministic evolution of damage.

International audience

International audience; L'objectif de cette étude est de déterminer les propriétés mécaniques intrinsèques d'un adhésif au sein des assemblages structuraux puis d'examiner l'évolution de ces propriétés en fonction de l'épaisseur des joints de colle. Nous nous plaçons dans le cas d'une résine époxy DGEBA-DDA, chargée ou non. Des essais mécaniques et ultrasonores ont été réalisés sur des assemblages de différentes épaisseurs. Dans le but de déterminer l'influence de la microstructure sur le comportement du joint, ces résultats d'essais ont été couplés à des analyses microscopiques fines au microscope électronique à balayage (MEB). La composition de cette microstructure (charges minérales lamellaires), occasionne un endommagement d'autant plus important que les épaisseurs de colle sont faibles. Les propriétés mécaniques mesurées sont donc très dépendantes de l'épaisseur des joints considérée.

International audience

International audience; In-service safety and reliable lifetime assessments are key challenges for high performance load bearing applications and require great care to be taken during their design. The design of composite material structures can be assisted by computational models. Many complex computational models have been developed for predicting the failure and lifetime analysis of structures. Critical structures such as high pressure composite cylinders require very accurate computational models, to understand the stochastic nature of the predicted structural response. The following issues limit the use of composite strength models for making reliable structural predictions:-The complex interaction between the fibres and matrix which governs the failure mechanism has not been accurately incorporated in composite strength models. -Lack of reliable constituent properties which are used as input for the models.There are several studies aimed at improving the state-of-the-art models. However, accurate constituent properties to be used as input for these models are rarely available. Authors usually do not comment on the uncertainty of the constituent properties reported, which is of major importance for stochastic simulations. Since fibres are the principal load bearing constituents of unidirectional composites, Islam et al. have quantified the uncertainty in the parameters of the fibre strength Weibull distribution arising during to the characterisation process, using a Monte-Carlo approach to capture the stochastic nature of the fibre strength behaviour. Strength of T700 carbon fibres, popularly used in composite pressure vessels, is used as reference. In this study, the influence of uncertainty in input fibre strength on model predictions has been evaluated. A composite strength model developed at Mines ParisTech was used. This model considers physical processes such as fibre failure and its interactions with the surroundingmatrix. It was first developed in 2005 and has been improved over the years to simulate different loading conditions during service of composite structures such as pressure vessels. The strength and lifetime of a composite structure (coupon) is simulated under two different practical loading conditions (monotonically increasing and sustained loading) to elucidate the sensitivity of different structural responses to the input fibre strength distribution. The calculated uncertainties in the shape (m) and scale (sigma_0) parameters of the fibre strength Weibull distribution were used as input for the models. The results are listed as follows:1) Monotonic loading:-The failure stress is seen to be significantly dependent on the scale parameter. The observed variation in the predicted failure stress is about 10% from the mean case, for an uncertainty of 10% in scale parameter.-The sensitivity of the model predictions to the shape parameter was insignificant.2) Sustained loading:-The time to failure of the composite specimen was also found to be strongly affected by uncertainties in the scale parameter. The calculated uncertainty of 10% in the input scale parameter resulted in a variation in the predicted lifetime of about 15-30%.-The calculated uncertainty of 25% in the shape parameter resulted in a variation of about 16% in the predicted lifetime of the specimen.The structural behaviour predicted by the model is found to be highly sensitive to the uncertainties in the input fibre strength distribution which arise during characterisation. The understanding of the constituent properties and their characterisation process needs to be improved, in order to improve the reliability of computational model predictions, so that the predictions can be used with confidence in industrial applications.