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This article presents an alternative manufacturing process for unidirectional tape natural fiber Polypropylene with Maleic Anhydride grafted PolyPropylene (PP + MAPP) thermoplastic composites that could enable large high performance parts to be built, while being recyclable at end of life. The performance target is flax/epoxy composites that are currently manufactured in shipyard. A parametric analysis (time and temperature) of vacuum-bag-only molding of flax unidirectional tapes-reinforced PP + MAPP matrix is conducted to find the best configuration regarding the final composite tensile properties, as no standard procedure exists. The physical phenomena induced by temperature, water sorption/desorption, and pressure during the manufacturing step are investigated. The best parameters for temperature and time are found to be at 180 °C during 45 min, resulting in a fiber volume fraction of 42.2 ± 1.5%, and between 1.9 ± 0.7% and 8.2 ± 1.8% of porosity, depending on the method of determination. Interestingly, tensile properties are in the same order of magnitude as conventionally manufactured continuous flax fiber-reinforced thermoset or thermoplastic composites. The article ends with a discussion of the accessibility and technical concerns of vacuum-bag-only molding, material selection, design, mechanical performance, and end of life.

International audience; Flax (Linum usitatissimum) lodging is an issue of great interest for industrial producers due to its economic impact; despite a strong varietal selection over around one century, this plant remains sensitive to lodging which represents a main technico-economic issue. To better understand lodging effects at the cell wall and stem scale, the cell wall composition dynamics during cell wall development and after a 90 degrees tilt bending stress is reported. Deep-UltraViolet (DUV) fluorescence emission dynamics recorded at the Synchrotron SOLEIL-DISCO beamline by multichannel autofluorescence imaging is addressed for five cellular wall types of flax stems after an artificially induced gravitropic reaction. The quantitative fluorescent profile intensities were computed after image analysis, and compared to the control flax stems, we reported a systematically higher average intensity fluorescence (probability >95%) for the 90 degrees tilted plants. Moreover, the average stem fluorescence intensities were significantly different among the 3 developmental stages, with the youngest stage (VS) exhibiting on average 30% and 20% less fluorescence than the medium (FG) and mature (M) stages, respectively. The flax stem response to tilt impacted the xylem cellular type, while the bast fibres were arguably less affected by the protein, and hydroxycinnamate contents. A complementary investigation was carried out on bast fibres by infrared microspectroscopy to explore the polysaccharide components not detected in DUV fluorescence, and significant modifications were monitored.

Flax shives have been proven to be relevant injection-moulded reinforcement materials with a stiffness that reaches 90% of that obtained with cut flax fibres. Nevertheless, little is known of the reinforcing mechanisms of this material. In this study, deeper insight is sought on the mechanical properties of flax shives, which originate mainly from the xylem tissue of the flax stems. The microstructure of flax xylem is investigated by nanotomography and image analysis as a function of stem height, combined with AFM measurements of the cell walls in Peak Force Quantitative Nano Mechanical indentation mode (20–22 GPa were measured for the S2 layer). The bending stiffness of peeled flax stems was furthermore investigated, and finally, an estimation of the flax shive particle stiffness was carried out and tested with a micromechanical approach in order to evaluate their reinforcing potential in injection-moulded composites.

International audience; PLA-flax non-woven composites are promising materials, coupling high performance and possible degradation at their end of life. To explore their ageing mechanisms during garden composting, microstructural investigations were carried out through scanning electron microscopy (SEM) and atomic force microscopy (AFM). We observe that flax fibres preferentially degrade ‘inwards’ from the edge to the core of the composite. In addition, progressive erosion of the cell walls occurs within the fibres themselves, ‘outwards’ from the central lumen to the periphery primary wall. This preferential degradation is reflected in the decrease in indentation modulus from around 23 GPa for fibres located in the preserved core of the composite to 3–4 GPa for the remaining outer-most cell wall crowns located at the edge of the sample that is in contact with the compost. Ageing of the PLA matrix is less drastic with a relatively stable indentation modulus. Nevertheless, a change in the PLA morphology, a significant decrease in its roughness and increase of porosity, can be observed towards the edge of the sample, in comparison to the core. This work highlights the important role of intrinsic fibre porosity, called lumen, which is suspected to be a major variable of the compost ageing process, providing pathways of entry for moisture and microorganisms that are involved in cell wall degradation.

International audience; Flax (Linum usitatissimum L.) is a plant of industrial interest. Its fibres have traditionally been used for textile applications and more recently, for composite reinforcement. To increase fibre yields, varietal selection has been used to develop varieties having high fibre content while retaining good resistance to lodging. This selection process has led to impressively slender structures of flax compared to other herbaceous plants. The present study focuses on the mechanical stability of flax related to its specific architecture. An anatomical study of transverse sections provides information about the architecture of flax stems, including the repartition of the internal reinforcing tissues being phloem fibres and xylem. Then, by using three-point bending tests, flexural modulus is evaluated along the stem. The safety factor (SF) against buckling for the plant was estimated based on Greenhill's model, taking into account gradients in diameter, load, and elastic modulus. Although flax plants have an unusually slender structure, they are mechanically stable. The stability of the plant is ensured by a high stem flexural modulus. This originates from an external ring composed of high-performance fibres, while an inner thick porous xylem provides the plant with a high resistance to local buckling. This is useful information for breeders, demonstrating that it is possible to keep increasing fibre yield without jeopardising plant stability.

International audience; Retting of fibrous plants such as flax is an essential step in the extraction of fibre bundles and their transformation into textiles and reinforcement fabrics for use in garments and composites. Dew-retting is traditionally performed from Northwest France to the Netherlands, as the climate is highly favourable for this process. Hemp is a plant that can be grown almost all over Europe with a low environmental impact. A retting step is also required to facilitate the separation of the hemp fibres before their transformation into textiles for garments or for 1D to 3D reinforcement composites, which requires thoroughly separated fibres. Dew-retting is currently used in flax production zones. The present work demonstrates that dew retting can be conducted under different climates on different soils, from dry Mediterranean environments up to the cooler regions of eastern France. If the ternary combination of moisture, temperature and solar radiation is appropriate, field retting (dew-retting) can be as short as about three weeks. In less favourable conditions, such as in dryer areas or when retting is performed late in the season after seed maturity (cooler temperatures), it lasts longer, but it can reach suitable levels. When conducted with care and with proper monitoring of the retting level, the dew-retting process does impact neither the tensile properties of elementary hemp fibres (by degrading crystalline cellulose I) nor the tensile properties of unidirectional and injected composite materials. Consequently, if extracted with a suitable process such as scutching and hackling, fibres suitable for load-bearing composites can be produced from dew-retted hemp stems produced in a wide range of climates and locations, therefore not limited to the conventional “dew retting zone” of flax production areas.

Flax lodging is an issue of great interest for producers due to its economic impact. To better understand its effects at the cell wall and stem scale, new knowledge regarding the cell wall composition dynamics during cell wall development and after a 90° tilt bending stress is reported. Deep-Ultra Violet fluorescence emission (DUV) dynamics recorded at the Synchrotron SOLEIL-DISCO beamline by multichannel autofluorescence imaging is reported for five cellular wall types of flax stems after an artificially induced gravitropic reaction. Three flax growth development stages, namely, the vegetative stage (VS), the fast growth (FG) and the mature stage (MS), were selected in normal plants, referred to as the control plants, or in gravitropic-induced response plants, referred to as 90° tilted plants.

Liquid composite moulding (LCM) of plant fibre composites has gained much attention for the development of structural biobased composites. To produce quality composites, better understanding of the resin impregnation process and flow behaviour in plant fibre reinforcements is vital. By reviewing the literature, we aim to identify key plant fibre reinforcement-specific factors that influence, if not govern, the mould filling stage during LCM of plant fibre composites. In particular, the differences in structure (physical and biochemical) for plant and synthetic fibres, their semi-products (i.e., yarns and rovings), and their mats and textiles are shown to have a perceptible effect on their compaction, in-plane permeability, and processing via LCM. In addition to examining the effects of dual-scale flow, resin absorption, (subsequent) fibre swelling, capillarity, and time-dependent saturated and unsaturated permeability that are specific to plant fibre reinforcements, we also review the various models utilised to predict and simulate resin impregnation during LCM of plant fibre composites.

Le renforcement des polymeres par des fibres naturelles permet de reduire les impacts environnementaux. Parmi les fibres naturelles, les fibres vegetales ayant un role structurel dans la nature presentent des proprietes mecaniques interessantes. Elles sont de plus facilement disponibles. Les fibres organiques peuvent etre utilisees, mais elles ont des proprietes plus variees. Compte tenu du developpement des connaissances, de la disponibilite de nouveaux demi-produits et de l'evolution de la legislation en termes de protection de l'environnement, les biocomposites sont amenes a se developper dans de nombreux secteurs d'activites.

Industrially and scientifically speaking, the flax (Linum usitatissimum L.) plant is of great interest. The specific architecture of its stem makes it an exceptional model of vertical stability and slenderness, allowing the stem to resist in a notable way to lodging and buckling in the event of external solicitations. This chapter, dedicated to the multi-scale mechanical characterization of flax, is divided into two main parts. First, the specific architecture of the flax stem is described in order to understand why this plant develops singular characteristics and what lessons can be drawn from it for material science. The second part of the chapter emphasizes on the characterization of the performances of flax fibres and cell walls using a multi-scale and top-down approach, from the stem to the cell wall level. Although this chapter contains only examples from analyzes on flax fibres, the tools and methods presented may be used to study different elementary fibres, provided that their morphologies allow it, such as hemp or nettle. For each investigation scale, main interests and potential mechanical information are listed, as well as limiting factors.

The automotive and aeronautic industries have become strategic sectors for North African countries. However, the future growth of these sectors will likely depend on the availability of raw materials such as plant fibres to reinforce polymers. In the framework of this paper, we reviewed the research that focused on alfa fibres, which are deemed to be one of the most available fibres in the Mediterranean area. This plant also plays an essential role in protecting the Mediterranean countries from desertification and desert encroachment. Throughout this review, we discussed and expounded the morphological, chemical compositions and the mechanical properties of the alfa fibres and we compared them to other plants fibres. Moreover, we surveyed several academic research works that used different alfa fibres as reinforcement of thermoset and thermoplastic matrices. In particular, we outlined that alfa fibre can constitute a potential reinforcement of polymer matrices in the North Africa countries.

Flax/Poly-(lactide) non-woven composites are an alternative to conventional glass/poly-(propylene), offering a potentially lower environmental impact solution for the automotive industry. To understand this complex material, its detailed architecture and void distribution are examined through 3D microtomography. Anisotropy in fibre orientation is also observed, further verified through off-axis tensile tests. Then micro-mechanics and laminate theory are used, taking into account fibre orientation distribution, to predict mechanical properties and compare with experimental measurements. Even though some deviation is observed at off-axis angles greater than 45°, we report that off-axis tensile tests (property) can be used to predict fibre orientation (structure) in industrial production facilities as a simple means of quality control and tailored design of new non-woven preforms.

International audience; Despite the wide development of biocomposites, their compressive behaviour is still not well understood. In this paper, the longitudinal compressive and tensile behaviour of unidirectional natural fibres is studied through a parametric analysis taking into account the nature of the fibre (flax or jute), the matrix (thermoplastic and thermoset PP, PP/MAPP, PA11, epoxy or acrylic), the fibre volume fraction and the fibre/matrix bond strength. In parallel with this approach, the quasi-static tensile behaviour is also investigated to allow comparisons. At low strains, the compressive and tensile moduli are closely similar. On the other hand, the compressive strength is systematically lower than the tensile strength whatever the fibre and matrix used. With a PP matrix, use of a coupling agent (MA) to improve the Interfacial Shear Strength (IFSS) leads to an increase of strength, which highlights the importance of this parameter. The compressive strength increases with the fibre volume fraction, but the maximum value remains lower than 140 MPa. Back calculation allows us to estimate the compressive strength of the flax fibres as 240 MPa, which appears as a current limit for the dimensioning of biocomposite structures.

Hemp (Cannabis Sativa) and flax (Linum Usitatissimum L) are the most commonly cultivated fibre plants in Europe. The use of their fibres covers a wide range of applications and requires a precise knowledge of their mechanical properties. The aim of this study is to investigate the contribution of fibres to the stiffness of flax and hemp dry stems subjected to bending during a simple three-point flexural test. The analysis is based on measuring the difference of bending stiffness between stems with fibres and stems without fibres after removal by manual peeling. Mechanical characterization shows that hemp and flax fibres contribute 54% and 71%, respectively, on average, to the flexural stiffness of a stem. Then, a microstructural analysis of stems is used to determine fibre distribution and geometry, which leads us to obtain longitudinal moduli of 33.9 ± 13.0 GPa and 55.8 ± 14.5 GPa for hemp and flax, respectively. These values are close to those obtained by tensile tests on elementary fibres (30.0 ± 13.3 GPa and 54.7 ± 13.2 GPa). These results encourage to propose the three-point bending test on stems as a standard test for the estimation of bast fibre stiffness.

Automated fiber placement (AFP), once limited to aerospace, is gaining acceptance and offers great potential for marine structures. This paper describes the influence of manufacturing defects, gaps, and overlaps, on the out-of-plane properties of carbon/epoxy composites manufactured by AFP. Apparent interlaminar shear strength measured by short beam shear tests was not affected by the presence of defects. However, the defects do affect delamination propagation. Under Mode I (tension) loading a small crack arrest effect is noted, resulting in higher apparent fracture energies, particularly for specimens manufactured using a caul plate. Under Mode II (in-plane shear) loading there is a more significant effect with increased fracture resistance, as stable propagation for specimens with small gaps changes to arrest with unstable propagation for larger gaps.

Biocomposites have major advantage in term of weight saving thanks to lower densities compared to conventional materials; this property also allows a reduction of environmental impacts which is beneficial for the automotive industry. In addition, they maintain rather good mechanical properties after recycling cycles which makes possible to consider other end-of-life scenarios than incineration or landfill. In this work, we studied two potential routes for the recycling of moulding wastes from poly (propylene)-flax nonwovens. By representing close to 25%-wt, these wastes are a major industrial problem, inducing additional recycling or treatment costs. In a first step, the parts were grinded and reincorporated at different weight fractions into virgin nonwovens. Up to 30%-wt of reincorporated wastes, results showed good mechanical performances for recycled nonwovens, especially in bending mode. In a second stage, scraps were grinded and then compounded for injection moulding applications. Their good rheological behavior and ability to conserve fibre lengths during extrusion made possible to formulate competitive materials in terms of tensile performances. (C) 2017 Elsevier Ltd. All rights reserved.

The natural character of plant fibres was often cited as a disadvantage which results in the scattering of their properties. This work aims at studying the influence of this scattering on the stiffness of an epoxy-flax fibres twill fabric ply. Elastic coefficients were estimated using an analytical model in an elliptical configuration based on the use of the classical laminate theory. Most of flax fibre elastic coefficients were obtained in the literature; in addition, a flax fibre Poisson's ratio of 0.498 was estimated from an experimentally inverse approach. Then, the use of the analytical model evidenced that the Young's moduli of woven composite were strongly correlated to longitudinal and transversal fibre ones; moreover, the fibre transversal modulus had a significant impact on the Poisson's ratios of the composite. As regards to the yarn configuration and architecture, we showed that the mean twisting angle significantly affected stiffness and Poisson's ratio of the composite; in the same way, the importance of waviness described by its waviness ratio was underlined whereas the scattering into yarn morphology had low influence on composite performances when the waviness ratio was high. Keywords: Flax fibre, Scattered properties, Woven composite, Elastic properties, Analytical model

International audience; Peak-Force Quantitative Nano-Mechanical property mapping (PF-QNM) was applied to explore the nano-mechanical properties of flax fibre cell walls in cross-section. After validation of the ability of PF-QNM to determine stiffness gradients in aramid fibres, measurements were performed on developing flax bast fibres. The presence of two layers with different indentation moduli implies their progressive development during thickening of the secondary cell wall. Finally, measurements were carried out on technical flax fibre cell wall; but, in this case, no significant stiffness gradient could be identified in the secondary S 2 layer.

International audience; The use of composite materials reinforced by flax fibres has been increasing steadily over the last 20 years. These fibres show attractive mechanical properties but also some particularities (naturally limited length, presence of a lumen, fibres grouped in bundles in the plant, complex surface properties and composition). An analysis of the available literature indicates that the quality of the composite materials studied is not always optimal (high porosity, incomplete impregnation, heterogeneous microstructure, variable fibre orientation). This paper reviews published data on the specific nature of flax fibres with respect to manufacturing of biocomposites (defined here as polymers reinforced by natural fibres). All the important steps in the process which influence final properties are analyzed, including the plant development, retting, fibre extraction, fibre treatment, preform preparation, available manufacturing processes, the impregnation step, fibre cell wall changes during processing and fibre/matrix adhesion. © 2018, Springer-Verlag France SAS, part of Springer Nature.

International audience; Flax fibres are a promising reinforcement in the development of biocomposites and are finding new applications in transport structures. However, there is a perceived problem with plant fibres related to the variability of the properties of these natural materials. This paper describes the factors which affect variability, from plant growth conditions to fibre sampling and testing. A large number of test results are presented (characterization of elementary fibres, bundles, assemblies of bundles, and unidirectional composites), and it is shown that provided fibre supply is carefully controlled, characterization procedures are appropriate, and manufacturing processes are optimal then excellent composite properties can be achieved with low variability.

This study investigates the evolution of hygromechanical properties of flax/PP nonwoven composites in a wide range of environmental Relative Humidity (RH) conditions from 10 to 98% RH. The influence of microstructure with various porosity content ( Φ = 5, 30, 50%) on the mechanical and hygroscopic behaviours is studied and compared to glass/PP composites as a baseline. No significant changes in the moisture content and mechanical properties of glass-fibre reinforced composites are observed. For flax fibre nonwoven composites, the porosity greatly impacts the kinetic of sorption with moisture saturation varying from 9 h to 15 days with decreasing voids. Hygroscopic expansion is hardly modified by the porosity content while anisotropic expansion is always observed. Tensile behaviour and properties are slightly changed over a range of 10–75%RH but negatively impacted between 75% and 98% RH. Interestingly, unlike the tangent tensile modulus and strain at rupture of flax/PP composites, the yield strength exhibits a non-monotonic trend with an optimal value over 50% RH; compressive stresses at the fibre/matrix interface induced by flax fibres hygroexpansion are proposed to explain this trend.

The hygro-mechanical behavior of single hemp fiber/epoxy interface was investigated through its characterization at the microscopic scale. Microdroplet debonding tests were developed in controlled humidity chambers to study the interfacial properties and adhesion mechanism of hemp/epoxy systems in in-use environmental conditions (from 9% to 98% relative humidity). The increase in environment relative humidity and therefore moisture content in the fiber/matrix system engenders a non-monotonic evolution of interfacial shear strength (IFSS) and post-debonding strength. Capillary adhesion illustrated with interfacial meniscus and attractive capillary force contribute to interfacial bond strength until RH = 50%. Increasing RH above 50%, corresponding to a specific amount of water molecules in the system, provokes a reduction of interactions between the hydroxyl groups on the fiber surface and the resin. Debonding behavior analysis associated to fiber/matrix interface fracture observation highlights a modification of the stress state at the interface when increasing relative humidity. The residual hygroscopic stresses generated during storage at various humidity are shown to be the major contributor of the optimal interface performances at 50% relative humidity (RH). The sensitivity to moisture of natural fibers could therefore be beneficial for improving biocomposite interface properties.

International audience; This paper investigates biochemical, morphological and mechanical properties of a large range of plant fibres explored with the same methods. Biochemical results clearly exhibit strong differences between gelatinous, i.e. flax and hemp, and xylan type, i.e. jute and kenaf, cell walls. These differences into parietal composition have an impact on cell wall stiffness, highlighted through nanoindentation and atomic force microscopy measurements, but also on fibre individualisation, due to variations into fibre bundles cohesion. In addition, the morphology and particularly the lumen size induces apparent density differences. Moreover, the influence of fibre morphology and properties is demonstrated on UD materials. Finally, longitudinal Young's modulus of each plant fibre batches is back-calculated from UD stiffness by an inverse method; the results obtained are in accordance with the values in the literature values, proving the interest of this method to estimate longitudinal Young's modulus of short plant fibres.

This article provides deeper insight into moisture sorption over a wide range of relative humidity (RH), from 9% to 98%, and its effect on the evolution of the tensile behaviour and performance of hemp/epoxy unidirectional biocomposites. Moisture sorption is associated with orthotropic hygroscopic expansion which is also related to the influence of fibre reinforcement distribution and the generation of residual stresses due to manufacturing. Tensile behaviour is significantly modified by moisture content, with a non-linear behaviour becoming more evidenced. The increase of moisture content causes exponential decay of tensile modulus, but a non-monotonic evolution of tensile strength and strain at break shows optimal properties at about 3%. The hygroscopic property evolution observed is responsible for the creation of compressive radial stresses at the fibre/matrix interface, which promotes load transfer and benefits the interface performance.

International audience; This study investigates the evolution of hygromechanical properties of flax/PP nonwoven composites in a wide range of environmental Relative Humidity conditions from 10 to 98% RH. The influence of microstructure with various porosity content (Φ = 5, 30, 50%) on the mechanical and hygroscopic behaviours is studied. The porosity greatly impacts the kinetic of sorption with moisture saturation varying from 9 hours to 15 days with decreasing voids. Tensile behaviour and properties are slightly changed over a range of 10-75% RH but negatively impacted between 75% and 98% RH. Interestingly, unlike the tangent tensile modulus and strain at rupture of flax/PP composites, the yield strength increases until 50% RH and stabilizes over this point; compressive stresses at the fibre/matrix interface induced by flax fibres hygroexpansion are proposed to explain this trend. A quasi-monotonous decrease is also observed in bending properties with the increase in water content in the material.; Ce travail a pour objectif d'étudier l'évolution des propriétés hygromécaniques des composites non-tissés lin/PP dans une large gamme d'humidité relative allant de 10 à 98% RH. L'influence de la microstructure sur le comportement hygroscopique et mécanique est étudiée avec des taux de porosité variables (Φ = 5, 30, 50%). La porosité a un impact important sur la cinétique de sorption avec une saturation en humidité variant de 9 heures à 15 jours avec moins de porosité. En traction, le comportement et les propriétés mécaniques sont légèrement modifiés sur une plage de 10-75%HR mais ont un impact négatif entre 75% et 98% RH. Il est intéressant de noter que, contrairement au module tangent de traction et à la déformation à la rupture des composites lin/PP, la contrainte maximale augmente dans un premier temps avant de se stabiliser pour de forts taux d'humidités. Ceci s'explique par l'importance des contraintes de compression à l'interface fibre/matrice induites par l'hygroexpansion des fibres de lin. En flexion, on observe une diminution quasi-monotone des propriétés avec l'augmentation de la teneur en eau dans le matériau.

International audience; Flax shives (FS) are the main by-product of the flax scutching industry and represent approximately 50 % of dry stem weight, making them a widely available resource having very low added value. In this study, raw flax shives have been fragmented by ball milling (sample FS50) and knife milling (samplesFS250 and FS500) and used as reinforcing material in polypropylene injected composites. The biochemical composition of reinforcing samples in monosaccharides and lignin shows no variation after comminution, but a 50 % drop in crystallinity index was measured between raw flax shives and the ball-milled sample. The morphological study of reinforcing materials (length, aspect ratio) after processing were correlated to composite tensile properties showing an increase in Young’s modulus with particle size, from 22.7 to 32.7 GPa for FS50 and FS500 respectively, the latter reaching 90% of the flax fibre reinforced composites. Flax shives provide promising mechanical performances.; Les anas, principaux co-produits de l’industrie du teillage de lin, sont une ressource disponible,à très faible valeur ajoutée et peu exploitée dans le domaine des biocomposites.Leur potentiel de renfort d’un composite de polypropylène injecté est étudié ici.Une première étape nécessaire de broyage (à boulet pour l’échantillon nommé anas 50 et à couteaux avec des grilles de 250 et 500 μm pour les échantillons anas 250 et 500respectivement) a été suivie d’une analyse morphologique. La composition biochimique des renforts en monosaccharides et en lignines n’évolue pas avec la méthode de broyage, mais l’indice de cristallinité chute progressivement de 50 % entre les anas bruts et les anas 50. Des biocomposites en polypropylène injectés avec30% en masse des différents lots d’anas ont été réalisés,et suite à une analyse morphologique (longueur, rapport d’aspect) après mise en œuvre,leurs propriétés mécaniques en traction ont été comparées à celles obtenues avec des matériaux renforcés par des fibres de lin et de la farine de bois. L’incorporation d’anas apporte une augmentation de la rigidité de 22.7 à 32.7 GPa pour les anas 50 et 500, ces derniers permettant d’atteindre 90% du module des composites renforcés par des fibres de lin.

International audience; Flax (Linum usitatissimum L.) fibres are commonly used as reinforcement of composite materials. Nevertheless, literature shows that the compressive strength of flax-based composites is rather modest compared with materials reinforced by synthetic fibres. The present article investigates the compressive strength of flax fibre bundles both within the stems and in unidirectional (UD) composites. In this way, an optimised arrangement of fibre bundles inside the plant is assumed. Damage mechanisms are found to be similar in the stem and within flax-based UD materials, namely by buckling of fibre bundles, a typical failure mechanism of UD composites. Inside the stems, this phenomenon is highlighted by nanotomography, which underlines the key role of the woody core in the buckling resistance of the plant. For UD, failure can also be studied by scanning electron microscopy (SEM). The same ranges of average compressive strength values are estimated for flax fibre bundles, being 206 MPa within the stem and 242 MPa within UD composites. Finally, this study highlights that, if a flax stem is an optimised natural structure, the compressive strength of flax fibre bundles seems to be a limiting factor for structural applications of flax-based composite materials.

International audience

International audience; Flax retting is a major bioprocess in the cultivation and extraction cycle of flax fibres. The aim of the present study is to improve the understanding of the evolution of fibre properties and ultrastructure caused by this process at the plant cell wall scale. Initially, investigations of the mechanical performances of the flax cell walls by Atomic Force Microscopy (AFM) in Peak Force mode revealed a significant increase (+33%) in the cell wall indentation modulus with retting time. Two complementary structural studies are presented here, namely using X-Ray Diffraction (XRD) and solid state Nuclear Magnetic Resonance (NMR). An estimation of the cellulose crystallinity index by XRD measurements, confirmed by NMR, shows an increase of 8% in crystallinity with retting mainly due to the disappearance of amorphous polymer. In addition, NMR investigations show a com-paction of inaccessible cell wall polymers, combined with an increase in the relaxation times of the C4 carbon. This densification provides a structural explanation for the observed improvement in mechanical performance of the secondary wall of flax fibres during the field retting process.

The last 80 years have seen significant changes in industrial technologies, with the development of composite materials being particularly striking. These were hardly present before the Second World War but were made possible by the availability of new polymers, fibre reinforcements and innovative manufacturing techniques. In parallel, the traditional flax farming practices for the textile industry have benefited from major improvements in seed selection, retting and fibre extraction methods. This aims of this paper are twofold; first to describe the early work to develop flax fibre reinforced composites and compare their mechanical properties with those of contemporary flax fibres. And second, to understand the impact of significant efforts made recently to improve flax fibre production. The simple substitution of synthetic fibres is no longer sufficient to justify their use; their environmental benefits must be demonstrated, through life cycle analyses to support sustainable societal choices, and these are also discussed.

International audience; The development of new plant fibre composites is a key point in the development of semi-structural biodegradable or biobased parts, especially for automotive applications. The aim of this original and innovating work is to study, at different scales, the recycling ability of a fully biodegradable L-Poly-(lactide)-Poly-(butylene-succinate)-flax (PLLA-PBS-flax) biocomposite. The biocomposites were manufactured by twin-screw extrusion followed by injection moulding, then the recycling behaviour was studied during successive injection moulding cycles. Firstly, we investigated the length of the flax fibre after compounding and injection, as well as the cell wall stiffness and hardness, by in-situ nano indentation tests. Secondly, we focused on the effects of recycling on thermal, rheological and tensile properties. We highlighted a severe evolution of the cell wall properties, especially concerning the polysaccharidic matrix after the first thermal cycle, nanoindentation properties remaining quasi-stable after this first drop. Furthermore, the biocomposites did not show any significant evolution of their mechanical performances during cycle three or four of the first injection cycles; after this plateau, the tensile strength and strain as well as impact energy were significantly altered due to the conjugated fibre length decrease and degradation of the PLLA, the latter being emphasized when the flax fibre is embedded. Nevertheless, this fully biodegradable composite exhibits a suitable recycling behaviour for 3 or 4 cycles, which is sufficient for industrial applications.

This paper reports the mechanical behaviour of novel biocomposites made up from PA11 and flax fibres. We find that, using various flax fibre loadings (i.e. volume fractions), it is possible to obtain bio-based composites with good and reliable tensile properties compared to other biocomposites such as PLA/Flax. Our investigations reveal that the thermal cycle process used for composite moulding has a significant impact on the fibre mechanical properties. Indeed, a 210 °C thermal cycle for 8 min induces a decrease in fibre stiffness and strength of 32.8 and 12.9%, respectively. A simple micromechanical model, i.e. mixing rule, is applied to estimate composite stiffness and strength based on heat-treated fibre properties. In the case of composite stiffness, our results indicate an underestimation of the Young’s modulus compared to experimental data, especially when the composite stiffness is taken in the first part of the stress–strain curve where the stiffness exhibit unstable values. This implies that the mechanical behaviour of flax fibre bundles compared to fibre bundles constrained within the polymer matrix is not the same, probably due to rotational locking, which prevents the micro fibrillar realignment of the cellulose leading to higher stiffness values.

Commingled plant and polypropylene fibres (PP) nonwovens are attractive for automotive industry because of their interesting mechanical performances combined with good acoustical properties. This double function is achieved thanks of controlled porosity within the composite material. Indeed, from a same material, different functions (acoustic or mechanical) can be obtained by only varying the compaction rate during moulding. This study aims to highlight the necessity of a very large porosity volume fraction (60%) to reach good acoustic properties and understand the mechanical effect behind it. By combining tensile testing, acoustic absorption measurement and scanning electron imaging analysis, the microstructure, acoustic and mechanical properties have been investigated and were found to be intimately related to the material porosity content. As expected, when increasing porosity level from 5 to 60%, the material behaviour change and its tensile properties (modulus, strength and elasticity domain) drop drastically to a point where the material is no longer elastic (70% porosity) due to a modification of the material microstructure involving different failure mechanisms. (C) 2016 Elsevier Ltd. All rights reserved.

This paper presents results from an experimental study of three types of non-woven preforms (needlepunched, spunlaced and mat manufactured using a paper-making process) intended as composite reinforcement. These are potentially very attractive for transport applications. First, the influence of processing on elementary fiber tensile properties is shown to be limited. Then the preforms are evaluated in polypropylene matrix composites and mechanical properties are determined. The structure of non-woven reinforcements is strongly dependent on the manufacturing route. By varying the fiber content it is shown that the most efficient reinforcement for flax fibers is the mat produced by paper processing. The new spunlaced reinforced composites are shown to have slightly lower tensile properties (15% lower strength, and 25% lower stiffness) compared to mat composites at equivalent volume fraction, but further optimization is possible for these materials. Based on the measured constituent properties micromechanics models have been used to estimate composite stiffness. A good correlation is obtained between test results and model predictions.

Automated fibre placement (AFP) has been used to manufacture flax/polypropylene biocomposites for the first time. This required preparation of tape with a calibrated width from impregnated fibre sheets. The unidirectional tapes showed polymer-rich zones on the surface. During manufacture the polypropylene (PP) is melted locally with a laser. A consolidation step in a hot press is then required to reduce void content. The flax fibres, composed of polysaccharides, have limited thermal resistance so optimization of the thermal cycle is necessary, but subsequent characterization of mechanical behaviour showed no evidence of property loss in spite of an additional melting cycle. AFP appears to be a promising manufacturing method for biocomposites. Keywords: Structural composites, AFP, Biocomposites, Flax, PP

This paper deals with the mechanical behaviour of lime hemp composites. LHC blocks have been produced by compression in a rigid die at a relatively high compression pressure. This process allows the production of LHC with a high proportion of hemp shiv. New mechanical parameters are proposed to compare experimental results of this study with those found in published literature. This paper shows that a high compaction pressure enhances the compressive strength and can offset a reduction of binder. Consequently, a new formula is proposed to predict the strength of LHC which depends on both the binder content and the compaction state of the shiv particles. The study leads to recommendations for the mix design of such composites.

Throughout this study, different plant fibres, used as polymer reinforcement, were tested by quasi-static tensile and nanoindentation tests to obtain their modulus using these two methods. Major differences of stiffness were found due to their different structures and morphologies. As example, tensile and nanoindentation moduli for sisal fibres were and 25.0±12.9 GPa and 10.2±1.7 GPa, respectively, whereas they were 52.4±13.2 GPa and 18.9±1.8 GPa for Eden flax fibres. A correlation between these two moduli was established; thus by an inverse method, we estimated the longitudinal stiffness of softwood fibre, for which the tensile test is delicate, mainly due to the short length of the fibre.