Flax fibres are increasingly explored as sustainable reinforcements in high-performance composites due to their remarkable stiffness and strength. However, their hygroscopic nature and complex hierarchical organisation pose challenges for dimensional stability and long-term durability. Among the intrinsic microstructural heterogeneities, kink-bands, localised deformations associated with cellulose misalignment and increased porosity, may affect water uptake and swelling. Despite their potential impact, the effect of kink-bands on water sorption and moisture-induced structural changes has not been directly investigated. This study investigates the influence of kink-band density on the hygroscopic behaviour of flax fibres using an integrated multi-scale approach combining Dynamic Vapour Sorption (DVS), Environmental Scanning Electron Microscopy (ESEM), X-ray microtomography, and solid-state Nuclear Magnetic Resonance (ss-NMR). Three flax fibre batches with distinct kink-band frequencies were prepared through controlled mechanical processing. DVS results revealed no significant difference in overall water sorption capacity between batches, indicating that kink-bands do not markedly affect global hygroscopicity. However, ESEM and X-ray tomography analyses highlighted heterogeneous swelling behaviour at the bundle and fibre scales, with kink-band regions exhibiting lower expansion than intact zones, suggesting a compensating effect of internal pores on local deformation. Solid-state NMR analyses revealed that, although fibril dimensions and overall crystallinity were unchanged, fibres with higher kink-band content exhibited increased water accessibility and subtle nanoscale polymer reorganisation. This suggests that kink-bands promote local water penetration and interactions with surrounding non-cellulosic components, without altering overall bulk hygroscopicity. Together, these results indicate that kink-bands modulate local moisture dynamics, providing new insights into the structural–functional relationships governing flax fibre behaviour in humid environments. They are also relevant for natural fibre composites, as understanding how kink-bands influence local moisture dynamics can inform predictions of dimensional stability, fibre–matrix interactions, and long-term durability.

Atomic Force Microscope (AFM) in mechanical mode, Raman and Second Harmonic Generation (SHG) microscopy were applied to explore the ultrastructure and nano-mechanical properties of kink-band regions in flax fibres. It is evidenced, that the longitudinal indentation modulus drastically drops (between -43 and -53%) in kink-band regions, mainly explained by pronounced changes in microfibrillar angle (MFA), from 0 to 6 degrees to 1921 degrees. At contrary, Raman investigations demonstrate a conservation of biochemical composition, along with a decrease in local crystallinity (between 11 and 57%), contributing also to the mechanical loss in kink-band regions. These micro-scale investigations provide inedited findings about the kink-band features and specificities, especially in term of local properties.

Plant fibres are promising reinforcements for bio-composites in additive manufacturing, but their use as long fibres remains limited, often reduced to short particles that underuse their potential. This study presents a customised yarn design that not only maintains fibre alignment parallel to the yarn axis but also ensures core resin impregnation. Commingling and wrap spinning techniques were used to produce four flax/PLA yarns with varying compositions. The manufacturing process and printing of unidirectional composite specimens are detailed. Tomography revealed up to 3.3 times lower intra-yarn porosity thanks to commingling, and tensile tests showed a modulus increase by a factor of 2.1 compared to similar previous works using conventional twisted yarns. These results pave the way for broader use of long flax fibres in 3D printing.

Defects in flax fibres limit the use of more sustainable load bearing composites in industry, which highlights the need for a thorough understanding of their nature. In this work, X-ray micro-tomography of flax revealed pores in the cell wall of elementary fibres: kink-bands pores and longitudinal pores, a previously unseen defect. Their morphology and organisation are examined, highlighting fibre deterioration and locally increased porosity up to 14.86 %. Finite element modelling under a 1.5 % tensile strain reveals that kink-band pores concentrate stress up to 7.15 times, while longitudinal pores reach 2.35 times compared to defect-free areas. Under tension, cracks are thus likely to initiate at kink-band defects, may propagate through longitudinal pores to other kink-bands, and lead to fibre and composite failure as these defects are favoured zones for crack initiation and propagation. In situ peeling of fibres due to knot tightening under scanning electron microscopy suggests interlaminar decohesion between cellulose macrofibrils as the origin of the longitudinal pores. The study explores hypotheses on the origin of these weak interfaces related to fibre growth and extraction processes. It provides insights for improving flax fibre properties and widening the use of more sustainable composites.

This work explores how the morphology of kink-band zones in flax fibres impacts the mechanical properties of the elementary fibres. Kink-bands are structural defects and are particularly sensitive to physical and biological stresses, on isolated fibres or in bio-based composite materials. To this end, a panel of archaeological samples from different time periods and preserved under different environmental conditions were selected and studied using synchrotron micro-tomography. It is demonstrated that although kink-bands are generally more numerous in ancient fibres, their degree of severity is sometimes less. This underlines the importance of fibre extraction methods, which are principally responsible for kink-band formation. The results also show that kink-band are weaknesses points, allowing rapid development of internal porosity (up to 25 %) when the fibres are used or stored in extreme environments, and that this porosity can also extend to healthy areas of the fibres. However, in some cases, even after millennia of conservation, it appears that the fibres can present morphologies comparable to modern samples, probably due to their good initial quality. Thanks to the findings of the present work, simplified schemes of degradation in kink-band zones, useable on single fibres but also in composite materials, are proposed. These results confirm the importance of fibre extraction processes on fibre quality and durability, and subsequent use for sustainable and high-performance composite materials and textiles.

Flax fibres offer performance capabilities comparable to glass fibres, thereby enhancing their potential in the biobased composites industry. However, these fibres have morphological defects affecting their mechanical features. In the present work, flax elementary fibres geometries with defects assessed by synchrotron X-ray microtomography were meshed to simulate a tensile test using finite element analysis. For the first time, the distribution of stresses in the vicinity of defects is revealed. The geometrical irregularities at the surface of the fibre and the delamination of cellulose layers within fibre cell wall turned out to concentrate stress up to 7.5 times compared to defect-free regions. These results demonstrate why flax fibres cannot reach their full potential in comparison to what could be expected from a structure mainly constituted from crystalline cellulose microfibrils, and why fracture in a composite is likely to initiate in those defect zones.

Residual shives have a major impact on the quality and performance of flax textile and composite preforms. Quantifying the content of shives in batches of scutched flax fibres is a time-consuming and operator-dependent process. Although the shive content can be estimated from the chemical composition, our aim here is to explore a series of effective, reliable and complementary approaches to quantifying shives. Various methods are presented: microscopic observations, analytical biochemistry (monosaccharides, lignins), indirect methods (infrared spectroscopy) and dynamic morphological analysis (QICPIC). A reference standard was first analysed and then compared with batches of flax tow fibre of unknown shive content from current industrial production. This approach shows that calibration curves can be established by applying each selected method to batches with known fibre and shive content. In addition, a database was generated to determine the shive content of industrial batches using partial least squares (PLS) regression. Finally, a detailed study of shives in industrial batches is presented, comparing the different analytical methods.

This paper deals with the study of the evolution of the durability of short hemp fibres reinforced composites subjected to long-term ageing. The compounds were stored for ten years in atmospheric conditions (natural ageing) prior to the study. Therefore, two kinds of specimen are studied. First, the aged compounds were used to moulded tensile specimens. The residual mechanical behaviour of natural aged hemp/PP composites (natural aged specimens) were compared to properties obtained 10 years ago. This storage showed that the hemp is more sensitive to time degradation than PP and was confirmed by the initial degradation of compounds pointed out by ATG. Then, injected natural aged polypropylene reinforced with several hemp fibre weight fractions compounds were subjected to two different protocols in order to characterize their durability (aged specimens). The specimens were subjected to water immersion at ambient temperature and storage in a climatic chamber maintained at 40 °C with a relative humidity of 98%. The fracture surfaces were observed with SEM to identify the physical phenomena leading to the observed loss of mechanical properties. A weight water sorption of 14% with respect to hemp weight fraction was, for the first time, determined as a critical water sorption for which the reinforcement effect becomes negligible. The physical phenomenon responsible for this critical absorption was finally investigated with Infrared Spectroscopy, showing the occurring of a dissolution of the fibres.

Flax fibres are valuable reinforcements for tomorrow's composites. However, defects called kink-bands, which mainly appear on fibres during the extraction and transformation phases, might affect their mechanical properties. Defects induced pores, within the kink-band are investigated in this work. They were morphologically explored using synchrotron phase-contrast X-ray microtomography, a technique that displays a sharp 3D representation of the pores. The study highlights the link between kink-bands and secondary cell wall ultrastructure. Pores are organised concentrically around the lumen, and their low thickness suggest that they are located at the interface between cellulose layers within S2 (G) layer. Moreover, the pores inclination with reference to the lumen axis follows the typical microfibrillar angle changes observed in the literature in the kink-band region. The volumes of the pores were measured, and a local increase in porosity was revealed in zones where defects are most severe along the fibre.

Single flax fibres develop morphological deformations, called kink-bands, which are considered to be areas of weakness. The correlation between the occurrence of kink-bands, fibre extraction method and fibre mechanical properties still needs to be clarified. To this end, an extensive multi-scale statistical study was conducted on 96 elementary fibres extracted from four distinct batches processed with different scutching and combing conditions. First, kink-bands were observed and quantified through polarized light microscopy (PLM), scanning electron microscopy (SEM) and synchrotron X-ray microtomography. PLM quantification demonstrated that uncombed fibres have more cortical residues and larger surface kink-bands with many small internal porosities. Then, the mechanical characteristics of each PLM-quantified single fibre were measured by tensile tests. A larger number of kink-bands was observed for the most intensively combed batch, 18.2 kink-bands/mm, as well as a surprisingly low average kink-band area, of the order of 158 mu m2 per kink-band. In addition, the same batch had a higher tensile strength and modulus than the other three batches with a negative Stearman correlation of 0.64 and - 0.78 with the proportion of defects, respectively. Finally, 74% of the fibres in this intensively combed batch, had a 'Type I' linear elastic behaviour, highlighting a significant change of behaviour in comparison to the uncombed batch, attributed to local reorientation and pre-stretching. The results suggest that changes in kinkbands structure are principally responsible for these evolutions; due to both new distribution of pores in kinkbands regions and reduction in kink-band size possibly induced by local cellulose realignment.

This paper proposes to assess, in a multi-scale experimental approach, the hygroscopic expansion coefficients of flax yarns in radial direction and of flax fibre reinforced composites in the three directions (longitudinal, transverse and out of plane). Results show a significant flax yarn radial swelling coefficient (βr=1.06) and a composite highly anisotropic swelling behaviour (βL=-0.02;βT=0.13;βT′=0.85). An analysis of conditioning until saturation at different relative humidities shows the dramatic effect of water content on the composite tensile behaviour. The evolutions of tangent modulus with respect to stress indicate the existence of two types of behaviour depending on water content within the material. As water content increases, the behaviour becomes predominantly plastic and a rigidification phenomenon is highlighted.

Based on experimental investigations on the influence of water content on time-dependent behaviour of flax reinforced epoxy composites, this paper proposes to model the viscoelastic–viscoplastic behaviour with hygromechanical coupling. From a rheological point of view, the proposed model is a combination of a linear spring (elasticity), a Kelvin–Voigt model (viscoelasticity) and a viscoplastic model described by kinematic hardening in which the evolution of the internal variable is collinear with the plastic deformation. The hygromechanical coupling is obtained via the dependence on water content of all the model parameters according to a power law. The developed model shows a good correlation between experimental data (tensile, creep and relaxation) and numerical predictions of the mechanical behaviour for low water contents (below 4.90%). However, the model shows its limits for high water contents (wc≥4.90%) and high loadings. Indeed, tensile tests have revealed the emergence of a stiffening phenomenon for high loadings according to water content. This phenomenon was not taken into account in the proposed model which could explain its limitations. This assumption is confirmed by comparing the evolution of experimental yield stress with the model prediction.

Plant fiber properties, such as morphological and mechanical, are characterized by a large dispersion. Thereby, a statistical analysis is needed to obtain consistent results. An experimental study, conducted on 50 flax yarns, shows that the flax yarn properties (Young's modulus, tensile strength and diameter) follow Gaussian distributions. This approach is obviously reliable, however time-consuming, to get relevant information. An alternative could be the identification of the yarn mechanical properties using an inverse approach, based on tensile tests conducted on flax fabric reinforcement. The aim of this study is to develop a numerical method that allows to identify the statistical distributions of flax yarn properties based on tensile tests conducted on fabric specimens. The proposed strategy relies on two assumptions. On the one hand, fabric is constituted of several yarns acting like springs in parallel. On the other hand, yarns are considered as brittle-elastic materials. Hence, a yarn breaks when the load reaches its failure strength, leading to a loading redistribution to the intact yarns. A comparison of the numerical and experimental results of flax fabric tensile behavior, and of the flax property statistical distributions allows to confirm the performance of this strategy. The results show that the flax fabric tensile behavior is correctly described by the proposed modeling strategy. The average and the standard deviation of the Young's modulus, the tensile strength and the diameter of flax yarns identified from fabric tensile tests via inverse approach are close to those obtained experimentally on individual yarns. Indeed, the measured average diameter of unitary yarns are 244.4±19.2μm and the fitted one is 2474.8±17.4μm, the fitting failure strength 292.3±33.4MPa are close to the experimental 271.2±47.5MPa. The identified Young's modulus is 9.4±0.9GPa is lower than the experimental 10.8±1.3GPa.

Based on experimental test results, flax fiber reinforced polymer composites are characterized by nonlinear visco-elastoplastic behavior. The aim of this work is to model the quasi-unidirectional flax fiber reinforced composite behavior through a three dimensional formulation with orthotropic elasticity and orthotropic plasticity using Hill criterion. The isotropic hardening and Johnson Cook parameters are identified from unidirectional tensile tests at different strain rates. The adjustment of Hill’s yield criterion is developed based on yield stresses obtained in tensile tests at different directions. The numerical integration of the constitutive equations is implemented in a user-defined material, UMAT subroutines for the commercial finite element code ABAQUS. Once model parameters are identified using tensile tests, the model needs to be validated by confronting it with other experimental results. That is why experimental and numerical three-point bending tests are carried out in order to validate the proposed model with tests that have not served for the identification. Finally, a numerical parametric study on low velocity impact of a flax/epoxy composite circular plate is investigated.

In order to analyse the influence of conditioning at relative humidity on the behaviour of flax fibre reinforced composites, a preliminary study on flax fabrics needs to be conducted. Therefore, this paper investigates the effect of conditioning on the tensile properties of non-impregnated flax fabrics at different humidities. These fabrics are used as reinforcement materials for semi-structural composite materials. The major drawback of natural fibre reinforced composites is moisture regain. Understanding how the moisture affects the behaviour of non-impregnated flax reinforcement is then crucial. The conditioning of the fabrics in different humid atmospheres made it possible to study the influence of the water content on the hygro-mechanical behaviour of flax reinforcement through tensile tests. The water content of samples is linked to the relative humidity of conditioning atmosphere through the isothermal sorption curves. This paper brings up the significant influence of water content on the stiffness and elongation at break of the fabric, while water content does not seem to have an influence on the breaking load which remains practically constant. Non-linear decrease of flax fabric rigidity and increase of its elongation at maximal force with the increase of water content are observed. It is concluded that water molecules act as a softening agent on flax fabrics. Two kinds of typical tensile curves are observed. Most of the tensile curves of samples show one unique linear region where the behaviour appears to be linear prior to the first yarn failure. For some samples in which water content is between 2.7% and 3.7%, the emergence of a two-linear region behaviour is pointed out. This two-linear region behaviour could be attributed to local heterogeneities. Indeed, this study shows that, at saturation, the amount of absorbed water is not uniform through flax fabric samples. The moisture uptake heterogeneities did not resorb with time after saturation. These local heterogeneities would induce a premature local plasticity in some regions of the flax fabric sample. These regions would present local weakness areas that could induce an exhibition of the two-linear region behaviour.

In the past decades, research was directed towards the use of plant fibres instead of synthetic fibres as reinforcement of composites. Due to their high specific mechanical properties coupled with low cost and their wide availability at the European scale, flax fibres could be considered as the most interesting plant fibres. Experimental tests carried out on flax fibre-reinforced composites have shown that these latter are characterized by a nonlinear viscoelastic–viscoplastic behaviour. In this paper, our work was focused on modelling the elastic–viscoplastic behaviour of a quasi-unidirectional flax fibre-reinforced composites. First of all, we developed a three-dimensional elastic–viscoplastic model taking into account the orthotropic elasticity and the anisotropic viscoplastic behaviour of quasi-unidirectional flax/epoxy composites. Then, based on tensile tests at different strain rates, we identified the isotropic hardening using an optimized exponential Johnson–Cook law. The model was validated against experimental data. Finally, we implemented the behaviour model in a UMAT procedure of the finite element code ABAQUS/Implicit and simulated low-velocity impact behaviour of a flax/epoxy circular plate. Simulation has shown that the impact velocity has a great influence on the behaviour of flax fibre-reinforced epoxy composite plate.

Parmi les modifications engendrees par l’absorption d’eau dans les materiaux, l’hygro-expansion est un parametre cle pour decrire les couplages hygro-mecaniques dans les composites polymeres renforces par des fibres vegetales. Cependant, peu d’etude experimentale sont proposees par la litterature. Cette etude se propose donc de determiner, dans une approche experimentale multi-echelle, les coefficients d’hygro-expansion des fils de lin et des composites renforcespar des fibres de lin (CRFL). Les resultats montrent un coefficient de gonflement radial des fils de lin important et nettement superieur a celui classiquement admis dans la litterature. De meme, ce papier met en evidence que les dilatations hygroscopiques des CRFL sont fortement anisotropes. L’etude montre un comportement non-lineaire des dilatations hygroscopiques des composites dans la direction des fils de chaine caracterise par un coefficient d’hygro-expansion negatif. Les coefficients de dilatations transversaux du composite sont inferieurs a celui radial des fils de lin expliques par l’effet bloquant de la matrice. De plus, les dilatations hygroscopiques dans la direction des fils de trame sont faibles. Ce resultat est explique par le coefficient d’hygro-expansion longitudinal des fils de trame negatif qui bloque les dilatations