# Development of a custom commingled flax/PLA wrapped yarn for additive manufacturing of long-fibre biocomposites

Delphine Quereilhac, Thomas, Raphaël Caillault, Svetlana Terekhina,
Mahadev Bar, Guillaume Morel, Marina Fazzini, Marwa Abida, Emmanuel de
Luycker, Pierre Ouagne

► To cite this version:

Delphine Quereilhac, Thomas, Raphaël Caillault, Svetlana Terekhina, Mahadev Bar, Guillaume Morel, et al.. Development of a custom commingled flax/PLA wrapped yarn for additive manufacturing of long-fibre biocomposites. Materials & Design, 2025, 258, pp.114628. 10.1016/j.matdes.2025.114628. hal-05413097

HAL Id: hal-05413097

https://hal.science/hal-05413097v1

Submitted on 12 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Development of a custom commingled flax/PLA wrapped yarn for additive manufacturing of long-fibre biocomposites

Delphine Quereilhac<sup>a, *, ID</sup>, Thomas Caillault<sup>b ID</sup>, Svetlana Terekhina<sup>b ID</sup>, Mahadev Bar<sup>a</sup>, Guillaume Morel<sup>a</sup>, Marina Fazzini<sup>a</sup>, Marwa Abida<sup>c</sup>, Emmanuel De Luycker<sup>a</sup>, Pierre Ouagne<sup>a ID</sup>

## ARTICLE INFO

### Keywords:

Flax fibres

Commingled yarn

Additive manufacturing

Bio-based composite

Mechanical properties

## ABSTRACT

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.

## 1. Introduction

Additive manufacturing of composite structures has been extensively investigated over the recent years, echoing the search for innovative, high-performance materials. This field of study brings together two manufacturing processes: composites and additive manufacturing. Composite materials combine the advantages of a matrix and a fibrous reinforcement, resulting in lightweight, strong and durable materials [1]. Additive manufacturing results in structures that are quick to implement, sometimes less expensive, more complex in terms of geometry while generating less waste [2,3]. Also known as three-dimension (3D) printing, it consists in adding successive layers of material to manufacture an object, from computer aided design (CAD) models.

Among the technologies available for additive manufacturing, fused deposition modelling (FDM) is the most encountered, because of the process simplicity, speed and low-cost [4]. However, the mechanical performance of 3D-printed thermoplastic parts is often limited due to processing-induced defects, such as voids caused by the high viscosity of thermoplastic resins compared to thermosetting alternatives [5]. A new field of research has therefore opened up to overcome this problem, for example by optimising printing processes such as bed temperatures,

deposition speeds, part orientation [6-8] or adding fillers like short carbon, flax or hemp fibres to create bonds between layers [9,10]. Unfortunately, these fillers, often in the form of very short discontinuous fibres (in the micrometre, millimetre range), do not exploit the full mechanical capacities of the fibres they are made of, in particular their remarkable tensile modulus and strength. This is why a real interest in printing continuous fibres (in the centimetre, decimetre range) has emerged for the design of higher-performance materials [11].

Unlike chopped fibre-reinforced composites, continuous fibre additive manufacturing requires more complex processes to supply both the matrix and the reinforcing filament. Two main approaches are being explored: (i) using a single feed with a pre-impregnated filament [12] or (ii) using separate feeds, where impregnation occurs either within a single nozzle that combines the thermoplastic polymer and the continuous fibre or through two distinct nozzles [13,14]. Most studies focus on synthetic fibres like carbon, glass, or aramid, which offer excellent mechanical properties but rely on non-renewable resources, have a high environmental impact, and pose challenges for end-of-life management [15]. To address these issues, it is possible to substitute these materials with bio-sourced materials for some non-structural applications, in particular annual plant fibres such as hemp or flax.

Flax has been used since ancient times to make textiles [16], and in

<footnote><sup>a</sup> Univ. de Technologie Tarbes Occitanie Pyrénées, LGP ENIT, F-65016 Tarbes, France</footnote>

<footnote><sup>b</sup> Arts et Métiers Institute of Technology, Campus Angers, LAMPA, F-49035 Angers, France</footnote>

<footnote><sup>c</sup> Univ. de Lorraine, UMR-CNRS 7239, LEM3, F-57078 Metz cedex 03, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: delphine.quereilhac@univ-ubs.fr (D. Quereilhac).</footnote>
the last century it has become part of the feedstock for reinforcing
composite materials [17]. There are two major advantages to this plant
fibre. Firstly, it is one of the most resistant among the large family of
plant fibres: the average Young's modulus, strength and elongation at
break are (52.5 ± 8.6) GPa, (945 ± 200) MPa and (2.07 ± 0.45) %,
respectively [18]. These intrinsic properties that are close to the one of
glass fibre when relative to their density, are due to the high content of
highly oriented crystalline cellulose constituting the bulk of the fibre
[19,20]. Secondly, the environmental impact of growing and processing
the fibre is lower than for synthetic fibres, making flax fibre a choice in
line with environmental concerns [21]. Despite their specific charac-
teristics (natural variability [22], sensitivity to water [23], presence of
structural defects [24,25]), expanding the use of long flax fibres in ad-
ditive manufacturing would be a major step toward designing innova-
tive and complex bio-based composite structures.

To achieve this, material processability and fibre-matrix compati-
bility must be carefully considered to ensure high fibre volume fraction,
low porosity, and optimal load transfer [26]. Initially, flax elementary
fibres are (35.5 ± 16.3) mm long and (24 ± 8) µm thick [27] and must
be processed to obtain yarns. To design a continuous structure, it is
therefore necessary to go through spinning process, which consists of
transforming a very large number of independent fibres or fibres bundles
into a single continuous homogeneous filament.

Yarn cohesion is traditionally achieved by twisting individual fibres together [28], a well-established process in the textile industry to make fibres suitable for weaving or knitting. Given this precedent, researchers exploring long-fibre 3D printing have naturally considered this approach. For instance, Matsuzaki et al. [29] used twisted jute yarn combined with polylactic acid (PLA) to create composites with superior properties compared to pure polymer-based materials. Similarly, Terekhina et al. [30,31] successfully modified a 3D printing system to print continuous bleached flax yarns originally dedicated to garment industry, demonstrating performance competitive with glass-fibre-reinforced composites. Le Duigou et al. [32] further advanced this field by developing an innovative co-extrusion process, coating a flax yarn core with a PLA sheath to produce long-fibre composites. These yarns were also at the basis of the creation of bio-inspired, humidity-responsive 4D-printed biocomposites capable of shape transformation [33,34] and energy dissipation [35].

While these studies have significantly advanced 3D-printed com-
posites with enhanced mechanical properties, twisted yarns still present
a major challenge: they inherently reduce the expected fibre perfor-
mance [36]. This occurs for two main reasons: i. the fibres are mis-
aligned with the composite's load-bearing axis, leading to inefficient
load transfer, and ii. the twisted fibres prevent the resin from pen-
trating and therefore impregnating the core of the composite, which
results in significant porosity [37].

This work investigates the benefits of an optimized yarn for 3D
printing composites. Flax fibres were blended with PLA fibres before
spinning to enhance flax core impregnation. The commingled sliver was
then helically wrapped with two PLA filaments, providing cohesion to
the fibres with minimal twist. These optimized yarns are then printed
following the custom process developed by Terekhina et al. [30]: during
printing, they are heated in the nozzle, inducing the melting of the PLA
matrix embedded within the yarn structure. Simultaneously, additional
molten PLA is co-extruded during deposition, ensuring enhanced matrix
continuity and fibre impregnation. The objective is to optimize yarn
configuration for maximum mechanical performance by adjusting the
volume fractions of flax, commingled PLA, and wrapped PLA. Sliver and
yarn morphology were analysed, pores quantified via X-ray tomogra-
phy, and tensile tests assessed mechanical properties and fracture pro-
files using SEM imagery.

## 2. Materials and methods

2.1. Materials

Long flax fibres used in this study were sourced from Van Robaeys Frères (Normandy, France). The flax plants from FELICIE crops were sown in March, harvested after 100 days, dew-retted for 4 weeks, and processed via scutching and hackling to remove the woody core and align the fibres. A drawing process then produced a uniform sliver with a linear mass of 10 KTex (10 g/m). The fibres were processed at a controlled temperature of 20 °C and a constant relative humidity of 65 %, without any chemical treatment.

PLA fibres for the commingling process in the form of slivers were
supplied by Peignage Dumortier (Tourcoing, France) with a melting
temperature of 159.28 °C, a crystallinity of 27.4 %, a staple length of 6
cm, and a sliver linear mass of 20 KTex (20 g/m). The PLA multifilament
for wrapping, sourced from the Technical University of Liberec (Czech
Republic), had a melting temperature of 163.15 °C, a crystallinity of 9.4
%, consisted of 32 filaments, and had a total linear mass of 76 dTex
(0.0076 g/m).

PLA filament used for the FFF in-nozzle impregnation technique was
supplied by 3DFil (Carpentras, France). Its glass transition and melting
temperatures were 60 °C and 160 °C respectively, and a crystallinity of
6.9 %.

## 2.2. Commingled roving preparation

Flax and PLA fibres were blended using a 6-roller gill drawing system (Linimpianti, Villa D'Almè, Italy) presented in Fig. 1a. The slivers were combined at the inlet, stretched, and progressively blended over six passes to create a homogeneous sliver with a linear mass of 4 KTex (~4.0 g/m). Specific mass ratios of flax and PLA fibres (62/38, 50/50, 57/43 w/w) were prepared to meet study requirements. The slivers were then converted into rovings using a roving frame machine (Linimpianti, Villa D'Almè, Italy) with a draft reducing the linear mass to 3.0 g/m and a minimal twist of 26 turns/m, to give them a sufficient tensile resistance necessary for the wrap spinning process.

The various fibre blends were analysed to evaluate the process effi-
ciency and blend quality. Roving cross-sections were examined using a
VHX-6000 digital microscope (Keyence Corporation, Osaka, Japan).
Fibre quantification was performed with Fiji software (open-source
distribution of National Institutes of Health ImageJ, USA) using the
Trainable Weka Segmentation module, which segments fibres based on
shape and colour differences. This method offers precise fibre volume
quantification and comparison with the target volume fractions.

2.3. Wrap spinning

To produce a yarn without relying on the conventional spinning
process, which involves twisting the fibres, a Gualchieri wrap spinning
machine (Gualchieri & Gualchieri, Prato, Italy) is used and described in
Fig. 1b. In this alternative method, the flax/PLA commingled roving is
guided through a hollow spindle, where it is wrapped in a thermoplastic
multi-filament arranged in a helical pattern. This wrapping increases the
roving cohesion, essentially binding the fibres together into a more
stable structure.

The wrapping process was performed twice with Z-twist and S-twist
to maintain yarn structure balance. Controlled parameters such as
drafting, speed, and thermoplastic amount ensured the yarn's apparent
diameter that is compatible with the printing nozzle size, as the rigid flax
yarn cannot be extruded during printing.

In this study, four yarns were produced (Table 1). The first, FWPLA (Flax Wrapped PLA), serves as the reference yarn with a pure flax core. Two similar yarns, CO60/30/10 and CO50/40/10, were also produced, differing slightly in the PLA content of their cores. The final yarn, CO50/30/20, features twice as much PLA wrapping as the others.
<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Schematic representation of the 6-roller gill drawing system (b) Schematic representation of the wrap spinning process.</figcaption></figure>

<table><caption>**Table 1**

Yarn nomenclature, composition and count.</caption><thead><tr><th>Yarn ID</th><th>Core V<sub>f</sub> (%)</th><th>Wrapping V<sub>f</sub> (%)</th><th>Count (tex)</th></tr></thead><tbody><tr><td>FWPLA</td><td>85 % flax</td><td>15 % PLA</td><td>257</td></tr><tr><td>CO60/30/10</td><td>60 % flax + 30 % PLA</td><td>10 % PLA</td><td>210</td></tr><tr><td>CO50/40/10</td><td>50 % flax + 40 % PLA</td><td>10 % PLA</td><td>194</td></tr><tr><td>CO50/30/20</td><td>50 % flax + 30 % PLA</td><td>20 % PLA</td><td>226</td></tr></tbody></table>

## 2.4. Additive manufacturing

The composite specimens were printed at the Laboratoire Angevin de Mécanique, Procédés et innovAtion (LAMPA, Angers, France) using a machine customised and adapted for flax printing by Terekhina et al. [30]. The machine used for in-nozzle impregnation is a TEVO Black Widow (TevoUp, Shen Zhen Shi, China). In this work, 1 mm flathead nozzle is employed to compact and compress the composite filament, thereby reducing the presence of voids within the composite [31]. The process for composite production involves the supply of the optimised flax yarn and PLA filament via two separate channels, followed by mixing in the small, heated zone (linear distance of 2 mm) before extrusion from a conic flat-head nozzle (Fig. 2a).

The inclination of the channel in the conical head nozzle for the continuous flax yarn, followed by the small size of the heated zone before extruding, is a crucial parameter for successful printing. It is indeed important to keep away the flax fibres from degradation at exceeding temperatures. Thus, the fibre remains in the heated zone for a very short moment (adjusted to approximately 200 ms to insure a total melting of all PLA). The mixture is then extruded from the nozzle and deposited onto a heated plate.

A machine-readable G-code obtained by a RepetierHost software has been modified to consider the printing morphology. Fig. 2b shows a schematic view of printing parameters considered in the present study [8]. The optimized hatch distance (H, distance between two layers of yarns) and the layer height parameters were varied depending on the yarn count considered at [0.5–0.55] and [0.45–0.5] mm going from thinner to thicker yarn count respectively. Following numerous experimental trial results presented in the following sections, the temperature of the nozzle and heating plate was set at 197 °C and 75 °C, respectively. The fibre/filament feeding, and nozzle head speed were both set at 0.0882 mm.mm⁻¹ and 200 mm.min⁻¹ respectively. This speed is sufficient to ensure a slower cooling process, which facilitates interdiffusion between printed filaments and layers and enhances the interlaminar properties of printed samples.

To enhance the mechanical properties of the specimens and remove

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) In-situ impregnation technique developed at LAMPA (b) Schematic representation of the main printing parameters that influence the mechanical properties of bio-composite.</figcaption></figure>
pores as much as possible, a thermal compression process (30 min at 180 °C) was performed after composite printing and cooling. Samples were positioned within moulds that permitted the retention of their original length and width while facilitating a reduction in their thickness from 1.5 mm to 1 mm. During thermal compression, no loss of material was observed. Only the reduction in thickness of the plate was noted, while the printed composite remained intact. Consequently, the volume fraction was calculated with the new sample dimensions (see below Table 3). The estimated fibre volume ratio $V_f$ of the flax/PLA composites was determined using Eq. (1):

$$V_f = \frac{1}{\frac{\rho_f}{\rho_m} \left( \frac{1}{\tau_m^{-1}} \right) + 1} \quad (1)$$

where $\rho_f$ and $\rho_m$ are the respective densities of the fibres and the matrix and $\tau_m^f$ the fibre mass fraction. $\tau_m^f$ is calculated with Eq. (2):

$$\tau_m^f = \frac{Ntex_f L_f}{m_c} \quad (2)$$

where N is the number of layers in the printed composite, $tex_f$ the yarn count, $L_f$ the length of yarn used in one layer and $m_c$ the total mass of the sample.

## 2.5. Composites characterisation

A tomographic analysis assessed the effect of commingling on composite porosity using an RX Solution EasyTom 150 machine (RX Solutions, Chavanod, France) with a 150 kV micro-focus X-ray generator. Imaging utilized detectors with resolutions up to 4032x2688 pixels and pixel sizes of 6.58 µm, managed by X-Act 64 software for precise 3D reconstruction. The reconstructed data was analysed using Avizo 2021.1 (Thermo Fisher Scientific, Waltham, USA). Pores were categorized as either process voids (between yarns, linked to deposition strategy) and intra-yarn voids (Fig. 3a). Due to similar densities, PLA and flax fibres were treated as a single entity. Tomographic scans were performed before and after thermal compression to visualize and quantify pore types accurately (Fig. 3b).

Composite mechanical properties were tested using an Instron 3366 Universal Testing Machine (Instron, Norwood, USA) with a 10 kN load cell and video extensometer. Tensile tests at 2 mm/min were conducted at 22 °C and 47 % humidity. Strains were recorded between two dark markers 50 mm apart. Specimen thickness and width were measured

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) Pores segmentation methodology (b) composite samples with matrix and fibres rendered transparent to highlight the internal pores.</figcaption></figure>

three times with a ±0.01 mm precision digital caliper to calculate cross-sectional areas. At least three specimens per configuration were tested to determine Young's modulus and tensile strength.

Experimental tensile properties are compared to the theoretical trend values. The theoretical Young's modulus $E_{th}$ is obtained by the rule of mixtures (if one considers the fibres as parallel to each other's in the direction of the yarn) as shown in Equation (3). The theoretical tensile strength $\sigma_{th}$ is estimated by accounting for the fibre and matrix contributions, fibre volume fraction, porosity, and load transfer efficiency, as detailed in Equation (4), under the assumption of quasi-brittle flax fibres and a quasi-ductile PLA matrix.

$$E_{th} = [E_f V_f + E_m (1 - V_f)] [1 - P] \# \quad (3)$$

$$\sigma_{th} = [\sigma_f V_f + (1 - V_f) \sigma_m E_f / E_m] [1 - P] \# \quad (4)$$

where $E_f$ and $\sigma_f$ are flax theoretical Young's modulus and tensile strength, respectively 50 GPa and 600 MPa, $E_m$ and $\sigma_m$ are PLA matrix theoretical Young's modulus and tensile strength, respectively 3 GPa and 60 MPa, $V_f$ is the flax fibres volume and P is the experimentally measured porosity of composite samples. Given the low torsion given to flax fibres and the parallelism with respect to the tensile axis, an elastic modulus of 50 GPa was considered for flax.

# 3. Results and discussion

## 3.1. Commingling efficiency

To assess the commingling process with parameters such as dispersion of fibres and fibre organisation within the yarn, optical analysis was performed on three flax/PLA slivers. Visual inspection showed a homogeneous blend with uniform colour and texture after six gill passes. To analyse core impregnation, digital microscopy micrographs display cross-sections for blends of 50/50 (Fig. 4a), 57/43 (Fig. 4b), and 62/38 (Fig. 4c).

The materials (flax fibres and PLA fibres) are easily distinguishable due to the differences in fibre shape and size: PLA fibres are round and uniform, while flax fibres are elliptical and vary in size. The lumen, a cavity running through the flax fibres, is visible in some cases, though the small associated pores are difficult to quantify without specialized tools [38]. Although the fibres appear to be well blended overall, the first thing to note is that the flax fibres are relatively concentrated in the form of technical fibres, which is defined as a bundle of about ten fibres bound together by a middle lamella (Fig. 4a yellow box). The separation of the bundles into elementary fibres is a complex process which can be facilitated by retting and chemical processes [39]. However, these methods may not be ideal for composite applications, as they can degrade the crystalline cellulose responsible for the high tensile properties of flax fibres. Drawing process also plays a role in this separation, but it appears that the several passes in this work were not sufficient to ensure an optimum separation of the fibres, which could be detrimental for the impregnation by the PLA resin. Moreover, it is possible to observe zones mainly composed of PLA fibres clusters (Fig. 4a blue box). This high PLA concentration areas can be explained by three hypotheses:

i. the blend in question (w/w 50/50 sample) is the one with the higher PLA volume fraction (50 %),

ii. the technical fibres cohesion prevents the penetration of the PLA fibres within the elementary flax fibres and,

iii. the diameter of the PLA fibres (22.53 ± 2.08) µm is larger than the diameter of the flax fibres (17.07 ± 5.84) µm, which again limits the mixing of the two materials.

Finally, for the w/w 57/43 sample, a more homogeneous blend can be observed (Fig. 4b red box), with individual PLA fibres surrounded by flax, whereas for the w/w 62/38 sample, wide zones of flax can be
<figure><img src="image_6.png" /><figcaption>Fig. 4. (a-c) Optical micrograph of sliver sections for each of the 3 different flax/PLA blend.</figcaption></figure>

identified (Fig. 4c green box).

Image analysis quantified flax and PLA volume content by examining sliver sections and fibres surface, and the corresponding weight content was then calculated using the materials densities (1.5 for flax and 1.26 for PLA). Results show an average flax contents of (48.06 ± 1.85) % for w/w 50/50, (56.65 ± 2.02) % for w/w 57/43, and (60.37 ± 2.77) % for w/w 62/38, slightly below targets. The discrepancy may result from flax fibre loss during gill machine drawing due to its sensitivity to hygroscopic conditions. Working under controlled conditions with increased humidity levels could help reduce these flax fibre losses. Corrected values will be used for composite fibre content measurements.

The blending process with gills machine participate to the individualisation of flax elementary fibres and thus produce a homogeneous mix of flax and PLA fibres. The fibres were passed through the gills six times, thus additional passes could further enhance fibre dispersion, reduce local heterogeneities and further improve load transfer between matrix and reinforcement. Improved flax retting and additional chemical or physical treatments could help to address this issue [40,41].

## 3.2. Yarn morphological properties

Digital micrographs reveal the specimens' external structures. The PLA wrapping filament appears white and slightly transparent. In the FWPLA sample, the beige core consists mostly of flax, with brown patches from poorly retted fibres (Fig. 5a). For CO60/30/10 and CO50/40/10 yarns, the light beige core reflects mixed flax and PLA fibres, with

surface cross-linking PLA filaments visible (Fig. 5c-d). In contrast, CO50/30/20 shows a fully PLA-covered surface, obscuring the core fibres (Fig. 5e).

Yarn sample diameters were measured via digital microscopy. Values are presented in Fig. 5g in a box plot form to display the variability. These apparent diameters include voids between untwisted fibres. As the yarn length cannot be increased, its diameter must match the nozzle size together with sufficient yarn tension to prevent fibre clogging within the nozzle during 3D printing with the device presented earlier.

Among the yarns, FWPLA showed the lower diameter variation, likely due to using uniform roving without commingling. This suggests that the variation in diameter may be influenced by both the quality and consistency of the roving and the effects of the wrapping process on the fibres. The PLA multi-filament wrapping can create a compressing effect at specific points, causing the fibres to swell slightly after a wrapping turn (Fig. 5b). Finally, the CO50/30/20 yarn showed the greatest diameter variability, likely caused by the local accumulation of PLA multi-filament around the yarn (Fig. 5f). However, since the PLA will melt during the additive manufacturing process, this variation is not expected to negatively affect the deposition process.

The target yarn diameter of 0.6 mm is thus achieved. This is a remarkable achievement given the absence of flax fibre twisting during manufacturing. Overall, while some diameter variability was observed, it is unlikely to create a significant issue in the context of 3D printing, especially considering that the material will be melted and reformed during deposition.

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a-f) Confocal micrographs of the yarn specimen with magnification (g) distribution of the apparent diameters of the yarns measured with optical microscopy.</figcaption></figure>
### 3.3. Determination of optimal printing parameters

Table 2 summarizes the tensile properties of the biocomposites after printing and before consolidation by thermal compression. The hatch distance and layer height were optimized for the selected printing speed and feed rate. For both comingled and dry flax yarn reinforced considered composites (CO50/40/10 and FWPLA respectively), the increasing hatch distance from 0.5 to 1.1 mm results in a significant reduction of tensile properties. The same trend is observed with increasing of layer height (from 0.5 to 1.1 for FWPLA) that is usually linked with increasing of pore distribution at the interface due to poorer inter-bonding strength leading to lower interfacial properties [30]. The chosen printing parameters for the rest of the study correspond to the configuration giving the best mechanical properties for each yarn, as shown in Table 2. However, the best tensile properties are found for the biocomposites CO60/30/10 and CO50/30/20 compared to FWPLA one (Table 2). That difference could be explained by more homogeneous distribution of the PLA polymer inside the flax yarn (for CO60/30/10), as also its better impregnation with more wrapped PLA (for CO50/30/20) and therefore, diffusion with melted PLA filament in the impregnation nozzle, regarding to the only dry flax wrapped PLA biocomposite. In the latter, the highest yarn count prevents the best impregnation of the fibres by the matrix and, consequently, reduces the mechanical properties.

### 3.4. Printing quality and porosity content

X-ray micro-computed tomography (X-ray µ-CT) was performed to assess the morphology of yarn layers and quantify porosity within the printed composites. In the tomographic images, material densities influence brightness: fibres appear the brightest, voids the darkest, and the polymer matrix is displayed as an intermediate grey level. The voxel size for the scans was set at 6.58 µm, which may limit the detection of the smallest pores. Despite this constraint, the comparative analysis between samples with varying compositions offers valuable informations about the distribution of fibres and voids. The tomography images provide a detailed view of the internal composite structure after consolidation, as shown in Fig. 6 for all four sample types.

First, the layout and shape of the yarns after printing are discussed. The longitudinal images of the samples show that the flax fibres are aligned with the direction of yarn deposition. This is due to the untwisted nature of the fibres, a result from the wrap-spinning process. The longitudinal sections, shown in Fig. 6a, c, e and g also highlight the quality of yarn positioning. While FWPLA yarns are mostly aligned with the printing axis, the positioning of the optimised yarns is less regular. This can be attributed to the incorporation of PLA fibres in the yarn core, which increases fibre mobility, creating a more flexible medium that

<caption>**Table 2**</caption>

Tensile properties for different printing parameters.

<table><caption>**Table 3**</caption><thead><tr><th>Yarn ID</th><th>V<sub>f</sub> (%)</th><th>H, mm</th><th>l<sub>b</sub>, mm</th><th>V, mm/min</th><th>Feed rate, mm/mm</th><th>E, GPa</th><th>σ<sub>ult</sub>, MPa</th></tr></thead><tbody><tr><td rowspan="5">FWPLA</td><td>36.35</td><td>0.55</td><td>0.5</td><td rowspan="5">200</td><td rowspan="5">0.0882</td><td>5.3</td><td>61 ±</td></tr><tr><td>± 1.5</td><td>1.1</td><td>0.5</td><td>± 1.1</td><td>5</td></tr><tr><td>35.75</td><td rowspan="3">0.55</td><td rowspan="3">1.1</td><td>2.0</td><td>61 ±</td></tr><tr><td>± 0.2</td><td>± 0.2</td><td>2</td></tr><tr><td>35.75</td><td>3.1</td><td>40 ±</td></tr><tr><td>± 0.2</td><td>± 0.2</td><td>± 0.3</td><td>2</td><td></td><td></td><td></td></tr><tr><td rowspan="2">CO60/<br>30/10</td><td>22.65</td><td>0.5</td><td>0.45</td><td rowspan="2">200</td><td rowspan="2">0.0882</td><td>7.8</td><td>80 ±</td></tr><tr><td>± 0.07</td><td></td><td></td><td>± 0.3</td><td>4</td></tr><tr><td rowspan="2">CO50/<br>40/10</td><td>17.55</td><td>0.5</td><td>0.45</td><td rowspan="2">200</td><td rowspan="2">0.0882</td><td>6.9</td><td>68 ±</td></tr><tr><td>± 0.21</td><td>0.55</td><td>0.5</td><td>± 0.2</td><td>2</td></tr><tr><td rowspan="4"></td><td>17.70</td><td rowspan="2">0.7</td><td rowspan="2">0.5</td><td rowspan="4">200</td><td rowspan="4"></td><td>6.0</td><td>61 ±</td></tr><tr><td>± 0.36</td><td>± 0.2</td><td>4</td></tr><tr><td>17.00</td><td>3.0</td><td>53 ±</td></tr><tr><td>± 0.30</td><td>± 0.2</td><td>1</td></tr><tr><td rowspan="2">CO50/<br>30/20</td><td>18.15</td><td rowspan="2">0.55</td><td rowspan="2">0.5</td><td rowspan="2">200</td><td rowspan="2"></td><td>7.4</td><td>69 ±</td></tr><tr><td>± 0.78</td><td>± 0.7</td><td>2</td></tr></tbody></table>

Theoretical flax volume fraction (%) of the composite samples and tensile properties with experimental Young's modulus (GPa) and ultimate tensile strength (MPa) compared to the theoretical values, and elongation at maximum strength (%).

<table><thead><tr><th>Sample ID</th><th>V<sub>f-th</sub> (%)</th><th>E<sub>exp</sub> (GPa)</th><th>E<sub>exp</sub> / E<sub>th</sub></th><th>σ<sub>exp</sub> (MPa)</th><th>σ<sub>exp</sub> / σ<sub>th</sub></th><th>ε<sub>ult</sub> (%)</th></tr></thead><tbody><tr><td rowspan="2">FWPLA</td><td>43.92</td><td>14.49</td><td rowspan="2">68.15 %</td><td>97.48 ±</td><td rowspan="2">38.20 %</td><td>1.57</td></tr><tr><td>± 0.95</td><td>± 0.52</td><td>8.75</td><td>± 0.12</td></tr><tr><td rowspan="2">CO60/<br>30/10</td><td>27.03 ±</td><td>10.96</td><td rowspan="2">73.35 %</td><td>110.25</td><td rowspan="2">61.49 %</td><td>1.22</td></tr><tr><td>0.21</td><td>± 0.01</td><td>± 6.55</td><td>± 0.48</td></tr><tr><td rowspan="2">CO50/<br>40/10</td><td>21.17 ±</td><td>8.97 ±</td><td rowspan="2">72.55 %</td><td>93.93 ±</td><td rowspan="2">63.31 %</td><td>1.16</td></tr><tr><td>0.21</td><td>0.86</td><td>3.56</td><td>± 0.27</td></tr><tr><td rowspan="2">CO50/<br>30/20</td><td>22.38 ±</td><td>12.20</td><td rowspan="2">93.34 %</td><td>92.64 ±</td><td rowspan="2">59.04 %</td><td>1.04</td></tr><tr><td>0.53</td><td>± 0.47</td><td>3.18</td><td>± 0.10</td></tr></tbody></table>

promotes yarn deformation. In contrast, the stiffer FWPLA yarn tends to maintain a straighter configuration, which may negatively affect the print quality of more complex shapes, such as curves. Fig. 6b, d, f and h show modifications of the initial cylindrical morphology. The elliptical shape of the yarns after printing, resulting from a reduction in the height of the specimen and their expansion in the width direction, is a consequence of the compression load applied by the nozzle.

Regarding the presence of voids, a considerable number of longitudinal pores are evidenced in the core and on the sides of the long fibre's yarns of FWPLA sample (Fig. 6a). The cross-section (Fig. 6b) corroborates the observation of a multitude of small pores between the fibres and around the yarn. The incorporation of PLA into the core of flax fibres (comingling), as achieved through the utilisation of optimised filaments, diminish the prevalence of pores. Indeed, the images of the CO60/30/10 (Fig. 6c, d) and CO50/40/10 (Fig. 6e, f) composites demonstrate a more uniform cross-section and exhibit reduced number of pores in the core of the yarns. This is particularly visible in the CO50/40/10 sample, which has a higher PLA content in the core of the filament and a markedly homogeneous cross-section. In this sample, the flax fibres are the most readily discernible due to their light shade (Fig. 6f). Nevertheless, pores are still discernible around the yarns, which appear to be the consequence of inadequate impregnation by the PLA incorporated during the printing process. The last sample, CO50/30/20 (Fig. 6g, e), which has a higher rate of PLA wrap to prevent this poor adhesion effect and the presence of pores surrounding, does not exhibit a discernible improvement. It can thus be inferred that the level of PLA fibres present in the core of the yarn has a favourable impact on the compatibility between the fibres and the matrix. To conclude on the optimum quantity of wrapping yarn, it is necessary to look at the porosity inside the samples.

Porosity was measured using image analysis of tomographic reconstructions for each sample, both before and after thermal compression (Fig. 7). A comparison of the two graphs shows that thermal compression significantly reduces voids, especially process-induced pores. This underscores the need to improve the deposition stage for better yarn application during printing. Currently, pressure is applied solely through the nozzle; incorporating a roller system [42] could reduce pores and achieve low porosity without additional consolidation.

For intra-yarn porosity, the trend is consistent across both graphs and supports the conclusions drawn from visual inspection of tomographic slice images. The FWPLA sample exhibits the highest porosity (9.75 % after consolidation), while the optimised yarns (comingled yarns) show a clear reduction in porosity as the PLA content increases, with values of 4.69 % for CO60/30/10 and 3.71 % for CO50/40/10. The CO50/30/20 sample has slightly lower intra-yarn porosity (2.87 %) compared to the other yarns. This may be due to the additional PLA surrounding the yarn, which melts during printing, enhancing impregnation by the remaining molten PLA during deposition.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** X-ray µ-CT image slices in the gauge section of (a, b) FWPLA specimen (c, d) CO60/30/10 specimen (e,f) CO50/40/10 specimen (g, h) CO50/30/20 specimen (a, c, e, g) longitudinal view and (b, d, f, h) transversal view. Images of samples after consolidation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Porosity measured after X-ray µ-CT segmentation for samples before and after consolidation.</figcaption></figure>

### 3.5. Composites tensile properties

The final theoretical composition of composite samples after printing and thermal compression is presented in Table 3, accompanied by the associated tensile properties.

In commingled yarns with originally 50 % weight ratio flax in the core, the final flax volume fraction is (21.17 ± 0.21) % for CO50/40/10 and (22.38 ± 0.53) % for CO50/30/20. This discrepancy may stem from

the manufacturing process (including the addition of molten PLA during printing) or irregularities along the yarn length. Fibre volume fraction calculations are based on the average linear mass (Eq. (2)) and can vary due to changes in apparent diameter (Fig. 5). The CO60/30/10 sample shows a higher flax fraction at (27.03 ± 0.21) % due to its greater initial flax content. The FWPLA sample exhibits the highest flax fraction at (43.92 ± 0.95) %, reflecting its flax-only core composition. The representative tensile curves for the four samples exhibit two distinct zones and non-linear behaviour (Fig. 8a).

This phenomenon is relatively common and frequently observed in the literature for composites comprising flax reinforcements. This non-linearity can be attributed to the scale of the fibre, whereby the cellulose fibrils undergo realignment. Furthermore, the viscous behaviour may be attributed to the presence of lignin and amorphous zones in the cellulose [43]. The issues of impregnation at the core of the yarns and the slippage of the yarns in relation to each other may also be considered as a potential explanation for the observed softening of the tension slope in the samples.

Fig. 8b presents the tensile Young's modulus and strength as a function of fibre volume fraction for post-consolidated composites. For comparison, trend lines from the rule of mixtures and relevant literature data are also included. Images of the fracture surfaces of the various composite test samples are presented in Fig. 9.

The FWPLA sample exhibits the highest mechanical properties. However, the gap between theoretical and experimental values remains significant, with the experimental tensile modulus and strength reaching only 68.15 % and 38.20 % of their expected values, respectively. This deviation can be attributed to internal pores (Fig. 6) and the occurrence of significant fibre debonding (Fig. 9a) caused by dry zones when subjected to tensile forces. As a result, the modulus decreases due to incomplete load transfer between the resin and fibres. Despite its high fibre content, the composite lacks efficient load transfer, leading to a substantial reduction in Young's modulus. Additionally, critical defects within the yarns (Fig. 7) likely contribute to the significantly lower experimental tensile strength compared to theoretical predictions.

The CO60/30/10 sample shows a higher ratio between theoretical and experimental values for both Young's modulus and ultimate tensile strength. This improvement can be attributed to the commingling effect, with 30 % PLA fibres in the yarn core reducing the discrepancy. However, fibre debonding remains evident (Fig. 9b), likely due to insufficient
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** (a) Stress–strain curves of the unidirectional composite. (b) Young's modulus (GPa) and stress at break (MPa) as a function of fibre volume fraction in this study. Trend values (rule of mixtures) and data from other studies on 3D-printed composites with natural long fibres are included for comparison.</figcaption></figure>

PLA within the core. Additionally, debonding and fibre pull-out are observed (Fig. 9e), caused by pores at the yarn-matrix interface.

For a similar fibre content, the difference in mechanical properties between the CO50/40/10 and CO50/30/20 samples can be attributed to the distinct PLA distribution within the yarns. The CO50/40/10 yarn, with a higher proportion of commingled PLA fibres, exhibits a sharper failure profile (Fig. 9c) and more fibre breaks (Fig. 9f), indicating

improved core impregnation and better load transfer between fibres and matrix. However, delamination between deposited layers is observed, which reduces load transfer within the composite and introduces defects that limit tensile strength.

In contrast, the CO50/30/20 sample shows no delamination, with only minor debonding (Fig. 9d). This is likely due to the higher amount of wrapped PLA, which enhances fibre cohesion and minimizes dry zones within and between yarns. As a result, the experimental-to-theoretical Young's modulus ratio is notably high and considered satisfactory. However, the ratio for tensile strength remains modest and requires further improvement. Large pores at the yarn-matrix interface, likely caused by the external PLA source, may be responsible for this limitation.

Thus, it appears that wrapping PLA around the yarns with the higher proportion leads to better mechanical load transfer between the flax fibres and the intra yarn and wrapped PLA because the yarn is tighter and contains less internal pores. Regarding the strength, the different levels of wrapping for the comingled yarns did not change the values of tensile strength very much. Large pores are visible at the interface between the PLA matrix from the yarn (wrapped PLA) and the PLA brought by the filament in the nozzle. This may be explained by the fact that different PLA grades are used in the present work. When fusing different PLA grades, variations in melting temperature and viscosity can lead to uneven flow, poor interlayer adhesion, and void formation. Thermal mismatches may introduce residual stresses, causing microcracks or delamination. Differences in crystallinity and molecular weight can further affect diffusion at the interface, preventing full polymer integration and leading to structural weaknesses [44,45].

## 4. Conclusion

A study on the additive manufacturing of optimised yarns composed of continuous flax fibres was conducted. The composition and structure of the yarns were controlled through spinning processes, allowing an investigation into their influence on the quality of the printed composite. The core impregnation of the flax fibres was achieved through the utilisation of the commingling process, while the preservation of fibre alignment was enabled by the integration of wrap spinning technology. The yarns were used as input material into a customised 3D printing machine, and composite specimens were produced. The composite manufactured from the different yarns were then compared. The main conclusions are proposed below:

* The process of commingling flax fibre with PLA using the gills system has produced a highly intimate and uniform mixture of the two materials. The careful integration of these materials not only enhances their compatibility but also optimises their individual properties, creating a more cohesive and functional material.

* The process of wrapped spinning has enabled the fibres to be kept as parallel as possible to the filament axis while a matrix is applied around the yarn, resulting in a small diameter for untwisted yarns that can be printed with precision.

* The incorporation of PLA both within and around the yarns, alongside the implementation of post-print consolidation techniques, has successfully reduced the porosity within the yarns leading to a good load transfer between the matrix and the flax fibres as illustrated by the high ratio between the experimental and the theoretical young's modulus for the best compromise of core comingling and wrapping by PLA fibres.

These findings confirm that incorporating matrix material within and around the yarn enhances fibre impregnation and overall composite performance. Refining the printing process with finer yarns could improve fibre alignment, reduce porosity, and enhance load transfer, ultimately increasing the tensile strength of the composite. Additionally, using an analogous PLA grade for commingling, wrapping and matrix
<figure><img src="image_11.png" /><figcaption>Fig. 9. Images revealing the fracture surfaces of the composite samples (a-d) confocal micrographs and (e, f) SEM micrographs.</figcaption></figure>

deposition would likely improve interfacial performances between the different PLA based components, minimizing pore formation at the yarn-matrix interface and between printed layers. This would lead to better mechanical integrity and more consistent properties. Future work could include flexural testing to further investigate the influence of fibre volume fraction and porosity on the mechanical behaviour of these 3D printed composites. It would probably show that comingling that has for effect to reduce the intra-yarn porosity is beneficial for improving bending properties. Finally, further research should investigate optimized processing parameters and the fabrication of complex geometries without post consolidation to expand the application range of flax fibre-reinforced biocomposites.

### CRediT authorship contribution statement

**Delphine Quereilhac:** Writing – original draft, Visualization, Investigation. **Thomas Caillault:** Writing – review & editing, Software, Investigation. **Svetlana Terekhina:** Writing – review & editing, Validation, Software, Resources, Methodology. **Mahadev Bar:** Writing – review & editing, Investigation, Conceptualization. **Guillaume Morel:** Writing – review & editing, Investigation. **Marina Fazzini:** Writing – review & editing, Investigation. **Marwa Abida:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization. **Emma-nuel De Luycker:** Writing – review & editing, Supervision, Resources, Conceptualization. **Pierre Ouagne:** Writing – review & editing, Supervision, Resources, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

The authors acknowledge the financial support from the French National Research Agency (ANR) through the FLOEME project ANR-21-CE10-0008. They also express their gratitude to Brigita Kolcavova from the Faculty of Textile Engineering at the Technical University of Liberec (Czechia) for generously providing the PLA multifilament used for yarn wrapping.

### Data availability

Data will be made available on request.

### References

[1] A. Boisseau, C. Peyrac, Long term durability of composites in marine environment: comparative study of fatigue behavior, Proc. Eng. 133 (2015) 535-544, https://doi.org/10.1016/j.proeng.2015.12.627.

[2] J.R.C. Dizon, A.H. Espera, Q. Chen, R.C. Advincula, Mechanical characterization of 3D-printed polymers, Addit. Manuf. 20 (2018) 44-67, https://doi.org/10.1016/j.addma.2017.12.002.

[3] T.D. Ngo, A. Kashani, G. Imbalzano, K.T.Q. Nguyen, D. Hui, Additive manufacturing (3D printing): a review of materials, methods, applications and challenges, Compos. B Eng. 143 (2018) 172-196, https://doi.org/10.1016/j.compositesb.2018.02.012.

[4] K.V. Wong, A. Hernandez, A review of additive manufacturing, ISRN Mech. Eng. 2012 (2012) 1-10, https://doi.org/10.5402/2012/208760.

[5] C.S. Lee, S.G. Kim, H.J. Kim, S.H. Ahn, Measurement of anisotropic compressive strength of rapid prototyping parts, J. Mater. Process. Technol. 187-188 (2007) 627-630, https://doi.org/10.1016/j.jmatprotec.2006.11.095.

[6] A. Ghorpade, K.P. Karunakaran, M.K. Tiwari, Selection of optimal part orientation in fused deposition modelling using swarm intelligence, Proc. Inst. Mech. Eng. B J. Eng. Manuf. 221 (2007) 1209-1219, https://doi.org/10.1243/09544054JEM771.

[7] S. Terekhina, T. Tatiana, E. Sergeï, S. Innokentiy, G. Laurent, H. Lamine, Flexural quasi-static and fatigue behaviours of fused filament deposited PA6 and PA12 polymers, Int. J. Adv. Manuf. Technol. 117 (2021) 2041-2048, https://doi.org/10.1007/s00170-021-07223-y.

[8] S. Terekhina, T. Tarasova, S. Egorov, I. Skornyakov, L. Guillaumat, M.L. Hattali, The effect of build orientation on both flexural quasi-static and fatigue behaviours of filament deposited PA6 polymer, Int. J. Fatigue 140 (2020) 105825, https://doi.org/10.1016/j.ijfatigue.2020.105825.
[9] A. Joseph Arockiam, K. Subramanian, R.G. Padmanabhan, R. Selvaraj, D.K. Bagal, S. Rajesh, A review on PLA with different fillers used as a filament in 3D printing, Mater. Today: Proc. 50 (2022) 2057–2064, https://doi.org/10.1016/j.matpr.2021.09.413.

[10] W. Zhong, F. Li, Z. Zhang, L. Song, Z. Li, Short fiber reinforced composites for fused deposition modeling, Mater. Sci. Eng. A 301 (2001) 125–130, https://doi.org/10.1016/S0921-5093(00)01810-4.

[11] P. Parandoush, D. Lin, A review on additive manufacturing of polymer-fiber composites, Compos. Struct. 182 (2017) 36–53, https://doi.org/10.1016/j.compstruct.2017.08.088.

[12] G.W. Melenka, B.K.O. Cheung, J.S. Schofield, M.R. Dawson, J.P. Carey, Evaluation and prediction of the tensile properties of continuous fiber-reinforced 3D printed structures, Compos. Struct. 153 (2016) 866–875, https://doi.org/10.1016/j.compstruct.2016.07.018.

[13] P. Bettini, G. Alitta, G. Sala, L. Di Landro, Fused deposition technique for continuous fiber reinforced thermoplastic, J. Mater. Eng. Perform 26 (2017) 843–848, https://doi.org/10.1007/s11665-016-2459-8.

[14] Z. Hou, X. Tian, J. Zhang, L. Zhe, Z. Zheng, D. Li, et al., Design and 3D printing of continuous fiber reinforced heterogeneous composites, Compos. Struct. 237 (2020) 111945, https://doi.org/10.1016/j.compstruct.2020.111945.

[15] A.N. Netravali, S. Chabba, Composites Get Greener, Fiber Science Program Cornell University, 2003.

[16] W. van Zeist, J.A.H. Bakker-Heeres, Evidence for linseed cultivation before 6000 bc, J. Archaeol. Sci. 2 (1975) 215–219, https://doi.org/10.1016/0305-4403(75)90059-X.

[17] E. Richely, A. Bourmaud, V. Placet, S. Guessasma, J. Beaugrand, A critical review of the ultrastructure, mechanics and modelling of flax fibres and their defects, Prog. Mater. Sci. 124 (2022) 100851, https://doi.org/10.1016/j.pmatsci.2021.100851.

[18] T. Jeannin, G. Arnold, A. Bourmaud, S. Corn, E. De Luycker, P.J.J. Dumont, et al., A round-robin study on the tensile characterization of single fibres: a multifactorial analysis and recommendations for more reliable results, Compos. A Appl. Sci. Manuf. 185 (2024) 108323, https://doi.org/10.1016/j.compositesa.2024.108323.

[19] T. Chernova, P. Mikshina, A. Petrova, N. Ibragimova, M. Ageeva, T. Gorshkova, Rhamnogalacturonan I with β-(1,4)-galactan side chains as an ever-present component of tertiary cell wall of plant fibers, IJMS 24 (2023) 17253, https://doi.org/10.3390/ijms242417253.

[20] E. Richely, A. Zarei, A. Melelli, D.K. Rajan, J. Govilas, X. Gabrion, et al., Measurement of microfibril angle in plant fibres: Comparison between X-ray diffraction, second harmonic generation and transmission ellipsometry microscopies, Composites, Part C: Open Access 11 (2023) 100355, https://doi.org/10.1016/j.jcomsc.2023.100355.

[21] A. Le Duigou, P. Davies, C. Bailey, environmental impact analysis of the production of flax fibres to be used as composite material reinforcement, J. Biobased. Mat. Bioenergy 5 (2011) 153–165, https://doi.org/10.1166/jbbm.2011.1116.

[22] D. Scida, A. Bourmaud, C. Bailey, Influence of the scattering of flax fibres properties on flax/epoxy woven ply stiffness, Mater. Des. 122 (2017) 136–145, https://doi.org/10.1016/j.matdes.2017.02.094.

[23] P. Chatron-Michaud, V. Trillaud, B. Lesaint, J. Adrien, J. Beaugrand, P. Steyer, Towards dynamic in situ observation of flax fibres under varying water contents through environmental SEM and X-ray tomography, Ind. Crop Prod. 215 (2024) 118654, https://doi.org/10.1016/j.indcrop.2024.118654.

[24] D. Quereilhac, E. De Luycker, S. Guessasma, M. Abida, J. Perrin, T. Weitkamp, et al., Synchrotron X-ray microtomography and finite element modelling to uncover flax fibre defect's role in tensile performances, Compos. A Appl. Sci. Manuf. 184 (2024) 108276, https://doi.org/10.1016/j.compositesa.2024.108276.

[25] L. Morgillo, C. Goudenhooft, A. Melelli, S. Durand, M. Abida, J. Beaugrand, et al., Impact of flax fibre kink-bands on local longitudinal mechanical properties and cell wall ultrastructure, Compos. A Appl. Sci. Manuf. 191 (2025) 108754, https://doi.org/10.1016/j.compositesa.2025.108754.

[26] B. Madsen, M. Aslan, H. Lilholt, Fractographic observations of the microstructural characteristics of flax fibre composites, Compos. Sci. Technol. 123 (2016) 151–162, https://doi.org/10.1016/j.compscitech.2015.12.003.

[27] K. Charlet, J.P. Jernot, S. Eve, M. Gomina, J. Bréard, Multi-scale morphological characterisation of flax: from the stem to the fibrils, Carbohydr. Polym. 82 (2010) 54–61, https://doi.org/10.1016/j.carbpol.2010.04.022.

[28] D.U. Shah, P.J. Schubel, M.J. Clifford, Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites, J. Compos. Mater. 47 (2013) 425–436, https://doi.org/10.1177/0021998312440737.

[29] R. Matsuzaki, M. Ueda, M. Namiki, T.-K. Jeong, H. Asahara, K. Horiguchi, et al., Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation, Sci. Rep. 6 (2016) 23058, https://doi.org/10.1038/srep23058.

[30] S. Terekhina, S. Egorov, T. Tarasova, I. Skornyakov, L. Guillaumat, M.L. Hattali, In-nozzle impregnation of continuous textile flax fiber/polyamide 6 composite during FFF process, Compos. A Appl. Sci. Manuf. 153 (2022) 106725, https://doi.org/10.1016/j.compositesa.2021.106725.

[31] Terekhina S, Hattali L. Three-dimensional printing of continuous plant fiber composites. Chapter 4 - Additive Manufacturing of Polymer-Based Composite Materials. Elsevier Ltd, 2024, p. 107–45.

[32] A. Le Duigou, A. Barbé, E. Guillou, M. Castro, 3D printing of continuous flax fibre reinforced biocomposites for structural applications, Mater. Des. 180 (2019) 107884, https://doi.org/10.1016/j.matdes.2019.107884.

[33] C. De Kergariou, A. Le Duigou, A. Perriman, F. Scarpa, Design space and manufacturing of programmable 4D printed continuous flax fibre polylactic acid composite hygromorphs, Mater. Des. 225 (2023) 111472, https://doi.org/10.1016/j.matdes.2022.111472.

[34] M. Josselin, M. Castro, N. Di Cesare, F. Scarpa, A. Le Duigou, Sustainable 4D printed meta biocomposite materials for programmable structural shape changing, Adv. Mater. 37 (2025), https://doi.org/10.1002/adma.202418656.

[35] P.L. Pichard, L. Maheo, J. Dirrenberger, M. Castro, U. Lafont, A. Le Duigou, Efficient energy-dissipative bioinspired architecture composite materials with high mechanical properties, Addit. Manuf. 109 (2025) 104875, https://doi.org/10.1016/j.addma.2025.104875.

[36] A. Le Duigou, D. Correa, M. Ueda, R. Matsuzaki, M. Castro, A review of 3D and 4D printing of natural fibre biocomposites, Mater. Des. 194 (2020) 108911, https://doi.org/10.1016/j.matdes.2020.108911.

[37] F. Bensadoun, I. Verpoest, J. Baets, P. Davies, M. Gomina, A. Kervoelen, et al., Impregnated fibre bundle test for natural fibres used in composites, J. Reinf. Plast. Compos. 16 (2017).

[38] E. Guillou, M. Bar, M. Scheel, T. Falher, T. Weitkamp, D.U. Shah, et al., Use of a commingling process for innovative flax fibre reinforced unidirectional composites, Compos. B Eng. 270 (2024) 111150, https://doi.org/10.1016/j.compositesb.2023.111150.

[39] B. Chabbert, J. Padovani, C. Djemiel, J. Ossemond, A. Lemaître, A. Yoshinaga, et al., Multimodal assessment of flax dew retting and its functional impact on fibers and natural fiber composites, Ind. Crop Prod. 148 (2020) 112255, https://doi.org/10.1016/j.indcrop.2020.112255.

[40] Z. Yang, Z. Zhang, Y. Long, K. Fu, Y. Li, Process optimization of continuous flax fiber/polylactic acid prepreg filaments toward high performance 3D-printed composites, Polym. Compos. 44 (2023) 6242–6253, https://doi.org/10.1002/pc.27559.

[41] I. Van De Weyenberg, J. Ivens, A. De Coster, B. Kino, E. Baetens, I. Verpoest, Influence of processing and chemical treatment of flax fibres on their composites, Compos. Sci. Technol. 63 (2003) 1241–1246, https://doi.org/10.1016/S0266-3538(03)00093-9.

[42] C. Bailey, A. Kervoelen, M. Lan, D. Cartié, A. Le Duigou, A. Bourmaud, et al., Flax/PP manufacture by automated fibre placement (AFP), Mater. Des. 94 (2016) 207–213, https://doi.org/10.1016/j.matdes.2016.01.011.

[43] A. Petrova, L. Kozlova, T. Chernova, T. Gorshkova, Development of the biomechanical system of the flax stem: Mutual game of primary and secondary growth, Ind. Crop Prod. 209 (2024) 117979, https://doi.org/10.1016/j.indcrop.2023.117979.

[44] A. Piontek, O. Vernaez, S. Kabasci, Compatibilization of poly(lactic acid) (PLA) and bio-based ethylene-propylene-diene-rubber (EPDM) via reactive extrusion with different coagents, Polymers 12 (2020) 605, https://doi.org/10.3390/polym12030605.

[45] G. Perego, G.D. Cella, C. Bastioli, Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties, J. Appl. Polym. Sci. 59 (1996) 37–43, https://doi.org/10.1002/(SICI)1097-4628(19960103)59:1<37::AID-APP6>3.0.CO;2-N.