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# Winter flax: Stem structure, fibre properties and reinforcement potential for composite materials

Coralie Buffet, Pierre d'Arras, Alain Bourmaud, Christophe Baley, Adele Hue

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Coralie Buffet, Pierre d'Arras, Alain Bourmaud, Christophe Baley, Adele Hue. Winter flax: Stem structure, fibre properties and reinforcement potential for composite materials. Industrial Crops and Products, 2024, 217, 10.1016/j.indcrop.2024.118831. hal-05061991

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https://ubs.hal.science/hal-05061991v1

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# Winter flax: Stem structure, fibre properties and reinforcement potential for composite materials

Adèle Hue<sup>a, b, *</sup>, Coralie Buffet<sup>a</sup>, Pierre D'Arras<sup>b</sup>, Alain Bourmaud<sup>a</sup>, Christophe Baley<sup>a</sup>

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

**Keywords:**

Winter flax

Fibre length

Mechanical properties

Composite reinforcement

## ABSTRACT

Climate change is characterized by rising temperatures and an increased risk of spring water deficit, both of which threaten the cultivation of textile flax. With the multiplication and intensification of these phenomena, fibre plant yields have become increasingly uncertain in recent years, leading to shorter stems as well as reduced yields. In this context, winter flax cultivation emerges as a solution to sustain the exploitation of this plant. Sown before winter, it can resist periods of spring water stress, owing to the early development of its root system, and is also protected from the summer heat. In this study, we investigate the stem architecture and fibres properties of two winter flax varieties, in comparison to spring flax. Cross-sections reveal that both winter and spring flax have a similar plant structure. The part located at mid-height of the winter stem contains the most fibres, accounting for around 23% of the biomass. At the middle of the stem, the elementary fibres of the Cirrus and Olga varieties have an average length of 20.9 ± 7.7 mm and 30.0 ± 10.8 mm, respectively. At this same plant height, winter flax fibres of the Cirrus and Olga varieties have an average Young's modulus of 63.6 ± 18.8 GPa and 57.6 ± 20.8 GPa, and an average strength at break of 1248 ± 554 MPa and 1187 ± 627 MPa, respectively. As a conclusion, winter flax has a similar length and mechanical properties to spring flax and therefore offers the same potential for textile applications. In addition, as demonstrated in this study, winter flax can be similarly useful as a reinforcement in composite materials.

## 1. Introduction

Flax fibre is increasingly attracting interest, particularly in the textile and composite materials industry. Due to its specific mechanical properties, dimensions and low density, it can compete with traditional fibres, particularly glass fibre, for the development of more eco-friendly materials. Almost 65% of the world's flax is cultivated in France (Cerfrance, 2023). However, this crop is increasingly affected by climate change. Flax is a sensitive plant that only grows in an oceanic climate, where temperatures do not exceed 30°C and annual rainfall is around 700 mm (Goudenhooft et al., 2019). Consequently, crops are mainly located in the coastal areas of northwestern France. But in recent years, the climatic conditions are no longer as favourable. Very few studies have been conducted on the climatic constraints on flax growing. Nevertheless, a recent study (Beauvais et al., 2022) has focused on the future of this crop in Normandy where, according to the scenarios presented by the IPCC (Intergovernmental Panel on Climate Change), temperatures could rise by up to 3°C by 2100. Sowing could occur

earlier, advancing the harvest date, thus allowing flax to escape the high summer temperatures. However, its growth will still take place in a warmer context, and flax will mature more quickly and with shorter stems since the plant will not have time to develop properly. In addition to the increase in temperature, flax crops will have to adapt to the intensification of spring droughts and be more resistant to water stress. Indeed, the total accumulated precipitation is not expected to decrease, but showers will be rarer and more intense. Water stress has a considerable impact on yields (Farooq et al., 2012). In one study, the yields of flax fell by 38% due to water deficit (Melelli et al., 2022). The biochemical composition of the fibres is also altered (Chemikosova et al., 2006), especially the lignin content, which is increased (Contro Malavasi et al., 2016; Hu et al., 2009), making the retting stage more difficult. Not all regions will be affected in the same way. Climatic variations are likely to be weaker near the English Channel coast, compared with inland areas where it is already becoming difficult to cultivate flax (Gate and Deudon, 2018). Beyond the increase in temperature and water stress, it is important to remember that many other

<footnote><sup>a</sup> Univ. Bretagne Sud, UMR CNRS 6027, IRDL, Lorient F-56100, France</footnote>

<footnote><sup>b</sup> Van Robaeys Frères, 83 rue Saint-Michel, Killem 59122, France</footnote>

<footnote>* Corresponding author at: Univ. Bretagne Sud, UMR CNRS 6027, IRDL, Lorient F-56100, France
E-mail address: adele.hue@univ-ubs.fr (A. Hue).</footnote>
factors influence plant growth and fibre properties. For instance, wind can exert mechanical pressure on growing flax stems, reducing the plant height (Biddington, 1986; Goudenhooft et al., 2018). Similarly, excessive seeding density can lead to increased competition among plants for available resources, thereby affecting their growth and development (Bourmaud et al., 2016).

Since the beginning of flax cultivation, the aim of varietal selection has been to increase resistance to diseases, ensure stability against lodging and, above all, improve fibre yield (Goudenhooft et al., 2017; Jankauskienė, 2014). Since the 20th century, flax fibre yields have been on the rise, but in recent years, they have become highly variable from one year to another due to climatic uncertainties. The Alliance for European Flax-Linen & Hemp estimates that fibre yields in 2023 should be halved compared with the 2022 harvest, dropping from 6 to 7 tonnes per hectare in a normal year to between 2 and 5 tonnes per hectare in 2023 (Alliance for European Flax-Linen and Hemp, 2023a, 2023b). Therefore, the current priority is to select new varieties that are more resistant to water stress and high temperatures (Heller et al., 2015; Mahfouze et al., 2017). However, this involves a long-term action, since the development of a new commercial variety would require approximatively 10 years. Treatments aiming to reduce plant evapotranspiration are also under study to enhance flax resistance to water stress (Heller et al., 2013; Quéro et al., 2015; Shivaraj et al., 2019).

One option to mitigate the impacts of climate change is to increase the cultivation of winter flax. Fibre flax is usually sown between March and April, to be pulled up in late June or early July and harvested after retting. The time interval between uprooting and harvesting is variable and depends on retting conditions. Winter flax is sown between September and November, depending on the crop location (Chaillet et al., 2023). Its growth is halted during the winter, when the plant can withstand temperatures as low as -15°C. By the arrival of spring, winter flax has already developed its root system, allowing it to draw water from deeper levels and be less affected by potential spring water deficits. It is harvested a month earlier than spring flax. In 2023, a total of more than 10,000 ha of winter flax were sown in France, Belgium and the Netherlands, compared to 147,000 ha for spring flax. Its yield was much higher than with spring flax, attaining between 6.5 and 7.5 tonnes of straw per hectare (Alliance for European Flax-Linen and Hemp, 2023b), indicating a likely increase in areas used for winter flax cultivation in the coming years. For the year 2024, winter flax crops are estimated to cover 30,000 ha out of a total estimated flax cultivation area of 160,000 ha across France, Belgium, and the Netherlands (Durand and Soverini, 2024).

The interest in flax is justified by the remarkable properties of its fibres, which contribute largely to the mechanical strength of the plant stems (Réquilé et al., 2018). These properties are explained by an optimized internal organization of the stem. Fig. 1a shows a cross-section of a flax stem, whose architecture is characterized by a

peripheral crown of flax fibres and a central woody core. The core is composed of xylem, known as shives, which can be utilized in various applications: thermal insulation, heating energy (as agrofuel) and structural reinforcement (Nuez et al., 2021). A more detailed examination of the plant's structure (Fig. 1b) shows that it is protected by an epidermal layer. Other types of tissues can be seen under the epidermis wall: cortical parenchyma, phloem (where numerous individual fibres are grouped into bundles), vascular cambium, primary and secondary xylem, and the pith. The distribution of these different tissues along the length of the stem has been studied by several researchers (Baley et al., 2018; Lefeuvre et al., 2015; Tiver, 1942). The amount of xylem decreases continuously from the base to the top of the stem, while the pith is smaller at the base and reaches a maximum towards the middle of the stem, where the bast fibres are the most abundant (Baley et al., 2018; Lefeuvre et al., 2015).

This study investigates the structure and the fibre properties of two adapted varieties of flax cultivated in winter. In a first step, the stem architecture of two batches of winter flax was analysed and compared with that of spring flax. Secondly, the mechanical properties of the elementary fibres of the studied winter flax were measured at different plant heights, in order to compare the results with those obtained for spring flax fibres. Finally, the reinforcement potential of these fibres incorporated within a unidirectional composite material was assessed through by tensile testing of single fibres and impregnated fibre bundle.

## 2. Materials and methods

### 2.1. Winter flax crops

Two varieties of winter fibre flax, Cirrus and Olga, marketed in 2016 by Terre de Lin, were cultivated by the flax scutching company Van Robaeys Frères (Killem, France). The Cirrus variety of flax studied here was cultivated in the Oise department during the winter of 2021/2022, and the Olga variety in the Nord department during the winter of 2022/2023. These two varieties were sown in mid-October and harvested in mid-June of the following year. These two full-achieved crops were chosen because a better representativity is obtained by studying two varieties over two years of cultivation. The straw from both batches was industrially scutched by Van Robaeys Frères after retting in the field. The studied fibres were manually extracted from the scutched fibres.

### 2.2. Analysis of internal stem architecture

The total height of the flax stem extends from the collar located above the roots to the tip of the inflorescence, while the technical height is limited at the first branch (denoted as $h_{tot}$ and $h_{tech}$, respectively, in Fig. 2). These two parameters were measured on 15 samples using a measuring tape. The stem diameter was measured with a caliper for the

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Transverse section of a spring flax stem, at mid-height (a) and corresponding schematic representation of the stem parts (b) (Baley et al., 2018).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Definition of the measured parameters and location of the five cross-sectional cuts along the flax stem.</figcaption></figure>

same number of samples. The histological study of flax stem cross-sections was carried out at 5 different heights in a single plant. For this purpose, the stems were preserved in a fixing solution (a mixture of 70%-wt ethanol/30%-wt distilled water). The stems were then immersed in an aqueous solution containing 5%-wt agarose. Cross-sectional cuts were prepared using a Leica VT1000 Vibratome (Leica Biosystems, Germany) with thicknesses ranging from 200 to 300 µm, suitable to observe thin cuts. The sections were observed under a Keyence VHX-7000 optical microscope and a Scanning Electron Microscope (SEM). SEM was performed using a JEOL JSM-IT500HR. The samples were glued on a carbon sticker and coated with gold using a sputter coater (Scancoat6) from Edward. Observations were conducted under high vacuum conditions with an acceleration voltage of 3 kV.

Analysis was performed using Image J® software to determine the number of elementary fibres per section, as well as their surface and the resulting equivalent diameter (minimum of 50 measurements). The biomass distribution in the plant height was also determined. Two percentages were evaluated: apparent tissue area relative to the total surface area of the stem (Fig. 3 A) and apparent fibre area relative to the surface area of the tissues (Fig. 3 B). Then, the remaining tissues were separated into two blocks, mainly consisting of xylem and epidermis (Fig. 3 C). Averages were then calculated for a minimum of three

sections.

## 2.3. Measurement of elementary fibre length

Elementary fibres were measured following the protocol developed by Ageeva et al., and adapted by Baley et al. (Ageeva et al., 2005; Baley et al., 2018). A bundle of fibres, approximately 15 cm long and extracted from the scutched fibres, was immersed for a few minutes in a solution containing 10% CrO₃ and 10% HNO₃ to facilitate the individualization of fibres. Subsequently, these fibres were manually extracted and measured using a Keyence VHX-7000 optical microscope. For each sampling zone (bottom, middle and top of the plant) of each variety, the average fibre length was determined from measurement of 25 fibres.

## 2.4. Tensile testing of elementary fibres

The tensile tests on elementary fibres were carried out under controlled atmosphere conditions with a temperature of 23 ± 2°C and a relative humidity of 50 ± 5%. A Fibre Dimensional Analysis System (FDAS) and a LEX820 tester (Diastron Ltd., Hampshire, UK) were used to measure the longitudinal mechanical properties of the elementary flax fibres. The fibres were bonded to a plastic support using a UV-curing adhesive (DYMAX, Wiesbaden, Germany). Firstly, the diameter of each fibre was measured using an ombroscopic sizing device (FDAS); the average value was calculated from ten measurements taken along the length of the fibre. Then, the LEX820 device was used to test individual fibres with a gauge length of 10 mm. The extension speed was set at 1 mm/min. Between 62 and 92 fibres were tested and validated for each batch.

## 2.5. Manufacture of unidirectional composites

When testing the mechanical properties of fibres at the elementary fibre scale, it is essential to check the representativity of the sample. This is because around a hundred fibres are tested at the most, whereas a single flax stem contains more than tens of thousands of fibres. Therefore, it is more interesting to use an IFBT (Individual Fibre Bundle Test) to study fibre properties at the scale of a unidirectional composite (Bensadoun et al., 2017). This test allows consideration of a larger number of fibres of varying lengths, thereby giving a more comprehensive estimate of the mechanical properties of the fibres. These tests also help investigate the reinforcement potential of the fibres within a composite material.

An EPOLAM 2020 epoxy resin composed of 75% resin and 25% hardener was prepared. Fibre bundles were extracted from the scutched fibres and cut to a length of 12 cm. The fibres were impregnated with the epoxy resin and placed in a mould consisting of two interlocking aluminum rails, opened at the ends to eliminate the excess of resin during the compression stage. The mould were left in the clamp for 24 hours at room temperature and then placed for 24 hours at 50°C before being demoulded. The specimen cross-section was 6×2 mm². Glass fibre tabs (± 45°) were glued to the ends with epoxy adhesive to

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Analysis of the different components of the flax stems in cross-section.</figcaption></figure>
prevent slipping and breaking at the grips during the tensile loading.

## 2.6. Tensile testing of unidirectional composites

Tensile tests were conducted in a controlled environment at 23 ± 2°C and 50 ± 5% relative humidity. The samples were stabilised in this environment for 2 weeks before conducting the tests. An Instron 5566 A tensile testing machine equipped with a 10 kN sensor was used, following the NF EN ISO 527-5 standard. The tensile speed was 1 mm/min in the longitudinal direction of the specimen. An Instron dynamic extensometer with an initial length of 25 mm was fixed to the specimen to measure its deformation.

# 3. Results and discussions

## 3.1. Growth weather conditions of studied winter flax

Fig. 4 presents the meteorological conditions of the two crops during growth. Rainfall during the 2021/2022 season, corresponding to the crop of the Cirrus variety, was lower than the following year. However, the rains were sufficiently regular to allow the full development of the plant, reaching an average total height of 82.8 ± 4.0 cm. The Olga variety, cultivated between 2022 and 2023, endured a very dry January and February, but the plants were protected from water stress by the high rainfall, during the resumption of vegetative growth in spring. The

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Daily precipitation (histograms) and accumulated precipitation (curves) (A and B); and daily minimum, average and maximum temperatures (C and D) for the two winter flax batches studied (Cirrus and Olga). Cumulative temperatures for the two varieties studied and for hypothetical spring flax (E). The shaded interval corresponds to the cumulative temperature range at which the fibres are mature.</figcaption></figure>
plants reached an average total height of 87.0 ± 5.7 cm. Regarding temperatures, the winter of 2021/2022 was on average colder than the winter of 2022/2023, but the plants did not undergo extremely cold conditions (< -10°C). However, the spring-summer of 2022 was slightly warmer than 2023. According to Bert, maturity for spring flax is reached when the sum of cumulative temperature is between 950 and 1100°C (Bert, 2013). This parameter is calculated using the following formula:

$$ \theta_i = \sum_{i=1}^{n} \left( \frac{\theta_{\text{max}_i} + \theta_{\text{min}_i}}{2} - 5 \right) \quad (1) $$

where $\theta_i$ is the cumulative temperature, $\theta_{\text{max}}$ and $\theta_{\text{min}}$ are the maximal and minimal temperature, respectively, expressed in °C and reached on day i. On the day of sowing, $i = 1$. Thus, this parameter indicates that maturity is reached for both winter varieties in June (Fig. 4 E), after about 240 days of vegetation, compared to 100–120 for spring flax.

If spring flax had been cultivated on the two studied plots instead of winter flax, the cumulative temperatures indicate that the plants would have reached maturity in July, following sowing at the end of March (Fig. 4 B). The spring flax would have been harvested a month later than the winter flax, and would not have received the same water supply. Indeed, during spring 2023, heavy rain in March and April benefited the winter flax but caused a delay in the sowing of spring flax. Consequently, a large part of the spring flax was only sown in early May. Indeed, 158 mm of rain fell between 1st May and 31st July 2023, but only 79 mm between 1st May and 30th June, during the plant's growing period. However, spring flax requires at least 110–150 mm of precipitation during its growing period for proper development (Heller and Byczyńska, 2015). This rainfall deficit was observed across most of the regions in France, Belgium and the Netherlands where flax is cultivated, explaining the poor yield obtained for the year 2023 (Alliance for European Flax-Linen and Hemp, 2023a). The flax plants were shorter due to the water stress they suffered.

## 3.2. Winter flax yield

According to the data from the Van Robaeya's Frères scutching company, the Cirrus variety, cultivated during the winter of 2021/2022, had a straw yield of 7.0 t/ha with a total fibre content (scutched fibres and tows) of 28.7%-wt. The Olga variety, cultivated during the winter of 2022/2023, had a straw yield of 6.8 t/ha with a fibre content of 31.6%-wt. These yields are satisfactory when compared to those obtained by Arslanoglu et al., for Rolin winter flax cultivated in Turkey over two consecutive years between 2018 and 2020 (Arslanoglu et al., 2022), with achieved biomass and fibre yields of up to 7.0 t/ha and 26%-wt, respectively.

In this study, winter flax yields are also equivalent to those obtained for spring flax. Under standard cultivation conditions in France, spring flax straw yields generally range between 6 and 7.5 t/ha, with a fibre content around 30%-wt (Chaillet et al., 2023; Doré and Varoquaux, 2006; Melelli et al., 2022).

## 3.3. Morphological analysis of winter flax cross-sections

Firstly, the two images (Fig. 5) are representative of the different stem samples, for both varieties Cirrus and Olga. These cross-sections obtained at mid-technical height of the stem show that two winter varieties have similar architectures. Indeed, the epidermis protecting the crown of bundles of individual fibres as well as the woody core (xylem) are well preserved. Table 1 summarizes the average parameter values for the Olga and Cirrus varieties. The tissue content of these two varieties is almost the same (on average 62.9% and 63.6% of the total stem for Olga and Cirrus, respectively), as is the fibre content values within the tissues. The results obtained in a previous study (Goudenhooft et al., 2017) show a slightly higher of tissue content mainly composed of xylem, with up to 84.6% tissue content in the Ariane variety of spring

<figure><img src="image_9.png" /><figcaption>Fig. 5. Images of stem sections at mid-height. The scale bar is 100 µm for both.</figcaption></figure>

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

Anatomical parameters of stem sections determined by image analysis.</caption><thead><tr><th>Variety</th><th>Total stem height (cm)</th><th>Stem diameter (mm)</th><th>Tissue content (% of stem area)</th><th>Fibre content (% of tissue area)</th><th>Fibres per section</th></tr></thead><tbody><tr><td>Olga</td><td>87.4 ± 7.2</td><td>1.60 ± 0.20</td><td>62.9 ± 0.9</td><td>23.7 ± 1.1</td><td>909 ± 82</td></tr><tr><td>Cirrus</td><td>82.8 ± 4.0</td><td>2.15 ± 0.37</td><td>63.6 ± 2.5</td><td>21.9 ± 1.9</td><td>1077 ± 67</td></tr></tbody></table>

flax. Moreover, by comparing the number of fibres per section, the values for both the Olga and Cirrus varieties are within the average of the four reference spring varieties from Goudenhooft et al., with values ranging between 828 and 1361 elementary fibres at mid-height of the stem. However, a number of factors need to be considered: the low representativity of the number of stems studied here in contrast to real crops (i.e. thousands of hectares), several years of cultivation and therefore different weather conditions (water stress, drought, degree of maturity), and finally, different varieties of flax. This would require more morphological data to complete this study. As mentioned above, the most important indicators remain the straw and fibre yields, and are therefore comparable with current figures for spring flax.

In the case of the Olga variety, the morphology of the stem is analysed at five different positions along its length. Position 3 represents the middle of the technical height, while position 1 is at least 2 cm from the collar (the most rigid part for cutting). The observations are presented in Fig. 6. Scanning electron microscopy was used to magnify a bundle of flax fibres in the cross-section at position 2 (see SEM image on Fig. 6). As commonly observed (Baley, 2002), the fibres have a polygonal geometry.

Table 2 reports some average values from the sections. The plant architecture remains unchanged along the stem. A general decrease in the plant diameter is then observed from bottom to top, reflecting its conical shape (Baley et al., 2018; Goudenhooft et al., 2017). The conical geometry is more pronounced at the top (Fig. 6 (5)). In addition, the least amount of plant tissue is found at mid-height of the stem, where the central gap is the largest. This gap is very small at the bottom, where the stem is fuller (87.7 ± 2.2% tissue content in relation to the total stem area). This provide greater stiffness, while supporting the plant's weight and ensuring its stability.

The histogram in Fig. 7 illustrates the distribution of various tissue along the stem. The plant tissue consists mainly of xylem, which also includes part of the phloem. The results confirm that the woody core is more thickly developed at the base of the stem. The mid-height section is characterized by the highest fibre content (23.7 ± 0.7%) including a maximum of 909 ± 82 fibres (Table 3). As previously noted, this feature is also observed in spring flax varieties. Secondly, as shown in Fig. 6 (5), the epidermis and cortical parenchyma are thicker. The upper part of the plant has more branches, explaining the development of this outer layer. Furthermore, the SEM image shown in Fig. 6 and the histological sections (Fig. 8 B) reveal the partial filling of some elementary fibres. In the
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Images of cross-sections as a function of flax stem height for Olga variety. Images from 1 to 5 were acquired using a digital microscope. The scale bar is 100 µm for all observations. SEM zoom-in view of tissue shown in image 2.</figcaption></figure>

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

Anatomical parameters of Olga plant determined by image analysis at different positions along stem.

<table><thead><tr><td>Position</td><td>Stem diameter (mm)</td><td>Tissue content (% of stem area)</td></tr></thead><tbody><tr><td>5</td><td>0.85 ± 0.13</td><td>66.3 ± 2.4</td></tr><tr><td>4</td><td>1.26 ± 0.16</td><td>70.3 ± 0.7</td></tr><tr><td>3</td><td>1.60 ± 0.20</td><td>62.9 ± 0.9</td></tr><tr><td>2</td><td>1.76 ± 0.20</td><td>66.8 ± 3.2</td></tr><tr><td>1</td><td>1.64 ± 0.17</td><td>87.7 ± 2.2</td></tr></tbody></table>

same section, fully filled fibres with small sized lumens are also observed (Fig. 8 A), illustrating a morphological diversity.

### 3.4. Mechanical properties of elementary fibres extracted at mid-height of the plant

Table 4 reports the tensile mechanical properties of elementary fibres located at the top, mid-height, and bottom of the plants for the two winter flax varieties, Cirrus and Olga. The Student's t-test was used to statistically compare the properties of the two winter flax varieties with each other, but not with the spring flax data from the literature, as the raw data are not available. Regarding the mechanical properties of fibres at mid-height plant, the two batches have comparable values of strength at break and ultimate elongation. The elementary fibres of the Cirrus variety show a Young's modulus that is statistically higher than for the fibres of the Olga variety (p<0.05). This can be explained by the smaller diameter of the fibres of the Cirrus variety (13.86 ± 3.39 µm) compared to the Olga variety (15.24 ± 4.14 µm). Indeed, the smaller the fibre diameter, the higher the Young's modulus, which is because fine fibres would contain fewer structural defects such as kink-bands.

Numerous studies have been carried out on the mechanical properties of elementary spring flax fibres. Fig. 9 compiles the average values of Young's modulus and strength at break obtained from 50 batches of

spring flax (Alix et al., 2012; Bailey and Bourmaud, 2014; Bourmaud et al., 2013, 2010; Charlet et al., 2009; Le Duigou et al., 2012; Martin et al., 2013; Pillin et al., 2011; Roussière et al., 2012) as well as from the two batches of winter flax in this study. The mechanical properties of winter flax are within the same range as those obtained for spring flax, and even rank among the top-performing batches. The same holds true for elongation at break; the values obtained for winter flax are in line with literature results for spring flax (Fig. 10). The difference in growing conditions between spring and winter flax, particularly exposure to low temperatures, do not have any impact on the properties of elementary fibres located at mid-height of the stem. These fibres represent the majority of the total fibre mass in the plant (Lefeuvre et al., 2015).

### 3.5. Mechanical properties of elementary fibres at different plant heights

Winter flax stops growing during the winter, reaching approximately 5 cm in height during this period. Consequently, the base of the plant is exposed to much lower temperatures than the rest of the plant, which develops in spring. It is therefore relevant to investigate the mechanical properties of elementary fibres at the base of the plant and compare the results with those obtained from the middle and upper parts of the plant, as well as with spring flax. The elementary fibres of Cirrus and Olga varieties were tested at different heights of the plant and compared to spring flax fibres from the literature (Baley et al., 2018; Lefeuvre et al., 2015) (Table 4).

Winter flax fibres at the bottom of the stem have a slightly larger diameter than those in the rest of the stem. The fibres with the smallest diameters are located at the top of the stem. Similar trends are observed within the spring flax varieties Eden and Marylin (Baley et al., 2018; Lefeuvre et al., 2015). Moreover, the fibres in the middle of the stem exhibit the best mechanical properties for both types of flax. For the Olga variety, fibres located at the top of the plant exhibit a comparable performance to those at mid-height (p>0.05). Fibres located at the base of the winter flax stems have the weakest mechanical properties. This observation can also be made for spring flax fibres. The winter weather
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Tissue distribution along the stem for the studied winter flax variety (Olga). Mean values are shown within each histogram bar.</figcaption></figure>

<table><caption>**Table 3**
Detailed anatomical parameters of Olga stem sections according to stem height determined by image analysis.</caption><thead><tr><th>Position</th><th>Epidermis +<br>Cortical<br>parenchyma<br>(% of tissue area)</th><th>Xylem + Phloem<br>(% of tissue area)</th><th>Fibre<br>content<br>(% of<br>tissue<br>area)</th><th>Fibres per<br>section</th></tr></thead><tbody><tr><td>5</td><td>26.2 ± 0.3</td><td>60.3 ± 0.8</td><td>13.5 ± 1.1</td><td>211 ± 10</td></tr><tr><td>4</td><td>14.7 ± 1.2</td><td>65.5 ± 2.3</td><td>20.1 ± 1.1</td><td>632 ± 34</td></tr><tr><td>3</td><td>14.3 ± 0.3</td><td>62.0 ± 0.6</td><td>23.7 ± 1.1</td><td>909 ± 82</td></tr><tr><td>2</td><td>15.5 ± 0.2</td><td>66.5 ± 1.3</td><td>17.9 ± 0.9</td><td>810 ± 9</td></tr><tr><td>1</td><td>18.4 ± 0.3</td><td>72.7 ± 0.2</td><td>9.0 ± 0.4</td><td>328 ± 39</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Images from the same cross-section at mid-height of the Olga stem, showing filled fibres (A) and hollow fibres (B). The scale bar is 100 µm for both.</figcaption></figure>

conditions experienced by the base of winter flax plants do not compromise their quality, which remains equivalent to that of elementary fibres located at the base of spring flax plants.

### 3.6. Elementary fibre length

Fig. 11 shows the lengths of elementary fibres located at the bottom, middle and top of winter flax scutched fibres. For each of the three

heights, the winter flax variety Cirrus has shorter fibres than the Olga variety. The average lengths of elementary fibres for Cirrus and Olga varieties are, respectively, 19.5 ± 10.6 mm and 21.4 ± 9.2 mm at the bottom; 20.9 ± 7.7 mm and 30.0 ± 10.8 mm at mid-height; 24.8 ± 12.3 mm and 30.8 ± 11.8 mm at the top of the stem. For both varieties, the lengths of the elementary fibres at the bottom and the top of the plant are statistically different (p<0.05), the shortest fibres being located at the bottom of the plant. Fibres located in the middle and upper parts of the plant have lengths that are not statistically different. The comparison of the two varieties reveals that the Olga variety has longer fibres at mid-height. This distribution of fibre length differs from that given for spring flax in the literature. Baley et al. reported average spring flax fibre lengths of 15.9 ± 5.4 mm, 35.5 ± 16.3 mm, and 17.2 ± 5.8 mm, at the bottom, middle, and top of stems, respectively (Baley et al., 2018). The top of winter flax plants appears to contain longer fibres than those located at the top of spring flax stems; in the case of winter flax, the seasonal-contrast is more drastic between the development of the bottom, middle and top of the plant than for spring flax. The development of the top fibres during the final growing stage, associated with significantly higher temperatures (Fig. 4), is probably in favour of longer fibres with a higher aspect ratio. This parameter is important to ensure correct spinning in the textile industry, as well as for facilitating an effective load transfer between matrix and fibres in composite materials. Therefore, the entirety of the fibres contained in winter flax stems offer a potential that is at least equivalent to spring flax, whether used for textiles or composite manufacturing. However, these results were obtained from a small number of fibres and need to be confirmed with additional measurements.

### 3.7. Reinforcement potential of winter flax fibres

Given the interest in flax fibres in the field of composite materials, it is useful to investigate the reinforcement potential of winter flax fibres within a unidirectional (UD) composite.

Table 5 presents the properties of UD composites made from fibres at the bottom, middle and top of Cirrus and Olga plants. The experimental values of the modulus of elasticity, the strength at break and the
<caption>**Table 4**</caption>

Tensile properties of elementary fibres from the two winter flax batches studied here (Cirrus and Olga), and two spring flax batches from the literature (Eden and Marylin), at three different heights of the plant. Values with the same superscript letter (a, b, c, etc.) can be considered as having no statistically significant difference (P>0.05).

<table><thead><tr><th>Localization of fibres along the stem</th><th>Fibre variety</th><th>Diameter of elementary flax fibres (μm)</th><th>Young's modulus (GPa)</th><th>Strength at break (MPa)</th><th>Ultimate Elongation (%)</th></tr></thead><tbody><tr><td rowspan="5">Top</td><td>Cirrus (this study)</td><td>12.6 ± 3.5<sup>a</sup></td><td>59.4 ± 19.8<sup>d</sup></td><td>1060 ± 547<sup>g</sup></td><td>1.7 ± 0.5<sup>i</sup></td></tr><tr><td>Olga (this study)</td><td>11.6 ± 3.3<sup>a</sup></td><td>62.7 ± 23.3<sup>d,e</sup></td><td>1098 ± 473<sup>g,h</sup></td><td>1.8 ± 0.4<sup>i</sup></td></tr><tr><td>Eden (Lefeuvre et al. 2015)</td><td>14.5 ± 2.5</td><td>50.8</td><td>760</td><td>-</td></tr><tr><td>Marylin (Baley et al. 2018)</td><td>-</td><td>50.1 ± 22.7</td><td>768 ± 363</td><td>1.8 ± 0.8</td></tr><tr><td>Cirrus (this study)</td><td>13.9 ± 3.4</td><td>63.6 ± 18.8<sup>e</sup></td><td>1248 ± 554<sup>h</sup></td><td>2.0 ± 0.5<sup>i</sup></td></tr><tr><td rowspan="5">Middle</td><td>Olga (this study)</td><td>15.2 ± 4.1<sup>b</sup></td><td>57.6 ± 20.8<sup>d</sup></td><td>1187 ± 627<sup>h</sup></td><td>2.0 ± 0.6<sup>i</sup></td></tr><tr><td>Eden (Lefeuvre et al. 2015)</td><td>16.7 ± 2.6</td><td>63.4</td><td>940</td><td>-</td></tr><tr><td>Marylin (Baley et al. 2018)</td><td>-</td><td>58.7 ± 21.3</td><td>911 ± 422</td><td>1.8 ± 0.6</td></tr><tr><td>Cirrus (this study)</td><td>15.5 ± 5.6<sup>b,c</sup></td><td>51.2 ± 19.4<sup>f</sup></td><td>926 ± 484</td><td>1.8 ± 0.6<sup>i</sup></td></tr><tr><td>Olga (this study)</td><td>16.8 ± 4.4<sup>c</sup></td><td>49.2 ± 17.3<sup>f</sup></td><td>755 ± 401</td><td>1.5 ± 0.5</td></tr><tr><td rowspan="5">Bottom</td><td>Eden (Lefeuvre et al. 2015)</td><td>17.3 ± 2.7</td><td>48.4</td><td>590</td><td>-</td></tr><tr><td>Marylin (Baley et al. 2018)</td><td>-</td><td>47.2 ± 18.1</td><td>872 ± 437</td><td>2.1 ± 0.8</td></tr></tbody></table>

elongation at break of the unidirectional composites are denoted as $E_{UD,exp}$, $\sigma_{UD,exp}$ and $\epsilon_{UD,exp}$, respectively. Firstly, one can notice that the volume fraction $V_f$ of fibres within the UD composites is not quite constant, since this parameter is difficult to control precisely, given that the UD composites are produced individually and at a small scale. Considering these variations in fibre volume fraction, the modulus of elasticity of the different batches are similar. The results obtained for the strength at break are more variable between the different batches. This is due to the wide dispersion of the mechanical properties of flax fibres, which means that they will not all break at the same time, and the failure of the UD composite is the result of cumulative damage. Fig. 12 illustrates the failure mode of UD composites. Since the elongation at break is correlated with stress, the results are also more variable. In the literature, spring flax/epoxy UD composites have a modulus of elasticity ranging from 22 to 40 GPa, a strength at break from 196 to 408 MPa and a maximum elongation between 0.7% and 1.3% (Coroller et al., 2013; Lefeuvre et al., 2015). UD composites made with winter or spring flax fibres thus exhibit similar mechanical properties. For the two varieties of winter flax studied here, the unidirectional composites with the best mechanical properties are those made with fibres from the mid-height of the plant. This supports the previous conclusion, based on tensile tests

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Average strength at break vs. average Young's modulus of elementary fibres for the two winter flax batches studied here compared with results on 50 spring flax batches extracted from the literature (Alix et al., 2012; Bailey and Bourmaud, 2014; Bourmaud et al., 2013, 2010; Charlet et al., 2009; Le Duigou et al., 2012; Martin et al., 2013; Pillin et al., 2011; Roussière et al., 2012).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Average elongation at break vs. average Young's modulus of elementary fibres for the two winter flax batches studied here compared with results on 50 spring flax batches extracted from the literature (Alix et al., 2012; Bailey and Bourmaud, 2014; Bourmaud et al., 2013, 2010; Charlet et al., 2009; Le Duigou et al., 2012; Martin et al., 2013; Pillin et al., 2011; Roussière et al., 2012).</figcaption></figure>

on individual fibres, that the best performing fibres are located at mid-height of the stem. The same observation has been made for spring flax (Lefeuvre et al., 2015) as for UD composites with a lower fibre content (Charlet et al., 2009).

The mixing law defined by Eq. (2) allows to estimate the Young's modulus and strength at break of fibres ($E_{L,f}$; $\sigma_{L,f}$) from that of the unidirectional composite. The porosity rate is not included in the equations as it is negligible.

$$E_{L,UD} = (E_{L,f} \times V_f) + (1 - V_f) \times E_m \quad (2)$$

$$\sigma_{UD} = k \cdot V_f \cdot \sigma_{L,f} + (1 - V_f) \cdot \sigma_{L,f} \cdot \frac{E_m}{E_{L,f}} \quad (3)$$

Fig. 13 compares Young's modulus of fibres obtained at the elementary fibre scale with values obtained from UD composites using the inverse mixing law. Fibres extracted from the bottom, the middle
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Lengths of elementary fibres from winter flax varieties Cirrus and Olga as a function of their position along the stem height of the plant. Boxes with the same superscript letter (a, b, c) can be considered as having no statistically significant difference (P>0.05).</figcaption></figure>

**Table 5**

Properties of unidirectional composites reinforced by fibres from winter flax varieties Cirrus and Olga, extracted from the top, middle, and bottom of scutched fibres with volume fraction $V_f$ and reinforcement efficiency factor k. Values with the same superscript letter (a, b, c, etc.) can be considered as having no statistically significant difference (P>0.05).

<table><thead><tr><th>Localization of fibres along the stem</th><th>Fibre variety</th><th>V<sub>f</sub> (%)</th><th>E<sub>UD,exp</sub> (GPa)</th><th>σ<sub>UD,exp</sub> (MPa)</th><th>ε<sub>UD,exp</sub> (%)</th><th>k</th></tr></thead><tbody><tr><td rowspan="2">Top</td><td rowspan="2">Cirrus</td><td>49 ± 4<sup>a</sup></td><td>25.5 ± 2.2<sup>c</sup></td><td>263 ± 16<sup>f</sup></td><td>1.4 ± 0.1</td><td rowspan="2">0.45<sup>1</sup></td></tr><tr><td>55 ± 5<sup>a,b</sup></td><td>26.9 ± 3.6<sup>c,d,e</sup></td><td>165 ± 12<sup>g</sup></td><td>0.9 ± 0.1<sup>1</sup></td></tr><tr><td rowspan="2">Middle</td><td rowspan="2">Cirrus</td><td>54 ± 6<sup>a,b</sup></td><td>32.5 ± 4.9<sup>d</sup></td><td>306 ± 34<sup>f</sup></td><td>1.1 ± 0.1<sup>1,k</sup></td><td rowspan="2">0.41<sup>1</sup></td></tr><tr><td>56 ± 3<sup>b</sup></td><td>30.8 ± 3.2<sup>d</sup></td><td>238 ± 18<sup>h</sup></td><td>1.1 ± 0.1<sup>1,j</sup></td></tr><tr><td rowspan="2">Bottom</td><td rowspan="2">Cirrus</td><td>52 ± 3<sup>a,b</sup></td><td>30.1 ± 2.7<sup>d</sup></td><td>243 ± 13<sup>h</sup></td><td>1.2 ± 0.1<sup>1</sup></td><td rowspan="2">0.45<sup>1</sup></td></tr><tr><td>49 ± 3<sup>a</sup></td><td>22.6 ± 2.3<sup>e</sup></td><td>187 ± 19<sup>g</sup></td><td>1.1 ± 0.1<sup>1,k</sup></td></tr></tbody></table>

and the top of the plant yield results that are comparable between the two characterization scales, for both varieties. Thus, the fibre Young's modulus obtained from the mixing law validates the elementary scale values, confirming the good representativity of the tests performed. However, for the two winter flax varieties studied here, tensile testing of UD composites yields properties of top fibres that are slightly worse than those obtained by tensile testing of top elementary fibres (see Table 4). This difference can be explained by the misalignment of top fibre bundles, or by the possible heterogeneity of fibre concentration within the composite. It could also be explained by the small number of individual fibres tested compared to the number of fibres in a plant. If the selected fibres inadvertently represent those with the best performance, then the properties of the batch are overestimated, and vice versa. Moreover, one can notice that the standard deviations of Young's modulus obtained at the elementary fibre scale are higher than those obtained from UD composites, which can be explained by the homogenizing effect of fibre properties within composite materials.

It is more difficult to estimate the strength at break of fibres from composite failure because this depends on many factors such as the state of division of the bundles, local fibre misorientation, the dispersion of mechanical properties of natural fibres, the quality of fibre-matrix interfaces, the porosity rate, the discontinuous nature of the reinforcement and the presence of residual cortical parenchyma and flax shives. Therefore, it is necessary to introduce a reinforcement efficiency factor k

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Representative picture of a UD composite after rupture (A) and SEM image representative of the fracture surface (B).</figcaption></figure>

into Eq. (3) of the mixing law. This factor is an empirical parameter that quantifies the loss of strength caused by the various factors mentioned above. The k factor is very low in the case of unidirectional composites made with fibres from the middle and top of the scutched flax plants of the Olga variety, likely due to poorer bundle division. For the other UD composites, the k factor is close to that of spring flax calculated for composites with an equivalent fibre content (Lefeuvre et al., 2015). In this latter study, k is equal to 0.53, 0.51 and 0.40 for UD made with fibres extracted from the top, middle and bottom of the plant, respectively. In another study conducted by Coroller et al., the k factor is higher, ranging from 0.54 to 0.69 for the same fibre volume fraction (Coroller et al., 2013). This can be explained by the fact that the spring flax fibres used to manufacture UD underwent hackling treatment which produces cleaner and better individualized fibres.

## 4. Conclusion

This study highlights the characteristics of winter flax to assess its mechanical potential and use in composite or textile industry. In this study, two varieties of winter flax, Cirrus and Olga, were cultivated for two consecutive years of successful harvests. In terms of plant architecture, the study reveals that winter flax stems have a structure similar to spring flax. The area with the highest fibre content is located at mid-height of the stem, and the xylem area is more significant in the base. The yields of these winter flax varieties are around 7 t/ha, equivalent to a spring flax crop under normal growing conditions. The elementary fibres of these two winter flax varieties are found to have mechanical properties equivalent to those of spring flax. Tensile tests at three different plant heights show that the fibres with the best mechanical properties are located at mid-height of the stem. Fibre lengths are also equivalent to spring flax data from the literature. Although fibres from the top of the winter flax stems appear longer compared to spring flax, this observation needs confirmation through further measurements. No morphological or mechanical differences in fibres are observed in the base of the stem, despite the different growing conditions. Therefore, winter flax demonstrates a good potential for various applications, mainly in the textile industry, but also as reinforcement in composite materials. Further studies are needed to compare flax plants and fibres obtained from the same variety and cultivated under winter and spring conditions in the same year. It would also be interesting to conduct
<figure><img src="image_17.png" /><figcaption>Fig. 13. Comparison of the fibre Young's modulus obtained from tensile tests on individual fibres and from tensile tests on UD composites using the inverse mixing law.</figcaption></figure>

further tests on winter flax, with variable soil and meteorological conditions, in order to confirm the good yields achieved over the last two years. Finally, it is important to consider the difficulties and risks that winter flax production can pose to farmers.

### CRediT authorship contribution statement

Christophe Baley: Writing - review & editing, Supervision, Conceptualization. Coralie Buffet: Writing - original draft, Visualization, Investigation. Pierre D'Arras: Supervision, Funding acquisition. Adèle Hue: Writing - original draft, Visualization, Investigation. Alain Bourmaud: Writing - review & editing, Supervision, 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.

### Data Availability

Data will be made available on request.

### Acknowledgements

The authors would like to thank the ANRT (Association Nationale de la Recherche Technologique) for funding the CIFRE agreement N° 2022/0380 as part of the LabCOM "NEW FLAX".

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