Maria Carolina Seghini
email: mariacarolina.seghini@uniroma1.it

Jacopo Tirillò
email: jacopo.tirillo@uniroma1.it

Maria Paola Bracciale
email: mariapaola.bracciale@uniroma1.it

Fabienne Touchard
email: fabienne.touchard@ensma.fr

Laurence Chocinski-Arnault

Antonio Zuorro
email: antonio.zuorro@uniroma1.it

Roberto Lavecchia
email: roberto.lavecchia@uniroma1.it

Fabrizio Sarasini
email: fabrizio.sarasini@uniroma1.it

Surface modification of flax yarns by enzymatic treatment and their interfacial adhesion with thermoset matrices

Keywords: flax fibres, enzymatic treatment, natural fibres, interface/interphase, surface modification treatments

The objective of this study was to assess the effects of commercially available and relatively inexpensive enzyme preparations based on endo 1,4-β-xylanase, pectinases and xyloglucanases on the thermal (TGA), morphological (SEM), chemical (FT-IR) and mechanical (single yarn tensile tests) properties of flax yarns. The preparation based on pectinases and xyloglucanases proved to be the best performing, resulting in effective removal of hydrophilic components such as hemicellulose and pectin, in individualization of yarns, and in increased thermal stability, at the expense of a reduction in mechanical properties depending on the treatment parameters. Single yarn fragmentation tests highlighted an improved interfacial adhesion after enzymatic treatment, with reduced debonding length values of 18 % for epoxy matrix and up to 36 % for a vinylester resin compared to untreated flax yarns.

Introduction

The development of high-performance natural fibre composites cannot neglect the optimization of the fibre/matrix interfacial adhesion [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF]. The importance of the fibre/matrix interface for the macroscopic response of fibre reinforced composites is well consolidated, and it is not an easy task to find an ideal interface able to optimize simultaneously all the macroscopic properties that are directly or indirectly affected by its characteristics, i.e., tensile strength, fracture toughness, resistance to creep, fatigue, dimensional stability and environmental degradation. This consideration leads to the full awareness that interfacial properties need to be optimized rather than maximized [START_REF] Karger-Kocsis | Recent advances in fiber/matrix interphase engineering for polymer composites[END_REF]. For these reasons, the improvement of the adhesion properties between natural fibres and polymer matrices can be based on the optimization of the chemistry of the sizing agents or on the surface modification of the fibres to increase the bonding and wettability by the matrix. In this regard, numerous strategies have been devised to address the limitations of natural fibres, mainly ascribed to their hydrophilic character that precludes a suitable interfacial adhesion with hydrophobic polymer matrices, but at the same time increases moisture absorption and related microbiological degradation [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF]. In this field, the engineering of fibres' surface for their effective incorporation in composite materials with reduced costs and environmental impact represents a noticeable challenge. With this in mind, many strategies that have been investigated over the years suffer from the need to handle large amounts of chemicals ( [START_REF] Bledzki | The effects of acetylation on properties of flax fibre and its polypropylene composites[END_REF][START_REF] Huo | Effect of surface treatments on interfacial properties of flax fiberreinforced composites[END_REF][START_REF] Amiri | Effect of Chemical Treatment of Flax Fiber and Resin Manipulation on Service Life of Their Composites Using Time-Temperature Superposition[END_REF][START_REF] Tserki | A study of the effect of acetylation and propionylation surface treatments on natural fibres[END_REF]) or complex equipment [START_REF] Bozaci | Effects of the atmospheric plasma treatments on surface and mechanical properties of flax fiber and adhesion between fiber-matrix for composite materials[END_REF][START_REF] Seghini | Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites[END_REF][START_REF] Yuan | Plasma treatment of sisal fibres and its effects on tensile strength and interfacial bonding[END_REF][START_REF] Oliveira | Surface Modification of Banana Fibers by DBD Plasma Treatment[END_REF][START_REF] Yuan | Effects of plasma treatment in enhancing the performance of woodfibre-polypropylene composites[END_REF], which can limit their full industrial exploitation. Another recent approach consists in decorating the fibres' surface with nanostructures to increase the surface area and the stress transfer with the polymer matrix [START_REF] Wang | Surface modification of ramie fibers with silanized CNTs through a simple spray-coating method[END_REF][START_REF] Sarker | High-Performance Graphene-Based Natural Fiber Composites[END_REF][START_REF] Wang | Grafting of nano-TiO2 onto flax fibers and the enhancement of the mechanical properties of the flax fiber and flax fiber/epoxy composite[END_REF], but this strategy usually involves several processing steps that make it less attractive for industrial applications. In [START_REF] Foruzanmehr | Physical and mechanical properties of PLA composites reinforced by TiO2 grafted flax fibers[END_REF], the authors reported the grafting of TiO2 on flax fibres with a view to increasing the mechanical properties of poly lactic acid-based composites. Fibres needed two steps, namely dewaxing and oxidation. In this regard, fibres were first treated with boiling acetone and then NaOH that was later neutralized by using a diluted citric acid solution. In the second step, the alcohol groups of the flax fibres were oxidized to carboxylic groups by using TEMPO and then finally immersed in the sol for the synthesis of TiO2 based on titanium isopropoxide as a precursor. A slight increase in the interlaminar shear strength (ILSS), in the thermal and moisture resistance of the resulting composites was achieved, but with a remarkable complexity.

An environmentally friendly substitute for chemical and physical methods is represented by the use of biological agents such as fungi [START_REF] Pickering | Interfacial Modification of Hemp Fiber Reinforced Composites Using Fungal and Alkali Treatment[END_REF][START_REF] Gulati | Fungal-modification of natural fibers: A novel method of treating natural fibers for composite reinforcement[END_REF][START_REF] Schirp | Production and characterization of natural fiber-reinforced thermoplastic composites using wheat straw modified with the fungusPleurotus ostreatus[END_REF] and enzymes [START_REF] Gustavsson | Modification of cellulose fiber surfaces by use of a lipase and a xyloglucan endotransglycosylase[END_REF][START_REF] George | Enzymatically treated natural fibres as reinforcing agents for biocomposite material: mechanical, thermal, and moisture absorption characterization[END_REF][START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF] or supercritical fluids [START_REF] Francois | Can supercritical carbon dioxide be suitable for the green pretreatment of plant fibres dedicated to composite applications[END_REF][START_REF] Seghini | Environmentally Friendly Surface Modification Treatment of Flax Fibers by Supercritical Carbon Dioxide[END_REF].

Biological modifications provide advantages over chemical and physical methods, including their selectivity, the lower energy input required and their recyclability and reusability. In particular, the use of enzyme technology in the field of textile and natural fibre modification is rapidly increasing.

The enzymatic treatment represents a green way to modify natural fibres, and in literature enzymatic treatments have been used with two final aims: (i) the extraction of bast fibres from non-fibrous tissues in the stems as an alternative to dew retting [START_REF] De Prez | Effect of enzymatic treatment of flax on fineness of fibers and mechanical performance of composites[END_REF][START_REF] De Prez | Flax treatment with strategic enzyme combinations: Effect on chemical fiber composition and ease of fiber extraction[END_REF], (ii) the surface modification of fibres already extracted in order to reduce hydrophilicity and increase thermal stability [START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF][START_REF] Liu | Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites[END_REF]. In these latter studies, the enzymatic treatment was reported to be able to catalyse the removal of undesirable substances like pectin, hemicelluloses, lignin, and waxes from the surface of the fibres improving fibre/matrix adhesion, moisture resistance and thermal stability. In fact, pectin and hemicellulose are the primary components contributing to surface polarity that promotes a hydrophilic behaviour and leads to moisture absorption. A careful selection of enzymes and their amounts is crucial in order to obtain an efficient treatment. The leading enzymes used in the fibre modification are: pectinases, used to hydrolyse pectin; hemicellulases (xylanases), able to destroy the hemicellulosic material; laccases, able to degrade lignin; cellulases, responsible for degradation of the β-1,4-glucosidic bonds in crystalline and amorphous cellulose [START_REF] De Prez | Enzymatic treatment of flax for use in composites[END_REF]. In some studies enzymes are combined with chelating agents, such as EDTA (ethylene diamine tetra-acetic acid) or EDTMPA (ethylene diamine tetra methylene phosphonic acid), whose role is to remove calcium ions from pectin in plant cell walls, thus making pectin soluble in many liquids and easily attacked by enzymes [START_REF] Stuart | Structural biocomposites from flax -Part I: Effect of bio-technical fibre modification on composite properties[END_REF][START_REF] Li | Hemp fibre reinforced composites using chelator and enzyme treatments[END_REF].

Different studies in literature investigated the effect of enzymatic treatments on the mechanical performance of composite materials. Bledzki et al. [START_REF] Bledzki | Polypropylene composites with enzyme modified abaca fibre[END_REF] addressed the effects of an enzymatic treatment (fungamix) on the properties of abaca fibres and their resulting composites based on a polypropylene matrix. The enzymatic treatment caused a decrease in the fibre roughness and pronounced fibrillation, while the tensile and flexural tests highlighted an increase of 5 % -45 % for the tensile strength and of 10 % -35 % for the flexural strength after the fibre modification. In addition, the moisture absorption of the composites was reduced by 20 % -45 %. George et al. [START_REF] George | Enzymatically treated natural fibres as reinforcing agents for biocomposite material: mechanical, thermal, and moisture absorption characterization[END_REF] tested different commercially available enzymes, namely xylanase, polygalacturonase, laccase, and xylanase + cellulase, on hemp and flax fibres embedded in a polypropylene matrix.

Polygalacturonase treatment caused the most significant decrease in hemicellulose and pectin for all treatments tested, which resulted in a better water resistance and thermal stability of the composite materials compared to the untreated ones. No significant influence on the mechanical properties of the composites was detected.

These available studies suggest that enzymatic treatments show potential to shape the fibre/matrix adhesion in natural fibre composites. In this framework, this study is one of the first that addresses the influence of commercially available and inexpensive enzyme preparations without the presence of any chemicals on morphological, thermal and mechanical properties of flax yarns intended for composite applications. In addition, this study reports an assessment of the interfacial adhesion with two thermoset matrices, namely epoxy and vinylester, performed by micromechanical single-fibre fragmentation tests (SFFT).

Materials and Methods

Raw materials

Flax (Linum usitatissimum) yarns were carefully extracted by hand from a 2×2 twill fabric (200 g/m 2 ) provided by Composites Evolution (Chesterfield, UK), which do not include any specific sizing formulation. The thermoset matrices used to assess the interfacial adhesion with flax yarns were an epoxy system (Epoxy Prime 27 infusion resin and PRIME 20 slow hardener, mixing ratio 100:28) supplied by Gurit (Newport, UK) and a styrene-free vinylester (Advalite VH-1207) provided by Reichhold (London, UK), which was cured by using a Norpol Peroxide PMEC N24 hardener with a mixing ratio of 1 phr.

Biochemical enzymatic treatment

In order to remove their pectin and hemicellulose content, flax fabrics and yarns were treated using two commercial enzyme preparations of relatively low cost. A Feedlyve AXC 1500L and a Peclyve EXG commercial enzyme solutions, supplied by Lyven SA (Colombelles, France), were selected. The Feedlyve AXC 1500L preparation is rich in endo 1,4-β-xylanase enzyme, which is able to degrade hemicellulose by attacking the xylan backbone arbitrarily that forms xylooligosaccharides [START_REF] De Prez | Enzymatic treatment of flax for use in composites[END_REF]. The Peclyve EXG preparation is instead rich in pectinases and xyloglucanases enzymes, which can remove both pectin and hemicellulose, respectively. The biochemical treatment was performed on both flax fabrics and flax yarns. Flax fabrics, with the dimension of 3×3 mm 2 , and flax yarns, with a length of 80 mm, were treated in small borosilicate lab jars, placed into a thermostatic and magnetically stirred water bath at the temperature of 50 °C. Different concentrations and times were selected, (Table 1). The optimum enzyme-specific pH values were used. Afterwards, drying was performed at 80 °C for 24 hours.

Characterization of wetting properties

The wetting properties of the flax fabric were addressed by a water drop test performed by using a video-based optical contact angle measuring instrument, OCA 15 Pro (DataPhysics GmbH, Filderstadt, Germany). Generally, this system is used in order to perform a contact angle measurement using the sessile drop method on homogeneous solid substrates. In this work, due to the non-homogeneous characteristics of flax fabrics, it was not possible to carry out a contact angle measurement and the wetting behaviour of the fabric was measured in terms of time for complete absorption of the liquid, i.e., water retention time. During the test, degassed water drops with a volume of 4 μl were applied to the flax fabric and the time until the applied drop is absorbed completely was measured.

Tensile testing of flax yarns

The mechanical properties of untreated and enzyme treated flax yarns were addressed by single yarn tensile tests in accordance with ASTM C-1557 at room temperature and using a Zwick/Roell Z010 machine (Ulm, Germany) equipped with a 1 kN load cell. Tests were carried out in displacement control with a cross-head speed of 2 mm/min. The flax yarns, after being extracted by hand from the fabric, were glued (Loctite™ Gel Superglue, Milan, Italy) onto a card tab with a central opening equal to the gauge length of 40 mm. 20-30 yarns, preliminary conditioned 45 °C for 24 h, were tested for each treatment, while the yarn diameter was calculated by optical microscopy as the average of at least ten measurements along the gauge length.

Fourier transform infrared spectroscopy analysis (FT-IR)

The surface modification of flax yarns caused by the enzymatic treatments was studied by Fourier-transform infrared (FT-IR) analysis, carried out with a Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Ettlingen, Germany) equipped with a single reflection Diamond ATR cell. Spectra were recorded with a 3 cm -1 spectral resolution in the mid-infrared range (400-4000 cm -1 ) using 512 scans.

Thermogravimetric analysis (TGA)

The thermal stability of untreated and treated flax yarns was investigated using a SetSys Evolution (Setaram Instrumentation, Caluire, France) thermogravimetric analyzer. The samples, placed in an alumina pan, were subjected to a heating rate of 10 °C/min up to a maximum temperature of 800 °C in a nitrogen atmosphere.

Single yarn fragmentation test

The interfacial properties of untreated and enzymatically treated flax yarns with epoxy and vinylester matrices were measured by using the single fiber fragmentation test (SFFT), which was adjusted to test single yarns, as previously reported in [START_REF] Seghini | Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites[END_REF][START_REF] Seghini | Environmentally Friendly Surface Modification Treatment of Flax Fibers by Supercritical Carbon Dioxide[END_REF][START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF]. Details of the mould and of the procedure used to manufacture the single yarn composite specimens can be found in [START_REF] Seghini | Environmentally Friendly Surface Modification Treatment of Flax Fibers by Supercritical Carbon Dioxide[END_REF]. Prior to matrix casting, flax yarns were conditioned at 45 °C for 24 h. Tests were carried out in an Instron E1000 ElectroPuls test machine (Norwood, MA, USA) with a load cell of 2 kN at a crosshead speed of 0.005 mm/min. The yarn diameter was evaluated as the average of at least ten measurements along the gauge length by optical microscopy (Zeiss, Oberkochen, Germany). The loading phase of the test was interrupted at specimen's failure or when the fragmentation saturation level was achieved, defined as no occurrence of new yarn breaks during a subsequent strain increase by 0.5 % [START_REF] Joffe | Interfacial shear strength of flax fiber/thermoset polymers estimated by fiber fragmentation tests[END_REF]. To achieve the saturation level, epoxy-and vinylester-based specimens were loaded up to strains larger than 9 % and 11 %, respectively. All flax yarns were characterized by a strain at failure at least three and two times smaller than the failure strain of both epoxy and vinylester resins, respectively, thus ensuring the conformity with the Kelly-Tyson model requirements [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum[END_REF].

X-ray microtomography

Single yarn composite specimens were analyzed by an UltraTom CT scanner (RX Solutions, France) with a resolution of 1.5 μm, an accelerating voltage of 50 kV, and a beam current of 157 μA.

Optical and FE-SEM observations

The flax yarn diameters, as well as the fragment lengths, were measured by using a ZEISS Axio Imager optical microscope (Oberkochen, Germany). The morphology of enzymatically treated and untreated flax yarns was investigated by scanning electron microscopy (FE-SEM) with a Zeiss Auriga (Oberkochen, Germany). All specimens were sputter-coated with chromium prior to FE-SEM observation.

Results and Discussion

Characterization of flax yarns after enzymatic treatment

In an initial phase of the work, the enzymatic biochemical treatments were carried out only on flax fabrics. For both Feedlyve and Peclyve enzymatic preparations, the fabrics were treated using a concentration of 0.5, 2.5 and 5 wt% and a duration of 2, 3 and 6 h. The aim of this first phase of the experimental investigation was to verify the actual effect of the two enzymatic preparations on the surface properties of flax and thus to select the solution able to produce the best results. Enzymes are able to modify the surface properties without significantly affecting the bulk properties [START_REF] Boisset | Optimized mixtures of recombinant Humicola insolens cellulases for the biodegradation of crystalline cellulose[END_REF]. It was supposed that the removal of hemicellulose and pectin from flax fibres would reduce their hydrophilic behaviour, making them more resistant to moisture and thermally stable. In this framework, the wetting properties of the untreated and treated flax fabrics were analysed by a water drop test. The representative values of absorption times are reported in Table 2. Due the presence of hydroxyl and strongly polar groups, the untreated flax fabrics showed a marked hydrophilic behaviour. Water retention times lower than 1 s were found for the as-received flax fabric. From the results reported in Table 2, it is possible to notice that flax fabrics displayed a wetting behaviour strongly dependent on the type of enzymatic preparation used. An increase in the hydrophobic behaviour is produced after the Peclyve solution. On the contrary, the flax fabrics treated using the Feedlyve preparation maintained their strong hydrophilic behaviour.

In Figure 1 De Prez et al. [START_REF] De Prez | Flax treatment with strategic enzyme combinations: Effect on chemical fiber composition and ease of fiber extraction[END_REF][START_REF] De Prez | Flax treatment with strategic enzyme combinations: Effect on fiber fineness and mechanical properties of composites[END_REF] investigated different enzyme combinations based on several pectinase and hemicellulase activities including up to three different enzymes applied to flax fibres, and discussed their effects on chemical fibre composition, ease of fibre extraction and fibre fineness. The FE-SEM micrographs reported in Figure 3 show the morphologies of the flax yarns after the different enzymatic treatments. Irrespective of treatment condition, flax yarn fibrillation and individualization were observed. It is worth highlighting that, the longer the process time and the greater the enzyme concentration, the higher the fibrillation of yarns. As reported by George et al. [START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF], the pectinase is the enzyme that contributes the most to the separation of fibres from their yarn. An enzymatic effect observed only at high solution concentration

is the progressive breakage of the flax fibres present at the outer surface of the yarn. This effect was found for the yarns treated for 4.5 h and 6 h using a Peclyve concentration of 10 wt% (Figure 4).

The effect of the biochemical treatments on the surface composition of flax yarns was studied by Fourier-Transform Infrared (FT-IR) spectroscopy. In order to assess the changes induced by the enzymatic treatment on the composition of flax yarns, the FT-IR analysis was carried out on the samples treated using the minimum (Figure 5) and maximum (Figure 6) concentration of Peclyve solution. The micrograph shows the damage on the outer fibres of the flax yarn after the enzymatic treatment Spectra appear to be similar, however the main effect produced by the enzymatic treatment is a strong reduction of the bands around 1736-1742 cm -1 that can be assigned to the carboxylic ester in the pectin and wax [START_REF] Wang | Characterization of bioscoured cotton fabrics using FT-IR ATR spectroscopy and microscopy techniques[END_REF] and to the carbonyl C=O stretching of acetyl groups of hemicellulose [START_REF] Haque | Physico-mechanical properties of chemically treated palm and coir fiber reinforced polypropylene composites[END_REF] and at 1596 cm -1 , 1512 cm -1 and 1451 cm -1 of the lignin [START_REF] Seghini | Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites[END_REF]. It is to be noted that the enzyme preparation used in the present study is not pure, therefore the presence of accessory enzymes besides the main components cannot be excluded. The removal of pectin bonded to hemicellulose might have triggered the rupture of the network, in line with the results in [START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF], where a reduction of hemicellulosic content in flax and hemp fibres was due to a treatment based only on pectinases (polygalacturonase and pectinmethylesterase). The reduction in the bands for hemicellulose, pectin and lignin was observed for flax yarns treated with both 2.5 wt% and 10 wt% of Peclyve solution, thus confirming that the enzymatic treatment was able to successfully degrade hydrophilic substances like hemicellulose and pectin from the surface of flax yarns. Comparing the infrared spectra obtained for the flax yarns enzymatically treated using a 2.5 wt%

Peclyve solution for different durations (Figure 5), it is possible to notice that the longer the treatment time, the greater the removal efficiency of the enzymatic solution. The lowest hemicellulose and pectin peaks were obtained after a treatment duration of 6 h (Figure 5d). These results are supported by the values of crystallinity index (CI) of cellulose calculated for the different flax yarns, which represents an important parameter describing the relative crystallinity of the fibres. The higher the CI, the greater the crystalline cellulose content and hence the greater the efficiency of the enzymatic treatment in extracting the amorphous components from flax [START_REF] Abidi | X-ray diffraction and FTIR investigations of cellulose deposition during cotton fiber development[END_REF][START_REF] Zhang | Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers[END_REF]. In this work a semiquantitative measurement of the crystallinity index of cellulose (Table 3) was performed by using the intensity ratio between the IR absorption bands at 1427 and 897 cm -1 (I1427/I897), which belong to the CH2 bending mode of cellulose and to the CH in β-glycosidic linkages between monosaccharides [START_REF] Abidi | Changes in the cell wall and cellulose content of developing cotton fibers investigated by FTIR spectroscopy[END_REF]. A progressive increase in the cellulose crystallinity index values was observed passing from 3 h to 6 h treatment. Concerning the infrared spectra obtained for the flax yarns treated using a 10 wt% Peclyve solution (Figure 6), an additional reduction in the hemicellulose and lignin bands occurred.

Negligible differences can be found between the infrared spectra of the flax yarns treated for 4.5 h and 6 h and the same cellulose CI value was measured. These results highlight that the use of the Peclyve solution with a concentration of 10 wt% allowed to obtain the maximum efficiency after 4.5 h of treatment. No further removal of amorphous components from the surface of flax fibres was produced using longer treatment times. Li and Pickering [START_REF] Li | Hemp fibre reinforced composites using chelator and enzyme treatments[END_REF] reported the same trend in hemp fibres treated with combinations of pectinase and laccase with and without the presence of a chelator (EDTMPA). With a progressive removal of amorphous non-cellulosic compounds, the authors found a significant increase in the crystallinity index evaluated by the Segal method after enzymatic treatment, which passed from 66.3 % to 86.3 % for the raw and treated hemp fibres, respectively. cellulose, respectively [START_REF] Tajvidi | Effect of fiber content and type, compatibilizer, and heating rate on thermogravimetric properties of natural fiber high density polyethylene composites[END_REF][START_REF] Van De Velde | Thermal and Mechanical Properties of Flax Fibres as Potential Composite Reinforcement[END_REF]. Hemicellulosic and pectic contents of lignocellulosic fibres decompose at a lower temperature than cellulose and lignin [START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF], therefore it is assumed that the removal of less thermally stable hemicellulose and pectin by enzymatic treatment, as confirmed by FT-IR measurements, would improve the thermal resistance of treated flax yarns. After the enzymatic treatment, a change in the thermal stability of the flax yarns occurred. The thermogravimetric analysis confirmed the enzymatic removal of hemicellulose and pectin components from the flax surface. In particular, the thermal stability of flax yarns treated using the maximum concentration of 10 wt% of Peclyve preparation was investigated. The thermograms reported in Figure 7 show that the peak related to water loss remains unchanged. However, the second step of thermal degradation changed.

In fact, contrary to the as-received flax, both yarns treated for 4.5 h and 6 h, do not show any step of weight loss associated with the hemicellulose and the pectin decomposition. From the mass loss vs.

temperature curves, it can be also seen that the onset degradation temperatures tend to increase after the enzymatic treatment, compared with the untreated flax yarns, again confirming the removal of thermally liable constituents [START_REF] Li | Hemp fibre reinforced composites using chelator and enzyme treatments[END_REF].

Table 4 summarizes the maximum degradation temperature and the temperature of 10 % weight loss for the untreated and the treated flax yarns. Enzymatically treated flax yarns displayed a maximum degradation temperature only slightly lower compared to that of the untreated ones, despite the removal of components that could result in a weaker macrostructure because of disorder in bonding within the network [START_REF] Bateman | Degradation of Plant Cell Walls and Membranes by Microbial Enzymes[END_REF]. An increase of the 10 % of weight loss temperature was obtained after both enzymatic treatments. In particular, the highest temperature value (325.5 °C) was obtained for the flax yarn after a 6 h treatment. It was not possible to detect by thermogravimetric analysis any significant effect related to the lignin removal. Lignin decomposes slower and over a broader temperature range (200-500 °C) compared to cellulose and the hemicellulose [START_REF] Brebu | Thermal degradation of lignin -a review[END_REF]. Flax fibres are usually characterized by a low amount of lignin, which is kept low because it enables an easier extraction of the fibres from the stem [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF]. Therefore, even untreated flax fibres did not display a marked degradation peak for the lignin in the 400 -500 °C temperature range.

The mechanical properties of neat and enzymatically treated flax yarns were addressed by single yarn tensile test and the results were statistically analyzed by using the Weibull distribution (eq. 1):

𝑃𝑟 𝜎 1 𝑒𝑥𝑝 (1)
where Pr(σ) represents the cumulative probability of failure as a function of applied stress σ, m is the Weibull modulus and σ0 is the characteristic strength. The failure probability (Pf) was estimated in accordance with equation 2:

𝑃 . ( 2
)
where N is the number of filaments tested and i is the rank of data point for each filament.

As shown in Table 5, the tensile strength and failure strain decreased after enzymatic treatments, in line with other studies on hemp fibres [START_REF] Liu | Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites[END_REF][START_REF] Li | Hemp fibre reinforced composites using chelator and enzyme treatments[END_REF]. The reduction in mechanical performance can be linked with the enzymatic effect of yarn fibrillation observed by FE-SEM analysis. A marked loss of tensile strength of about 58.5 % was obtained after a treatment with the 10 wt% Peclyve solution for 6 h. This tensile strength drop can be connected to the progressive breakage of the fibres present at the outer surface of the yarn produced at high solution concentration (Figure 4). Another phenomenon able to explain the reduction in the mechanical performance of the yarns is likely to be the enzymatic removal of non-cellulosic constituents of flax, such as pectin, hemicellulose and lignin.

Although they are not structural constituents of the fibre as cellulose, the non-cellulosic components have the role to keep linked together the cellulose microfibrils. A similar result was found by Liu et al. [START_REF] Liu | Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites[END_REF]. A reduction in the tensile properties of hemp fibres was reported after the enzymatic treatment using the polygalacturonase enzyme. The authors proposed a "composite model" to explain the mechanical behaviour of plant fibres and the effect of the enzymatic preparation on their properties. The hemp fibre can be assimilated to a composite material, in which the cellulose microfibrils are the reinforcing fibres and the non-cellulosic components are the matrix. According to this model, the mechanical performance of the composite is controlled not only by the cellulose microfibrils but also by their adhesion with the non-cellulosic matrix. The enzymatic degradation of the non-cellulosic matrix produced during the treatment results in a reduction in fibre/matrix interface strength and in an overall decrease in the tensile properties of the fibres. Similar conclusions have been found on pectinase treated hemp fibres by Saleem et al. [START_REF] Saleem | Treating bast fibres with pectinase improves mechanical characteristics of reinforced thermoplastic composites[END_REF].

In addition, lignin is a structural component and its degradation can result in a decrease in fibre tensile strength. The removal of amorphous non-cellulosic components from flax fibres has been confirmed by both FT-IR and thermogravimetric analysis. Several studies in literature investigated the relationships between the tensile properties and the cellulose crystallinity index for plant fibres [START_REF] Gassan | Alkali treatment of jute fibers: Relationship between structure and mechanical properties[END_REF][START_REF] Sawpan | Effect of various chemical treatments on the fibre structure and tensile properties of industrial hemp fibres[END_REF]. In general, an increase in tensile strength and Young's modulus with increased crystallinity index was reported. As described previously, in this study an increase in the cellulose crystallinity index was observed after the different enzymatic treatments. The reduction in the tensile strength of enzymatically treated flax yarns can be related to an excessive degradation of the structural components of fibre, such as lignin. A similar behaviour was found by Mwaikambo et al. [START_REF] Mwaikambo | Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials II. Sisal fibres[END_REF] for alkali treated sisal fibres. A decrease in tensile strength and Young's modulus of sisal fibres with increased crystallinity index was reported. The authors suggested that the alkali treatment severity had a large influence on tensile properties and that higher levels of treatment can produce a marked degradation of the fibres. The results of the single yarn tensile tests were analysed by using the Weibull distribution (Table 5) and the corresponding curves are reported in Figure 8. Both untreated and enzymatically treated flax yarns displayed a quasi-linear trend, pointing out that the final failure was controlled by a unique population of defects and that the treatment had an influence on the concentration and severity of flaws but not on their nature.

Fibre/matrix interfacial assessment by single yarn fragmentation test

The interfacial adhesion analysis was focused on two specific treatment conditions. In fact, according to the wetting test results and tensile strength values, single yarn fragmentation tests were performed on flax yarns treated using 5 wt% and 10 wt% of Peclyve enzymatic preparation for 4.5 h and 6 h. The 5 wt% -4.5h condition was selected because it allowed to obtain changes in the wetting properties of flax yarns without significantly affecting their mechanical performance. Indeed, this enzymatic treatment condition produced a decrease in the tensile strength of the yarns of approximately 27 %. On the other hand, the condition of 10 wt% for 6h was selected in order to assess the effects of the maximum concentration of Peclyve solution and of the maximum treatment duration on the adhesion quality between flax yarns and thermoset matrices. At least 10 fragmentation tests were carried out for each family of single yarn composites. The experimental procedure has been thoroughly described in previous publications [START_REF] Seghini | Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites[END_REF][START_REF] Seghini | Environmentally Friendly Surface Modification Treatment of Flax Fibers by Supercritical Carbon Dioxide[END_REF][START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF]. A comparison between the critical filament lengths measured for the untreated and the enzymatically treated single yarn composites is shown in Table 6. The results highlight that following the enzymatic treatments, a modest reduction in the critical yarn length in both epoxy and vinylester matrices occurred. In particular, the best adhesion quality was obtained after the treatment performed using the maximum concentration value of 10 wt% and the maximum treatment duration of 6 h. In contrast, the flax/epoxy and the flax/vinylester systems with the flax yarn treated using the 5 wt% of Peclyve solution for 4.5 h showed a critical yarn length almost equal to that obtained for the untreated single yarn composites. Unfortunately, the yarn critical length cannot be used as the exclusive parameter for evaluating the interfacial adhesion, as it strictly depends on the yarn strength that has been affected by the biochemical treatment [START_REF] Fraser | Evaluation of surface treatments for fibers in composite materials[END_REF].

Nevertheless, a comparison between the two matrix systems is possible, and the results suggest that flax/epoxy samples are characterized by smaller values of critical lengths than the flax/vinylester ones. As already described for alternative flax yarn treatments [START_REF] Seghini | Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites[END_REF][START_REF] Seghini | Environmentally Friendly Surface Modification Treatment of Flax Fibers by Supercritical Carbon Dioxide[END_REF][START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF], this behaviour is related to the significant incompatibly between the highly hydrophilic flax yarns and the fully dispersive vinylester resin used in the present study.

Another parameter usually reported in interfacial studies is the interfacial shear strength (IFSS), which was calculated in accordance with the Kelly and Tyson model [START_REF] Kelly | Tensile properties of fibre-reinforced metals: Copper/tungsten and copper/molybdenum[END_REF]. The strength value at the gauge length was obtained through extrapolation of the Weibull cumulative distribution function [START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF], and the corresponding values are summarized in Table 7. By comparing the results found after the enzymatic treatment with those obtained for the as-received flax yarns, it is possible to highlight that a reduction in the IFSS values was produced after both surface modification treatments. This is especially true for the flax yarns enzymatically treated using the 10 wt% of Peclyve solution for 6 h.

A poor interfacial adhesion between the treated flax yarns and both epoxy and vinylester matrices could be inferred from these results. However, as highlighted previously for the critical yarn length values, it is important to emphasize that the IFSS results are strictly dependent on the strong reduction in the mechanical properties of flax yarns caused by the biochemical treatments. After the 10 wt% Peclyve -6h treatment, the tensile strength at a length equal to the critical yarn length was reduced by 50 %, resulting in a strong decrease in the interfacial shear strength value with the epoxy and the vinylester resin. The main problem with this kind of test for yarns is related to the correct measurement of the cross-sectional area of the yarn. In fact a yarn includes voids among the technical fibres that during the impregnation stage are replaced by the matrix. This suggests that the IFSS values in the present work can only be used for a qualitative screening of the different flax yarns. Nevertheless, once again the better compatibility of flax yarns with the epoxy matrix can be confirmed. A more reliable assessment of the level of interfacial adhesion can be based on another parameter, i.e. the debonding length [START_REF] Kim | a Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites[END_REF]. An extended debonding zone is typical for weak interfacial bonds. In addition, this parameter does not depend on the constraints that are required to be satisfied for a sound single fibre fragmentation test (SFFT). Values included in Table 6 show reduced debonding lengths for the enzymatically treated yarns compared to the untreated ones, thus fostering a better interfacial adhesion of flax yarns with the epoxy and the vinylester matrices. The decrease in debonding length found for the enzymatically treated flax yarn with both epoxy and vinylester matrices can be linked to the increase in hydrophobic behaviour produced by the Peclyve enzymatic solution and better fibre individualization that could promote wettability of the resin. Similar debonding length values were measured for both the 5 wt% -4.5 h and 10 wt% -6 h treatment conditions. A decrease in debonding length of about 18 % was found for both enzymatically treated flax yarns in the epoxy-based composites. This result indicates that the different solution concentrations and treatment durations used affect in the same way the adhesion quality between the flax yarn and the epoxy matrix. Different are the results found for the flax yarn/vinylester system.

In this case, the enzymatic treatment performed with a 10 wt% of Peclyve solution for 6 h produces better adhesion results than the treatment performed using the 5 wt% Peclyve -4.5 h condition. If compared with the as-received flax yarn/vinylester system, a decrease in debonding length of about 36 % was found for the Peclyve 10 wt% -6 h flax yarn/vinylester composite.

The improved interfacial adhesion can be also inferred from micro-CT micrographs. This analysis was carried out only for the best performing treatment, i.e. the Peclyve 10 wt% -6 h (Figure 9). where the failure is mostly inside the flax yarn without evidence of extensive debonding, if compared to the untreated case [START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF].

Conclusions

This study demonstrated that commercially available and inexpensive enzyme preparations

  are compared the time sequences of the water drop tests carried out for fabrics treated using the Feedlyve and Peclyve enzymatic solutions at 5 wt% and for a duration of 6 h. A maximum value of water retention time of 50 s was obtained after the treatment performed using the Peclyve solution. On the contrary, the flax fabric treated by the Feedlyve solution showed a water retention time lower than 1 s. According to George et al.[START_REF] George | Surface and thermal characterization of natural fibres treated with enzymes[END_REF], the increase in hydrophobic behaviour may be explained as a consequence of the removal of pectin and hemicellulose fractions on the fibres' surface produced by both pectinases and xyloglucanases enzymes. The better results obtained by the Peclyve solution can be linked to the synergic action of pectinases and xyloglucanases enzymes. In contrast, the only endo 1,4-β-xylanase enzyme present in the Feedlyve preparation was not able to induce substantial modifications of the flax properties.

Figure 1 .

 1 Figure 1. Representative time sequences of the water drop test performed onto the flax fabrics treated using the Feedlyve and Peclyve enzymatic solutions at 5 wt% and for a duration of 6 hThe authors identified some synergic combinations that resulted in enhanced cellulose content (polygalacturonase + xylanase), while combinations with three enzymes showed significant improvements in fibre fineness compared to the single enzyme treatments. It is hypothesized that when pectinases and hemicellulases are combined, the polymer network surrounding the fibre should be degraded more thoroughly. In the present study, the removal of pectin and hemicellulose components resulted in a decrease in the surface polarity of flax fabrics, reducing the presence of hydroxyl groups distributed on their surface. Based on these results, the Peclyve preparation was chosen as enzymatic solution able to successfully modify the surface properties of flax fibres. The subsequent enzymatic treatments were performed on flax yarns using this preparation and to analyse in detail the action of the Peclyve solution, different solution concentrations (2.5 -5 -7.5 -10 wt%) and treatment times (3, 4.5 and 6 h) were investigated. The water drop test, despite being useful for a qualitative analysis of the wetting behaviour of the different flax fabrics, was characterized by a large data dispersion, hence it proved to be inadequate to identify a precise relationship between process times and enzyme concentrations to be used.The changes in the surface morphology of flax yarns produced by the Peclyve solution were compared with those of the untreated flax yarns. Differences between the lateral surface of the asreceived and treated flax yarns using the Peclyve solution at 5 wt% for 6 h are reported in Figure2.One of the main effects observed after the enzymatic treatment is the removal of a substantial part of the impurities present on the untreated fibres, such as fat and waxes that bind the flax bundles together, ensuring good mechanical properties of the yarn[START_REF] Abdel-Halim | Enhancing hydrophilicity of bioscoured flax fabric by emulsification post-treatment[END_REF].

Figure 2 .

 2 Figure 2. Comparison between the lateral surface of the untreated flax fibres (A) and the flax fibres treated using the 5 wt% Peclyve enzymatic solution for 6h (B)

Figure 3 .

 3 Figure 3. SEM micrographs of the flax yarns as a function of the different enzymatic treatments with Peclyve preparation: (A) 2.5 wt%-4.5 h; (B) 2.5 wt% -6 h; (C) 5 wt% -4.5 h; (D) 5 wt% -6 h; (E) 7.5 wt% -4.5 h; (F) 7.5 wt% -6 h; (G) 10 wt% -4.5 h; (H) 10 wt% -6 h

Figure 4 .

 4 Figure 4. SEM micrograph of the flax yarn treated with the 10 wt% Peclyve solution for a duration of 4.5 h.

Figure 5 .Figure 6 .

 56 Figure 5. Infrared spectra of as-received (a) and enzymatically treated flax yarns using a 2.5 wt% Peclyve solution for 3 h (b), 4.5 h (c) and 6 h (d)

Figure 7

 7 Figure 7 displays the representative thermographs expressed as the mass loss and derivative mass loss for neat and enzymatically treated flax yarns.

Figure 7 .

 7 Figure 7. Derivative thermogravimetry (dTG) curves and TGA thermograms of flax yarns before and after enzymatic treatment Neat flax yarns were characterized by the well-known three stages of degradation that depend on the different thermal stabilities of their constituents, i.e. cellulose, hemicellulose and lignin. While the first mass loss in the range 60-120 °C is due to the absorbed water, the additional two peaks at about 240-280 °C and 340-360 °C are ascribed to degradation of pectin and hemicellulose and to

Figure 8 .

 8 Figure 8. Weibull distribution plots of strength measured for untreated flax yarns and enzymatically treated flax yarns for a duration of 4.5 h and 6 h

Figure 9 .

 9 Figure 9. Representative debonding length observation by micro computed tomography. High resolution micro-CT images for Peclyve EXG 10 wt% -6h treated flax/epoxy (A) and flax/vinylester (B) single yarn samples

  based on pectinases and xyloglucanases have a synergistic effect in removing the hydrophilic pectic and hemicellulosic materials from flax yarns, which was confirmed by FT-IR and TGA analyses. From a microstructural point of view, the biochemical treatment resulted in a cleaning effect of fiber surface with an increase in thermal stability and a reduction in mechanical properties depending on the enzyme concentration and treatment duration. By taking advantage of the reduced hydrophilic behaviour and increased individualization of flax yarns, an improvement in fibre/matrix compatibility with epoxy and vinylester matrices was obtained. After exposure to enzymatic treatments, single yarn fragmentation tests highlighted smaller debonding and critical yarn length values, especially with the fully dispersive vinylester matrix. Enzymes, being relatively inexpensive, recyclable and less energy intensive, represent a green and promising surface modifying agent for natural fibres intended for composite applications.

Table 1 .

 1 Concentrations of the enzyme preparations and process parameters used.

Table 2 .

 2 Water absorption time values obtained for the untreated and enzymatically treated flax fabric surface.

	Flax

fabric Absorption time t [s] Absorption time t after 2 h [s] Absorption time t after 3 h [s] Absorption time t after 6 h [s]

	Neat	t<1	-	-	-
	Peclyve 0.5 wt%	-	1	1.2	5.5
	Peclyve 2.5 wt%	-	1.4	2.2	1.5
	Peclyve 5 wt%	-	1.5	35	50
	Feedlyve 0.5 wt%	-	t < 2	t < 1	1
	Feedlyve 2.5 wt%	-	t < 2	1	t < 1
	Feedlyve 5 wt%	-	1	t < 1	t < 1

Table 3 .

 3 The crystallinity index of cellulose for the untreated and enzymatically treated flax yarns using different times and concentrations of Peclyve preparation.

	Flax Yarn	Treatment	Cellulose
		Time	Crystallinity
			index (CI)
	Neat	-	1.01
	Peclyve 2.5 wt%	3 h	1
	Peclyve 2.5 wt%	4.5 h	1.15
	Peclyve 2.5 wt%	6 h	1.24
	Peclyve 10 wt%	3 h	1.36
	Peclyve 10 wt%	4.5 h	1.54
	Peclyve 10 wt%	6 h	1.54

Table 4 .

 4 Results of the thermogravimetric analysis for the different types of flax yarns.

		Maximum degradation temperature [°C]	Temperature of 10 % weight loss [°C]
	Flax yarn (neat)	357.2	311.3
	Peclyve 10 wt% -4.5 h	355.7	322.8
	Peclyve 10 wt% -6 h	356.7	325.5

Table 5 .

 5 Tensile properties of untreated and enzymatically treated flax yarns. In the table are also included the parameters of the Weibull distribution for the tensile strength.

	Flax Yarn	Fmax	Diameter	σf	εf	Characteristic	Weibull
		[N]	[μm]	[MPa]	[%]	strength σ0	Modulus m
						[MPa]
	Neat	19.8 ± 4.8	327 ± 95	236 ± 57	3.4 ± 0.42	257.4	5
	Peclyve 2.5
	wt% -4.5 h	20.2 ± 3.5	376 ± 69	181 ± 31	3.55 ± 0.49	194	6.8
	Peclyve 5 wt% -4.5 h	16.9 ± 3.6	353 ± 59	172 ± 36	2.99 ± 0.47	186	5.5
	Peclyve 7.5 wt% -4.5 h	16.2 ± 4	348 ± 77	170 ± 41	2.95 ± 0.41	186	4.8
	Peclyve 10 wt% -4.5 h	14.8 ± 3.5	390 ± 78	123 ± 29	3.04 ± 0.47	134	5
	Peclyve 2.5 wt% -6 h	14.1 ± 3.4	382 ± 61	123 ± 29	3.03 ± 0.59	134	4.6
	Peclyve 5 wt% -6 h	13.7 ± 2.1	379 ± 63	121 ± 19	3.19 ± 0.55	129	7.6
	Peclyve 7.5 wt% -6 h	11.7 ± 3.6	370 ± 74	108 ± 33	2.93 ± 0.6	120	3.5
	Peclyve 10	11.1 ± 2.8	362 ± 63	107 ± 27	3.02 ± 0.54	118	4.3
	wt% -6 h

Table 6 .

 6 Critical fragment length and debonding length values for the different single yarn composites.

	Matrix	Flax yarn	lc [μm]	ldebonding [μm]
		Neat	2687 ± 631	444 ± 49
	Epoxy	Peclyve 5 wt% -4.5 h	2628 ± 531	353 ± 89
		Peclyve 10 wt% -6 h	2511 ± 203	365 ± 46
		Neat	3942 ± 807	830 ± 343
	Vinylester	Peclyve 5 wt% -4.5 h	3962 ± 243	628 ± 128
		Peclyve 10 wt% -6 h	3554 ± 776	529 ± 65

Table 7 .

 7 Strength values at critical fragment length and IFSS values for the untreated and enzymatically treated flax/epoxy and flax/vinylester systems.

	Matrix	Flax Yarn	σ f(lc) [MPa]	IFSS [MPa]
		Neat	406 ± 18	19.3 ± 3.7
	Epoxy	Peclyve 5 wt% -4.5 h	283 ± 10	18.1 ± 2.7
		Peclyve 10 wt% -6 h	203 ± 3	12.2 ± 1.1
		Neat	376 ± 15	13.9 ± 2.8
	Vinylester	Peclyve 5 wt% -4.5 h	262 ± 2	10. 5 ± 1.4
		Peclyve 10 wt% -6 h	188 ± 9	7.8 ± 2

Funding: This research received no external funding

Conflicts of Interest:

The authors declare no conflict of interest.


Maria Carolina Seghini

Fabienne Touchard
email: fabienne.touchard@ensma.fr

Fabrizio Sarasini

Laurence Chocinski-Arnault

Maria Rosaria Ricciardi

Vincenza Antonucci

Jacopo Tirillo

Jacopo Tirillò

Fatigue behaviour of flax-basalt/epoxy hybrid composites in

Keywords: Polymer matrix composites, S-N curves, Fatigue modelling, Fractography, Damage

come

Introduction

In the last two decades, the development of plant fibre reinforced polymer matrix composites has attracted great interest due to their properties and environmentally friendly character. In particular, lignocellulosic fibres such as flax, hemp, jute, sisal and abaca have been proposed as substitutes of traditional glass fibres [START_REF] Kiruthika | A review on physico-mechanical properties of bast fibre reinforced polymer composites[END_REF]. These fibres offer several advantages over conventional synthetic ones, such as low density, low cost, good specific mechanical properties, wide availability and biodegradability [START_REF] Gholampour | A review of natural fiber composites: properties, modification and processing techniques, characterization, applications[END_REF]. However, despite their benefits, the natural fibres have also some drawbacks. Indeed, they show high moisture sensitivity, low thermal stability, low microbial resistance and poor compatibility with polymeric matrices, which makes their use inconvenient in semi-or structural applications [START_REF] Koronis | Green composites: A review of adequate materials for automotive applications[END_REF]. Therefore, the development of hybrid composite materials, by introducing different types of fibres aiming at overcoming the drawbacks of composites reinforced with only one type of fibres, can represent a very attractive way of solving this problem. In this regard, hybridisation of plies of plant and synthetic fibres represents a significant strategy to reduce weight and carbon footprint of traditional composite materials, while retaining a sufficient mechanical performance to permit structural applications. In particular, it has been shown that hybrid composite materials can improve impact and bending resistance in comparison with non-hybrid ones [START_REF] Chapman | Effects of Hybridisation on the Low Velocity Falling Weight Impact and Flexural Properties of Flax-Carbon/Epoxy Hybrid Composites[END_REF][START_REF] Margabandu | Experimental evaluation and numerical validation of bending and impact behaviours of hybrid composites with various stacking arrangements[END_REF][START_REF] Supian | Hybrid reinforced thermoset polymer composite in energy absorption tube application: A review[END_REF][START_REF] Bakar | The crashworthiness performance of stacking sequence on filament wound hybrid composite energy absorption tube subjected to quasi-static compression load[END_REF]. According to Dong [START_REF] Dong | Review of Natural Fibre Reinforced Hybrid Composites[END_REF], hybrid composites including plant fibres were fabricated most frequently with glass fibres. Several works have given an overview on the different properties of glass-plant hybrid composites [START_REF] Sanjay | Characterization and properties of natural fiber polymer composites: A comprehensive review[END_REF][START_REF] Rajesh | Mechanical and dynamic mechanical behaviour of novel glass-natural fibre intraply woven polyester composites[END_REF][START_REF] Mochane | Recent progress on natural fiber hybrid composites for advanced applications : A review[END_REF]. In recent years, there has been substantial interest in the application of basalt fibres as reinforcement of polymer matrices mainly as replacement for glass fibres in view of their advantages in terms of environmental cost relative to their chemical and mechanical properties [START_REF] Chairman | Mechanical properties of basalt fabric plain and twill weave reinforced epoxy composites[END_REF][START_REF] Chowdhury | Analysis of failure modes for a non-crimp basalt fiber reinforced epoxy composite under flexural and interlaminar shear loading[END_REF][START_REF] Matykiewicz | On the impact of flax fibers as an internallayer on the properties of basaltepoxy composites modified with silanized basalt powder[END_REF]. This has triggered the development of hybrids including plant fibre and basalt fibre laminates [START_REF] Dhakal | Effect of basalt fibre hybridisation on post-impact mechanical behaviour of hemp fibre reinforced composites[END_REF][START_REF] Ricciardi | Effect of hybridization on the impact properties of flax/basalt epoxy composites: Influence of the stacking sequence[END_REF][START_REF] Papa | Impact behaviour of hybrid basalt/flax twill laminates[END_REF][START_REF] Fiore | Effect of external basalt layers on durability behaviour of flax reinforced composites[END_REF][START_REF] Sarasini | Quasi-Static and Low-Velocity Impact Behavior of Intraply Hybrid Flax/Basalt Composites[END_REF][START_REF] Matykiewicz | Hybrid Epoxy Composites Reinforced with Flax Fiber and Basalt Fiber[END_REF].

It is known that fatigue loadings are responsible for many, if not most, failures in engineering structures [START_REF] Wang | A damage model for the fatigue life of elastomeric materials[END_REF]. Indeed, dynamic repetitive loadings (vibration, rotation, wind and wave action, turbulence, pressurisation, etc.) at levels much lower than ultimate strengths can result in sudden and catastrophic failure due to internal damage accumulation over a period of time. Hence, the fatigue behaviour of the material is vital in almost all engineering applications [START_REF] Amiri | Rapid determination of fatigue failure based on temperature evolution: fully reversed bending load[END_REF]. On one hand, there are several studies addressing the fatigue behaviour of 100 % plant fibre composites or 100 % basalt fibre composites. For example, Bensadoun et al. [START_REF] Bensadoun | Fatigue behaviour assessment of flax-epoxy composites[END_REF] compared the tension-tension fatigue behaviour of flax fibre composites for nine different reinforcement architectures combined with epoxy matrix: one random mat, two cross-ply laminate configurations and six textiles (a plain weave, three twills with various twist values, a quasi-unidirectional and a unidirectional fabrics). They showed that composites displaying high static strength have longer fatigue lifetimes, even if, as the loading level decreases, all textile and cross-ply laminates achieve comparable lifetimes. In their review, Mahboob and Bougherara [START_REF] Mahboob | Fatigue of flax-epoxy and other plant fibre composites: Critical review and analysis[END_REF] reported that fatigue testing parameters (e.g. frequency, loading ratio) and structural variables (e.g. off-axis plies, moisture content, fibre crimp, 'out-of-plane' weaves in fabric) are all influencing longevity of plant fibre composites. Regarding the moisture content, Barbiere et al. [START_REF] Barbière | Influence of moisture and drying on fatigue damage mechanisms in a woven hemp/epoxy composite: Acoustic emission and micro-CT analysis[END_REF] have recently demonstrated the influence of the storage and the testing conditions on fatigue behaviour of a woven hemp/epoxy composite. Liang et al. [START_REF] Liang | A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites[END_REF], in their study, showed that the [±45] 3S flax/epoxy specimens offer better specific fatigue endurance than similar glass/epoxy laminates, which was not the case for the [0/90] 3S specimens. On the opposite, for basalt/epoxy composites, Dorigato et al. [START_REF] Dorigato | Fatigue resistance of basalt fibers-reinforced laminates[END_REF] found superior performances of basalt/epoxy laminates with respect to the corresponding glass/epoxy composites, with an improved capability of sustaining progressive damaging and slightly higher damping properties. Zhao et al. and Wang et al. [START_REF] Zhao | Fatigue behavior and failure mechanism of basalt FRP composites under long-term cyclic loads[END_REF][START_REF] Wang | Static and fatigue behavior of basalt fiber-reinforced thermoplastic epoxy composites[END_REF] have also demonstrated the fatigue performances of basalt/epoxy composites. In particular, Wang et al. [START_REF] Wang | Static and fatigue behavior of basalt fiber-reinforced thermoplastic epoxy composites[END_REF] compared the fatigue behaviour of basalt composites with thermosetting or thermoplastic epoxy matrix and observed that the basalt fibre-reinforced thermosetting epoxy polymer composite has similar static strength and similar fatigue life at high-stress levels than the thermoplastic epoxy-based one. However, at lowstress levels, the fatigue life for the latter is much higher than that of thermosetting epoxy-based one.

On the other hand, there are only few studies on the fatigue behaviour of hybrid composites with continuous reinforcements. Sharba et al. carried out a study on the effect of kenaf fibre orientation on fatigue life of a glass-kenaf hybrid composite [START_REF] Sharba | Partial replacement of glass fiber by woven kenaf in hybrid composites and its effect on monotonic and fatigue properties[END_REF]. Mostafa performed fatigue tests on glass-jute hybrid composites [START_REF] Mostafa | Tensile and fatigue properties of Jute-Glass hybrid fibre reinforced epoxy composites[END_REF], Sivakumar et al. studied glass-kenaf hybrid composites [START_REF] Sivakumar | Fatigue Life Behaviour of Glass/Kenaf Woven-Ply Polymer Hybrid Biocomposites[END_REF] and Salman et al.

worked on Kevlar-kenaf hybrid composites [START_REF] Salman | Tension-Compression Fatigue Behavior of Plain Woven Kenaf/Kevlar Hybrid Composites[END_REF]. The benefits of basalt hybridisation in flax based laminates have been well documented in literature in terms of quasi-static [START_REF] Yuanyuan | Hybrid basalt/flax fibers reinforced polymer composites and their use in confinement of concrete cylinders[END_REF][START_REF] El-Baky | Mechanical characterization of hybrid composites based on flax, basalt and glass fibers[END_REF][START_REF] Fiore | Effect of external basalt layers on durability behaviour of flax reinforced composites[END_REF] and low velocity impact properties [START_REF] Papa | Impact behaviour of hybrid basalt/flax twill laminates[END_REF][START_REF] Živković | Influence of moisture absorption on the impact properties of flax, basalt and hybrid flax/basalt fiber reinforced green composites[END_REF]. Nevertheless, to the best of authors' knowledge, there are no published results concerning the fatigue behaviour of basalt-plant hybrid composites. The aim of the current study is to bridge this gap. Basalt and flax fibres are chosen for their eco-friendly character along with their good mechanical performances. With a view to semi-structural applications, for example in the transport sector, woven reinforcement is selected, being also better suited to resist accidental impact events. At first, an optimisation of the curing cycle was made in order to reach the highest possible value of the glass transition temperature for the epoxy resin. Then, three different kinds of woven composites were manufactured: two non-hybrid composites with only flax reinforcement or basalt reinforcement, and a basalt-flax hybrid composite. Quasi-static tensile tests and tension-tension fatigue tests were performed on samples with two orientations, 0°/90° and ± 45°. The application of a fatigue life model is proposed, while damage mechanisms are investigated by means of infrared thermography and scanning electron microscopy.

Materials and methods

Materials

Concerning the plant reinforcement, a Biotex flax fabric (Linum usitatissimum), supplied by Composites Evolution (UK), was used. This fabric is commercialised without any specific sizing. It is a 2×2 twill fabric with a fibre areal weight of 200 g/m 2 . Each flax yarn has a diameter of about 327 m and is made of several elementary flax fibres twisted together. A typical elementary flax fibre has a diameter of about 25 m [START_REF] Seghini | Interfacial adhesion assessment in flax/epoxy and in flax/vinylester composites by single yarn fragmentation test: Correlation with micro-CT analysis[END_REF]. For the mineral reinforcement, a basalt woven fabric, supplied by Basaltex (Belgium), was selected. It is a plain woven fabric (220 g/m 2 ), made of 100 % BCF (basalt continuous filament) roving. One basalt fibre has a nominal diameter of 13 m [START_REF] Seghini | Engineering the interfacial adhesion in basalt/epoxy composites by plasma polymerization[END_REF]. For all the composites, the same epoxy resin was used. It is the PRIME 27 infusion resin with the PRIME 20 slow hardener, both delivered by GURIT. The mixing ratio was 100:28 by weight.

Three kinds of epoxy matrix woven composites were manufactured, all with the same target thickness: 2.5 ± 0.3mm. Two non-hybrid composites were elaborated as reference materials: a flax composite (100 % flax), made of 4 plies, and a basalt composite (100 % basalt), made of 16 plies. The hybrid composite (50 % of flax plies and 50 % of basalt plies) consisted of 8 plies with a symmetrical intercalated sequence: [(basalt/flax) 2 ] s . This stacking sequence has been chosen because optimum mechanical properties can be achieved by placing high strength fibres as the skin layers [START_REF] Nunna | A review on mechanical behavior of natural fiber based hybrid composites[END_REF]. For instance, Gupta et al. have compared the mechanical properties of flax fibre composite, glass fibre composite, glass-flax-glass hybrid (GFGH) composite and flax-glass-flax hybrid (FGFH) composite.

They reported that the stacking sequence GFGH was superior to the FGFH one in terms of tensile, compressive, flexural and impact strengths [START_REF] Gupta | Experimental investigation of the behavior of dual fiber hybrid composite under different stacking sequence[END_REF].

The different composite plates were produced by vacuum infusion. The fabrication process started with cleaning the mould plate and waxing it with a release agent so that proper demoulding can be achieved after curing. Then the dry plies were stacked on the mould plate to make the proper laminates. Peel ply, sealing tape and resin tubing were added and the whole set up was vacuum bagged. Then the resin was infused and impregnated the fabrics. The fibre volume fraction V f and the porosity volume fraction V p were determined for each type of composite by weighing and measuring samples. For the basalt composite, V f is equal to 55 ± 1 %, and it is 26 ± 0.6 % for the flax one. The fibre volume fraction is 33 ± 0.7 % for the flax-basalt hybrid composite, with 20 % of flax fibres and 13 % of basalt fibres. The average measured value of V p for all the tested samples is 7.5 ± 2 %. In order to assess the influence of fibre orientation on the mechanical properties of the composites, specimens were cut in two different directions in the manufactured plates. Thus, two stacking sequences were studied: a first sequence, identified as 0°/90°, is characterised by the warp yarns aligned along the loading direction; the second sequence, identified as ± 45°, corresponds to an orientation of the warp yarns at 45 degrees with respect to the loading direction of the composite. Each sample has a rectangular shape with an overall length of 150 mm, a gauge length of 60 mm and a width of 25 mm.

In order to produce a valid failure mode during fatigue tests, i.e. within the gauge length of the specimens, different tab configurations were tried, with various materials (aluminium, glass/PP composite and glass/epoxy composite) and geometries. A final configuration of glass/epoxy tabs with a length of 40.5 mm, a thickness of 1.3 mm and a taper angle of 15° was chosen. The ADEKIT H9951 structural epoxy adhesive, supplied by Axon, was used for the tab bonding (Fig. 1).

Differential Scanning Calorimetry (DSC) measurements

DSC measurements were performed using the differential scanning calorimeter Q20 from TA Instruments on pure resin samples. For each test, two aluminium pans were prepared: the first one is empty and is used as the reference and the second one is filled with the matrix sample (between 5 mg and 10 mg of material were used). The method used was as follows: a first heating cycle from 20 °C to 150 °C, an isotherm of 2 minutes at 150 °C, a cooling cycle from 150 °C to 20 °C, an isotherm of 2 minutes at 20 °C and finally a second heating cycle identical to the previous one. All heating cycles were carried out with a rate of 10 °C/min.

Tensile testing

A tensile characterisation of the basalt/epoxy, flax/epoxy and basalt-flax/epoxy composite materials was carried out by performing tests at room temperature (ASTM D 3039). An Instron 1195 machine equipped with a 100 kN load cell and with manual wedge action grips was used. For each test, a crosshead speed of 2.5 mm/min was used and the sample deformation was evaluated using an axial clip-on extensometer.

Fatigue testing

Fatigue tests were performed using an Instron 8874 servo-hydraulic bi-axial testing machine equipped with a 25kN load cell and screw grips (Fig. 2). Tests were realised according to the ASTM D 3479 standard test method. Constant amplitude loads were applied in a sinusoidal waveform at a frequency of 5 Hz under load-control. Tension-tension mode fatigue tests were carried out with a stress ratio (R) of 0.1. Fatigue tests were performed on the three composites for both 0°/90° and ± 45° orientations.

Different levels of maximum stress  max were used, from 25 % to 85 % of the tensile strength  f . Some samples were not broken after more than 3×10 6 cycles. In that cases, fatigue tests were stopped and are indicated with an arrow in S-N curves.

Infrared camera

A qualitative analysis of the temperature evolution at the specimen surface was performed by a Fluke Ti300 + Thermal Camera, with an acquisition rate of 9 Hz. This camera is equipped with the LaserSharp™ AutoFocus, ensuring focused images. The temperature range is -20 °C to 650 °C with a measurement accuracy of ±2 °C at 25 °C and a detector resolution of 320×240pixels.

FE-SEM observations

After fatigue tests, the morphology of the fracture surfaces of the different composites was investigated by a Field Emission -Scanning Electron Microscope (FE-SEM): JEOL JSM-7000F. All specimens were sputter coated with gold prior to FE-SEM observation.

Results and discussion

Optimisation of the curing cycle

In order to reach the maximum level of crosslinking for the epoxy resin and thus to generate optimal mechanical properties of the resin, an optimisation of the curing process was carried out. The glass transition temperatures, T g , were determined by DSC measurements on various epoxy samples. Fig. 3 shows an example of DSC thermogram obtained with an uncured epoxy sample. The endothermic peak visible at about 50 °C during the first heating cycle can be referred to as the physical aging phenomenon of the resin. Indeed, the storage of polymers below their glass transition temperature results in a gradual process of molecular relaxation in which the polymer chains form denser regions. This endothermic peak is thus due to the enthalpic recovery of the polymer. It demonstrates that the T g value for the uncured resin is about 50 °C. From the DSC results shown in Fig. 4, it is possible to see that, although such curing treatments are suggested in the technical data sheet of the manufacturer, the presence of an exothermic phenomenon in both DSC curves during the first heating cycle (red arrows in Fig. 4) indicates that such curing schedules are not sufficient to complete the resin crosslinking and thus to maximise its mechanical properties. Therefore, other curing protocols were investigated. Finally, the curing process with a duration of 7 hours at 90 °C was selected. It can be seen in Fig. 5 that, after this curing process, the exothermic phenomenon is completely eliminated from DSC curves and so the crosslinking of the resin can be considered almost complete. This curing treatment leads to a T g of about 88.9 °C. All the composite plates were thus cured with this procedure of 7 h at 90 °C.

Tensile characteristics

Quasi-static tensile tests were carried out on a minimum of three specimens for each type of composite: basalt, flax and flax-basalt hybrid materials with the two orientations, 0°/90° and ± 45°.

Examples of stress-strain curves obtained for 0°/90° and ± 45° flax-basalt hybrid composites are plotted in Fig. 6. For the 0°/90° orientation, the tensile curve is quasi linear up to failure, driven by the behaviour of longitudinal fibres. On the contrary, for the ± 45° orientation, the matrix-dominated behaviour leads to a more ductile aspect of the curve (Fig. 6). The mean values of tensile strengths and moduli for all the tested composites are given in Table 1. The results of the present study are consistent with these values, as for a fibre volume fraction of 26 %, the tensile strength is 106 MPa and the elastic modulus is 6 GPa for the 0°/90° orientation (Table 1). For the 100 % basalt composite, measured values are similar to those obtained by Dorigato et al. [START_REF] Dorigato | Fatigue resistance of basalt fibers-reinforced laminates[END_REF] for a fibre volume fraction of 61 %, while it is of 55 % in the present study: 600 MPa vs 635 MPa for the tensile strength, and 29.6 GPa vs. 28.8 GPa for the Young's modulus of the 0°/90° orientation. 1). This is in accordance with the results obtained in tensile loading for carbon-natural fibres hybrid composites [START_REF] Santulli | Mechanical and Impact Damage Analysis on Carbon / Natural Fibers Hybrid Composites: A Review[END_REF] or for jute-glass hybrid composites [START_REF] Mostafa | Tensile and fatigue properties of Jute-Glass hybrid fibre reinforced epoxy composites[END_REF].

Wöhler curves

Fatigue tests were performed on each type of composite: basalt, flax and flax-basalt hybrid materials for the two orientations 0°/90° and ± 45°. Between eight and twelve samples were tested for each configuration, with two or three tests for given stress levels. The average number of fatigue failure cycles is given in Table 2 for the 0°/90° orientation, and in Table 3 for the ± 45° one. The corresponding Wöhler curves are plotted in Fig. 7. The points related to the non-broken samples are indicated by an arrow. It can be seen in Fig. 7 that there is a slight and usual dispersion in fatigue lifetime for all the tested materials. As expected, for a given number of fatigue failure cycles, the maximum stress is lower for flax composites than for basalt composites, and intermediate for flax-basalt composites, in accordance with the quasi-static tensile strengths (Table 1). Moreover, fatigue results demonstrate that, for each type of composite, for a given maximum fatigue stress value, the fatigue life for the ±45° orientation is far lower than for the 0°/90° one. Fig. 7. Wӧhler curves for basalt, flax and hybrid composites with 0°/90° and ±45° orientations.

In order to compare the fatigue sensitivity of the different materials, normalised Wöhler curves are reported in Fig. 8 and Fig. 9 for the 0°/90° and ±45° orientations, respectively. In these figures, the xaxis is the decimal logarithm of the number of cycles to failure for each sample, while the y-axis reports the ratio between the maximum applied stress and the tensile strength. The points related to the non-broken samples are again indicated by an arrow.

The main aspect emerging from Fig. 8 and9 is the positive effect of hybridisation when comparing the normalised S-N curves for these different composites. For both 0°/90° and ±45° orientations, the hybrid composites exhibit an intermediate fatigue behaviour between those of basalt/epoxy and flax/epoxy composites. The fatigue resistance of basalt/epoxy composites is enhanced by flax fibre hybridisation. From Fig. 8 and 9 it is possible to point out that the hybridisation allows to obtain a higher number of cycles to failure than those of basalt fibre reinforced composites. This result may be related to a positive action of flax fibres. Several studies present in literature have compared the fatigue behaviour of natural fibre reinforced composites with that of glass fibre composites. Most of these works highlighted that the glass reinforced composites display a higher absolute resistance to fatigue than the natural fibre reinforced composites [START_REF] Mahboob | Fatigue of flax-epoxy and other plant fibre composites: Critical review and analysis[END_REF]. This is mainly due to the superior tensile strength of glass fibre composites compared to plant fibre composites. Shah et al. [START_REF] Shah | Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams[END_REF] performed a comparison between the fatigue behaviour of plant fibre (jute, hemp and flax) polyester matrix composites with the behaviour of a glass/polyester composite. The authors reported that the fatigue strength degradation rates are lower in plant fibre composites than in glass fibre composites. A slower fatigue stiffness degradation of plant fibres compared to that of glass fibres and the ability of plant fibre to absorb and deflect cracks, thanks to their complex microstructure, can be considered as possible explanations of the fatigue behaviour of natural fibre reinforced composites. This better fatigue resistance of natural fibre composites allows the positive effect of hybridisation that is visible in Fig. 8 and9: by adding flax fibres, hybrid composites show better fatigue resistance than pure basalt composites. Same behaviour was observed for other hybrid composites in the literature. For example, it has been shown that kenaf-glass [START_REF] Sharba | Partial replacement of glass fiber by woven kenaf in hybrid composites and its effect on monotonic and fatigue properties[END_REF] or jute-glass [START_REF] Mostafa | Tensile and fatigue properties of Jute-Glass hybrid fibre reinforced epoxy composites[END_REF] hybrid composites exhibit lower fatigue strength but higher fatigue resistance than pure glass composites.

𝛼 • 𝑁 1 𝜎 , 𝜎 𝑚𝑎𝑥 1 𝐹 1 𝑅 . | | 𝜎 , 𝜎 𝑚𝑎𝑥 . | | (3)
The parameters α and β have to be optimised in order to ensure that the experimental data converge on a straight line passing through the origin when plotting the right-hand term against N f β -1. The optimisation scheme used is described in Fig. 10. The α and β values determined for the different composites are reported in Table 4 .

Fig. 10. Optimisation scheme for the determination of α and β (from [START_REF] Malpot | Fatigue behaviour of a thermoplastic composite reinforced with woven glass fibres for automotive application[END_REF]). From Fig. 11 it is possible to observe that the model is able to correctly predict the fatigue behaviour of flax fibre and hybrid composites. A good correspondence between the experimental values and the model is reported. On the contrary, a different result is found for the basalt composites for both 0°/90° and the ±45° orientations. In particular, the latter configuration exhibits the worst results, specifically at the lower values of maximum applied stress. To better understand this phenomenon and the fatigue behaviour of the different composites, an analysis of the damage was performed.

Damage analysis

A thermal camera has been used in order to obtain the distribution of the temperature on the surface of tested specimens. When a composite is deformed or damaged, a part of the energy is transformed in an irreversible way into heat [START_REF] Toubal | Damage evolution and infrared thermography in woven composite laminates under fatigue loading[END_REF]. In this study, a qualitative analysis of the temperature evolution at the specimens' surface was performed by a thermal camera, with acquisition rate of 9 Hz. Temperature measurements performed on woven hemp/epoxy composites reported that the highest temperature gradient on the specimens' face was obtained for the highest applied fatigue loading values [START_REF] Vasconcellos | Tension-tension fatigue behaviour of woven hemp fibre reinforced epoxy composite: A multi-instrumented damage analysis[END_REF]. In addition, the authors highlighted that the temperature gradient is more significant for the ±45° laminates than for the 0°/90° ones. For these reasons, in order to address the worst heating conditions, in the present study, measurements were carried out during fatigue tests performed at the stress level of 85 % of the ultimate tensile stress and for the composite samples with the ±45° orientation. Results

show the progressive appearance of hot areas in localised zones on specimen's surface, as shown in Fig. 12, where images were taken right before the specimen's failure. The basalt composite exhibits an excessive overheating, reaching a maximum temperature of 98.5 °C. It demonstrates that, for this configuration, an excessive internal heating occurs during fatigue testing. This phenomenon could explain why the fatigue life model cannot fit with the Wöhler curves obtained for the basalt composites, especially for the ±45° orientation. Indeed, the model proposed by Epaarachchi and Clausen is based on the assumption that the temperature of the specimen is constant or close to constant during test [START_REF] Epaarachchi | An empirical model for fatigue behavior prediction of glass fibre-reinforced plastic composites for various stress ratios and test frequencies[END_REF]. debonding is also found for flax/epoxy composites (Fig. 14). An extensive flax yarn pull-out was observed, where the matrix crack runs around the yarns. Fig. 14B shows a matrix transverse crack spreading around the flax fibre, producing a subsequent debonding at the fibre-matrix interface.

Through cyclic loading, the matrix cracks can grow and initiate local debonding between the flax yarns thus causing a faster decrease in resistance of the laminate [START_REF] Bensadoun | Fatigue behaviour assessment of flax-epoxy composites[END_REF]. In the case of flax fibre composites, where twist and crimp are present compared to the basalt composite counterparts where only the crimp is present, extra matrix cracking can be initiated due to the more complex stress state in the twisted yarns oriented along the testing direction. The flax yarns can try to "untwist" creating shear and normal stresses relative to the fibre orientation and resulting in additional micro-cracks at the fibre/matrix interface. The FE-SEM observations also highlighted the presence of porosities, clearly visible in Fig. 14A. In hybrid composites, pull-out and debonding phenomena of both basalt fibres and flax yarns can be seen (Fig. 15), without evidence of significant delaminations at basalt/flax interfaces. These results

show that interfacial adhesion is a key-parameter for fatigue performance of all the composites, hybrids and non-hybrids.

Fig. 1 .

 1 Fig. 1. a) a 0°/90° flax composite sample before testing, b) the glass/epoxy tabs bonded on a ± 45° flax-basalt composite sample, c) a 0°/90° basalt composite sample after tensile failure.

Fig. 2 .

 2 Fig. 2. The fatigue testing machine (A) and the grips configuration used (B).

Fig. 3 .

 3 Fig. 3. DSC thermogram of the uncured epoxy resin.Two curing protocols were then tested: one with a duration of 16 hours at 50 °C and one with a duration of 7 hours at 65 °C.

Fig. 4 .

 4 Fig. 4. DSC thermograms for the epoxy resin after the curing process at 50 °C during 16 hours (a), and at 65 °C during 7 hours (b).

Fig. 5 .

 5 Fig. 5. DSC thermograms for the epoxy resin after the curing treatment at 90 °C during 7 hours.

Fig. 6 .

 6 Fig. 6. Examples of quasi-static tensile stress-strain curves for the 0°/90° and the ± 45° flax-basalt hybrid composites.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Normalised Wӧhler curves for basalt, flax and hybrid composites with the 0°/90° orientation.

Fig. 11 .

 11 Fig. 11. Normalised Wӧhler curves and curves of the Epaarachchi and Clausen model for basalt, flax and hybrid composites with 0°/90° and ±45° orientations.

Fig. 11

 11 Fig. 11 shows the Wöhler curves of the experimental results for each type of composite and the corresponding Epaarachchi and Clausen model curves, represented by the solid lines. The dashed lines represent the dispersion intervals, which are calculated from the dispersion of the tensile strength.

Fig. 12 .

 12 Fig. 12. Temperature fields on specimen's surface measured by infrared camera during fatigue tests just before the failure for ±45° laminates at the stress level of 85 %  f .

Fig. 13 .

 13 Fig. 13. FE-SEM micrographs of the basalt/epoxy composites after fatigue failure at stress level of 45 %  f : (A-B) 0°/90° and (C-D) ±45° laminates.

Fig. 14 .

 14 Fig. 14. FE-SEM micrographs of the flax/epoxy composites after fatigue failure at stress level of 45 %  f : (A-B) 0°/90° and (C-D) ±45° laminates.

Fig. 15 .

 15 Fig. 15. FE-SEM micrographs of the basalt-flax hybrid composites after fatigue failure at stress level of 45 %  f : (A-B) 0°/90° and (C-D) ±45° laminates.

Table 1 .

 1 Tensile strengths and moduli of the hybrid and non-hybrid composite materials for the two orientations.

	Orientation	Material	f [MPa]	E [GPa]
		Basalt	635 ± 50	28.8 ± 0.2
	0°/90°	Flax	106.3 ± 5.2	5.8 ± 0.1
		Flax-basalt hybrid	190.9 ± 6.4	13.2 ± 0.1
		Basalt	154.3 ± 5.4	14.6 ± 0.5
	± 45°	Flax	63.6 ± 0.6	3.7 ± 0.1
		Flax-basalt hybrid	118.4 ± 1.7	6.8 ± 0.8

Results in Table

1

also show that, for 0°/90° and ± 45° orientations, the flax fibre composites exhibit a significantly lower tensile strength  f and elastic modulus E than the basalt composites, while the hybrid composites show an intermediate behaviour (Table

Table 2 .

 2 The average number of fatigue failure cycles for hybrid and non-hybrid 0°/90° composites.

			N f
	 max			Flax-basalt
		Basalt	Flax
				hybrid
	85 %  f,0°	73	17	167
	65 %  f,0°	231	3165	3214
	45 %  f,0°	804	69480	16167
	35 %  f,0°	8001	1097894	330527
	30 %  f,0°	69965	/	/
	25 %  f,0°	2852941	>3145098	>10000000

Table 3 .

 3 The average number of fatigue failure cycles for hybrid and non-hybrid ±45° composites.

			N f
	 max			Flax-basalt
		Basalt	Flax
				hybrid
	85 %  f,45°	106	2370	210
	65 %  f,45°	382	21690	1541
	50 %  f,45°	1010	813931	57596
	45 %  f,45°	7014	1535002	268131
	35 %  f,45°	2924442	>3933076	2832228

Table 4 .

 4 The α and β values of the fatigue life model, calculated for basalt, flax and hybrid composites with 0°/90° and ±45° orientations.

	Orientation	Material	α	β
		Basalt	0.129	0.473
	0°/90°	Flax	0.095	0.323
		Hybrid	0.121	0.338
		Basalt	0.655	0.044
	± 45°	Flax	0.043	0.320
		Hybrid	0.245	0.210

Modelling of fatigue life

Several models have been proposed over the years in order to predict the fatigue resistance of composite materials characterised by different stacking sequences, different types of fibres and matrices and under different loading conditions [START_REF] Degrieck | Fatigue Damage Modelling of Fibre-reinforced Composite Materials: Review[END_REF][START_REF] Sevenois | Fatigue Damage Modeling Techniques for Textile Composites: Review and Comparison With Unidirectional Composite Modeling Techniques[END_REF]. Among these, it is possible to find the model proposed by D'Amore et al., based on the residual strength degradation of the composite [START_REF] D'amore | Effect of stress ratio on the flexural fatigue behaviour of continuous strand mat reinforced plastics[END_REF]. This model was developed for glass-mat-reinforced thermoset based composites, but it was proved to be able to successfully predict the fatigue behaviour of continuous glass fibre reinforced composites [START_REF] Caprino | Flexural fatigue behaviour of randomcontinous-fibre-reinforced thermoplastic composites[END_REF][START_REF] Caprino | Fatigue lifetime of glass fabric/epoxy composites[END_REF] and woven hemp/epoxy composites [START_REF] Barbière | Influence of moisture and drying on fatigue damage mechanisms in a woven hemp/epoxy composite: Acoustic emission and micro-CT analysis[END_REF]. The model proposed by D'Amore et al. was used as a basis for the development of the Epaarachchi and Clausen model [START_REF] Epaarachchi | An empirical model for fatigue behavior prediction of glass fibre-reinforced plastic composites for various stress ratios and test frequencies[END_REF], used in the present study. The starting point of the model is a deterministic equation to measure the rate of strength degradation (Eq. 1):

where σ is the residual strength after N cycles, C 1 and m 1 are material constants and N is the number of fatigue cycles. From Eq. 1 and assuming that the fatigue failure occurs when the residual strength remaining in the sample in the direction of applied load is equal to σ max , the number of cycles required to degrade the material strength from σ f (the ultimate tensile stress) to σ max (the maximum applied stress), corresponding to the number of cycles at failure N f , can be expressed using Eq. 2:

where σ max is the maximum fatigue stress, σ f,θ is the tensile strength for the orientation θ, F is the frequency, R is the stress ratio, θ is the smallest angle of fibres between the loading direction and the fibre direction and α and β are the two parameters to be determined. As it can be seen in Eq. 2, this model takes into account the frequency, the stress ratio and the fibre orientation in the composite. In order to identify the two parameters, α and β, Eq. 2 has to be written as Eq. 3: