Michaël Berges

Romain Léger

Vincent Placet
email: vincent.placet@univ-fcomte.fr

Véronique Person

Stéphane Corn

Xavier Gabrion

Jérôme Rousseau

Emmanuel Ramasso

Patrick Ienny

Stéphane Fontaine

Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates

Keywords: A. Polymer-Matrix Composites, A. Natural fibres, B. Mechanical properties, C. Moisture ρ m : matrix density W f : fibre weight fraction V f : fibre volume fraction

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

NOMENCLATURE

Mechanics

σ L : tensile strength in fibre direction σ T : tensile strength in transverse direction ε L : tensile strain at failure in fibre direction ε T : tensile strain at failure in transverse direction ν LT , ν TL : Poisson's ratios (determined using linear regression between 0.01% and 0.15% axial strain) E 𝐿 𝑚1 : axial apparent modulus determined under monotonic tensile loading using linear regression between 0.01% and 0.15% strain E 𝐿 𝑚2 : axial apparent modulus determined under monotonic tensile loading using linear regression between 0.4% and strain at failure E 𝑇 𝑚 : transverse apparent modulus determined under monotonic tensile loading using linear regression between 0.01% and 0.15% strain 𝐸 𝐿 * : axial dynamic modulus determined from the peak-to-peak amplitudes of strain and stress during fatigue tests E 𝐿 𝑑 : axial dynamic elastic modulus determined using free vibration analysis ξ: damping ratio determined using free vibration

INTRODUCTION

Due to the concerns about environment, sustainability issues, new rules and regulations, biobased materials have raised growing interest from numerous industries [START_REF] Shah | Developing plant fibre composites for structural applications by optimising composite parameters: a critical review[END_REF]. The considerable attraction of the worldwide research community and the rapid growth in innovation is warranted to the unique intrinsic properties such as biodegradability, availability, environmental friendliness, and physico-mechanical properties of this group of materials. [START_REF] Thakur | Processing and characterization of natural cellulose fibers/thermoset polymer composites[END_REF][START_REF] Thakur | Review: Raw Natural Fiber-Based Polymer Composites[END_REF][START_REF] Pickering | A review of recent developments in natural fibre composites and their mechanical performance[END_REF].

Thanks to outstanding research accomplishments and recent development efforts, plant fibres reinforced composites have been developed. They are usually dedicated to non-structural applications, i.e. where high loading capacity is not required. These applications take place in automotive, aerospace, packaging and building industries.

Fibre with high mechanical properties can be extracted from plants, such as flax, hemp, sisal for example. They are good candidates to be used as composite reinforcements for lightweight structural applications. Up-to-date critical reviews of literature [START_REF] Shah | Developing plant fibre composites for structural applications by optimising composite parameters: a critical review[END_REF][START_REF] Summerscales | A review of bast fibres and their composites. Part 2 -Composites[END_REF][START_REF] Dittenber | Critical review of recent publications on use of natural composites in infrastructure[END_REF][START_REF] Bhowmick | Mechanical properties of natural fibre-reinforced composites[END_REF][START_REF] Huber | A critical review of all-cellulose composites[END_REF][START_REF] Koronis | Green composites: A review of adequate materials for automotive applications[END_REF][START_REF] Mantia | Green composites: A brief review[END_REF] show that several technological and scientific barriers still remain to be knocked down in order to obtain fully optimised plant fibre composites (PFCs) for structural applications. One of the main question concerns the long-term durability of such composites. Indeed, for some intended applications, PFCs will be heavily exposed to moisture, various temperatures and UV radiations. These environmental loadings can strongly affect their mechanical properties and therefore play a critical role in the service life of PFCs.

Up to now, the influence of temperature on the mechanical properties of plant fibres and PFC has been mainly characterized with a view to improving the composites manufacturing processes, in particular for thermoplastic matrices [START_REF] Liang | Thermo-compression forming of flax fibre-reinforced polyamide 6 composites: influence of the fibre thermal degradation on mechanical properties[END_REF][START_REF] Gourier | Mechanical analysis of elementary flax fibre tensile properties after different thermal cycles[END_REF][START_REF] Placet | Diameter dependence of the apparent tensile modulus of hemp fibres: A morphological, structural or ultrastructural effect?[END_REF].

Moisture absorption is a phenomenon affecting most of polymer composites when placed in humid atmosphere or immersed in water. Water uptake can lead to a change in the physical, chemical, mechanical and dimensional properties of the matrix (plasticization, hydrolysis, swelling as a function of its affinity with water). It can also degrade fibre-matrix interface. This results in a decrease in the stress transfer efficiency and thus in the mechanical properties of the composite material. Contrary to glass or carbon fibres, plant fibres have a high affinity with water. This property gives composite materials a high water uptake capacity. Water vapour or liquid water can thus be easily absorbed by PFCs through fibres. Into the wall of plant fibres, water is absorbed through hydrogen bonds. It induces breaks into the secondary interactions between cellulose macromolecules, it also causes swelling and modifies the mechanical properties of the fibres [START_REF] Placet | Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres[END_REF][START_REF] Davies | Effect of Environmental Relative Humidity and Damage on the Tensile Properties of Flax and Nettle Fibers[END_REF][START_REF] Stamboulis | Effects of environmental conditions on mechanical and physical properties of flax fibers[END_REF][START_REF] Symington | Tensile Testing of Cellulose Based Natural Fibers for Structural Composite Applications[END_REF]. It is therefore of paramount importance to characterize moisture absorption and its influence on the long-term behaviour of PFCs.

In the literature, several studies have been focused on this topic for different types of PFCs [START_REF] Mokhothu | Review on hygroscopic aging of cellulose fibres and their biocomposites[END_REF]. Le Duigou et al. [START_REF] Duigou | Seawater ageing of flax/poly(lactic acid) biocomposites[END_REF][START_REF] Duigou | Long term immersion in natural seawater of Flax/PLA biocomposite[END_REF][START_REF] Duigou | In-situ evaluation of flax fibre degradation during water ageing[END_REF] studied the influence of sea water immersion and ageing on flax PLA biocomposites. They showed a linear relationship between water uptake and loss of mechanical properties [START_REF] Duigou | Long term immersion in natural seawater of Flax/PLA biocomposite[END_REF]. They also pointed out, using in situ nano-indentation, that the fibre wall rigidity decreases when immersion time increases [START_REF] Duigou | In-situ evaluation of flax fibre degradation during water ageing[END_REF].

For hemp fibre reinforced polyester composites, water absorption was found to follow a Fickian behaviour at room temperature and a non-Fickian one at high temperatures [START_REF] Dhakal | Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites[END_REF]. A non-Fickian behaviour was also observed for hemp-polypropylene composite [START_REF] Toubal | Hygrothermal effect on moisture kinetics and mechanical properties of hemp/polypropylene composite: Experimental and numerical studies[END_REF]. For those composite materials, authors also showed that the time required to reach water saturation decreases significantly when the immersion temperature increases. In addition, the tensile and flexural properties of these hemp-polyester composites were found to decrease as the percentage of moisture uptake increases [START_REF] Dhakal | Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites[END_REF].

In the case of flax-epoxy composites, several studies pointed out that the evolution of water absorption follows a Fickian law. Moreover, kinetics of water uptake is clearly influenced by the temperature level [START_REF] Scida | Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite[END_REF][START_REF] Munoz | Water Absorption Behaviour and Its Effect on the Mechanical Properties of Flax Fibre Reinforced Bioepoxy Composites[END_REF]. Scida et al. [START_REF] Scida | Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite[END_REF] showed that water vapour sorption induces a significant reduction of the Young's modulus but only a slight decrease in strength. They also noticed a change in the shape of the tensile curve. Munoz et al. [START_REF] Munoz | Water Absorption Behaviour and Its Effect on the Mechanical Properties of Flax Fibre Reinforced Bioepoxy Composites[END_REF] showed that water uptake increases when fibre weight content increases. Moreover, they noticed that, while the tensile modulus decreases with the absorption of water, the tensile strength of wet specimens is higher than that of dry samples.

This review on PFCs also shows that the influence of moisture on durability has been mainly studied from a quasi-static behaviour perspective. Only few papers deal with fatigue [START_REF] Sodoke | Hygrothermal effects on fatigue behavior of natural fibers composites[END_REF] or vibration [START_REF] Duc | The role of twist and crimp on the vibration behaviour of flax fibre composites[END_REF][START_REF] Guen | Tailoring the vibration damping behaviour of flax fibre-reinforced epoxy composite laminates via polyol additions[END_REF] behaviours.

These lacks need to be filled in order that plant fibres can be considered as viable reinforcement for composite materials in structural applications.

Fatigue of flax-epoxy composites has been recently studied under ambient conditions [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF][START_REF] Shah | Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams[END_REF][START_REF] Liang | A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites[END_REF][START_REF] Liang | Properties evolution of flax/epoxy composites under fatigue loading[END_REF][START_REF] Asgarinia | Tension-tension fatigue behaviour of woven flax/epoxy composites[END_REF][START_REF] Ueki | Fatigue behavior of uni-directional flax fibre/epoxy composites[END_REF][START_REF] Bensadoun | Residual properties and damage evolution of flax-epoxy composites subjected to fatigue loading[END_REF][START_REF] Shah | Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading[END_REF]. Results show that these composite materials exhibit good fatigue performance. The latter are considered comparable to those measured on composites reinforced with glass fibres. This points out the potential of flax epoxy composites to be implemented in load-bearing applications. Bensadoun et al. [START_REF] Bensadoun | Residual properties and damage evolution of flax-epoxy composites subjected to fatigue loading[END_REF] underlined that the fibre architecture has a strong effect on the fatigue behaviour of these composite materials. Hence, best fatigue characteristics are reached when using fibres with architectures that lead to the highest static strengths and moduli. Despite these recent works on fatigue, the influence of moisture absorption and environmental loading has not been yet considered.

Thus, the purpose of this paper is to investigate and quantify the influence of water vapour absorption on the mechanical properties of flax fibres reinforced epoxy composites, using quasi static, fatigue and vibration tests.

This work was conducted as part of the scientific collaboration between three French laboratories (DRIVE, FEMTO-ST and C2MA), which are members of a research consortium called BioComp. This consortium groups several partners with expertise in the field of mechanics of fibre and reinforcements, manufacturing and characterization of plant reinforcements, polymers and composites, biochemistry as well as chemistry of interfaces.

MATERIAL AND METHODS

Composite material

Composite biobased-materials have been fabricated from prepregs flax fibres and epoxy resin. FlaxPreg UD 150 was supplied by Lineo company (France). The flax fabric has an areal weight of 154 g/m 2 . It contains 95% of warp yarns and 5% of weft yarns in weight. The epoxy resin is an Epoxy Huntsman LY 5150. The fibre fraction is approximately 50% in weight before curing.

Composite fabrication

Six plates (400 mm x 400 mm x 2.2 mm) were fabricated in DRIVE lab using a thermo-compression process under specific pressure and temperature cycle (see Fig. 1). These plates consist in 10 elementary unidirectional plies. Each plate has been characterized in terms of porosity and fibre contents according to the ASTM D 3171-99 standard. In order to evaluate the fibre, matrix and voids volume fractions, samples of 30 mm×25 mm were cut from the plates. The mass 𝑀, thickness h and area A of each specimen were measured.

The density of the composite 𝜌 𝑐 and the fibre weight fraction in specimen 𝑊 𝑓 were first determined using the following equations:

𝜌 𝑐 = 𝑀 ℎ.𝐴 𝑊 𝑓 = 𝐴 𝑟 .𝑛.𝐴 𝑀 ( 1
)
where n is the number of plies of the tested laminate and 𝐴 𝑟 is the mass of one ply per unit area. Then, the fibre volume fraction 𝑉 𝑓 , matrix volume fraction 𝑉 𝑚 and void volume fraction 𝑉 𝑣 were calculated using Eq. 2.

𝑉 𝑓 = 𝑊 𝑓 .

𝜌 𝑐 𝜌 𝑓 𝑉 𝑚 = (1 -𝑊 𝑓 ). 𝜌 𝑐 𝜌 𝑓 𝑉 𝑣 = 1 -𝑉 𝑓 -𝑉 𝑚 (2)
where 𝜌 𝑓 and 𝜌 𝑚 represent respectively the fibre and matrix densities.

A scanning electron microscope (Hitachi S-3000N) has also been used to observe the cross-section areas of the produced materials. Images have been recorded under partial vacuum with back-scattered electrons. A specific attention was dedicated to the study and the distribution of the voids (Fig. 2). On such an image, the warp yarns (white areas, which represent approximately 95% of the reinforcement structure), the epoxy matrix (grey areas)

and the voids (black areas) can be easily identified. The presence of weft yarns, which represent approximately 5% of the reinforcement structure, is also easily observed. A summary of the plate properties is presented in table 1.

Sample preparation

Rectangular specimens (250 mm in length and 25 mm in width) were sawn with a high-speed diamond cutting machine, in the fibre direction (0°) for one part, and perpendicularly to the fibre direction (90°) for the other part. After surface preparation (sanding then cleaning with acetone), aluminium tabs (50 mm in length, 25 mm in width and 3 mm in thickness) have been bonded to the test specimens dedicated to quasi-static and fatigue tests. DRIVE used a two-component adhesive (Huntsman Araldite 2015) and performed a heating of the assemblies at 40 °C for 2 hours with a slight pressure applied to the tabs. FEMTO-ST and C2MA used a Loctite Super Glue 3 adhesive to bond specimen and aluminium tabs.

Samples used for monotonic tests were also prepared for strain measurement. DRIVE equipped samples with 2 strain gages (Kyowa KFG-10-120-C1-11) in axial and transverse direction. Samples were speckled at C2MA in order to estimate the in plane strain tensor by digital image correlation [START_REF] Christmann | Mechanical behaviour at large strain of polycarbonate nanocomposites during uniaxial tensile test[END_REF].

Hygrothermal conditioning

After fabrication, samples were stored in a climatic chamber at 21°C and 50% RH before mechanical testing.

This hygroscopic state was named "DRY". Samples were also aged in a climatic chamber at 70°C and 85% RH for more than 2 weeks. During this period, relative weight uptake, dimensions, dynamic tensile modulus and damping were monitored. A Mettler-Toledo AT200 balance was used for weight monitoring (precision of 0.001 g), a calliper with a precision of 10 -2 mm was used to evaluate the swelling and a specific "home" set-up (presented in a following section) was employed to carry out the dynamic properties of aged specimens. At the end of the two-week monitoring period, an equilibrium state named WVS (Water Vapour Saturated state) has been achieved.

Mechanical testing

Monotonic tensile tests (MTT)

The tensile properties were determined by the three partners (DRIVE, FEMTO-ST and C2MA) using their own testing procedures. Results will be later discussed in terms of robustness and repeatability. Relevance of existing norms for composite materials will be discussed. Actually, the ISO 527-4 standard recommends to measure the elastic modulus by linearization of strain between 0.05 and 0.25%. However, it is known that plant fibre composites present a strongly non-linear behaviour at low strain. These recommendations should therefore be questioned [START_REF] Liang | Quasi-static behaviour and damage assessment of flax/epoxy composites[END_REF][START_REF] Poilâne | Polymer reinforced by flax fibres as a viscoelastoplastic material[END_REF][START_REF] Andersons | Modeling the nonlinear deformation of flax-fiber-reinforced polymer matrix laminates in active loading[END_REF]. When displaying the derivative of stress as a function of strain, the tangent modulus can be plotted (Fig. 3). On this figure, tangent modulus is not constant and presents a strong variation for these strains levels. As a consequence, we have chosen to evaluate this modulus in a similar and reduced strain domain (0.01-0.15%) to compare results. Strain measurements techniques in each lab are precisely described below.

MTT1 (DRIVE):

Uniaxial tensile measurements were carried out in both direction 0° and 90°on a tensile testing machine (MTS Criterion C45) equipped with a load cell of 100 kN.

The speed of the tests was 2 mm/min. Ten specimens (five by tested direction) were tested until failure. Young's modulus and Poisson's ratios were calculated between two levels of longitudinal deformation: = 0,01% and = 0,15% .

MTT2 (C2MA):

Young modulus and ultimate tensile stress and strain were determined by uniaxial tensile tests on a MTS testing machine (model Criterion C45.105) equipped with a 100 kN load cell. Samples were loaded at a crosshead speed of 1 mm/min. Modulus was evaluated in the strain interval of 0.01-0.15% using linear regression.

For each set of samples (DRY and WVS) oriented at 0°, 3 specimens were tested, while for samples oriented at 90°, 5 specimens were tested.

MTT3 (FEMTO-ST):

A hydraulic testing machine (Instron 8501) equipped with a load sensor with a measurement range of 100 kN was used to carry out tensile tests in the fibre direction. A 5 kN load sensor was used for the transverse direction.

The axial and transverse strains were measured using clip-on extensometers, with respective gauge length of 45 mm and 25 mm. The tensile stress was computed by dividing the applied load by the initial cross-section of the specimen. Samples were subjected to a displacement control; with a 2 mm.min -1 moving crosshead speed.

Tests were carried out up to failure. For the determination of the Young's modulus, considering that the materials do not have a strong linear relationship between stress and strain, even at very low stresses, a tangent method was used. The tangent modulus was expressed as a function of the axial strain. Two apparent moduli were also determined using linear regression respectively between 0.01% and 0.15% of strain (E 𝐿 𝑚1 ) and between 0.4% and strain at failure (E 𝐿 𝑚2 ).

For each set of samples, 3 to 6 specimens were tested and results are presented in terms of average and standard deviation values.

Tensile-tensile fatigue tests

The tensile-tensile fatigue tests were performed under a triangular waveform loading at a frequency of 5 Hz, using a load amplitude control mode, and a ratio between minimum and maximum stress (R) of 0.1. The experimental configuration (loading machine, specimen shape and preparation) was similar to monotonic tensile tests (MTT3, FEMTO tests). Fatigue tests were performed in the fibre direction for DRY and WVS specimens. Four levels of maximum stress were applied. The levels were 80%, 65%, 50% and 40% of the quasistatic tensile failure stress (at 2 mm.min -1 ) for the DRY specimens and approximately 75%, 60%, 55% and 45 % for the VWS specimens. At least three replicates were tested at each stress level. Tests were stopped at failure.

Tests were carried out under controlled hygrothermal conditions (T = 21 °C ±3 °C, RH = 50% ±5%). For WVS specimens, it was necessary to keep the water-saturated state during fatigue loading. Thus, the specimens were covered after the hygrothermal treatment with a flexible coating film. This one was elaborated from an aqueous mixture of iron oxide-filled acrylic resin. The mass of each specimen was measured after hygrothermal treatment and also before and after fatigue loading. The mass variation was checked to be negligible and the water-saturation state has been assumed to be constant during tests.

Peak-to-peak and average amplitudes of load and strain sensors were recorded for each cycle. A complete cycle was also recorded as a function of a logarithmic progression whose common difference depends on the stress level and thus fatigue test duration.

Free vibration tests

The viscoelastic parameters (elastic modulus and loss factor) of composite samples were determined using vibration analysis. This technique, which aims at studying the dynamic behaviour of a structure thanks to its natural vibration modes [START_REF] Corn | Vibration analysis techniques for detecting filler-matrix decohesion in composites[END_REF], is non-destructive. It is thus well suited for monitoring materials during water sorption or ageing.

For this study, the set-up simulates free boundary conditions by supporting the sample with soft suspensions.

The sample, a 250 mm x 25 mm beam, is excited in free vibration by an impulse hammer (Kistler 9722A2000).

Its response is measured by an accelerometer (Kistler 8778A500). An additional steel mass (m0 = mL = 86 g) was clamped at each end of the beam in order to bring down the natural frequency f1 of its first "tensioncompression" mode into a measurable frequency range (100-9000 Hz).

The resonant frequencies of the sample are given by the series of peaks in the frequency response function (Fig. 4). The software MODAN®, developed by the FEMTO-ST (Franche-Comté University, Besançon, France), was used to extract eigen frequencies and damping ratios from the frequency response function (by using a curve fitting technique).

Then, derived from the equation of the elastodynamic equilibrium of the tension-compression mode, the expression of the axial (i.e. longitudinal) elastic modulus E 𝐿 𝑑 is given by expression (3):

𝐸 𝐿 𝑑 = 𝜌 𝐿 2 𝑓 1 2 ( 𝛽 1 𝜋 ) 2 (3)
with β1 being the lowest positive root of equation ( 4):

𝛽 1 . tan 𝛽 1 = 𝑚 𝑀 𝑡 ( 4
)
where Mt is the total added mass (172 g), L and m being respectively the length and the mass of the part of the sample located between these two clamped masses, and 𝜌 its density.

The damping ratio ( ξ) of the first tenson-compression mode is extracted from the impulse response in the vicinity of the corresponding resonance peak thanks to MODAN®.

Before hygrothermal conditioning 10 specimens were tested for each plate, while the moduli of 6 specimens were followed throughout water vapour until saturation.

Optical and SEM observations

Pre-failure and post-failure microscopic observations of the cross-section of some DRY and WVS specimens were performed to inspect the defects and damage progress in the composite when submitted to fatigue loading.

These clichés allow to characterize the influence of water vapour sorption. Small coupons were cut in the tensile specimens with a diamond blade and embedded in a PMMA resin. The embedded coupons were grinded and polished. The observations were done using an optical microscope (Nikon Eclipse LV150) and an environmental SEM (FEI Quanta 450W).

Acoustic emission (AE)

Some of the monotonic and fatigue tests were instrumented with acoustic emission measurements. A twochannel data acquisition system (Euro Physical Acoustics, MISTRAS Group) was used. This system comprises miniature piezoelectric sensors (-80) with a resonance frequency ranging from 250-325 kHz, preamplifiers with a gain of 20 or 40 dB, a 20-1000 kHz filter, a PCI card with a 5 MHz sampling rate. The sensors were coupled onto the specimen faces using silicone grease. They were placed at the gauge extremities with a distance of 130 mm between their centres. System calibration was performed after installing the transducers on the specimen and before each test, using a pencil lead break procedure.

AE signals treatment and data processing and interpretation were done using home-made methodology and software. This methodology was developed in FEMTO-ST laboratory during the last years to reliably analyze and interpret AE data when collected in complex experimental configurations, such as during fatigue tests. This methodology relies on two steps.

- kHz, centroid frequency, peak frequency and weighted peak frequency). This method is widely described in [START_REF] Kharrat | A signal processing approach for enhanced Acoustic Emission data analysis in high activity systems: Application to organic matrix composites[END_REF]. Denoising was performed using discrete-wavelet transform. Hits' determination and separation was achieved using the conventional amplitude threshold-based technique. A threshold of -The second step is focused on the data clustering, which involves the classification of the AE hits into groups (clusters) representing the AE sources. The proposed approach is a robust-to-parameterization and unsupervised method based on clustering fusion. It is based on the idea that, using unsupervised pattern recognition techniques, several parametrizations may be suitable for a given dataset but it is difficult to precisely find an optimal one. Therefore, the clustering fusion makes possible the determination of a robust result with uncertainty quantification.

In this second step, all combinations of subsets of 4 features from the previous list (leading 2380 cases) are considered separately for clustering. The algorithm of Gustafson-Kessel is used [START_REF] Ramasso | Unsupervised Consensus Clustering of Acoustic Emission Time-Series for Robust Damage Sequence Estimation in Composites[END_REF]. A preselection of the most relevant partitions (among the 2380 ones) is automatically conducted using an entropy-based measure calculated on the clusters' proportions. This method is described in a previous paper [START_REF] Ramasso | Unsupervised Consensus Clustering of Acoustic Emission Time-Series for Robust Damage Sequence Estimation in Composites[END_REF]. For those multifarious subsets, the number of clusters in the Gustafson-Kessel algorithm was varied from 2 to 8. To select the best number of clusters, a second criterion based on both the mutual information and a set of boostrapped ensembles computed on clusters' proportions was applied. This method was initially suggested in [START_REF] Fred | Combining multiple clusterings using evidence accumulation[END_REF][START_REF] Kuncheva | Decision templates for multiple classifier fusion: an experimental comparison[END_REF] and adapted for AE data processing in [START_REF] Ramasso | Unsupervised Consensus Clustering of Acoustic Emission Time-Series for Robust Damage Sequence Estimation in Composites[END_REF]. This method thus allows to make use of multifarious parameterizations (subsets of features, number of clusters and other parameters used in clustering) and to ensure robust results particularly for large datasets. Finally, the set of partitions can be used to estimate confidence bounds on the final partition which is of particular interest for AE data processing under uncertainty.

RESULTS AND DISCUSSION

Water Vapour absorption behaviour Fig. 5 displays the water uptake of samples in the humid environment described before. The diffusion kinetics follows a one dimensional Fickian behaviour (solid lines in Figure 5) as presented by Scida et al. [START_REF] Scida | Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite[END_REF]. After 6 days, the average mass uptake reached at equilibrium Mm is approximately 3.3 ± 0.2%. The coefficient of diffusion D is estimated at 6.47x10 -6 ± 0.14x10 -6 m 2 .s -1 . It is interesting to notice the difference in kinetics observed for P5 specimen. This specimen shows a lower mass uptake at equilibrium. This phenomenon can be explained by its lower void ratio (6.3%) in comparison to other specimens from P2 and P6 (with a void ratio of respectively 7.2 and 8.1% -see Tab. 1).

Globally, the void contents of the plates manufactured during this study are comprised between 4.9 and 8.6%.

This result is in agreement with the work of Meredith et al. [START_REF] Meredith | On the static and dynamic properties of flax and Cordenka epoxy composites[END_REF] which highlighted a high void content with Lineo Flaxpregs (up to 10%). The authors suggested a lack of resin in their prepreg. The same issue is likely to be encountered in our case. Furthermore, we can suppose that air bubbles were trapped between the layers during the lay-up. Moreover, during the process, two other phenomena shall occur: (i) the yarns represent obstacles and limit the evacuation of air during the lay-up process (ii) the resin mainly flows into the yarns because of lower internal capillary pressure, as pointed out by Shah et al. in [START_REF] Shah | Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites[END_REF].

Influence of moisture absorption on the mechanical behaviour

Monotonic tensile behaviour

Fig. 6 shows the typical stress-strain curves under tensile loading of UD composite laminates in the fibre direction and perpendicularly to the fibre direction for DRY specimens. All results obtained by the three partners are given in Table 2. The mean values of tensile moduli (approximately 30 GPa in the fibre direction and 4.7 GPa in the transverse direction), tensile strength (approximately 320 MPa in the fibre direction and 30 MPa in the transverse direction) and strain at failure (approximately 1.5% in the fibre direction and 1% in the transverse direction) are in good agreement with data of literature collected on the same type of flax-epoxy UD [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF][START_REF] Poilâne | Polymer reinforced by flax fibres as a viscoelastoplastic material[END_REF]. In the longitudinal direction, a really good agreement is also observed between the mean values and coefficients of variation (CoV) determined by the three different laboratories involved in this study. The difference in mean values is generally comprised between 5 and 10% between labs for these tensile properties.

Readers have to keep in mind that the experimental devices and some of the techniques (strain measurements, tabs gluing…) were different between the different labs. The loading rate was also two times slower in MTT2 when compared to MTT1 and MTT3. Despite these differences in the experimental implementation of the tests, the obtained values are quite similar. This reflects the good homogeneity of material properties obtained with this material and manufacturing process. The CoV is lower than 6% for the strength, 10% for the rigidity for each lab. Fig. 7 displays the same curves for samples extracted in the fibre direction before (DRY) and after water vapour saturation (WVS). For DRY specimens, the tensile curve is strongly non-linear. Its shape is identical that the one depicted by other authors for the same type of UD material [START_REF] Scida | Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite[END_REF][START_REF] Liang | A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites[END_REF][START_REF] Poilâne | Polymer reinforced by flax fibres as a viscoelastoplastic material[END_REF][START_REF] Shah | Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites[END_REF][START_REF] Baets | Determination of the optimal flax fibre preparation for use in UD flax-epoxy composites[END_REF]. Poilâne et al. [START_REF] Poilâne | Polymer reinforced by flax fibres as a viscoelastoplastic material[END_REF] showed that this non-linear behaviour can be attributed to time-dependent mechanisms and well represented by a viscoelastoplastic model. The microstructural origin of such non-linearity remains an open question in the scientific community, and could be attributed to reorganization mechanisms at the scale of yarns and of fibre wall.

Fig. 7 also shows that water absorption induces a change in the stress-strain curve of samples extracted in the fibre direction. An increase of failure strains, a decrease of the Young's modulus as well as a slight increase of the Poisson's ratio are observed (Tab.2). These evolutions can be attributed to the plasticization of both matrix and fibres by water molecules. WVS specimens exhibit slightly superior tensile strengths to DRY specimens.

Munoz and Garcia-Manrique [START_REF] Munoz | Water Absorption Behaviour and Its Effect on the Mechanical Properties of Flax Fibre Reinforced Bioepoxy Composites[END_REF] also observed a higher tensile strength of flax-epoxy composites after immersion. They attributed this behaviour to the swelling of flax fibres in the composite material (as a result of water absorption). This phenomenon was assumed to induce a stronger interfacial bonding between the fibres and the matrix. Water sorption also increases the macromolecules mobility [START_REF] Scida | Influence of hygrothermal ageing on the damage mechanisms of flax-fibre reinforced epoxy composite[END_REF].

On Fig. 8, another interesting feature can be observed and concerns the tangent modulus. For DRY specimens, this modulus is decreasing towards a finite value (E 𝐿 𝑚2 ). Concerning WVS specimens, this modulus is slightly increasing at large longitudinal strains. Such a phenomenon has already been mentioned by other authors [START_REF] Charlet | Tensile deformation of a flax fiber[END_REF][START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF]. This could be attributed to the alignment of cellulosic microfibrils or to the crystallisation of amorphous cellulose in the wall of flax fibres.

Tensile-tensile fatigue behaviour

S-N curves for tensile-tensile fatigue tests on DRY and WVS specimens are presented in Fig. 9. For each studied load level, and particularly for WVS specimens, there is a slight dispersion in the maximum tensile stress. This is due to the variation in cross-section of the specimens. Results show that both sets of specimens (DRY and WVS) present a gradual decline in fatigue strength with increasing number of fatigue cycles. For a given loading level, dispersion in fatigue life duration is observed. The order of magnitude of this dispersion is in agreement with data collected on flax-epoxy laminates [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF][START_REF] Shah | Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams[END_REF][START_REF] Liang | Properties evolution of flax/epoxy composites under fatigue loading[END_REF]. The fatigue strength as a function of number of cycles can be fitted by the Wöhler law. The fatigue strength for a high number of cycles (1 million) is approximatively 140 MPa for DRY specimens. This value represents approximately 42 % of the monotonic tensile strength. It is in agreement with the works El Sawi et al. [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF]. They found a fatigue strength (for 6 million cycles) in the range of 45 to 50% of ultimate tensile stress for the same type of unidirectional flax-epoxy laminates.

Moreover, both mean strain 𝜀 𝑚𝑒𝑎𝑛 and dynamic modulus 𝐸 𝐿 * have been calculated at each fatigue cycle. The evolution of these values is plotted in Figs. 10a and10b.

Under the lowest stress levels (40, 50 and 65% of ultimate tensile stress), the mean strain is strongly increasing until a life fraction comprised between 0.05 and 0.1. Following this increasing, the mean strain reaches a plateau. Just before failure, a sudden increasing is observed. Under highest stress level (80% of ultimate tensile stress, see Fig. 10a), the slope of the second stage is higher than for the lower stress level up to 0.4 of life fraction. The mean strain tends to be stabilized at this level after this point.

This increase in mean strain as a function of fatigue cycles could be attributed to a time-dependent behaviour.

Indeed, in plant-based composite, both matrix and fibres can develop viscoelastic behaviour. Recently, it has been shown that bast fibres are characterized by a significant time-dependent behaviour when submitted to tensile testing [START_REF] Guicheret-Retel | Creep behaviour of single hemp fibres. Part II: Influence of loading level, moisture content and moisture variation[END_REF][START_REF] Cisse | Creep behaviour of single hemp fibres. Part I: viscoelastic properties and their scattering under constant climate[END_REF]. In addition to this behaviour, irreversible phenomena such as damage or plastic effects can occur. These dependences were mentioned by El Sawi et al. [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF]. They showed a positive correlation between the mean strain evolution and cracks ratios in a laminate.

Moreover, Fig. 10b clearly shows a raise in stiffness when the number of cycles increases. The initial dynamic modulus is approximately 34 GPa for the tested specimen. For the higher stress levels (65% and 80% of ultimate tensile stress), a slight decrease in dynamic modulus can be observed during the first fatigue cycles (up to 33 GPa). During the second phase of fatigue cycling, a clear increase in stiffness is observed. It overpasses the initial stiffness. Under the lower stress levels (40% and 50% of ultimate tensile stress), a sharp increase is observed in the early part of the fatigue test. A plateau is then reached until the failure of the specimen. The stiffness increase for these stress levels is of approximately 7 to 8.5%. These values are in agreement with those reported by Liang et al. [START_REF] Liang | Properties evolution of flax/epoxy composites under fatigue loading[END_REF] and El Sawi et al. [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF].

This stiffness increase under cyclic loading can be attributed to various phenomena, at different scales, such as flax fibre hardening or fibre re-orientation. Plant fibres have been observed to become stiffer when they are submitted to cyclic tensile loading [START_REF] Placet | Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres[END_REF][START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF][START_REF] Baley | Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase[END_REF][START_REF] Placet | Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the manufacturing of high performance composites[END_REF][START_REF] Fda | An experimental investigation of the fatigue behavior of sisal fibers[END_REF]. This was attributed both to cellulose microfibrils reorientation [START_REF] Placet | Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres[END_REF][START_REF] Placet | Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations[END_REF][START_REF] Fdr | Nonlinear tensile behaviour of elementary hemp fibres. Part II: Modelling using an anisotropic viscoelastic constitutive law in a material rotating frame[END_REF] and stress-induced crystallization of amorphous cellulose in the fibre wall. This phenomena has also been observed and recently highlighted by Shah [START_REF] Shah | Damage in biocomposites: Stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading[END_REF] and Bensadoun et al. [START_REF] Bensadoun | Residual properties and damage evolution of flax-epoxy composites subjected to fatigue loading[END_REF] in the case of flax-epoxy composites.

So, the tensile fatigue behaviour of unidirectional flax-epoxy laminates seems to result from the combination of antagonistic mechanisms. Hence, since creep and damage induce an increase in mean strain and a decrease in apparent stiffness, fibres hardening, due to microstructural mechanisms, seem to induce a slight increase of the laminates stiffness.

Results (Figs 9, 10c, 10d) show that the fatigue strength is significantly influenced by water sorption. Regarding WVS specimens, the initial dynamic modulus is 22 GPa, against 33 GPa for DRY specimens. Contrary to all expectations, the fatigue strength for a high number of cycles is higher for WVS specimens than for DRY ones.

In the case of WVS specimens, the fatigue strength reaches 155 MPa (47% of the monotonic tensile strength), see Fig. 9. Moreover, the mean strain is higher when testing DRY specimens (Fig. 10c). This phenomenon is assumed to be due to the plasticization of the laminate. The evolution of the mean strain (Fig. 10c) and dynamic modulus (Fig. 10d) is similar to the one observed for DRY specimens. The stiffness respectively increases by 22% and 32% under maximum stresses levels of 180 MPa (55% of ultimate tensile stress) and 155 MPa (45% of ultimate tensile stress). This result clearly shows that the laminate hardening phenomenon is activated by water sorption. An activation of such a hardening phenomenon by water sorption has already been observed under cyclic loading of plant fibres [START_REF] Placet | Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres[END_REF].

Fig. 11 proposes typical stress-strain hysteresis loops of various fatigue cycles (n°1, 125, 3125 and 78 125) of DRY (maximum dynamic stress = 50% of monotonic tensile strength) and WVS specimens (maximum dynamic stress = 55% of monotonic tensile strength). Hysteresis loops provide a valuable insight into the material stiffness and internal friction and their evolution during fatigue [START_REF] Towo | Fatigue of sisal fibre reinforced composites: Constant-life diagrams and hysteresis loop capture[END_REF]. The loop area is indeed proportional to the dissipated energy. Results exhibit hysteresis loops with diminishing area as the number of cycles increases, for both DRY and WVS specimens, as well as an increase of the dynamic modulus. This decrease in loop area could be explained by a decrease in damage activity when the number of fatigue cycles is increasing. This hypothesis was proposed by Towo and Ansell for sisal fibre reinforced composites [START_REF] Towo | Fatigue of sisal fibre reinforced composites: Constant-life diagrams and hysteresis loop capture[END_REF]. Fig. 12 shows the evolution of the AE activity (i.e. the cumulated number of AE hits) as a function of the fatigue cycles for the different stress levels. Results point out that the damage activity is generally high during the first stage of cycling, except for the fatigue stress level equal to 50% of the monotonic tensile strength. For this stress level, the damage activity is almost constant during fatigue test. This result confirms that damage activity could partly explain the dissipated energy (and the significant area of the hysteresis loop) that is observed during the first stage of fatigue tests. But, it cannot be the only explanation since, for 50% loading level, the damage activity is constant (Fig. 12c) and the loop area still decreases when fatigue cycles is increasing (Fig. 11a). Thus, this decrease in loop area could also comes from a decrease in the damping capacity of flax fibres, resulting from microstructural rearrangement within the fibre wall, such as cellulose microfibrils reorientation. Such a reorientation of the cellulose microfibrils could explain both the hardening phenomenon (increase in rigidity in the fibre direction) and the decrease in damping capacity. The viscoelasticity is indeed higher in the transverse direction of cellulose MFs than in the axial one. It was shown previously for plant fibres that the increase in stiffness under cyclic tensile loading is also accompanied with a decrease in damping capacity [START_REF] Placet | Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the manufacturing of high performance composites[END_REF].

On Fig. 11, it can also be observed that the dissipated energy is higher for water saturated specimens. It can be explained by the increase in damping capacity provided by water sorption of both matrix and flax fibres.

Fatigue damage

The fatigue damage in such flax-epoxy laminates has been studied by El Sawi et al. [START_REF] Sawi | An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates[END_REF]. They reported microcracks inside the bundles of fibres, between the elementary fibres. These cracks induce, at the early stage of lifetime, delamination between adjacent fibres. With increasing number of cycles, at 2/3 of fatigue life, they showed that such cracks propagate inside the bundle and through the matrix. Failures at the interface between bundles and matrix were also observed at this stage.

In the present work, we observed the same damage kinetics in the DRY specimens submitted to fatigue loading (Fig. 13 a ,b, c andd). More surprisingly, for the WVS specimens, cracks were not observed inside the bundles of fibres but in the matrix and more precisely at the interface between the yarns of fibres and the matrix (Fig. 13 e,f and g). This difference could be explained by the swelling of individual fibres inside the bundles, induced by water sorption in the fibre wall, when the composite are exposed to humidity. This sorption-induced swelling of fibres could result in a stronger strength at the interface between individual fibres inside the bundles and thus explain the higher fatigue strength of WVS specimens when compared to DRY specimens. Then, the tensile strength of WVS laminates would be more driven by the strength at the interface between the yarns and the matrix.

These images and observations are useful to inspect the defects and damage progress in the composite when submitted to fatigue loading. However, results have to be considered with great care considering the potential influence of sample preparation (including composite cutting, coupons embedding, grinding and polishing) on the damage state. The mode of visualisation is also important and may involve interaction between the probe and the composite, and more particularly the plant fibres. SEM observations can, for example, induce deformation or shrinkage, and as a consequence damage, due to the use of low vacuum conditions, and produce significant surface modifications due to electron beam damage.

Thus, damage during fatigue tests has also been assessed using acoustic emission. Data collected on WVS specimens are not relevant for comparison purpose, since the presence of a cover layer highly distorts and attenuates the waves that are recorded at the surface of the composite materials. For this reason, only the results collected on the DRY specimens are considered in this study.

The clustering fusion methodology was applied on the fatigue datasets recorded at the different level of stress.

Results are presented on Fig. 14 and Tab. 3 for a specimen tested at 65% of the monotonic tensile strength. The number of clusters was automatically determined to 5 using the proposed methodology. One of the main difficulty with unsupervised pattern recognition techniques is to label clusters, i.e. to assign each cluster to the corresponding acoustic source. This was partially done using in situ or/and post mortem optical observations and prior knowledge of acoustic signature of plant fibres and PFCs damage [START_REF] De Vasconcellos | Tension-tension fatigue behaviour of woven hemp fibre reinforced epoxy composite: A multi-instrumented damage analysis[END_REF][START_REF] Rosa | Acoustic emission for monitoring the mechanical behaviour of natural fibre composites: A literature review[END_REF].

Cluster 1, which first starts, is characterized by relatively low amplitude (50 dB) and energy (1% of the total AE energy), despite comprising 89% of the AE hits. This cluster was assigned to external sources, including electromagnetic and mechanical noises (rubbing and friction) which are mainly generated by the actuating systems, electronic equipment and the surrounding environment. The second cluster is made of higher amplitudes (87 dB) and represents about 10% of the released acoustic energy. It begins early (before cycle n°150). Cluster 3 and 4 started around cycles n°300 to 400 and are characterized by amplitude close to 96 and 102 dB while representing 80% of the total released energy. These 3 clusters are certainly related to cracks initiation and propagation in the matrix and within the bundles of fibres, fibre debonding and individual fibre breakage. However, the available results are not enough to reliably assign these clusters to their respective damage families or sub-families, and clearly deserve further investigations using 3D inspection techniques such as tomography.

The fifth source of AE hits is activated from around cycle n° 750 until the end of the test. It has the highest amplitudes (107 dB). The energy released by this source represents 9.4% of the total AE energy with only 0.06% of the total number of AE hits which means that the AE events related to this source are highly energetic.

This source may be related to the breakage of bundles of fibres or yarns. This was correlated with visual observations during fatigue tests.

Even if additional work is still needed to ensure damage assessment of PFCs under fatigue loading, these results illustrate yet the capacity of the proposed AE methodology to discriminate damage families and subfamilies. They also show that during the first hundred fatigue cycles, an intense damage activity is observed.

It is mainly related to cracks propagation in the matrix and at the interface between individual fibres. Such dissipative mechanisms could explain partly the significant area of the hysteresis loops measured in the first stage of fatigue tests.

Dynamic behaviour

Dynamic elastic modulus 𝐸 𝐿 * and damping ratio evaluated for DRY and WVS samples are given in Table 4.

Dynamic elastic moduli are greater than quasi-static moduli because of the viscoelastic character of PFC. As a comparison, values are plotted on Fig. 8. An extrapolation of the quasi-static tangent modulus, measured during monotonic tests, at low strain logically tends towards the dynamic elastic modulus value. It can be underlined that in this dynamic technique, the specimen is submitted to tensile -compression loadings and that the behaviour of plant fibre composite is generally different in compression and tension [START_REF] Van Vuure | Compressive properties of natural fibre composites[END_REF]. After water vapour saturation, dynamic elastic modulus is decreasing by 20%. However, damping ratio is increasing by 50%. This increase in damping ratio provided by water sorption and measured using free vibration test is consistent with the increase in damping capacity observed on the same material under the previous tensile-tensile fatigue tests. This is assumed to be due to the plasticization of both matrix and fibres by water molecules.

CONCLUSIONS

This study focused on the consequences of water vapour sorption of unidirectional flax-epoxy laminates.

Mechanical investigations concerned monotonic, fatigue and dynamic behaviours. The motivation of this work was to evaluate the durability of biobased laminates for structural applications. The goal was particularly to characterize and understand the influence of moisture absorption and weathering on the mechanical behaviour and long-term behaviour of PFCs. It is necessary to safely and reliably introduce them in the structural composite market.

Results show that, when exposed to hygrothermal conditioning at 70°C and 85% RH, the diffusion kinetics of UD flax-epoxy composites follows a one dimensional Fickian behaviour. Water vapour sorption is showed to induce a significant change in the shape of the tensile stress-strain curve, a decrease in dynamic elastic modulus and a large increase in damping ratio (approximately 50%). Contrary to all expectations, water saturation does not degrade the monotonic tensile strength of such a flax-epoxy composites. Moreover, an increase in the fatigue strength is clearly observed for wet specimens. Water sorption seems to activate some hardening phenomena. This is a benefit both for the static and fatigue strength. This result is of paramount importance for biobased composites end-users. Future of this work will help to quantify the influence of time of exposure to such hygrothermal conditions (i.e. ageing) on the mechanical behaviours of PFCs. The final purpose is to understand how microstructural mechanisms help to harden such biobased laminates when they are submitted to moisture and mechanical loading. b.

Specimens σ L (MPa) σ T (MPa) ε L (%) ε T (%) E 𝐿 𝑚1 (GPa) E 𝐿 𝑚2 (GPa) E 𝑇 𝑚 (GPa) LT (-) TL (-) DRY MTT1 311

  35 dB was used. The timing parameters: PDT (Peak Definition Time) = 60 sec; HDT (Hit Definition Time) = 120 sec and HLT (Hit Lock Time) =300 sec were identified by means of preliminary calibration measurements.

Fig. 14 shows

 14 Fig.14shows the activity within each cluster. The solid lines represent the best result of clustering fusion. The colored envelopes describe the uncertainty resulting from clustering parametrization. Tab. 3 provides the clusters labels, their thresholds, amplitude range and cumulated acoustic energy. The threshold represents the number of cycles before the first appearance of a given cluster, i.e. of an acoustic

Figure 1 :Figure 2 :

 12 Figure 1: Pressure and temperature cycle used for curing materials

Figure 3 :

 3 Figure 3: About the measurement of modulus in a plant fibre composite

Figure 4 :Figure 5 -Figure 6 -

 456 Figure 4: Typical impulse spectrum of the free vibration response of the composite samples

Figure 7 :Figure 8 :

 78 Figure 7: Tensile stress-strain curves for 0°UD specimens before and after aging

Figure 9

 9 Figure 9: S-N curves DRY and WVS specimens

Figure 11 :

 11 Figure 11: Hysteresis loops for different cycles (n°1, 125, 3125 and 78 125) of: a. DRY (maximum dynamic stress = 50% of monotonic tensile strength) and b. WVS specimens (maximum dynamic stress = 55% of monotonic tensile strength).

Figure 12 :

 12 Figure 12: AE activity (i.e. cumulated number of AE hits) versus number of fatigue cycles. DRY specimens tested at a fatigue maximum stress equal to 80% (a.), 65% (b.), 50% (c.) and 40% (d.) of the monotonic tensile failure stress.

Figure 13 :

 13 Figure 13: Micrographs of the cross section surface of laminates after fatigue failure for DRY specimen (a, b, c, d) and WVS specimens (e, f, g).

Figure 14 :

 14 Figure 14: Activity in each cluster versus number of fatigue cycles for a DRY specimen tested at 65% of the monotonic tensile strength. The solid lines represent the best result of clustering fusion. The coloured envelopes describe the uncertainty resulting from clustering parametrization. a. Total fatigue lifetime. b. Zoom on the first 1400 cycles.

  The first step includes: (i) a real time AE data streaming, (ii) AE hits' determination and separation and (iii) conventional AE features extraction (MARSE energy, amplitude, average frequency, root mean square, average signal level, count-to-peak, reverberation frequency, initiation frequency, signal

strength, absolute energy, partial power in [0 250] kHz, [250 500] kHz, [500 750] kHz and [750 1000]

Table 1 : Plate thickness, porosity and fibre/matrix volume fractions

 1

	Plates	Thickness	Fibre volume	Matrix volume	Void volume	Dedicated to
		(mm)	ratio (%)	ratio (%)	ratio (%)
	P1	2.19	45.0	46.4	8.6	Monotonic
						(MTT3) and
						fatigue tests
						(DRY
						specimens)
	P2	2.18	45.4	47.4	7.2	Fatigue tests
						(WVS
						specimens)
	P3	2.18	46.1	49	4.9	Monotonic
						(MTT1)tests
						(DRY
						specimens)
	P4	2.16	45.6	47.7	6.7	Monotonic
	P5	2.17	45.7	48.0	6.3	(MTT2) and vibratory tests
	P6	2.17	45.8	46.1	8.1	(DRY and
						WVS
						specimens)

Table 2 : Properties obtained in monotonic tensile tests (mean value ±standard deviation)

 2

Table 3 -Clusters labels and AE source thresholds for a DRY specimen tested under fatigue loading at 65% of the tensile monotonic strength. Failure was observed after 15 068 cycles.

 3

				31.2	1.50	0.96	31.6		4.71	0.36	0.07
			±13.9	±0.84	±0.12	±0.08	±0.96		±0.33	±0.02	±0.01
		MTT2	319 ±20	30.0 ±1.3	1.59 ±0.14	0.99 ±0.16	30.4 ±2.7	/	4.69 ±0.74	0.34 ±0.02	/
		MTT3	327	27.7	1.45	1	32.9	19.6	4.80	0.36	0.06
			±9.8	±0.3	±0.15	±0.15	±3.16	±3.16	±0.25	±0.02	±0.006
	WVS	MTT2	320 ±20	25.9 ±1.5	2.32 ±0.14	1.80 ±0.21	19.6±2.0	/	3 ±0.27	0.37 ±0.02	/

Table 4 -Properties obtained from free vibration tests

 4

	Specimens	E 𝐿 𝑑 (GPa)	ξ (%)
	DRY	33.9	0.87
		±0.6	±0.07
	WVS	27.2	1.33
		±0.9	±0.09

Acknowledgements

The authors thank the Fondation de Coopération Scientifique Bourgogne Franche-Comté for its financial support through the project Bonus Qualité Recherche « Durabilité des composites biosourcés » (FCS-BFC BQR2014-19). The authors also express theirs thanks to Marina Raschetti from FEMTO-ST for the MEB support.


Alexandre Robert
email: arobert@mnhn.fr

Colin Fontaine

Simon Veron

Anne-Christine Monnet

Marine Legrand

Joanne Clavel

Stéphane Chantepie

Denis Couvet

Frédéric Ducarme

Benoît Fontaine

Frédéric Jiguet

Isabelle Le Viol

Jonathan Rolland

François Sarrazin

Céline Teplitsky

Maud Mouchet

Fixism and conservation science

Keywords: ethics, biodiversity, species, evolutionary potential, Anthropocene Running head: Fixism and conservation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Modern biology is anchored in evolutionary principles according to which speciation, extinction, and character evolution shape the patterns of biological diversity (hereafter biodiversity) and their changes through time. Evolutionism opposes fixism, the idea that species do not change, recently defined as "the doctrine that the most important properties of the members of any species, those properties allowing for their classification as members of this species, cannot vary beyond definite limits" [START_REF] Barberousse | Species from Darwin onward[END_REF].

Although fixism is considered by most biologists an obsolete theory [START_REF] Dobzhansky | Nothing in Biology Makes Sense Except in the Light of Evolution[END_REF], recent publications suggest that a growing sector of conservation science partly relies on a fixist view of the living world. Most critics do not claim that conservation scientists themselves are fixist, but they denounce a pattern-oriented view of biodiversity conservation based primarily on the study and management of species as static typological units. Although most biologists recognize that individual variation is ubiquitous in nature and is the basis for evolutionary change, typological thinking is still pervasive [START_REF] Ashley | Evolutionary enlightened management[END_REF]. Such a typological approach is associated with the mistaken view that species are relatively fixed, independent entities [START_REF] Ashley | Evolutionary enlightened management[END_REF][START_REF] Diniz-Filho | Darwinian shortfalls in biodiversity conservation[END_REF]) rather than interacting evolutionary units [START_REF] Rojas | The species problem and conservation: what are we protecting?[END_REF]) and that their diversity and distributions are static biological phenomena [START_REF] Winker | The Crumbling Infrastructure of Biodiversity: The Avian Example[END_REF]. This static conservation paradigm [START_REF] Hannah | Conservation of biodiversity in a changing climate[END_REF] has been the subject of much criticism [START_REF] Ibisch | Biodiversity needs the help of global change managers, not museum-keepers[END_REF][START_REF] Toledo | The shifting of ecological restoration benchmarks and their social impacts: digging deeper into pleistocene re-wilding[END_REF][START_REF] Harmsen | What Are We Trying to Conserve?[END_REF][START_REF] Harris | Noah's Ark Conservation Will Not Preserve Threatened Ecological Communities under Climate Change[END_REF]. The apparent difficulty of framing policies to preserve dynamic processes rather than objects (threatened biodiversity) implies that most conservation strategies are inconsistent with an evolutionary perspective [START_REF] Smith | The preservation of process: the missing element of conservation programs[END_REF][START_REF] Mace | Evolutionary biology and practical conservation: bridging a widening gap[END_REF][START_REF] Grant | Evolution and Biodiversity: The evolutionary basis of biodiversity and its potential for adaptation to global change[END_REF]. Associated with these criticisms is the view that conservation efforts to save some threatened species are grounded in the past [START_REF] Ibisch | Biodiversity needs the help of global change managers, not museum-keepers[END_REF], reductionist, artificial, and scientifically inconsistent [START_REF] Kareiva | What is conservation science?[END_REF]. Finally, it has been argued that evolutionary biology has little impact on practical conservation because of the limited participation of evolutionary biologists in conservation science and the ignorance (or neglect) of evolutionary processes by managers. Thus, for over 20 years, biologists have repeatedly claimed that conservation needs to put more emphasis on evolution(e.g., [START_REF] Smith | The preservation of process: the missing element of conservation programs[END_REF][START_REF] Hannah | Conservation of biodiversity in a changing climate[END_REF][START_REF] Stockwell | Contemporary evolution meets conservation biology[END_REF][START_REF] Mace | Evolutionary biology and practical conservation: bridging a widening gap[END_REF][START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF][START_REF] Carroll | Applying evolutionary biology to address global challenges[END_REF].

Conservation science is interdisciplinary and draws on natural and social sciences to protect biodiversity components and processes from deleterious anthropogenic activities and to build mutually beneficial relationships between humans and biodiversity. An evolutionary conservation perspective has existed for several decades. In 1985 Soulé stated that evolution is not only the one and only framework with which to describe and understand biological processes but also one of the primary ethical postulates for conservation. Along with the acknowledgment that the drivers of the current biodiversity crisis are likely to disrupt some evolutionary processes [START_REF] Palumbi | The Evolution Explosion: How Humans Cause Rapid Evolutionary Change[END_REF][START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF], conservation science has provided concepts and tools based on evolutionary knowledge. For instance, conservation genetics [START_REF] Frankham | Introduction to Conservation Genetics[END_REF] addresses genetic deterioration in wild and captive populations, future evolutionary potential, and the designation of evolutionarily significant units, which has direct consequences for planning and management in a diversity of conservation approaches, from translocations to protected areas.

We reviewed the main criticisms of conservation science regarding its presumed fixist component and attempt to demonstrate that most practices of conservation incorporate the contemporary understanding of dynamic ecological and evolutionary processes and that the evolutionary-based postulates constructed over 30 years ago provide a way to solve most apparent contradictions between biodiversity conservation and evolution.

We considered the most recurrent criticism of conservation science: most methods used to protect biodiversity are intended to save species from extinction, which apparently contradicts our knowledge of evolutionary processes. We also addressed the problem of the shifting frame of reference when seeking to conserve an evolving biodiversity in dynamic environments and human societies and under changing perceptions. Finally, we examined how evolutionarily driven goals of conservation are affected by the ultimate motivations underlying these goals. Consideration of these 3 topics together allowed us to emphasize important distinctions among general conservation goals, the ultimate motivations underlying these goals, and the practices used to achieve them. We suggest, in particular, that fixist methods are sometimes used to serve evolutionarily sound conservation goals and vice versa.

Saving species, genes, or functions

The species problem

The species has long been the basic unit in conservation biology, notably in the process of defining conservation status (e.g., IUCN 2014). Classifications of threatened species are directly connected to legislation in many countries and often considered in triage discussions and in defining protected areas based on the idea that species-rich areas or those with threatened species should be prioritised. Other types of conservation actions, such as translocations or forms of legal protection, are directly implemented for particular species, and academic research in conservation biology is often focused on the species level [START_REF] Fazey | What do conservation biologists publish?[END_REF].

Two lines of arguments feed the criticisms of the species approach in conservation science.

The first is related to the complexity, limits, and artificiality of the species definition, which has been one of the most controversial topics in biology for over 150 years. The complexity of the species definition is linked to the multiplicity of the approaches to species identification (i.e., the species concepts) [START_REF] Hey | On the failure of modern species concepts Trends in[END_REF]. Although the criteria inherent to species concepts are assumed to reflect the divergence of evolutionary lineages ( [START_REF] Queiroz | Species concepts and species delimitation[END_REF], the relevance of species as important functional or evolutionary units has been questioned [START_REF] Winter | Phylogenetic diversity and nature conservation: where are we?[END_REF], and there is still much debate about species recognition and nomenclatural rules for most biodiversity (viruses, archaea, bacteria, fungi, etc.). Thus, some authors consider the use of species in conservation science is partly unjustified [START_REF] Rojas | The species problem and conservation: what are we protecting?[END_REF]).

The second line of argument is related to the apparent contradiction between species conservation and evolutionary dynamics [START_REF] Grant | Evolution and Biodiversity: The evolutionary basis of biodiversity and its potential for adaptation to global change[END_REF]. First, species evolve continuously, and species extinction is part of the evolutionary process, but conserving species implies incorrectly treating them as fixed entities. Second, counter-arguments on the irreversibility of species extinction to justify their conservation (e.g., [START_REF] Hunter | The complementary niches of anthropocentric and biocentric conservationists[END_REF]) are fixist arguments that ignore intraspecific genetic variation and the fact that irreversible loss of genetic variation is part of the evolutionary process (e.g., from an evolutionary perspective, the death of 1 individual is also an irreversible phenomenon). This general dispraise is amplified because both scientific interest and public support for conservation efforts focus on certain species, such as large, charismatic vertebrates [START_REF] Fazey | What do conservation biologists publish?[END_REF]. [START_REF] Kareiva | What is conservation science?[END_REF] describe concern for local charismatic species threatened with extinction as "nostalgia" for "the world as it once was" and suggest that conservation efforts are wrongly directed to biodiversity units that are no longer adapted to the current environment.

Evolutionary versus human time scale and justifications for the species approach

Consistent with some seminal conservation biology publications [START_REF] Frankel | Genetic conservation -our evolutionary responsibility[END_REF][START_REF] Soulé | What is conservation biology?[END_REF], many biologists agree that maintaining evolutionary potential and processes is a primary concern of conservation science (e.g., [START_REF] Crandall | Considering evolutionary processes in conservation biology[END_REF][START_REF] Stockwell | Contemporary evolution meets conservation biology[END_REF][START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF]. However, assuming that conservation biology should ultimately seek to restore the evolutionary balance of speciation and extinction events rather than save particular species, saving species remains one pragmatic response of conservationists to the biodiversity crisis for several reasons. First, there is a strong discrepancy between the time scale of macroevolutionary processes (e.g., extinction or speciation) and the time scale of human influence. The current species extinction rate is much higher than background extinction rates [START_REF] Barnosky | Has the Earth's sixth mass extinction already arrived?[END_REF], and the anthropogenic causes of most extinction events are widely acknowledged [START_REF] Brook | Synergies among extinction drivers under global change[END_REF]. The period of strong human influence is very short relative to the average longevity of species (a few hundred years versus 1 million years or more) [START_REF] Jenkins | Species extinction[END_REF]Hunters 1996;[START_REF] Brook | Synergies among extinction drivers under global change[END_REF]. Thus, most species threatened by human activities would not become extinct during the human time scale in the absence of these activities and species will experience human influence during only a small portion of their existence, a period during which there would be little opportunity to adapt to environmental change. For comparison, some previous mass extinction events, such as the Cenomanian-Turonian crisis that lasted 500,000 years, may have generated adaptive responses in the affected populations [START_REF] Erwin | The end and the beginning: recoveries from mass extinctions[END_REF]. Saving particular species from humaninduced extinction is a way to reduce the global rate of untimely extinctions [START_REF] Soulé | What is conservation biology?[END_REF] and thus minimise humanity's effect on evolutionary trajectories via both direct extinction effects of the target species at the phylogenetic level (i.e., at the level of the evolutionary history) and secondary effects mediated by community processes and coevolution [START_REF] Lawrence | Species interactions alter evolutionary responses to a novel environment[END_REF]. Therefore, saving a species is both a fixist goal in itself in the short term and a relevant means to achieve evolutionarily sound goals in the longer term.

Second, human pressure and feedback (e.g., in response to exploitation) act at the population or species scale, and the population is the level at which scientists are able to make reliable and sound predictions about persistence that can be reasonably extended to the scale of the entire species. Thus, threatened-species listing constitutes a valuable way to better characterise the extinction risk at the phylogenetic level, for example. Further, key microevolutionary processes, such as genetic deterioration and adaptation to global changes [START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF], have been historically described and theorised at the population, metapopulation, and species levels, and numerous evolutionary biologists and phylogenetic systematists consider that, at least for vertebrates, species are important evolutionary units [START_REF] Cracraft | Speciation and its ontology: the empirical consequences of alternative species concepts for understanding patterns and processes of differentiation[END_REF]).

Finally, although some higher taxa (e.g., insects, mollusks, and fungi) are clearly neglected in biodiversity conservation policies and research, the focus on large charismatic species has been justified by functional and pragmatic arguments. For example, among groups of vertebrates, large species are particularly vulnerable to extinction [START_REF] Fritz | Phylogenetic diversity does not capture body size variation at risk in the world's mammals[END_REF] and have large impacts on communities and ecosystems [START_REF] Ripple | Status and ecological effects of the world's largest carnivores[END_REF]. Such flagship species can also be used to leverage strong support because species are, in the realm of human perception, the most immediate and direct way to perceive and apprehend an element of biodiversity.

Conservation phylogenetics, communities, and functions

The species is one conservation unit among others, and modern conservation science embraces both intraspecies and multispecies approaches to address biodiversity loss. Since the 1990s, scientists have proposed a phylogenetic approach to conservation to prioritise protecting evolutionarily distinct groups or areas that encompass the richness of the tree of life [START_REF] Faith | Conservation evaluation and phylogenetic diversity[END_REF]. Projections of biodiversity loss show a loss of phylogenetic diversity for many groups that is higher than expected according to a model of random extinctions [START_REF] Jono | Threat diversity will erode mammalian phylogenetic diversity in the near future[END_REF].

Conservation biology has also begun to embrace community-level approaches, which has been fuelled by the development of network ecology, in which the dependencies among species within communities are explicitly considered. Such approaches allow for the identification of ecological interactions among species to determine the response of target species to perturbations or conservation strategies and have begun to stimulate approaches that focus on entire ecological communities, including the patterns of interaction among species [START_REF] Forup | The restoration of ecological interactions: plant-pollinator networks on ancient and restored heathlands[END_REF], rather than target species.

In addition to the species, phylogenetic, and community approaches, the biodiversityecosystem functioning framework offers another alternative to species-centred approaches to explicitly link species to ecological processes and fluxes. This framework relies on the observation that one species can perform more than one ecological function; species are not ecologically equivalent; and species can be functionally redundant. Functional ecologists have defined the taxon-free concept of functional types (i.e., groups of species having similar ecological effects) or, more recently, functional entities (i.e., unique combinations of functional traits [START_REF] Mouillot | Functionak over-redundancy and high functional vulnerability in global fish faunas on tropical[END_REF]).

Recently, attempts have been made to integrate these approaches by introducing the use of phylogenetic relatedness to understand the ecological and evolutionary processes underlying community assembly [START_REF] Pavoine | Measuring biodiversity to explain community assembly: a unified approach[END_REF] under the assumption that phylogenetic signal can be informative regarding functional diversity, ecosystem functioning [START_REF] Faith | Biodiversity and evolutionary history: useful extensions of the phylogenetic diversity assessment framework[END_REF], but see [START_REF] Winter | Phylogenetic diversity and nature conservation: where are we?[END_REF], and the similarity of the ecological roles of species [START_REF] Webb | Phylogenies and Community Ecology[END_REF]. Although practical applications of the phylogenetic, community, and functional frameworks are still rare, these approaches have recently moved from pure theory to early stages of implementation (e.g., [START_REF] Isaac | Mammals on the EDGE: conservation priorities based on threat and phylogeny[END_REF][START_REF] Laity | Phylodiversity to inform conservation policy: An Australian example[END_REF].

Reference states and processes

The benchmark problem

Using reference states to compare current biodiversity with past biodiversity is a common practice in conservation biology. Reference states, or benchmarks, can be either natural or human modified [START_REF] Willis | What is natural? The need for a longterm perspective in biodiversity conservation[END_REF], and time references as diverse as prehumans, preagriculture, the year 1500, and the year 1970 have been used in conservation studies.

Conservation approaches as diverse as establishing protected areas, restoring past systems, and ex situ conservation rely on this benchmark approach in which reference states are used to define conservation goals. Three major conceptual problems and criticisms have been raised considering these approaches. The first basic problem with the benchmark approach is that the preservation of pristine, prehuman landscapes is impossible if such landscapes no longer exist. The term Anthropocene refers to a model in which humans have changed Earth's surface systems sufficiently to affect the biosphere on a scale comparable with some past geologic epochs or periods. If human-induced changes in land cover, biogeochemical cycling, and climate are global [START_REF] Corlett | The Anthropocene concept in ecology and conservation[END_REF], establishing undisturbed protected areas is impossible and useless [START_REF] Thomas | Anthropocene park? no alternative[END_REF].

The second problem is that biodiversity benchmarks are necessarily arbitrary. Ecosystems evolve continuously, implying that they have no original state to be considered as a reference [START_REF] Toledo | The shifting of ecological restoration benchmarks and their social impacts: digging deeper into pleistocene re-wilding[END_REF]; any arbitrary reference state is affected by the human perception of the environment, which changes over human generations [START_REF] Pauly | Anecdotes and the shifting baseline syndrome of fisheries[END_REF]; and valuing a particular system at a particular time is nontrivial if the system changes continuously [START_REF] Hale | Clowning around with conservation: Adaptation, reparation, and the new substitution problem[END_REF]). This problem is particularly acute in the context of species or ecosystem restoration. Seddon et al. (2014) note, for example, that the term rewilding has been widely and variously misused, with meanings ranging from the reintroduction of any recently extirpated species to the proposed introduction of megafauna to replace species lost 13,000 years ago (Pleistocene rewilding). This semantic discrepancy reflects the heterogeneity of the systems, states, and times of reference used in restoration ecology and echoes the conceptual difficulty of assessing what is wild or natural [START_REF] Willis | What is natural? The need for a longterm perspective in biodiversity conservation[END_REF].

The third and most important criticism of the benchmark approach is that it is used to achieve and maintain patterns (a predefined optimum) rather than processes [START_REF] Harmsen | What Are We Trying to Conserve?[END_REF] and ignores evolutionary and environmental dynamics [START_REF] Thomas | Anthropocene park? no alternative[END_REF][START_REF] Toledo | The shifting of ecological restoration benchmarks and their social impacts: digging deeper into pleistocene re-wilding[END_REF].

For example, ex situ conservation techniques aimed at protecting endangered species outside their natural habitat, in captive animal populations, botanic gardens, seed banks, or cryogenic zoos, raise the concern of freezing evolutionary processes. Such approaches focus on some fixed evolutionary patrimony but do not allow for the conservation of evolutionary processes.

Similar concerns have been raised to criticise more interventionist actions aimed at restoring past systems, such as Pleistocene rewilding and deep deextinction. Deep deextinction aims to use synthetic biology to revive long-extinct species, such as the woolly mammoth or the dodo [START_REF] Seddon | Reversing defaunation: Restoring species in a changing world[END_REF], and to reestablish them in natural environments. The expected evolutionary benefit of deextinction is low [START_REF] Robert | De-extinction and evolution[END_REF], and some authors consider it an unnatural and hubristic reversal of natural selection [START_REF] Sandler | The ethics of reviving long extinct species[END_REF]. Most of these criticisms are also directed toward the Pleistocene rewilding approach, which has been qualified as ignorant of landscape and human dynamics, species interactions, and coevolution [START_REF] Toledo | The shifting of ecological restoration benchmarks and their social impacts: digging deeper into pleistocene re-wilding[END_REF].

Necessity of the benchmark approach

The benchmark approach remains an important component of the scientific method; neutral models, null hypotheses, starting points, and control groups are necessary references to assess, understand, and quantify effects and correlations. In biodiversity conservation, benchmarks are necessary to quantify biodiversity with a different perspective than conservation actions themselves, and it is an equally essential prerequisite to legitimate actions with facts rather than concepts alone. Most of the criticisms raised do not disagree with the benchmark approach per se; rather, they are aimed at its primary application to fixed patterns rather than processes. If one considers conservation actions from a broad perspective by considering proximal practices within their larger goals, some criticisms vanish.

As discussed above, conservation biology is not a response to the extinction of particular species; rather, it seeks to address the extinction process affecting biodiversity at the global scale and to analyze this pattern within long-term evolutionary dynamics.

Although subject to uncertainty and highly debated [START_REF] Costello | Can we name Earth's species before they go extinct[END_REF], the global rate of species extinction is one of the most emblematic diagnoses made by conservation scientists, and this rate is always compared with its benchmarks (e.g., global extinction rates during times of normal background extinction; previous, non-human mass extinctions; and even extinction dynamics within particular clades) (Quental & Marshall 2010). Similarly, current patterns of extinction selectivity in the phylogeny (i.e., the relative vulnerability of taxa to extinction) are compared with "basal" selectivity [START_REF] Barnosky | Has the Earth's sixth mass extinction already arrived?[END_REF]. Here, benchmarks used by conservationists are not based on reference states but on reference macroevolutionary processes, such as extinction dynamics. Thus, the reference to natural or prehuman patterns and processes is essential not only to justify biodiversity conservation as a whole (such as natural and human-induced climate variability require disentanglement in the climatewarming debate) but also to implement evolutionarily meaningful actions [START_REF] Willis | What is natural? The need for a longterm perspective in biodiversity conservation[END_REF].

The study of climate change has profoundly influenced conservation science since the early 2000s had has led to a shift from a preservationist philosophy to a more anticipatory model of action (Minteer & Collins 2012), which relies on processes (such as range shift or adaptation) rather than fixed reference states and on establishing new connections between conservationists and evolutionary biologists [START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF]. The developing theory of adaptation to climate change (e.g., [START_REF] Chevin | Adaptation, plasticity, and extinction in a changing environment: towards a predictive theory[END_REF]) is anchored in the general framework of the human, environmental, ecological, and evolutionary scenarios of the future. This dynamic and future-oriented view of biodiversity does not contradict past and current conservation practices, such as reserve establishment, restoration, and ex situ approaches.

Although no one reasonably denies that humans have affected the entire planet [START_REF] Thomas | Anthropocene park? no alternative[END_REF], many conservationists agree that establishing reserves remains one of the best and most efficient tools to slow biodiversity loss [START_REF] Hunter | The complementary niches of anthropocentric and biocentric conservationists[END_REF], and recent results suggest that protected areas can mitigate climate-change impacts on biological communities [START_REF] Gaüzère | Can protected areas mitigate the impacts of climate change on bird's species and communities?[END_REF]. The design and management of protected areas now address processes such as ecological interactions and adaptation (Lawson 2013). Outside these protected areas, it has been suggested that evolutionary dynamics in human landscapes ought to be accepted [START_REF] Sarrazin | Evolution in the Anthropocene[END_REF] in order to reduce anthropogenic directional selective pressures on biodiversity. This implies, in particular, that the dynamics of the ordinary biodiversity (e.g., common birds or pollinator insects that are not listed as threatened but may be rapidly declining and may have already accumulated climatic debts) should be considered.

Similarly, ex situ conservation is not an end in itself ; rather, it aims to establish "dynamic reserves of evolutionary potential" [START_REF] Stockwell | Contemporary evolution meets conservation biology[END_REF] to create or restore populations or ecosystems in the wild [START_REF] Robert | Captive breeding genetics and reintroduction success[END_REF]. Minimisation of the loss of genetic variation, selection relaxation, and adaptation to captivity are addressed via the management of the size and connectivity of captive populations and the implementation of gene flow from wild to captive populations. Finally, most ecological restoration approaches are not intended to return to some arbitrary historical state but rather promote adaptation [START_REF] Aitken | Assisted gene flow to facilitate local adaptation to climate change[END_REF].

Conservation values and evolution

Evolutionary trajectories

The general objective of conserving evolutionary processes [START_REF] Crandall | Considering evolutionary processes in conservation biology[END_REF] can be translated into concrete conservation practices at relatively short-term (e.g., decades or centuries) and small phylogenetic scales (e.g., at the species level) because such objectives can be related to the potential for, for example, a given species to avoid extinction by responding evolutionarily to environmental changes and because existing genetic variation is the primary fuel for contemporary evolution [START_REF] Hendry | Evolutionary biology in biodiversity science, conservation, and policy: a call to action[END_REF]. However, at the global macroevolutionary scale, the maintenance of evolutionary processes becomes tautological because there is no alternative to evolution. Species extinctions and even mass extinctions are parts of the evolutionary processes that shape biodiversity [START_REF] Erwin | The end and the beginning: recoveries from mass extinctions[END_REF]. One corollary is that conservation scientists must necessarily make choices about the evolutionary processes and trajectories that they intend to favor for the future. These choices depend on the ultimate motivation to conserve biodiversity.

Historical conservation biology (hereafter classical conservation) relies on biocentric ethical postulates related to the intrinsic value of species [START_REF] Vucetich | Evaluating whether nature's intrinsic value is an axiom of or anathema to conservation[END_REF], according to which species have a right to continued existence [START_REF] Doak | What is the future of Conservation?[END_REF]. Over the last 30 years, more utilitarian and anthropocentric arguments have enriched the values of conservation science [START_REF] Mace | Whose conservation[END_REF]). These values rely on the collective benefits of biodiversity functions to humans, known as ecosystem services. The science of ecosystem services has benefited from ecological research on the role of biodiversity in ecosystem properties and functions [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF] and is anchored in the new ecology [START_REF] Schmitz | The New Ecology: Rethinking a Science for the Anthropocene[END_REF] and Anthropocene [START_REF] Lövbrand | Who speaks for the future of Earth? How critical social science can extend the conversation on the Anthropocene[END_REF] research fields. These fields focus on the interdependencies between humans and the natural world, emphasizing the importance of conserving species diversity because it offers a portfolio of options to keep global socioecological systems resilient in the face of environmental change. The Anthropocene research also raises important challenges in the fields of the humanities and social sciences by questioning the reflexivity of the power of humans to shape the environment, the separation of humans and nature [START_REF] Tidball | Urgent biophilia: human-nature interactions and biological attractions in disaster resilience[END_REF], and the social consequences of the dominance of humans on the planet [START_REF] Palsson | Reconceptualising the 'Anthropos' in the Anthropocene[END_REF].

Along with the rise of these new motivations and research questions, the multidisciplinary field of conservation science has progressively replaced conservation biology through the integration of nonbiological disciplines in conservation research, such as climate sciences, public policy, economics, sociology, and psychology.

The various and complementary points of view on biodiversity conservation have begun to confront each other [START_REF] Tallis | Working together: a call for inclusive conservation[END_REF] through debate about the "new conservation", which advocates a primarily human-centred conservation ethic focused on human development [START_REF] Kareiva | What is conservation science?[END_REF] and economic growth [START_REF] Nordhaus | Break Through: From the Death of Environmentalism to the Politics of Possibility[END_REF]. Advocates of the new conservation argue that some classical conservation practices rely on fixism, asserting for example that these practices are anachronistic and scientifically unsupportable [START_REF] Doak | What is the future of Conservation?[END_REF]. Whereas much of the opposition between these two conservation movements relies on an ideological component as well as on mutual misunderstanding and caricature [START_REF] Hunter | The complementary niches of anthropocentric and biocentric conservationists[END_REF], their implementation would favour distinct evolutionary processes and biodiversity trajectories. From an evolutionary perspective, the ultimate aim of most classical conservation advocates is to minimise the evolutionary footprint of humans on biodiversity by reducing human pressure on the environment, stopping rapid population and species extinctions, and promoting autonomous biodiversity responses to environmental changes. Such goals will necessarily generate different evolutionary trajectories [START_REF] Sarrazin | Evolution in the Anthropocene[END_REF] relative to the new conservation's moving target of providing ecosystem services to a fast-growing population whose requirements change through time.

Human-biodiversity trade-off

An important point raised by the two opposing movements is the role of the relationship between biodiversity conservation and human well-being. Partisans of the new conservation indicate the existence of trade-offs between conservation actions and human well-being, development, and economic growth [START_REF] Kareiva | What is conservation science?[END_REF][START_REF] Nordhaus | Break Through: From the Death of Environmentalism to the Politics of Possibility[END_REF]. Classical conservationists use a rhetoric of resource sharing between humans and other species, which is consistent with these trade-off arguments, and some advocate reducing human numbers and economic growth [START_REF] Noss | Humanity's domination of nature is part of the problem: A response to Kareiva and Marvier[END_REF].

The supposed trade-off between humans and biodiversity conservation implicitly fuels the arguments regarding the fixist component of classical conservation, which prioritises species that are no longer adapted to the Anthropocene over human development. New conservationists indicate the artificiality of some classical conservation practices [START_REF] Kareiva | What is conservation science?[END_REF]) that are considered an expression of human values [START_REF] Sarkar | Environmental Philosophy: From Theory to Practice[END_REF]). However, these criticisms are incorrectly based on a rhetoric of the naturalization of human development via the market economy, yet such development is related to political and societal choices (Picketti 2014) that are no less expressions of human values than the biocentric ethic of classical conservation. Most authors agree that human well-being generally benefits from biodiversity conservation and that human development is more complex than economic growth alone; it depends heavily on the ability to manage finite energetic resources available on Earth, avoid catastrophic human extinction, and mitigate the deleterious effects of human demographics and economic growth. Thus, some authors advocate limiting human demands on the biosphere for both biocentric and anthropocentric reasons [START_REF] Noss | Humanity's domination of nature is part of the problem: A response to Kareiva and Marvier[END_REF].

Conclusions

The functional and normative postulates constructed over 30 years ago [START_REF] Soulé | What is conservation biology?[END_REF] provide a way to solve many apparent contradictions between biodiversity conservation and evolution because conservation efforts are not directed toward fixed patterns or products but are, rather, directed toward evolutionary processes; scientists do not need fixed reference states to conserve biodiversity because they already use evolutionary processes as references; these references are intimately linked to knowledge of the components, mechanisms, and history of life since its origin, which implies that short-term human history falls outside evolution.

Although such an approach to conservation is based on an ethic primarily centred on the intrinsic value of biodiversity, the rise of more anthropocentric perspectives has broadened its interdisciplinary component and has attracted more interest and support from society. The important debate between the classical and new conservation movements not only questions humanity's relationship with biodiversity and its evolution but also emphasizes the political foundations and implications of all conservation practices. One important issue is to ensure that the ideological and political components of this opposition do not weaken the voices of conservation scientists or distract them from their original goals. In particular, we must accept that conservation science relies on human values and that this does not make it a fixist discipline.

Acknowledgments C.T. and S.C. were funded by the French Agence Nationale de la Recherche (grant ANR-12-ADAP-0006). We thank E. Porcher, G. Lois, M. Jeanmougin, R. Julliard, A.-C. Prévot, A.C. Torres, L. Mermet, and two anonymous reviewers for helpful comments.