Keywords: woven composites, water aging, acoustic emission, tomography, damage factor, fracture surface

Biocomposites are being developed for structural applications, particularly in the automotive industry. The use of plant fibers as reinforcement is a first step towards the development of ecomaterials but it is necessary to use also a more eco-friendly matrix. In this paper, woven hemp composites impregnated with two different eco-polymers are compared: hemp/Greenpoxy and hemp/Elium. Greenpoxy is a partially bio-based thermoset resin and Elium is a recyclable thermoplastic polymer. The influence of various hygrothermal aging conditions on the mechanical properties of these composites is studied and results are compared with the behavior of samples stored at ambient temperature and humidity. Water immersion was carried out at three different temperatures: 21°C, 60°C and 70°C. A Fick's law was used to model the water uptake. Modulated DSC was performed to measure the evolution of the glass transition temperatures. Repeated progressive tensile loading tests were carried out and instrumented with acoustic emission monitoring. The evolution of the damage factor, the residual strains and the acoustic events was analyzed. Fracture surface analysis and microtomography observations were also performed. Damage mechanisms are discussed. Results show that mechanical properties are highly affected by the hygrothermal aging whatever the matrix, but the increase in temperature affects the two composites differently.

Influence of hygrothermal aging on mechanical properties and damage mechanisms of hemp-reinforced biocomposites

Quentin Drouhet*, Fabienne Touchard and Laurence Chocinski-Arnault

Introduction

The interest in natural fibers as reinforcements in composite materials is growing. Indeed, manufacturers are increasingly considering the use of biocomposites due to their good specific properties compared to those of glass-reinforced composites and due to environmental policies [START_REF] Zwawi | A Review on Natural Fiber Bio-Composites, Surface Modifications and Applications[END_REF][START_REF] Barnat-Hunek | Properties of Hemp-Flax Composites for Use in the Building Industry[END_REF][START_REF] Faruk | Biocomposites Reinforced with Natural Fibers: 2000-2010[END_REF]). Among the different natural fibers, hemp fibers are ones of the most environment-friendly industrial fibers for bio-composites [START_REF] Marrot | Properties of Frost-Retted Hemp Fibers for the Reinforcement of Composites[END_REF][START_REF] Manaia | Industrial Hemp Fibers: An Overview[END_REF][START_REF] Madsen | Hemp Yarn Reinforced Composites -I. Yarn Characteristics[END_REF]). One of the advantages of extracting hemp fibers is that it is possible to use all parts of the plant for the production of different products -hemp seeds, their shells and hemp shives -, thus maximizing the added value of this resource which is the concept of sustainable development. To obtain biocomposites that may be used in semi-structural parts, it is necessary to use continuous reinforcements and a polymer matrix with good rigidity [START_REF] Lotfi | Natural Fiber-Reinforced Composites: A Review on Material, Manufacturing, and Machinability[END_REF]. Therefore, several studies deal with biocomposites made of thermoset resin and unidirectional (UD) or woven natural fibers [START_REF] Borrello | Manufacturing and Characterization of Hemp•reinforced Epoxy Composites[END_REF][START_REF] Neves | Comparative Mechanical Properties between Biocomposites of Epoxy and Polyester Matrices Reinforced by Hemp Fiber[END_REF][START_REF] Krishnasamy | Vibration and Wear Characteristics of Aloevera/Flax/Hemp Woven Fiber Epoxy Composite Reinforced with Wire Mesh and BaSO 4[END_REF][START_REF] Shah | Damage in Biocomposites: Stiffness Evolution of Aligned Plant Fibre Composites during Monotonic and Cyclic Fatigue Loading[END_REF][START_REF] Bensadoun | Fatigue Behaviour Assessment of Flax-Epoxy Composites[END_REF][START_REF] De Vasconcellos | Tension-Tension Fatigue Behaviour of Woven Hemp Fibre Reinforced Epoxy Composite: A Multi-Instrumented Damage Analysis[END_REF][START_REF] Sawi | An Investigation of the Damage Mechanisms and Fatigue Life Diagrams of Flax Fiber-Reinforced Polymer Laminates[END_REF][START_REF] Newman | Auto-Accelerative Water Damage in an Epoxy Composite Reinforced with Plain-Weave Flax Fabric[END_REF]. However, most of the thermoset resins used come from fossil resources and they are all non-recyclable. So alternative polymers, more eco-friendly and with a stiffness similar to that of classical thermoset resins have been developed. In particular, in this study, the two polymers used are Greenpoxy 56 and Elium 188. Greenpoxy 56 is a partially bio-based epoxide thermoset resin from Sicomin, which allows manufacturing more bio-sourced composites [START_REF] Essid | Comparison of the Properties of Flax Shives Based Particleboards Prepared Using Binders of Bio-Based Lignin and Partially Bio-Based Epoxy Resin[END_REF][START_REF] Gbadeyan | Mechanical, Microstructure, and Dynamic Mechanical Analysis of Nano-Shell and Plant Fiber Hybrid Biocomposite[END_REF]. The second matrix is a thermoplastic resin called Elium 188 from Arkema. It is a liquid methylmethacrylate based polymer, which is, in this study, thermoplastic. Thus, it can be thermoformed and recycled [START_REF] Allagui | Effect of Recycling Cycles on the Mechanical and Damping Properties of Flax Fibre Reinforced Elium Composite: Experimental and Numerical Studies[END_REF][START_REF] Khalili | Ramie Fabric Elium® Composites with Flame Retardant Coating: Flammability, Smoke, Viscoelastic and Mechanical Properties[END_REF]. Composites studied in this work are all made of the same hemp fabric impregnated with either Greenpoxy 56 or Elium 188.

One of the major concerns to be considered for biocomposites is their long-term performance in outdoor applications. Weathering conditions such as humidity and temperature can affect adversely on the service life of these composites. Natural fibers in composites have a hydrophilic behavior and the matrix is comparatively hydrophobic. This issue has consequences on mechanical properties, in particular on interfacial cohesion [START_REF] Perrier | Quantitative Analysis by Micro-CT of Damage during Tensile Test in a Woven Hemp/Epoxy Composite after Water Ageing[END_REF]. Many studies have already shown that the moisture absorption in natural fiber reinforced composites implies a decrease in strength and rigidity {Citation} . Some of them have studied the effect of the water temperature on the absorption behavior [START_REF] Sala | Influence of the Stress Level and Hygrothermal Conditions on the Creep/Recovery Behaviour of High-Grade Flax and Hemp Fibre Reinforced GreenPoxy Matrix Composites[END_REF][START_REF] Scida | Influence of Hygrothermal Ageing on the Damage Mechanisms of Flax-Fibre Reinforced Epoxy Composite[END_REF]. Nevertheless, to the best of authors' knowledge, there are no published results concerning the damage mechanisms due to tensile loading after hygrothermal aging for hemp/Greenpoxy and hemp/Elium composites.

To deeply analyze damage mechanisms, different techniques are used in this work: scanning electron microscopy (SEM), acoustic emission (AE) and micro computed tomography (micro-CT). In literature, SEM is frequently used for fracture surface analysis [START_REF] Huang | Durability of Plant Fiber Reinforced Alkali Activated Composites[END_REF][START_REF] Langhorst | Technical Agave Fiber Tensile Performance: The Effects of Fiber Heat-Treatment[END_REF]) and AE has already been successfully applied in order to record damage evolution in a variety of biocomposite materials [START_REF] Jani | Influence of Natural Filler on Mechanical Properties of Hemp/Kevlar Hybrid Green Composite and Analysis of Change in Material Behavior Using Acoustic Emission[END_REF][START_REF] Bartos | Reinforcement of Polypropylene with Alkali-Treated Sugarcane Bagasse Fibers: Mechanism and Consequences[END_REF][START_REF] Mahi | Damage Mechanisms Characterization of Flax Fibers-Reinforced Composites with Interleaved Natural Viscoelastic Layer Using Acoustic Emission Analysis[END_REF]). X-ray micro-CT with high resolution allows specimen observation in three dimensions. Only few studies deal with the use of X-ray micro-CT on plant fiber composites, for example for the measurement of orientation and dimension of short fibers in composites [START_REF] Madra | A Clustering Method for Analysis of Morphology of Short Natural Fibers in Composites Based on X-Ray Microtomography[END_REF], or for damage quantification in a woven hemp/epoxy composite [START_REF] Perrier | Quantitative Analysis by Micro-CT of Damage during Tensile Test in a Woven Hemp/Epoxy Composite after Water Ageing[END_REF]. In a previous work, the association of micro-CT and acoustic emission to study the fatigue behavior of hemp/epoxy composites has shown the complementarity of these two techniques for damage analysis [START_REF] Barbière | Influence of Moisture and Drying on Fatigue Damage Mechanisms in a Woven Hemp/Epoxy Composite: Acoustic Emission and Micro-CT Analysis[END_REF]). In the present study, a supplementary method for damage analysis is also used: the macroscopic damage factor is determined from the secant modulus measured during repeated progressive tensile loadings. This type of tests consists of cyclically loading the material with increasing intensity until it breaks. It allows extracting the secant modulus at each cycle from the stress-strain curve. It was initially proposed by Lemaître (1985), and then applied for studying the modulus evolution in plant fiber composites [START_REF] Shah | Damage in Biocomposites: Stiffness Evolution of Aligned Plant Fibre Composites during Monotonic and Cyclic Fatigue Loading[END_REF][START_REF] Richard | A Viscoelastoplastic Stiffening Model for Plant Fibre Unidirectional Reinforced Composite Behaviour under Monotonic and Cyclic Tensile Loading[END_REF].

In this paper, the aim is to investigate the consequences of hygrothermal aging on mechanical behavior of hemp/Greenpoxy and hemp/Elium composites. Specimens are subjected to different water-aging conditions: they are immersed in water at 21°C, 60°C or 70°C until saturation. All results are compared with the reference condition where samples are stored at ambient temperature and humidity. Progressive cyclic loading tests are then carried out with an AE set up to evaluate macroscopic and microstructural damage. Then, the damage analysis is completed with post-failure observations by SEM and micro-CT.

Materials and methods

Tested materials

All the studied composites are made of seven plain-woven plies of hemp fabric with an areal density of 290 ± 10 g/m². Two different matrices are used. One matrix is a partially bio-based thermoset polymer named Greenpoxy 56, provided by Sicomin. It is an epoxy resin, which contains 56% of bio-based carbon atoms. The second matrix used in this study is a thermoplastic polymer, Elium 188, developed by the company Arkema. Composite plates were manufactured by vacuum infusion. The mass and volume fractions of fibers were determined by geometry and weight measurements. The fiber volume fraction is equal to 39.7 ± 1.6% for the hemp/Elium plates and 41.8 ± 1.7% for the hemp/Greenpoxy plates, which corresponds in mass fraction to 44.6 ± 0.3% and 46.5 ± 1.9%, respectively. After manufacturing, hemp/Elium plates were heat treated at 80°C for 4 hours and hemp/Greenpoxy plates were cured with the following cycle: 24 hours at room temperature, 24 hours at 40°C, 16 hours at 60°C, 8 hours at 80°C and 30 minutes at 95°C. Rectangular samples were cut from each plate to get ±45° reinforcement orientation. Overall dimensions of samples are 140 mm in length, 20 mm in width and 4 mm in thickness.

Aging conditions

In this study, four aging conditions were performed on the samples to evaluate the influence of moisture absorption and immersion temperature. "Ambient storage" is the reference conditioning, samples were stored at room temperature and humidity (about 21 ± 2°C and 48 ± 5% of relative humidity, respectively). The three following aging conditions consisted in immersing samples in water at different temperatures until the maximum of water uptake: "Saturated at 21°C", "Saturated at 60°C" and "Saturated at 70°C". Tensile tests were carried out just after water saturation. During the immersion, the water uptake of samples was measured with a precision balance (precision of 0.1 mg). Weight gain 𝑀𝑀 𝑡𝑡 was evaluated by Equation 1:

𝑀𝑀 𝑡𝑡 = 100 × 𝑚𝑚(𝑡𝑡) -𝑚𝑚 0 𝑚𝑚 0 (1)
where 𝑚𝑚(𝑡𝑡) is the weight of the sample at time 𝑡𝑡 and 𝑚𝑚 0 is the initial weight.

The diffusion of water in a solid is usually described by the one-dimensional Fick's model.

With this model, the water uptake can be expressed by Equation 2 [START_REF] Shen | Moisture Absorption and Desorption of Composite Materials[END_REF]:

𝑀𝑀 𝑡𝑡 𝑀𝑀 ∞ = 1 - 8 𝜋𝜋 2 � 1 (2𝑛𝑛 + 1) 2 exp � -𝐷𝐷(2𝑛𝑛 + 1) 2 𝜋𝜋 2 𝑡𝑡 ℎ 2 � ∞ 𝑛𝑛=0 (2)
where 𝑀𝑀 ∞ is the maximum of water uptake, 𝐷𝐷 is the Fick's diffusion coefficient (mm²/s), ℎ is the thickness (mm) of the sample and 𝑡𝑡 is the time (s). This expression can be approximated by the Equation 3 and the Fick's diffusion coefficient is expressed by Equation 4:

𝑀𝑀 𝑡𝑡 𝑀𝑀 ∞ = 1 -exp �-7.3 � 𝐷𝐷𝑡𝑡 ℎ 2 � 3/4 � (3) 𝐷𝐷 = 𝜋𝜋 � ℎ𝑘𝑘 4𝑀𝑀 ∞ � 2 (4)
where 𝑘𝑘 is the initial slope of the 𝑀𝑀 𝑡𝑡 -√𝑡𝑡 curve.

Experimental procedures

Differential Scanning Calorimetry (DSC) measurements

Biocomposites glass transitions were determined using a differential scanning calorimeter Q20 from the company TA Instruments. Modulated Differential Scanning Calorimetry (M-DSC) was carried out in order to detect glass transition in presence of water in the sample. Indeed, it is necessary to separate reversing and non-reversing heat flows to measure Tg. The non-reversing heat flow shows kinetic events such as cold crystallization, physical aging and evaporation of water. With classical DSC, some of these events may hide the glass transition. The M-DSC allows to obtain the reversing part of the heat flow, which is related to the heat capacity of the material, and thus to identify the glass transition. Modulated DSC was used to detect more easily the glass transition temperature (Tg). In modulated DSC, the temperature variation is nonlinear and it is expressed as follows (Equation 5):

𝑑𝑑𝑑𝑑 𝑑𝑑𝑡𝑡 = 𝑏𝑏 + 𝐴𝐴 2𝜋𝜋 𝑃𝑃 cos ( 2𝜋𝜋 𝑃𝑃 𝑡𝑡) (5)
where 𝑏𝑏 is the heat ramp rate (°C/min), 𝐴𝐴 is the modulation amplitude and 𝑃𝑃 is the period of the signal. The M-DSC method used, in order to detect the Tg whatever the conditioning of the material, was based on the following parameters: 𝑏𝑏 = 10 °𝐶𝐶/𝑚𝑚𝑚𝑚𝑛𝑛, 𝐴𝐴 = 0.796 °𝐶𝐶 and 𝑃𝑃 = 30 𝑠𝑠.

Repeated progressive tensile loading

Mechanical tests were carried out using an Instron 5982 tensile testing machine with a crosshead speed of 0.5mm/min. Strain was measured by a 25mm gauge length extensometer.

Section area values of samples were measured just before each mechanical test. Therefore, the possible variations of sample dimensions due to the different environmental conditions were taken into account. To analyze the modulus evolution in samples during tensile tests, repeated progressive loading was applied. It made possible to determine the secant modulus at each loading-unloading cycle (Figure 1) and to determine a macroscopic damage coefficient, the damage factor d, defined by Equation 6 [START_REF] Lemaitre | A Continuous Damage Mechanics Model for Ductile Fracture[END_REF]:

𝑑𝑑 = 1 - 𝐸𝐸 𝜎𝜎 𝑖𝑖 𝐸𝐸 0 (6)
where 𝐸𝐸 0 is the Young's modulus, 𝐸𝐸 𝜎𝜎 𝑖𝑖 is the secant modulus measured from the cycle number 𝑚𝑚 of the repeated progressive tensile test. For each loading-unloading, the residual strain 𝜖𝜖 𝑟𝑟𝑟𝑟 can also be extracted as shown in Figure 1.

Acoustic emission

Acoustic emission (AE) monitoring was carried out by using the AE system PCI-2 from the company Mistras Group. Two Micro-80 sensors, with a resonant frequency of 300 kHz and a contact diameter of 10 mm, were placed at the specimen gauge extremities. They were separated with a distance of 65 mm and the coupled agent was Blu-tack. An amplitude threshold of 33 dB was chosen. Each test was preceded by a speed test and by the pencil lead break procedure.

The first one consists in measuring the speed of an acoustic wave for the material tested. The second one is used to check the speed and ensure the right location of acoustic emissions. Postprocessing was done using a multi-parametric identification, based on the k-means algorithm.

The k-means algorithm aims to partition data into k clusters by minimizing the Euclidian distance between each value and the nearest center. This algorithm is unsupervised, which means that the number of clusters k has to be known a priori. It is an iterative algorithm, which allocates each value to the closest centroid. In this study, data were acoustic events and five descriptors were chosen among all the AE parameters: amplitude, duration, rise time, absolute energy and number of counts. Then, each cluster was associated with a damage mechanism such as fiber breaks, matrix cracking or debonding at the fiber/matrix interface (Ben [START_REF] Ameur | Investigation and Identification of Damage Mechanisms of Unidirectional Carbon/Flax Hybrid Composites Using Acoustic Emission[END_REF][START_REF] Barbière | Influence of Moisture and Drying on Fatigue Damage Mechanisms in a Woven Hemp/Epoxy Composite: Acoustic Emission and Micro-CT Analysis[END_REF].

Scanning Electron Microscopy (SEM)

Fracture surfaces were investigated by scanning electron microscope VEGA3 from the company TEACAN. All specimens were sputter coated with gold before SEM observations.

The fracture surfaces of coated samples were analyzed with secondary electrons and an operating voltage of 7 kV.

Micro-CT

Image acquisition was performed using an UltraTom CT scanner manufactured by RX Solutions (France). The system consists of a Hamamatsu micro focus sealed X-ray tube operating at a voltage range of 20-150 kV and an amperage range of 0-500 µA. The maximum achievable power is about 75 W. X-rays, generated by the source, diverge at an angle providing a cone beam. Various geometric magnifications can be obtained by moving the sample close to the source to provide a high-resolution mode. In this study, a 15-µm resolution and a voltage of 50 kV were used. The image acquisition time was about four hours per sample. The reconstruction was performed using an algorithm based on the filtered back-projection procedure for Feldcamp cone-beam geometry.

Results and discussion

Water uptake, evolution of glass transition temperature and mechanical properties are studied for the "Ambient storage", "Saturated at 60°C" and "Saturated at 70°C" aging conditions. Then, damage analysis presents a comparison between ambient storage and saturated at 60°C aging condition.

Water uptake and Fick's law

The water uptake curves of the samples immersed in water at different temperatures are shown in Figure 2. Moisture absorption is normalized by fiber volume fraction in order to take into account the variations in fiber content between all specimens. In addition, the water uptake is plotted as a function of the square root of time (in seconds) divided by the thickness (in millimeters) of each sample. First, it appears that the diffusion properties of hemp-reinforced biocomposites follow a Fickian behavior whatever the temperature of the water. Results also show that the temperature has different effects depending on the polymer matrix. On one hand, the immersion temperature has little influence on the maximum of water uptake in hemp/Greenpoxy samples. On the other hand, maximum moisture content increases with the immersion temperature for hemp/Elium material. These behaviors can be explained by the fact that the nature and the molecular structure of the matrices are different. Table 1 presents the values obtained for the maximum moisture absorption normalized by the fiber volume fraction for both materials immersed in water at room temperature (21°C), 60°C and 70°C. It can be seen that for hemp/Elium specimens, the saturation absorption increases by 39% when the immersion temperature is raised to 60°C and by 80% when it is raised to 70°C. The diffusion coefficient D, defined in equation 4, depends on the slope of the 𝑀𝑀 𝑡𝑡 -√𝑡𝑡 curve. Table 2 presents the obtained D values of both hemp/Elium and hemp/Greenpoxy samples for immersion temperatures of 21°C, 60°C and 70°C. Results show that whatever the temperature, the Fick's diffusion coefficients for hemp/Elium samples are higher than for hemp/Greenpoxy ones. It means that hemp/Elium absorb water more easily than hemp/Greenpoxy. It also appears that the increase in D values with the water temperature is very significant for both composites and is similar whatever the temperature value: almost +500% for hemp/Elium material and about +900% for hemp/Greenpoxy composite. In order to evaluate the kinetics of absorption, the duration needed to reach 90% of the maximum absorption value has been determined. This duration, noted 𝑡𝑡 90% , corresponds to an absorption of 90% of the maximum water uptake for each material and condition. It is calculated from Equation 3 with 𝑀𝑀 𝑡𝑡 = 0.9 × 𝑀𝑀 ∞ . Then, 𝑡𝑡 90% in days follows Equation 7 and the results are given in Table 3.

𝑡𝑡 90% = 1 3600 * 24 × ℎ 2 𝐷𝐷 × � -ln(0.1) 7.3 � 4/3 (7)
Table 3 shows that the kinetics of absorption for hemp/Elium composite is faster than the one for hemp/Greenpoxy material for immersion in water at ambient temperature. 90% of the saturation in water is reached in two weeks for hemp/Elium samples and in about 25 days for hemp/Greenpoxy ones. When the temperature is raised to 60°C or 70°C, the duration to reach 90% of maximum water content decreases drastically to about two days whatever the material.

These results show the significant influence of the matrix choice concerning the hygrothermal behavior of biocomposites. Indeed, at ambient temperature, hemp/Elium biocomposite absorbs water more rapidly than the hemp/Greenpoxy one, but reaches a lower water uptake at saturation. However, the maximum water uptake of hemp/Elium is significantly increasing when the temperature is raised to 60°C or 70°C, while it remains almost constant for hemp/Greenpoxy.

Evolution of glass transition temperature

Glass transition temperatures (Tg) were measured by using Modulated Differential Scanning Calorimetry (M-DSC) for both biocomposites for three aging conditions: ambient storage, saturated at 21°C and saturated at 60°C. Figure 3 presents the reversing heat flow-temperature curves obtained for the samples studied. Results show that the Tg values in ambient condition are about 91°C for hemp/Elium composite and 74°C for hemp/Greenpoxy one. For both biocomposites, Tg values decrease with the water uptake and the temperature of the water.

Deviations from ambient storage are significant (Table 4). The Tg values of hemp/Elium and hemp/Greenpoxy samples immersed in water at room temperature decrease by about 10.1% and 7.9% respectively. This is the consequence of the plasticizing phenomenon due to chemical reaction of the water molecules in the polymers [START_REF] Capiel | Water Diffusion and Hydrolysis Effect on the Structure and Dynamics of Epoxy-Anhydride Networks[END_REF]. Concerning the water aging at 60°C, the Tg values decrease again and the deviations from ambient storage are higher than those obtained after a water aging at 21°C: -14.5% for hemp/Elium and -11.7% for hemp/Greenpoxy. It can be explained by the fact that increasing the temperature allows for more intense molecular activity and can therefore produce a higher plasticizing effect on matrices than immersion at room temperature. It has to be noticed that final Tg values were still above the immersion temperature of 60°C for both biocomposites.

Influence on mechanical properties

Tensile behavior

Tensile tests were carried out to characterize the mechanical properties of [±45]7 hemp/Elium and hemp/Greenpoxy specimens for the different aging conditions: ambient storage, saturated at 21°C, saturated at 60°C and saturated at 70°C. The obtained stress-strain curves are shown in Figure 4. The tensile properties of the biocomposites have been determined following ASTM D 3039. Young's moduli E0 of both biocomposites for different aging conditions compared with ambient storage are summarized in Figure 5. Strength values and longitudinal strain at failure are presented in Table 5. For ambient storage, results show that hemp/Elium and hemp/Greenpoxy composites have almost the same Young's modulus E0 and the same maximal stress σm. However, the strain at failure seems to be slightly higher for the hemp/Elium samples, which are made of a thermoplastic matrix, than for the hemp/Greenpoxy specimens. It appears that the water aging degrades the mechanical properties of these hemp-reinforced biocomposites. For the water aging at 21°C, hemp/Elium samples are less affected than hemp/Greenpoxy ones. Indeed, hemp/Greenpoxy Young's modulus is divided by more than two compared to the reference one (ambient storage), while there is "only" a decrease of 36.2% in the case of hemp/Elium (Figure 5). Concerning the maximal stress, hemp/Elium and hemp/Greenpoxy samples show a decrease of 7% and 19% respectively, while their strain at failure significantly increases with water aging in comparison with reference values. This evolution of the mechanical properties after water aging can be explained by the plasticization of the matrix coupled with a degradation of the hemp yarn and of the yarn/matrix interface. For water aging at 60°C and 70°C, the higher the temperature, the greater the loss of Young's modulus and strength (Figure 5 and Table 5). Concerning the strain at failure, despite the high dispersion of the values, it appears that it increases with the immersion temperature for hemp/Elium while it remains constant for the hemp/Greenpoxy (Table 5). It is interesting to notice that the Young's modulus evolution is directly linked with the one of the water uptake at saturation, whatever the material (Table 1): when 𝑀𝑀 ∞ /𝑉𝑉 𝑓𝑓 increases, the Young's modulus value decreases, and when 𝑀𝑀 ∞ /𝑉𝑉 𝑓𝑓 remains constant, E0 is almost similar. It demonstrates that, whatever the immersion temperature, the rigidity is linked to the amount of absorbed water in the biocomposite.

Damage factor and residual strain

In this part, the evolution of the damage factor and of the residual strain during the repeated progressive tensile loading tests is discussed. Figure 6 presents the damage factor d versus the normalized stress (𝜎𝜎/𝜎𝜎 𝑚𝑚 ) for both biocomposites with ambient storage or with the different water aging conditions. Results show that there are two types of evolution of d depending on the conditioning. The concavity of the plotted curves for 60°C and 70°C water aging conditions differs from the one obtained in the other cases. The initial kinetics of the damage factor increase is much higher for water aging at 60°C and 70°C. This can be explained by the greater modulus degradation with these severe conditions. Then, for higher applied stress level, a significant slowdown in the damage factor growth is observed. This means that the secant modulus does not decrease anymore for high stress values, or even slightly increases in the case of hemp/Elium (Figure 6a). Studies are underway to better understand this phenomenon.

Nevertheless, we already know that fiber reorientation during tests plays an important role, especially when the material is more deformed and when the matrix is plasticized. A damage threshold has been determined to characterize the appearance of damage mechanisms in the biocomposites samples: it is defined as the applied stress level when damage factor reaches 5%.

The obtained damage threshold values are presented in Figure 7. It can be seen that, for ambient storage, the damage threshold of hemp/Elium specimens is lower than the hemp/Greenpoxy one. Indeed, a tensile stress of 10MPa is sufficient to initiate damage in hemp/Elium, whereas it is necessary to reach about 19MPa to start damaging hemp/Greenpoxy, within the meaning of modulus degradation. When the biocomposites are immersed in water, whatever the temperature, the damage threshold drops. It is about 6.5MPa for hemp/Elium (-35%) and 8.5MPa for hemp/Greenpoxy (-54%). In all cases, the thermoplastic composite has a lower damage threshold than the thermoset one. The evolutions of residual strains were also studied and are presented in Figure 8. Results show that, for all the hygrothermal aging conditions, residual strains are more significant and become earlier in hemp/Elium than in hemp/Greenpoxy. Moreover, it can be seen that an increase in immersion temperature implies an increase in the residual strains and a decrease in the stress triggering the appearance of residual strains. This has to be related to the higher degradation of the biocomposites in severe conditions: the matrix is more plasticized, natural fibers and interfaces are also more affected.

Damage analysis

The analysis of damage mechanisms for both materials, hemp/Elium and hemp/Greenpoxy, is focused on two different types of aging: ambient storage or immersion in water at 60°C until saturation.

In situ measurement by acoustic emission

Acoustic emissions were recorded during repeated progressive tensile loading tests. Figure 9a presents the normalized cumulative number of acoustic emission events as a function of the normalized applied stress. To better readability, the few AE events corresponding to the unload phases of tensile tests were hidden. Curves shapes can be described as follows: a first phase where the damage is very low, a transition where damage mechanisms are activated, and then a rapid growth of these mechanisms until the failure of the samples. To quantify the transition, a damage threshold was chosen corresponding to the applied stress for which the normalized cumulative number of AE events reaches 5%. The values obtained for these EA damage thresholds are presented in Figure 9b. Results show that, for ambient storage, activation of damage mechanisms begins for hemp/Elium samples at a stress of 43MPa whereas it happens at a stress of 54MPa for hemp/Greenpoxy specimens. When the material is saturated in water at 60°C, the AE damage threshold of hemp/Elium and hemp/Greenpoxy specimens drops to a value of 17MPa and 12MPa respectively. It is interesting to compare these results with those obtained by measuring the values of the damage factor threshold (Figure 7). It can be seen that the threshold values are different, which can be explained by the fact that the measurement methods used are very different: the threshold value is obtained from the evolution of the secant modulus in Figure 7, whereas it is determined from the evolution of the number of acoustic events in Figure 9b. It is thus not the same "damage" that it is measured with these two complementary techniques: the evolution of the secant modulus gives the macroscopic mechanical damage, while the number of AE events corresponds to the physical damage releasing energy during the test. Nevertheless, trends are following the same conclusions: the damage threshold sharply decreases after hygrothermal aging for both materials, showing that this type of aging accelerates the damage occurrence in the biocomposites. Moreover, it appears that, with the two methods of measurement, hemp/Elium material seems to be less affected by the hygrothermal aging than hemp/Greenpoxy one. Post-processing of AE data was then carried out to associate each AE cluster with a type of damage mechanism: matrix cracking, fiber/matrix interface debonding and fiber breaks. Figure 10 presents the proportion of each damage mechanism in number (Figure 10a) and in energy (Figure 10b). It allows to compare the contribution of the three types of damage to the final cumulative AE event number or to the final cumulative AE energy. Results in Figure 10a show that, whatever the biocomposite and the aging condition, the damage contribution in number is quite similar. The numbers of AE events due to matrix cracks and debonding are dominant: about 40% for each. The fiber break contribution to the AE event number is less than 20%. These proportions have to be related to the ±45° orientation of the studied samples. With such orientation, matrix and interfaces are the most mechanically loaded and the fiber breaks occur only at the very end of the test. Concerning the damage contribution in energy, the rates are also similar whatever the material and the conditioning (Figure 10b). Logically, the proportion of fiber breaks is largely predominant with about 85% of the total AE energy due to fiber failures. The contribution of interface damage is between 10% and 17% and the rest is due to resin cracking. The comparison between Figure 10a and Figure 10b shows how different the contribution of each type of damage is in the total AE event number or in the total AE energy. Fiber breaks are the most energetic type of damage, but the least numerous, while matrix cracking is the least energetic but accounts for over 40% of the number of events.

Damage observations

Fracture surfaces were observed by Scanning Electron Microscope (SEM) after the repeated progressive tensile loading tests for both materials and aging conditions (Figure 11).

Observations do not show many differences between hemp/Elium and hemp/Greenpoxy samples. For both materials, the comparison between "ambient storage" and "water saturated at 60°C" reveal that fiber/matrix debonding is much more developed after water aging at 60°C than in the case of ambient storage. This is due to the degradation of the interfaces by the presence of water. This phenomenon has contributed to the higher strain at failure, the lower Young's modulus and the lower strength that were measured (Figure 5 and Table 5). It has also contributed to the earlier activation of the physical and mechanical damage (Figures 7 and9).

Micro-tomographies were also carried out after failure in order to perform 3D observations of the damage. Figure 12 presents examples of pictures obtained for both materials and both aging conditions in the plane of the fabric plies and on the edge of the specimens. The images shown are from the core of each biocomposite. Firstly, the micro-CT observations show the low porosity content of both materials, due to the manufacturing process by vacuum infusion. They also show that the shape of the macroscopic failure is quite similar in all cases. The failure starts along a yarn with a 45° angle relative to the X-axis, and propagates by slipping from yarn to yarn. Moreover, Figure 12 shows that the same type of damage appears whatever the material and the conditioning: cracks initiate at a yarn/matrix interface and propagate from an interface to the next one. Each crack is made of a coalescence of partial debondings at hemp yarn/matrix interfaces, which can lead to a macro-crack crossing the sample thickness. The damage distribution inside the coupons is also the same in all cases: cracks are more numerous and open at the free edges of the samples than in the center of the specimens. Concerning the quantity of damage, micro-CT scans show that damage is far more developed in the samples submitted to hygrothermal aging than in the ambient stored ones. Cracks become longer and wider and debonding is widespread, as it can be seen in the zoomed-in images (Figure 12). These results

show that, even if the damage type is not modified by the hygrothermal aging, the damage quantity is significantly increased. This increase in damage quantity explains the poor mechanical characteristics that were measured for both materials after hygrothermal aging (Figure 5 and Table 5).

Conclusion

In this study, the influence of different hygrothermal aging conditions on mechanical properties and damage mechanisms was studied for two woven [(±45)]7 hemp-reinforced biocomposites:

hemp/Elium and hemp/Greenpoxy. The aging conditions consisted in immersing samples in water until the maximum of water uptake at different temperatures: 21°C, 60°C and 70°C. It was shown that water absorption is correctly described by a Fickian model whatever the material and the immersion temperature. For hemp/Elium samples, water uptake at saturation increases with the immersion temperature whereas for hemp/Greenpoxy specimens it is almost constant. However, concerning the diffusion coefficient, it increases twice as much for hemp/Greenpoxy samples as for hemp/Elium ones when the water is heated. The plasticizing effect of water on matrices is also accentuated by the temperature increase: glass transition temperatures decrease with the immersion temperature for the two biocomposites.

Repeated progressive tensile loading tests instrumented with an axial extensometer and an acoustic emission system were also carried out and showed significant differences in the behavior of the samples after aging or after storage under ambient conditions. Mechanical properties of both biocomposites drop with hygrothermal aging. An in-depth study was conducted on damage mechanisms for "Ambient storage" and "Saturated at 60°C" conditions.

Figure 1 .

 1 Figure 1. Scheme of a repeated progressive tensile test curve and method of determination of

Figure 2 .

 2 Figure 2. Experimental moisture absorption up to saturation for different immersion

Figure 3 .

 3 Figure 3. DSC thermogramms of hemp/Elium (blue) and hemp/Greenpoxy (red) for three aging

Figure 4 .

 4 Figure 4. Tensile stress-strain curves after different aging conditions of [±45]7 composites: a)

Figure 6 .

 6 Figure 6. Damage factor d as a function of normalized stress (𝜎𝜎/𝜎𝜎 𝑚𝑚 ) for the different aging conditions a) for hemp/Elium and b) for hemp/Greenpoxy.

Figure 7 .

 7 Figure 7. Damage threshold for the different aging conditions of hemp/Elium and

Figure 8 .

 8 Figure 8. Residual strains µri as a function of normalized stress (𝜎𝜎/𝜎𝜎 𝑚𝑚 ) for the different aging conditions a) for hemp/Elium and b) for hemp/Greenpoxy.

Figure 9 .

 9 Figure 9. Acoustic emission results for hemp/Elium and hemp/Greenpoxy composites

Figure 10 .

 10 Figure 10. Post-processing of acoustic emission results for hemp/Elium and hemp/Greenpoxy

Figure 11 .

 11 Figure 11. SEM images of fracture surfaces of hemp/Elium and hemp/Greenpoxy composites

Figure 12 .

 12 Figure 12. Micro-CT images and focuses on damaged areas of hemp/Elium and

Table 1 .

 1 The mechanical damage threshold and the AE damage threshold significantly decrease after aging, but hemp/Elium specimens are slightly less affected than hemp/Greenpoxy ones. The damage observations by SEM and micro-CT demonstrated that hygrothermal aging does not have any effect on the type of damage but it increases the quantity and the severity of damage: fiber/matrix debonding is widespread and cracks become longer and wider.This study also allows compare various techniques to analyze damage mechanisms after hygrothermal aging. The damage factor, determined from the evolution of the secant modulus, gives information on mechanical degradation of the material. Acoustic emission records damage releasing energy during the test, and micro-CT provides 3D observations of damage in the samples. Results show that these three techniques give different information and are complementary, allowing a detailed analysis of damage mechanisms. It reveals that the choice of the eco-friendly matrix in hemp composites will influence the biocomposite behaviour after hygrothermal aging. Water uptake at saturation normalized by fiber volume fraction as a function of material and water immersion temperature, and deviation from immersion in water at room temperature.

	Water uptake at saturation normalized by fiber volume fraction
		21°C	60 °C	Deviation	70 °C	Deviation
				from 21°C		from 21°C
	Hemp/	0.139 ± 0.004 0.193 ± 0.001	+39%	0.249 ± 0.001	+80%
	Elium
	Hemp/	0.211 ± 0.005 0.193 ± 0.001	-9%	0.199 ± 0.001	-5%
	Greenpoxy

Table 2 .

 2 Fick diffusion coefficient D as a function of material and water immersion temperature, and deviation from immersion in water at room temperature.

			Diffusion coefficient (10 -6 mm²/s)
		21°C	60 °C	Deviation	70 °C	Deviation
				from 21°C		from 21°C
	Hemp/	2.54 ± 0.28	15.2 ± 0.6	+496%	14.8 ± 0.6	+483%
	Elium
	Hemp/	1.22 ± 0.49	12.7 ± 0.1	+937%	12.3 ± 0.1	+908%
	Greenpoxy

Table 3 .

 3 Time 𝑡𝑡 90% (in days) to reach 90% of the maximum water absorption value as a function of material and water immersion temperature, and variation from immersion in water at room temperature.

		Duration to reach 90% of water saturation (in days)
		21 °C 60 °C Deviation from 21 °C 70 °C Deviation from 21 °C
	Hemp/	13.8	2.2	-84%	2.1	-85%
	Elium
	Hemp/	24.6	2.0	-92%	2.5	-90%
	Greenpoxy

Table 4 .

 4 Glass transition temperature Tg measured for three aging conditions: ambient storage, saturated at 21°C and saturated at 60°C, and deviation from ambient storage.

			Glass transition temperature (°C)
		Ambient	Saturated	Deviation from	Saturated	Deviation from
		storage	at 21 °C	ambient storage	at 60 °C	ambient storage
	Hemp/ Elium	91.4	82.2	-10.1%	78.1	-14.5%
	Hemp/ Greenpoxy	73.5	67.7	-7.9%	64.9	-11.7%

Acknowledgments

This work was partially funded by the French Government program "Investissements d'Avenir" (EQUIPEX GAP, reference ANR-11-EQPX-0018) and by the CPER FEDER project of Région Nouvelle Aquitaine. It pertains to the French Government program "Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010). The authors also thank Akema for the manufacture of the Elium-based composites.

Declaration of interest statement

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