Taiqu Liu

Pauline Butaud

Vincent Placet

Morvan Ouisse

Damping behavior of plant fiber composites: a review

Keywords: Damping, Loss factor, Dynamic mechanical analysis, Plant fiber, Composites, Energy dissipation

This paper reviews the damping characteristics of plant fiber composites (PFCs) with a particular attention regarding their performances with respect to synthetic fiber composites (SFCs). Indeed, PFCs become more and more popular in many application fields. Their specificities when compared to synthetic fibers, such as glass fibers, make them good candidate to improve the damping behavior of composite materials and structures. The influence of meso and micro scale parameters as well as surrounding conditions is reviewed in the present paper. Contradictory reports are sometimes found and the existing knowledge on the damping behavior of PFCs is sometimes deficient or ambiguous. Some key points like variability, hierarchical aspects and sensitivity of the mechanical properties are thus discussed. This review provides a first reference for the factors that affect damping properties in PFCs to be used in engineering applications in various fields including automotive, aerospace, music devices. It also highlights the current shortcomings on knowledge on damping of PFCs. The Ashby diagram built from the data available in literature constitutes a first tool to select materials considering the compromise between loss factor and stiffness that can be used for engineering design considerations.

Introduction

The invention of mechanical equipment accelerated the process of industrial development. Human requirements for machinery were not limited to high-efficiency characteristics but began to pay more attention to comfort performances and safety.

Therefore, engineers began to look for different kinds of damping materials to reduce the effects of vibration and noise [START_REF] Chung | Structural composite materials tailored for damping[END_REF]2]. The use of damping materials improves people's living and working conditions and creates quiet and comfortable surroundings. With the development of the petrochemical industry, the oil began to expand from the original fuel source to the use of by-products [3][4][START_REF] Zhang | Fully biodegradable natural fiber composites from renewable resources: All-plant fiber composites[END_REF]. Resins, asphalt, and rubber began to enter the industrial field of large-scale applications, especially for reducing vibration [START_REF] Zeng | Stiffness and Damping Ratio of Rubber-Modified Asphalt Mixes: Potential Vibration Attenuation for High-Speed Railway Trackbeds[END_REF]. However, they cannot be used alone due to their low stiffness. In most cases, they are used together with wood or metal sheets in sandwich structures to make up for the shortcomings of each other. A sandwich structure has characteristics of sound insulation and vibration damping, which cannot be provided by a single material in addition to the strength properties compared with pure wood board or metal plate [7][START_REF] Li | A review on vibration damping in sandwich composite structures[END_REF][START_REF] Wang | Damping of honeycomb sandwich beams[END_REF][START_REF] Araújo | Damping optimization of viscoelastic laminated sandwich composite structures[END_REF]. With the development of highstrength fibers such as glass fibers and carbon fibers, attempts have been made to mix fibers and polymers in a specific ratio to manufacture fiber-reinforced composites [START_REF] Mallick | Fiber-reinforced composites: materials, manufacturing, and design[END_REF]. It has been found that this type of material has outstanding performances in specific strength, specific modulus, fatigue strength, impact resistance, damping and devisable characteristics than pure metal material or polymers [START_REF] Chandra | Damping studies in fiber-reinforced composites -a review[END_REF][START_REF] Safri | Impact behaviour of hybrid composites for structural applications: A review[END_REF]. In particular, people currently want to reduce energy consumption by using some lightweight materials, and the advantages of composite materials in this respect make it a significant trend to replace traditional materials, especially in the fields of aerospace, transportation, wind power, etc. [START_REF] Hine | Validating a micromechanical modelling scheme for predicting the five independent viscoelastic constants of unidirectional carbon fibre composites[END_REF][START_REF] Grunenfelder | Effect of prepreg format on defect control in out-ofautoclave processing[END_REF][START_REF] Liu | Dynamic shear-lag model for understanding the role of matrix in energy dissipation in fiber-reinforced composites[END_REF]. Many studies have focused on increasing strength, modulus, and enhanced crashworthiness as structural components when composite materials began to be of interest [START_REF] Nguyen | A potential natural energy absorption material -Coconut mesocarp: Part A: Experimental investigations on mechanical properties[END_REF]. At present, composite materials are also intended to improve the damping performances of the structures while retaining the other primary structural functions.

However, the large-scale application of petroleum-based compounds has also brought about some adverse effects. Engineers should now consider environmental impact at each stage of the life cycle during the implementation of damping materials since petroleum-based products are difficult to be degraded by nature [START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos Part A[END_REF]. Plant fibers become more and more concerned because of abundant reserves, while being renewable, low cost, quickly obtained and processed, degradable, lightweight, possess relatively high specific modulus, and other advantages [3,[START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos Part A[END_REF][START_REF] Hardiman | A review of key developments and pertinent issues in nanoindentation testing of fibre reinforced plastic microstructures[END_REF][START_REF] Yan | Flax fibre and its composites -A review[END_REF]. The properties of many plant fibers coming from hemp, flax, jute, ramie, kenaf, banana, agave, doum palm, pine cone, etc.

are investigated [START_REF] Thiruchitrambalam | Review on kenaf fiber composites[END_REF][START_REF] Shah | Natural fibre composites: Comprehensive Ashby-type materials selection charts[END_REF][START_REF] Flax | Linum usitatissimum L.) fibre reinforced polymer composite materials: A review on preparation, properties and prospects[END_REF][START_REF] Yahaya | Mechanical performance of woven kenaf-Kevlar hybrid composites[END_REF]. Plant fibers become a sustainable material of choice in automotive, aerospace, musical instruments, and other applications. In particular, their role in automotive parts includes ceilings, coat racks, seatbacks, instrument panels [START_REF] Khalfallah | Innovative flax tapes reinforced Acrodur biocomposites: A new alternative for automotive applications[END_REF][START_REF] Hagnell | The economic and mechanical potential of closed loop material usage and recycling of fibre-reinforced composite materials[END_REF].

Up to now, plant fiber-reinforced composites (PFCs) have been mainly used for mass saving as a low-cost and sustainable solution. PFCs can also overcome the mechanical and physico-chemical properties of conventional composite materials to a certain extent, and they could solve some critical problems that cannot be addressed by traditional materials in engineering structures. Some of their intrinsic properties, for instance their natural damping, can be exploited to implement new and advanced functionalities in structures. Indeed, literature clearly points out that the loss factor of PFCs is generally much higher than SFCs. PFCs have reported values of loss factors comprised between 0.7% and 14%, while values are typically comprised between 0.24% and 2.5% for SFCs. Loss factor and storage modulus (or Young's modulus) at ambient temperature were collected from literature and plotted in a stiffness-loss map as proposed by Lakes et al. [START_REF] Brodt | Composite Materials Which Exhibit High Stiffness and High Viscoelastic Damping[END_REF][START_REF] Lakes | Viscoelastic solids[END_REF] for different material families (Figure 2). Due to their internal structures, metals exhibit high stiffness and low loss factors. On the opposite, the chemical composition of polymers results in low stiffness and relatively high loss factors. The combination of components in composite materials is currently the best way to provide compromises between stiffness and loss factor. In this category, PFCs globally perform better than SFCs in terms of damping.

The sources of energy dissipation in fiber reinforced polymer composites are quite well described and documented in literature [START_REF] Chandra | Damping studies in fiber-reinforced composites -a review[END_REF][START_REF] Gibson | Damping characteristics of composite materials and structures[END_REF][START_REF] Chandra | Damping studies in fiber-reinforced composites-a review[END_REF][START_REF] Gibson | Micromechanical modeling of damping in composites including interphase effects[END_REF][START_REF] Nelson | Interfacial slip and damping in fibre reinforced composites[END_REF]. They are mainly: (1) viscoelastic nature of the matrix and/or fiber materials, (2) damping due to interphase, (3) damping due to inelastic and irreversible behaviors such as damage and/or plasticity. By contrast, the damping behavior of PFCs, even if already documented [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Duc | The role of twist and crimp on the vibration behaviour of flax fibre composites[END_REF] is not fully covered. Furthermore, various effects on damping are observed when plant fibers are introduced in polymer matrices depending on the nature of the polymer, stiffness, textile architecture and yarn lengths [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF]. The physics underlying their particular behavior is not yet fully understood and require additional research efforts. Also, the length scales corresponding to all dissipation mechanisms that may occur in these multiscale materials can result in damping occurring at various time (or frequency) scales. Therefore, this paper aims at reviewing the current knowledge on the damping behavior of PFCs in order to draw the needs for the future research activities, and to evaluate their potentialities to reach specific levels of damping of composite materials. All over the paper, the term damping is used to describe the physical mechanisms corresponding to energy dissipation occurring when the materials are subjected to cyclic deformations, while the term loss factor refers to the ratio of the energy dissipated per cycle to the maximum strain energy stored in the material during the cycle, which is widely used to describe the damping performance of materials and structures.

In this paper, we propose to review the existing studies on the damping behavior of PFCs. The classical experimental techniques used to characterize the damping behavior of composite materials are first discussed in section 2. Section 3 proposes to review the studies available in the open literature. The analysis is performed using different key parameters owing to the mesoscale (including reinforcement type, stacking sequence), microscale (fiber, matrix, interface/interphase and porosities) and related to testing and environmental conditions (moisture and temperature). Section 4 discusses the current limitations in the existing studies. Finally, conclusions are given in section 5.

On the experimental techniques for the characterization of composite damping

In this section, the most widely used damping characterization techniques such as DMA tests, modal analyses and wave number-based approaches are briefly described.

Quasi-static and low frequency characterization: DMA

One of the most widely used non-resonance techniques for damping characterization is Dynamic Mechanical Analysis (DMA). It is usually used to characterize viscoelastic materials with low stiffness such as polymers, or organic composite materials, and is widely used for the rheological analysis of polymers and elastomers, especially in the fields of chemistry and materials science [START_REF] Menrad | Dynamic Mechanical Analysis: A Practical Introduction[END_REF]. In particular, their glass transition temperatures can be identified through temperature sweep curves at different frequencies.

The storage modulus (E'), loss modulus (E'' ) and loss factor (tan δ), are usually identified from DMA tests to describe the viscoelastic properties of materials for various temperatures [START_REF] Margem | The dynamic-Mechanical behavior of epoxy matrix composites reinforced with ramie fibers. 65th[END_REF][START_REF] Mylsamy | The mechanical properties, deformation and thermomechanical properties of alkali treated and untreated Agave continuous fibre reinforced epoxy composites[END_REF][START_REF] Menrad | Dynamic Mechanical Analysis: A Practical Introduction[END_REF][START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF][START_REF] Martí Nez-Herná Ndez | Dynamical-mechanical and thermal analysis of polymeric composites reinforced with keratin biofibers from chicken feathers[END_REF][START_REF] Singh | Characterization of the damping behavior of a nanoindentation instrument for carrying out dynamic experiments[END_REF]. The temperature range of interest is generally investigated either through temperature ramps or temperature steps. In the latter case, isothermal conditions are used often with a temperature stabilized stage of several minutes to make sure that the sample has reached a homogeneous temperature distribution [START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF][START_REF] Martí Nez-Herná Ndez | Dynamical-mechanical and thermal analysis of polymeric composites reinforced with keratin biofibers from chicken feathers[END_REF][START_REF] Kuzak | Dynamic mechanical analysis of fiber-reinforced phenolics[END_REF].

Harmonic excitation is usually set between 0.01 Hz and 100 Hz in most existing studies, since DMA apparatus always exhibit mechanical resonances in the higher frequency range, that affect the measurement [START_REF] Kumar | Studies on mechanical, thermal and dynamic mechanical properties of untreated (raw) and treated coconut sheath fiber reinforced epoxy composites[END_REF][START_REF] Placet | Is dynamic mechanical analysis (DMA) a non-resonance technique[END_REF][START_REF] Bhudolia | Enhanced vibration damping and dynamic mechanical characteristics of composites with novel pseudo-thermoset matrix system[END_REF][START_REF] Wielage | Processing of natural-fibre reinforced polymers and the resulting dynamic-mechanical properties[END_REF]. In addition, another essential factor that needs to be considered in DMA is the ratio of the stiffness of the sample to that of the apparatus: the stiffness of the sample should be much smaller than the stiffness of the system to obtain accurate test results, especially for storage modulus [START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF]. Furthermore, the deformation of the sample should be kept in the linear viscoelastic range to meet the theoretical requirements [START_REF] Menrad | Dynamic Mechanical Analysis: A Practical Introduction[END_REF]. Despite of these limitations in terms of frequency, DMA remains a popular technique for the characterization of damping, in particular because the Time-Temperature Superposition (TTS) principle, which is verified for a large set of polymers and composites, can be used to estimate their damping and stiffness properties in the higher frequency range [START_REF] Menrad | Dynamic Mechanical Analysis: A Practical Introduction[END_REF][START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF].

Low to mid-frequency characterization: modal analysis

Modal analysis is another common method for damping identification. Natural frequencies, damping ratios, and modal shapes of composite structures are estimated at resonances by using an external excitation source within a specific frequency range [START_REF] Araújo | Damping optimization of viscoelastic laminated sandwich composite structures[END_REF][START_REF] Theotokoglou | Analytical, experimental and numerical approach of storage and loss moduli of fibre reinforced epoxy composites[END_REF][START_REF] Gibson | Modal vibration response measurements for characterization of composite materials and structures[END_REF]. These methods are efficient on frequency ranges starting at the first eigenfrequency of the structure, up to mid-frequency range which is typically reached when the -3dB bandwidth of subsequent modes are superimposed one to another, hence depending not only on the materials, but also on the geometry and the boundary conditions. Several excitation signals and boundary conditions can be used for resonance testing. Existing techniques for damping measurement using logarithmic decrement decreasing free vibrations of beams are reported [START_REF] Rueppel | Damping of carbon fibre and flax fibre angleply composite laminates[END_REF][START_REF] Gibson | Modal vibration response measurements for characterization of composite materials and structures[END_REF][START_REF] Mishra | An experimental approach to free vibration response of woven fiber composite plates under free-free boundary condition[END_REF][START_REF] Ouisse | Damping control for improvement of acoustic black hole effect[END_REF][START_REF] Matter | Numerical-experimental identification of the elastic and damping properties in composite plates[END_REF][START_REF] Alexander | Hygrothermal effect on natural frequency and damping characteristics of basalt/epoxy composites[END_REF]. The test configuration is important: several aspects are discussed in the literature, such as the location of the excitation, boundary conditions, adhesion of the accelerometers, and interference of the measurement [START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF][START_REF] Theotokoglou | Analytical, experimental and numerical approach of storage and loss moduli of fibre reinforced epoxy composites[END_REF][START_REF] Gibson | Modal vibration response measurements for characterization of composite materials and structures[END_REF][START_REF] López-Aenlle | Mechanical characterization of polyvinil butyral from static and modal tests on laminated glass beams[END_REF][START_REF] Rao | Dynamic analysis and damping of composite structures embedded with viscoelastic layers[END_REF]. Specific techniques for composite structures are proposed for beams on complex shapes [START_REF]Standard Test Method for Measuring Vibration-Damping Properties of Materials n[END_REF][START_REF] Oberst | Über die Dä mpfung der Biegeschwingungen dünner Bleche durch fest haftende Belä ge[END_REF][START_REF] Koruk | Identification and removal of adverse effects of non-contact electromagnetic excitation in Oberst Beam Test Method[END_REF][START_REF] Viala | Identification of the anisotropic elastic and damping properties of complex shape composite parts using an inverse method based on finite element model updating and 3D velocity fields measurements (FEMU-3DVF): Application to bio-based composite violin sou[END_REF].

Since there is usually no heating or cooling device used in the modal analysis tests, the samples are sometimes placed in a constant temperature oven to maintain the required test temperature [7,[START_REF] Butaud | Identification of the viscoelastic properties of the tBA/PEGDMA polymer from multi-loading modes conducted over a wide frequency-temperature scale range[END_REF]. However, these settings can generally not be used to reach high temperatures because most instruments cannot tolerate excessive temperatures.

High frequency characterization: wave number-based approaches

Marchetti et al. review several wave number based approaches used for the characterization of the dynamic properties of composite structures in frequency ranges where modal analysis approaches become unpractical as the increased modal density is too large [START_REF] Ege | High-resolution modal analysis[END_REF][START_REF] Marchetti | On the structural dynamics of laminated composite plates and sandwich structures; a new perspective on damping identification[END_REF]. The loss factor and storage modulus can be computed from the natural wave number obtained from the high frequency analysis.

However, this type of characterization has not been widely applied for PFCs at this time, so this frequency range is not addressed in this review paper. The works by Zhang et al. and Duval et al. can be mentioned as first inputs to be completed in future researches with additional data related to damping properties of PFCs in high frequencies [START_REF] Zhang | Effect of natural fibre reinforcement on the sound and vibration damping properties of bio-composites compression moulded by nonwoven mats[END_REF][START_REF] Duval | Vibro-acoustic properties of a very long flax fibers reinforced thermoset "flaxpreg" light sandwich[END_REF].

Review of studies on the damping behavior of PFCs

Mesoscale parameters

This section discusses the effects of mesoscale parameters (features of laminate) on damping, given the issues of reinforcement architecture and stacking sequence.

Reinforcement type

Plant fibers can be processed into many kinds of reinforcements such as short fibers, nonwoven, non-crimp fabrics, and woven fabrics. It leads to composite meso-structures with significant differences. This part summarizes the effect of different kinds of reinforcement types on damping performances.

For short fiber composites, Senthil Kumar et al. [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF] investigated the influence of fiber length and weight percentage on the free vibration characteristics. Their study concerned banana fiber and sisal fiber polyester composites. Results show that the fiber content influences more the free vibration behavior that the fiber length. This is attributed to the shear behavior at the fiber ends. The authors also claim that the damping of banana fiber is higher than that of sisal fiber owing to the smaller diameter of banana fiber and a thicker interface could appear, as reported by Bledzki et al [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF].

Sreenivasan et al. reported that for Sansevieria cylindrica fiber reinforced polyester matrix composites the dynamic characteristics are significantly influenced by the increase in fiber length and fiber loading but not in a geometric progression [START_REF] Sreenivasan | Dynamic mechanical and thermogravimetric analysis of Sansevieria cylindrica/polyester composite: Effect of fiber length, fiber loading and chemical treatment[END_REF] In contrast, the loss factor of short synthetic fiber composites are found higher than that of long fiber composites, due to the long fibers that limit the movement of polymer molecules [START_REF] Subramanian | Effect of reinforced fiber length on the damping performance of thermoplastic composites[END_REF][START_REF] Harris | Study of carbon fibre surface treatments by dynamic mechanical analysis[END_REF][START_REF] Rezaei | Effect of fiber length on thermomechanical properties of short carbon fiber reinforced polypropylene composites[END_REF].

The fiber-matrix interface is considered as a significant source of energy dissipation of discontinuous synthetic fiber based composites since short fiber increase more fiber ends and fiber-matrix interfaces [START_REF] Subramanian | Effect of reinforced fiber length on the damping performance of thermoplastic composites[END_REF]. However, the comparisons of discontinuous, short and long plant fiber composites under the same condition are rarely reported.

When woven reinforcements are considered, most of the authors report a reduction of damping level compared to unidirectional reinforcements (UD) in transverse direction such as tapes [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Cihan | Mechanical and dynamic performance of woven flax/E-glass hybrid composites[END_REF][START_REF] Audibert | Mechanical characterization and damage mechanism of a new flax-Kevlar hybrid/epoxy composite[END_REF][START_REF] Rouf | Effect of fabric weaves on the dynamic response of two-dimensional woven fabric composites[END_REF][START_REF] Alkbir | Fibre properties and crashworthiness parameters of natural fibre-reinforced composite structure: A literature review[END_REF]. Among the different weave patterns investigated, the loss factor in huckaback-type woven composites is found to be higher than that of plain, satin, twill, and basket because their performances depend on the interlace between the warp and weft directions, which increases the interaction between fiber and matrix [START_REF] Rajesh | Dynamic mechanical analysis and free vibration behavior of intra-ply woven natural fiber hybrid polymer composite[END_REF]. Also, twisted yarns generally induce decrease Young's modulus because of the induced crimp but increase the damping through enhanced inter-yarn friction [START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF].

However, the existing reports have not found any significant effect of long fibers on damping when compared to short fibers and continuous reinforcements [START_REF] Saha | Study of Jute Fiber Reinforced Polyester Composites by Dynamic Mechanical Analysis[END_REF][START_REF] Zhang | Effect of natural fibre reinforcement on the sound and vibration damping properties of bio-composites compression moulded by nonwoven mats[END_REF]. Some research efforts focused on the comparison of the effects of these three types of reinforcement on damping performance are still required.

Stacking sequence

The effect on damping of different stacking sequences, using some common arrangements such as 0º, 45º, and 90º are investigated by several authors. When it comes to symmetric layups, composites often show lower damping level in longitudinal direction when 90º at the outer layer and the highest for that when 0º at the outer layer [START_REF] Senthilkumar | Static and dynamic properties of sisal fiber polyester composites -Effect of interlaminar fiber orientation[END_REF]. The authors indicate that this is related to the flexural properties of the composite structure. In particular, the shearing effect is found to enhance resistance to fiber mobility and increase the effective load transfer between fiber and matrix in turn [START_REF] Pothan | Tensile and Flexural Behavior of Sisal Fabric/Polyester Textile Composites Prepared by Resin Transfer Molding Technique[END_REF].

Stacking sequences with long UD fibers and randomly oriented short fibers are also studied. Results show the stacking sequence does not influence the dynamic mechanical properties (including storage modulus, loss factor) of the studied PFCs in longitudinal direction [START_REF] Fiore | The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites[END_REF]. Some research on hybrid fibers (flax and E-glass fibers woven fabric) show that the best damping performance are obtained in longitudinal direction when flax fibers are distributed in the outer layer [START_REF] Cihan | Mechanical and dynamic performance of woven flax/E-glass hybrid composites[END_REF]. Y. Li et al. find the damping properties in hybrid composites (flax and carbon fiber) are greatly influenced by the position of the flax fiber layers, which are supposed to dissipate more energy than those with synthetic fibers in fiber direction [START_REF] Li | Multi-scaled enhancement of damping property for carbon fiber reinforced composites[END_REF]. Contrary results have also been reported in the latest literature [START_REF] Ramraji | Mechanical and free vibration properties of skin and core designed basalt woven intertwined with flax layered polymeric laminates[END_REF] for hybrid composites made of basalt and flax woven fabrics (0/90 orientation). Authors observed that the maximum damping is obtained when basalt is in the outer layer [START_REF] Ramraji | Mechanical and free vibration properties of skin and core designed basalt woven intertwined with flax layered polymeric laminates[END_REF][START_REF] Rajesh | Free Vibration Characteristics of Banana/Sisal Natural Fibers Reinforced Hybrid Polymer Composite Beam[END_REF].

Microscale parameters

This section discusses the effect of microscale parameters on damping, such as fiber type, fiber orientation, fiber volume fraction and microstructure. Meanwhile, treatments of fiber, interface between fiber and matrix, and porosities are also discussed.

Fiber

Fiber type

This section summarizes the damping characteristics of composites with different types of fibers. As expected, the Young's modulus of the composites in the longitudinal direction is significantly higher than the one of the neat epoxy. The stiffening factor varies as a function of the fiber type. For the considered results in Figure 3 (a), it is approximately 25 times with carbon fibers, 8 times with glass fibers and at least 5 times with flax fibers, compared with neat epoxy. It can also be observed that the fiber type does have a significant effect on the damping properties (fiber volume fraction can also be a factor, but most authors have not stated it in the literature). The addition of synthetic fibers into epoxy induces a decrease of the damping in the longitudinal direction. For the considered results in Figure 3 (b), it is approximately decreasing of 40% and 20% with carbon fibers and glass fibers, respectively, compared with neat epoxy. This is attributed to stress transfers from the matrix to the fibers and by the fact that the presence of stiff fibers limits the chain mobility in the in the matrix, which implies that the friction of the intermolecular chain is reduced [START_REF] Essabir | Dynamic mechanical thermal behavior analysis of doum fibers reinforced polypropylene composites[END_REF][START_REF] Pan | Kenaf fiber/poly(ɛ-caprolactone) biocomposite with enhanced crystallization rate and mechanical properties[END_REF][START_REF] Hwang | Contribution of interlaminar stresses to damping in thick laminated composites under uniaxial extension[END_REF]. Conversely, the addition of plant fibers increases damping. It is approximately increasing of 70% with flax fibers as it can be seen in Figure 3 (b). This is attributed to the friction at the interface but also may be due to the intrinsic damping capacity of the fibers themselves [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Duc | The role of twist and crimp on the vibration behaviour of flax fibre composites[END_REF][START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF]. However, the damping is also sometimes reported decreased in flax/epoxy composites [START_REF] El-Hafidi | Determination of dynamic properties of flax fibres reinforced laminate using vibration measurements[END_REF] compared with that of pure epoxy [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF] but this comparison may ignore the impact of different test methods. Figure 3 also points out that the loss factor obtained with flax fibers is high than with sisal. Anyway, more tests including a large variety of fibers in the form of continuous UD reinforcement and with a same matrix and similar volume fractions are recommended in order to better evaluate the influence of the fiber type on the damping of their composites.

It was also shown by Hadiji et al. that nonwoven composites reinforced by plant fibers present higher loss factors compared with those of glass-based composites [START_REF] Zahari | Mechanical Properties and Water Absorption Behavior of Polypropylene / Ijuk Fiber Composite by Using Silane Treatment[END_REF].

The loss factors of polypropylene composites based on non-woven hemp, flax and kenaf reinforcements are 2 to 25 times higher than glass-PP composites. Among the tested plant fibers, it can be observed that the higher damping is obtained with hemp and flax.

In addition, some authors also show that the incorporation of ramie fiber in epoxy tends to increase the damping due to weak adhesion which indicates low interfacial shear stress [START_REF] Margem | The dynamic-Mechanical behavior of epoxy matrix composites reinforced with ramie fibers. 65th[END_REF]. Another reason that may explain the diverse damping with these plant fibers is the difference in inherent morphology of fiber surface [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF].

Hybrid plant fiber composites are also reported regarding damping performance.

Results show that hybrid plant fiber composites with banana/coconut sheath or kenaf/bamboo possess higher loss factors compared to single fiber composites [START_REF] Ismail | Void content, tensile, vibration and acoustic properties of kenaf/bamboo fiber reinforced epoxy hybrid composites[END_REF].

Hybrid fibers combine the advantages of their components and achieve superior performances that cannot be obtained from only one type and authors also claim damping values are higher for all hybrid composites possibly due to greater energy dissipation and restricted molecular mobility at the interface [START_REF] Chandradass | Effect of nanoclay addition on vibration properties of glass fibre reinforced vinyl ester composites[END_REF][START_REF] Kumar | Layering pattern effects on vibrational behavior of coconut sheath/banana fiber hybrid composites[END_REF][START_REF] Sathishkumar | Hybrid fiber reinforced polymer composites-a review n[END_REF].

Fiber microstructure

Plant fibers differentiate from the conventional ones in terms of composition, and microstructure. Indeed, plant fibers often have unique microstructures and morphologies, notably different cell wall layers and a complex cross-sectional area which varies along the fiber length [START_REF] Le Duigou | Hygroscopic expansion: A key point to describe natural fibre/polymer matrix interface bond strength[END_REF][START_REF] Bourmaud | Impact of the seeding rate on flax stem stability and the mechanical properties of elementary fibres[END_REF][START_REF] Baley | Industrial Crops & Products Variability of mechanical properties of flax fibres for composite reinforcement . A review[END_REF][START_REF] Li | An overview of structural-functional-integrated composites based on the hierarchical microstructures of plant fibers[END_REF]. In additional to this complex morphology, plant fibers have also a polymer-based composition and a very hierarchical organization with different layers and sublayers made of a mixture of carbohydrates and polyphenols [START_REF] Hosseinaei | Effects of decreasing carbohydrate content on properties of wood strands[END_REF][START_REF] Goubet | Polysaccharide analysis using carbohydrate gel electrophoresis. A method to study plant cell wall polysaccharides and polysaccharide hydrolases[END_REF]. It imparts them viscoelastic properties [START_REF] Keryvin | Analysis of flax fibres viscoelastic behaviour at micro and nano scales[END_REF][START_REF] Cisse | Creep behaviour of single hemp fibres. Part I: viscoelastic properties and their scattering under constant climate[END_REF][START_REF] Van Hazendonk | Structural analysis of acetylated hemicellulose polysaccharides from fibre flax (Linum usitatissimum L.)[END_REF][START_REF] Biagiotti | A review on natural fibre-based composites -Part I: Structure, processing and properties of vegetable fibres[END_REF][START_REF] Bismarck | Surface characterization of flax, hemp and cellulose fibers; Surface properties and the water uptake behavior[END_REF]. These particularities lead also to specific static and fatigue behavior which were quite widely studied in literature, including nonlinearities [START_REF] Jeannin | Influence of hydrothermal ageing on the fatigue behaviour of a unidirectional flax-epoxy laminate[END_REF][START_REF] Jeannin | About the fatigue endurance of unidirectional flaxepoxy composite laminates[END_REF], stiffening effects [START_REF] Haggui | Static and fatigue characterization of flax fiber reinforced thermoplastic composites by acoustic emission[END_REF] and moisture activation of some mechanisms [START_REF] Launay | Modelling the influence of temperature and relative humidity on the time-dependent mechanical behaviour of a short glass fibre reinforced polyamide[END_REF][START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF][START_REF] Berthelot | Damping analysis of composite materials and structures[END_REF].

It could also be at the origin of specific energy dissipation mechanisms and particular damping behavior. Only few studies investigate the damping behavior of plant fibers [START_REF] Placet | Characterization of the thermo-mechanical behaviour of Hemp fibres intended for the manufacturing of high performance composites[END_REF][START_REF] Bourmaud | Rigidity analysis of polypropylene/vegetal fibre composites after recycling[END_REF][START_REF] Perrier | Effect of water ageing on nanoindentation response of single hemp yarn/epoxy composites[END_REF][START_REF] Khelfa | IDENTIFICATION DES PROPRIÉTÉS D'ÉLASTICITÉ ET D'AMORTISSEMENT D'UNE FIBRE ISOLÉE ANISOTROPE PAR ULTRASONS LASER Ouverture au cas des fibres naturelles JURY[END_REF] and, unfortunately, the influence of the fiber microstructure on the damping of the PFCs themselves has not been studied so far.

PFCs are made of single individual fibers but also bundle of fibers. Friction at the interface between individual fibers within the fiber bundle, internal friction within fiber wall (between the heterogeneous polymers constituting the wall and particularly between the rigid cellulose microfibrils and the amorphous polymers in which they are embedded) [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Duc | The role of twist and crimp on the vibration behaviour of flax fibre composites[END_REF][START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF] are also potential sources of damping. Additionally, plant fibers have a finite length when compared with synthetic fibers. The effect of such discontinuities, even for continuous reinforcement, on the damping behavior of PFCs can be questioned and deserves to be investigated in the future.

Treatments

Several investigations on the effect of pre-treatments on the plant fibers to achieve better mechanical performances in PFCs have been reported. This section summarizes the treatment methods and their effects on composite damping, including functionalization using nanotubes, chemical treatments, etc. which may change the interface state.

Carbon nanotubes (CNTs) are proposed to enhance the damping property of PFCs and SFCs [START_REF] Latibari | Investigation of interfacial damping nanotube-based composite[END_REF]. Damping is further enhanced by the stick-slip action of CNTs that takes place at the CNTs/matrix interfaces. Besides, the penetrated CNTs interact with microfibrils in the S2 cell wall of plant fibers, leading to effective stress transfer from matrix to microfibrils, which contributes to energy dissipation and enhancement of damping property [START_REF] Li | Multi-scaled enhancement of damping property for carbon fiber reinforced composites[END_REF]. In contrast, some authors also claim the presence of stiff fibers limits the chain mobility in the matrix, which implies that the friction of the intermolecular chain is reduced [START_REF] Essabir | Dynamic mechanical thermal behavior analysis of doum fibers reinforced polypropylene composites[END_REF][START_REF] Pan | Kenaf fiber/poly(ɛ-caprolactone) biocomposite with enhanced crystallization rate and mechanical properties[END_REF][START_REF] Hwang | Contribution of interlaminar stresses to damping in thick laminated composites under uniaxial extension[END_REF].

Other results report the effect of microfibers. The addition of macro/microfibers decreases the damping characteristics of PFCs and increased storage modulus, as the added fibers act as barriers to the free movement of the macromolecular chains. As a contrast, the unfilled matrix has the highest damping ratio, indicating a significant degree of mobility [START_REF] Joseph | Dynamic mechanical properties of oil palm microfibril-reinforced natural rubber composites[END_REF].

It demonstrated that the improved interaction exerted by the chemical treatments makes the PFCs and SFCs more compatible and with better adhesion than the untreated fiber composites [START_REF] George | Thermogravimetric and dynamic mechanical thermal analysis of pineapple fibre reinforced polyethylene composites[END_REF][START_REF] Zhao | Influence of polyurethane dispersion as surface treatment on mechanical, thermal and dynamic mechanical properties of laminated woven carbon-fiber-reinforced polyamide 6 composites[END_REF]. Moreover, some authors claim that a high-quality interface tends to lower energy dissipation, showing a lower value of the damping peak [START_REF] Fay | Effect of morphology, crosslink density, and miscibility on interpenetrating polymer network damping effectiveness[END_REF][START_REF] Yu | Effect of fiber surface-treatments on the properties of poly(lactic acid)/ramie composites[END_REF].

The effect of chemical treatments such as acid, alkali, ethanol, and silane agents are studied. Chemical modifications cause the hemicellulose removal, which increases the number of hydrogen bonds between the modified fibers and the matrix [START_REF] Zierdt | Sustainable woodplastic composites from bio-based polyamide 11 and chemically modified beech fibers[END_REF]. The alkali and potassium permanganate treatment of PFCs leads to higher damping when compared to other earlier reports [START_REF] Mohanty | Dynamic mechanical and thermal properties of MAPE treated jute/HDPE composites[END_REF][START_REF] Gassan | Possibilities for improving the mechanical properties of jute/epoxy composites by alkali treatment of fibres[END_REF]. The authors explain that the damping characteristics of heterogeneous systems is not only based on interfacial bonding but also depends on different parameters like change in interfacial thickness, bending of fibers, broken fibers, matrix cracking and formation of cavities due to the fiber pullout [START_REF] Cihan | Mechanical and dynamic performance of woven flax/E-glass hybrid composites[END_REF].

Different chemical reaction times show not much difference in the height of the loss factor's peak [START_REF] Saha | Study of Jute Fiber Reinforced Polyester Composites by Dynamic Mechanical Analysis[END_REF]. Some authors claim that the reduction of the amplitude of the loss factor's peaks means well-combined load capacity, due to a good stress transfer between interface [START_REF] Jawaid | Effect of jute fibre loading on tensile and dynamic mechanical properties of oil palm epoxy composites[END_REF][START_REF] Pothan | Dynamic mechanical analysis of banana fiber reinforced polyester composites[END_REF]. Silane-treated fiber composites lead to better fiber/matrix interactions than other treatments [START_REF] Dayo | The influence of different chemical treatments on the hemp fiber/polybenzoxazine based green composites: Mechanical, thermal and water absorption properties[END_REF][START_REF] Atiqah | Dynamic mechanical properties of sugar palm/glass fiber reinforced thermoplastic polyurethane hybrid composites[END_REF]. Alkali and silane treated surfaces are rough, which are formed by the elimination of lignin and hemicellulose compounds. A rough surface enhances fiber/matrix adhesion and increases both the glass transition temperature and loss factor during glassy state [START_REF] Dayo | The influence of different chemical treatments on the hemp fiber/polybenzoxazine based green composites: Mechanical, thermal and water absorption properties[END_REF][START_REF] Li | Chemical treatments of natural fiber for use in natural fiberreinforced composites: A review[END_REF]. This can be also explained as the combination of the shear stress concentration at the fiber end and the additional viscoelastic energy dissipated in the matrix material [START_REF] Atiqah | Dynamic mechanical properties of sugar palm/glass fiber reinforced thermoplastic polyurethane hybrid composites[END_REF][START_REF] Zhou | Effect of silane treatment on microstructure of sisal fibers[END_REF][START_REF] Zahari | Mechanical Properties and Water Absorption Behavior of Polypropylene / Ijuk Fiber Composite by Using Silane Treatment[END_REF].

Besides, Yadav & Gupta find fiber coating (PLA+ Chloroform) followed by chemical treatments can improve the damping at ambient temperature, and could also be considered as a practical approach to improve the performance of composite materials for advanced applications [START_REF] Yadav | Development and characterization of jute composites for sustainable product: Effect of chemical treatments and polymer coating[END_REF].

In general, the effect caused by treatment on composite damping is based on the changes of the fiber/matrix interface. The quality of the interface determines the change in damping, but sometimes there are still conflicting conclusions.

Fiber volume fraction

PFCs with a single type of reinforcement in the form of short fibers are studied to investigate the influence of the fiber volume fraction on damping properties. Sathishkumar et al. showed that the damping, measured using free vibration technique, increases with the fiber content (up to 50 wt%) for sisal, while it decreases for banana fiber composites [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF]. This result is attributed to the difference in inherent morphology of fiber surface [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF]. Etaati et al. also investigated the influence of the fiber volume fraction on the damping behavior of short hemp fibers reinforced polypropylene composites [START_REF] Etaati | Vibration damping characteristics of short hemp fibre thermoplastic composites[END_REF]. They reported that composite with 30 wt% noil hemp fiber shows the highest damping capacity among all investigated composites, for fiber volume fractions comprised between 0 and 60%. Tajvidi et al. indicated that the presence of higher fiber contents can considerably reduce damping, indicating that the composite material is more elastic at higher fiber contents [START_REF] Tajvidi | Mechanical performance of hemp fiber polypropylene composites at different operating temperatures[END_REF]. The interface area increases with the number of incorporated fibers, which leads to stronger interactions. Therefore, the molecular mobility of the polymer decreases and the mechanical loss that overcome intermolecular chain friction is reduced [START_REF] Salleh | Effects of extrusion temperature on the rheological, dynamic mechanical and tensile properties of kenaf fiber/HDPE composites[END_REF]. As previously mentioned, other reports show the dynamic characteristics are significantly influenced by the increase in fiber length and fiber loading by the changes of interface but not in a geometric progression, as in the case Sansevieria cylindrica fiber reinforced polyester matrix composites [START_REF] Sreenivasan | Dynamic mechanical and thermogravimetric analysis of Sansevieria cylindrica/polyester composite: Effect of fiber length, fiber loading and chemical treatment[END_REF].

For non-woven composites, the loss factor of flax/PP composites is observed to decrease of approximately 20% when increasing the flax weight ratio from 30 to 70%. This is attributed to the superior contribution of PP on damping [START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF].

Increasing the fraction of synthetic fibers in hybrid fiber composites (flax and carbon fiber) reduces damping [START_REF] Guen | The damping-modulus relationship in flax-carbon fibre hybrid composites[END_REF], but there are also reports of increase damping [START_REF] Sathishkumar | Vibration and viscoelastic characteristics of sisal fiber reinforced polyester composite[END_REF], without clear physical explanation of this observation.

In summary, the impact of fiber content on damping often presents conflicting conclusions in the existing literature considering the different types of fibers used and their architecture. Therefore, dedicated experimental studies as well as modelling approaches need to be established to explain these inconsistent conclusions in future research.

Fiber orientation

Different fiber orientations can be used during the design of composite laminates and structures. Damping performance of PFCs with different fiber orientations has been studied in the past decades.

The loss factor of flax/GreenPoxy 56 (GP56) composites are tested from 0º to 90º fiber orientation using a modal method [START_REF] Daoud | Numerical and experimental characterization of the dynamic properties of flax fiber reinforced composites[END_REF]. Results show that the loss factor decreases as the frequency increases. This is attributed to the high internal friction between cellulose and hemicellulose caused by the flax fibers microstructure, especially at low frequency [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] El-Hafidi | Determination of dynamic properties of flax fibres reinforced laminate using vibration measurements[END_REF][START_REF] Assarar | Evaluation of the damping of hybrid carbonflax reinforced composites[END_REF][START_REF] Daoud | Numerical and experimental characterization of the dynamic properties of flax fiber reinforced composites[END_REF]. A maximum loss factor is always found at 70º -75º fiber orientation for different frequencies.

Figure . 4 Variation of loss factor as function of fiber orientation in UD composites in longitudinal direction using modal tests (a) Flax/GP56 [START_REF] Daoud | Characterization of the vibrational behaviour of flax fibre reinforced composites with an interleaved natural viscoelastic layer[END_REF], (b) Flax/PP [START_REF] Rahman | Vibration damping of flax fibre-reinforced polypropylene composites[END_REF], (c) Flax/Epoxy [START_REF] Rueppel | Damping of carbon fibre and flax fibre angleply composite laminates[END_REF], (d) Glass/Epoxy [START_REF] Berthelot | Damping analysis of unidirectional glass and Kevlar fibre composites[END_REF] In other reports, the loss factor is found to increase at first, and then to decrease with the increasing fiber angles in flax fiber-reinforced polypropylene (PP) [START_REF] Rahman | Vibration damping of flax fibre-reinforced polypropylene composites[END_REF]. Some authors show that the maximum loss factor is obtained around 45º fiber orientation, which is similar to glass fiber composites and carbon fiber composites [START_REF] Berthelot | Damping analysis of unidirectional glass and Kevlar fibre composites[END_REF][START_REF] Gao | Modeling of the damping properties of unidirectional carbon fibre composites[END_REF][START_REF] Adams | Damping in advanced polymer-matrix composites[END_REF]. This phenomenon is attributed to the in-plane shear strain energy of fiber-reinforced composites which is the maximum at this fiber orientation [START_REF] Hwang | Contribution of interlaminar stresses to damping in thick laminated composites under uniaxial extension[END_REF]. However, the global trend is that the loss factor of the fiber orientation in 90º is higher than that of 0º, as shown in Figure 4, in which (a) and (b) are measured around 500 Hz, (c) is obtained around 300-400 Hz and (d) is measured around 300 Hz.

Unlike the unidirectional PFCs, loss factor varies slightly from the longitudinal direction (0°) to the cross direction (90°) in nonwoven PFCs [START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF]. Indeed, the anisotropy level is less pronounced than in UD composites It should be emphasized that testing UD composites out of longitudinal and tangential directions requires strong attention on the experimental setup in order to guarantee the homogeneity of the strain and stress fields in the sample. Moreover, the identified properties correspond to coupled information between the material properties corresponding to the principal directions. For this reason, it may be preferable to focus only on the principal directions when using DMA-like tests, or use free-free vibration tests that are not affected by out-of-axis boundary conditions. Additionally, purely UD panels are rarely used in practical applications where cross ply composites are preferred, whose properties can be identified from longitudinal and tangential data.

Matrix

This section summarizes some research results on the dynamic mechanical properties of conventional polymers and their PFCs. Figure 5 summaries some dynamic mechanical properties of pure matrices, which are widely used in industrial production. Thermoset polymers, such as epoxy, are the most widely used matrix for PFCs and SFCs due to the excellent adhesion of resin and the long lifecycle. However, thermoset polymers tend to be more brittle and less tough than thermoplastics [START_REF] Chung | Structural composite materials tailored for damping[END_REF]. The reason is that high loss factor values are associated with ease of movement of side chains, functional groups, segments, pendant groups, and even entire molecules in the polymer. Moreover, the loss factor is reduced by the presence of negatively charged atoms (such as oxygen and nitrogen) in the molecules, which reduces the motion of hydrogen bonding [START_REF] Fu | Vibration reduction ability of polymers, particularly polymethylmethacrylate and polytetrafluoroethylene[END_REF]. This phenomenon is also interpreted as a mechanism for damping in polymer blends provided by network and interfaces [START_REF] Tung | Vibration damping with urethane/acrylate simultaneous semi-interpenetrating polymer networks[END_REF].

Although thermoplastic polymers exhibit higher energy dissipation than thermosets, thermosets are often preferred due to their higher stiffness and better adhesion properties [START_REF] Chung | Structural composite materials tailored for damping[END_REF][START_REF] Treviso | Damping in composite materials: Properties and models[END_REF]. Besides, results with particle additions are also reported. The mixture of agar particles restricts the mobility of the chains, and this performance reduces the sharpness and the maximum value of loss factor. The viscosity is substantially enhanced by fillers at a low shear rate, and in this case, the rheological behavior is utterly dependent on the composition of the polymer in the interfacial region [START_REF] Liang | Mechanical and thermal properties of poly(butylene succinate)/plant fiber biodegradable composite[END_REF]. It is also reported that the incorporation of solid fillers into the polymer matrix could increase or decrease the damping of the polymer, depending on the quality of fiber-matrix bonding [START_REF] Saha | Study of Jute Fiber Reinforced Polyester Composites by Dynamic Mechanical Analysis[END_REF][START_REF] Wielage | Processing of natural-fibre reinforced polymers and the resulting dynamic-mechanical properties[END_REF][START_REF] Jonoobi | Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion[END_REF][START_REF] Landel | Mechanical properties of polymers and composites[END_REF]. Also, it can be observed that the damping factor decreases with the increase in bio-filler contents because the rigid particles restrain the mobility of the polymer molecules, raise the storage modulus, and reduce the loss factor [START_REF] Essabir | Morphological, Structural, Thermal and Tensile Properties of High Density Polyethylene Composites Reinforced with Treated Argan Nut Shell Particles[END_REF]. The Young's modulus and loss factor of the more widespread thermoset (epoxy), thermoplastic (Polylactic acid 4 (PLA 4), and Polypropylene (PP)) polymers used for flax composites are reported in Figure 6 [27], for both pure resin and UD flax composites.

Although the reinforcement is the same, their global damping is quite different. The addition of plant fiber in epoxy, PLA4, and PP show a distinctly different trend in loss factor, but a total increase in modulus. This is because of the interactions between fiber and matrix, and aspect discussed in the following section. It could also depends on the contribution of the internal friction in the fibers to the overall damping response [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF].

Interface / interphase

As abovementioned, interfaces play a critical role in the damping properties of composites. Their properties depend not only on the manufacturing process but also on the treatments, which have been discussed in the fiber treatment section, as shown in Figure 7. This section focuses on the original interface.

Figure. 7 Parameters related to the interface properties

It is reported that a composite with weak interface bonding tends to dissipate more energy than that with good interface bonding [START_REF] Essabir | Dynamic mechanical thermal behavior analysis of doum fibers reinforced polypropylene composites[END_REF][START_REF] Geethamma | Dynamic mechanical behavior of short coir fiber reinforced natural rubber composites[END_REF]. However, other reports show that an increased damping can be obtained often by improvement of fiber/matrix adhesion, which may activate the damping phenomena such as intra-cell wall friction between the cellulose microfibrils and the hemicellulose/lignin matrix in each cell wall and inter-cell wall friction between the cell walls [START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF].

In most cases, higher resin contents for most organic-based composites should lead to higher damping due to its viscoelastic properties. However, in some cases, the reduction of the matrix fraction increases the damping. This is due to the interface thickness and interface stiffness that also play an essential role in the damping mechanisms [START_REF] Chandra | Damping studies in fiber-reinforced composites -a review[END_REF].

It was previously reported that the incorporation of stiff fibers affects the damping behavior of matrices by changing the movement of polymer chains [START_REF] Harris | Study of carbon fibre surface treatments by dynamic mechanical analysis[END_REF][START_REF] Datta | Mechanical and dynamic mechanical properties of jute fibersnovolac-epoxy composite laminates[END_REF][START_REF] Wingard | Crosslinking of an epoxy with a mixed amine as a function of stoichiometry. II. Final properties via dynamic mechanical spectroscopy[END_REF][START_REF] Otaigbe | Dynamic mechanical response of a thermoplastic sheet molding compound-glass fiber composite[END_REF]. With certain thermoset systems, the proximity of the stiff fibers and preferential adsorption and/or absorption of diffusible constituents, in particular low molecular weight curatives, on the fiber surface or in the fiber wall may impose a relatively high crosslink density, decreasing locally the damping behavior of the resin. At the same time, this may lead to some softening of the matrix in the zone next to the interface because of the depletion of the curative [START_REF] Datta | Mechanical and dynamic mechanical properties of jute fibersnovolac-epoxy composite laminates[END_REF]. This is particularly conceivable with plant fibers, which have a certain affinity and/or absorption ability with the curatives. Plant fiber reinforcements are also generally made up of yarns of elementary fibers. The friction mechanisms between fibers (intra-yarn friction) as well as the friction between the yarns (inter-yarn friction) could increase the intrinsic damping with respect to that obtained with synthetic fibers [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF]. Some studies show that the loss factor and stiffness of interleaf films play an essential role in the loss factor of the interleaved laminates at the test temperature [START_REF] Kishi | Damping properties of thermoplasticelastomer interleaved carbon fiber-reinforced epoxy composites[END_REF].

Porosities

Porosity is inevitable during the manufacture of composite materials, in particular with plant fibers. However, its influence on the damping behavior of PFCs is poorly discussed in literature. A report on hybrid fiber composites (SFCs+PFCs) describes the effect of the existence of voids on damping characteristics. The damping is found to be not sensitive to the void content. This might be due to the small void content in the samples reducing the contribution [START_REF] Cihan | Mechanical and dynamic performance of woven flax/E-glass hybrid composites[END_REF]. For non-woven PP composites, a recent study by Hadiji et al shows that the loss factor increases by 108.7% when the porosity changes from 9 to 64% based on modal analysis [START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF]. This is attributed to poor adhesion between fiber and matrix leading to more energy dissipation [START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF][START_REF] Merotte | Mechanical and acoustic behaviour of porosity controlled randomly dispersed flax/PP biocomposite[END_REF].

Additional research on this topic regarding different types of PFCs, such as woven pattern, matrix (thermoset or thermoplastic), etc., is necessary since not enough conclusions have been reached at this time.

Testing and surrounding conditions

Testing technique and frequency-dependent

The testing techniques may have an influence on the determined values of the loss factors [START_REF] Butaud | Identification of the viscoelastic properties of the tBA/PEGDMA polymer from multi-loading modes conducted over a wide frequency-temperature scale range[END_REF]. So, the damping results from DMA tests and modal analysis tests are compared in some studies [START_REF] Rueppel | Damping of carbon fibre and flax fibre angleply composite laminates[END_REF][START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF][START_REF] Butaud | Identification of the viscoelastic properties of the tBA/PEGDMA polymer from multi-loading modes conducted over a wide frequency-temperature scale range[END_REF]. Regarding PFCs, Rueppel et al. describe damping measurement tests with three different strategies: DMA, logarithmic decrement measurement method (LDM) and vibration beam measurements (VBM) [START_REF] Rueppel | Damping of carbon fibre and flax fibre angleply composite laminates[END_REF]. The values obtained from DMA and VBM have a significant difference, which is attributed to air resistance effects as the amplitude of vibration is larger during VBM [START_REF] Rueppel | Damping of carbon fibre and flax fibre angleply composite laminates[END_REF][START_REF] Zoghaib | Damping analysis of a free aluminum plate[END_REF]. LDM provides non-linear decay for the materials and the authors recommend to carefully consider initial parts of the displacement curve during tests, especially for high damping materials. It is then essential to take into account the experimental techniques when comparing damping properties of different materials.

The damping properties may vary as the changes of frequency. The loss factor of UD or twill flax fabric reinforced epoxy composites are observed decreasing trend for low frequencies (<500 Hz) and then stabilization for higher frequencies (500 -2000 Hz) [START_REF] Daoud | Characterization of the vibrational behaviour of flax fibre reinforced composites with an interleaved natural viscoelastic layer[END_REF][START_REF] Assarar | Evaluation of the damping of hybrid carbonflax reinforced composites[END_REF][START_REF] Daoud | Numerical and experimental characterization of the dynamic properties of flax fiber reinforced composites[END_REF][START_REF] Cheour | Effect of water ageing on the mechanical and damping properties of flax-fibre reinforced composite materials[END_REF]. Assarar and Daoud explain that the vibration behavior at low frequencies is result from the internal friction between cellulose and hemicellulose in the plant fiber and this kind of friction is more pronounced at low frequencies [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Daoud | Characterization of the vibrational behaviour of flax fibre reinforced composites with an interleaved natural viscoelastic layer[END_REF][START_REF] Assarar | Evaluation of the damping of hybrid carbonflax reinforced composites[END_REF]].

However, UD flax reinforced polypropylene or epoxy composites are observed an slight increasing trend at low frequencies (<1000 Hz) [START_REF] El-Hafidi | Determination of dynamic properties of flax fibres reinforced laminate using vibration measurements[END_REF][START_REF] Rahman | Vibration damping of flax fibre-reinforced polypropylene composites[END_REF][START_REF] Ameur | Damping analysis of unidirectional carbon/flax fiber hybrid composites[END_REF]. In addition, the damping properties of UD flax/PA11 composites were also obtained over a large frequency range (2000 -10000 Hz) and it was difficult to derive a trend due to the coupling of the plate vibration with aerodynamic phenomena [START_REF] Amenini | Dynamical properties of flax fibre reinforced PA11 over a large frequency range[END_REF]. Therefore, an experimental technique that can eliminate the influence of air and show the contribution of each component (fiber, matrix and interface) on damping properties as a function of frequency is proposed in the future.

Environmental conditions

Researchers have also paid attention to the influence of some external factors in addition to the inherent factors of PFCs components. In this section, the effects of external environment such as water aging or moisture content, temperature and various coupling conditions is summarized.

Moisture

The environment in which PFC materials are serviced is sometimes harsh, and in most cases, the environment is accompanied by changes in moisture content.

Plant fibers are sensitive to moisture temperature due to the hydrophilicity of some of their wall constituents and also due to their hollow morphology [START_REF] Zhang | Influence of some factors on the damping property of fiberreinforced epoxy composites at low temperature[END_REF][START_REF] Pé Ron | Study of hygroscopic stresses in asymmetric biocomposite laminates[END_REF][START_REF] Senthilrajan | Ageing and Its Influence on Vibration Characteristics of Jute/Polyester Composites[END_REF][START_REF] Rong | The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites[END_REF]. Therefore, their hygroscopic properties and effects need to be studied if plant fiber composite materials are to be used in engineering fields. Many factors affect water absorption characteristics in PFCs. External factors such as temperature, manufacturing features such as fiber fraction, fiber orientation, size and percentage of voids, interface factors such as exposed area, surface treatment, component hydrophilicity, and bonding quality of interfaces between fiber-matrix are proved to be critical influencing factors [START_REF] Lu | Improving moisture durability of flax fibre composites by using nondry fibres[END_REF][START_REF] Saxena | Sisal Fiber Based Polymer Composites and Their Applications[END_REF][START_REF] Machado | Impact of high moisture conditions on the serviceability performance of wood plastic composite decks[END_REF].

Generally, the absorption of water in PFCs is started by water entering the plant fiber through capillary transport. Materials with micro-crack defects also accelerate the diffusion of water. Plant fibers absorb water and cause the fibers to swell, leading to micro-cracks in the fiber-matrix interface area [START_REF] Bal | Effect of Changing Environments on Microstructure of HDPE Polymer[END_REF][START_REF] Cheng | Microstructural changes in wood plastic composites due to wetting and re-drying evaluated by X-ray microtomography[END_REF]. Moreover, this diffusion is enhanced by the aging of the material itself [START_REF] Machado | Impact of high moisture conditions on the serviceability performance of wood plastic composite decks[END_REF], which causes the deformation and mechanical properties of the PFCs to decline [START_REF] Arul Selvan | Mechanical properties of fragrant screwpine fiber reinforced unsaturated polyester composite: Effect of fiber length, fiber treatment and water absorption[END_REF][START_REF] Hamid | Effect of antioxidants and fire retardants as mineral fillers on the physical and mechanical properties of high loading hybrid biocomposites reinforced with rice husks[END_REF]. Many studies have shown that good interfacial properties between fiber reinforcement and matrix or better moisture absorption resistance can reduce the effect of moisture absorption on plant fiber [START_REF] Arul Selvan | Mechanical properties of fragrant screwpine fiber reinforced unsaturated polyester composite: Effect of fiber length, fiber treatment and water absorption[END_REF]. Damping is generally observed increasing with the increasing relative humidity in PFCs at the expense of Young's modulus. Damping of wood fiber composite is more sensitive to relative humidity than Young's modulus, changing by 26% and -13% from dry to humid conditions, respectively, as shown in Figure 8 (a) [START_REF] Bogren | Dynamic-Mechanical Properties of Wood-Fiber Reinforced Polylactide: Experimental Characterization and Micromechanical Modeling[END_REF]. Berges et al. indicate a 50% increasing of damping ratio after water vapor saturation of flaxtape/epoxy composites [START_REF] Berges | Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates[END_REF]. Reports on synthetic fiber composites are also available, but the effect of relative humidity on stiffness is not significant [START_REF] Launay | Modelling the influence of temperature and relative humidity on the time-dependent mechanical behaviour of a short glass fibre reinforced polyamide[END_REF][START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF]. In addition, the matrix of a composite material usually exhibits plasticization and swelling when exposed to moisture.

Damping is very sensitive to changes in stiffness of the outer layer due to the plasticization of macromolecular networks, which exacerbates energy dissipation [START_REF] Fraga | Relationship between dynamic mechanical properties and water absorption of unsaturated polyester and vinyl ester glass fiber composites[END_REF].

In addition, the moisture present in the areas at the interfaces increases friction losses [START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF].

PFCs are not only affected by the matrix when dealing with wet environment compared to SFCs, but the changes in fiber molecules also need to be understood. The dynamic FT-IR technique can be used since the traditional macro-mechanical tests cannot provide information about the stress transfer between the fiber and the matrix [START_REF] Almgren | Effects of Moisture on Dynamic Mechanical Properties of Wood Fiber Composites Studied by Dynamic FT-IR Spectroscopy[END_REF]. As moisture is transported from the plant fibers to the interface between fiber and matrix, the ability to transfer stress between the fibers and the matrix is reduced [START_REF] Almgren | Effects of Moisture on Dynamic Mechanical Properties of Wood Fiber Composites Studied by Dynamic FT-IR Spectroscopy[END_REF]. The matrix bears more loads, and the in-phase contribution of the matrix increases relatively to that of cellulose [START_REF] Almgren | Effects of Moisture on Dynamic Mechanical Properties of Wood Fiber Composites Studied by Dynamic FT-IR Spectroscopy[END_REF]. The energy dissipation is related to the strain energy of the fiber and the friction between the different components is increased as water absorption [START_REF] Berthelot | Damping analysis of composite materials and structures[END_REF].

The effect of fiber orientation changes have also been studied in UD composites.

The sensitivity of damping in different fiber orientations to the moisture decreases gradually from 0°, 90° to 45° [START_REF] Cheour | Effect of water ageing on the mechanical and damping properties of flax-fibre reinforced composite materials[END_REF]. The PFCs laminates with 90º outer layers are profoundly affected by moisture meaning more sensitive effect on damping [START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF].

Therefore, this situation should be avoided in the design of composite materials if it is intended to be used in high humidity environments. However, another reason to explain the effect of stacking sequences, is that different fiber orientations have a different amount of water absorption before the specimen reaches saturation, which are not taken into account when discussing the effect on damping. Similar research is also reported in synthetic fiber composites [START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF]. In this work, a dehydrated flax composites specimen after water absorption is compared with the original specimen. Although a 15% decrease in the bending modulus is observed, the author claim that the damping performance is reversible because the damping in PFCs was mainly driven by the water content in the fiber and fiber friction. The effects of cracks and interface failures are found to be negligible [START_REF] Cheour | Effect of water ageing on the mechanical and damping properties of flax-fibre reinforced composite materials[END_REF]. Several authors claim that this behavior appears to be unrecoverable in glass fiber reinforced polymer composites because the damping of SFCs is mainly determined by the damping of the matrix and the interface [START_REF] Chandra | Damping studies in fiber-reinforced composites -a review[END_REF][START_REF] Gu | Dynamic mechanical analysis of the seawater treated glass/polyester composites[END_REF].

Damping performance under seawater or strong acids are also studied in addition to freshwater or pure water environments [START_REF] Mazuki | Degradation of dynamic mechanical properties of pultruded kenaf fiber reinforced composites after immersion in various solutions[END_REF]. Research shows that plant fibers are more susceptible to acids than synthetic fibers [START_REF] Ghasemzadeh | Dynamic mechanical study of epoxy, epoxy/glass, and glass/epoxy/wood hybrid composites aged in various media[END_REF]. Besides, a silane agent is proved to decrease the water absorption, which is caused by reducing the chance of hydrogen bonding between free -OH groups in cellulose and water molecules [START_REF] Dayo | The influence of different chemical treatments on the hemp fiber/polybenzoxazine based green composites: Mechanical, thermal and water absorption properties[END_REF]. V. Fiore et al. also claim that NaHCO3 treatment show beneficial effect on damping properties of flax composites but not jute composites during exposition to salt-fog environments which is strictly related to the fiber's chemical composition [START_REF] Fiore | Dynamic Mechanical Behavior Analysis of Flax/Jute Fiber-Reinforced Composites under Salt-Fog Spray Environment[END_REF].

In general, many reports on the effects of moisture on SFCs behavior are available, while researches related to PFCs have focused more on monotonic mechanical behavior.

The effect of water-heat coupling on the damping using different types of PFCs needs more research.

Temperature

The dynamic mechanical properties of organic-based composites are also strongly sensitive to temperature. Temperature is the first factor that affects damping properties in various external environments [START_REF] Mizumachi | Study of Polymer Blends as a Vibration Damper[END_REF].

Below the glass transition temperature, the loss factor increases with temperature attributed to matrix softening [START_REF] Subramanian | Effect of reinforced fiber length on the damping performance of thermoplastic composites[END_REF]. The free volume and space of the molecule's internal movement increase when the temperature rises, which causes the storage modulus and loss modulus to decrease. An ideal damping material should have a wider transition region and higher peaks in loss factor. However, as the matrix of composite materials, the stiffness decreases significantly in the transition zone, which requires engineers to find a suitable compromise between stiffness and damping.

Besides, damping performances are strongly related to glass transition temperature (Tg). The incorporation of plant fibers in the matrix generally induces a shift of Tg toward higher temperatures and a reduction of the loss factor peak due to the restriction of the movements of the matrix chains. This suggests an increase in stiffness of the fiber-matrix interfacial zone, however, contradictory effects have sometimes been observed [START_REF] Saha | Study of Jute Fiber Reinforced Polyester Composites by Dynamic Mechanical Analysis[END_REF][START_REF] Ray | Dynamic mechanical and thermal analysis of vinylester-resinmatrix composites reinforced with untreated and alkali-treated jute fibres[END_REF][START_REF] Pothan | Dynamic mechanical analysis of banana fiber reinforced polyester composites[END_REF][START_REF] Huda | Wood-fiber-reinforced poly(lactic acid) composites: Evaluation of the physicomechanical and morphological properties[END_REF][START_REF] Montazeri | Viscoelastic properties and determination of free volume fraction of multi-walled carbon nanotube/epoxy composite using dynamic mechanical thermal analysis[END_REF][START_REF] Chua | Dynamic mechanical analysis studies of the interphase[END_REF]. It strongly depends on the matrix type, its affinity with plant fiber and the resulting stiffness properties at the interface between fibers and matrices.

Besides, some results report the damping properties of flax/epoxy composites during thermal shock cycling conditions from -40 ℃ to 28 ℃ [START_REF] Kollia | Effect of thermal shock cycling on the quasi-static and dynamic flexural properties of flax fabric-epoxy matrix laminates[END_REF]. The maximum decrease in the loss factor observed is 8%. In addition, it is reduced by about 50% for storage modulus, and dynamic mechanical properties reach an equilibrium state due to microdamage saturation after 100 thermal shock cycles. The glass transition temperature (Tg) is not affected by the thermal shock cycling conditions. However, most of the results available up to now provide a combine effect, temperature increase and specimen drying since it is really difficult using traditional experimental methods (such as DMA tests) to maintain the moisture content equilibrium within PFCs samples while changing the temperature. Hence, more researches are suggested in the future efforts to decorrelate the effect of temperature and moisture content.

Table 1 Main features in mesoscale, microscale and surrounding condition parameters

Parameters

Damping root Reference

Mesoscale parameters

Fiber length

The ratio of fiber length to diameter, surface contact area [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF][START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Subramanian | Effect of reinforced fiber length on the damping performance of thermoplastic composites[END_REF][START_REF] Harris | Study of carbon fibre surface treatments by dynamic mechanical analysis[END_REF][START_REF] Rezaei | Effect of fiber length on thermomechanical properties of short carbon fiber reinforced polypropylene composites[END_REF] Weave patterns Interlace between the warp and weft directions [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Cihan | Mechanical and dynamic performance of woven flax/E-glass hybrid composites[END_REF][START_REF] Audibert | Mechanical characterization and damage mechanism of a new flax-Kevlar hybrid/epoxy composite[END_REF][START_REF] Rouf | Effect of fabric weaves on the dynamic response of two-dimensional woven fabric composites[END_REF][START_REF] Alkbir | Fibre properties and crashworthiness parameters of natural fibre-reinforced composite structure: A literature review[END_REF][START_REF] Rajesh | Dynamic mechanical analysis and free vibration behavior of intra-ply woven natural fiber hybrid polymer composite[END_REF][START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF] Stacking sequence Shearing effect, effective load transfer [START_REF] Li | Multi-scaled enhancement of damping property for carbon fiber reinforced composites[END_REF][START_REF] Pothan | Tensile and Flexural Behavior of Sisal Fabric/Polyester Textile Composites Prepared by Resin Transfer Molding Technique[END_REF][START_REF] Ramraji | Mechanical and free vibration properties of skin and core designed basalt woven intertwined with flax layered polymeric laminates[END_REF][START_REF] Rajesh | Free Vibration Characteristics of Banana/Sisal Natural Fibers Reinforced Hybrid Polymer Composite Beam[END_REF]

Microscale parameters

Fiber type Stress transfers, intrinsic damping capacity of plant fiber [START_REF] Essabir | Dynamic mechanical thermal behavior analysis of doum fibers reinforced polypropylene composites[END_REF][START_REF] Pan | Kenaf fiber/poly(ɛ-caprolactone) biocomposite with enhanced crystallization rate and mechanical properties[END_REF][START_REF] Hwang | Contribution of interlaminar stresses to damping in thick laminated composites under uniaxial extension[END_REF] , [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Duc | The role of twist and crimp on the vibration behaviour of flax fibre composites[END_REF][START_REF] Duc | Dynamic mechanical properties of epoxy/flax fibre composites[END_REF] Fiber orientation In-plane shear strain energy [START_REF] Hwang | Contribution of interlaminar stresses to damping in thick laminated composites under uniaxial extension[END_REF] Fiber volume fraction Increase interface or restrict the mobility of matrix [START_REF] Kumar | Synergy of fiber length and content on free vibration and damping behavior of natural fiber reinforced polyester composite beams[END_REF][START_REF] Salleh | Effects of extrusion temperature on the rheological, dynamic mechanical and tensile properties of kenaf fiber/HDPE composites[END_REF] Treatments Stress transfer, quality of interface [START_REF] Li | Multi-scaled enhancement of damping property for carbon fiber reinforced composites[END_REF][START_REF] Fay | Effect of morphology, crosslink density, and miscibility on interpenetrating polymer network damping effectiveness[END_REF][START_REF] Yu | Effect of fiber surface-treatments on the properties of poly(lactic acid)/ramie composites[END_REF] Matrix Molecular structure, interactions at the interfaces [START_REF] Duc | Damping of thermoset and thermoplastic flax fibre composites[END_REF][START_REF] Fu | Vibration reduction ability of polymers, particularly polymethylmethacrylate and polytetrafluoroethylene[END_REF][START_REF] Tung | Vibration damping with urethane/acrylate simultaneous semi-interpenetrating polymer networks[END_REF] Interface Fiber/matrix adhesion [START_REF] Essabir | Dynamic mechanical thermal behavior analysis of doum fibers reinforced polypropylene composites[END_REF][START_REF] Geethamma | Dynamic mechanical behavior of short coir fiber reinforced natural rubber composites[END_REF] Porosities Not enough study in porosities -

Surrounding conditions

Moisture effect

Friction losses caused by interface damage [START_REF] Zai | Effect of moisture absorption on damping and dynamic stiffness of carbon fiber/epoxy composites[END_REF][START_REF] Dayo | The influence of different chemical treatments on the hemp fiber/polybenzoxazine based green composites: Mechanical, thermal and water absorption properties[END_REF][START_REF] Bal | Effect of Changing Environments on Microstructure of HDPE Polymer[END_REF][START_REF] Cheng | Microstructural changes in wood plastic composites due to wetting and re-drying evaluated by X-ray microtomography[END_REF] Temperature Internal movement of molecule chain, changes in microstructure of plant fiber [START_REF] Subramanian | Effect of reinforced fiber length on the damping performance of thermoplastic composites[END_REF][START_REF] Kollia | Effect of thermal shock cycling on the quasi-static and dynamic flexural properties of flax fabric-epoxy matrix laminates[END_REF] In this section, the effect from mesoscale parameters (reinforcement type, stacking sequence), microscale parameters (fiber, matrix, interface and porosities) and surrounding conditions have been discussed. The main features for damping root are summarized in Table 1.

Limitations of existing PFC damping studies

(1) Porosity -The influence of porosity level has been recently investigated for PFCs made of non-woven fabrics and thermoplastic polymer [START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF]. However, results remain poor in literature in particular for short fibre composites as well as woven fabric based composites. Additional research on this topic regarding different type of PFCs with different matrices (thermoset or thermoplastic) is necessary since not enough conclusion has been reached at this time. In addition to the porosity level, the influence of the size and distribution of porosities should be investigated.

(2) Environmental conditions -The effect of hygrothermal coupling on the damping using different types of PFCs needs more research. At present time, the influence of environmental conditions is generally investigated using DMA tests, involving moisture content variation when sweeping temperature. The use of vibration tests is also recommended in the future to obtain direct measurements in a mid-frequency range.

(3) Characterization at the microscale and multi-scale approach -For the characterization of damping, a large number of reports focus on macro-and meso-scales, while studies on microscale are currently rarely seen. However, this is required to map the damping in the different constituents: plant fiber wall, surrounding matrix and its interface, to better understand the influence of microscale parameters on the damping at the macroscale. A particular attention has to be paid on the time scales related to each dissipation phenomena occurring at the various space scale.

(4) Wideband frequency and experimental technique effects -Evolution with frequency -Most of the results obtained for non-woven composites as well as non-crimp and woven composites show that the loss factor varies slightly with the frequency [START_REF] Amenini | Dynamical properties of flax fibre reinforced PA11 over a large frequency range[END_REF][START_REF] Hadiji | Damping analysis of nonwoven natural fibre-reinforced polypropylene composites used in automotive interior parts[END_REF]. Based on these results, it is however sometimes difficult to derive a trend. The combination of data collected using different experimental techniques on a same PFCs is suggested to observe the trend of the loss factor over a wide range of frequencies.

Comparison of experimental techniques -The comparison of different test methods

for specific PFCs under the same conditions to know their influential would also constitute valuable analysis for future research since many other influential parameters vary from one study to another.

Use of additional techniques -In parallel to the classical DMA and vibration techniques, other methods such as ultrasonic testing, nano-indentation, and scanning micro-deformation microscopy have been investigated for the damping characterization of polymer [START_REF] Baley | Industrial Crops & Products Variability of mechanical properties of flax fibres for composite reinforcement . A review[END_REF]. These techniques could also be used for PFCs. Although they are also limited by frequency and temperature, they can complement the limitations of other experiments in multi-scale [START_REF] Butaud | Investigations on the frequency and temperature effects on mechanical properties of a shape memory polymer (Veriflex)[END_REF][START_REF] Butaud | Identification of the viscoelastic properties of the tBA/PEGDMA polymer from multi-loading modes conducted over a wide frequency-temperature scale range[END_REF]. Wave number-based approaches can be an optional method to deal with high frequency range issues.

(5) Fiber length and microstructure -Even if the influence of fiber length on the damping properties has been already investigated for short-fiber composites, a more in-depth study is necessary to better comprehend the influence of fiber length, fiber ends and discontinuities on the damping behavior, in particular in non-crimp fabric composites.

For such composites, the influence of fiber type and fiber microstructural features should also be studied. Some effects of fatigue on damping performance are already reported. The loss factor is shown to decrease substantially in the first cycles, then it slightly decreases, then it stabilizes before the final failure [START_REF] Haggui | Static and fatigue characterization of flax fiber reinforced thermoplastic composites by acoustic emission[END_REF]. This deserves to be explained since one may expect an increase of the damping capacity with the damage creation and propagation.

The effect of various coupling conditions such as fatigue, moisture, temperature, etc. on the damping properties of PFCs should be studied in the future.

Besides, different parameter configurations during the composite manufacturing process also have an impact on the damping performance. A study finds that higher pressures appear to reduce the damping ratio due to alterations in the fiber-matrix bond [START_REF] Ashworth | Mechanical and damping properties of resin transfer moulded jute-carbon hybrid composites[END_REF]. The influence of parameters in the manufacturing process can be considered in the future.

Conclusions

This article critically reviews many factors that affect the damping properties of PFCs in mesoscale parameters, microscale parameters, surrounding conditions, etc. based on recent research reports. Literature shows that PFCs have values of loss factor comprised between 0.7% and 14%, while values are comprised between 0.24% and 2.5%

for SFCs. Therefore, the damping capacity of PFCs is generally much higher than SFCs.

The damping range is also more widespread. This is linked to the wide variety of fibers and their hierarchical organization and complex composition. The literature review points out also sometimes contradictory results. This is attributed to the wide variety of plant fiber composites studied, involving various types of plant fibers organized in different reinforcement architectures embedded in a very broad set of polymer matrices. This variety prevents sometimes from reaching a consensus and establishing generic conclusion. The review also shows some gaps in knowledge to bridge in the future.

The main conclusions are the following:

(a) The damping characteristics of PFCs are unique because of their microstructural and morphological properties, linked to their polymeric nature, moisture sensitivity, complex interface, finite length when compared to SFCs. Quantitative analysis of the influence of microstructure on damping performance is rarely seen, although there have been many studies on static mechanical properties.

(b) The diameter-length ratio of plant fiber has a significant effect on the damping of PFCs and different reinforcement types have different trends. The outer layer in the stacking sequence has a considerable effect on damping.

(c) Interface properties between fibers and matrices have a significant effect on damping performance with sometimes contradictory interpretations. Additional studies and knowledge are necessary to shield light of this complex issue.

(d) The special damping mechanisms for plant fiber composites is mainly due to intracell and inter-cell wall friction, intra-yarn, and inter-yarn friction, and fiber/matrix sliding. The effect of treatments on composite damping is caused by the change of interfacial properties between fiber and matrix.

(e) Plant fiber composites are more sensitive to moisture content than synthetic fiber composites because of the mismatch of the moisture expansion coefficients between the matrix and the fiber, which would induce a modification of the interfacial properties.

(f) Future work can expand on these issues regarding the effect on damping properties such as comparison of multiscale experimental method, different reinforcement type, surrounding conditions, parameters in the manufacturing process, etc.

Declaration of competing interest

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

Figure. 2

 2 Figure. 2 Ashby diagram: loss factor vs. modulus (summarized from refs. [10,27-62],

Figure. 3

 3 Figure. 3 (a) Young's modulus and (b) loss factor of unidirectional (UD) fiber

Figure. 5

 5 Figure. 5 Loss factor and storage modulus of different polymers at 1 Hz and ambient

Figure. 6

 6 Figure. 6 Young's modulus (a) and loss factor (b) of flax composites in the

Figure. 8

 8 Figure. 8 Young's modulus and loss factor of (a) wood fiber composite and (b) different

( 6 )( 7 )

 67 Stress level effect -Since most of the PFCs exhibit a nonlinear static behavior as a function of the stress level, it would be also interesting to verify the linearity of the damping behavior as a function of the stress level. Other factors -Composite materials face fatigue issues during long-term service.

paper.

Acknowledgements

The authors express their gratitude for the funding received from the Bio Based Industries Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No. 744349-SSUCHY project and the funding received from the Region Bourgogne-Franche-Comté under grant agreement No. 2016Y-06124-D-BCOMP project. Financial support from the EUR EIPHI Graduate school (contract "ANR-17-EURE-0002") is also gratefully acknowledged.