Accepted Manuscript

Damping of thermoset and thermoplastic flax fibre composites

F. Duc, P.E. Bourban, C.J.G. Plummer, J.-A.E. Månson

PII: S1359-835X(14)00118-3

DOI: http://dx.doi.org/10.1016/j.compositesa.2014.04.016

Reference: JCOMA 3607

To appear in: Composites: Part A

Received Date: 29 July 2013

Revised Date: 14 April 2014

Accepted Date: 21 April 2014

Please cite this article as: Duc, F., Bourban, P.E., Plummer, C.J.G., Månson, J.-A.E., Damping of thermoset and

thermoplastic flax fibre composites, Composites: Part A (2014), doi: http://dx.doi.org/10.1016/j.compositesa.

2014.04.016

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Damping of thermoset and thermoplastic flax fibre

composites

F. Duca, P.E. Bourban∗,a, C. J. G. Plummera, J.-A.E. Mansona

aLaboratoire de Technologie des Composites et Polymeres (LTC), Ecole PolytechniqueFederale de Lausanne (EPFL), Station 12, CH-1015 Lausanne, Switzerland

Abstract

The mechanical and damping properties of unidirectional (UD) and twill

2/2 flax fibre (FF) reinforced thermoset (epoxy) and thermoplastic (polypropy-

lene (PP) and polylactic acid (PLA)) composites containing 40 vol% of fibres

have been compared with those of carbon (CF) and glass (GF) fibre reinforced

epoxy composites. Thanks to the relatively low density of the FF, the specific

mechanical properties of the UD FF based composites were comparable with

those of the GF epoxy composites. The composites reinforced with FF also

showed improved damping as reflected by dynamic mechanical analysis with

respect to composites reinforced with synthetic CF and GF. For example,

the addition of UD FF to epoxy led to an approximately 100 % increase in

loss factor with respect to both the matrix and GF reinforced epoxy. FF/PP

showed the highest damping at 25 ◦C and 1 Hz of all the composites investi-

gated (tanδ = 0.033). However the best compromise between stiffness and

damping was obtained with FF reinforced semicrystalline PLA.

Key words:

∗Corresponding author. Tel.: +41 21 693 58 06; fax: +41 21 693 58 80Email address: pierre-etienne.bourban@epfl.ch (P.E. Bourban)

Preprint submitted to Composites Part A April 26, 2014

Natural fibres, A. Polymer-matrix composites (PMCs), B. Mechanical

properties, B. Vibration, D. Mechanical testing

1. Introduction

The use of natural fibres (NF) dates back to at least 8000 BC, at which

time linen and hemp fabrics are known to have existed. These and many

other types of NF have since been widely used for clothes, ropes, canvas,

paper, pottery, etc. Moreover, the potential of plant fibres as reinforcements

for composite materials was recognized as long as 3000 years ago, when straw

reinforced clay was first used by the Egyptians as a building material [1].

Owing to the improved performance and reduced price of technical fibres

(glass, aramid, carbon etc.), the use of the NF in composites had almost ceased

by the middle of the 20th century [1, 2]. However, increased environmental

awareness, dwindling non-renewable raw materials and a growing global

waste problem have led to a renaissance in NF from sustainable resources, and

stimulated interest in so-called biocomposites. NF may be classified according

to their origins, i.e. whether they are derived from plants, animals or minerals

[3]. Of these, plant fibres (mainly bast and leaf types) are the most widely

used as reinforcements in biocomposites. Such fibres are themselves composite

materials, being made up of an amorphous lignin and/or hemicellulose matrix

reinforced with crystalline cellulose microfibrils [3]. Certain types of fibre

also contain small quantities of pectin. The properties of each constituent

contribute to the overall properties of the fibre.

Plant-based NF typically have poorer mechanical properties than synthetic

fibres [3, 4]. However, owing to their lower density, their specific properties

2

may be comparable to, or even better than those of e.g. glass fibres (GF).

This represents one of the main advantages of NF composites in applications

for which property requirements include weight reduction. NF have other

advantages, however. They are not only derived from low cost renewable

raw materials but are also biodegradable, CO2 neutral and nonabrasive.

They cause less dermal and respiratory irritation than synthetic fibres, and

enhance energy recovery during their end-of-life treatment by incineration.

The intrinsic damping properties of NF are also of key interest, notably in

sports equipment [5], where a good balance between damping and stiffness

is important for providing the athlete with optimum feel and control. As

a consequence, tennis rackets, bicycle frames and skis incorporating NF

composites are now entering the market.

Flax fibres (FF) are currently the most common NF for composite appli-

cations. However only 6% of annual FF production is used in composites [5].

In industry, FF are typically used with epoxy (EP) resins, and EP reinforced

with FF or chemically treated FF is also widely described in the literature

[6–15]. For example, van de Weyenberg et al. report a Young’s modulus

(E) and a tensile strength (σmax) of 28 GPa and 133 MPa respectively for

an unidirectional (UD) EP composite containing 40 vol% FF. As ecological

impact becomes increasingly important in the development of new materials,

thermoplastic and even bio-based matrices combined with NF are also gaining

interest. Moreover, thermoplastic matrices may also show improved damping

behaviour with respect to EP [16]. Polypropylene (PP) is currently the ther-

moplastic matrix most widely used with NF [17–23]. However, polylactide

(PLA) has also recently been combined with various NF, including jute, FF,

3

hemp, bamboo, wood flour and wood fibres [24–28].

While the damping of NF composites is of increasing interest in industry, to

the authors’ knowledge, there has so far been little detailed comparison of the

damping performance of NF composites with composites based on synthetic

fibres. There are many different definitions and ways of measuring damping.

For example, the loss factor, the quality factor, the specific damping capacity,

the logarithmic decrement or the damping ratio all provide information on

damping properties, and indeed are explicitly linked at low damping levels

[29]. Dynamic mechanical analysis (DMA) is a particularly convenient way

to measure the loss factor and hence assess the damping performance of

composites. Talib et al. [30] studied the dynamic mechanical properties at 1

Hz of 0 to 60 wt% randomly oriented kenaf fibre reinforced PLA composites.

The damping peak (tan δ) of composites containing more than 50 wt% kenaf

fibres showed a decrease in amplitude with respect to that for neat PLA.

However the amplitude of the tan δ peak increased sharply at low fibre

contents. Wielage et al. [18] considered the dynamic mechanical properties of

FF, hemp and GF reinforced PP composites. They again found that the loss

factor decreases as the fibre content is increased. Even so, at a fibre content

of 30 wt%, the loss factor was significantly higher for FF than for GF.

The aim of the present study is to quantitatively determine and directly

compare the damping properties of thermoset and thermoplastic composites

reinforced with UD and twill 2/2 (TW) FF fabric. The properties measured for

the FF/EP composites will first be compared with those of the currently most

widespread composite materials, i.e. carbon fibre-reinforced EP (CF/EP)

and GF/EP. A discussion will then be given of the behaviour of FF-based

4

thermoplastic composites, with the aim of assessing the effect of the matrix

on the damping properties. The main objective is to quantitatively determine

the damping performance of NF composites and thus to identify the damping

mechanisms induced by NF.

2. Experimental details

2.1. Fibres and polymer matrices

2.1.1. Carbon fibres

UD and TW woven CF fabrics were purchased from Swiss-Composite in

Fraubrunnen, Switzerland. The fibre average weight (FAW) of the UD and

TW fabrics were 270 g/m2 and 200 g/m2 respectively.

2.1.2. Glass fibres

UD and TW woven GF fabrics were purchased from Swiss-Composite. The

FAW of the UD and TW fabrics were 220 g/m2 and 280 g/m2 respectively.

2.1.3. Flax fibres

UD and TW woven FF fabrics were purchased from LINEO, in Meulebeke,

Belgium. The yarns used to produce the TW fabric consisted of FF assembled

by torsion, which introduces significant crimp.

The fibres were nei eth r treated nor sized to modify their original surface

conditions. The two only fabrics containing fibres and yarns pre-coated with

epoxy were the FlaxPly-E UD and TW used for the FF/EP composites. The

FAW of the UD FlaxPly-E and TW FlaxPly-E were 180 g/m2 and 300 g/m2

respectively.

5

Dry FF woven fabrics (FlaxDry) (UD and TW) were used for the thermo-

plastic based FF composites. The FAW of the UD FlaxDry and TW FlaxDry

were 180 g/m2 and 300 g/m2 respectively. In order to reduce moisture and

thus formation of defects in the composite parts, the fibres were always dried

for 12 hours at 60 ◦C before processing.

2.1.4. Epoxy matrix

The EP resin L-235, purchased from Swiss-Composite, was used as a

thermoset resin. The hardener was Epoxy-Harter 236 from the same company.

Its density was 0.99 g/cm3. E and elongation at break were 3.5 GPa and 1.4

% respectively.

2.1.5. Polypropylene matrix

In order to ensure good impregnation of the woven FF fabrics, a low

viscosity PP homopolymer, Molpen HP500V from LyondellBasell industries,

was chosen as a thermoplastic resin. Its density was 0.910 [g/cm3]. E and

elongation at break were 2.0 GPa and 20 % respectively.

2.1.6. Polylactide matrix

To investigate the performance of novel sustainable ”green composites”

based on NF and biopolymers, two different grades of PLA from NatureWorks

LLC (USA) were selected for their processing properties:

(i) PLA 2002D (PLA2), a semicrystalline grade, with a density of 1.24

g/cm3, and E and elongation at break of 3.6 GPa and 5.5 %, respectively.

(ii) PLA 4032D (PLA4), a less crystalline grade, with a density of 1.24

g/cm3, and E and elongation at break of 3.6 GPa and 7 %, respectively.

6

Following previous work [31], the hygroscopic PLA granules were dried

in a vacuum oven at 35 ◦C for one day and then overnight at 65 ◦C, with a

vacuum of at least 200 mbar, so as to limit the moisture content to below

250 ppm, and hence prevent degradation during subsequent processing.

The thermal properties of the polymers were determined by Differential

Scanning Calorimetry (DSC) temperature scans at a rate of 10 ◦C/min. The

glass transition temperature (Tg) (from cooling scans) and, where relevant,

the melting temperature (Tm) (from heating scans) are given in table 1.

Table 1: Tg (from cooling scans) and Tm (from heating scans) measured byDSC at heating and cooling rate of 10 ◦C/min for the different matrices.

Materials Tg [◦C] Tm [◦C]EP 77.4 −PLA2 47.0 154.8PLA4 50.7 171.2PP (≈−10) 170.9

2.2. Composite processing

2.2.1. Resin Transfer Molding (RTM)

RTM was used to produce CF, GF and FF/EP composite plates. A

volume fibre fraction of 40 % was used throughout.

The final dimensions of the plates were 260×260×2.5 mm3. Prior to

impregnation, the mold and the resin were preheated to 60 ◦C in order to

reduce the resin viscosity. A vacuum pressure of 0.8 bar and a constant

injection pressure of 0.7 bar were applied during impregnation. The plates

were then cured in the mould at 40 ◦C for twelve hours.

7

2.2.2. Compression molding

Compression moulding (CM) was used to produce 140×60×2.5 mm3

rectangular FF/PP and FF/PLA plates, again with a fibre volume fraction

of 40 % throughout. PP and PLA sheets of 1 mm thick were prepared using

a Twin Screw Prism extruder, with the temperatures of the four heating

zones (in the direction of extrusion) set to 160 ◦C, 160 ◦C, 150 ◦C and 150

◦C, and 190 ◦C, 190 ◦C, 180 ◦C and 180 ◦C for PP and PLA respectively.

The sheets were stacked with interleaved woven FF fabrics and consolidated

using a Fontijne press. In order to prevent degradation of the FF [32], the

processing cycle was adapted to minimize exposure of the fibres to elevated

temperatures. The mould was first preheated to 180 ◦C for one hour to obtain

an homogeneous temperature distribution. Then the FF/polymer stack was

placed in the mold. A pressure of 5 bar was applied for five minutes at 180

◦C. The mold was then cooled under pressure to room temperature over 10

minutes.

Table 2 summarizes the different composites produced for this study and

their thermal properties and density are given in table 3. The densities were

generally similar for the UD and TW composites.

2.3. Mechanical properties

The mechanical properties of the polymers and composites were deter-

mined from tensile tests at 1 mm·min−1 using a screw driven tensile test

machine from UTS Testsysteme GmbH, equipped with a 100 kN load cell

and extensometers and following the norms ASTM D638 and ASTM D3039

for polymers and composites respectively. All the composite specimens had

a width of 20 mm and a thickness of approximately 2.5 mm. The length

8

Table 2: The different composites and abbreviations used in this study.

Fibres Matrices Fabrics Process AbbreviationsCF EP UD RTM CF EP UDCF EP TW RTM CF EP TWGF EP UD RTM GF EP UDGF EP TW RTM GF EP TWFF EP UD RTM FF EP UDFF EP TW RTM FF EP TWFF PP UD CM FF PP UDFF PP TW CM FF PP TWFF PLA2 UD CM FF PLA2 UDFF PLA2 TW CM FF PLA2 TWFF PLA4 UD CM FF PLA4 UDFF PLA4 TW CM FF PLA4 TW

Table 3: Tg (from cooling scans) and Tm (from heating scans) measured atheating and cooling rate of 10◦C/min and density for the composites.

Materials Tg [◦C] Tm [◦C] Density [g/cm3]CF EP 73.2 − 1.38GF EP 70.5 − 1.73FF EP 68.3 − 1.21FF PLA2 46.9 148.1 1.29FF PLA4 48 167 1.33FF PP (≈−10) 166.3 1.10

9

of the EP-based samples was 240 mm. The clamped length was 50 mm (at

either end) and the gage length was 60 mm. The corresponding values for

the thermoplastic-based specimens were 130 mm for the length, 30 mm for

the clamped length and 40 mm for gage length. Three specimens of each

material were tested.

E was determined by linear extrapolation of the stress-strain curve between

0.05 and 0.15 % of strain.

2.4. Damping properties

DMA (Q800 from TA Instruments) was used in the single cantilever

mode to characterize the damping behaviour of the composites under flexural

conditions of sollicitation. Specimens of 35 mm in length (in the direction

of the fibres for the UD composites), 10 mm in width and approximately

2.5 mm in thickness were used throughout. Temperature sweep tests (TS)

were made to evaluate the damping behaviour of the composites at specific

frequencies. Each composite was tested at three different frequencies (0.1 Hz,

1 Hz and 100 Hz). At each frequency, the composites were heated from -40

◦C to 120 ◦C at 2 ◦C/min at a constant deformation of 0.01 %. A soak time

of 2 minutes was applied at -40 ◦C and the preload force was set to 0.0001N.

All tests were repeated twice with different samples.

3. Results and discussion

3.1. Mechanical properties

3.1.1. Stress-strain curves

Figure 1 shows typical stress-strain curves for the neat EP resin and the

CF EP UD, GF EP UD and FF EP UD composites. The CF EP UD and

10

the GF EP UD showed the highest σmax, while the FF EP UD showed a

slightly higher elongation at break than the former, albeit less than that for

the neat EP.

0

200

400

600

800

1000

1200

1400

0

0.5 1

1.5 2

Stre

ss [M

Pa]

Strain [%]

CF_EP_UD

GF_EP_UD

FF_EP_UD

EP

Figure 1: Stress-strain curves obtained from tensile tests on unreinforced EPand CF EP UD, GF EP UD and FF EP UD composites.

All the EP UD composites showed characteristic stress-strain curves with

an initial linear elastic regime followed by a point of inflection and a non-linear

regime at high strains, as is seen most clearly for CF EP UD in Figure 1.

This transition occured at about 0.6 % strain for CF EP UD and GF EP UD

and at about 0.2 % for FF EP UD. The form of the curves is explained by the

nature of the reinforcement. As pointed out in section 2.1.3, the FF yarns in

the FF fabrics are assembled by torsion. The axial stiffness of the fibre yarns

depends strongly on the compression and friction between the twisted fibres.

With increasing axial load, the fibres may slip within the yarns, decreasing

the effective stiffness.

11

3.1.2. Mechanical properties

E and σmax of the polymers and composites are given in table 4, and

the specific moduli (Es) and strains at break of the composites are shown in

figure 2(a) to (d).

Table 4: E and σmax of the unreinforced polymers and the UD and TWcomposites. The standard deviations are given in brackets (Std).

Materials E (Std) σmax (Std)[GPa] [MPa]

Pol

ym

ers EP 3.5 (0.3) 43.3 (14.0)

PP 2.0 (0.3) 28.0 (6.5)PLA2 3.6 (0.1) 56.3 (0.2)PLA4 3.6 (0.1) 65.8 (0.1)

UD

com

pos

ites

CF EP 101.2 (6.6) 1207.7 (47.72)GF EP 35.4 (1.7) 514.2 (83.7)FF EP 20.2 (2.0) 258.8 (3.1)FF PP 17.4 (3.7) 215.4 (23.4)FF PLA2 18.2 (2.5) 240.0 (13.4)FF PLA4 18.3 (0.7) 234.8 (19.7)

TW

com

pos

ites

CF EP 28.4 (0.3) 607.7 (28.6)GF EP 18.3 (2.9) 245.5 (7.4)FF EP 7.9 (0.7) 85.1 (5.5)FF PP 5.8 (0.3) 64.5 (2.0)FF PLA2 8.8 (0.6) 85.8 (5.3)FF PLA4 8.8 (0.1) 79.0 (0.1)

The highest values of E, σmax and Es were obtained for the CF reinforced

composites. Indeed, the specific mechanical properties of the CF reinforced

composites were also considerably better than those of the FF reinforced com-

posites. As expected, E and Es were generally higher for the UD composites

than for the TW composites, owing to the higher degree of fibre orientation

in the former, but the TW composites showed relatively high strains at break.

12

0

5

10

15

20

25

30

Spe

cific

You

ng's

mod

ulus

[GP

a cm

3 /g]

EpoxyCF GF FF

PP PLA2 PLA4

UD composites73.6 (4.8)

(a)0

0.5

1

1.5

2

Stra

in a

t bre

ak [%

]

UD composites

EpoxyCF GF FF

PP PLA2 PLA4

(b)

0

2

4

6

8

10

12

14

Spe

cific

You

ng's

mod

ulus

[GP

a cm

3 /g] 20.47 (0.22) TW composites

EpoxyCF GF FF

PP PLA2 PLA4

(c)0

1

2

3

4

5

Stra

in a

t bre

ak [%

]

TW composites

EpoxyGF FF

PP PLA2 PLA4

(d)

Figure 2: Specific Young’s moduli and strains at break of UD ((a) and (b))and TW ((c) and (d)) composites.

13

The absolute values of E and σmax obtained for the FF EP UD composites

were similar to those reported by Weyenberg et al. [11] and Bensadoun et al.

[33] for a FF EP UD composite with the same fibre content, bearing in mind

that there may be differences in the characteristics of the bundled fibres used

in each case [5]. They were also substantially lower than the values for the

GF EP UD composites. However, Es was only 18 % lower in FF EP UD than

in GF EP UD. In view of the experimental scatter, the stiffness obtained

with FF was considered to be comparable with that obtained with GF.

As seen from table 4, EP, PLA2 and PLA4 showed significantly higher E

and σmax than PP. Addition of 40 vol% of FF UD nevertheless resulted in

similar specific stiffness in the fibre direction, again, in view of the experimen-

tal scatter. Given the relatively low E of the PP matrix, the relative increase

in the Es was much greater upon addition of FF for PP composite (636 %)

than for the other polymers (about 375 %).

The TW composites showed somewhat contrasting behaviour. E and

σmax for FF EP TW were much lower than those for GF EP TW and the

corresponding decrease in Es was 38 %. This trend was seen in all the TW

composites and may be explained by the higher crimp in the FF TW woven

fabrics than in the GF TW and FF UD woven fabrics. Verpoest et al. [5],

have demonstrated that a higher crimp in twisted yarns may result in poorer

composite mechanical properties owing to the greater misorientation within

the yarns. On the other hand, the crimp effect may also contribute to the

higher strain at break obtained with FF TW fabrics. The larger contribution

of the matrix strain in TW composites also explains the higher strains at

break of the thermoplastic composites.

14

Of the FF reinforced thermoplastic composites, those based on PLA2

gave the best results, showing similar or even improved properties with

respect to the FF EP based composites, while the more amorphous PLA

(PLA4) gave somewhat lower σmax. However, the lowest values of E and

σmax were obtained with the PP composites, presumably reflecting the lower

stiffness and tensile strength of the PP matrix. The FF PLA2 UD composites

showed E and σmax that were respectively 5 % and 11 % higher than for the

FF PP UD composites. As a sustainable biocomposite, FF PLA therefore

has competitive mechanical performance.

3.2. Damping properties

In this section representative results from DMA temperature sweep tests at

different frequencies on FF PP TW are first considered. The results obtained

at 1 Hz will then be used to compare the different materials.

Temperature sweep (TS) test. The evolution of the storage modulus, loss

modulus and loss factor with temperature at three different frequencies is

shown in figure 3 for FF PP TW. These results are consistent with those

obtained by Wielage et al. [18]. The peaks in the loss modulus and loss factor

at around 0 ◦C shifted to higher temperatures as the frequency increased as

expected for a classical viscoelastic response. This peak corresponds to the

onset of long range cooperative mobility in the amorphous phase, resulting in

a decrease in the storage modulus and an increase in the loss factor and loss

modulus. At temperatures above this transition, the damping properties were

dominated by the matrix, resulting in increased damping at low frequencies,

for which more relaxation process are activated at a given temperature.

15

However at temperatures below this transition the trend was reversed, with

an increase in energy dissipation with frequency, suggesting the fibres and/or

fibre/matrix interactions to have greater influence on the damping properties.

For each polymer, this transition temperature was defined as TTrans and

correspond to the temperature of the loss factor peak just below the Tg of

the amorphous phase measured in DMA temperature scans at 1 Hz.

100

1000

104

-0.04

0

0.04

0.08

0.12

0.16

-50 0 50 100

150

0.1 Hz1 Hz100 Hz

Sto

rage

(E'),

loss

(E'')

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss modulus

Loss factor

FF_PP_TW

Figure 3: Evolution of the storage modulus, loss modulus and loss factor ofFF PP TW with temperature at three different frequencies.

Epoxy based composites at 1Hz. The storage moduli and loss factors for the

UD and TW EP based composites are shown in figure 4(a) and (b) respectively.

As seen in figure 4(a), on adding 40 vol% UD fibres to the EP matrix, the

storage modulus generally increased, and the intensity of the loss factor peak

decreased. Moreover TTrans, which was characterized by a sharp drop in

storage modulus and a peak in loss factor, was higher in the composites than

in the matrix. This may be due to a higher degree of matrix cure in the

presence of fibres. That the EP may not have been completely cured after

16

processing in the absence of fibres is consistent with the sharp increase in the

storage modulus with temperature above TTrans (figure 4(a)).

Consistent with the results of the tensile tests, at temperatures below

TTrans, the storage moduli of FF EP UD and FF EP TW were lower than

those of the respective GF and CF reinforced EP, implying the properties of

the fibres to dominate the response under these conditions. Above TTrans,

better adhesion between the FF and EP than between CF and EP or GF

and EP, owing to the rougher surface of the FF, may have contributed to

the observed behaviour [1]. The consequently limited sliding between FF and

the matrix would account for both its relatively high storage modulus above

TTrans and the reduced intensity of the loss factor peak. The importance of

the fibre/matrix sliding on the damping properties has been demonstrated by

Khan et al. [34], who showed that increased sliding obtained by incorporating

CF nanotubes in CF reinforced EP composites, increased damping, even at

temperatures below TTrans.

Thermoplastic based composites at 1Hz. The storage moduli and loss factors

at 1 Hz are shown as a function of temperature for all the thermoplastic-

based composites in figures 5 and 6. FF PP UD showed markedly different

behaviour to FF EP UD, the sharp drop in storage modulus characteristic of

the glassy/rubbery transition in the EP composites being absent in the PP-

based composites owing to its relatively high degree of crystallinity. Instead,

this transition marks a transition to a regime in which the storage modulus

decreases gradually with increasing temperature. It follows that at high

temperature. FF PP UD showed a higher storage modulus than FF EP UD.

The storage modulus of FF PLA composites showed a glassy plateau at

17

1

10

100

1000

104

0

0.5

1

1.5

2

2.5

3

-50 0 50 100

150

CF_EPGF_EPFF_EPEP

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

UD composites

Storage modulus

Loss factor

(a)

1

10

100

1000

104

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-50 0 50 100

150

CF_EPGF_EPFF_EP

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss factor

TW composites

(b)

Figure 4: Storage modulus and loss factor at 1 Hz as a function of temperaturefor (a) UD and (b) TW EP based composites.

18

low temperature followed by a sharp drop at TTrans as with FF EP UD. How-

ever, on further increasing the temperature, the storage modulus increased,

presumably owing to crystallization during the scan.

10

100

1000

104

0

0.05

0.1

0.15

0.2

-50 0 50 100

150

FF_PPPP

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss factor

UD composites

(a)

0.001

0.01

0.1

1

10

100

1000

104

0

1

2

3

4

5

-50 0 50 100

150

FF_PLA2FF_PLA4PLA2PLA4

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss factor

UD composites

(b)

Figure 5: Storage modulus and loss factor at 1 Hz of UD (a) PP and (b) PLAbased composites.

Consistent with the results of the tensile tests, the TW composites showed

lower storage moduli than the UD composites (figure 6). The main loss factor

peak was also generally weaker for the TW composites, owing to a decrease

19

1

10

100

1000

104

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-50 0 50 100

150

FF_EPFF_PPFF_PLA2FF_PLA4

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss factor

UD composites

(a)

10

100

1000

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100

150

FF_EPFF_PPFF_PLA2FF_PLA4

Sto

rage

mod

ulus

[MP

a]Loss factor

Temperature [°C]

Storage modulus

Loss factor

TW composites

(b)

Figure 6: Storage modulus and loss factor at 1 Hz of (a) UD and (b) TW FFbased composites.

20

in the fibre/matrix sliding length. In a TW weave, the distance between

the intersections between warp and weft yarns limits the length available for

fibre/matrix sliding.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Loss

fact

or a

t 1H

z an

d 25

°C

Epoxy PP PLA2 PLA4

(a)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Loss

fact

or a

t 1H

z an

d 25

°C

EpoxyCF GF FF

PP PLA2 PLA4

UD composites

(b)0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Loss

fact

or a

t 1H

z an

d 25

°C

TW composites

EpoxyCF GF FF

PP PLA2 PLA4

(c)

Figure 7: Loss factor at 1 Hz and 25 ◦C of (a) the polymer matrices, (b) UDand (c) TW composites.

All composites at 1Hz and 25 ◦C. Figure 7 summarizes the loss factor at

1 Hz and 25 ◦C for all the polymers and composites, allowing quantitative

comparison and illustrating the influence of the matrix properties on the

composite damping performance. At 25 ◦C, all the polymers are in their

glassy state, with the exception of PP.

21

A marked increase in damping when using FF instead of CF or GF may

be inferred from figure 7(b) and (c). However, while the evolution of the

storage modulus was dominated by the fibre type, that of the loss factor

also depended strongly on the matrix. Thus, the addition of FF to PP

resulted in a decrease in loss factor, indicating the damping properties of the

matrix to be superior to those of the fibres. On the other hand, addition of

FF to EP increased damping significantly. Thus, at 25 ◦C, the loss factors

for FF EP UD and FF EP TW were 117 % and 232 % greater than for

GF EP UD and GF EP TW, respectively. Similarly, when FF was used

instead of CF in the UD EP composites, the loss factor increased by 201

%. Moreover, unlike CF and GF, addition of FF to the EP increased the

damping with respect to that in the neat EP. The loss factor increased from

0.015 to about 0.030. This improvement is thought to be directly linked to

the FF architecture. NF used for composite applications are generally made

up of yarns of elementary fibres, each being composed of cell walls in which

rigid cellulose microfibrills are embedded in a soft lignin and hemi-cellulose

matrix. These cell walls consist of several layers differing in composition, the

ratio between cellulose and lignin/hemicellulose, and the orientation of the

cellulose microfibrils. The resulting structure promotes dissipation of energy

through friction between cellulose and hemicellulose in each cell wall, called

intra-cell wall friction, and friction between the cell wall, called inter-cell

wall friction. These friction mechanisms increase the intrinsic damping with

respect to that obtained with synthetic fibres. The use of FF yarns as a

reinforcement in composites may also contribute to damping through friction

between the fibres within the yarns, called intra-yarn friction, and friction

22

between the yarns, called inter-yarn friction.

At 25 ◦C, PLA is below its glass transition temperature and hence has

properties similar to EP. However, addition of FF resulted in little change in

the loss factor for UD composites. This might be accounted for by weaker

interactions between PLA and FF than between EP and FF, such that less

load is transmitted to the fibres during deformation, reducing the contribution

of the internal friction in the fibres to the overall damping response. In the

case of TW composites, weaker adhesion may also promote energy dissipation

through inter-yarn friction, increasing the loss factor at 25 ◦C of FF PLA2 TW

and FF PLA4 TW with respect to that in FF PLA2 UD and FF PLA4 UD

respectively. With better adhesion between the fibres and the matrix, the

fibre/matrix sliding effect dominates as already discussed in the context of

figure 4 and figure 6. In this case, the loss factor was higher for FF EP UD

than for FF EP TW.

The comparison of the loss factors obtained at 25◦C and at TTrans for

UD EP and PLA based composites at a frequency of 1 Hz, shown in figure 8,

highlights the different mechanisms that contribute to the damping behaviour

of the composites. At room temperature the nature of the fibres and the

quality of the interface dominated. Thus, the use of FF instead of GF in EP

composites increased damping, owing to the intrinsic nature of the FF and the

yarns, although the specific mechanical properties remained similar (figure 2).

On the other hand, the FF PLA2 UD biocomposite showed similar damping

to the synthetic GF EP UD composite. The limited damping in this case

may be due to limited adhesion between FF and PLA2, as already discussed

in section 3.2, resulting in poor load transfer between the matrix and the

23

fibres. Intra-fibre damping phenomena are thus less strongly activated. At

TTrans, the matrix properties became dominant. Even so, the addition of FF

instead of GF to EP reduces the loss factor, because the rougher surface of

the FF decreases friction and sliding between the matrix and the fibres.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0

0.5

1

1.5

2

2.5T=25°CT=T

Trans

Loss

fact

or a

t 1 H

z an

d 25

°CLoss factor at 1 H

z and TTransUD composites

EpoxyGF

PLA2FF FF

Figure 8: Loss factor values obtained at 25 ◦C and TTrans for UD EP andPLA based composites

4. Conclusion

At a fixed volume fraction of unidirectional fibres, the low density of

flax resulted in specific mechanical properties in epoxy, polypropylene and

polylactide composites that are comparable with those of glass fibre reinforced

epoxy composites.

The damping properties obtained with the flax fibre reinforced composites

were also generally better at around room temperature than those of the

carbon and glass fibre reinforced composites. Indeed at 1 Hz and 25 ◦C,

addition of unidirectional flax fibre to epoxy led to an approximately 100 %

increase in loss factor with respect to both the matrix and GF reinforced

epoxy.

24

However the best compromise between stiffness and damping was obtained

with flax fibre reinforced semicrystalline PLA, which also has the advantage

of being produced entirely from renewable resources and biodegradability.

With regard to the underlying mechanisms, complex multiscale phenomena

are known to control the tanδ of composite materials. Thus, the nature of the

matrix, the stiffness increases due to the fibres, the textile architecture and

the yarn lengths available for fibre sliding and friction have all been identified

in this study to be important considerations for damping. The next step

towards a better understanding of these phenomena will be to investigate

in more detail the role of the interface quality, as well as intra-yarn and

intra-fibre damping phenomena. At the same time the work will be extended

to other types of test and vibration conditions, in order to gain insight

into the damping performance of natural fibre composites under conditions

representative of real applications.

5. Acknowledgments

The authors acknowledge Thomas Chenal and Lara Arietano for their

contribution to this work, and Dr. Christian Neagu and Dr. Yves Leterrier

for fruitful discussions and advice.

25

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