Static and fatigue characterization of flax fiber reinforced thermoplastic composites by acoustic emission

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Static and fatigue characterization of flax fiber reinforced thermoplastic composites by acoustic emission ⁎

Mondher Hagguia,b, , Abderrahim El Mahia, Zouhaier Jendlic, Ali Akroutb, Mohamed Haddarb a

Acoustics Laboratory of Maine University (LAUM), UMR CNRS 6613, Maine University, Av. O. Messiaen, 72085 Le Mans cedex 9, France Laboratory of Mechanics Modeling and Production (LA2MP), National School of Engineers of Sfax, University of Sfax, BP N° 1173, 3038 Sfax, Tunisia c ESTACA’Lab – Pôle Mécanique des Structures Composites et Environnement, Parc Universitaire, Laval-Change, rue Georges Charpak, BP-76121, 53061 Laval Cedex 9, France b

A R T I C LE I N FO

A B S T R A C T

Keywords: Flax fibers Thermoplastic Fatigue Damage Acoustic emission

The objective of this study is to investigate the mechanical behaviour and to provide a complete set of static and fatigue loading data on thermoplastic composites based on flax fiber. The manufacturing technique used for preparing the composite material samples was the liquid resin infusion. Since the inclusion of natural fiber in the polymer induces nonlinear behaviour within the structure, it is essential to proceed by an experimental approach. Firstly, static tensile tests were carried out to identify the composite mechanical properties and the damage and rupture mechanisms that occur during loading. Secondly, cyclic tensile test were realized to investigate macroscopic damage. Then, fatigue tensile behaviour was studied by evaluating the material stiffness degradation which is regularly used to identify failure criterion. The fatigue results were summarized in a S-N curve. All experimental tests were coupled with the acoustic emission technique which leads to identify the major mechanisms that contribute to damage and its kinetics of appearance.

1. Introduction Today, the flax fiber composites are used in many industrial sectors such as in automotive, aerospace, sport and leisure. Bio-composites have large potential as structural materials due to the high biodegradability, eco-friendliness, reduced costs, as well as acoustic and thermal insulation [1–4]. Therefore, a large amount of research was conducted over the last decade to investigate the mechanical properties of flax composite subjected to static and fatigue loading. During fatigue testing, damage in natural fiber composites and its accumulation depend on many factors such as fiber volume fraction, fiber-matrix interface, specimen shape, and the applied stress level and direction. The inclusion of natural fibers in polymers introduces several challenges such as the nonlinear behaviour. In addition, the processing method is often an important factor that influences the mechanical properties and interfacial characteristics of the composites. Thus, suitable processing techniques and parameters must be carefully selected in order to produce the optimum composite, in particular, with the least amount of porosity. Liang et al. [5], in a comparative study with equivalent [0/90]3S laminates configuration, has shown that fatigue properties of the glass fiber composites are higher than those of flax fiber composites. The

⁎

author attributes this effect to the higher difference in their ultimate tensile strength (UTS). However, for [ ± 45]3S specimens, resistances become comparable below a loading level equal to 64 MPa. In the same context, Shah et al. [6] have compared the strength degradation rates in fatigue test of flax/polyester and glass/polyester composites. They found, from a power–law regression equations applied to the S-N curves, that the degradation rate of natural fiber composites curve are lower than that of glass reinforced composites. Gassan [7] has studied the tension-tension fatigue behaviour of composites made of flax and Jute yarns and woven as reinforcements for epoxy, polyester and polypropylene resin. It was emphasized that many parameters could affect the life times of composites such as the mechanical property of fiber, the textile architecture, the amount of fiber and fiber matrix adhesion. In case of woven reinforcement, the damage propagation becomes more rapid and loading to failure is lower than it is in unidirectional composites. Silva et al. [8] have studied the fatigue behaviour of sisal fibers. They have found that for all fibers, fatigue life exceeded 106 cycles at a loading rate below 50% UTS. Due to the nonlinear behaviour of natural composites, the evolution of stiffness during static and fatigue testing is an added complication in the analysis of damage propagation. Many studies have linked stiffness

Corresponding author at: Acoustics Laboratory of Maine University (LAUM), UMR CNRS 6613, Maine University, Av. O. Messiaen, 72085 Le Mans cedex 9, France. E-mail address: [email protected] (M. Haggui).

https://doi.org/10.1016/j.apacoust.2018.03.011 Received 29 June 2017; Received in revised form 20 February 2018; Accepted 10 March 2018 0003-682X/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Haggui, M., Applied Acoustics (2018), https://doi.org/10.1016/j.apacoust.2018.03.011

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2.2. Experimental set up

degradation to crack density [9,10]. Shah [11] has investigated the mechanical behaviour of bio-composite through quasi-static and cyclic fatigue testing. He has found that stiffness evolves in two phases in monotonic testing and three phases for cyclic tests. It was observed that the residual stiffness stabilized after 0.4% of applied strain in monotonic tests. However, for cyclic loading it stabilized after 1.2% of applied strain. Furthermore, the author has found that biaxial flax composites experience a 50% reduction in stiffness under failure, unlike unidirectional that reaches just10%. Characterizing the damage in composite by stiffness evaluation seems to be not enough to describe the different mechanisms that contribute to failure. Therefore, non-destructive testing should be applied not only to identify the presence of defects but also to quantify its effect on the overall structure. The acoustic emission study in real time associated to microscopic analyses could be an effective method to investigate mechanisms sources of damage in composite. Monti et al. [12,13] studied the damage mechanism for different configurations of flax/Elium composites subjected to static and dynamic testing. Microscopic analysis and AE processing show that matrix cracking and fiber-matrix debonding are the first registered defects. Then, fiber pulls out and fiber breakage accumulates to cause the final failure. This damage scenario was cited in a study realized by El Mahi et al. [14] in which deals with damage analysis by AE technique of a flax/epoxy eco-composite. Also Romhány et al. [15] found that AE processing could be used to identify the failure mode sequence of flax fiber composites during static test. Furthermore, they estimated the amplitude of AE events released by the flax fibers. The aim of this study is to investigate the mechanical behaviour of tow configurations of flax fiber/Elium composite subjected to static and fatigue loading. In first time, it deals with static tensile testing to evaluate mechanical properties and cyclic tensile tests to quantify the macroscopic damage. Second, fatigue tensile-tensile tests under failure were performed to characterize the lifetime performance of the material. AE technique was used to pick up the different damage mechanisms that occur under static and dynamic tests. During cyclic testing, energy dissipation and loss factor were evaluated from the hysteresis loop, they considered as representative parameters of damage.

Tensile and cyclic tensile tests were carried out with a tensile machine equipped with a 100 kN load cell. The strains in the tensile direction were measured by means of an extensometer. The tensile specimens have rectangular geometries (25 × 250 × 3 mm). All prepared unidirectional and cross-ply laminates [02/902]s configuration were tested under uniaxial loading, according to the standard test method ASTM D3039M (Fig. 1). The Young’s modulus E is determined by the slope of the linear portion of the σ-ε curve and the ultimate tensile strength of each sample is determined from the maximum load supported before failure. Longitudinal and transversal values were obtained respectively from UD-0° and UD-90° specimens. The tests were carried out at least 5 times for each type of specimen and the results were averaged. The same configurations of specimens were tested in cyclic tensiontension Fatigue according to ASTM D3479 standard. The tests were carried on at controlled load using a sinusoidal waveform with a 10 Hz frequency (Fig. 2). The temperature during the test was measured and ΔT° does not exceed 7 °C. Different level of loading was applied for each configuration. Moreover, the tests were followed by AE using two sensors with bandwidth range of 100 kHz to 1 MHz, which are held on the specimens

Grips Test specimen AE sensors

Extensometer

2. Materials and experimental set up 2.1. Materials The material considered in this study is a natural fiber composite. It is made of continuous unidirectional flax fibers, developed by the company LINEO, and an innovative liquid acrylic thermoplastic resin called Elium. This Elium resin produced by ARKEMA can be processed with techniques conventionally used for thermosetting resins. Furthermore, due to its thermoplastic nature, the Elium resin is postformable and potentially recyclable. Flax fibers were cut and then dried in an oven for about 1 h at 110 °C. This procedure is a good compromise between drying the fibers and preserving their mechanical properties [16]. The plies are then stacked in desired sequence on a plane and waxed mold. The fibers are inserted between tow peel plies and then coated by a micro-perforated film. The assembly is then placed in a waterproof cover, attached to the mold by suitable sealant tapes. The vacuum is achieved by a pipe inserted in a plug and connected to a spiral duct outlet. The arrival of resin is provided by an inlet pipe, allowing a front advanced resin. Before the infusion, the maximum vacuum is ensured for 5 h to allow degassing. The infusion is then performed at a 0.5 bar depression. Once the preform is fully impregnated, the resin inlet is stopped and the vacuum is maintained until the ambient temperature under the exothermic peak. The plate is then removed from the mold, and conditioned at ambient temperature.

Fig. 1. Experimental tensile test.

Load

Fmax

Fmin 0

T1

T2

Time

T3

Fig. 2. Profile of loading signal in cyclic test.

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and connected to an EPA (Euro Physical Acoustic) acquisition system. During the tests, the signals received by the sensors with a sampling frequency of 5 MHz are amplified using a 40 dB gain preamplifier, processed by the PCI card and converted into different AE parameters.

Table 1 The mechanical properties of unidirectional and crossply Flax fiber reinforced composite. Elastic properties

3. Results and discussion 3.1. Tensile test 3.1.1. Mechanical properties The typical stress-strain responses of unidirectional and cross-ply laminate samples are given by Fig. 3. All configuration curves exhibit a nonlinear trend and evolve in three consecutive stages. First one is linear until 0.1% of strain and refers to elastic and reversible mechanical behaviour of the bio-composite. The second stage is a nonlinear phase that could be associated to the creation and evolution of damage mechanisms [17,18]. However, in the third stage, a linear and relatively inelastic behaviour is observed. This may correspond to the intensification and development of damage prior to the total specimen failure. The mechanical properties of the unidirectional and cross-ply bio-composite are presented in Table 1. (EL, σLrupt), (ET, σTrupt) and E45 were respectively deduced from tensile tests performed on UD-0°, UD90° and UD-45° specimens. However, the strength σLTrup was determined from a test applied on a cross-ply composite CR-(0/90).

Stress (MPa)

200

Value

Components

Value

EL (GPa) ET (GPa) E45 (GPa) νLT

23 3.2 3.65 0.35

σLrupt (MPa) σTrupt (MPa) σLTrupt (MPa)

210 7.5 115

Rij (D & B ) =

150

0,2

0,4

0,6

0,8

1,0

1,2

1 k

n

∑ i=1

max ⎜⎛ i ⎝

d i + dj ⎞ ⎟ dij ⎠

(1)

where k is the number of classes, di and dj are respectively the average distance in the ith and jth class and dij the average distance between i and j classes. Results were reported by plotting amplitude and number of vector of hits versus time for different configurations (Figs. 5 and 6). Four classes were obtained for unidirectional 0° and cross ply laminates. However 3 classes are present in the UD-45° and UD-90° specimens. Class A includes the first events appearing in the chronology of the specimens. The signal distributions are relatively similar for the four types of specimens and the five temporal descriptors. The amplitude is in range of [40–50 dB]. Therefore, it can be associated with the mechanisms of matrix micro-cracking [12,27]. Class B also appears for all configurations at an amplitude between [45–60 dB]. It can be attributed to fiber/matrix debonding. The third class C is estimated as the fiber pull out. It represents the last event preceding failure for the UD45° and UD-90°. In this class, the acoustic waves reach amplitude from 65 to 80 dB. The last class D contains usually few events with a high energetic rate and amplitude up to 78 dB. The appearance of these events announces the rupture of the specimen. This class does not exist for the UD-90° and UD-45° specimens. Therefore, it seems consistent to attribute them to breaks in fiber and fiber bundles.

100

50

1,4

Strain (%)

(a) 140 120 100

Stress (MPa)

Components

3.1.2. Damage analysis In order to investigate the source of nonlinear behaviour and the different damage mechanisms within the studied materials, it is very useful to apply non-destructive technique during mechanical tests such as the AE. Generally, AE data can be inspected and analyzed by two ways: waveform-based analysis or parameter based analysis. The former is used in this study thanks to the high capabilities of acquisition components, while the latter is considered as traditional one. Fig. 4 illustrates a typical AE waveform and its parameters. To distinguish signal from noise, the acquisition threshold parameter must be determined before the start of each test using the Pencil Lead Breaking (PLB) technique [19]. After processing data, NOESIS software [20] was used to classify the acoustic events. Amplitude, duration, rise time, energy and numbers of counts to peak were considered as effective parameters for data clustering. The K-Mean algorithm [21] was used for the unsupervised pattern recognition [22–25]. The optimal number of class was obtained by having the minimal average value of Davies and Bouldin coefficient Rij(D&B) [26] which is given by the relation below:

250

0 0,0

Strength properties

80 60 40

3.2. Cyclic tensile test 20 0 0,0

0,2

0,4

0,6

0,8

1,0

1,2

3.2.1. Stress-strain response Cyclic tensile tests (charge-discharge) were carried out for tow configurations of the designed flax/Elium Composite in order to investigate and quantify the macroscopic damage [28]. The applied stress is amplified periodically until failure. The stress/strain response is given by Fig. 7. The stress strain curves have a shape of hysteresis loops. Its slope

1,4

Strain (%)

(b) Fig. 3. Composites stress/strain response in tensile test: (a) UD-0° and (b) [02/902]S.

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Fig. 4. Characteristic of AE signals.

10

100 Class A Class B Class C Class D Load

14

Class D

12

90

Amplitude (dB)

Class B Class C Load

80

10

70

8 6

60

Load (KN)

Amplitude (dB)

90

80

9 8 7 6 5

70

4 60

3

4 50

0

20

40

60

80

40

1.4

75

Load

1.2

70

100 120

140

0

160

1 Class A

65

0.8

60

0.6

55

0.4

50

0.2

45 25

80

80

Class A Class B Class C

1

20

60

(b)

70

40 15

40

(a)

Amplitude (dB)

75

20

time (s)

1.6

80

0

time (s)

85

Amplitude (dB)

1

0 100 120 140 160 180

Load (KN)

40

2

50

2

30

35

40

45

Load (KN)

16 Class A

0.9

Class B Class C Load

0.8 0.7

65

0.6

60

0.5 0.4

55

0.3 50

0.2

45

0 50

40 15

0.1 20

25

30

time (s)

time (s)

(c)

(d)

35

40

45

Fig. 5. Distribution of amplitude and Load versus time during static tensile tests: (a) UD 0°, (b) CR[02/902]s, (c) UD45° and (d) UD90°.

4

0

Load (KN)

100

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cumulative number of vector of hits

cumulative number of vector of hits

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Class A Class B Class C Class D

1800 1600 1400 1200 1000 800 600 400 200 0

0

20

40

60

80

100

120

140

500

0

20

40

60

80

100

(a)

(b) cumulative number of vector of hits

cumulative number of vector of hits

1000

time (s)

400 300 200 100

25

1500

time (s)

Class A Class B Class C

20

2000

0

500

0

2500

160

700 600

Class A Class B Class C Class D

3000

30

35

40

45

140

160

300 Class A Class B Class C

250 200 150 100 50 0

50

120

20

25

30

time (s)

time (s)

(c)

(d)

35

40

45

Fig. 6. Cumulative number of vector of hits versus time during static tensile tests: (a) UD 0°, (b) CR[02/902]s, (c) UD45° and (d) UD90°.

normalized residual stiffness Ed/E0 as a function of applied strain for the studied materials (Eq. (2)) [30].

decreases after each charge-discharge cycle until material failure. The area inside the hysteresis loops increases progressively, indicating the viscosity behaviour within the composite. In this case, the hysteresis loops areas in the cross-ply curves are wide rather than it for the UD-0°. Thus the dissipated energy during test is greater for the cross-ply laminate and the damage in it is reached quickly.

E Dmacro = 1−⎛ d ⎞ ⎝ E0 ⎠ ⎜

⎟

(2)

where E0 and Ed are respectively the Young’s modulus of virgin and damaged material. The damage during cyclic tensile test versus strain evolves differently through many stages for each configuration. For the UD-0° specimens, it takes place via three phases: An increase in a high rate in the first cycles until 0.3% of strain. The correlation of this finding with AE results, given in Fig. 8, makes it clear that matrix cracking and fibermatrix debonding are the major damage mechanisms present at this stage. Then the curve slope change until 1.2% of strain. Here, the creation of an additional mechanism is estimated, which could be the fiber pull out. The third phase in curve corresponds to the two last cycles. It is caused by the fiber breakage as it represented in the AE results. However for the cross-ply laminates the damage takes place in a high rate in the first cycles and then becomes slowly in the end of test, this is due to the accumulation of different mechanisms in the first stage such matrix micro-cracking, fiber-matrix debonding and fiber pull out then the fibers breakage begin in the three last cycles.

3.2.2. Damage analysis AE processing was applied during cyclic tensile tests with the same approach and with the same classification method developed in static tensile tests. The amplitude of events and the cumulative number of hits were registered and given as function of time respectively in Figs. 8 and 9. In the plots of amplitude versus time each cycle is given by a group of events indicating the charging phase followed by a vacant area containing few events corresponding to the discharge. This could be useful to distinguish different damage mechanisms in each cycle. The tow material configurations have the same chronological arise. However, in the plots of cumulative vector of hits (Fig. 9), it is clearly observed that the development of hits for all classes in cross-ply laminates is increasing by steps gradually unlike in the unidirectional composite in which the progress of hits is sharply. This may due to the high amount of flax fibers in the loading direction [29]. To evaluate the stiffness degradation of the composite, the curves presented in Fig. 10 describe the evolution of damage given by

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100

Class A Class B Class C Class D

Amplitude (dB)

90 80 70 60 50 40

0

500

1000

1500

2000

2500

time (s)

(a)

85 80

Class A Class B Class C Class D

Amplitude (dB)

75 70 65 60 55 50 45 40 200

400

600

800

1000 1200

1400 1600

1800

time (s) Fig. 7. Stress/strain response under cyclic tensile test: (a) UD 0° and (b) CR[02/902]s.

(b) Fig. 8. Distribution of amplitude versus time during cyclic tensile tests: (a) UD 0° and (b) CR[02/902]s.

3.3. Fatigue behaviour 3.3.1. Stiffness degradation In order to investigate the tension-tension fatigue behaviour of the flax fiber Elium composites, tests were performed under controlled load and with a loading ratio R (Eq. (3)) equal to 0.1 [31].

R=

σmin = 0.1 σmax

Fig. 11 shows the progress of the displacement (dmax/d0max), represented by E/E0, as function of the number of cycles to failure for the UD-0° and the cross-ply laminate [02/902]s using semi logarithmic scale. For the two configurations, it was observed that the variation of stiffness takes place through two stages. For the first one, a drastic reduction especially in the first 100 cycles is considered. For the second one, the decreasing of stiffness becomes more slowly. The fracture of the UD specimens corresponds to a drop of approximately 25% of the maximum displacement. However, for the cross-ply laminates the decreasing in stiffness reach about 30%. This reveals the influence of architecture of composites on the stiffness evolution, which could be explained by visco-plastic behaviour of flax fiber in the loading direction. The orientation of the fibers shows a great influence on the stiffness properties and the way that damage develops through time. Like in conventional composites; off-axis plies in flax composites present more matrix stressed regions that induce matrix cracking [31].

(3)

where σmax and σmin are respectively the maximum and minimum applied stress. Different levels (r) (Eq. (4)) of the ultimate tensile strength were applied for the two configurations of the materials: UD and cross-ply laminates. The fatigue tests were achieved until failure.

r=

σmax σLrupt

(4)

where σmax is the maximum applied stress and σLrupt is the maximum stress to failure. The evaluation of the composite stiffness degradation by the means of cyclic loading is considered as an effective method to investigate the damage progression. Through the experimental tests, the evolution of the maximum displacement dmax for each cycle was recorded. The degradation is represented in percent by referring the maximum displacement at each cycle to the maximum displacement at first cycle.

3.3.2. Acoustic emission analysis Due to the nonlinear behaviour of the natural composite, a detailed analysis by AE could be a good solution to identify the type of damage mechanism inside the material during fatigue tests.

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0,25

10000

Class A Class B Class C Class D

0,20

8000

Dmacro

cumulative number of vector of hits

12000

6000

0,15 Exp results

0,10

Curve fit

4000 0,05 2000 0,00 0,0

0 0

500

1000

1500

2000

0,2

0,4

2500

0,6

time (s)

1,2

1,4

0,25 Class A Class B Class C Class D

0,20

3500 3000

Dmacro

cumulative number of vector of hits

4000

1,0

(a)

(a) 4500

0,8

strain (%)

2500

0,15

0,10

2000

Exp results Curve fit

1500

0,05

1000 500 0 0

0,00 0,0 200

400

600

800

0,2

0,4

0,6

0,8

1,0

1,2

1,4

strain (%)

1000 1200 1400 1600 1800

(b)

time (s)

(b)

Fig. 10. Macroscopic damage versus strain during cyclic tensile test: (a) UD 0° and (b) CR [02/902]s.

Fig. 9. Cumulative number of vector of hits versus time during cyclic tensile tests: (a) UD 0° and (b) CR[02/902]s.

Section 3.3.1. Each damage mechanism is characterized by an amplitude range. Matrix cracking (class A) occurs at 45–59 dB, fiber- matrix debonding (class B) in range of 45–59 dB, fiber pull out (class C) at 65–80 dB and fiber breakage goes above 78 dB. Fig. 13 allows us to know about the difference in amount between damage mechanisms. It is clear that the hits becoming from matrix cracking and fiber-matrix debonding are widely greater than hits collected from the other mechanisms. Upon cyclic loading, fiber matrix adhesion or weak interfacial properties is considered as an important factor that contributes to damage especially for the cross-ply configuration since the fiber/matrix debonding and frictional sliding develop upon crack extension however, fibers remain relatively intact and propagate the crack [7]. Unlike cross-ply laminates, for the unidirectional specimens the fiber breakages do not appear from the beginning and the fiber–matrix debonding is more important. The acoustic signal amplitude varies with the different modes of failure: AE amplitude range from 45 to 57 dB corresponds to matrix micro-cracking, 48–68 dB to interfacial debonding, 62–78 dB to pull-out and up to 77 dB to fiber fracture. All results in unidirectional loading have shown the presence of four classes in AE data, which define the damage mechanisms. However, in reversible loading damage develops throw several mechanisms such as fiber buckling and delamination. Generally, in compressive fatigue

Like in static tests, AE processing were performed with application of unsupervised classification methodology detailed before. The optimal number of class was obtained by the K-means method and with the minimum Rij(D&B) factor. The amplitude threshold was 45 dB for the unidirectional and cross ply specimens. The results are presented by the curves of amplitude versus number of cycles and Number of vector of hits versus number of cycles. Tests were performed at a loading ratio about 65% of ultimate tensile strength. Fig. 12 shows four classes for both configurations. Referring to the identification developed before, results allow perceiving a first class A corresponding to matrix micro-cracking with a high number of events, a second class B which can be attributed to fiber/matrix debonding then a third one C estimated as fiber pull out and the fourth class characterized by the highest amplitude and corresponding to fiber breakage. For the cross ply laminates, it is clearly observed in Fig. 12b that all the damage mechanisms appear from the first stage and with a high rate of acoustic activity. This activity corresponds especially to the initiation and the multiplication of micro-cracks. Then acoustic activity decrease during the test until the failure stage which is caused by a high amount of fiber breakage and pull out, unlike in static tensile test, the fiber breakage appear from the first stage. This could explain the degradation in stiffness presented through two phases previously described in

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85 r=0.5 r=0.6 r=0.66 r=0.73 r=0.8 r=0.86

95

E/E0 (%)

A B C D

75

90 85 80

70 65 60 55

75 70

Class Class Class Class

80

Amplitude (dB)

100

50 0

10

1

10

2

10

3

10

4

10

5

10

6

10

45

Number of cycles

2

(a)

4

6

8

Number of cycles

10

12 4 x 10

(a) 100

r=0.95 r=0.89 r=0.83 r=0.78 r=0.72 r=0.66

90

Class Class Class Class

A B C D

85

Amplitude (dB)

E/E0 (%)

90

95

80

70

80 75 70 65 60

60

0

10

1

10

2

10

3

10

4

10

5

10

55

6

10

50

Number of cycles

45

(b)

1

2

3

4

5

6

Number of cycles

Fig. 11. Stiffness degradation: (a) UD 0° and (b) CR[02/902]s.

7 4 x 10

(b) Fig. 12. Distribution of amplitude versus time under fatigue tests: (a) UD 0° and (b) CR [02/902]s,

tests, clustering of the AE data leads to distinguish number of classes less than it in case of unidirectional loading. Hua Su [32] has coupled AE technique with axial compression test applied on E-glass/Epoxy. He has found that AE events can be divided into three stages. First matrix yield band forms, then fiber buckling occurs and finally fiber breaks without inter-fiber matrix crack.

create delamination between adjacent fiber-cells, then micro-cracks attempt the cell wall layers resulting the weakness of link between all the component of the structural system.

3.3.3. Hysteresis loop To study the loading displacement behaviour during tension-tension fatigue tests, 100 experimental points were reported in real time for each loading-unloading cycle. Fig. 14 shows the plots at given cycles: 10, 102, 103, 104, 5.104 and 105 at a loading ratio about 65% of ultimate tensile strength. All represented curves have the form of a hysteresis loop, which are translated along the displacement axis to show its attitude. It appears that the hysteresis loop size is decreasing for the two configurations. The first loops are wider than the last ones, which are becoming slimmer. This may refers to high damage activity during the first stage of fatigue life, especially at the fiber-matrix interface. In the second stage, generally density of matrix micro-cracks goes into a saturation state [33]. The flax fibers also contribute progressively to the damage sequences due to their complex microstructure [34–36]. In this context, Silva et al. [8] have shown, by analyzing the fatigue behaviour of sisal fiber, that the damage kinetics in plant fibers begins by changing in the fiber-cells shape from circular to an oval one. Also, the created micro-cracks in the secondary wall evolve to the middle lamellae and

3.3.4. Energy dissipation and loss factor The area enclosed by the loop curve is generally used to evaluate the dissipated energy in the material in one cycle. This loop expands with the number of strain-stress cycle and could be a useful means to know about the material state. Two types of energy could be reported from the hysteresis loop. The dissipated energy Ed which corresponds to the area inside the loop and the maximum potential energy Ep given by the loading part [37]. A simple trapezoidal summation was used to evaluate numerically the dissipated and potential energies Fig. 15. For each cycle, Ep and Ed are given by [37]:

Ed =

Ep =

n

1 2

∑

1 2

∑

(di + 1−di ){[f (di + 1) + f (di )]−[g (di + 1) + g (di )]}

i=1

(5)

n i=1

(di + 1−di )[f (di + 1) + f (di )]

(6)

The energy dissipated versus the number of cycles for two configurations at a maximal loading equal to 65% of the static strength is 8

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10000

5

Class A Class B Class C Class D

3.5 3 2.5 2 1.5

10 2

0,8

1,0

1,2

10 3

10 4 5.10 4

10 5

6000

4000

2000

0 0,0

0.5

0,2

0,4

0,6

1,4

1,6

1,8

2,0

Displacement (mm) 2

0

6

4

10

8

(a)

12 x 10

Number of cycles

4

(a) x 10

2.5

7000

5

Class A Class B Class C Class D

2

1

6000

10

10

2

10

3

10

4

5.10

4 10 5

5000

Load (N)

cumulative number of vector of hits

10

1

0

1.5

4000 3000 2000

1

1000 0 0,0

0.5

0,2

0,4

0,6

0,8

1,0

1,2

1,4

Displacement (mm) 0

(b) 0

1

2

3

4

5

Number of cycles

6

7 x 10

Fig. 14. Hysteresis curves for different cycles: (a) UD 0° and (b) CR[02/902]s.

4

(b)

tested configuration. This allows concluding that this parameter could be an appropriate indicator of damage kinetics and accumulation in fatigue tests.

Fig. 13. Distribution of cumulative number of hits versus time under fatigue tensile tests (a) UD 0° and (b) CR[02/902]s.

3.3.5. S–N curves As illustrated in Fig. 18, after plotting the Wohler S-N curves for cross-ply and unidirectional composites, linear-law regression equation could be a representative means to describe the life time fatigue for each material.

given in Fig. 16. The plots show a similar tendency for all configurations. A rapid drop in the first cycles until 2 × 104 cycles than a gradual decrease is registered. The unidirectional composite dissipate more energy than the cross-ply one. This could be explained by the high energy needed during testing it. A measure of temperature carried on during fatigue tests for both materials has shown that ΔT° for the UD-0° is greater two time than it for the cross-ply. This could reveal the coherence of experimental results presented in Fig. 16. The evaluation of the two types of energy allows evaluating the loss factor, which is presented as function of the number of cycles. It is given by [37]:

η=

1

8000

Load (N)

cumulative number of vector of hits

x 10

σmax = b−a. log(Nr )

where σmax is the maximum applied stress, Nr is the number of cycles to failure. Eq. (8) yields a linear S-N curve on a log scale. It shows that under tension–tension load regime, the unidirectional composite along the loading direction has high fatigue strength capacities comparing it to the cross-ply laminates especially at a high rate of loading. This could be explained by the high ultimate tensile strength. However, decreasing in maximal stress rate the UD specimens’ life times becomes more sensitive. For about 50% of the ultimate tensile strength (UTS) the two configurations get failure after a comparable number of cycles. Hence, the fiber orientation has a great impact on the degradation rate registered during fatigue testing. It has been reported that for UD0°, a steeply decreasing slope reveals a strong influence of fatigue

Ep 2πEd

(8)

(7)

Fig. 17 presents the loss factor as a function of the number of cycles. It is shown that the loss factor gets a substantially drop in the first cycles then it slightly decreases. However, it stabilizes before the final failure. This could be referred to the degradability in the state of the composite during the fatigue test. This seems appropriate to the stiffness degradation evaluated in Section 3.3.2. The results are similar for both

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Fig. 15. Maximum potential energy (Ep) and dissipated energy (Ed).

700 UD 0°

600

[02/902]S

Fig. 18. S-N Curves.

Ed (mJ)

500

characterization, was applied to the test protocol. The results allow distinguishing four classes of acoustic events during testing to failure. Each one corresponds to a damage mechanism. Namely, the matrix micro-cracking, the fiber-matrix debonding, the fiber pull out and finally the fiber breakage. Composite configuration has shown a great impact on the kinetics and density of damage at static and cyclic loading. Monitoring the stiffness variation and dissipated energy, during fatigue test, were considered as a compromising means to describe the state of the specimen. It was clearly observed that unidirectional composite dissipates more energy than the cross ply one. However, they have a comparable attitude in stiffness degradation. In order to evaluate the materials lifetime, the S-N curve were presented. The results have shown that flax/Elium composites exhibit good cyclic performance especially in case of unidirectional configuration. This study confirms the growing interest of researchers towards plant fiber composites.

400 300 200 100

0

2,0x10

4

4,0x10

4

6,0x10

4

8,0x10

4

5

10

5

1,2x10

Number of cycles Fig. 16. Dissipated energy versus number of cycles.

Acknowledgments The authors would like to thank Pierre GERARD from ARKEMA for the supply and help with the processing of the Elium resin. References [1] Väisänen T, Das O, Tomppo L. A review on new bio-based constituents for natural fibre-polymer composites. J Cleaner Prod 2017;149(15):582–96. [2] Fatima S, Mohanty AR. Acoustical and fire-retardant properties of jute composite materials. Appl Acoust 2011;72:108–14. [3] Zini E, Scandola M. Green composites: an overview. Polym Compos 2011;32:1905–15. [4] Dunne R, Desai D, Sadiku R. A review of natural fibres, their sustainability and automotive applications. J Reinf Plast Compos 2016;35:1–10. [5] Liang S, Gning PB, Guillaumat L. A comparative study of fatigue behaviour of flax/ epoxy and glass/epoxy composites. Compos Sci Technol 2012;72(5):535–43. [6] Shah DU, Schubel PJ, Clifford MJ, Licence P. Fatigue life evaluation of aligned plant fibre composites through S-N curves and constant-life diagrams. Compos Sci Technol 2013;74:139–49. [7] Gassan J. A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites. Compos A Appl Sci Manuf 2002;33:369–74. [8] Silva FA, Chawla N, Filho RDT. An experimental investigation of the fatigue behaviour of sisal fibres. Mater Sci Eng A 2009;516:90–5. [9] Highsmith A, Reifsnider KL. Stiffness-reduction mechanisms in composite laminates. In: Reifsnider K, editor. Damage in composite materials. Baltimore, Maryland, USA: ASTM; 1982. [10] Jendli Z, Fitoussi J, Meraghni F, Baptiste D. Micromechanical analysis of strain rate effect on damage evolution in Sheet Moulding Compound composites. Compos A Appl Sci Manuf 2004;35(7–8):779–85. [11] Shah DU. Damage in biocomposites: stiffness evolution of aligned plant fibre composites during monotonic and cyclic fatigue loading. Composites: Part A 2016;83:160–8. [12] Monti A, El Mahi A, Jendli Z, Guillaumat L. Mechanical behaviour and damage mechanisms analysis of a flax-fibre reinforced composite by acoustic emission. Compos A Appl Sci Manuf 2016;90:100–10.

Fig. 17. Loss factor versus number of cycles.

damage occurring at the fiber level (fiber breakage, fiber/bundle debonding, etc.), whereas a gradual slope for the cross-ply samples reflects a matrix dominated damage mechanism [23]. The experimental results show that the endurance limit of the UD composite clearly exceeds the ones of cross-ply laminate. 4. Conclusion The failure and damage behaviour of flax fiber reinforced Elium matrix composite have been developed in the present study. Static and cyclic tensile tests were carried out for different prepared configurations. The flax fiber composite has shown a nonlinear response, which induces a complexity in understanding the damage behaviour of the material. Thus, AE technique, considered as a significant means of 10

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