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Degradation of bending properties of flax fiber reinforced polymer after natural aging and accelerated aging
Construction and Building Materials 240 (2020) 117909
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Degradation of bending properties of flax fiber reinforced polymer after natural aging and accelerated aging Xiaomeng Wang a, Michal Petru˚ a,⇑ a
Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, Studentska 2, Liberec 461 17, Czech Republic
h i g h l i g h t s The bending properties of FFRP under natural aging conditions are measured. The relationship between natural aging and accelerated aging is studied. The residual mechanical property model may be applied in life prediction of FFRP.
a r t i c l e
i n f o
Article history: Received 18 April 2019 Received in revised form 9 November 2019 Accepted 18 December 2019
Keywords: Flax fiber Composite Bending performance Natural aging Accelerated aging
a b s t r a c t In this paper, the effect of natural aging and accelerated aging on the bending properties of flax fiber reinforced polymer (FFRP) is studied. Test results show that the flexural strength decreases by 11.2%, 14.9%, 15.5%, and the flexural modulus decreases by 21.3%, 32.3%, 35.8%, after 60 days, 120 days and 180 days of natural aging, respectively. Then accelerated test is carried out to predict the long-term performance of FFRP. The results show that short-term exposure test (just considering the temperature and humidity) cannot achieve the same effect on mechanical property degradation as natural aging. A modified residual mechanical property model of FFRP is established according to the test results. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Eco-friendly fiber reinforced polymer (FRP) has gained much attention recently because of its low density, low cost and abundant sources [1–4]. Flax FRP is a commonly used eco-friendly FRP in various fields, such as automobile, packaging, and building construction [5,6]. However, the porous structure of flax fiber leads to high water absorbability [7–9]. Assarar et al. [10] and Duigou et al. [11,12] reported that the weakening of fiber/matrix interface is the main damage mechanism caused by water aging of the FFRP. Li et al. [13] found that cracks of the resin matrix around the fiber after water aging is caused by the swelling volume difference between the flax fiber and matrix. Water leads to leaching of pectin, hemicellulose and weakly crystallized fibers in the flax fibers, which also results in delamination of the fiber/matrix interface. Regazzi et al. [14] simulated the water aging process of FFRP. They noted that the performance degradation caused by swelling is
⇑ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Petru˚). https://doi.org/10.1016/j.conbuildmat.2019.117909 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
reversible after desorption, but mechanical property degradation caused by interface delamination is permanent and irreversible. They proposed that the elastic modulus of FFRP can be predicted according to the water content. Scida et al. [15] reported that hygrothermal aging does not induce other mechanisms of damage but only accelerates the process of degradation, particularly concerning the flax fibers, helped by the plasticisation of the bonds and the reorientation of the microfibrils. Durability is one of the key issues that should be considered in engineering applications of FFRP. Hristozov et al. [16] investigated the durability of FFRP soaked in water, salt water and alkaline solution. The results showed that the tensile strength of flax decreased significantly with the increase of temperature and exposure time, and the effect of salt and alkali solution on the mechanical properties of FFRP was greater than that of water. Yan et al. [17,18] investigated the effects of UV, water, seawater and alkali solution on the tensile and flexural properties of FFRP. Discoloration of FFRP was observed for aging samples. Degradation of fiber/matrix interface is one of the main reasons for the decrease of tensile properties of FFRP after aging. Yan et al. [17,18] suggested that fiber surface treatment and HFRP (hybrid fiber reinforced polymer with flax
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2
fiber and other fibers, such as glass fiber or carbon fiber) can help to improve its durability. Hallonet et al. [19] compared the performance of FFRP and GFRP after aging. After aging, the porosity of FFRP is slightly higher than that of GFRP, and the increase of porosity is mainly concentrated in flax fiber bundles. After 250 aging cycles, the modulus of FFRP returns to 85% or even 100% of the initial value. However, the modulus of GFRP remains stable. Mak et al. [20] studied the influence of manufacturing methods (wet layup (WL) and vacuum bag molding (VB)) on the mechanical properties of FFRP. The results show that the strength and modulus of VB samples are larger than that of WL samples before aging. However, VB samples demonstrated a higher susceptibility to environmental degradation than WL samples. Most of the aging tests focus on the degradation of tensile properties of FFRP, researches on bending properties of FFRP is limited. Structures made by FFRP may only bear tensile stress, but also bear bending stress (such as furniture panel, car roof and frame) [21–24]. Therefore, the bending test carried out in this paper may help to increase the durability database of FFRP. 1.1. Life prediction models of FRP Mak et al. [20] proposed a linearized model of the Arrhenius equation to predict the long-term property of FFRP. The residual mechanical property f ðpÞ at time t is
f ðpÞ ¼ Kt
ð1Þ n
where K is the reaction rate, and K ¼ A0 eRT . A0 is material constant, n is activation energy, R is the gas constant, T is the temperature (Kelvin). A life prediction model can be derived by taking the logarithm of both sides of Eq. (1)
lnt ¼ ln½f ðpÞ=A0 þ
E ¼ a þ bT 1 RT
ð2Þ
When applying this model to composites, the following assumptions must be made: (1) Degradation of material properties is mainly caused by a chemical degradation mode; (2) High temperature does not change the degradation mode of the material; (3) FRP is subjected to a water environment [20,25]. Wiederhorn et al. [26] reported that the mechanical property degradation of composites depends on two factors. One is the interaction between composites and the external environment, the other is the crack propagation resistance of the material. The degradation rate of composites under a hydrothermal environment is defined as
v ¼ a/f e
n RT
brk
e RT
ð3Þ
where / is relative humidity, f is the interaction factor, rk is the stress factor, a and b are material parameters. In this model, the influence of temperature, humidity and stress on service life of the material are assumed to be independent. Bulmanis et al. [27] proposed a residual strength model of fiber reinforced composite after natural aging as
S ¼ S0 þ gð1 ekt Þ blnð1 þ utÞ
ð4Þ
where S is the strength at time t, t is the aging time, S0 is the initial strength, g and b are material parameters, k and u are parameters of the material and the ambient medium. Research [28–30] shows that this model agrees well with the aging law of composites. 1.2. The relationship between natural aging and accelerated aging Natural aging tests can directly reflect the changes of the mechanical properties of FFRP in a service environment. However,
most research [15,31–33] on the durability of FFRP focusses on accelerated aging tests due to their convenience and time saving. Using both the natural aging and accelerated aging approaches to study the changes in mechanical properties of FFRP has been rarely reported up to now. Accelerated aging tests are useful to predict the long-term performance of FFRP only if such tests can be correlated with natural aging. Therefore, it is important to find the conversion relationship between accelerated aging and natural aging. According to the Manual of Composite Structural Design [34], the acceleration factor kt is
kt ¼
t1 eC=ðT 2 /2 Þ ¼ t2 eC=ðT 1 /1 Þ
ð5Þ
where t 1 is the natural aging time, t 2 is the accelerated aging time, T 1 and /1 are the temperature and relative humidity under natural aging, T 2 and /2 are the temperature and relative humidity under accelerated aging, C is an experimental coefficient, such that whenT 2 60 , C = 46.1, when T 2 ＞60 , C = 81.5 [34]. Another acceleration coefficient proposed by the Manual of Composite Structural Design [34] is:
1=kt ¼
t 2 D1 ¼ t 1 D2
D ¼ D0 expðC=TÞ
ð6Þ ð7Þ
In this model, the quotient of two diffusion coefficients is taken as the acceleration coefficient, where D0 and C are the diffusionrelated constants at room temperature and they can be obtained according to Manual of Composite Structural Design [34]. Shin et al. [35] proposed an acceleration factor a to predict the life of composites as
a¼
tn ta
ð8Þ
where tn and t a are the natural aging time and accelerated aging time of FRP under the same damage degradation condition. The acceleration factor a can be used to predict the long-term performance of the composite and achieve the same effect on mechanical property degradation through a short-term exposure test. In this study, natural aging tests of FFRP were carried out first, and a residual mechanical property model was proposed according to the test results. Then accelerated aging tests were carried out to study the correlation between accelerated aging and natural aging. The accelerated test results were compared with natural aging test results. 2. Material and test method The unidirectional flax fabric is shown in Fig. 1(a). The elastic modulus and tensile strength of flax fiber are 28.5 GPa and 429.6 MPa, respectively. The density of flax fiber is 1.4 g/cm3. The elastic modulus and tensile strength of epoxy resin are 3.5 GPa and 73.4 MPa, respectively. The density of epoxy is 1.4 g/cm3. FFRP was laminated using the wet hand-layup method as shown in Fig. 1. The epoxy impregnated flax fabrics were placed in the same orientation and then placed in compression between two steel plates. After curing for two weeks, FFRP plates were cut into a rectangular form by using a diamond saw blade. The FFRP specimen is composed of approximately 30% fiber in volume. The dimensions of the FFRP specimens (Fig. 1 (d)) are 50.0 mm 10.0 mm 1.4 mm, according to the dimension of the test device and Chinese standards GB/T 1449-2005 [36]. Because of the size effect, the strength of larger specimens is usually less than smaller specimens due to the increased probability of defects. The relationship between bending strength of FRP with different sizes can be obtained by the formulas proposed by Nader et al. [37]. The aging tests were carried out according to Chinese standards GB/T 25732008 [38]. The natural aging specimens were placed on a frame facing south in an open space, away from obstacles. The horizontal inclination of the frame was
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
3
Fig. 1. Fabrication of FFRP specimen: (a) Unidirectional flax fabric; (b) Hand lay-up with epoxy; (c) FFRP plate after curing; (d) cut into desired shape.
P Specimen
40mm
10mm
10mm
Specimen
(a) Test setup
(b) Loading schematic diagram Fig. 2. Bending test of FFRP.
45°. The hygrothermal aging specimens were soaked in distilled water at 60 °C in an environmental test chamber. The specimens were periodically taken out and tested under three-point loading to assess the flexural properties (Fig. 2). All experimental values were obtained by an average value of five specimens. The loading rate was 2 mm/min. Load and deflection were recorded by the test device. The flexure strength r and flexure modulus E were calculated as
r¼ E¼
3P l 2w h
ð9Þ
2
DP l
3
ð10Þ
3
4w h Df
Table 1 Average temperature and humidity of each natural aging period. Period Temperature/°C Humidity/%
Average value Standard deviation Average value Standard deviation
Period Temperature/°C Humidity/%
Average value Standard deviation Average value Standard deviation
1
2
3
4
5
6
7
8
9
0.8 2.3 60.5 5.6
6.0 2.4 66.7 13.3
9.0 2.1 65.6 14.7
10.1 4.4 70.1 12.4
10.9 4.8 82.6 5.7
18.8 1.9 63.2 3.8
16.7 5.2 67.3 13.0
17.8 4.0 64.1 13.1
20.0 3.0 81.0 8.1
10
11
12
13
14
15
16
17
18
19.8 1.4 75.6 11.1
24.8 3.8 83.7 7.9
22.4 2.1 72.3 13.5
24.0 1.8 72.8 12.1
26.7 1.7 66.6 8.2
27.6 3.0 73.5 10.1
28.1 1.6 84.7 6.9
30.8 1.9 75.8 6.6
31.1 2.5 79.6 8.3
Table 2 Bending properties of FFRP after natural aging. Bending properties
Aging time/day
0
60
120
180
Bending modulus/GPa
Average value Standard deviation
12.1 0.5
9.5 1.2
8.2 1.6
7.7 1.0
Bending strength/MPa
Average value Standard deviation
182.1 3.9
161.7 7.5
155.0 13.5
153.9 10.2
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
4 Table 3 Acceleration factor kt . Period
1
2 76
3
1.5 10
8
4 5
5 4
6 3
7 2
8 2
9 2
kt
5.6 10
2.5 10
2.6 10
2.2 10
2.5 10
3.9 10
Period
10
11
12
13
14
15
16
3.3 10 17
18
kt
58.6
12.8
40.7
26.7
24.7
13.9
7.8
8.3
6.8
39.7
Table 4 Accelerated aging time corresponding to each natural aging period. 1
2
3
4
5
6
7
8
9
Time/h
4.3 1075
1.6 106
9.6 104
9.2 103
0.1
1.0
0.6
0.7
6.1
Period
10
11
12
13
14
15
16
17
18
Time/h
4.1
18.7
5.9
9.0
9.8
17.3
30.4
28.9
35.1
Flexural strength / MPa
Period
190
Natural aging
180
accelerated aging
170
3. Results and discussion
160
3.1. Natural aging test
150 140 0
30
60
90
120
150
180
210
Aging time /day (a) Flexural strength 13
Flexural modulus / GPa
where P is the ultimate load, l is the span, w is the width of the specimen, h is the thickness of the specimen. DP is the load increment, Df is the increment of midspan deflection. As the flexure modulus decreases slightly with the increase of load, the initial flexure modulus is adopted.
Natural aging
12
accelerated aging
11 10 9 8 7 6 5 0
30
60
90
120
150
180
210
Aging time /day
r ¼ 182:1 þ 10:5ð1 e2:710
(b) Flexural modulus Fig. 3. Comparation of bending properties of FFRP under natural aging and hygrothermal aging.
Aging time
Natural aging tests were carried out in Nanjing from February 1st, 2018 to July 30th, 2018. The daily temperature and humidity during the natural aging test were recorded. Every ten days were set as an aging period. The experiment lasted 180 days and was divided into 18 aging periods. The average temperature and humidity of each period are shown in Table 1. The bending properties of FFRP after natural aging are shown in Table 2. For the value of p < 0.05, a statistically significant decrease in mechanical property was observed at 60 days of aging, but not for aging specimen after 60 days. Compared with the reference group, the average flexural strength decreases by 11.2%, 14.9%, 15.5%, and the average flexural modulus decreases by 21.3%, 32.3%, 35.8%, after 60 days, 120 days and 180 days of natural aging, respectively. By fitting the test results with Eq. (4) using the Levenberg-Marquardt and Universal Global Optimization algorithms [39,40], the residual strength r and flexure modulus E of FFRP are obtained as - 3
t
Þ 10:1lnð1 þ 0:14tÞ
3 E ¼ 12:1 þ 7:9 1 e1:910 t 4:4lnð1 þ 0:02tÞ
0 days
60 days
120 days
180 days
0h
1.1 h
37.2 h
167.5 h
Natural aging Aging time Accelerated aging Fig. 4. Comparation of the effect of aging on FFRP specimens.
ð11Þ ð12Þ
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
(ten days) can be obtained as t2 ¼ tk1t , where t2 is the accelerated
3.2. Accelerated aging test Accelerated aging test was carried out at 60 °C with a relative humidity of 100%. To determine the correlation of accelerated aging test to natural aging test, the acceleration factor C=ðT /
Þ
kt ¼ eC=ðT 21 /21 Þ in Eq. (5) is calculated and listed in Table 3. Then the e
accelerated aging time corresponding to each natural aging period
Fiberfracture
(a) Reference specimen Fiber pull out
5
Gap between fiber and matrix
Matrix crack
aging time, t1 is the natural aging time. The accelerated aging time corresponding to each natural aging period is shown in Table 4. The accelerated aging time corresponding to 60 days (6 periods), 120 days (12 periods), and 180 days (18 periods) of natural aging are 1.1 h, 37.2 h, and 167.9 h, respectively. The comparisons between the flexural strength and flexural modulus of FFRP under natural aging and corresponding accelerated aging are shown in Fig. 3. For the values of p < 0.05, a statistically significant decrease in mechanical property was not observed at 1.08 h of accelerated aging, but after 1.08 h, a statistically significant decrease was observed. The color of the specimens has changed obviously after aging, and the boundary between the fiber and matrix is more and more obvious, as shown in Fig. 4. Muasher et al. [41] reported that color fading after aging is caused by oxidation of lignin. As shown in Fig. 5, the failure mode of the reference specimen is mainly fiber fracture, and the bond between the flax fiber and epoxy matrix is firm. However, after natural aging and hygrothermal aging, obvious gap was observed between the fiber and matrix, which indicates the degradation of fiber/matrix interface. Compared with the reference specimen, flax fibers were pulled out more frequently after aging. The surface of the pulled fibers was smooth, and there were less matrix impurities around, which was also evidence of poor fiber/matrix adhesion. SEM studies confirmed the degradation of fiber/matrix interface after both natural aging and hygrothermal aging, which affects the mechanical properties of the FFRP. The average bending strength of FFRP under accelerated aging is 20.3%, 16.7%, and 13.5% higher than that of 60, 120, 180 days of natural aging. Accelerated aging test cannot achieve the same effect on mechanical property degradation as natural aging. It mainly because only two factors (temperature and humidity) are considered in the accelerated test, but the specimens under natural aging were also influenced by ultraviolet rays, rainwater, oxygen and ozone, microorganisms, et al. Besides, the aging mechanism of accelerated aging and natural aging materials is different. The degradation mechanism of FRP caused by freeze-thaw cycling under natural aging is different from that caused by hydrothermal aging in the lab. In Eq. (4), the influence of various environmental factors on material property degradation of FRP is considered as a combination. In order to reflect various factors in the aging environment, an improved formula is presented as
X S ¼ S0 þ g 1 ekT ðxi Þ bi lnð1 þ ui T ðxi ÞÞ
(b) Nature aging specimen after 180 days of exposure Fiber pull out
Gap between fiber and matrix
where xi is the ith environmental aging factor, T ðxi Þ is the ith equivalent aging time of xi . T ðxi Þ ¼ Xv t, where v is the severity index of xi , X is the benchmark index, t is the aging time. For the accelerated aging test of FFRP at a constant temperature, v is the moisture absorption, X is the saturated moisture content. T ðxÞ is the equivalent moisture absorption time. The moisture content of FFRP under accelerated aging is listed in Table 5. By fitting the test results with Eq. (13), the residual strength r and flexure modulus E of FFRP under accelerated aging are obtained as
r ¼ 182:1 þ 6:4 1 e5:610 E ¼ 12:1 þ 4:1 1 e5:710
(c) Hygrothermal aging specimen after 167.9 h of exposure Fig. 5. SEM image of fracture surface of FFRP.
ð13Þ
- 2
- 2
T ðxÞ
T ðxÞ
4:1lnð1 þ 4:3T ðxÞÞ
1:4lnð1 þ 4:6T ðxÞÞ
ð14Þ ð15Þ
Table 5 Moisture content of FFRP under accelerated aging. Accelerated aging time/h Moisture content/%
0h 0.0
1.1 h 1.4
37.2 h 7.2
167.5 h 7.6
384.0 h 7.8
6
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
4. Conclusion The experimental study carried out in this paper aimed at increasing the durability database and developing a better understanding of the effect of natural aging and accelerated aging on the mechanical behavior of flax fiber reinforced composite. After 60 days, 120 days and 180 days of natural aging, the flexural strength of FFRP decreases by 11.2%, 14.9%, 15.5%, and the flexural modulus decreases by 21.3%, 32.3%, 35.8%, respectively. The SEM images of the fracture surface of FFRP show an obvious change of failure morphology after aging. The failure mode of the reference specimen is mainly fiber fracture. After natural aging and hygrothermal aging, fiber pull-out was more frequently, and a gap between the fiber and matrix was observed, which proves that aging had an adverse effect on the fiber/matrix interface. Although the short-term accelerated test was tried to simulate long-term aging, the results show that accelerated aging and natural aging have different effects on the degradation properties of FFRP. The main reason is that only limited aging factors are considered in accelerated aging, while natural aging is caused by a combination of various environmental factors. In addition, the mechanisms of natural aging and accelerated aging are different. Therefore, a modified residual mechanical property model of FFRP considering independent environmental factor is proposed. Although this model is established for FFRP under accelerated aging, this model may have potential applications in life predictions of other FRPs. Author contributions Xiaomeng Wang performed the experiments and wrote the manuscript supported by Michal Petru. 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 paper. Acknowledgements The result was obtained through the financial support of the Ministry of Education, Youth and Sports of the Czech Republic and the European Union (European Structural and Investment Funds – Operational Programme Research, Development and Education) in the frames of the project ‘‘Modular platform for autonomous chassis of specialized electric vehicles for freight and equipment transportation”, Reg. No. CZ.02.1.01/0.0/0.0/16_ 025/0007293. References [1] D.B. Dittenber, H.V.S. Gangarao, Critical review of recent publications on use of natural composites in infrastructure, Compos. A Appl. Sci. Manuf. 43 (8) (2012) 1419–1429. [2] A. Shahzad, Hemp fiber and its composites – a review, J. Compos. Mater. 46 (8) (2012) 973–986. [3] H. Ventura, J. Claramunt, M.A. Rodríguez-Pérez, M. Ardanuy, Effects of hydrothermal aging on the water uptake and tensile properties of PHB/flax fabric biocomposites, Polym. Degrad. Stab. 142 (2017) 129–138. [4] M. Berges, R. Léger, V. Placet, V. Person, S. Corn, X. Gabrion, J. Rousseau, E. Ramasso, P. Ienny, S. Fontaine, Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates, Compos. A Appl. Sci. Manuf. 88 (2016) 165–177. [5] K.L. Pickering, M.G.A. Efendy, T.M. Le, A review of recent developments in natural fibre composites and their mechanical performance, J. Compos. Part A 83 (2016) 98–112. [6] L. Yan, C. Nawawi, J. Krishnan, Flax fibre and its composites – a review, J. Compos. Part B 56 (1) (2014) 296–317.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Degradation of bending properties of flax fiber reinforced polymer after natural aging and accelerated aging Xiaomeng Wang a, Michal Petru˚ a,⇑ a
Institute for Nanomaterials, Advanced Technologies and Innovation, Technical University of Liberec, Studentska 2, Liberec 461 17, Czech Republic
h i g h l i g h t s The bending properties of FFRP under natural aging conditions are measured. The relationship between natural aging and accelerated aging is studied. The residual mechanical property model may be applied in life prediction of FFRP.
a r t i c l e
i n f o
Article history: Received 18 April 2019 Received in revised form 9 November 2019 Accepted 18 December 2019
Keywords: Flax fiber Composite Bending performance Natural aging Accelerated aging
a b s t r a c t In this paper, the effect of natural aging and accelerated aging on the bending properties of flax fiber reinforced polymer (FFRP) is studied. Test results show that the flexural strength decreases by 11.2%, 14.9%, 15.5%, and the flexural modulus decreases by 21.3%, 32.3%, 35.8%, after 60 days, 120 days and 180 days of natural aging, respectively. Then accelerated test is carried out to predict the long-term performance of FFRP. The results show that short-term exposure test (just considering the temperature and humidity) cannot achieve the same effect on mechanical property degradation as natural aging. A modified residual mechanical property model of FFRP is established according to the test results. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Eco-friendly fiber reinforced polymer (FRP) has gained much attention recently because of its low density, low cost and abundant sources [1–4]. Flax FRP is a commonly used eco-friendly FRP in various fields, such as automobile, packaging, and building construction [5,6]. However, the porous structure of flax fiber leads to high water absorbability [7–9]. Assarar et al. [10] and Duigou et al. [11,12] reported that the weakening of fiber/matrix interface is the main damage mechanism caused by water aging of the FFRP. Li et al. [13] found that cracks of the resin matrix around the fiber after water aging is caused by the swelling volume difference between the flax fiber and matrix. Water leads to leaching of pectin, hemicellulose and weakly crystallized fibers in the flax fibers, which also results in delamination of the fiber/matrix interface. Regazzi et al. [14] simulated the water aging process of FFRP. They noted that the performance degradation caused by swelling is
⇑ Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Petru˚). https://doi.org/10.1016/j.conbuildmat.2019.117909 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
reversible after desorption, but mechanical property degradation caused by interface delamination is permanent and irreversible. They proposed that the elastic modulus of FFRP can be predicted according to the water content. Scida et al. [15] reported that hygrothermal aging does not induce other mechanisms of damage but only accelerates the process of degradation, particularly concerning the flax fibers, helped by the plasticisation of the bonds and the reorientation of the microfibrils. Durability is one of the key issues that should be considered in engineering applications of FFRP. Hristozov et al. [16] investigated the durability of FFRP soaked in water, salt water and alkaline solution. The results showed that the tensile strength of flax decreased significantly with the increase of temperature and exposure time, and the effect of salt and alkali solution on the mechanical properties of FFRP was greater than that of water. Yan et al. [17,18] investigated the effects of UV, water, seawater and alkali solution on the tensile and flexural properties of FFRP. Discoloration of FFRP was observed for aging samples. Degradation of fiber/matrix interface is one of the main reasons for the decrease of tensile properties of FFRP after aging. Yan et al. [17,18] suggested that fiber surface treatment and HFRP (hybrid fiber reinforced polymer with flax
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
2
fiber and other fibers, such as glass fiber or carbon fiber) can help to improve its durability. Hallonet et al. [19] compared the performance of FFRP and GFRP after aging. After aging, the porosity of FFRP is slightly higher than that of GFRP, and the increase of porosity is mainly concentrated in flax fiber bundles. After 250 aging cycles, the modulus of FFRP returns to 85% or even 100% of the initial value. However, the modulus of GFRP remains stable. Mak et al. [20] studied the influence of manufacturing methods (wet layup (WL) and vacuum bag molding (VB)) on the mechanical properties of FFRP. The results show that the strength and modulus of VB samples are larger than that of WL samples before aging. However, VB samples demonstrated a higher susceptibility to environmental degradation than WL samples. Most of the aging tests focus on the degradation of tensile properties of FFRP, researches on bending properties of FFRP is limited. Structures made by FFRP may only bear tensile stress, but also bear bending stress (such as furniture panel, car roof and frame) [21–24]. Therefore, the bending test carried out in this paper may help to increase the durability database of FFRP. 1.1. Life prediction models of FRP Mak et al. [20] proposed a linearized model of the Arrhenius equation to predict the long-term property of FFRP. The residual mechanical property f ðpÞ at time t is
f ðpÞ ¼ Kt
ð1Þ n
where K is the reaction rate, and K ¼ A0 eRT . A0 is material constant, n is activation energy, R is the gas constant, T is the temperature (Kelvin). A life prediction model can be derived by taking the logarithm of both sides of Eq. (1)
lnt ¼ ln½f ðpÞ=A0 þ
E ¼ a þ bT 1 RT
ð2Þ
When applying this model to composites, the following assumptions must be made: (1) Degradation of material properties is mainly caused by a chemical degradation mode; (2) High temperature does not change the degradation mode of the material; (3) FRP is subjected to a water environment [20,25]. Wiederhorn et al. [26] reported that the mechanical property degradation of composites depends on two factors. One is the interaction between composites and the external environment, the other is the crack propagation resistance of the material. The degradation rate of composites under a hydrothermal environment is defined as
v ¼ a/f e
n RT
brk
e RT
ð3Þ
where / is relative humidity, f is the interaction factor, rk is the stress factor, a and b are material parameters. In this model, the influence of temperature, humidity and stress on service life of the material are assumed to be independent. Bulmanis et al. [27] proposed a residual strength model of fiber reinforced composite after natural aging as
S ¼ S0 þ gð1 ekt Þ blnð1 þ utÞ
ð4Þ
where S is the strength at time t, t is the aging time, S0 is the initial strength, g and b are material parameters, k and u are parameters of the material and the ambient medium. Research [28–30] shows that this model agrees well with the aging law of composites. 1.2. The relationship between natural aging and accelerated aging Natural aging tests can directly reflect the changes of the mechanical properties of FFRP in a service environment. However,
most research [15,31–33] on the durability of FFRP focusses on accelerated aging tests due to their convenience and time saving. Using both the natural aging and accelerated aging approaches to study the changes in mechanical properties of FFRP has been rarely reported up to now. Accelerated aging tests are useful to predict the long-term performance of FFRP only if such tests can be correlated with natural aging. Therefore, it is important to find the conversion relationship between accelerated aging and natural aging. According to the Manual of Composite Structural Design [34], the acceleration factor kt is
kt ¼
t1 eC=ðT 2 /2 Þ ¼ t2 eC=ðT 1 /1 Þ
ð5Þ
where t 1 is the natural aging time, t 2 is the accelerated aging time, T 1 and /1 are the temperature and relative humidity under natural aging, T 2 and /2 are the temperature and relative humidity under accelerated aging, C is an experimental coefficient, such that whenT 2 60 , C = 46.1, when T 2 ＞60 , C = 81.5 [34]. Another acceleration coefficient proposed by the Manual of Composite Structural Design [34] is:
1=kt ¼
t 2 D1 ¼ t 1 D2
D ¼ D0 expðC=TÞ
ð6Þ ð7Þ
In this model, the quotient of two diffusion coefficients is taken as the acceleration coefficient, where D0 and C are the diffusionrelated constants at room temperature and they can be obtained according to Manual of Composite Structural Design [34]. Shin et al. [35] proposed an acceleration factor a to predict the life of composites as
a¼
tn ta
ð8Þ
where tn and t a are the natural aging time and accelerated aging time of FRP under the same damage degradation condition. The acceleration factor a can be used to predict the long-term performance of the composite and achieve the same effect on mechanical property degradation through a short-term exposure test. In this study, natural aging tests of FFRP were carried out first, and a residual mechanical property model was proposed according to the test results. Then accelerated aging tests were carried out to study the correlation between accelerated aging and natural aging. The accelerated test results were compared with natural aging test results. 2. Material and test method The unidirectional flax fabric is shown in Fig. 1(a). The elastic modulus and tensile strength of flax fiber are 28.5 GPa and 429.6 MPa, respectively. The density of flax fiber is 1.4 g/cm3. The elastic modulus and tensile strength of epoxy resin are 3.5 GPa and 73.4 MPa, respectively. The density of epoxy is 1.4 g/cm3. FFRP was laminated using the wet hand-layup method as shown in Fig. 1. The epoxy impregnated flax fabrics were placed in the same orientation and then placed in compression between two steel plates. After curing for two weeks, FFRP plates were cut into a rectangular form by using a diamond saw blade. The FFRP specimen is composed of approximately 30% fiber in volume. The dimensions of the FFRP specimens (Fig. 1 (d)) are 50.0 mm 10.0 mm 1.4 mm, according to the dimension of the test device and Chinese standards GB/T 1449-2005 [36]. Because of the size effect, the strength of larger specimens is usually less than smaller specimens due to the increased probability of defects. The relationship between bending strength of FRP with different sizes can be obtained by the formulas proposed by Nader et al. [37]. The aging tests were carried out according to Chinese standards GB/T 25732008 [38]. The natural aging specimens were placed on a frame facing south in an open space, away from obstacles. The horizontal inclination of the frame was
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
3
Fig. 1. Fabrication of FFRP specimen: (a) Unidirectional flax fabric; (b) Hand lay-up with epoxy; (c) FFRP plate after curing; (d) cut into desired shape.
P Specimen
40mm
10mm
10mm
Specimen
(a) Test setup
(b) Loading schematic diagram Fig. 2. Bending test of FFRP.
45°. The hygrothermal aging specimens were soaked in distilled water at 60 °C in an environmental test chamber. The specimens were periodically taken out and tested under three-point loading to assess the flexural properties (Fig. 2). All experimental values were obtained by an average value of five specimens. The loading rate was 2 mm/min. Load and deflection were recorded by the test device. The flexure strength r and flexure modulus E were calculated as
r¼ E¼
3P l 2w h
ð9Þ
2
DP l
3
ð10Þ
3
4w h Df
Table 1 Average temperature and humidity of each natural aging period. Period Temperature/°C Humidity/%
Average value Standard deviation Average value Standard deviation
Period Temperature/°C Humidity/%
Average value Standard deviation Average value Standard deviation
1
2
3
4
5
6
7
8
9
0.8 2.3 60.5 5.6
6.0 2.4 66.7 13.3
9.0 2.1 65.6 14.7
10.1 4.4 70.1 12.4
10.9 4.8 82.6 5.7
18.8 1.9 63.2 3.8
16.7 5.2 67.3 13.0
17.8 4.0 64.1 13.1
20.0 3.0 81.0 8.1
10
11
12
13
14
15
16
17
18
19.8 1.4 75.6 11.1
24.8 3.8 83.7 7.9
22.4 2.1 72.3 13.5
24.0 1.8 72.8 12.1
26.7 1.7 66.6 8.2
27.6 3.0 73.5 10.1
28.1 1.6 84.7 6.9
30.8 1.9 75.8 6.6
31.1 2.5 79.6 8.3
Table 2 Bending properties of FFRP after natural aging. Bending properties
Aging time/day
0
60
120
180
Bending modulus/GPa
Average value Standard deviation
12.1 0.5
9.5 1.2
8.2 1.6
7.7 1.0
Bending strength/MPa
Average value Standard deviation
182.1 3.9
161.7 7.5
155.0 13.5
153.9 10.2
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
4 Table 3 Acceleration factor kt . Period
1
2 76
3
1.5 10
8
4 5
5 4
6 3
7 2
8 2
9 2
kt
5.6 10
2.5 10
2.6 10
2.2 10
2.5 10
3.9 10
Period
10
11
12
13
14
15
16
3.3 10 17
18
kt
58.6
12.8
40.7
26.7
24.7
13.9
7.8
8.3
6.8
39.7
Table 4 Accelerated aging time corresponding to each natural aging period. 1
2
3
4
5
6
7
8
9
Time/h
4.3 1075
1.6 106
9.6 104
9.2 103
0.1
1.0
0.6
0.7
6.1
Period
10
11
12
13
14
15
16
17
18
Time/h
4.1
18.7
5.9
9.0
9.8
17.3
30.4
28.9
35.1
Flexural strength / MPa
Period
190
Natural aging
180
accelerated aging
170
3. Results and discussion
160
3.1. Natural aging test
150 140 0
30
60
90
120
150
180
210
Aging time /day (a) Flexural strength 13
Flexural modulus / GPa
where P is the ultimate load, l is the span, w is the width of the specimen, h is the thickness of the specimen. DP is the load increment, Df is the increment of midspan deflection. As the flexure modulus decreases slightly with the increase of load, the initial flexure modulus is adopted.
Natural aging
12
accelerated aging
11 10 9 8 7 6 5 0
30
60
90
120
150
180
210
Aging time /day
r ¼ 182:1 þ 10:5ð1 e2:710
(b) Flexural modulus Fig. 3. Comparation of bending properties of FFRP under natural aging and hygrothermal aging.
Aging time
Natural aging tests were carried out in Nanjing from February 1st, 2018 to July 30th, 2018. The daily temperature and humidity during the natural aging test were recorded. Every ten days were set as an aging period. The experiment lasted 180 days and was divided into 18 aging periods. The average temperature and humidity of each period are shown in Table 1. The bending properties of FFRP after natural aging are shown in Table 2. For the value of p < 0.05, a statistically significant decrease in mechanical property was observed at 60 days of aging, but not for aging specimen after 60 days. Compared with the reference group, the average flexural strength decreases by 11.2%, 14.9%, 15.5%, and the average flexural modulus decreases by 21.3%, 32.3%, 35.8%, after 60 days, 120 days and 180 days of natural aging, respectively. By fitting the test results with Eq. (4) using the Levenberg-Marquardt and Universal Global Optimization algorithms [39,40], the residual strength r and flexure modulus E of FFRP are obtained as - 3
t
Þ 10:1lnð1 þ 0:14tÞ
3 E ¼ 12:1 þ 7:9 1 e1:910 t 4:4lnð1 þ 0:02tÞ
0 days
60 days
120 days
180 days
0h
1.1 h
37.2 h
167.5 h
Natural aging Aging time Accelerated aging Fig. 4. Comparation of the effect of aging on FFRP specimens.
ð11Þ ð12Þ
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
(ten days) can be obtained as t2 ¼ tk1t , where t2 is the accelerated
3.2. Accelerated aging test Accelerated aging test was carried out at 60 °C with a relative humidity of 100%. To determine the correlation of accelerated aging test to natural aging test, the acceleration factor C=ðT /
Þ
kt ¼ eC=ðT 21 /21 Þ in Eq. (5) is calculated and listed in Table 3. Then the e
accelerated aging time corresponding to each natural aging period
Fiberfracture
(a) Reference specimen Fiber pull out
5
Gap between fiber and matrix
Matrix crack
aging time, t1 is the natural aging time. The accelerated aging time corresponding to each natural aging period is shown in Table 4. The accelerated aging time corresponding to 60 days (6 periods), 120 days (12 periods), and 180 days (18 periods) of natural aging are 1.1 h, 37.2 h, and 167.9 h, respectively. The comparisons between the flexural strength and flexural modulus of FFRP under natural aging and corresponding accelerated aging are shown in Fig. 3. For the values of p < 0.05, a statistically significant decrease in mechanical property was not observed at 1.08 h of accelerated aging, but after 1.08 h, a statistically significant decrease was observed. The color of the specimens has changed obviously after aging, and the boundary between the fiber and matrix is more and more obvious, as shown in Fig. 4. Muasher et al. [41] reported that color fading after aging is caused by oxidation of lignin. As shown in Fig. 5, the failure mode of the reference specimen is mainly fiber fracture, and the bond between the flax fiber and epoxy matrix is firm. However, after natural aging and hygrothermal aging, obvious gap was observed between the fiber and matrix, which indicates the degradation of fiber/matrix interface. Compared with the reference specimen, flax fibers were pulled out more frequently after aging. The surface of the pulled fibers was smooth, and there were less matrix impurities around, which was also evidence of poor fiber/matrix adhesion. SEM studies confirmed the degradation of fiber/matrix interface after both natural aging and hygrothermal aging, which affects the mechanical properties of the FFRP. The average bending strength of FFRP under accelerated aging is 20.3%, 16.7%, and 13.5% higher than that of 60, 120, 180 days of natural aging. Accelerated aging test cannot achieve the same effect on mechanical property degradation as natural aging. It mainly because only two factors (temperature and humidity) are considered in the accelerated test, but the specimens under natural aging were also influenced by ultraviolet rays, rainwater, oxygen and ozone, microorganisms, et al. Besides, the aging mechanism of accelerated aging and natural aging materials is different. The degradation mechanism of FRP caused by freeze-thaw cycling under natural aging is different from that caused by hydrothermal aging in the lab. In Eq. (4), the influence of various environmental factors on material property degradation of FRP is considered as a combination. In order to reflect various factors in the aging environment, an improved formula is presented as
X S ¼ S0 þ g 1 ekT ðxi Þ bi lnð1 þ ui T ðxi ÞÞ
(b) Nature aging specimen after 180 days of exposure Fiber pull out
Gap between fiber and matrix
where xi is the ith environmental aging factor, T ðxi Þ is the ith equivalent aging time of xi . T ðxi Þ ¼ Xv t, where v is the severity index of xi , X is the benchmark index, t is the aging time. For the accelerated aging test of FFRP at a constant temperature, v is the moisture absorption, X is the saturated moisture content. T ðxÞ is the equivalent moisture absorption time. The moisture content of FFRP under accelerated aging is listed in Table 5. By fitting the test results with Eq. (13), the residual strength r and flexure modulus E of FFRP under accelerated aging are obtained as
r ¼ 182:1 þ 6:4 1 e5:610 E ¼ 12:1 þ 4:1 1 e5:710
(c) Hygrothermal aging specimen after 167.9 h of exposure Fig. 5. SEM image of fracture surface of FFRP.
ð13Þ
- 2
- 2
T ðxÞ
T ðxÞ
4:1lnð1 þ 4:3T ðxÞÞ
1:4lnð1 þ 4:6T ðxÞÞ
ð14Þ ð15Þ
Table 5 Moisture content of FFRP under accelerated aging. Accelerated aging time/h Moisture content/%
0h 0.0
1.1 h 1.4
37.2 h 7.2
167.5 h 7.6
384.0 h 7.8
6
˚ / Construction and Building Materials 240 (2020) 117909 X. Wang, M. Petru
4. Conclusion The experimental study carried out in this paper aimed at increasing the durability database and developing a better understanding of the effect of natural aging and accelerated aging on the mechanical behavior of flax fiber reinforced composite. After 60 days, 120 days and 180 days of natural aging, the flexural strength of FFRP decreases by 11.2%, 14.9%, 15.5%, and the flexural modulus decreases by 21.3%, 32.3%, 35.8%, respectively. The SEM images of the fracture surface of FFRP show an obvious change of failure morphology after aging. The failure mode of the reference specimen is mainly fiber fracture. After natural aging and hygrothermal aging, fiber pull-out was more frequently, and a gap between the fiber and matrix was observed, which proves that aging had an adverse effect on the fiber/matrix interface. Although the short-term accelerated test was tried to simulate long-term aging, the results show that accelerated aging and natural aging have different effects on the degradation properties of FFRP. The main reason is that only limited aging factors are considered in accelerated aging, while natural aging is caused by a combination of various environmental factors. In addition, the mechanisms of natural aging and accelerated aging are different. Therefore, a modified residual mechanical property model of FFRP considering independent environmental factor is proposed. Although this model is established for FFRP under accelerated aging, this model may have potential applications in life predictions of other FRPs. Author contributions Xiaomeng Wang performed the experiments and wrote the manuscript supported by Michal Petru. 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 paper. Acknowledgements The result was obtained through the financial support of the Ministry of Education, Youth and Sports of the Czech Republic and the European Union (European Structural and Investment Funds – Operational Programme Research, Development and Education) in the frames of the project ‘‘Modular platform for autonomous chassis of specialized electric vehicles for freight and equipment transportation”, Reg. No. CZ.02.1.01/0.0/0.0/16_ 025/0007293. References [1] D.B. Dittenber, H.V.S. Gangarao, Critical review of recent publications on use of natural composites in infrastructure, Compos. A Appl. Sci. Manuf. 43 (8) (2012) 1419–1429. [2] A. Shahzad, Hemp fiber and its composites – a review, J. Compos. Mater. 46 (8) (2012) 973–986. [3] H. Ventura, J. Claramunt, M.A. Rodríguez-Pérez, M. Ardanuy, Effects of hydrothermal aging on the water uptake and tensile properties of PHB/flax fabric biocomposites, Polym. Degrad. Stab. 142 (2017) 129–138. [4] M. Berges, R. Léger, V. Placet, V. Person, S. Corn, X. Gabrion, J. Rousseau, E. Ramasso, P. Ienny, S. Fontaine, Influence of moisture uptake on the static, cyclic and dynamic behaviour of unidirectional flax fibre-reinforced epoxy laminates, Compos. A Appl. Sci. Manuf. 88 (2016) 165–177. [5] K.L. Pickering, M.G.A. Efendy, T.M. Le, A review of recent developments in natural fibre composites and their mechanical performance, J. Compos. Part A 83 (2016) 98–112. [6] L. Yan, C. Nawawi, J. Krishnan, Flax fibre and its composites – a review, J. Compos. Part B 56 (1) (2014) 296–317.
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