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glass fiber reinforced hybrid composites
Composites Science and Technology 88 (2013) 172–177
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Tensile and interfacial properties of unidirectional flax/glass fiber reinforced hybrid composites Yongli Zhang, Yan Li ⇑, Hao Ma, Tao Yu School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2013 Received in revised form 18 August 2013 Accepted 27 August 2013 Available online 11 September 2013 Keywords: Natural fiber A. Hybrid composites B. Mechanical properties B. Fracture toughness D. Scanning electron microscopy (SEM)
a b s t r a c t This paper studied the mechanical behaviors of unidirectional flax and glass fiber reinforced hybrid composites with the aim of investigation on the hybrid effects of the composites made by natural and synthetic fibers. The tensile properties of the hybrid composites were improved with the increasing of glass fiber content. A modified model for calculating the tensile strength was given based on the hybrid effect of tensile failure strain. The stacking sequence was shown to obviously influence the tensile strength and tensile failure strain, but not the tensile modulus. The fracture toughness and interlaminar shear strength of the hybrid composites were even higher than those of glass fiber reinforced composites due to the excellent hybrid performance of the hybrid interface. These macro-scale results have been correlated with the twist flax yarn structure, rough surface of flax fiber and fiber bridging between flax fiber layers and glass fiber layers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the usage of natural fibers as a replacement for synthetic fibers such as carbon and glass fibers in composite materials has gained interest among researchers throughout the world. Extensive studies on natural fibers, such as sisal [1], jute [2,3] and flax [4,5], showed natural fibers has the potential to be an effective reinforcement for composite materials. The renewed interest of natural fibers over synthetic fibers was that they are abundant in nature and are also renewable raw materials. The usage of natural fibers also provided a healthier working condition than that of glass fibers [6]. Furthermore, natural fibers offer good thermal properties and excellent acoustic performance. These advantages made natural fibers gain applications in automotive, packaging and construction industries [7]. However, the products made from natural fiber composite were still limited to the non-structure or sub-structure applications, for example, the interiors of cars due to their relatively poor mechanical properties [8]. Different approaches have been attempted to increase the mechanical properties of natural fiber reinforced composites, such as chemical or physical modifications of the matrix, fibers or both of the components. Mohanty et al. [9] found that alkali treatment increased the bending strength of jute/biopol composites by 30%. Xie et al. [10] reviewed the influence of silane coupling agents used for ⇑ Corresponding author. Address: School of Aerospace Engineering and Applied Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86 021 65985919; fax: +86 021 65983950. E-mail address: [email protected] (Y. Li). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.08.037
natural fiber/polymer composites. Besides, hybridizing the natural fibers with a stronger synthetic fiber could significantly improve the strength and stiffness of the natural fiber reinforced composites [11]. Earlier works done on the natural fiber/glass fiber hybrid composites basically focused on the short fibers [12]. Nayak et al. [13] proved that hybridization with short glass fiber, the storage modulus of short bamboo fiber reinforced polypropylene composites could be improved. Velmurugan and Manikandan [14] reported addition of glass fiber to palmyra fiber in the matrix could increase the mechanical properties and decrease the moisture absorption of the composites. The comparison on the mechanical properties of different natural fibers and E-glass fiber was shown in Table 1. It was seen that flax fibers possess superior mechanical properties over other natural fibers. The tensile properties and elongation at break of natural fibers are all lower than those of E-glass fiber. However, the density of natural fibers are almost 1/2 of that of glass fiber. Therefore, hybridizing flax fibers with glass fibers might yield a material with interesting properties (lighter in weight, higher in strength and modulus and greener than synthetic materials) and the new hybrid effect might be revealed from the point of views of properties matches of the component fibers and the structure characteristics of natural fiber yarns. In this work, unidirectional flax fibers and glass fibers were selected to make the hybrid composite laminates so that the hybrid effects could be revealed more easily. The mechanical properties, such as tensile, interlaminar shear and interlaminar fracture toughness properties of the hybrid composite laminates, were studied. The influence of hybrid ratio and stacking sequence were
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Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177 Table 1 Mechanical properties of natural fibers and glass fiber [12,15]. Fibers
Flax Hemp Sisal Jute E-glass
Table 3 Hybrid composites with different stacking sequences.
Density (g/cm3)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation at break (%)
1.2 1.48 1.33 1.46 2.5
800–1500 550–900 600–700 400–800 2000–3500
60–80 70 38 10–30 70
1.2–1.6 1.6 2–3 1.8 2.5
investigated and the hybrid mechanisms were revealed with the aid of the Scanning Electronic Microscopy (SEM) observations. 2. Materials and experimental 2.1. Materials The unidirectional flax fabric was supplied by Belgium Lone Company, which had a density of 1.20–1.25 g/cm3 and aerial weight of 200 g/m2. The unidirectional glass fabric was supplied by Zhejiang Mengtai Composites Company, which also had an aerial weight of 200 g/m2. Phenolic resin was supplied by the Institute of Chemistry, Chinese Academy of Sciences. 2.2. Fabrication of composite laminates The composite laminates were manufactured by compression molding. The curing pressure was 1.8 MPa for obtaining the least amount of voids in the composites. The curing temperature was 140 °C for 1 h to get 100% curing of the resin. Six types of unidirectional hybrid composites with different hybrid ratios shown in Table 2 were made to investigate the effect of hybrid ratio on the tensile properties of the composites which included one flax fiber reinforced polymer composite (FFRP), 4 types of flax/glass fiber reinforced hybrid polymer composites (HFRP) and one glass fiber reinforced polymer composite (GFRP). The fiber volume fractions of flax and glass in the composites were varying while the total fiber volume fractions of the composites were kept the same which were around 67%. Another three types of hybrid composites were made to investigate the influence of stacking sequences on the tensile properties of the hybrid composites shown in Table 3. The total fiber volume fraction and fiber volume fractions for each component fibers were all the same (total fiber: 67%; flax fiber: 35%; glass fiber: 32%). 3. Experimental Tensile properties of the composites were measured based on ASTM D3039 and the test speed was 3 mm/min. The nominal Table 2 Hybrid composite laminates with different hybrid ratios. Designation
Volume fraction ratio (flax/glass)
Ply number ratio (flax/glass)
FFRP
100/0
10/0
2G8F
86/14
8/2
4G6F
69/31
6/4
6G4F
50/50
4/6
8G2F
27/73
2/8
GFRP
0/100
0/10
GFRP ply;
, FFRP ply.
Stacking sequence
Designation
Stacking sequence
GF GGFF GGGGFFFF GFRP ply;
, FFRP ply.
dimensions of the specimens were 15 mm 1 mm 250 mm. Short beam shear tests were performed based on ASTM D2344 to measure the interlaminar shear strength of the composites. The test speed was 1 mm/min and the span-to-depth ratio was 4:1. The sizes of the specimens were 12 mm 6 mm 32 mm. Mode I interlaminar fracture toughness were measured according to ASTM D5528-01 with the test speed of 2 mm/min. All the mechanical tests were carried out by a universal mechanical testing machine, CSS-44010, manufactured by Changchun Testing Machine Institute, China. In each case, five specimens were tested to obtain the average values. The micro-structures and the failure modes of the composites were observed with a SEM (PHILIPS XL30 FEG). The surfaces were coated with gold before observation. 4. Results and discussion 4.1. Tensile behaviors and properties of flax/glass fiber reinforced hybrid composites 4.1.1. Tensile behaviors and properties of the hybrid composites with different hybrid ratios The tensile modulus of unidirectional flax/glass fiber reinforced hybrid composites, shown in Fig. 1a, increased with the increasing of the relative volume fraction of glass fibers. The theoretical values predicted by the Rule of Mixture (ROM) were also shown in the same figure. It can be seen that ROM prediction values essentially agreed with the experimental values as expected since there was strain compatibility throughout the hybrid composites for measuring the modulus (i.e., initial elastic deformation) and glass fibers acted as an improvement in resulting in better stiffness for the hybrid composites [16]. The effect of hybrid ratio on the tensile strength of flax/glass fiber reinforced hybrid composites was shown in Fig. 1b. The tensile strength increased with the increasing of the relative volume content of glass fibers. However, the tensile strength of the HFRP did not obey to the ROM since the fibers with low elongation were expected to break when the failure strain was reached [12,17]. The stress–strain curves of the FFRP, GFRP and HFRP, shown in Fig. 2a, indicated that FFRP possessed lower strength and smaller tensile failure elongation compared to those of GFRP. Therefore the tensile behavior of the hybrid composites could be divided into two types due to the differences in failure elongation of flax fiber and glass fiber, shown in Eq. (1) [18]. If the volume fraction of flax fibers in the hybrid composites was high, the hybrid composites would fail when the tensile strain reached the failure elongation of FFRP. However, if the volume fraction of glass fibers in the hybrid composites was high, the FFRP phase would also fail at first. But the hybrid composites would still keep their integrity until the failure of the GFRP phase occurred due to the bigger failure elongation of glass fiber.
rHT ¼
ð1 V m Þðrf V f þ ef Eg V g Þ; ð1 V m Þrg V g ;
V g V crit
V g V crit
ð1Þ
where rHT was the tensile strength of the HFRP. rg, Eg and Vg were the tensile strength, tensile modulus and relative volume fractions of glass fiber. ef, rf and Vf are the tensile failure strain, tensile strength and relative volume fractions of flax fiber. Vm and Vcrit were
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Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177
Fig. 1. Tensile properties of flax/glass fiber reinforced hybrid composites with different hybrid ratio (a) tensile modulus (b) tensile strength.
the volume fraction of matrix and critical relative volume fraction of glass fiber respectively. The critical glass fiber volume fraction of the hybrid composites was calculated as 49.6% by equating the two equations in Eq. (1). The theoretical tensile strength thus obtained were listed in Fig. 1b. A big difference between the theoretical values and the experimental results was observed. Therefore, the synergistic effect of failure strain between flax fiber and glass fiber must play an important role on the failure of the hybrid composites and should be considered. A linear relationship could be easily observed from the tensile failure strain of the composites against the hybrid ratio in Fig. 2b. Therefore, the tensile failure strain of the HFRP eh may be predicted by the following equation:
eh ¼ ef V f þ eg V g
ð2Þ
where ef and eg were the tensile failure strains of the FFRP and GFRP, respectively. So the tensile strength of the hybrid composites could be predicted by the modified Eq. (3) based on the hybrid effect of failure strain and the results were also given in Fig. 1b. A good consistency between the modified theoretical values and the experimental data could be observed. Therefore, as long as the contents of the component materials were known, the tensile strength of unidirectional natural fiber and synthetic fiber reinforced hybrid composites could be predicted by Eq. (3).
rHT ¼ ð1 V m Þeh ðEg V g þ Ef V f Þ
ð3Þ
4.1.2. Tensile behaviors and properties of the hybrid composites with different stacking sequences The effects of fiber layer stacking sequence on the tensile properties of flax/glass fiber reinforced hybrid composites were investi-
Fig. 2. Tensile stress–strain curves and failure strain of flax/glass fiber reinforced hybrid composites with different hybrid ratios (a) tensile stress–strain curves (b) tensile failure strain.
gated. All the hybrid composites had the same total fiber volume fraction and fiber volume fractions of flax and glass fibers in the composites were all the same. Therefore, only the stacking sequence would be the matter. The influence of stacking sequence on the tensile properties of the hybrid composites was shown in Table 4. It could be seen that the tensile modulus for all the hybrid composites were almost the same. Tensile modulus was obtained from the elastic stage of the tensile stress strain curve. In this stage, no damages would occur. The interface did not play a major role for the elastic modulus. Therefore the stacking sequences showed no additional influence on the tensile modulus of the HFRP if the fiber volume fractions of glass fiber and flax fiber in the hybrid composites were the same [17]. However, the stacking sequence of the FFRP plies and the GFRP plies showed bigger influence on the tensile strength and tensile failure strains of the hybrid composites. The tensile strength and failure strain of GF type composite were the highest among the three types of hybrid composites. More fiber layers interaction between flax fiber plies and glass fiber plies (i.e., more FFRP and GFRP
Table 4 Tensile properties of flax/glass fiber reinforced hybrid composites with different stacking sequences. Composites
Tensile modulus (GPa)
Tensile strength (MPa)
Failure strain (%)
GF GGFF GGGGFFFF
40.1 ± 1.7 40.8 ± 1.4 39.7 ± 0.6
450.1 ± 16.5 412.5 ± 12.7 392.5 ± 20.0
1.09 ± 0.02 0.99 ± 0.04 0.96 ± 0.06
Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177
175
Fig. 3. Fiber bridging between flax fiber and glass fiber on the tensile specimen of GF type hybrid composite.
Fig. 4. Interlaminar shear strength of the GFRP, FFRP and HFRP.
interfaces) led to higher tensile strength and bigger failure strain. From the observations of the failure modes of the GF type composites shown in Fig. 3, the rough surface of flax fiber and the twisted flax yarn structure played very obvious bridging roles on the adhesion between FFRP plies and GFRP plies. Hence improved stress transfer efficiency on the hybrid interface was obtained. More hybrid interfaces led to higher tensile strength and tensile failure strain.
Fig. 6. Delamination resistance curves (R curves) from DCB Test of FFRP, GFRP and HFRP.
4.2. Interlaminar properties of flax/glass fiber reinforced hybrid composites The interlaminar shear strength and interlaminar fracture toughness of GFRP, FFRP and HFRP were investigated. All the com-
Fig. 5. SEM of (a) flax yarn, (b) glass fabric, (c) flax fiber and (d) glass fiber.
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Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177
Fig. 7. Failure specimens of (a) GFRP, (b) FFRP and (c) HFRP.
posites had the same total fiber volume fraction (67%). The fiber volume fraction of flax fiber in the HFRP was 35% and the layup sequence in the HFRP was one layer glass fiber and one layer flax fiber interply hybrid. Fig. 4 shows the interlaminar shear strength of FFRP, GFRP and HFRP measured by short beam shear tests. It could be seen that the HFRP possessed the highest shear strength, compared with those of GFRP and FFRP. Improvements on interlaminar shear strength by hybridizing flax and glass fibers significantly depended on the fiber bridging between glass fibers and flax fibers which was mainly caused by the twisted flax yarn structure and rough surface of flax fiber compared to their glass counterparts, as shown in Fig. 5. The flax yarns were made by twisting a bundle of short single flax fibers for industry application requirements as the lengths of most natural fibers are less than 20 mm due to their natural growth characteristics. Therefore, some flax single fibers stretched from the flax yarns which made the flax yarn structure like a tree full of branches (Fig. 5a). However, the glass fiber yarns showed the
regular-knit structure (Fig. 5b). And flax fiber (Fig. 5c) showed much rougher surfaces than that of the glass fiber (Fig. 5d). All of above produced large amount of fiber bridging and fiber entangling within flax/glass fiber hybrid interface and the out-layers flax fibers served as the Z-directional reinforcement, thus contributed to the increased interlaminar shear strength of HFRP. Fig. 6 shows the delamination resistance curves (R-curves) of FFRP, GFRP and HFRP obtained from double cantilever beam (DCB) tests. The strain energy release rate (GI) values obtained from the R-curves were 0.4 kJ/m2 for GFRP which was the lowest and 0.55 kJ/m2 and 0.56 kJ/m2 for FFRP and HFRP, respectively. It could also be seen that more energy was needed for the initiation of the cracks for HFRP, FFRP compared with GFRP. Fig. 7 compared the fractured specimens of FFRP, GFRP and HFRP. Very clean delaminated surfaces of GFRP (Fig. 7a) were observed with almost no fiber bridging due to the regular-knit structures of glass fiber fabrics. An easier path for crack propagation was expected. Large amount flax fiber bridgings occurred on the fracture surface of FFRP (Fig. 7b) resulted in higher fracture toughness. A completely different fracture surfaces for HFRP specimens compared to FFRP and GFRP have been observed (Fig. 7c). It was seen that the twisted flax yarn structures and rough surface of flax fibers led to a large number of fibers bridging between flax yarns, flax fibers and glass fibers, which contributed to the improved fracture toughness of HFRP [19]. The failure surface of HFRP was observed by SEM as shown in Fig. 8. There were lots of glass fibers torn out form the fractured surface of glass fiber layer (Fig. 8a) and plenty of glass fibers bonded on the fractured surface of flax fiber layer (Fig. 8b) for HFRP due to the easier separation of glass fibers in their fabrics compared with FFRP, in which not only the polymer matrix bonding forces but also the mechanical forces caused by the twist contributed to the connection between the single flax fibers. Thus, the torn out glass fibers played as the bridgings between the flax fiber layer and glass fiber layer, which led to the increased interlaminar fracture toughness.
5. Conclusions The tensile properties of the flax/glass fiber reinforced hybrid composites were improved with the increasing of glass fiber content. It was found that the tensile modulus of the HFRP followed the ROM very well. And a theoretical model used to predict the tensile
Fig. 8. SEM photographs of mode I delamination fracture interface for HFRP.
Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177
strength of flax/glass fiber reinforced hybrid composites was proposed based on the hybrid effect of tensile failure strain. The stacking sequence showed great influence on the tensile strength of flax/glass fiber reinforced hybrid composites, but not on the tensile modulus if the fiber volume in the HFRP were the same. More different fiber interactions or more different phases interfaces led to higher tensile strength and longer tensile failure strain. The interlaminar shear strength and the interlaminar fracture toughness of flax/glass fiber reinforced hybrid composites were higher than those of GFRP. The twist flax yarn structure and the rough surface of flax fibers led to remarkable fibers bridging between flax fibers, flax yarn and glass fibers, thus improved the interlaminar properties of HFRP. Acknowledgement This project is supported by the National Basic Research Program of China (‘‘973’’ Program) (Grant No. 2010CB631105). References [1] Barreto ACH, Rosa DS, Fechine PBA, Mazzetto SE. Properties of sisal fibers treated by alkali solution and their application into cardanol-based biocomposites. Compos A Appl Sci 2011;42:492–500. [2] Behera Ajaya Kumar, Avancha Sridevi, Basak Ratan Kumar, Sen Ramkrishna, Adhikari Basudam. Fabrication and characterizations of biodegradable jute reinforced soy based green composites. Carbohydr Polym 2012;88:329–35. [3] Plackett David, Andersen Tom Løgstrup, Pedersen Walther Batsberg, Nielsen Lotte. Biodegradable composites based on L-polylactideand jute fibres. Compos Sci Technol 2003;63:1287–96. [4] Liang S, Gning PB, Guillaumat L. A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites. Compos Sci Technol 2012;72:535–43.
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[5] Andersons J, Joffe R. Estimation of the tensile strength of an oriented flax fiberreinforced polymer composite. Compos A – Appl Sci 2011;42:1229–35. [6] Joshi SV, Drzal LT, Mohanty AK, Arora S. Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos Part A – Appl Sci 2004;35:371–6. [7] Davoodi MM, Sapuan SM, Ahmad D, Ali Aidy, Khalina A, Jonoobi Mehdi. Mechanical properties of hybrid kenaf/glass reinforced epoxy composite for passenger car bumper beam. Mater Des 2010;31(10):4927–32. [8] Li Yan, Mai Yiu-Wing, Ye Lin. Sisal fibre and its composites: a review of recent developments. Compos Sci Technol 2000;60:2037–55. [9] Mohanty AK, Khan Mubarak A, Hinrichsen G. Surface modification of jute and its influence on performance of biodegradable jute-fabric/biopol composites. Compos Sci Technol 2000;60(11):15–24. [10] Xie Yanjun, Hill Callum AS, Xiao Zefang, Militz Holger, Mai Carsten. Silane coupling agents used for natural fiber/polymer composites: a review. Compos Part A – Appl Sci 2010;41:806–19. [11] Jawaid M, Abdul Khalil HPS. Cellulosic synthetic fibre reinforced polymer hybrid composites: a review. Carbohydr Polym 2011;86:1–18. [12] Jarukumjorn Kasama, Suppakarn Nitinat. Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites. Compos B – Eng 2009;40:623–7. [13] Nayak Sanjay K, Mohanty Smita, Samal Sushanta K. Influence of short bamboo/ glass fiber on the thermal, dynamic mechanical and rheological properties of polypropylene hybrid composites. Mater Sci Eng A 2009;523:32–8. [14] Velmurugan R, Manikandan V. Mechanical properties of palmyra/glass fiber hybrid composites. Compos Part A – Appl Sci 2007;38:2216–26. [15] Wambua Paul, Ivens Jan, Verpoest Ignaas. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63:1259–64. [16] Qiu Yiping, Schwartz Peter. Micromechanical behavior of Kevlar-149/S-glass hybrid seven-fiber microcomposites: I. Tensile strength of the hybrid composite. Compos Sci Technol 1993;47(3):289–301. [17] Kretsis G. A review of the tensile, compressive, flexural and shear properties of hybrid fibre-reinforced plastics. Composites 1987;18(1):13–23. [18] Khatri C Subhash, Koczak Michael J. Thick-section as4-graphite/e-glass/pps hybrid composites: Part I. Tensile behavior. Compos Sci Technol 1996;56(2):181–92. [19] Le Duigou Antoine, Davies Peter, Baley Christophe. Macroscopic analysis of interfacial properties of flax/PLA biocomposites. Compos Sci Technol 2010;70:1612–20.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Tensile and interfacial properties of unidirectional flax/glass fiber reinforced hybrid composites Yongli Zhang, Yan Li ⇑, Hao Ma, Tao Yu School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, PR China
a r t i c l e
i n f o
Article history: Received 14 April 2013 Received in revised form 18 August 2013 Accepted 27 August 2013 Available online 11 September 2013 Keywords: Natural fiber A. Hybrid composites B. Mechanical properties B. Fracture toughness D. Scanning electron microscopy (SEM)
a b s t r a c t This paper studied the mechanical behaviors of unidirectional flax and glass fiber reinforced hybrid composites with the aim of investigation on the hybrid effects of the composites made by natural and synthetic fibers. The tensile properties of the hybrid composites were improved with the increasing of glass fiber content. A modified model for calculating the tensile strength was given based on the hybrid effect of tensile failure strain. The stacking sequence was shown to obviously influence the tensile strength and tensile failure strain, but not the tensile modulus. The fracture toughness and interlaminar shear strength of the hybrid composites were even higher than those of glass fiber reinforced composites due to the excellent hybrid performance of the hybrid interface. These macro-scale results have been correlated with the twist flax yarn structure, rough surface of flax fiber and fiber bridging between flax fiber layers and glass fiber layers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, the usage of natural fibers as a replacement for synthetic fibers such as carbon and glass fibers in composite materials has gained interest among researchers throughout the world. Extensive studies on natural fibers, such as sisal [1], jute [2,3] and flax [4,5], showed natural fibers has the potential to be an effective reinforcement for composite materials. The renewed interest of natural fibers over synthetic fibers was that they are abundant in nature and are also renewable raw materials. The usage of natural fibers also provided a healthier working condition than that of glass fibers [6]. Furthermore, natural fibers offer good thermal properties and excellent acoustic performance. These advantages made natural fibers gain applications in automotive, packaging and construction industries [7]. However, the products made from natural fiber composite were still limited to the non-structure or sub-structure applications, for example, the interiors of cars due to their relatively poor mechanical properties [8]. Different approaches have been attempted to increase the mechanical properties of natural fiber reinforced composites, such as chemical or physical modifications of the matrix, fibers or both of the components. Mohanty et al. [9] found that alkali treatment increased the bending strength of jute/biopol composites by 30%. Xie et al. [10] reviewed the influence of silane coupling agents used for ⇑ Corresponding author. Address: School of Aerospace Engineering and Applied Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86 021 65985919; fax: +86 021 65983950. E-mail address: [email protected] (Y. Li). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.08.037
natural fiber/polymer composites. Besides, hybridizing the natural fibers with a stronger synthetic fiber could significantly improve the strength and stiffness of the natural fiber reinforced composites [11]. Earlier works done on the natural fiber/glass fiber hybrid composites basically focused on the short fibers [12]. Nayak et al. [13] proved that hybridization with short glass fiber, the storage modulus of short bamboo fiber reinforced polypropylene composites could be improved. Velmurugan and Manikandan [14] reported addition of glass fiber to palmyra fiber in the matrix could increase the mechanical properties and decrease the moisture absorption of the composites. The comparison on the mechanical properties of different natural fibers and E-glass fiber was shown in Table 1. It was seen that flax fibers possess superior mechanical properties over other natural fibers. The tensile properties and elongation at break of natural fibers are all lower than those of E-glass fiber. However, the density of natural fibers are almost 1/2 of that of glass fiber. Therefore, hybridizing flax fibers with glass fibers might yield a material with interesting properties (lighter in weight, higher in strength and modulus and greener than synthetic materials) and the new hybrid effect might be revealed from the point of views of properties matches of the component fibers and the structure characteristics of natural fiber yarns. In this work, unidirectional flax fibers and glass fibers were selected to make the hybrid composite laminates so that the hybrid effects could be revealed more easily. The mechanical properties, such as tensile, interlaminar shear and interlaminar fracture toughness properties of the hybrid composite laminates, were studied. The influence of hybrid ratio and stacking sequence were
173
Y. Zhang et al. / Composites Science and Technology 88 (2013) 172–177 Table 1 Mechanical properties of natural fibers and glass fiber [12,15]. Fibers
Flax Hemp Sisal Jute E-glass
Table 3 Hybrid composites with different stacking sequences.
Density (g/cm3)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation at break (%)
1.2 1.48 1.33 1.46 2.5
800–1500 550–900 600–700 400–800 2000–3500
60–80 70 38 10–30 70
1.2–1.6 1.6 2–3 1.8 2.5
investigated and the hybrid mechanisms were revealed with the aid of the Scanning Electronic Microscopy (SEM) observations. 2. Materials and experimental 2.1. Materials The unidirectional flax fabric was supplied by Belgium Lone Company, which had a density of 1.20–1.25 g/cm3 and aerial weight of 200 g/m2. The unidirectional glass fabric was supplied by Zhejiang Mengtai Composites Company, which also had an aerial weight of 200 g/m2. Phenolic resin was supplied by the Institute of Chemistry, Chinese Academy of Sciences. 2.2. Fabrication of composite laminates The composite laminates were manufactured by compression molding. The curing pressure was 1.8 MPa for obtaining the least amount of voids in the composites. The curing temperature was 140 °C for 1 h to get 100% curing of the resin. Six types of unidirectional hybrid composites with different hybrid ratios shown in Table 2 were made to investigate the effect of hybrid ratio on the tensile properties of the composites which included one flax fiber reinforced polymer composite (FFRP), 4 types of flax/glass fiber reinforced hybrid polymer composites (HFRP) and one glass fiber reinforced polymer composite (GFRP). The fiber volume fractions of flax and glass in the composites were varying while the total fiber volume fractions of the composites were kept the same which were around 67%. Another three types of hybrid composites were made to investigate the influence of stacking sequences on the tensile properties of the hybrid composites shown in Table 3. The total fiber volume fraction and fiber volume fractions for each component fibers were all the same (total fiber: 67%; flax fiber: 35%; glass fiber: 32%). 3. Experimental Tensile properties of the composites were measured based on ASTM D3039 and the test speed was 3 mm/min. The nominal Table 2 Hybrid composite laminates with different hybrid ratios. Designation
Volume fraction ratio (flax/glass)
Ply number ratio (flax/glass)
FFRP
100/0
10/0
2G8F
86/14
8/2
4G6F
69/31
6/4
6G4F
50/50
4/6
8G2F
27/73
2/8
GFRP
0/100
0/10
GFRP ply;
, FFRP ply.
Stacking sequence
Designation
Stacking sequence
GF GGFF GGGGFFFF GFRP ply;
, FFRP ply.
dimensions of the specimens were 15 mm 1 mm 250 mm. Short beam shear tests were performed based on ASTM D2344 to measure the interlaminar shear strength of the composites. The test speed was 1 mm/min and the span-to-depth ratio was 4:1. The sizes of the specimens were 12 mm 6 mm 32 mm. Mode I interlaminar fracture toughness were measured according to ASTM D5528-01 with the test speed of 2 mm/min. All the mechanical tests were carried out by a universal mechanical testing machine, CSS-44010, manufactured by Changchun Testing Machine Institute, China. In each case, five specimens were tested to obtain the average values. The micro-structures and the failure modes of the composites were observed with a SEM (PHILIPS XL30 FEG). The surfaces were coated with gold before observation. 4. Results and discussion 4.1. Tensile behaviors and properties of flax/glass fiber reinforced hybrid composites 4.1.1. Tensile behaviors and properties of the hybrid composites with different hybrid ratios The tensile modulus of unidirectional flax/glass fiber reinforced hybrid composites, shown in Fig. 1a, increased with the increasing of the relative volume fraction of glass fibers. The theoretical values predicted by the Rule of Mixture (ROM) were also shown in the same figure. It can be seen that ROM prediction values essentially agreed with the experimental values as expected since there was strain compatibility throughout the hybrid composites for measuring the modulus (i.e., initial elastic deformation) and glass fibers acted as an improvement in resulting in better stiffness for the hybrid composites [16]. The effect of hybrid ratio on the tensile strength of flax/glass fiber reinforced hybrid composites was shown in Fig. 1b. The tensile strength increased with the increasing of the relative volume content of glass fibers. However, the tensile strength of the HFRP did not obey to the ROM since the fibers with low elongation were expected to break when the failure strain was reached [12,17]. The stress–strain curves of the FFRP, GFRP and HFRP, shown in Fig. 2a, indicated that FFRP possessed lower strength and smaller tensile failure elongation compared to those of GFRP. Therefore the tensile behavior of the hybrid composites could be divided into two types due to the differences in failure elongation of flax fiber and glass fiber, shown in Eq. (1) [18]. If the volume fraction of flax fibers in the hybrid composites was high, the hybrid composites would fail when the tensile strain reached the failure elongation of FFRP. However, if the volume fraction of glass fibers in the hybrid composites was high, the FFRP phase would also fail at first. But the hybrid composites would still keep their integrity until the failure of the GFRP phase occurred due to the bigger failure elongation of glass fiber.
rHT ¼
ð1 V m Þðrf V f þ ef Eg V g Þ; ð1 V m Þrg V g ;
V g V crit
V g V crit
ð1Þ
where rHT was the tensile strength of the HFRP. rg, Eg and Vg were the tensile strength, tensile modulus and relative volume fractions of glass fiber. ef, rf and Vf are the tensile failure strain, tensile strength and relative volume fractions of flax fiber. Vm and Vcrit were
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Fig. 1. Tensile properties of flax/glass fiber reinforced hybrid composites with different hybrid ratio (a) tensile modulus (b) tensile strength.
the volume fraction of matrix and critical relative volume fraction of glass fiber respectively. The critical glass fiber volume fraction of the hybrid composites was calculated as 49.6% by equating the two equations in Eq. (1). The theoretical tensile strength thus obtained were listed in Fig. 1b. A big difference between the theoretical values and the experimental results was observed. Therefore, the synergistic effect of failure strain between flax fiber and glass fiber must play an important role on the failure of the hybrid composites and should be considered. A linear relationship could be easily observed from the tensile failure strain of the composites against the hybrid ratio in Fig. 2b. Therefore, the tensile failure strain of the HFRP eh may be predicted by the following equation:
eh ¼ ef V f þ eg V g
ð2Þ
where ef and eg were the tensile failure strains of the FFRP and GFRP, respectively. So the tensile strength of the hybrid composites could be predicted by the modified Eq. (3) based on the hybrid effect of failure strain and the results were also given in Fig. 1b. A good consistency between the modified theoretical values and the experimental data could be observed. Therefore, as long as the contents of the component materials were known, the tensile strength of unidirectional natural fiber and synthetic fiber reinforced hybrid composites could be predicted by Eq. (3).
rHT ¼ ð1 V m Þeh ðEg V g þ Ef V f Þ
ð3Þ
4.1.2. Tensile behaviors and properties of the hybrid composites with different stacking sequences The effects of fiber layer stacking sequence on the tensile properties of flax/glass fiber reinforced hybrid composites were investi-
Fig. 2. Tensile stress–strain curves and failure strain of flax/glass fiber reinforced hybrid composites with different hybrid ratios (a) tensile stress–strain curves (b) tensile failure strain.
gated. All the hybrid composites had the same total fiber volume fraction and fiber volume fractions of flax and glass fibers in the composites were all the same. Therefore, only the stacking sequence would be the matter. The influence of stacking sequence on the tensile properties of the hybrid composites was shown in Table 4. It could be seen that the tensile modulus for all the hybrid composites were almost the same. Tensile modulus was obtained from the elastic stage of the tensile stress strain curve. In this stage, no damages would occur. The interface did not play a major role for the elastic modulus. Therefore the stacking sequences showed no additional influence on the tensile modulus of the HFRP if the fiber volume fractions of glass fiber and flax fiber in the hybrid composites were the same [17]. However, the stacking sequence of the FFRP plies and the GFRP plies showed bigger influence on the tensile strength and tensile failure strains of the hybrid composites. The tensile strength and failure strain of GF type composite were the highest among the three types of hybrid composites. More fiber layers interaction between flax fiber plies and glass fiber plies (i.e., more FFRP and GFRP
Table 4 Tensile properties of flax/glass fiber reinforced hybrid composites with different stacking sequences. Composites
Tensile modulus (GPa)
Tensile strength (MPa)
Failure strain (%)
GF GGFF GGGGFFFF
40.1 ± 1.7 40.8 ± 1.4 39.7 ± 0.6
450.1 ± 16.5 412.5 ± 12.7 392.5 ± 20.0
1.09 ± 0.02 0.99 ± 0.04 0.96 ± 0.06
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Fig. 3. Fiber bridging between flax fiber and glass fiber on the tensile specimen of GF type hybrid composite.
Fig. 4. Interlaminar shear strength of the GFRP, FFRP and HFRP.
interfaces) led to higher tensile strength and bigger failure strain. From the observations of the failure modes of the GF type composites shown in Fig. 3, the rough surface of flax fiber and the twisted flax yarn structure played very obvious bridging roles on the adhesion between FFRP plies and GFRP plies. Hence improved stress transfer efficiency on the hybrid interface was obtained. More hybrid interfaces led to higher tensile strength and tensile failure strain.
Fig. 6. Delamination resistance curves (R curves) from DCB Test of FFRP, GFRP and HFRP.
4.2. Interlaminar properties of flax/glass fiber reinforced hybrid composites The interlaminar shear strength and interlaminar fracture toughness of GFRP, FFRP and HFRP were investigated. All the com-
Fig. 5. SEM of (a) flax yarn, (b) glass fabric, (c) flax fiber and (d) glass fiber.
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Fig. 7. Failure specimens of (a) GFRP, (b) FFRP and (c) HFRP.
posites had the same total fiber volume fraction (67%). The fiber volume fraction of flax fiber in the HFRP was 35% and the layup sequence in the HFRP was one layer glass fiber and one layer flax fiber interply hybrid. Fig. 4 shows the interlaminar shear strength of FFRP, GFRP and HFRP measured by short beam shear tests. It could be seen that the HFRP possessed the highest shear strength, compared with those of GFRP and FFRP. Improvements on interlaminar shear strength by hybridizing flax and glass fibers significantly depended on the fiber bridging between glass fibers and flax fibers which was mainly caused by the twisted flax yarn structure and rough surface of flax fiber compared to their glass counterparts, as shown in Fig. 5. The flax yarns were made by twisting a bundle of short single flax fibers for industry application requirements as the lengths of most natural fibers are less than 20 mm due to their natural growth characteristics. Therefore, some flax single fibers stretched from the flax yarns which made the flax yarn structure like a tree full of branches (Fig. 5a). However, the glass fiber yarns showed the
regular-knit structure (Fig. 5b). And flax fiber (Fig. 5c) showed much rougher surfaces than that of the glass fiber (Fig. 5d). All of above produced large amount of fiber bridging and fiber entangling within flax/glass fiber hybrid interface and the out-layers flax fibers served as the Z-directional reinforcement, thus contributed to the increased interlaminar shear strength of HFRP. Fig. 6 shows the delamination resistance curves (R-curves) of FFRP, GFRP and HFRP obtained from double cantilever beam (DCB) tests. The strain energy release rate (GI) values obtained from the R-curves were 0.4 kJ/m2 for GFRP which was the lowest and 0.55 kJ/m2 and 0.56 kJ/m2 for FFRP and HFRP, respectively. It could also be seen that more energy was needed for the initiation of the cracks for HFRP, FFRP compared with GFRP. Fig. 7 compared the fractured specimens of FFRP, GFRP and HFRP. Very clean delaminated surfaces of GFRP (Fig. 7a) were observed with almost no fiber bridging due to the regular-knit structures of glass fiber fabrics. An easier path for crack propagation was expected. Large amount flax fiber bridgings occurred on the fracture surface of FFRP (Fig. 7b) resulted in higher fracture toughness. A completely different fracture surfaces for HFRP specimens compared to FFRP and GFRP have been observed (Fig. 7c). It was seen that the twisted flax yarn structures and rough surface of flax fibers led to a large number of fibers bridging between flax yarns, flax fibers and glass fibers, which contributed to the improved fracture toughness of HFRP [19]. The failure surface of HFRP was observed by SEM as shown in Fig. 8. There were lots of glass fibers torn out form the fractured surface of glass fiber layer (Fig. 8a) and plenty of glass fibers bonded on the fractured surface of flax fiber layer (Fig. 8b) for HFRP due to the easier separation of glass fibers in their fabrics compared with FFRP, in which not only the polymer matrix bonding forces but also the mechanical forces caused by the twist contributed to the connection between the single flax fibers. Thus, the torn out glass fibers played as the bridgings between the flax fiber layer and glass fiber layer, which led to the increased interlaminar fracture toughness.
5. Conclusions The tensile properties of the flax/glass fiber reinforced hybrid composites were improved with the increasing of glass fiber content. It was found that the tensile modulus of the HFRP followed the ROM very well. And a theoretical model used to predict the tensile
Fig. 8. SEM photographs of mode I delamination fracture interface for HFRP.
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strength of flax/glass fiber reinforced hybrid composites was proposed based on the hybrid effect of tensile failure strain. The stacking sequence showed great influence on the tensile strength of flax/glass fiber reinforced hybrid composites, but not on the tensile modulus if the fiber volume in the HFRP were the same. More different fiber interactions or more different phases interfaces led to higher tensile strength and longer tensile failure strain. The interlaminar shear strength and the interlaminar fracture toughness of flax/glass fiber reinforced hybrid composites were higher than those of GFRP. The twist flax yarn structure and the rough surface of flax fibers led to remarkable fibers bridging between flax fibers, flax yarn and glass fibers, thus improved the interlaminar properties of HFRP. Acknowledgement This project is supported by the National Basic Research Program of China (‘‘973’’ Program) (Grant No. 2010CB631105). References [1] Barreto ACH, Rosa DS, Fechine PBA, Mazzetto SE. Properties of sisal fibers treated by alkali solution and their application into cardanol-based biocomposites. Compos A Appl Sci 2011;42:492–500. [2] Behera Ajaya Kumar, Avancha Sridevi, Basak Ratan Kumar, Sen Ramkrishna, Adhikari Basudam. Fabrication and characterizations of biodegradable jute reinforced soy based green composites. Carbohydr Polym 2012;88:329–35. [3] Plackett David, Andersen Tom Løgstrup, Pedersen Walther Batsberg, Nielsen Lotte. Biodegradable composites based on L-polylactideand jute fibres. Compos Sci Technol 2003;63:1287–96. [4] Liang S, Gning PB, Guillaumat L. A comparative study of fatigue behaviour of flax/epoxy and glass/epoxy composites. Compos Sci Technol 2012;72:535–43.
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