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The effect of PVDF nanofibers on mode-I fracture toughness of composite materials
Composites: Part B 72 (2015) 213–216
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of PVDF nanofibers on mode-I fracture toughness of composite materials H. Saghafi ⇑, T. Brugo, G. Minak, A. Zucchelli Department of Industrial Engineering (DIN), Alma Mater Studiorum, Università di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 23 August 2014 Received in revised form 3 December 2014 Accepted 9 December 2014 Available online 15 December 2014 Keywords: A. Laminates B. Fracture toughness D. Mechanical testing Interleaving
a b s t r a c t In this study, the fracture behavior of carbon/epoxy laminates interleaved by polyvinylidene fluoride (PVDF) nanofibers is investigated. For this aim, a mode-I fracture test is conducted on virgin and modified laminates. Unlike the results of other studies, it is shown that PVDF nanofibers can increase mode-I fracture toughness (GI) noticeably in a specific situation. The results show that GI is enhanced about 43% and 36% in initiation and propagation stages of the fracture, respectively, using PVDF nanofibers. The morphology of the fractured surface is also presented for investigating the mechanism of toughening. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resin is commonly used as a thermosetting matrix for many polymeric composite materials, but its low toughness restricts its application to structural components. For example, significant damage and delamination can occur in the structure under a very low-velocity-impact (even under 10 J) [1]. So far, many attempts have been made to increase the delamination resistance of epoxy-based composite and a novel method has finally been introduced using thermoplastic polymers to modify the epoxy matrix [2,3]. The toughening material can be either used in the form of film, fiber, or particle, placed between the laminate layers [4–6]. Using film is easier than particles, since obtaining a uniform distribution of particles in the matrix, or between composite layers, is a very complicated process, while nanofibrers can be easily uniformly inserted between the layers. Moreover, nanofibrous toughening materials are more effective than film in toughening the epoxy [7]. So far, various kinds of polymeric nanofibers have been applied to increase the fracture toughness of composite materials: polysulfone (PSF) [7], Nylon 6 [8], Phenoxy [9], Nylon 6,6 [10–12], poly(e-caprolactone) (PCL) [13], polyvinylidene fluoride (PVDF) [14] and etc. Although several studies have been conducted for many of these nanofibers, only a few research papers have been reported in the literature about the toughening effect of PVDF on
⇑ Corresponding author. E-mail address: Hamed.saghafi[email protected] (H. Saghafi). http://dx.doi.org/10.1016/j.compositesb.2014.12.015 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.
epoxy [13–15]. Zhang et al. [13] used three different electrospun nanofibers: PCL, PVDF, and PAN between composite layers, and compared their effect and considered their toughening mechanism. Their results showed that only PCL can toughen epoxy. Magniez et al. [14] employed low and high molecular weight PVDF for investigating its efficiency in mode-I and mode-II fracture toughness of carbon/epoxy composite materials. They showed that the use of both types of PVDF can increase mode-II fracture toughness but at the same time it slightly decreases fracture toughness in mode-I. Furthermore, they showed that molecular weight slightly affected the PVDF crystal forms and the morphology of the fractured surface; at the same time it did not significantly affect mode-I and mode-II fracture toughness. The difference of the study of Zhang and that of Magniez is in the curing processes. In the first one [13], specimens were cured at 150 °C and so PVDF did not melt; on the other hand, in [14] the curing temperature was 175 °C, which caused the fibers to melt. Results showed that fracture toughness enhanced as the curing temperature increased and nanofibers melted. The researchers in [13,14] used a very thick nanofibrous mat (about 70 lm) and, as mentioned above, the curing temperatures in [13,14] were below and above the PVDF’s melting point, respectively. Therefore in this paper, it is shown that, by changing the thickness of the nanofibrous mat, curing and electrospinning process, the fracture toughness in mode-I can be increased by incorporating PVDF nanofibers. The effect of all these parameters is introduced in the following sections. The SEM pictures of the fractured interfaces will then be used to explain the role of the nanofibers in the interface.
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2. Materials and method
3. Results and discussion
2.1. Materials
Fig. 2 presents the fracture behavior of the reference and modified laminates in mode-I fracture loading. As seen in Fig. 2A, the trend of both the curves is almost the same until fracture initiation. When the fracture starts to propagate, and during its propagation, interleaved laminate always tolerates a force higher than the reference laminate. Fig. 2B shows the variation of mode-I fracture toughness during crack propagation for both the non-modified and modified composite laminates. According to the curves, GI is minimum at the start of fracture and after that increases with a very low slope. Black lines fitted on the curves in Fig. 2B are used to consider the general behavior of GI during crack propagation. The initial fracture toughness is much higher in modified laminate when compared with the reference one. Table 1 summarizes the parameters that can be obtained from the fracture tests. Interleaving PVDF nanofibers increased GI about 43% and 36% in fracture initiation and crack propagation stages, respectively. The nanomodified specimens’ maximum GI and maximum force were increased by 36% and 23%, respectively, compared to virgin specimens. The results obtained in this study are in contrast to two other studies [13,14] in which the use of PVDF did not increase fracture toughness in mode-I composite laminate specimens. In [13], modeI fracture toughness of specimens was not affected by interlaying PVDF nanofibers in the laminate, while in [14] the GI decreased by about 20%. The differences between the outcomes of this study with [13,14] are due to the following parameters: 1 – curing process, 2 – the thickness of nanofibrous membrane, 3 – the crystallization type of PVDF after electrospinning and curing processes. Magniez et al. [14] cured the specimens at 150 °C for 10 min while the melting point of PVDF is about 165–170 °C and so the nanofibers were visible in the morphology of the fractured surface shown in the paper. Zhang et al. [13] used a different curing process: 125 °C for 1.5 h and 175 °C for 2 h, which caused the nanofibers to melt. By increasing the curing temperature from 150 °C to 175 °C, the effect of PVDF nanofibers on fracture toughness increased from 20% to 0% as mentioned above. The thickness of the nanofibrous membrane in both of these studies was about 70 lm. So far, many studies have been conducted regarding the effect of electrospinning and curing processes on PVDF crystallization [17]. Zheng et al. [17] showed that parameters such as the solvents, electrospinning temperature, feeding rate, and tip-to-collector distance could affect the crystal phases (a, b, and c) of PVDF fibers during the electrospinning process. Magniez et al. [14] foresaw that fracture toughness in mode-I can be improved probably by increasing the a phase in PVDF, but so far no studies have been done in this regard. They only used two kinds of PVDF with different molecular weights, which eventually caused a small difference in the form of phase crystals. The thickness of the nanofibrous mat is another important factor that can affect the toughening of the epoxy. As described in [13], PVDF is immiscible with epoxy. So, when the thickness of the nanomat mat is very thick and nanofibers are melted, it is difficult to bond the layers. However, reducing the nanomat’s thickness allows the epoxy to fully penetrate the nanofibrous mat, which is highly porous, and to bond the layers, letting the PVDF play its toughening role. There is some evidence on this topic in other studies [14,18]. Nylon 6,6 is a polymer whose melting point is around 260 °C. Palazzetti et al. [18] interleaved Nylon 6,6 nanofibers between carbon/epoxy layers and cured the laminates at 150 °C. The morphology of the fractured surface showed that the nanofibers are completely visible between layers after the curing process. They considered the effect of mat thickness on GI. They found that increasing the thickness leads to decreasing fracture
Carbon/epoxy prepreg (twill 2/2 240 gsm) supplied by Impregnatex Composite Srl (Milan, Italy) was used as composite material. PVDF (SolefÒ 6008) provided by Solvay in the form of powder was used to produce electrospun nanofibers. Dimethyl sulfoxide and Aceton provided by Sigma Aldrich were used as solvents. 2.2. Producing nanofibers The nanofibrous membranes were prepared by electrospinning 15% (w/v) PVDF solutions in a mixture of Dimethyl sulfoxide (DMSO) and Acetone (30:70 v/v). The electrospinning process was carried out under the following conditions: applied voltage 12 kV, feed rate 0.01 mL/min, distance between the collector and tip of the needle 12 cm, at room temperature. The average fiber diameter was 500 ± 110 nm and the final thickness of the membrane was 30 ± 3 mm (see Fig. 1). 2.3. Laminate fabrication and test method The specimens were manufactured stacking 14 plies of preimpregnated woven fabric. A crack was initiated by inserting a 15 lm Teflon sheet in the mid-layer of the specimens; PVDF nanofibers were placed in the same interface. As shown in [14], if nanofibers melt, they are more effective than those which do not, therefore the curing process should overcome the melting point of the PVDF, which is 165–170 °C, according to the Solvay data sheet. For this reason, a prepreg that requires a high temperature curing process was chosen. In particular, the temperature followed a 4-step cycle: (1) from room temperature to 170 °C at 1 °C/min, (2) 1 h at 170 °C, (3) from 170 to 190 °C at 1 °C/min, (4) 20 min at 190 °C. The specimen dimensions are given by the ASTM D5528 [16] standard: width B = 20 mm, length L = 140 mm, nominal thickness t = 4.2 mm, and initial crack length a = 60 mm. Energy release rate for mode-I fracture testing (GI) is obtained from the beam theory presented in the same ASTM standard [16]:
GI ¼
3Pd 2Ba
ð1Þ
where P is the load and d is the displacement. It should be mentioned that for each configuration (reference and interleaved laminates), 3 specimens were fabricated and tested.
Fig. 1. SEM micrograph of PVDF nanofibers.
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Fig. 2. (A) Force – displacement and (B) mode-I fracture toughness – crack length curves for non-modified and PVDF modified laminates.
Table 1 Fracture parameters obtained from mode-I fracture tests.
Non-modified laminate PVDF-modified laminate Variation (%)
GI – initiation (J/m2)
GI – propagation (J/m2)
Maximum GI (J/m2)
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
254 362 +42.5
5 56
307 418 +36.2
17 12
349 476 +36.4
24 13
32.2 39.5 +22.7
2.0 0.5
toughness. Magniez et al. [14] also compared the effectiveness of PVDF on fracture behavior of composite laminates in forms of film and nanofiber. They showed that toughening of carbon fiber/epoxy composite by PVDF nanofibrous mats prepared by electrospinning was more effective than PVDF film, because the porous nature of the nanofibers did not hinder resin flow like a film could do [14]. In this study, it was found that PVDF nanofibers can increase mode-I fracture toughness of 43%. All the factors mentioned in this paper can be the reason for fracture toughness improvement, but understanding the effect of each one needs more focus in this regard. The main goal of the paper is presenting the fact that PVDF nanofibers have the capability to increase mode-I fracture toughness, differently to what was thought before. Anyway, nanofibrous mat thickness and PVDF crystallization type probably have the most important effect among the other factors, and further studies are planned in these directions. The morphology of the fractured surface is shown in Fig. 3. The cured PVDF is a brown material between the mid-plane of composite laminate, which because of the porosity of the nanofibrous mat can be passed through by the epoxy. A combination of brittle and
(A)
Maximum force (N)
ductile regions in the fractured surface can be observed. The brittle region shows the area in which the crack propagates near the fibers and so there is only matrix; in the ductile region, instead, the crack propagates through the blend of melted PVDF and epoxy and so a plastic deformation is visible. The rough and irregular shape of the fractured surface in the ductile region proves that a plastic zone existed in front of the crack tip during crack propagation. The mechanism of improving fracture toughness using PVDF nanofibers depends on plastic deformation of the ductile region, while non-modified samples consist only of the brittle fracture (without any or a little plastic deformation) [14]. The Plastic constraint at the crack tip in modified laminates absorbed more energy in comparison with the reference laminates during crack propagation; therefore it finally leads to an increase of fracture toughness. It is worth mentioning that some marks of nanofibers produced during the curing process can be seen in Fig. 3B. The formation of these marks can be described as follows: as a pre-preg composite was used in this study, the epoxy was partially cured in the factory. This initial curing process changed the liquid state of the epoxy to B-stage in which the epoxy is like a gel. After interleaving the
(B)
Ductile Region
Brittle Region
Fig. 3. Morphology of the fractured surface: (A) ductile and brittle regions and (B) more magnification of the ductile region.
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nanofibers and starting the main curing process, the epoxy started to be liquid again. In this stage, the epoxy could penetrate into the nanofibrous mat. After a specific time (Gel-time), the epoxy became gel-like while the nanofibers were still solid. Since the PVDF nanofibers melted around 170 °C and at this temperature the state of epoxy is almost solid (this temperature is near the end of the curing process), the marks seen in the picture are the position of the melted nanofibers among the solidified (cured) epoxy. 4. Conclusion In this study, electrospun polyvinylidene fluoride (PVDF) nanofibers are used to increase the mode-I fracture toughness (GI) of carbon/epoxy laminates. It had been shown before that PVDF was not a suitable choice for increasing GI, but this study proved that PVDF has the ability to enhance GI. SEM pictures of the fractured surface showed that plastic deformation of the matrix and melted PVDF play the main role in the fracture toughness enhancing process. Acknowledgements The authors thank the University of Bologna for providing a scholarship for this research and also the SpinBow company for manufacturing a suitable electrospinning machine for producing nanofibers. References [1] Saghafi H, Minak G, Zucchelli A. Effect of preload on the impact response of curved composite panels. Compos Part B – Eng 2014;60:74–81. [2] Yun NG, Won YG, Kim SC. Toughening of carbon fiber/epoxy composite by inserting polysulfone film to form morphology spectrum. Polymer 2004;45:6953–8. [3] Davoodi MM, Sapuan SM, Ahmad D, Aidy A, Khalina A, Jonoobi M. Effect of polybutylene terephthalate (PBT) on impact property improvement of hybrid kenaf/glass epoxy composite. Mater Lett 2012;67:5–7.
[4] Kelkar AD, Mohan R, Bolick R, Shendokar S. Effect of nanoparticles and nanofibers on mode-I fracture toughness of fiber glass reinforced polymeric matrix composites. Mater Sci Eng B – Solid 2010;168:85–9. [5] Li G, Li P, Yu Y, Jia X, Zhang S, Yang X, et al. Novel carbon fiber/epoxy composite toughened by electrospun polysulfone nanofibers. Mater Lett 2008;62:511–4. [6] Wong DWY, Lin L, McGrail PT, Peijs T, Hogg PJ. Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres. Compos Part A – Appl S 2010;41:759–67. [7] Li G, Li P, Zhang C, Yu Y, Liu H, Zhang S, et al. Inhomogeneous toughening of carbon fiber/epoxy composite using electrospun polysulfone nanofibrous membranes by in situ phase separation. Compos Sci Technol 2008;68:987–94. [8] De Schoenmaker B, Van der Heijden S, De Baere I, Van Paepegem W, De Clerck K. Effect of electrospun polyamide 6 nanofibres on the mechanical properties of a glass fibre/epoxy composite. Polym Test 2013;32:1495–501. [9] Magniez K, Chaffraix T, Fox B. Toughening of a carbon-fibre composite using electrospun poly(hydroxyether of bisphenol A) nanofibrous membranes through inverse phase separation and inter-domain etherification. Materials 2011;4:1967–84. [10] Saghafi H, Zucchelli A, Palazzetti R, Minak G. The effect of interleaved composite nanofibrous mats on delamination behavior of polymeric composite materials. Compos Struct 2014;109:41–7. [11] Palazzetti R, Trendafilova I, Zucchelli A. The self-reinforcing effect of Nylon 6,6 nano-fibres on CFRP composite laminates subjected to low velocity impact. Compos Struct 2013;106:661–71. [12] Alessi S, Di Filippo M, Dispenza C, Focarete ML, Gualandi C, Palazzetti R, Pitarresi G, Zucchelli A. Effects of Nylon 6,6 nanofibrous mats on thermal properties and delamination behavior of high performance CFRP laminates. Poly Compos. doi: 10.1002/pc.23035. [13] Zhang J, Yang T, Lin T, Wang CH. Phase morphology of nanofibre interlayers: critical factor for toughening carbon/epoxy composites. Compos Sci Technol 2012;72:256–62. [14] Magniez K, De Lavigne C, Fox BL. The effects of molecular weight and polymorphism on the fracture and thermo-mechanical properties of a carbonfibre composite modified by electrospun poly (vinylidene fluoride) membrane. Polymer 2010;51:2585–96. [15] Kim JK, Robertson RE. Toughening of thermoset polymers by rigid crystalline particle. J Mater Sci 1992;27:161–74. [16] ASTM D5528. Standard test method for mode-I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites. Annual book of ASTM standards; 2007. [17] Zheng J, He A, Li J, Han CC. Polymorphism control of poly(vinylidene fluoride) through electrospinning. Macromol Rapid Commun, Macromol Rapid Commun 2007;28:2159–62. [18] Palazzetti R, Yan X, Zucchelli A. Influence of geometrical features of electrospun Nylon 6,6 interleave on the CFRP laminates mechanical properties. Poly Compos 2013;35:137–50.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of PVDF nanofibers on mode-I fracture toughness of composite materials H. Saghafi ⇑, T. Brugo, G. Minak, A. Zucchelli Department of Industrial Engineering (DIN), Alma Mater Studiorum, Università di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 23 August 2014 Received in revised form 3 December 2014 Accepted 9 December 2014 Available online 15 December 2014 Keywords: A. Laminates B. Fracture toughness D. Mechanical testing Interleaving
a b s t r a c t In this study, the fracture behavior of carbon/epoxy laminates interleaved by polyvinylidene fluoride (PVDF) nanofibers is investigated. For this aim, a mode-I fracture test is conducted on virgin and modified laminates. Unlike the results of other studies, it is shown that PVDF nanofibers can increase mode-I fracture toughness (GI) noticeably in a specific situation. The results show that GI is enhanced about 43% and 36% in initiation and propagation stages of the fracture, respectively, using PVDF nanofibers. The morphology of the fractured surface is also presented for investigating the mechanism of toughening. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Epoxy resin is commonly used as a thermosetting matrix for many polymeric composite materials, but its low toughness restricts its application to structural components. For example, significant damage and delamination can occur in the structure under a very low-velocity-impact (even under 10 J) [1]. So far, many attempts have been made to increase the delamination resistance of epoxy-based composite and a novel method has finally been introduced using thermoplastic polymers to modify the epoxy matrix [2,3]. The toughening material can be either used in the form of film, fiber, or particle, placed between the laminate layers [4–6]. Using film is easier than particles, since obtaining a uniform distribution of particles in the matrix, or between composite layers, is a very complicated process, while nanofibrers can be easily uniformly inserted between the layers. Moreover, nanofibrous toughening materials are more effective than film in toughening the epoxy [7]. So far, various kinds of polymeric nanofibers have been applied to increase the fracture toughness of composite materials: polysulfone (PSF) [7], Nylon 6 [8], Phenoxy [9], Nylon 6,6 [10–12], poly(e-caprolactone) (PCL) [13], polyvinylidene fluoride (PVDF) [14] and etc. Although several studies have been conducted for many of these nanofibers, only a few research papers have been reported in the literature about the toughening effect of PVDF on
⇑ Corresponding author. E-mail address: Hamed.saghafi[email protected] (H. Saghafi). http://dx.doi.org/10.1016/j.compositesb.2014.12.015 1359-8368/Ó 2014 Elsevier Ltd. All rights reserved.
epoxy [13–15]. Zhang et al. [13] used three different electrospun nanofibers: PCL, PVDF, and PAN between composite layers, and compared their effect and considered their toughening mechanism. Their results showed that only PCL can toughen epoxy. Magniez et al. [14] employed low and high molecular weight PVDF for investigating its efficiency in mode-I and mode-II fracture toughness of carbon/epoxy composite materials. They showed that the use of both types of PVDF can increase mode-II fracture toughness but at the same time it slightly decreases fracture toughness in mode-I. Furthermore, they showed that molecular weight slightly affected the PVDF crystal forms and the morphology of the fractured surface; at the same time it did not significantly affect mode-I and mode-II fracture toughness. The difference of the study of Zhang and that of Magniez is in the curing processes. In the first one [13], specimens were cured at 150 °C and so PVDF did not melt; on the other hand, in [14] the curing temperature was 175 °C, which caused the fibers to melt. Results showed that fracture toughness enhanced as the curing temperature increased and nanofibers melted. The researchers in [13,14] used a very thick nanofibrous mat (about 70 lm) and, as mentioned above, the curing temperatures in [13,14] were below and above the PVDF’s melting point, respectively. Therefore in this paper, it is shown that, by changing the thickness of the nanofibrous mat, curing and electrospinning process, the fracture toughness in mode-I can be increased by incorporating PVDF nanofibers. The effect of all these parameters is introduced in the following sections. The SEM pictures of the fractured interfaces will then be used to explain the role of the nanofibers in the interface.
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2. Materials and method
3. Results and discussion
2.1. Materials
Fig. 2 presents the fracture behavior of the reference and modified laminates in mode-I fracture loading. As seen in Fig. 2A, the trend of both the curves is almost the same until fracture initiation. When the fracture starts to propagate, and during its propagation, interleaved laminate always tolerates a force higher than the reference laminate. Fig. 2B shows the variation of mode-I fracture toughness during crack propagation for both the non-modified and modified composite laminates. According to the curves, GI is minimum at the start of fracture and after that increases with a very low slope. Black lines fitted on the curves in Fig. 2B are used to consider the general behavior of GI during crack propagation. The initial fracture toughness is much higher in modified laminate when compared with the reference one. Table 1 summarizes the parameters that can be obtained from the fracture tests. Interleaving PVDF nanofibers increased GI about 43% and 36% in fracture initiation and crack propagation stages, respectively. The nanomodified specimens’ maximum GI and maximum force were increased by 36% and 23%, respectively, compared to virgin specimens. The results obtained in this study are in contrast to two other studies [13,14] in which the use of PVDF did not increase fracture toughness in mode-I composite laminate specimens. In [13], modeI fracture toughness of specimens was not affected by interlaying PVDF nanofibers in the laminate, while in [14] the GI decreased by about 20%. The differences between the outcomes of this study with [13,14] are due to the following parameters: 1 – curing process, 2 – the thickness of nanofibrous membrane, 3 – the crystallization type of PVDF after electrospinning and curing processes. Magniez et al. [14] cured the specimens at 150 °C for 10 min while the melting point of PVDF is about 165–170 °C and so the nanofibers were visible in the morphology of the fractured surface shown in the paper. Zhang et al. [13] used a different curing process: 125 °C for 1.5 h and 175 °C for 2 h, which caused the nanofibers to melt. By increasing the curing temperature from 150 °C to 175 °C, the effect of PVDF nanofibers on fracture toughness increased from 20% to 0% as mentioned above. The thickness of the nanofibrous membrane in both of these studies was about 70 lm. So far, many studies have been conducted regarding the effect of electrospinning and curing processes on PVDF crystallization [17]. Zheng et al. [17] showed that parameters such as the solvents, electrospinning temperature, feeding rate, and tip-to-collector distance could affect the crystal phases (a, b, and c) of PVDF fibers during the electrospinning process. Magniez et al. [14] foresaw that fracture toughness in mode-I can be improved probably by increasing the a phase in PVDF, but so far no studies have been done in this regard. They only used two kinds of PVDF with different molecular weights, which eventually caused a small difference in the form of phase crystals. The thickness of the nanofibrous mat is another important factor that can affect the toughening of the epoxy. As described in [13], PVDF is immiscible with epoxy. So, when the thickness of the nanomat mat is very thick and nanofibers are melted, it is difficult to bond the layers. However, reducing the nanomat’s thickness allows the epoxy to fully penetrate the nanofibrous mat, which is highly porous, and to bond the layers, letting the PVDF play its toughening role. There is some evidence on this topic in other studies [14,18]. Nylon 6,6 is a polymer whose melting point is around 260 °C. Palazzetti et al. [18] interleaved Nylon 6,6 nanofibers between carbon/epoxy layers and cured the laminates at 150 °C. The morphology of the fractured surface showed that the nanofibers are completely visible between layers after the curing process. They considered the effect of mat thickness on GI. They found that increasing the thickness leads to decreasing fracture
Carbon/epoxy prepreg (twill 2/2 240 gsm) supplied by Impregnatex Composite Srl (Milan, Italy) was used as composite material. PVDF (SolefÒ 6008) provided by Solvay in the form of powder was used to produce electrospun nanofibers. Dimethyl sulfoxide and Aceton provided by Sigma Aldrich were used as solvents. 2.2. Producing nanofibers The nanofibrous membranes were prepared by electrospinning 15% (w/v) PVDF solutions in a mixture of Dimethyl sulfoxide (DMSO) and Acetone (30:70 v/v). The electrospinning process was carried out under the following conditions: applied voltage 12 kV, feed rate 0.01 mL/min, distance between the collector and tip of the needle 12 cm, at room temperature. The average fiber diameter was 500 ± 110 nm and the final thickness of the membrane was 30 ± 3 mm (see Fig. 1). 2.3. Laminate fabrication and test method The specimens were manufactured stacking 14 plies of preimpregnated woven fabric. A crack was initiated by inserting a 15 lm Teflon sheet in the mid-layer of the specimens; PVDF nanofibers were placed in the same interface. As shown in [14], if nanofibers melt, they are more effective than those which do not, therefore the curing process should overcome the melting point of the PVDF, which is 165–170 °C, according to the Solvay data sheet. For this reason, a prepreg that requires a high temperature curing process was chosen. In particular, the temperature followed a 4-step cycle: (1) from room temperature to 170 °C at 1 °C/min, (2) 1 h at 170 °C, (3) from 170 to 190 °C at 1 °C/min, (4) 20 min at 190 °C. The specimen dimensions are given by the ASTM D5528 [16] standard: width B = 20 mm, length L = 140 mm, nominal thickness t = 4.2 mm, and initial crack length a = 60 mm. Energy release rate for mode-I fracture testing (GI) is obtained from the beam theory presented in the same ASTM standard [16]:
GI ¼
3Pd 2Ba
ð1Þ
where P is the load and d is the displacement. It should be mentioned that for each configuration (reference and interleaved laminates), 3 specimens were fabricated and tested.
Fig. 1. SEM micrograph of PVDF nanofibers.
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Fig. 2. (A) Force – displacement and (B) mode-I fracture toughness – crack length curves for non-modified and PVDF modified laminates.
Table 1 Fracture parameters obtained from mode-I fracture tests.
Non-modified laminate PVDF-modified laminate Variation (%)
GI – initiation (J/m2)
GI – propagation (J/m2)
Maximum GI (J/m2)
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
254 362 +42.5
5 56
307 418 +36.2
17 12
349 476 +36.4
24 13
32.2 39.5 +22.7
2.0 0.5
toughness. Magniez et al. [14] also compared the effectiveness of PVDF on fracture behavior of composite laminates in forms of film and nanofiber. They showed that toughening of carbon fiber/epoxy composite by PVDF nanofibrous mats prepared by electrospinning was more effective than PVDF film, because the porous nature of the nanofibers did not hinder resin flow like a film could do [14]. In this study, it was found that PVDF nanofibers can increase mode-I fracture toughness of 43%. All the factors mentioned in this paper can be the reason for fracture toughness improvement, but understanding the effect of each one needs more focus in this regard. The main goal of the paper is presenting the fact that PVDF nanofibers have the capability to increase mode-I fracture toughness, differently to what was thought before. Anyway, nanofibrous mat thickness and PVDF crystallization type probably have the most important effect among the other factors, and further studies are planned in these directions. The morphology of the fractured surface is shown in Fig. 3. The cured PVDF is a brown material between the mid-plane of composite laminate, which because of the porosity of the nanofibrous mat can be passed through by the epoxy. A combination of brittle and
(A)
Maximum force (N)
ductile regions in the fractured surface can be observed. The brittle region shows the area in which the crack propagates near the fibers and so there is only matrix; in the ductile region, instead, the crack propagates through the blend of melted PVDF and epoxy and so a plastic deformation is visible. The rough and irregular shape of the fractured surface in the ductile region proves that a plastic zone existed in front of the crack tip during crack propagation. The mechanism of improving fracture toughness using PVDF nanofibers depends on plastic deformation of the ductile region, while non-modified samples consist only of the brittle fracture (without any or a little plastic deformation) [14]. The Plastic constraint at the crack tip in modified laminates absorbed more energy in comparison with the reference laminates during crack propagation; therefore it finally leads to an increase of fracture toughness. It is worth mentioning that some marks of nanofibers produced during the curing process can be seen in Fig. 3B. The formation of these marks can be described as follows: as a pre-preg composite was used in this study, the epoxy was partially cured in the factory. This initial curing process changed the liquid state of the epoxy to B-stage in which the epoxy is like a gel. After interleaving the
(B)
Ductile Region
Brittle Region
Fig. 3. Morphology of the fractured surface: (A) ductile and brittle regions and (B) more magnification of the ductile region.
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nanofibers and starting the main curing process, the epoxy started to be liquid again. In this stage, the epoxy could penetrate into the nanofibrous mat. After a specific time (Gel-time), the epoxy became gel-like while the nanofibers were still solid. Since the PVDF nanofibers melted around 170 °C and at this temperature the state of epoxy is almost solid (this temperature is near the end of the curing process), the marks seen in the picture are the position of the melted nanofibers among the solidified (cured) epoxy. 4. Conclusion In this study, electrospun polyvinylidene fluoride (PVDF) nanofibers are used to increase the mode-I fracture toughness (GI) of carbon/epoxy laminates. It had been shown before that PVDF was not a suitable choice for increasing GI, but this study proved that PVDF has the ability to enhance GI. SEM pictures of the fractured surface showed that plastic deformation of the matrix and melted PVDF play the main role in the fracture toughness enhancing process. Acknowledgements The authors thank the University of Bologna for providing a scholarship for this research and also the SpinBow company for manufacturing a suitable electrospinning machine for producing nanofibers. References [1] Saghafi H, Minak G, Zucchelli A. Effect of preload on the impact response of curved composite panels. Compos Part B – Eng 2014;60:74–81. [2] Yun NG, Won YG, Kim SC. Toughening of carbon fiber/epoxy composite by inserting polysulfone film to form morphology spectrum. Polymer 2004;45:6953–8. [3] Davoodi MM, Sapuan SM, Ahmad D, Aidy A, Khalina A, Jonoobi M. Effect of polybutylene terephthalate (PBT) on impact property improvement of hybrid kenaf/glass epoxy composite. Mater Lett 2012;67:5–7.
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