Flexural and compression response of woven E-glass/polyester–CNF nanophased composites

Composites: Part A 42 (2011) 1774–1782

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Flexural and compression response of woven E-glass/polyester–CNF nanophased composites M.K. Hossain ⇑, M.E. Hossain, M.V. Hosur, S. Jeelani Center for Advanced Materials, Tuskegee University, Tuskegee, AL 36088, United States

a r t i c l e

i n f o

Article history: Received 30 December 2010 Received in revised form 26 July 2011 Accepted 29 July 2011 Available online 6 August 2011 Keywords: A. Glass fibers B. Mechanical properties D. Electron microscopy

a b s t r a c t A significant improvement in fiber reinforced polymeric composite (FRPC) materials can be obtained by incorporating a very small amount of nanofiller in the matrix material. In this work, an ultrasonic liquid processor was used to infuse carbon nanofiber (CNF) into the polyester matrix which was then mixed with catalyst using a mechanical agitator. Both conventional and CNF-filled glass-fiber reinforced polyester composites (GRPC) were fabricated using the vacuum assisted resin transfer molding (VARTM) process. Excellent dispersion of CNFs into the polyester resin was observed from the scanning electron microscopy (SEM) micrographs. Flexural and quasi-static tests were performed for investigating the mechanical responses. Fracture surface was examined using optical microscopy (OM) and SEM. Flexure tests performed on the conventional GRPC, 0.1–0.4 wt.% CNF-filled GRPC showed up to 49% and 31% increase in the flexural strength and modulus, respectively, compared to the conventional one with increasing loading of CNFs up to 0.2 wt.%. Similar trend was seen in quasi-static compression properties. SEM evaluation revealed relatively less damage in the tested fracture surfaces of the nanophased composites in terms of matrix failure, fiber breakage, matrix–fiber debonding, and delamination, compared to the conventional one. This might be the result of better interfacial interaction between matrix and fibers, due to the presence of CNFs. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Fiber reinforced polymeric matrix composites have become attractive structural materials in aerospace, marine, armored vehicles, automobile, railways, civil engineering structures, sporting goods, etc. due to their high specific strength and stiffness [1]. Most widely used reinforcement is glass fiber because of its low cost, high tensile and impact strength, light weight, and good corrosion resistance. Polyester resin reinforced with glass fiber is the material of choice for applications in marine, structural, automobile, and railway industry. However, these composites have some limitations including the matrix dominated properties which often limit their extensive applications. A significant improvement in the properties of engineering structural polymeric composite materials can be achieved by incorporating a small amount of organic or inorganic fillers at nanoscale level due to the promising nature of nanoparticles as reinforcements in the polymer matrix. Hence, the objective of this research work is to improve fiber reinforced polyester matrix composite properties with uniform dispersion of carbon nanofiber (CNF). Numerous studies have revealed the potential enhancement in properties and performances of matrix materials in which nano

⇑ Corresponding author. Tel.: +1 334 727 8128; fax: +1 334 724 4224. E-mail address: [email protected] (M.K. Hossain). 1359-835X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2011.07.033

and micro-scale particles were integrated. Several methods have been used to incorporate nanoparticles like nanoclay, single- and multi-walled carbon nanotubes, carbon nanofibers, etc. to amend the matrix properties of composite materials. Pinnavaia and coworkers [2–4] showed the possibility of significant improvement in the tensile strength and modulus of epoxy by adding organophilic montmorillonite into Diglycidylether of Bisphenol A (DGEBA). Schmidt [5], Novak [6], Usuki et al. [7] and Mark [8] showed better dispersion of Al2O3 particles into organic polymer. Hussain et al. [9] established the feasibility of dispersing nanoparticles in epoxy matrix and investigated their effect on the mechanical properties of carbon fiber reinforced polymer composites. Haque et al. [10] concluded that 1 wt.% nanosilicates addition in the S2-glass/ epoxy–clay nanocomposites resulted in an improvement of 44%, 24%, and 23% in the interlaminar shear strength, flexural strength, and fracture toughness, respectively. Carbon nanofiber reinforced polymer composites using vapor grown carbon nanofiber (VGCNF) have shown increased mechanical properties through better interfacial adhesion and fiber alignment. Rana et al. [11] achieved uniform dispersion of CNFs into epoxy resin applying a combination of ultrasonication and the use of solvent and surfactants. Iwahori et al. [12] showed 45% and over 17% improvement in the tensile modulus and strength, respectively, in the epoxy composites dispersing 10 wt.% of the AR10 and AR50 CNFs; this improvement was attributed to

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increased crack propagation resistance by bridging effect of CNF. CNF dispersed resin was impregnated into the carbon fiber and cured by hot press to fabricate carbon fiber reinforced composites in their study. Xu et al. [13] have confirmed that infusion of VGCNF had little influence on the flexural properties and glass transition temperature but increased the storage modulus of the VGCNF/vinyl ester composite using different types of VGCNF. Pervin et al. [14] and Morales et al. [15] used high speed dispersion mixer to disperse CNFs into SC-15 epoxy matrix material. Researchers have dispersed different types of nanoparticles into the virgin polymeric matrix using acoustic cavitation [16–25]. Eskin concluded in his papers [20–22] that acoustic cavitation is one of the efficient ways to disperse nanoparticles into polymeric materials. Chisholm et al. [18] reported that using ultrasonic liquid processor leads to homogeneous mix of epoxy polymer and nanoparticles like SiC. Hsiao and Gangireddy [23] used acetone and sonication mixing method for the dispersion of CNF in unsaturated polyester resin. Sadeghian et al. [25] enhanced mode-I delamination resistance by 100% with addition of 1 wt.% CNFs to polyester/glass fiber composites through sonication method. Miyagawa et al. [26] investigated thermophysical and mechanical properties of amine-cured epoxy/clay nanocomposites. Relative to the value of neat epoxy below the Tg, the storage and tensile moduli were 50% greater at 10 wt.% (6.0 vol.%) of clay nanoplatelets. The storage modulus increased due to the increase in the clay wt.%. Addition of only 2.5 wt.% of organoclay yielded 13% increase in storage modulus at 30 °C. Pervin et al. [14] documented significant improvement in thermal and mechanical properties of carbon nanofiber reinforced SC-15 epoxy. Chowdhury et al. [27] noticed improvement in thermal properties such as glass transition temperature and storage modulus of woven carbon/nanoclay-epoxy laminates. Morales et al. [15] used high speed dispersion mixer to disperse CNF into matrix and observed improvement in flexural and tensile strengths with addition of 1 wt.% CNFs in the glass/polyester composite by injection molding for better adhesion between matrix and glass fibers due to the presence of CNFs. To the best of our knowledge, no study was reported in the open literature that explained the optimum amount of CNFs needed to obtain maximum improvement in flexural and compressive properties of glass/polyester–CNF nanocomposites. In this work, glass reinforced polyester nanocomposites with 0.1–0.4 wt.% CNFs loading and conventional composite were fabricated using the vacuum assisted resin transfer molding (VARTM) process. Glass/polyester nanocomposites were characterized through flexural and quasistatic compression tests, optical microscopy, and SEM analysis. The experimental results were used to assess the influence of CNFs on the properties of glass/polyester nanocomposites. 2. Experimental 2.1. Materials selection Commercially available B-440 premium polyester resin and styrene from US Composite Inc., West Palm Beach, Florida, USA, heat treated PR-24 CNF from Pyrograf Products Inc., an affiliate of Applied Sciences, Inc., Cedarville, Ohio, USA, and plain weave E-glass fiber from fiberglasssite, Kingsville, Maryland, USA were considered as matrix, thinner, nanoparticle, and reinforcement, respectively, in the current study. Polyester resin contains two-part: part-A (polyester resin) and hardener part-B (MEKP – methyl ethyl ketone peroxide). 2.2. Resin preparation In this study, sonication was performed using a high intensity ultrasonic irradiation (Ti-horn, 20 kHz Sonics Vibra Cell, Sonics

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Mandmaterials, Inc., USA) for 90 min by adding 0.1–0.4 wt.% CNFs into polyester that had already styrene in it. However, an additional 10 wt.% of styrene was used to reduce the viscosity to facilitate fabrication of composite panel using the VARTM process [28– 32]. The mixing process was carried out in a pulse mode of 30 s on/ 15 s off at an amplitude of 50%. To reduce the void formation, desiccation was carried out using Brand Tech Vacuum system for about 90–120 min. Once the bubbles were completely removed from the mix, 0.7 wt.% catalyst was mixed using a high-speed mechanical stirrer for about 2–3 min and vacuum was again applied for about 6–8 min to degasify the bubbles produced during the catalyst mixing. In parallel, neat polyester samples were also fabricated. 2.3. Composite fabrication Both conventional and nanophased E-glass/polyester–CNF composites were manufactured by the VARTM process. Vacuum was maintained until the end of cure to remove any volatiles generated during the polymerization, while keeping the pressure of one atmosphere. The panels were cured for about 12–15 h at room temperature and then thermally post cured at 110 °C for 3 h in a mechanical convection oven [21]. The fiber volume fraction for the nanophased glass reinforced polyester composites fabricated by VARTM was found to be around 56% and reasonable limit of the void content (3–4%). 2.4. Scanning electron microscopy (SEM) SEM studies were carried out to examine change in the microstructure due to addition of CNFs. SEM analyses were carried out using JEOL JSM 5800. The samples were positioned on a sample holder with a silver paint and coated with gold to prevent charge build-up by the electron absorbed by specimen. 2.5. Flexure Test Flexural tests under three-point bend configuration were performed using Zwick Roell testing unit according to the ASTM D790–02 standard [33] to evaluate flexural modulus and strength of each of the material systems of the polymer nanocomposites and its laminates. The machines were run under displacement control mode at a crosshead speed of 2.0 mm/min [14] and tests were performed at room temperature. The span to depth ratio was maintained at 16:1. The maximum stress at failure on the tension side of a flexural specimen was considered as the flexural strength of the material. Flexural modulus was calculated from the slope of the stress–strain plot. Five samples of each type were tested. The average values and standard deviation of flexural strength and modulus were determined. 2.6. Quasi-Static Compression Test In order to investigate the quasi-static compression response, the specimens were tested in the thickness direction using servohydraulically controlled Material Testing System (MTS) machine. The ASTM D 695–10 standard was followed for this quasi-static compression test. The capacity of the MTS machine is approximately 10,000.00 kg. The test was carried out at the displacement control mode with the crosshead speed of 1.27 mm/min. In order to maintain evenly distributed compressive loading, each specimen was sanded and polished so that the opposite faces were parallel to each another. Test Ware-SX software was used to develop a program which controls the test conditions and records both the load and crosshead displacement data. The load–deflection data recorded by the data acquisition system was converted to the

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stress–strain curve after dividing load by initial cross-sectional area of the specimen and deflection by specimen thickness. 2.7. Optical Microscopy An optical microscope was used to investigate the failure mode and crack propagation of the failed specimen. The optical microscopy was performed using Olympus SZX16. The Olympus SZX16 provides a large zoom ratio of 16.4:1. With this seamless zoom ratio combined with the most comprehensive range of parfocal objectives (0.5x, 1.0x, 1.6x & 2.0x), the SZX16 can be used to take micrograph from a macro-view to a micro-view allowing visualization of whole organism down to fine microscopic structures. The fractured surfaces were exposed to the optical microscope using polarized light. Fig. 2. SEM micrograph of 0.2 wt.% CNF-loaded polyester matrix. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. Scanning electron microscopy (SEM) analysis The SEM micrographs of as-received PR-24 CNF, the neat polyester matrix, and the 0.2 wt.% CNFs infused polyester matrix are shown in Figs. 1a, b and 2, respectively. From the micrograph of 0.2 wt.% CNF-filled polyester, excellent dispersion of CNFs was found. Only broken ends of CNFs were observed near the surface. Some CNFs were broken in a brittle manner and some were pulled out. Strong attractive fiber van der Waals forces cause CNFs to agglomerate, which reduces the strength of the nanocomposite by stress concentration effect. Agglomerates of CNFs, called nanoropes, are difficult to separate and infiltrate with matrix. They entangle and form nest-like structures due to their curvature and high aspect ratios. Both disagglomeration and dispersion in resins depend on the relative van der Waals forces, curvature, and on the relative surface energy of CNFs versus that of the resin. To overcome attractive forces, researchers have been extensively using mechanical energy, intense ultrasonication, and high speed shearing. Some rebundling of the aggregates is possible even after discontinuation of the external force [34]. However, optimal loading and uniform dispersion of CNFs in matrix are the key parameters to promote better nanofiber–matrix interface properties to reach an efficient load transfer between the two constituents of the nanocomposite [35,36]. In order to further investigate the dispersion of CNFs in the polyester, concentrated nitric acid was added on the cleavage surfaces to partly unveil the CNFs formerly covered by the polyester. The etched surfaces were then studied by SEM (Figs. 2, 3a and 3b). It can be easily observed that the interfacial

bonding between the CNF and matrix was very compact which would allow CNFs to be anchored in the embedding matrix [37]. In the current study, uniform dispersion of 0.2 wt.% CNFs into the polyester resin was achieved using the sonication mixing method for 90 min. High magnification SEM micrograph in Fig. 3b clearly exhibits that CNFs are well separated and uniformly

Fig. 3a. SEM micrograph of acid-etching 0.2 wt.% CNF-loaded polyester. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 1. SEM micrograph of (a) as-received PR-24 CNF (b) neat polyester matrix.

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Fig. 3b. SEM micrograph of acid-etching 0.2 wt.% CNF-loaded polyester. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 5. Flexural stress–strain plot of polyester samples with different wt.% of CNF.

embedded in the 0.2% polyester resin system. These CNFs are likely to interlock and entangle with the polymer chains in the matrix [37]. Thus, addition of CNFs enhanced the crosslinking between polymer chains and provided better interfacial bonding. Fig. 4a and b shows the woven glass reinforced polyester laminates with 0.2 wt.% CNFs. It was found that the resin was distributed uniformly over the fabric and the interfacial bonding between matrix and fiber was very good. Resin flow and impregnation of the glass fibers were observed in the SEM micrographs. Clear resin matrix adhesion is present in these micrographs and glass fibers are observed to be embedded within the matrix. Good matrix-fiber wetting was achieved and resin is also visible in between the glass fiber filaments. It appears that better interfacial bonding between the nanophased polymer matrix and glass fiber is present due to the presence of CNFs [38]. The fiber volume fraction as determined from matrix digestion method for the nanophased glass reinforced polyester composites fabricated by VARTM was found to be around 56%.

3.2. Flexural test results Flexural tests were performed to evaluate the bulk stiffness and strength of neat and nanophased polyester samples and their fiber reinforced laminates. Typical stress–strain behaviors from the flexural tests are shown in Figs. 5 and 6. The positive effect of CNFs was evident from these stress–strain plots with improved strength and stiffness in the CNF-loaded nanophased composites. Flexural

Strength, modulus, and the strain at maximum strength for all nanophased samples were larger than those of the neat samples due to the addition of CNFs as CNF has high aspect ratio which can prevent crack generation and propagation in the polyester matrix. The average properties of the neat polyester and CNF-filled polyester (CNF-FP) obtained from these tests are shown in Fig. 5. In all cases, the samples failed rapidly after experiencing the maximum load showing induced brittle nature of failure due to the addition of CNFs. From the analysis, it was found that the 0.2 wt.% CNFs loading and 90 min sonication showed the optimal condition. The 0.2 wt.% CNF-loaded polyester samples enhanced the flexural strength and modulus by about 88% and 16%, respectively, compared to the neat polyester samples. The failure strain also increased significantly with the addition of CNFs into the polyester matrix system. The flexural properties were slightly decreased at higher CNF content. It might be due to the development of CNFs micro aggregates in various regions of the polymer matrix, which act as areas of weakness. In essence, better dispersion methods are needed for higher CNFs loading. Flexure tests were performed on the unfilled, 0.1–0.4 wt.% CNFfilled glass reinforced polyester composites (GRPC) to evaluate their bulk stiffness and strength. Their typical stress–strain behaviors are shown in Fig. 6. It is clear from these stress–strain curves that all the nanophased composites showed significant improvement in the mechanical properties up to the 0.2 wt.% of CNFs loading, beyond which there was a decreasing trend. The curves showed considerable nonlinear deformation before reaching the

Fig. 4. 0.2 wt.% CNF-loaded GRPC laminates (a and b).

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Fig. 6. Flexural stress–strain plot of GRPC laminates with different wt.% of CNF.

Table 1 Flexural test results of GRPC laminates with different wt.% of CNF. Glass reinforced polyester composites

Flexural strength (MPa)

Gain in strength (%)

Flexural modulus (GPa)

Gain in modulus (%)

Conventional GRP 0.1% CNF-LGRP 0.2% CNF-LGRP 0.3% CNF-LGRP 0.4% CNF-LGRP

174 ± 5.8 228 ± 9.4 260 ± 4.3 248 ± 8.3 220 ± 5.2

– 31 49 43 26

16 ± 0.8 19 ± 0.5 21 ± 1.3 20 ± 1.7 18 ± 0.1

– 19 31 25 13

(a)) 0.1 wt.% % CNF F-L LGR RPC C laamiinatte

(c) 0.3 wt.% CNF-LGRPC laminate

Fig. 7. Stress–strain curves of the conventional and CNF-loaded GRPC.

Table 2 Quasi-static (strain rate 10 Composite name

3

) results of conventional and CNF-loaded GRPC.

Max. Stress (MPa)

Neat GRP 80.72 ± 6.27 0.1 wt.% CNF-FGRP 106.98 ± 1.65 0.2 wt.% CNF-FGRP 115.71 ± 3.25 0.3 wt.% CNF-FGRP 95.4 ± 2.75

Improvement Modulus (%) (GPA)

Improvement (%)

– 32.50 43.35 18.19

– 46.03 60.05 35.98

3.78 ± 0.25 5.52 ± 0.48 6.05 ± 0.27 5.14 ± 0.44

(bb) 0.2 wt.% CN NF-LG GRP PC lam minaate

(d) Conventional GRPC laminate

Fig. 8. Optical micrograph of flexural tested samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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compression stiffness and strength. Five samples were tested for each condition and the average properties were obtained from these tests. Their typical quasi-static (low strain rate, 10 3 s 1) stress–strain behaviors are shown in Fig. 7. The highest value of stress is termed as the peak stress or maximum stress and the corresponding strain value is hereafter mentioned as the strain at maximum stress. The modulus is determined from the slope of the linear portion of the engineering stress versus strain curve. From the stress–strain curves, it was found that the incorporation of CNFs enhanced the stress and modulus of the E-glass/polyester composite. The 0.2 wt.% CNF-loaded laminates showed the best improvement in the stress and modulus. CNF acts as crack propagation resistance. There was a slight improvement in the strain at the peak stress due to the addition of CNFs. In all the cases, after reaching maximum stress, the samples were failed. However, brittle failure was observed in each type of laminate sample and no obvious yield point was found. It was evident that the 0.2 wt.% CNF-loaded GRPC showed the maximum enhancement in the compressive strength and modulus by about 43% and 60%, respectively, compared to the conventional GRPC samples. This enhancement was also consistent with Ma et al. [39] investigations on the polyester/carbon nanofiber composites. Compressive failure in polymeric fibers occurs by yielding which consequences in the development and spread of kinks [40]. CNF might act as a barrier for kinks spread, thus ensuring the enhanced compressive strength. The summarized quasi-static (low strain rate) compression results of neat GRP and CNF-filled GRP (FGRP) composites are given in Table 2. 3.4. Fracture analysis

Fig. 9. (a) Bridging effect at the interface region of the long glass fiber, CNF, and the resin, and (b) 0.2 wt.% CNF-loaded polyester matrix stacked with glass fabric after fractured laminate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

maximum stress and also the irregularities in the curves were attributed to random fiber breakage during loading. However, more or less ductility was observed in each type of laminate sample and cracking noise was heard while the individual fiber broke or the inter-layer delaminated, but no obvious yield point was found. Most of the samples failed around the mid-point of the sample. From the resultant data, it was explored that the 0.2 wt.% CNFs was the optimum amount for this material system to achieve the maximum flexural modulus and strength. The three point bending tests results of GRP and CNF-filled GRP composites are summarized in Table 1. The 0.2 wt.% CNF-loaded nanophased GRPC system showed approximately 49% and 31% increase in the flexural strength and modulus, respectively. These improvements for CNFs are also consistent with Movva et al. investigations [32]. There might be several possible reasons for the better mechanical properties observed in the CNF infused glass fabric reinforced polyester laminates. Firstly, CNF increases the strength and modulus of the polyester matrix, which was observed in the CNF-loaded polyester in this study. Secondly, the presence of CNFs increases the crack propagation resistance and prevents crack generation by bridging effect at the interface region of the long glass fiber, CNF, and polyester matrix. Moreover, CNF has high aspect ratio, which improves the strength and modulus [12,32]. 3.3. Quasi-static compression tests Quasi-static tests were performed on the neat, 0.1–0.3 wt.% CNF-filled glass reinforced polyester composites to evaluate their

Results from the OM and SEM study substantiate the quantitative results obtained through flexural and compressive tests. Optical microscopy studies were carried out on the flexural tested samples of the conventional and nanophased GRPC composites. Fig. 8a–d illustrate the fracture surfaces of 0.1–0.3 wt.% CNF-loaded GRPC (CNF-LGRPC) and conventional GRPC samples, respectively. Fiber breakage and little kinking were found in the 0.1 wt.% and 0.2 wt.% CNF loaded samples shown in Fig. 8a and b, respectively. On the other hand, fiber pullout followed by fiber breakage was observed in the 0.3 wt.% loaded and conventional samples illustrated in Fig. 8c and d, respectively. It was very clear from these micrographs that the inter-layer delamination was also observable in the conventional and 0.3 wt.% CNF-loaded GRP (CNF-LGRP) composites. As CNF loading increases beyond 0.2 wt.%, CNFs start to agglomerate and these agglomerations produce stress concentration which might act as crack initiation sites [41]. It was evident from the optical micrographs that the 0.2 wt.% CNF system promotes good interfacial bonding between the fiber and matrix. From the SEM micrograph taken at higher magnification as shown in Fig. 9a, excellent bridging effect in the interfacial region of the long glass fiber, CNF, and matrix was observed. CNF has high aspect ratio which can prevent crack propagation and crack generation resulting in improved performance. Some resin was stacked on the fractured glass fiber as shown in Fig. 9b, which represents the better adhesion due to the addition of CNF. The presence of polyester adhering to the fiber surface also suggests that the interfacial adhesion is stronger than matrix strength in nanophased composites [42]. Thus, it is evident from these micrographs that CNFs are anchored with both resin and fiber tightly that promotes a better interfacial bonding between the matrix and fiber. Better fiber–matrix interfacial bonding, and CNFs’ crack generation and propagation resistance result in higher strength in nanocomposites. On the other hand, the addition of the CNFs led to an improvement in the modulus of elasticity of the nanophased composites. This is attributed to the stiffened matrix of these composites

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00.1 wt..% CN NF-lloaded GRPC

00.3 wt..% CNF-lloadedd GRPC

0.2 wt..% CN NF-lloaded GRPC

C Connventioonall GRPC

Fig. 10. Fracture of the GRPC at quasi-static tests. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Fracture of (a) conventional, and (b) 0.2 wt.% CNF-loaded GRPC.

(Fig. 5). The interfacial area between the resin matrix and CNFs was increased because of the high aspect ratio of the CNFs, which in turn led to better mechanical properties [43]. The nanoparticles also act as reinforcing element and bear the load in the composite material system [44]. Again, both CNFs and fibers are stronger than matrix. Thus, when load is applied to the composite structures, matrix starts to crack first and stress is then transferred from the lower modulus matrix to the CNFs to the long fiber by bridging effect and ultimately the composites’ properties enhance. The thought derived previously for the enhancement in the mechanical properties with addition of small weight percentages of CNF (0.2 wt.%) in this material system was well justified. During service life, composite structures might encounter high stresses resulting in crack propagation through fiber matrix interfaces. Therefore, stronger adhesion between fiber and matrix, higher strength, and higher toughened matrix are desired. Improvement of flexural strength by addition of nanofillers into the matrix was expected to be observed for several reasons. Young’s modulus of the

second phase dispersed particles is higher than that of the matrix and thus stress transfer from the matrix to the particles will take place. As a result the strength of the composites is increased. Strong interfacial bonding between the fiber and matrix also contributes to higher flexural strength. Dispersed filler particles act as mechanical interlocking between fiber and matrix which creates a high friction coefficient. Finally, a mixed mode of fracture (flexural and shear) occurs under bending-load conditions. After an initial failure of fibers at the tensile side of the specimen, cracks are deflected parallel to the fibers and also to the applied load direction. The stress–strain curve in Fig. 6 shows a sharp increment with increasing load before reaching the maximum stress and then irregularities and staggered decrease in stress were observed for both conventional and nanophased composites. However, the initial load and the crack arrest area are higher in nanophased composites which lead to high energy absorbing mechanisms [42]. The fracture behavior of tested samples at the quasi-static strain rate of 10 3/s is shown in optical micrographs in Fig. 10.

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At first, some cracks were initiated in the samples followed by the kink band formation and crack propagation upon reaching the maximum peak stress, catastrophic load drop was observed and samples failed in the direction of compression loading (Fig. 10). It is evident from these micrographs that matrix cracking, kinking, and fiber breakage dominated the failure modes of 1–2 wt.% CNFloaded nanophased composites whereas matrix cracking and delamination were mostly observed in the 0.3 wt.% CNF-infused and conventional composites. The sonication process was unable to break the agglomerations of the 0.3 wt.% CNF-loaded composites completely. The modulus, a low deformation property, was not affected by the high stress concentrations caused by the agglomerated particles. However, the strength was reduced by initiating early failure in the matrix [35]. This explains the decrease in compressive strength observed in the 0.3% CNF-loaded glass fiber reinforced composite. It has also been reported that even at low concentration of nanoparticles the fracture energy of polyester nanocomposites could be doubled and prevent large scale fragmentation of polyester matrix [44]. This behavior is clearly seen from OM micrographs. For better understanding, fracture morphology of samples was studied using higher magnification SEM micrographs. The SEM micrographs of the fractured surfaces of the conventional and 0.2 wt.% CNF-loaded GRPC are illustrated in Fig. 11. For conventional composite shown in Fig. 11a, the surface of the fiber was clean, and no matrix adhered to the fiber. The fracture surface of the matrix was flat, and some cracks were seen in the matrix side near the fiber–matrix interface. Resin appears not to protrude from the surface of fibers. These results indicate that the interfacial bonding between the fiber and matrix was weak. The fracture surface of the nanophased composite (Fig. 11b) shows that the surface of the matrix was rougher than that of neat composite. CNFs were observed to be randomly but uniformly distributed in the matrix. The resin appears to cling to fibers well. The strengthened matrix held the glass fabrics together. The protrusion of the resin from the surface of the fibers accounts for the increase in fracture toughness of the samples. Moreover, the resin appears to be sticking to the fiber surface giving rise to a significant plastic deformation [45]. The plastic deformation enhances mechanical properties significantly in the nanophased composites (Figs. 6 and 7).

4. Conclusions Carbon nanofibers (CNF) were used as nanoparticle fillers in woven glass fiber-reinforced polyester composites. Better dispersion of CNFs was observed in the 0.2 wt.% CNF-loaded polyester resin and the fiber volume fraction for the nanophased GRPC fabricated by VARTM was found around 56% and reasonable limit of the void content. CNF infusion even at quite low concentrations enhanced the mechanical properties of the system. This investigation showed that CNF can be used without difficulty to modify the conventional fiber reinforced composite materials with the following outcomes:  Flexural test results of polyester samples with 0.2 wt.% CNFs indicated a maximum improvement in strength and modulus of about 88% and 16%, respectively, whereas glass reinforced polyester composite samples with 0.2 wt.% enhanced 49% and 31%, respectively.  GRPC laminates showed maximum enhancement in compressive strength and modulus by about 43% and 60%, respectively, with addition of CNFs up to 0.2 wt.% compared to neat GRPC samples.  The bridging effect of CNFs was observed in SEM micrographs of nanophased GRPC.

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 Optical micrographs of flexural fractured samples revealed interlayer delamination in conventional composite. However, no delamination was found in CNF-loaded GRPC for better interfacial interaction between fiber and matrix due to the presence of CNF.

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