Composites: Part A 42 (2011) 1774–1782

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Composites: Part A

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

M.K. Hossain ⇑, M.E. Hossain, M.V. Hosur, S. JeelaniCenter for Advanced Materials, Tuskegee University, Tuskegee, AL 36088, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 December 2010Received in revised form 26 July 2011Accepted 29 July 2011Available online 6 August 2011

Keywords:A. Glass fibersB. Mechanical propertiesD. Electron microscopy

1359-835X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.compositesa.2011.07.033

⇑ Corresponding author. Tel.: +1 334 727 8128; faxE-mail address: hossainm@tuskegee.edu (M.K. Hos

A significant improvement in fiber reinforced polymeric composite (FRPC) materials can be obtained byincorporating a very small amount of nanofiller in the matrix material. In this work, an ultrasonic liquidprocessor was used to infuse carbon nanofiber (CNF) into the polyester matrix which was then mixedwith catalyst using a mechanical agitator. Both conventional and CNF-filled glass-fiber reinforced polyes-ter composites (GRPC) were fabricated using the vacuum assisted resin transfer molding (VARTM) pro-cess. Excellent dispersion of CNFs into the polyester resin was observed from the scanning electronmicroscopy (SEM) micrographs. Flexural and quasi-static tests were performed for investigating themechanical responses. Fracture surface was examined using optical microscopy (OM) and SEM. Flexuretests 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 withincreasing 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 com-posites in terms of matrix failure, fiber breakage, matrix–fiber debonding, and delamination, comparedto the conventional one. This might be the result of better interfacial interaction between matrix andfibers, due to the presence of CNFs.

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1. Introduction

Fiber reinforced polymeric matrix composites have becomeattractive structural materials in aerospace, marine, armored vehi-cles, automobile, railways, civil engineering structures, sportinggoods, etc. due to their high specific strength and stiffness [1]. Mostwidely used reinforcement is glass fiber because of its low cost, hightensile and impact strength, light weight, and good corrosion resis-tance. Polyester resin reinforced with glass fiber is the material ofchoice for applications in marine, structural, automobile, and railwayindustry. However, these composites have some limitations includingthe matrix dominated properties which often limit their extensiveapplications. A significant improvement in the properties of engineer-ing structural polymeric composite materials can be achieved byincorporating a small amount of organic or inorganic fillers at nano-scale level due to the promising nature of nanoparticles as reinforce-ments in the polymer matrix. Hence, the objective of this researchwork is to improve fiber reinforced polyester matrix composite prop-erties with uniform dispersion of carbon nanofiber (CNF).

Numerous studies have revealed the potential enhancement inproperties and performances of matrix materials in which nano

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: +1 334 724 4224.sain).

and micro-scale particles were integrated. Several methods havebeen used to incorporate nanoparticles like nanoclay, single- andmulti-walled carbon nanotubes, carbon nanofibers, etc. to amendthe matrix properties of composite materials. Pinnavaia and co-workers [2–4] showed the possibility of significant improvementin the tensile strength and modulus of epoxy by adding organo-philic montmorillonite into Diglycidylether of Bisphenol A (DGE-BA). Schmidt [5], Novak [6], Usuki et al. [7] and Mark [8] showedbetter dispersion of Al2O3 particles into organic polymer. Hussainet al. [9] established the feasibility of dispersing nanoparticles inepoxy matrix and investigated their effect on the mechanical prop-erties 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 vaporgrown carbon nanofiber (VGCNF) have shown increased mechani-cal properties through better interfacial adhesion and fiber align-ment. Rana et al. [11] achieved uniform dispersion of CNFs intoepoxy resin applying a combination of ultrasonication and theuse 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 theAR10 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 andcured by hot press to fabricate carbon fiber reinforced compositesin their study. Xu et al. [13] have confirmed that infusion of VGCNFhad little influence on the flexural properties and glass transitiontemperature but increased the storage modulus of the VGCNF/vi-nyl ester composite using different types of VGCNF. Pervin et al.[14] and Morales et al. [15] used high speed dispersion mixer todisperse CNFs into SC-15 epoxy matrix material.

Researchers have dispersed different types of nanoparticles into thevirgin polymeric matrix using acoustic cavitation [16–25]. Eskin con-cluded in his papers [20–22] that acoustic cavitation is one of the effi-cient ways to disperse nanoparticles into polymeric materials.Chisholm et al. [18] reported that using ultrasonic liquid processorleads to homogeneous mix of epoxy polymer and nanoparticles likeSiC. Hsiao and Gangireddy [23] used acetone and sonication mixingmethod for the dispersion of CNF in unsaturated polyester resin. Sade-ghian et al. [25] enhanced mode-I delamination resistance by 100%with addition of 1 wt.% CNFs to polyester/glass fiber compositesthrough sonication method. Miyagawa et al. [26] investigated thermo-physical and mechanical properties of amine-cured epoxy/clay nano-composites. Relative to the value of neat epoxy below the Tg, thestorage and tensile moduli were 50% greater at 10 wt.% (6.0 vol.%) ofclay nanoplatelets. The storage modulus increased due to the increasein the clay wt.%. Addition of only 2.5 wt.% of organoclay yielded 13% in-crease in storage modulus at 30 �C. Pervin et al. [14] documented sig-nificant improvement in thermal and mechanical properties ofcarbon nanofiber reinforced SC-15 epoxy. Chowdhury et al. [27] no-ticed improvement in thermal properties such as glass transition tem-perature and storage modulus of woven carbon/nanoclay-epoxylaminates. Morales et al. [15] used high speed dispersion mixer to dis-perse CNF into matrix and observed improvement in flexural and ten-sile strengths with addition of 1 wt.% CNFs in the glass/polyestercomposite by injection molding for better adhesion between matrixand glass fibers due to the presence of CNFs.

To the best of our knowledge, no study was reported in the openliterature that explained the optimum amount of CNFs needed toobtain maximum improvement in flexural and compressive prop-erties of glass/polyester–CNF nanocomposites. In this work, glassreinforced polyester nanocomposites with 0.1–0.4 wt.% CNFs load-ing and conventional composite were fabricated using the vacuumassisted resin transfer molding (VARTM) process. Glass/polyesternanocomposites were characterized through flexural and quasi-static compression tests, optical microscopy, and SEM analysis.The experimental results were used to assess the influence of CNFson the properties of glass/polyester nanocomposites.

2. Experimental

2.1. Materials selection

Commercially available B-440 premium polyester resin and sty-rene from US Composite Inc., West Palm Beach, Florida, USA, heattreated PR-24 CNF from Pyrograf Products Inc., an affiliate of Ap-plied Sciences, Inc., Cedarville, Ohio, USA, and plain weave E-glassfiber from fiberglasssite, Kingsville, Maryland, USA were consid-ered as matrix, thinner, nanoparticle, and reinforcement, respec-tively, in the current study. Polyester resin contains two-part:part-A (polyester resin) and hardener part-B (MEKP – methyl ethylketone peroxide).

2.2. Resin preparation

In this study, sonication was performed using a high intensityultrasonic irradiation (Ti-horn, 20 kHz Sonics Vibra Cell, Sonics

Mandmaterials, Inc., USA) for 90 min by adding 0.1–0.4 wt.% CNFsinto polyester that had already styrene in it. However, an addi-tional 10 wt.% of styrene was used to reduce the viscosity to facil-itate 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, des-iccation was carried out using Brand Tech Vacuum system forabout 90–120 min. Once the bubbles were completely removedfrom the mix, 0.7 wt.% catalyst was mixed using a high-speedmechanical stirrer for about 2–3 min and vacuum was again ap-plied for about 6–8 min to degasify the bubbles produced duringthe catalyst mixing. In parallel, neat polyester samples were alsofabricated.

2.3. Composite fabrication

Both conventional and nanophased E-glass/polyester–CNF com-posites were manufactured by the VARTM process. Vacuum wasmaintained until the end of cure to remove any volatiles generatedduring the polymerization, while keeping the pressure of oneatmosphere. The panels were cured for about 12–15 h at roomtemperature and then thermally post cured at 110 �C for 3 h in amechanical convection oven [21]. The fiber volume fraction forthe nanophased glass reinforced polyester composites fabricatedby VARTM was found to be around 56% and reasonable limit ofthe void content (3–4%).

2.4. Scanning electron microscopy (SEM)

SEM studies were carried out to examine change in the micro-structure due to addition of CNFs. SEM analyses were carried outusing JEOL JSM 5800. The samples were positioned on a sampleholder with a silver paint and coated with gold to prevent chargebuild-up by the electron absorbed by specimen.

2.5. Flexure Test

Flexural tests under three-point bend configuration were per-formed using Zwick Roell testing unit according to the ASTMD790–02 standard [33] to evaluate flexural modulus and strengthof each of the material systems of the polymer nanocompositesand its laminates. The machines were run under displacement con-trol mode at a crosshead speed of 2.0 mm/min [14] and tests wereperformed at room temperature. The span to depth ratio was main-tained at 16:1. The maximum stress at failure on the tension side ofa flexural specimen was considered as the flexural strength of thematerial. Flexural modulus was calculated from the slope of thestress–strain plot. Five samples of each type were tested. The aver-age values and standard deviation of flexural strength and moduluswere 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 servo-hydraulically controlled Material Testing System (MTS) machine.The ASTM D 695–10 standard was followed for this quasi-staticcompression test. The capacity of the MTS machine is approxi-mately 10,000.00 kg. The test was carried out at the displacementcontrol mode with the crosshead speed of 1.27 mm/min. In orderto maintain evenly distributed compressive loading, each speci-men was sanded and polished so that the opposite faces were par-allel to each another. Test Ware-SX software was used to develop aprogram which controls the test conditions and records both theload and crosshead displacement data. The load–deflection datarecorded by the data acquisition system was converted to the

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stress–strain curve after dividing load by initial cross-sectionalarea of the specimen and deflection by specimen thickness.

2.7. Optical Microscopy

An optical microscope was used to investigate the failure modeand crack propagation of the failed specimen. The optical micros-copy was performed using Olympus SZX16. The Olympus SZX16provides a large zoom ratio of 16.4:1. With this seamless zoom ra-tio combined with the most comprehensive range of parfocalobjectives (0.5x, 1.0x, 1.6x & 2.0x), the SZX16 can be used to takemicrograph from a macro-view to a micro-view allowing visualiza-tion of whole organism down to fine microscopic structures. Thefractured surfaces were exposed to the optical microscope usingpolarized light.

Fig. 2. SEM micrograph of 0.2 wt.% CNF-loaded polyester matrix. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

Fig. 3a. SEM micrograph of acid-etching 0.2 wt.% CNF-loaded polyester. (Forinterpretation of the references to color in this figure legend, the reader is referredto 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 polyes-ter matrix, and the 0.2 wt.% CNFs infused polyester matrix are shownin Figs. 1a, b and 2, respectively. From the micrograph of 0.2 wt.%CNF-filled polyester, excellent dispersion of CNFs was found. Onlybroken ends of CNFs were observed near the surface. Some CNFswere broken in a brittle manner and some were pulled out.

Strong attractive fiber van der Waals forces cause CNFs toagglomerate, which reduces the strength of the nanocompositeby stress concentration effect. Agglomerates of CNFs, called nano-ropes, are difficult to separate and infiltrate with matrix. Theyentangle and form nest-like structures due to their curvature andhigh aspect ratios. Both disagglomeration and dispersion in resinsdepend on the relative van der Waals forces, curvature, and on therelative surface energy of CNFs versus that of the resin. To over-come attractive forces, researchers have been extensively usingmechanical energy, intense ultrasonication, and high speed shear-ing. Some rebundling of the aggregates is possible even after dis-continuation of the external force [34]. However, optimal loadingand uniform dispersion of CNFs in matrix are the key parametersto promote better nanofiber–matrix interface properties to reachan efficient load transfer between the two constituents of thenanocomposite [35,36]. In order to further investigate the disper-sion of CNFs in the polyester, concentrated nitric acid was addedon the cleavage surfaces to partly unveil the CNFs formerly coveredby the polyester. The etched surfaces were then studied by SEM(Figs. 2, 3a and 3b). It can be easily observed that the interfacial

Fig. 1. SEM micrograph of (a) as-received

bonding between the CNF and matrix was very compact whichwould allow CNFs to be anchored in the embedding matrix [37].

In the current study, uniform dispersion of 0.2 wt.% CNFs intothe polyester resin was achieved using the sonication mixingmethod for 90 min. High magnification SEM micrograph inFig. 3b clearly exhibits that CNFs are well separated and uniformly

PR-24 CNF (b) neat polyester matrix.

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

Fig. 5. Flexural stress–strain plot of polyester samples with different wt.% of CNF.

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embedded in the 0.2% polyester resin system. These CNFs are likelyto interlock and entangle with the polymer chains in the matrix[37]. Thus, addition of CNFs enhanced the crosslinking betweenpolymer chains and provided better interfacial bonding.

Fig. 4a and b shows the woven glass reinforced polyester lami-nates with 0.2 wt.% CNFs. It was found that the resin was distrib-uted uniformly over the fabric and the interfacial bondingbetween matrix and fiber was very good. Resin flow and impregna-tion of the glass fibers were observed in the SEM micrographs.Clear resin matrix adhesion is present in these micrographs andglass fibers are observed to be embedded within the matrix. Goodmatrix-fiber wetting was achieved and resin is also visible in be-tween the glass fiber filaments. It appears that better interfacialbonding between the nanophased polymer matrix and glass fiberis present due to the presence of CNFs [38]. The fiber volume frac-tion as determined from matrix digestion method for the nanoph-ased glass reinforced polyester composites fabricated by VARTMwas found to be around 56%.

3.2. Flexural test results

Flexural tests were performed to evaluate the bulk stiffness andstrength of neat and nanophased polyester samples and their fiberreinforced laminates. Typical stress–strain behaviors from the flex-ural tests are shown in Figs. 5 and 6. The positive effect of CNFs wasevident from these stress–strain plots with improved strength andstiffness in the CNF-loaded nanophased composites. Flexural

Fig. 4. 0.2 wt.% CNF-loaded G

Strength, modulus, and the strain at maximum strength for all nan-ophased samples were larger than those of the neat samples due tothe addition of CNFs as CNF has high aspect ratio which can pre-vent crack generation and propagation in the polyester matrix.The average properties of the neat polyester and CNF-filled polyes-ter (CNF-FP) obtained from these tests are shown in Fig. 5. In allcases, the samples failed rapidly after experiencing the maximumload showing induced brittle nature of failure due to the additionof CNFs. From the analysis, it was found that the 0.2 wt.% CNFsloading and 90 min sonication showed the optimal condition. The0.2 wt.% CNF-loaded polyester samples enhanced the flexuralstrength and modulus by about 88% and 16%, respectively, com-pared to the neat polyester samples. The failure strain also in-creased significantly with the addition of CNFs into the polyestermatrix system. The flexural properties were slightly decreased athigher CNF content. It might be due to the development of CNFsmicro aggregates in various regions of the polymer matrix, whichact as areas of weakness. In essence, better dispersion methodsare needed for higher CNFs loading.

Flexure tests were performed on the unfilled, 0.1–0.4 wt.% CNF-filled glass reinforced polyester composites (GRPC) to evaluatetheir bulk stiffness and strength. Their typical stress–strain behav-iors are shown in Fig. 6. It is clear from these stress–strain curvesthat all the nanophased composites showed significant improve-ment in the mechanical properties up to the 0.2 wt.% of CNFs load-ing, beyond which there was a decreasing trend. The curvesshowed considerable nonlinear deformation before reaching the

RPC laminates (a and b).

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

Table 1Flexural test results of GRPC laminates with different wt.% of CNF.

Glass reinforcedpolyestercomposites

Flexuralstrength(MPa)

Gain instrength(%)

Flexuralmodulus(GPa)

Gain inmodulus(%)

Conventional GRP 174 ± 5.8 – 16 ± 0.8 –0.1% CNF-LGRP 228 ± 9.4 31 19 ± 0.5 190.2% CNF-LGRP 260 ± 4.3 49 21 ± 1.3 310.3% CNF-LGRP 248 ± 8.3 43 20 ± 1.7 250.4% CNF-LGRP 220 ± 5.2 26 18 ± 0.1 13

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

Table 2Quasi-static (strain rate 10�3) results of conventional and CNF-loaded GRPC.

Composite name Max. Stress(MPa)

Improvement(%)

Modulus(GPA)

Improvement(%)

Neat GRP 80.72 ± 6.27 – 3.78 ± 0.25 –0.1 wt.% CNF-FGRP 106.98 ± 1.65 32.50 5.52 ± 0.48 46.030.2 wt.% CNF-FGRP 115.71 ± 3.25 43.35 6.05 ± 0.27 60.050.3 wt.% CNF-FGRP 95.4 ± 2.75 18.19 5.14 ± 0.44 35.98

(a)) 0.1 wwt.%% CCNFF-LLGRRPCC laamiinatte (bb) 00.2 wt.% CNNF-LGGRPPC llamminaate

(c) 0.3 wt.% CNF-LGRPC laminate (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 thisarticle.)

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

M.K. Hossain et al. / Composites: Part A 42 (2011) 1774–1782 1779

maximum stress and also the irregularities in the curves wereattributed to random fiber breakage during loading. However,more or less ductility was observed in each type of laminate sam-ple and cracking noise was heard while the individual fiber brokeor the inter-layer delaminated, but no obvious yield point wasfound. Most of the samples failed around the mid-point of the sam-ple. From the resultant data, it was explored that the 0.2 wt.% CNFswas the optimum amount for this material system to achieve themaximum flexural modulus and strength. The three point bendingtests results of GRP and CNF-filled GRP composites are summarizedin Table 1. The 0.2 wt.% CNF-loaded nanophased GRPC systemshowed approximately 49% and 31% increase in the flexuralstrength and modulus, respectively. These improvements for CNFsare also consistent with Movva et al. investigations [32]. Theremight be several possible reasons for the better mechanical prop-erties observed in the CNF infused glass fabric reinforced polyesterlaminates. Firstly, CNF increases the strength and modulus of thepolyester matrix, which was observed in the CNF-loaded polyesterin this study. Secondly, the presence of CNFs increases the crackpropagation resistance and prevents crack generation by bridgingeffect at the interface region of the long glass fiber, CNF, and poly-ester matrix. Moreover, CNF has high aspect ratio, which improvesthe 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

compression stiffness and strength. Five samples were tested foreach condition and the average properties were obtained fromthese tests. Their typical quasi-static (low strain rate, 10�3 s�1)stress–strain behaviors are shown in Fig. 7. The highest value ofstress is termed as the peak stress or maximum stress and the cor-responding strain value is hereafter mentioned as the strain atmaximum stress. The modulus is determined from the slope ofthe linear portion of the engineering stress versus strain curve.

From the stress–strain curves, it was found that the incorpora-tion of CNFs enhanced the stress and modulus of the E-glass/poly-ester composite. The 0.2 wt.% CNF-loaded laminates showed thebest improvement in the stress and modulus. CNF acts as crackpropagation resistance. There was a slight improvement in thestrain 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 noobvious yield point was found. It was evident that the 0.2 wt.%CNF-loaded GRPC showed the maximum enhancement in the com-pressive strength and modulus by about 43% and 60%, respectively,compared to the conventional GRPC samples. This enhancementwas also consistent with Ma et al. [39] investigations on the poly-ester/carbon nanofiber composites. Compressive failure in poly-meric fibers occurs by yielding which consequences in thedevelopment and spread of kinks [40]. CNF might act as a barrierfor kinks spread, thus ensuring the enhanced compressivestrength. The summarized quasi-static (low strain rate) compres-sion results of neat GRP and CNF-filled GRP (FGRP) compositesare given in Table 2.

3.4. Fracture analysis

Results from the OM and SEM study substantiate the quantita-tive results obtained through flexural and compressive tests. Opti-cal microscopy studies were carried out on the flexural testedsamples of the conventional and nanophased GRPC composites.Fig. 8a–d illustrate the fracture surfaces of 0.1–0.3 wt.% CNF-loadedGRPC (CNF-LGRPC) and conventional GRPC samples, respectively.Fiber breakage and little kinking were found in the 0.1 wt.% and0.2 wt.% CNF loaded samples shown in Fig. 8a and b, respectively.On the other hand, fiber pullout followed by fiber breakage was ob-served in the 0.3 wt.% loaded and conventional samples illustratedin Fig. 8c and d, respectively. It was very clear from these micro-graphs that the inter-layer delamination was also observable inthe conventional and 0.3 wt.% CNF-loaded GRP (CNF-LGRP) com-posites. As CNF loading increases beyond 0.2 wt.%, CNFs start toagglomerate and these agglomerations produce stress concentra-tion which might act as crack initiation sites [41]. It was evidentfrom the optical micrographs that the 0.2 wt.% CNF system pro-motes good interfacial bonding between the fiber and matrix.

From the SEM micrograph taken at higher magnification asshown in Fig. 9a, excellent bridging effect in the interfacial regionof the long glass fiber, CNF, and matrix was observed. CNF has highaspect ratio which can prevent crack propagation and crack gener-ation resulting in improved performance. Some resin was stackedon the fractured glass fiber as shown in Fig. 9b, which representsthe better adhesion due to the addition of CNF. The presence ofpolyester adhering to the fiber surface also suggests that the inter-facial adhesion is stronger than matrix strength in nanophasedcomposites [42]. Thus, it is evident from these micrographs thatCNFs are anchored with both resin and fiber tightly that promotesa better interfacial bonding between the matrix and fiber. Better fi-ber–matrix interfacial bonding, and CNFs’ crack generation andpropagation resistance result in higher strength in nanocompos-ites. On the other hand, the addition of the CNFs led to an improve-ment in the modulus of elasticity of the nanophased composites.This is attributed to the stiffened matrix of these composites

0

0

0.1

0.3

wt.

wt.

.%

.%

CN

CN

NF-l

F-l

loa

loa

ded

ded

G

d G

RP

RP

C

C

0.2

C

wt.

Con

.%

nven

CN

tio

NF-l

onal

load

l GR

ed

P

GR

C

PC

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.

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(Fig. 5). The interfacial area between the resin matrix and CNFs wasincreased because of the high aspect ratio of the CNFs, which inturn led to better mechanical properties [43]. The nanoparticlesalso act as reinforcing element and bear the load in the compositematerial system [44]. Again, both CNFs and fibers are stronger thanmatrix. Thus, when load is applied to the composite structures,matrix starts to crack first and stress is then transferred from thelower modulus matrix to the CNFs to the long fiber by bridging ef-fect and ultimately the composites’ properties enhance. Thethought derived previously for the enhancement in the mechanicalproperties 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 highstresses resulting in crack propagation through fiber matrix inter-faces. Therefore, stronger adhesion between fiber and matrix, higherstrength, and higher toughened matrix are desired. Improvement offlexural strength by addition of nanofillers into the matrix was ex-pected to be observed for several reasons. Young’s modulus of the

second phase dispersed particles is higher than that of the matrixand thus stress transfer from the matrix to the particles will takeplace. As a result the strength of the composites is increased. Stronginterfacial bonding between the fiber and matrix also contributes tohigher flexural strength. Dispersed filler particles act as mechanicalinterlocking between fiber and matrix which creates a high frictioncoefficient. Finally, a mixed mode of fracture (flexural and shear) oc-curs under bending-load conditions. After an initial failure of fibersat the tensile side of the specimen, cracks are deflected parallel to thefibers and also to the applied load direction. The stress–strain curvein Fig. 6 shows a sharp increment with increasing load beforereaching the maximum stress and then irregularities and staggereddecrease in stress were observed for both conventional and nanoph-ased composites. However, the initial load and the crack arrest areaare higher in nanophased composites which lead to high energyabsorbing mechanisms [42].

The fracture behavior of tested samples at the quasi-staticstrain rate of 10�3/s is shown in optical micrographs in Fig. 10.

M.K. Hossain et al. / Composites: Part A 42 (2011) 1774–1782 1781

At first, some cracks were initiated in the samples followed by thekink band formation and crack propagation upon reaching themaximum peak stress, catastrophic load drop was observed andsamples failed in the direction of compression loading (Fig. 10). Itis evident from these micrographs that matrix cracking, kinking,and fiber breakage dominated the failure modes of 1–2 wt.% CNF-loaded nanophased composites whereas matrix cracking anddelamination were mostly observed in the 0.3 wt.% CNF-infusedand conventional composites. The sonication process was unableto break the agglomerations of the 0.3 wt.% CNF-loaded compositescompletely. The modulus, a low deformation property, was not af-fected by the high stress concentrations caused by the agglomer-ated particles. However, the strength was reduced by initiatingearly failure in the matrix [35]. This explains the decrease in com-pressive strength observed in the 0.3% CNF-loaded glass fiber rein-forced composite. It has also been reported that even at lowconcentration of nanoparticles the fracture energy of polyesternanocomposites could be doubled and prevent large scale frag-mentation of polyester matrix [44]. This behavior is clearly seenfrom OM micrographs.

For better understanding, fracture morphology of samples wasstudied using higher magnification SEM micrographs. The SEMmicrographs of the fractured surfaces of the conventional and0.2 wt.% CNF-loaded GRPC are illustrated in Fig. 11. For conven-tional composite shown in Fig. 11a, the surface of the fiber wasclean, and no matrix adhered to the fiber. The fracture surface ofthe matrix was flat, and some cracks were seen in the matrix sidenear the fiber–matrix interface. Resin appears not to protrude fromthe surface of fibers. These results indicate that the interfacialbonding between the fiber and matrix was weak. The fracture sur-face of the nanophased composite (Fig. 11b) shows that the surfaceof the matrix was rougher than that of neat composite. CNFs wereobserved to be randomly but uniformly distributed in the matrix.The resin appears to cling to fibers well. The strengthened matrixheld the glass fabrics together. The protrusion of the resin fromthe surface of the fibers accounts for the increase in fracture tough-ness of the samples. Moreover, the resin appears to be sticking tothe fiber surface giving rise to a significant plastic deformation[45]. The plastic deformation enhances mechanical properties sig-nificantly in the nanophased composites (Figs. 6 and 7).

4. Conclusions

Carbon nanofibers (CNF) were used as nanoparticle fillers inwoven glass fiber-reinforced polyester composites. Better disper-sion of CNFs was observed in the 0.2 wt.% CNF-loaded polyester re-sin and the fiber volume fraction for the nanophased GRPCfabricated by VARTM was found around 56% and reasonable limitof the void content. CNF infusion even at quite low concentrationsenhanced the mechanical properties of the system. This investiga-tion showed that CNF can be used without difficulty to modify theconventional fiber reinforced composite materials with the follow-ing outcomes:

� Flexural test results of polyester samples with 0.2 wt.% CNFsindicated a maximum improvement in strength and modulusof about 88% and 16%, respectively, whereas glass reinforcedpolyester composite samples with 0.2 wt.% enhanced 49% and31%, respectively.� GRPC laminates showed maximum enhancement in compres-

sive strength and modulus by about 43% and 60%, respectively,with addition of CNFs up to 0.2 wt.% compared to neat GRPCsamples.� The bridging effect of CNFs was observed in SEM micrographs of

nanophased GRPC.

� Optical micrographs of flexural fractured samples revealed inter-layer delamination in conventional composite. However, no delam-ination was found in CNF-loaded GRPC for better interfacial interac-tion between fiber and matrix due to the presence of CNF.

Acknowledgements

The authors greatfully acknowledge the funding agency DOE,NSF-EPSCoR, NSF-RISE, and NSF-CREST for providing financial sup-port to perform this research.

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