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  Flexural fatigue behavior of synthesized graphene/carbon-nanofiber/epoxy hybrid nanocomposites M.M. Shokrieh a, ⇑ , M. Esmkhani a , A.R. Haghighatkhah a , Z. Zhao b a Composites Research Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology,Tehran 16846-13114, Iran b School of Chemical Engineering, Shandong University of Technology, 255049 Zibo, Shandong, PR China a r t i c l e i n f o  Article history: Received 8 February 2014Accepted 8 May 2014Available online 2 June 2014 Keywords: Flexural bending fatigueHybrid nanocompositesGraphene nanosheetCarbon nanofiber a b s t r a c t In the present research, effects of adding a combination of synthesized graphene nanosheets and carbonnanofibers (CNFs) on the flexural fatigue behavior of epoxy polymer have been investigated. Graphenenanosheets are synthesized based on a changing magnetic field. The flexural bending fatigue life of 0.5wt.% of graphene/CNF/epoxy hybrid nanocomposites has been considered at room temperature.The samples were subjected to different displacement amplitudes fatigue loadings. Due to the additionof hybrid nanoparticles, a remarkable improvement in fatigue life of epoxy resin was observed in com-parisonwithresultsobtainedbyadding0.25wt.%grapheneor0.25wt.%CNFintotheresin.Experimentalobservations show that at a strength ratio equal to 43% by using 0.5wt.% of hybrid nanoparticles;37.3-fold improvement in flexural bending fatigue life of the neat epoxy was observed. While, enhance-ment of adding only graphene or CNF was 27.4 and 24-fold, respectively.   2014 Elsevier Ltd. All rights reserved. 1. Introduction The flexural fatigue behavior of composites and nanocompos-ites has been carried out by many researchers [1–4]. For compos-ites under displacement-controlled condition, Paepegem andDegrieck developed an experimental setup for bending fatigueloading [1]. They adopted a residual stiffness model whichdescribes the fatigue damage behavior of the composite material[2]. Also, Paepegem and Degrieck [3] used a finite element approach for composites fatigue life prediction. El Mahi et al. [4]studied the flexural fatigue behavior of the sandwich compositematerials using three-point bend test and the derived approachpermitted to predict the fatigue life of the sandwich compositematerials while avoiding the large number of experiments thatwould normally required in fatigue testing. A survey in the avail-able literature reveals that the addition of nanoparticles canimprove the fatigue behavior of composites under displacementcontrol loading and has been carried out by many researchers[5–8]. Ramkumar and Gnanamoorthy [5] studied the stiffness and flexural fatigue life improvements of polymer matrixreinforced nanocomposites with nanoclay. They described theeffect of adding nanoclay fillers on the flexural fatigue responseof Polyamide-6 (PA6). Rajeesh et al. [6] considered the influenceof humidity on the flexural fatigue behavior of commercial gradepolyamide-6 granules and hectorite clay nanocomposites.Timmaraju et al. [7] considered the influence of the environmenton the flexural fatigue behavior of polyamide 66/hectorite nano-composites. They also found the effect of initial imbibed moisturecontent on the flexural fatigue behavior of polyamide 66/hectoritenanocomposites conducted under deflection control method usinga custom-built, table-top flexural fatigue test rig at a laboratorycondition [8].In the literature, it was also found that the presence of multi-nanoparticles in composites improves the properties of nanocomposites. Some researchers used hybrid fillers in order tohave a perfect potential of bothfillers. For instanceas a first group,a combination of micro rubber and nanosilica has been used toimprove the fracture toughness and fatigue behavior of  [9–15].Liang and Pearson [9] used two different sizes of nanosilica (NS)particles, 20nm and 80nm in diameter, and carboxyl terminatedbutadiene acrylonitrile (CTBN) which was blended into a lightlycross-linked, DGEBA/piperidine epoxy system in order to investi-gate the toughening mechanisms. It was shown that addition of small amount of NS particles into CTBN, caused increase of thefracturetoughness.Manjunathaetal.[10–15]investigatedthefati-gue behavior of reinforced composites by adding a combination of microrubber and nano-silica particles intoepoxy matrixinseveralstates. For instance, they [10] studied the tensile fatigue behavior http://dx.doi.org/10.1016/j.matdes.2014.05.0400261-3069/   2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel./fax: +98 21 7720 8127. E-mail address:  Shokrieh@iust.ac.ir (M.M. Shokrieh).Materials and Design 62 (2014) 401–408 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes  of modified micron-rubber and nano-silica particle epoxy poly-mers. They [11] also addressed the tensile fatigue behavior of aglass-fiber reinforced-plastic (GFRP) with participation of rubbermicro-particles and silica nano-particles. They [12] also observedthe enhanced capability to withstand longer crack lengths, due tothe improved toughness together with the retarded crack growthrate, to enhance the total fatigue life of the hybrid-modified epoxypolymer. Also, Manjunatha et al. [13] enhanced the fatiguebehavior of fiber reinforced plastic composites by means of 9wt.% of rubber microparticles and 10wt.% of silica nanoparticlesand showed the fatigue life under WISPERX load sequence wasabout 4–5 times higher than that of the neat composites.Manjunatha et al. [14] also used another hybridization of car-boxyl-terminated butadiene–acrylonitrile rubber microparticlesand silica nanoparticles to increase the tensile fatigue behavior of GFRP composites at a stress ratio equal to 0.1. Manjunatha et al.[15] conductedfatiguecrackgrowthtest onathermosettingepoxypolymer which was hybrid-modified by incorporating 9wt.% of CTBN rubber micro particles and 10wt.% of silica nano-particles.The fatigue crack growth rate of the hybrid epoxy polymer wasobserved to be significantly lower than that of the unmodifiedepoxy polymer.Inthenextcategory,applyingcarbonnanotubes(CNTs)withdif-ferentnanoparticlesashybridfillersweretakenintoaccountintheliterature[16–19]toimprovethefatiguebehavior,mechanical andelectrical properties of reinforced composites. Böger et al. [16]appointed silica and MWCNT hybrid nanoparticles to increase thehigh cycle fatigue life of epoxy laminates and finally reported thatthe life was increased by several orders of magnitude in numberof load cycles. Fritzsche et al. [17] investigatedthe CNT based elas-tomer-hybrid-nanocomposites prepared by melt mixing andshowed promising results in electrical, mechanical andfracture-mechanical properties. Witt et al. [18] improved mechan-ical properties such as tensile strength and strain to failure of aconductive silicone rubber composite using both CNTs and carbonblack (CB). Al-Saleh Mohammed and Walaa Saadeh [19] fabricatedananostructuredhybridpolymericmaterialsbasedonCNTs,CBandCNFs and investigated electrical properties and electromagneticinterferenceshieldingeffectivenessintheX-bandfrequencyrange.The other various hybrid nanoparticles were discussed in theliterature are considered here as the last category [20,21]. Jenet al. [20] applied hybrid Magnesium/carbon fiber to increase thefatigue life of nanocomposite laminates. On the other hand, apply-ing carbon nanotubes (CNT) and graphite nanoplatelets (GNPs) toepoxy nanocomposites was shown by Li et al. [21]. It was repre-sented that the flexural mechanical as well as electrical propertiesof the neat resin was marginally changed by hybridization.This survey reveals that the effect of hybrid particles is mostlypositive and can improve the static and dynamic properties of composites. However, it is figured out that in case of displacementcontrol fatigue loading condition, there is a lack of researchon thisissue for hybrid nanofillers/epoxy nanocomposites. Therefore, inthe present research, the flexural fatigue behavior of graphene/CNF/epoxy hybrid nanocomposites under displacement controlflexural loading is investigated and compared with those of thepure epoxy resin. 2. Materials specification  2.1. Epoxy resin In the present research, ML-526 (Bisphenol-A) epoxy resin wasselectedbecauseofitslowviscosityandextensiveindustrialappli-cations to fabricate the specimens. The low viscosity of the matrixmakes the dispersion of additives easier. Physical and mechanicalproperties of ML-526 epoxy resin are shown in Table 1. The curingagent was HA-11 (Polyamine). The ML-526 resin and the HA-11polyamine hardener were supplied by Mokarrar Company, Iran.  2.2. Nanoparticles In this research, graphene nanoplatelets (GPL) and CNF areutilized as carbon based nanofillers. The graphene nanoplatelets(GPL)weresynthesizedwithastirringgrindingdrivenbychangingthemagneticfieldasshowninFig.1.Thesteelneedleswithaweakmagnetism are used as grinding media and four NdFeB permanentmagnets are inserted into a motor-driven disc (Fig. 1). When thedisc is made of steel, the magnetic stainless steel needles areattracted by the permanent magnets (Fig. 1a and b). By increasingtherotationalspeed,themagneticstainlesssteelneedlesflyupandcollide with each other with a high frequency under the changingattraction and repulsion forces of the high speed rotating perma-nent magnets (Fig. 1c). When a rigid grinding chamber filled witha certain amount of graphite powder is set on the disc, there arehigh frequent collisions and shears between the grinding chamberand magnetic stainless steel needles, which can finally result in astrong collision and shear forces. Graphite in the chamber will becrushed into ultra-fine powder under the action of these strongforces and then the powder will be prepared efficiently. Physicalproperties of synthesized graphene powders are shown in Table 2.The TEM image of the synthesized GPL powder is shown in Fig. 2.The D, G and 2D bands of Raman spectra of the synthesized GPLspowder are demonstrated in Fig. 3.The CNF was supplied by Grupo Antolin SL, Spain. The physicalpropertiesofCNFarerepresentedinTable3. TheScanningElectronMicroscopy (SEM) and Transmission Electron Microscopy (TEM)images of CNF nanoparticles are illustrated in Fig. 4. 3. Specimen preparation The polymer nanocomposite reinforced with 0.5wt.% of graphene/CNF hybrid nanoparticles/epoxy nanocomposites wasprepared as described below. Firstly, epoxy resin was mixed with0.25wt.%CNFandstirredfor10minat2000rpmandthenthemix-turewassonicatedvia14mmdiameterprobe-sonicator(HielscherUP400S)atoutputpowerof200Wand12kHzfrequency.Themix-ture was sonicated for 60min. It is worth mentioning that duringthe sonication, the mixture container was kept by the aid of ice-bathtopreventtheoverheatingofthesuspensiontokeepthetem-perature around 40  C. Secondly, suspension was mixed with0.25wt.% GPL under same condition within 30min by the sonica-tion. After sonication, the hardener at a ratio of 15:100 was addedto the mixtureand stirred gently for 5min. Then, it was vacuumedat 1mbar for 10min to remove any trapped air. Six samples wereprepared and cured at room temperature for 48h and followed by2h at 80  C and 1h at 110  C for post curing.The approach was used to disperse GPL/CNF hybrid nanoparti-clesintoepoxyresin,isadoptedfromacombinationofsupplemen-tary research [22]. Time for sonication depends on the fillercontents and has been defined based on experiments until fillersremain intact. For CNF fillers, Shokrieh et al. [22] investigated thesuitable time for sonication versus contents of the filler andpointedoutfor0.25wt.%CNFmaterials,theoptimumvalueofson-ication with regard to Fig. 5, was found around 90min with thesame compartment and conditions. Also, the optimum sonicationtime for 0.25wt.% GPL was equal to 30min. In addition, to inspectthe dispersion state of nanofillers, a newtechnique based on scan-ning electron microscopy, which utilizes the burn-off test, wasintroduced to visualize the dispersion state of nanofillers [23]. 402  M.M. Shokrieh et al./Materials and Design 62 (2014) 401–408  4. Calculation of the bending stress In this study, high cycle fatigue properties of nanocompositesaremeasuredbyamodifiedcantileverbeambendingtest.Atypicalfatigue life test specimen for the cantilever beam bending test isshown in Fig. 6. The presented specimen is designed based onASTM: B593-96standardand the method presentedby Ramkumarand Gnanamoorthy [5]. The wide end of the specimen is clampedto a bed plate, while the narrow end is cyclically deflected (see,Fig. 6(a)). To catch reliable results of the flexural fatigue strength,the gage area of the specimen is designed based on the stress con-centration concept (Fig. 6(b)).The stress concentration of the critical location of the specimenhas the maximum magnitude; therefore the failure will start fromthis area. For the wedge-shaped beam as applied specimen, thecross section is not uniform and defined by means of a parametercalled ‘‘local B’’ according to Eq. (1). (Fig. 7): B ð  x Þ¼ B 0 L 0 : ð L 0   x Þ ð 1 Þ where  L 0  is the length of the specimen and  B 0  is the width at thebase of the wedge-shaped beam. Therefore, the magnitude of thesecondmomentof areaof thecrosssectiondependsonthepositionalong the  x -axis as Eq. (2): I  ð  x Þ¼ B 0  : ð L 0   x Þ : H  3 12 : L 0 ð 2 Þ where H   isthethicknessofthebeam.Finally,themaximumtensionor compression stress at a given cross section for small  Table 1 Properties of ML-526 epoxy resin. Physical properties Mechanical propertiesViscosity at 25  C (centipoise) Glass transition temperature (  C) Tensile modulus (GPa) Tensile strength (MPa)1190 72 2.6 60 Fig. 1.  GPL synthesis method. (a and b) Still condition; (c) moving condition.  Table 2 GPL nanoparticles specifications. Nanoparticle Diameter (nm) Thickness (nm) Specific surface area (m 2 /g)GPL 40–120 3–5 500 Fig. 2.  The transmission–electron microscopy (TEM) of the synthesized graphenenanoplatelets. Fig. 3.  Raman spectra of synthesized graphene nanoplatelets, D, G and 2D bands.  Table 3 CNF specifications. Properties Unit ValueFiber diameter (TEM) nm 20–80Fiber length (SEM)  l m >30Bulk density g/cc >1.97Apparent density g/cc 0.060Surface energy mJ/m 2  100Graphitization degree %   70Electrical resistivity  O m 1  10  3 Metallic particles content % 6–8 M.M. Shokrieh et al./Materials and Design 62 (2014) 401–408  403  displacements within elastic deformation behavior is calculatedaccording to the following equation [24]: r max  ¼  z  0  E   H L 20 ð 3 Þ where  r max  is the maximum stress,  H   is the thickness of the beam,  z  0  is the displacement at point  x  = L 0  and  E   is the Young’s modulus.The relation between the displacement  z  0  at the tip and themaximum stress  r max  for a small deformation is linear. 5. Test equipment 5.1. Static testing instruments The Santam universal testing machine STM-150 was utilized toperform bending tests in accordance with the ASTM:D790. Thecross-head speed for bending tests was 16mm/min. To analyzehybrid nanoparticles, gold sputtered samples were used. Thefield-emission scanning electron microscopy (FESEM) photographswere taken by using Zeiss-Germany Sigma microscope. 5.2. Experimental setup for flexural bending fatigue Thepureepoxyandreinforcedpolymerspecimensaremountedinto a fixed cantilever, constant deflection type fatigue testingmachine. The machine called  BFM-110  is designed and manufac-tured based on a developed version of a testing machine designedbyPaepegemandDegrieck[1]andshowninFig.8.Thespecimenis heldatoneend,actingasacantileverbeamandcycleduntilacom-pletefailureisachieved.Thenumberofcyclestofailureisrecorded Fig. 4.  (a) SEM and (b) TEM images of CNF, prepared by Grupo Antolin SL, Spain. Fig. 5.  Viscosity (mPas) versus sonication time (min) of 0.25wt.% CNF/epoxynanocomposites [22]. Fig. 6.  (a) Schematic of specimen clamping procedure and (b) schematic picture of the bending fatigue specimen. Fig. 7.  Schematic view of a beam, with coordinates.404  M.M. Shokrieh et al./Materials and Design 62 (2014) 401–408
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