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The effects of nanoclay inclusion on cyclic fatigue behavior and residual properties of carbon fiber-reinforced composites (CFRPs) after fatigue have been studied. The tension–tension cyclic fatigue tests are conducted at various load levels to establish the S-N curve. The residual strength and modulus are measured at different stages of fatigue cycles. The scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM) are employed to characterize the underlying fatigue damage mechanisms and progressive damage growth. The incorporation of nanoclay into CFRP composites not only improves the mechanical properties of the composite in static loading, but also the fatigue life for a given cyclic load level and the residual mechanical properties after a given period of cyclic fatigue. The corresponding fatigue damage area is significantly reduced due to nanoclay. Nanoclay serves to suppress and delay delamination damage growth and eventual failure by improving the fiber/matrix interfacial bond and through the formation of nanoclay-induced dimples.
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  Fatigue damage behaviors of carbon fiber-reinforced epoxy compositescontaining nanoclay Shafi Ullah Khan a , Arshad Munir b , Rizwan Hussain b , Jang-Kyo Kim a, ⇑ a Department of Mechanical Engineering, Hong Kong University of Science and Technology Clear Water Bay Kowloon, Hong Kong  b National Engineering and Scientific Commission, P.O. Box 2801, Islamabad, Pakistan a r t i c l e i n f o  Article history: Received 13 February 2010Received in revised form 19 June 2010Accepted 5 August 2010Available online 11 August 2010 Keywords: A. NanoclayA. Carbon fiberB. FatigueB. Interfacial strengthC. Damage tolerance a b s t r a c t The effects of nanoclay inclusion on cyclic fatigue behavior and residual properties of carbon fiber-rein-forced composites (CFRPs) after fatigue have been studied. The tension–tension cyclic fatigue tests areconducted at various load levels to establish the S-N curve. The residual strength and modulus are mea-suredat different stages of fatiguecycles. Thescanningelectronmicroscopy(SEM) andscanningacousticmicroscopy(SAM)areemployedtocharacterizetheunderlyingfatiguedamagemechanismsandprogres-sivedamagegrowth. TheincorporationofnanoclayintoCFRPcompositesnotonlyimprovesthemechan-ical properties of the composite in static loading, but also the fatigue life for a given cyclic load level andthe residual mechanical properties after a given period of cyclic fatigue. The corresponding fatigue dam-age area is significantly reduced due to nanoclay. Nanoclay serves to suppress and delay delaminationdamage growth and eventual failure by improving the fiber/matrix interfacial bond and through the for-mation of nanoclay-induced dimples.   2010 Elsevier Ltd. All rights reserved. 1. Introduction Carbon fiber-reinforced composites (CFRPs) are widely used asstructural material in load bearing applications because of highstrength and stiffness, dimensional and thermal stability, and cor-rosionresistance.Fatigueisknowntobeoneoftheprimaryreasonsfor failure in many structural materials, including CFRPs [1–3].When subject to cyclic loading, CFRPs exhibit gradual degradationofthemechanicalandstructuralperformanceasaresultofdamageaccumulation. The nature of fatigue damage in CFRPs is very com-plicated and is quite different from those of isotropic materials.The damage states are closely related to the anisotropy and heter-ogeneity which leads to the formation of different stress levelsdepending on the lay-up sequence and orientation of laminate.ThefatiguedamagemodesinCFRPsincludecombinationsof inter-facialdebonding,matrixcracking,delamination,fiberbreakage,etc.Early work on unidirectional CFRP laminates under tensile fatigueloading displayed a high degree of resistance before sudden cata-strophic failure [4]. However, when the matrix was more highlyloaded such as laminates with off-axis fiber orientations, the re-sponsewascompletelydifferent: thereweremultiplemechanismsof failure throughout the material involving combinations of fiberand matrix damage interaction. The fatigue behavior of on-axisspecimenswasinfluencedbythestochasticbreakageofbrittlefiberbundles,whereasthatofoff-axisangle-plywasstronglyaffectedbytheinelasticsheardeformationandcrackpropagationoftheductilepolymer matrix [5]. Similar conclusions were drawn in a recentstudy where the failure modes were as much related to the cyclicstress as to the off-axis angle [6]. For on-axis specimensthe failuremodes were fiber-dominated and matrix-dominated when highand low cyclic stresses, respectively, were applied. In sharp con-trast, for off-axis specimens the failure mode was always matrix-dominated irrespective of the stress level.Thermosetting epoxy resin systems are widely employed asmatrix materials for composites in many fields such as aerospace,automotive and microelectronics. Toughening of epoxies has beenone of the topics most extensively studied because of the brittlenature of epoxies and their widespread applications for engineer-ing components. Understanding the fatigue crack propagationbehaviors of epoxy composites has been of great importance be-cause such composites are often used for engineering componentsthat are subject to cyclic loading. Curtis [7] found that the tough-ened resin system can improve the tensile fatigue response inthe low cycle fatigue regime, while in the high-cycle fatigue rangethe fatigue performance of the toughened epoxy is inferior to thatof standard epoxy-based composites. Epoxy matrices with a highductility exhibited a higher compressive fatigue resistance [8].The mode I delamination fatigue crack growth was studied of interlayer/interleaf-toughened CFRP laminates [9]. The heteroge-neous interlayer with fine polyamide particles increased the crackgrowth resistance. 0266-3538/$ - see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2010.08.004 ⇑ Corresponding author. Tel.: +852 23587207; fax: +852 23581543. E-mail address:  mejkkim@ust.hk (J.-K. Kim).Composites Science and Technology 70 (2010) 2077–2085 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech  Although many research efforts have been directed towardunderstanding the mechanisms of fatigue in polymer matrix com-posites, the effects of nanoparticles on their fatigueperformanceisstill not fully understood. The addition of 1wt% of carbon nano-tubes (CNTs) to the matrix of glass fiber-epoxy composite lami-nates improved their high-cycle fatigue life by a remarkable 60–250% [10]. Even more impressively, the addition of 2 and 5wt%multi-walled CNTs enhanced the fatigue performance of physio-logically maintained methyl methacrylate–styrene copolymer(MMA-co-sty) by 565 and 593%, respectively [11]. Zhang et al.[3] demonstrated an order of magnitude reductionin fatigue crackpropagation rate for an epoxy system with the addition of 0.5wt%of CNTs. Thecrack-tipbridgingandfrictional pull-outmechanismswere responsible for the suppression of fatigue in the nanocom-posite. Other types of nanofillers also gave rise to improved frac-ture properties. For example, the introduction of SiO 2  particlesincreased both the initiation fracture toughness and the corre-sponding cyclic fatigue behavior of epoxy [12]. Al 2 O 3  and TIO 2 nanoparticlesimprovedtheflexuralstrength,stiffnessandfracturetoughnessaswellasthefatiguecrackpropagationresistanceoftheepoxy [13]. The incorporation of organoclay in polyurethane elas-tomers showed significantly improved fatigue life in addition tomore than 150% increase in static strength and failure strain [14].Many studies have devoted to improving the mechanical prop-erties of fiber-reinforced composites by adding nanoclay. In addi-tion to mechanical properties, clay-epoxy nanocomposites haveshown wide array of property improvements with only very lowfractions of clay, including the enhanced thermal stability[15,16], reduced moisture and gas permittivity [17] and superior flame retardancy [18]. The nanoclay, in particular, exhibited ame-liorating effects on fracture and fatigue resistance of carbon fibercomposites(CFRPs):e.g. increasedmode1delaminationresistance[19], enhanced impact damage resistance and tolerance [20] and better static and impact fracture toughness [21]. However, veryfewstudies have appearedinthe openliteratureonfatigueperfor-mance of hybrid CFRP composites containing nanoclay. As a con-tinuation of our previous studies on clay-CFRP hybrid composites[19–21], this work specifically studies the fatigue performance of CFRP composites affected by the incorporation of nanoclay. TheS-N curves and the residual properties of hybrid composites aftertension–tension cyclic loads of different levels were specificallyevaluated. 2. Experiments  2.1. Materials and fabrication of composite laminates The laminate composites were fabricated from unidirectionalcarbonfiberandorganoclayfilledepoxyresin.Theepoxyresinsys-temwasbasicallythesameasthatemployedinourpreviousstud-ies [19–21]: a diglycidyl ether of bisphenol A (DGEBA) epoxy(Epon828, supplied by Shell Corp) mixed with 1,3-phenylenedi-amine (supplied by Aldrich) hardener at a ratio of 100:14.5 byweight. Unidirectional carbonfabric (suppliedby Taiwanelectricalinsulators) with a unit weight of 200g/m 2 was used as the mainreinforcement for composite laminates. The organoclay, NanomerI30P (supplied by Nanocor), is an octadeclyamine modified mont-morillonite suitable for dispersion in epoxy resins [17]. Theorganoclay was dried overnight at 75  C in an oven prior to use.The epoxy in a glass beaker was heated at 75  C to lower the vis-cosity and the organoclay was added. The organoclay contentwas varied between 0, 3, and 5wt% of the epoxy resin-hardenermixture. Mixing was conducted at a shear rate of 3000rpm for1h using a high speed shear mixer (Ross Mixer). The mixturewas subjected to sonication using an ultrasonicator (Branson2510)atanultrahighfrequencyfor 3htofurtherdispersetheclay,whilemaintainingtheresintemperatureat75  Cusingahotwaterbath. After sonication, the translucent color of the epoxy/clay mix-ture indicates uniform distribution of organoclays, partly confirm-ing the efficiency of the sonication conditions used. The mixturewas degassed in a vacuum oven followed by addition of curingagent, and the mixture was stirred while avoiding the formationof bubbles. Twelve ply laminates of 30cm square were preparedby hand lay-up of carbon fabrics with a stacking sequence [0/90] 3S onasteelmouldplate.Tokeepfabricswellaligned,necessaryprecautions were takenduring handlay-up. The moldedlaminateswere wrapped with bleeders and peel plies within Teflon dam allaround, which was cured at 80  C for 2h and at 150  C for 8h, fol-lowed by post-cure at 160  C for 2h in a vacuum hot press (Tech-nical Machine Product Corp). The high cure temperatureexcursions for long durations were aimed at complete cure of theresin. The cured composite laminates were cut, by a diamondwheel, at 45   off-axis directions to obtain a resultant stacking se-quence of [±45] 3S . Introduction of clay into epoxy inevitably in-creases the viscosity of the resin, which may result in compositelaminates thicker than those without clay. To lower the viscosityand thus to avoid the thickness variation, the resin was heated to75  C during the whole processing steps, including shear mixing,sonication and degassing, as well as before hand lay-up after mix-ing with the hardener. A uniform laminate thickness and a con-stant fiber volume fraction were further assured through the useof Teflon dams of required thickness and a constant pressure of 0.32MPa during curing. The volume fraction of carbon fibers,  V   f  ,was consistently maintainedat about 0.55 for boththecompositeswith and without nanoclay, which was determined from theknown weights and densities of the composite constituents.  2.2. Characterization, static and cyclic fatigue tests The static tensile tests were conducted according to the specifi-cation, ASTM D3039, on a universal testing machine (MTS Sintex10/D) to determine the tensile strength and modulus. Rectangularspecimens of 230mm long  20mm wide  2.5mm thick wereloaded at a crosshead speed of 2mm/min. An extensometer withgauge length of 25mm was attached to the specimen to monitorthe strain during loading.Thetension–tensioncyclicfatiguetestswereconductedaccord-ing to the specification, ASTM D3479, on a universal testing ma-chine (25 KN servo-hydraulic Instron 1300). The tests wereconducted at room temperature on a load control mode at a stressratio of 0.1, and with constant-amplitude sine-wave loading. Todetermine the fatigue S-N curves, the maximumstress levels werekept at 80, 70, 60 and 45% of the corresponding ultimate tensilestrength(UTS) of the composite. Atest frequencyof 2Hz was usedwhich was lowenough to minimize the effect of adiabatic heating.Rectangular specimens, 230mm long, 20mm wide and 2.5mmthick, were cut from the composite plates, and end tabs made of glass fabrics and 40mm long were bonded at both ends of thespecimen to avoid failure around the gripping device during thetests. At least four specimens were tested for each set of loadingconditions. The residual properties of the composites were mea-sured after different periods of fatigue loading at a maximum loadequivalentto60%oftheultimatetensilestrengthofthecomposite.Static tensile tests were conducted on the pre-cycled specimens tomeasure the residual tensile strength and modulus. The tests fol-lowedthe same procedureas those for the static tensile propertieson virgin specimens.The scanning electron microscopy (SEM) was used to examinethe surface morphologies of the static and fatigue fractured speci-mens and thus to identify the different failure mechanisms in-volved in CFRPs with and without nanoclay. The scanning 2078  S.U. Khan et al./Composites Science and Technology 70 (2010) 2077–2085  acoustic microscopy (SAM, Sonix Micro-Scan System) was em-ployed to characterize the progressive fatigue damage growth atdifferent stages of fatigue cycles. A focused acoustic beam wasscanned over the damaged laminate using a transducer equippedwith a 35MHz probe in a through-transmission mode. Both theneat CFRP composites and the hybrid composites containing5wt% nanoclay were examined before loading and after 5K,10K, 20K, 25K and 30K fatigue cycles. For ease of understandingonly two colors, black and grey, were used to present the damagestate and a threshold value of 10% was used as the border line be-tween the two colors. 3. Results and discussion  3.1. Static tensile properties Fig. 1 presents a typical TEM image of nanocomposites with5wt% clay content, indicating a mixture of full intercalation andpartial exfoliation. Representative stress–strain curves obtainedfromthestatictensiletestsareshowninFig.2.Allmaterialsexhib-ited a typical bilinear stress–strain behavior before failure. As re-ported previously [22–24], the tensile stress–strain curves forangle-plyspecimensarenon-linearduetothesignificantcontribu-tion of the polymer matrix. It is clearly seen that both the yieldstrength and the failure strain increased with increasing the claycontent. Fig. 3 summarizes the static tensile strength and modulusof clay-CFRP hybrid composites containing varying clay contents.Both the tensile strength and modulus increase continuously withincreasing clay content, which is again a reflection of the compos-ite property significantly affected by the matrix property. Thisobservation is generally consistent with the flexural properties re-portedearlier[21]althoughtheflexuralstrengthtendedtobemar-ginally reduced at a high clay content due to the potential lack of dispersion of clay.The fracture surface morphologies as shown in Fig. 4 exhibitedsharp contrast between the composites without and with 5wt%nanoclay. Interfacial debonding between the fiber and matrix, aswell as limited deformation of matrix material are the major fail-ure mechanisms observed in the composites without clay. Thefracture surface was generally smooth and featureless indicatingbrittle failure. Meanwhile, the clay-CFRP hybrid composites re-vealed improved fiber–matrix interfacial bonding due to the pres-ence of nanoclay in the matrix material that maximizes the stresstransfer between matrix and fiber. The modified epoxy adheredwell to thelongcarbonfibersandthe fracturesurface was rougherandtextured,quitesimilartothoseobservedfromtheinterlaminarfracture surfaces [19]. It is thought that the octadecylamine modi-fier used for I.30P organoclay had alkyl and amine groups that arefunctionally compatible with carbon fibers to give rise to strongadhesion [20]. Similar amine groups have been extensively usedtofunctionalizecarbonnanotubes/nanofibersforpolymercompos- Fig. 1.  Typical TEM image of nanocomposite containing 5wt.% clay, showingdispersion state of nanoclay. Fig. 2.  Representative stress–strain curves of clay-CFRP hybrid composites. Fig. 3.  Tensile properties of clay-CFRP hybrid composites containing varying clay contents. S.U. Khan et al./Composites Science and Technology 70 (2010) 2077–2085  2079  ite applications, which may also be responsible for the improvedadhesion between the modified epoxy and ultra-high molecularweight polyethylene fibers [25]. It is well known that the proper-ties of the composites with [±45] S  ply orientation are dominatedby the in-plane shear properties, which is further confirmed bythegeneralviewofthefailedspecimenshowninFig.5a. Theinter-laminar shear and in-plane shear are considered to be the matrixdominated properties [26]. It was noted (Fig. 2) that the presence of nanoclay in the matrix not only increased the apparent yieldstress but also the strain to failure, which is consistent with theprevious observations of improved interlaminar shear strength(ILSS) [19,27] and in-plane shear strength of fiber composites [28].  3.2. Fatigue life and residual strength Fig. 6 presents the S-N data of clay-CFRP hybrid composite atvarying clay contents. For the same level of maximum appliedstress, the clay-CFRP hybridcompositeexhibitedmuch longer fati-gue life than the composite without clay at all the stress levelstested. A maximum improvement of about 74% in fatigue lifewas achieved with 3wt% clay when cyclic fatigue was carriedout at a load equivalent to 45% of the tensile strength of the spec-imen. Results indicatedthat nanoclaymodificationproducedmoreimprovement in fatigue life at a low stress or a high cycle regimeand was less potent in improving life at high stress levels. At high Fig. 4.  Fracture surface morphologies of CFRP composites: (a and b) without clay and (c and d) with 5wt.% clay. Fig. 5.  Images of the failed specimens from (a) static tension and (b) cyclic fatigue.2080  S.U. Khan et al./Composites Science and Technology 70 (2010) 2077–2085
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