2006PlaseiedFatemiASMEHwaii17043

8/8/2019 2006PlaseiedFatemiASMEHwaii17043

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1Copyright2006byASME

ProceedingsofMN2006MultifunctionalNanocomposites2006

September20-22,2006,Honolulu,Hawai

MECHANICALPROPRTIESANDDEFORMATIONBEHAVIOROFACARBONNANOFIBERPOLYMER

COMPOSITEMATERIAL

AtousaPlaseied1,AliFatemi

2

1GraduateResearchAssistant,TheUniversityofToledo,Toledo,Ohio, [email protected]

2Professor,TheUniversityofToledo,Toledo,Ohio, [email protected]

ABSTRACT

Tensile behavior of a carbon nanofiber reinforced vinyl esterpolymercompositewasstudiedusingdog-boneshapedspecimensto

obtain itsmechanical properties. PyrografIII which isa veryfine,highlygraphiticandyetlowcostcarbonnanofiberwasusedasthefiber material. Vinyl ester with low molecular weight which wasusedasthematrixmaterialisathermosetwithhightensilestrengthatroom temperature. When small amounts of carbon nanofibers arecombined withvinyl ester, the stiffness of the resulting compositecanimproveif the fiber-matrix adhesion is good. Themechanicalproperties can improve further after surface treatment(functionalization)ofcarbonnanofibers.Thissurfacetreatmentaddssome functionalgroupschemicallyto thenanofiberssurfacewhichincreasestheadhesionbetweennanofiberandmatrixresin.

Understanding themechanicalbehavior of these composites iscrucial to their effective application. In thisresearch thestiffness,strength,and tensile deformation behaviorof these nanocompositeswere investigated. The effects of matrix curing systems and

composition, strain rate, nanofiber concentration, nanofiber surfacetreatment and environment suchas low and high temperatures andhumidity were also characterized. Based on the mechanicalproperties simplemodelswereusedto representtensilestress-strainanddeformationbehaviorsofthenanocomposite.Theexperimentalresultswerealsoappliedtothesemodelstoexaminetheirpredictivecapability.

NOMENCLATURESymbol Description Unit

a,b,c,d MaterialsConstants D1,D2MaterialsConstantsGPa,MPa e EngineeringStrain%E TensileSecantModulusat1%Strain GPa

Ec NanocompositeModulusofElasticity GPaEf NanofiberModulusofElasticity GPaEm MatrixModulusofElasticity GPaK StrengthCoefficient MPalf/df NanofiberAspectRatio m,n MaterialsConstants n StrainHardeningExponent S EngineeringStress MPaSy YieldStrength MPaT Temperature CVf NanofiberVolumeFraction %Wf NanofiberWeightPercent %

GreekSymbols Truestrain %

& StrainRate 1/s

e TrueElasticStrain %p TruePlasticStrain %

TrueStress MPa

PoissonsRatio

Abbreviations ASI AppliedSciencesInc PC PostCuring RH RoomHumidity R-O Ramberg-OsgoodModel RT RoomTemperature St Styrene VE VinylEster VF FunctionalizedFiberNanocomposite VO OxidizedFiberNanocomposite

wt% WeightPercent INTRODUCTION

The principal objective of the overall project was to assessdemonstrateand describean integrated methodology fordesignandmanufacturing optimization of commercial nanofiber reinforcedpolymer composites with high performance, affordable cost andenvironmental compatibility. The material requirements of highstiffnessandstrengthand low-densityleadtothe materialchoiceocarbonfiberreinforcedpolymermatrixcompositematerials.Anewproduction method uses vapor grown carbon fiber technology tomakecompositematerialscapableofprovidingimprovedmechanicapropertieswithhighelectricalandthermalconductivity.Thegainsiphysical properties of carbon nanofiber composite materials withsignificant reductions in production cost will have a substantia

impact on the weight, reliability and cost of aerospace vehiclecomponents, electronicsand automobiles (http://www.ml.afrl.af.mil2003).

Very little work has been performed on the mechanicaproperties of vinyl ester polymer and composites reinforced withvaporgrowncarbon nanofibers. The goal ofthis research was toinvestigatestiffness,strength,and tensiledeformationofvinylestepolymer and carbon nanofiber reinforced vinyl ester compositematerials. It was also intended to examine available modelsparticularly for deformation and stress-strain behavior of thesematerials. Understanding environmental effects (high and lowtemperatures and moisture) and the role of material and

MECHANICAL PROPERTIES AND DEFORMATION BEHAVIOR OF A CARBON NANOFIBER

POLYMER COMPOSITE MATERIAL

MN2006-17043
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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manufacturingparameterssuchasfibervolumefraction,fibersurfacetreatment, and matrix material (composition and curing systems)werealsoincludedintheresearchprogram.

EXPERIMENTALDESCRIPTION

The composite materials were made from polymer matrix(thermosetvinylester)and carbonnanofibersas thereinforcementsforthepolymermatrixbycastinginspecialmolds.Dogbone-shapedspecimens were manufactured based on the requirements formechanicaltesting. Threesetsofmaterialsconsistingofvinylesterpolymer, conventional composite with oxidized carbon nanofiberswithout nanofiber surface treatment, and nanocomposite withfunctionalized carbon nanofibers were used for these mechanicaltesting. Some specimens were also fabricated to compare themechanical properties of nanocomposites with different volumefractionof carbonnanofibers. TensileexperimentswereconductedbasedonASTMstandards.

TESTMATERIALSANDSPECIMENSHETRON 942/35(65wt% Vinyl ester 35wt% Styrene)from

Ashland Chemical Company was used as the polymer material.Recommended composition ranges for commercial application arebetween 75wt% Vinyl ester - 25wt% Styrene (by evaporation ofstyrene)and 55wt% Vinyl ester - 45wt% Styrene(by dilution withstyrene). The polymer material used in this research had a

compositionof45wt%Styreneand55wt%Vinylester.Thedensityofthevinylesterresinwasmeasuredtobe1.04gr/cm3andthe1%secantmodulusofelasticitywasmeasuredtobe3.4GPa.

Nanofibersusedin thisstudywerePR-24-PSOXL.D.withoutCVDcarboncoatingprovidedbyAppliedSciencesInc(ASI).Thesenanofiberswithadiameterofabout100nmandanaspectratio(lf/df)of200werefunctionalizedbytheChemicalEngineeringGroupattheUniversity of Toledo. The nanofibers can have a rangeof elasticmodulusfrom240GPa(TibbettsandMcHugh,1999;Tandonetal.,2002) to 600 GPa and an average density of 1.7 gr/cm3(http://www.apsci.com/asi-research.html, 2005). The carbonnanofiberswereaddedintovinylesterresinanddispersedinsidetheresinbysonication.

TESTEQUIPMENT

MTS closed-loop servo-hydraulic axial load frame inconjunction with an INSTRON Fast-Track 8800 digital servo-controllerwasusedtoconductmechanicaltests.Theloadcellhadacapacityof100kN.Thegrippingsystemusedwasawedgeactiontypesuitableforflatspecimens.Thesmoothgrippingfacesusedfortheexperimentsweremodifiedbyinsertinganemerypaperbetweenjaw face andthe test specimenwiththe roughpapersurfacefacingthetestspecimen. Anoptimizedgrippressurewasusedforholdingthespecimensthroughoutexperiments. Thedesignof thehydraulicgrips ensured alignment of the test specimen in the direction ofappliedstrainwithoutanysignificantbending.Anytwistingofthespecimenswasavoidedbyusingananti-rotationdevicemountedonthelowergripsarm.

Total strain wascontrolled for all tests using an extensometerrated as ASTM E83 (2002) class B2. The calibration of the

extensometerwasverifiedusingdisplacementapparatuscontainingamicrometer barrel in divisions of 0.00254 mm (0.0001 in). Theextensometerhadagagelengthof12.7mm(0.5in)andwascapableofmeasuringstrainsupto15%.Inordertoprotectthespecimenssurface from the knife edges of the extensometer, three layers oftransparent tape were used to cushion the attachment. Theextensometer was carefully positioned at the center section of thespecimensuniformgagesection.

Testswereconductedatlow,room,andhightemperaturesandroom humidity. The relative humidity was monitored using aprecision hydrometer. Testing at low and high temperatures was

performedinsideanenvironmentalchamber(MTSModel651)andtemperatures were controlled using a precision thermocoupleattached to the center of the specimens. For lowering t hetemperaturesbelowroomtemperature,liquidnitrogenwasused.

EXPERIMENTALPROCEDURESTensiletestingwasbasedonASTMstandardsD638(1989)and

D3039(1989).ThedogbonespecimenswiththegeometryshowninFigure1 wereclampedin thehydraulicallyoperated wedgegripsaboth ends and pulled at one of the clamped ends at constanelongation(displacementcontroltest).ItisrecommendedbyASTM

D638thatthestrainrateshouldbesuchthatruptureoccursin0.5to5minutes. Therefore,thestandardtensiletestswereconductedataconstant displacement rate of 0.042 mm/sec (0.1 in/mincorrespondingtoastrainrateof0.001/s.

Figure 1: Specimen configuration for tensile test(Dimensionsareinmm).

Since tensile properties vary with speed and environment otesting, these tests were conducted at different temperatureshumidity,and strainrates. Tensiletestsatcoldtemperatures(-35Cand-10C), roomtemperature(23C) andhigh temperatures(50C75C,and100C)wereperformedinsideanenvironmentalchamber.These tests were conducted at low (0.0001/s) to high (1/s) strainrates.Sometestswerealsoperformedonwater-soakedspecimensatroomtemperaturebasedonASTMstandardD570(1998).

EXPERIMENTALRESULTSThere are many variables affecting mechanical properties o

nanocomposites. Tensile strength and modulus depend on matrixcuringsystemsandcomposition,fibervolumefraction,fibersurfacetreatment,temperature,humidity,andstrainrate.Thehigherratesoloadingcorrespondtolowertemperatures.Testresultswereanalyzedtoinvestigatetheeffectsofmatrixcuringandcomposition,nanofibevolume fraction, strain rate and temperature on tensile mechanicabehavior of nanocomposites. To show these effects, at least tworepeat tests were conducted. There wasa scatterof lessthan 20%between mechanical propertiesof repeated tests and in most casegood repeatability of materials behavior was observed. In thfollowing plots, however, only one representative curve for eachconditiontestedisshownforbetterclarityincomparisons.

MATRIXCOMPOSITIONANDCURINGEFFECTS

Tworesin compositions were chosen with 45%VE-55%Stand55%VE-45%St. In additionto affectingmechanicalproperties,thecomposition also affects the ease of adding and dispersing carbonnanofibersintotheneatresin,duetoachangeinviscosity.StressstraincurvesinFigure2showthatforspecimenswithoutpostcuringbyincreasingthe styrene content, the strength and stiffness of thneatresinincrease.Thereismoredifferencebetweenthemechanicaproperties without post curing, because in this case the vinyl esteresin is not completely cross-linked and the stiffness and strengthshowlowvalues.Therefore,postcuringofvinylesterplaquesafteroomtemperaturecuringwasnecessary.


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Room temperature curing procedure was used in order toprovideenoughtimeforthenanofiberstodispersewellthroughvinylesterresin.Differentpostcuringprocedureswerefollowed.Figure3showsthatdirectpostcuringathightemperaturedecreasesductility,whilesteppostcuring(1/2hrat57C,1/2hrat63C,1hrat71C,2hrs at 82C, and 2 hrs at 150C) provides higher ductility. Thestiffness and strength are nearly identical by using different postcuringprocedures.

Forthisstudy,thecompositionof55%VE-45%Stwasselectedbased on more common industrial application as well as suitableviscosity for nanofiber addition. The plaques were curedat room

temperaturefollowedbystephightemperaturepostcuring.

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100

0 1 2 3 4 5 6 7 8 9 10 11

Strain(%)

Stress(MPa)

45%VE-55%Stw/oPC

55%VE-45%Stw/oPC

45%VE-55%Stw/PCat150Cfor2hrs

55%VE-45%Stw/PCat150Cfor2hrs

Figure 2:Stress - strainbehavior ofspecimens with

compositionsof 45wt% VE 55wt% St and 55wt% VE45wt%Stwithoutpostcuringandwithpostcuringat150Cfor2hrsatstrainrateof0.001/s.

0

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30

40

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80

90

100

0 1 2 3 4 5 6 7 8 9 10

Strain(%)

Stress(MPa)

RTcure&PCat80Cfor2hrs

RTcure&PCat120Cfor2hrs

RTcure&stepPC

Figure 3:Stress strainbehaviorofspecimenswith

room temperature curing and different post curingprocessesonspecimensatstrainrateof0.001/s.

NANOFIBERVOLUMEFRACTIONEFFECTS Nanocomposites with 0.5 to 2 wt% functionalized carbonnanofiberswere prepared for tensile tests at roomtemperature and0.001/s strainrate. Stress-straincurves obtainedfor these materialareshowninFigure4.Ascanbeseeninthisfigure,byincreasingthe nanofiber weight percent up to2 wt%, tensile properties weredecreased.

Therewasanincreaseof12%inmodulusofelasticitybyaddinonly0.5wt%functionalizedcarbonnanofibersanda slightincreaseofabout4% byadding 1 wt% nanofiberintovinyl ester. Tensilestrength did not increase by adding nanofibers, since the materia

showed more brittle behaviorunder tensileloadingas comparedtothe neat resin. Addition of 2 wt% nanofibers decreased all themechanicalpropertiesoftheneatresineventhemodulusofelasticity

0

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100

0 1 2 3 4 5 6 7 8 9 10

Strain(%)

Stress(MPa)

VF(0.5%),RT,RH,0.001/s

VF(1%),RT,RH,0.001/s

VF(2%),RT,RH,0.001/s

VE,RT,RH,0.001/s

Figure 4: Effect of weight percent of functionalizecarbon nanofibers on stress strain behavior onanocomposite at room temperature and strain rate o0.001/s.

STRAINRATEEFFECTSThe effect o f strain rate on stress-strain behavior o

nanocompositeatroomtemperatureisshowninFigure5.Thestressstrain curvesfor nanocomposite showthatmechanicalpropertiesothismaterialarestronglystrainratedependent.

InFigure5,tensilestrengthandmodulusofelasticityincreasedwithincreasingstrainrate,whileelongation(ductility)ofthemateriadecreasedbyincreasingthestrainrate.Thismaterialshowedabrittlbehaviorinallstrainratesatroomtemperature.


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0

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100

0 1 2 3 4 5 6

Strain(%)

S

tress(MPa)

VF,RT,RH,0.0001/s

VF,RT,RH,0.001/s

VF,RT,RH,0.01/s

VF,RT,RH,0.1/s

VF,RT,RH,1/s

Figure 5: Effect of strain rate on stress strain

behavior of nanocomposite with 0.5 wt% functionalizedcarbonnanofiberatroomtemperatureandhumidity.

ENVIRONMENTALEFFECTS

Theshapeof thestress-straincurvefornanocompositechangeswith changes in temperature and humidity. In the composite, thematrix-dominated properties are more affected by increasingtemperaturethanthefiber-dominatedproperties.

Temperatureeffects:Temperature effects on stress-strainbehavior of nanocomposite at constant strain rate of 0.001/s andtemperatures of -35 C, -10C, 23C, 50C, 75C, and 100C areshowninFigure6.Fromthisfigure,itcanbeseenthatthetensilemodulusandstrengthofthismaterialaredecreasedwithincreasingtemperature,whileductilityincreasedwithincreasingtemperature.

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100

0 2 4 6 8 10 12 14 16

Strain(%)

Stress(MPa)

VF,-35C,RH,0.001/s

VF,-10C,RH,0.001/s

VF,RT,RH,0.001/s

VF,50C,RH,0.001/s

VF,75C,RH,0.001/s

VF,100C,RH,0.001/s

Figure 6:Stress strain behaviorof nanocompositewith 0.5 wt% functionalized carbon nanofiber at roomhumidity,strainrateof0.001/s,anddifferenttemperatures.

Moisture effects: The increases in weight of nanocompositespecimens were 0.01 gror 0.13%after oneday, 0.03 gror 0.39%afteroneweek, and0.07gror 0.85%afterone monthexposure todistilled water. The stress-strain curves in Figure 7 show thathumidity absorption decreases the mechanical properties ofnanocompositeatRT.Thedecrease,however,doesnotappeartobe

consistentasafunctionofmoisturecontent.Thismaybeduetotestvariabilityandscatter,withmoretestsrequiredforconclusiveresults

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Strain(%)

Stress

(MPa)

VF,RT,RH,0.001/s

VF,RT,watersoaked(24hrs),0.001/s



Figure7:Effectofhumidityonstressstrainbehavioof nanocomposite with 0.5 wt% functionalized carbonanofiberatroomtemperatureandstrainrateof0.001/s.

FIBERSURFACETREATMENTEFFECTSCarbonnanofiberswereaddedtothevinylestermaterialintwo

different conditions, with and without surface treatment. Surfacemodificationshad apositiveimpacton themechanicalpropertiesonanocomposite. Figure 8 shows the stress-strain behavior onanocompositeswithandwithoutfunctionalization.Nanocompositewithoxidizedcarbonnanofibershadsimilarstiffnessascomparedtothe nanocomposite with functionalized nanofibers. The differencbetweentwomaterialswasintheverylowductilityoftheoxidizefibernanocomposite.Thismaterialstensilefractureoccurredatverlowstrain,evenathighertemperatures.Thiscouldbeduetopooradhesion of nanofibers to the matrix material. Functionalizednanofiberscouldprovidebondingsitestothepolymermatrixsothattheloadcouldbetransferredtothenanofibersandpreventseparation

betweenthepolymersurfacesandnanofibers.

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Strain(%)

Stress(MPa)

VF,RT,RH,0.001/s

VF,50C,RH,0.001/s

VF,100C,RH,0.001/s

VO,RT,RH,0.001/s

VO,50C,RH,0.001/s

VO,100C,RH,0.001/s

Figure 8: Comparison between stress straibehavior of nanocomposite with 0.5 wt% oxidized an0.5 wt% functionalized carbon nanofibers at roomhumidity,strainrateof0.001/s,anddifferenttemperaturesfromRTto100C.


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ANALYSISANDMODELINGPREDICTIONOF TENSILEMODULUSANDCOMPARISONWITHEXPERIMENTALRESULTS InthisstudythecommonlyusedHalpin-TsaimodelaswellasCox model were used to predict elastic modulus of thenanocomposite made from vinyl ester matrix and functionalizedcarbon nanofibers. Halpin-Tsai equations for tensile modulus ofrandomlyorientedshortfibercompositesaregivenby:

mE

fVT

fVT

fVL

fVLfdfl

cE

+

+

+

=

1

21

8

5

1

)/(21

8

3 (1)

where:)/(2)/(

1)/(

fdflmEfE

mEfE

L+

= (2)

and 2)/(

1)/(

+

=

mEfE

mEfE

T (3)

TheHalpin-Tsaimodelprovidesempirical relationships thatenablethepropertiesofacompositematerialtobeexpressedintermsoftheproperties of the matrix and reinforcing fibers together with their

proportionsandgeometry(Qianetal.2000;Wangetal.,2004).Coxmodelisanothermodeltopredictthemodulusofelasticityofcompositeswithrandomlyoriented fibers. Thismodelestimatesstresstransferto thefibersusingparameter.Combiningthiswiththeruleofmixturesyieldsthecompositemodulusofelasticityas:

fEfVmEfVcE)/tanh1()1( += (4)

where:)4/ln(.)1(

fVfE

mE

fd

fl

+

= (5)

and=1/6forfibersrandomlyorientedinthree-dimensions(Tibbets

andMcHugh,1999;Lake,2001).Inthesemodelsitisassumedthatfibersarestraightandhavea

goodbondingand dispersioninside thematrix material. Assumingplanarisotropyforthenanocompositeused,a0.5%secantmodulusofelasticityof3.5GPaandPoissonsratioof0.4fortheresin,andmodulusofelasticityof400GPaforthecarbonnanofibers,atVf =0.5%the modulusof elasticityof carbonnanofiberreinforcedvinylesterresinispredictedtobe4.01GPabytheHalpin-Tsaimodeland3.67GPabytheCoxmodel.Comparedtotheexperimentalvalueof3.77GPa,theCoxmodelprovidescloserestimation.However,thedifferences between the experimental results and both modelpredictions, as well as predicted values between the two modelsincreasebyincreasingthevolumefractionofcarbonnanofibersandby usinga higher value of nanofiberelastic modulus. In addition,bothmodels predict an increasein tensilemodulus with increasingfiber volume fraction,whereas experimentalobservations presentedearliersuggestadecreaseintensilemodulus.Itshouldbenotedthatthe two aforementioned models were developed based on theassumptions of homogeneity and isotropy of fiber and matrixmaterials, straight fibers, and for relatively high fiber volumefractions.Someoftheseassumptionsdonotagreewiththenanofiberand nanocomposites used in this investigation. For example thenonofiberswerenotstraightandthefibervolumefractionswerelow.

TENSILESTRESS-STRAINBEHAVIORMODELINGTwo models, Ramberg-Osgood (Ricks, 2005) and Menge

(Schmachtenberg and Menges, 1985), were used to predict thedeformation behavior of nanocomposite material. As explainedearlier, nanocomposites mechanical properties and deformationbehavior are strain rate and temperature dependent. Viscoelasticbehaviorof thematerialalso shows thisstrainratedependency. Ishould be noted that in both of these models used here, constanstrainratedatawereused.

Inorderto findtherelationshipbetweenmechanical propertiewithstrainrate andtemperature, theaverage mechanical propertie

obtained from tensiletests wereused. Mechanical propertieswerefoundtohavealinearrelationshipwithtemperatureandlogarithmicstrainrate.Abestfitlinebyusingleastsquaresmethodwasobtainedforthe data. Themechanicalproperties (modulus of elasticity andyield strength) can be estimated using the following equations tomodeltheseproperties:

)961.0(15.4201.0)log(2.02=+= RTE & (6)

)919.0(62.75327.0)log(35.22=+= RTyS & (7)

These equations correlated well with the experimental data adifferentstrainratesandtemperatures.

Thetruestress()-truestrain()plotisoftenrepresentedbytheRamberg-Osgoodequation:

npeKE

1

)( +=+= (8)

Thestrengthcoefficient,K,andstrainhardeningexponent, n,aretheintercept andslopeofthebestline fitto true stress () versus true

plasticstrain(p)datainlog-logscale:n

pK )( = (9)

Ramberg-OsgoodmodelbasedonEquation(8)wasusedtorepresentthe stress-strain behavior of nanocomposite up to ultimate tensilestrength. In Equation (8), constantsK and n were obtained fromEquation(9)usingexperimentaldata.Itwasobservedthattherewasagoodcorrelationbetween KandSyandbetweennandEbyfittinganexponentialcurvetothedata.

ThefollowingequationswereobtainedbasedontheRamberg

Osgoodmodel: )9135.0()04.0exp(27.20

2== RySK (10)

)811.0()4271.0exp(0503.02== REn (11)

Intheseequations,SyandEcouldbesubstitutedbyEquations(7)and(6),respectively. Therefore,Kandn could be relatedto thestrainrateandtemperatureoftesting. Using Equations (6), (7), (8), (10), and (11), stress-strainbehavior of nanocomposite at different temperatures and constanstrainrateof0.001/swasmodeled.Thesecurvesarecomparedwiththecurvesbasedon KandnobtainedfromexperimentaldatashowninFigure9.

Mengesmodelwasanothermodelusedinthisstudytodescribethe deformation behavior of nanocomposite. A parabolic function

was used to describe the stress-strain behavior. Therefore, thefollowing equation was proposed based on Menges mode(SchmachtenbergandMenges,1985):

2

21eDeDS = (12)

Thisparabolicfunctionwasfittedtotheexperimentaldatato obtainconstantsD1andD2.Theseconstantswereshowntobeafunctionostrain rate and temperature. This equation would permit anapproximationofstress-strainbehavioruptothetensilestrengthothematerial.


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It w as o bserved t hatD1 andD2 had linear relationship withEand Sy, respectively. The followingequations were obtained fromdatafits:

)9866.0(6103.08997.012=+= RED (13)

)8506.0(5845409.45222=+= RySD (14)

UsingEquations(6),(7),(12),(13),and(14),stress-strainbehaviorofnanocompositeatdifferenttemperaturesandconstantstrainrateof0.001/s was modeled. These predicted stress-strain curves arecompared with the curves based on D1 and D2 obtained from

experimentaldatashowninFigure9.In Figure 9, comparisons between experimental results with

predicted models usingRamberg-Osgood and Menges model showthat Menges model can reasonably well predict the deformationbehaviorofnanocompositewithvinylestermatrixinthetemperaturerangeof-35to100Candstrainrateof0.001/s.

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0 2 4 6 8Strain(%)

Stress(MPa)

VF,-35C,0.001/s,R-OVF,-10C,0.001/s,R-OVF,RT,0.001/s,R-OVF,50C,0.001/s,R-OVF,75C,0.001/s,R-OVF,100C,0.001/s,R-OVF,-35C,0.001/s,Model1VF,-10C,0.001/s,Model1VF,RT,0.001/s,Model1VF,50C,0.001/s,Model1VF,75C,0.001/s,Model1VF,100C,0.001/s,Model1VF,-35C,0.001/s,Menges

VF,-10C,0.001/s,MengesVF,RT,0.001/s,MengesVF,50C,0.001/s,MengesVF,75C,0.001/s,MengesVF,100C,0.001/s,MengesVF,-35C,0.001/s,Model2VF,-10C,0.001/s,Model2VF,RT,0.001/s,Model2VF,50C,0.001/s,Model2VF,75C,0.001/s,Model2VF,100C,0.001/s,Model2

Figure9:Comparisonbetweenstress-strainbehavior

of nanocomposite obtained from Ramberg-Osgood andMengesmodels at different temperatures anda constantstrainrateof0.001/s.

CONCLUSIONS

Mechanical properties of functionalized nanocompositesarestronglystrainrateandtemperaturedependent.

Humidit y decreases the mechanical properties ofnanocompositeatroomtemperature.

Matrixcuringatroomtemperatureandsteppostcuringathigh temperature wereshownto besuperiorto the othercuringsystemsformakingnanofiberreinforcedvinylesterpolymercomposites.

Tensile experiments on nanocomposites with differentweightpercentnanofibersupto2wt%showedthehighesttensilestiffnessfor0.5wt%functionalizednanofibers.

Chemicalnanofibersurfacefunctionalizationwasshowntobe an effective means of improving the dispersion of

nanofibersinsidethepolymermatrix.Itwasalsoshowntoincrease the strength and ductility of the resultantnanocomposite.

At room temperature the stiffness of the nanocompositeshowedanincreaseupto12%byadditionoffunctionalizedcarbon nanofiberinto vinyl ester resin while theductilitydecreased.Athighertemperaturesof50Cand75Cbothstrengthandstiffnessincreased,buttheductilitydecreased.Moreincreaseinmechanicalpropertiescanbeexpectedbyusingweakermatrixmaterialascomparedtothethermosetvinylesterused.

Halpin-Tsai and Cox models were used to predict themodulus of elasticity of nanocomposite. Cox modeshowed close agreement with the experimental result fonanocompositewithnanofibervolumefractionof0.5%.

Ramberg-Osgood and Menges models were used for thmodelingofstress-strainbehaviorof nanocompositeup toultimate tensile strength of the material. Menges modeshowedabetterpredictionofnanocompositesdeformationbehavioratalltemperaturesatagivenstrainrateof0.001/s

ACKNOWLEDGEMENTSFinancial support was provided by Army Research Office (AROunderGrantNo.DAAD19-03-1-0012.Thehelponthisprojectfromthe program manager at ARO, Dr. Stepp, is acknowledged. WthankDr.Afjeh,Dr.Coleman,Dr.Hu,Mr.Premo,Mr.Jaegly,MrGrivanos, and Mr. Borges for their help and contributions to thiproject.

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