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411Polymers & Polymer Composites, Vol. 20, No. 5, 2012

Mechanical Properties of Kevlar-49 Fibre Reinforced Thermoplastic Composites

1. INTRODUCTION

Thermosetting plastic reinforced composites are commonly used over a number of years. However, their delamination tendency and property degradation have stimulated interest to develop toughened composites using thermoplastic-based matrices. The latter type has been receiving increasing attention for use in aerospace, automobile, and defence industries due to their good impact resistance1, high damage tolerance, good durability, high recyclability2, as well as shorter processing time. Although, thermoplastic materialsoffer the possibility of producing composites with various mechanical properties ranging from ductile to brittle ones their application to

structural components and elements is stillimmaturebecauseofthelackofrelevantexperimentaldata.Inaddition,thermoplastic composite materials have high viscosity which makes itdifficulttoimpregnateresinintofibrebundles3.

The principal advantages and disadvantages of glass and carbon fibres as reinforcements in epoxyresinsarewellknown.Kevlarisusedfor rubber reinforcement as in armour jackets.Kevlar-49fibrehasatensilestrength comparable with that of carbon fibre,amodulusbetweenthoseofglassandcarbonfibresandalowerdensitythan both.

Interfacialpropertiesoffibresareoffundamental importance in designing

composites. Chemical treatment is known to be a very efficientmethod for improving the interfacial adhesion of organic fibre-reinforcedcomposites4 that leads to increase in mechanical properties of the composites5.However,theKevlar-49fibre reinforced composites show poor interfacial adhesion between fibre and the thermoplastic matrixresin due to low surface energy and chemicallyinertsurfaceofthefibre.In order to improve the interfacial adhesion,extensivestudieshavebeenperformed6. Wu et al.7 investigated the effect of plasma and chemical treatment ofKevlarfibresonthemechanicalandfracture properties of Kevlar/epoxycomposites.

E f f o r t s t o u n d e r s t a n d t h e mechanical behaviour of fibre reinforced composites follow two extremes, namely, continuum andmicromechanical approaches. In the continuum approach, the composite is


K.K. Herbert Yeung and K.P. RaoDepartmentofMechanicalandBiomedicalEngineering,CityUniversityofHongKong,HongKongSAR,China

Received: 29 January 2010, Accepted: 3 October 2011

SUMMARYConventionalmaterialsmadeof glass and carbonfibres in thermoset resins are very frequently employed.Recently,thermoplasticmatrixcompositesarebeingdevelopedtoimprovethetoughnessanddamagetoleranceof composite laminates. The ductility of thermoplastic resins implies a more pronounced plasticity in service. Kevlarfibrehasatensilestrengthcomparablewiththatofcarbonfibre,amodulusbetweenthoseofglassandcarbonfibresandalowerdensitythanboth.Kevlar-49reinforcedcompositesareincreasinglydemandedinavarietyofcommercialapplicationsbuttheirunderstandingisrelativelynewwhencomparedtotraditionalfibres.Simplemicromechanicsbasedmodelssuchastheruleofmixtures,inverseruleofmixtures(ROM),Halpin-TsaiandXu-ReifsnidermodelsareemployedtopredictthemechanicalpropertiesofKevlar-49fibrecompositesformulations.Inthisstudy,themechanicalpropertiesincludingtensile,compressiveandflexuralstrengthandmodulusofKevlar-49thermoplasticbasedcompositeswereexperimentallyobtainedandcomparedwiththetheoreticalpredictions.Asystematiccostinganalysisofrespectivethermoplasticcompositeshasbeenmadeforthe provision of composite selection guidelines for designers.

Keywords: Composites, Thermoplastic, Kevlar, Mechanical properties, Micromechanics

©SmithersRapraTechnology,2012

412 Polymers & Polymer Composites, Vol. 20, No. 5, 2012

K.K. Herbert Yeung and K.P. Rao

considered as a homogeneous material havingauniform(average)property,ignoring the property difference between each constituent8. On the other hand, the micromechanical approach considers the composite as the combination of various materials and derives overall properties utilizing homogenization procedures based on the individual properties of constituents. The micromechanical approach is useful to optimize the design parameters of composites.

A general designer or manufacturerrequiresasimpleandreliablemethodto estimate the mechanical properties of composites for relevant applications, mainly based on the fundamental constituents’ properties. The present work is specifically targeted towardsestablishing a methodology for economical production of composites. Thespecificobjectivesof this studyare: (i) to evaluate the mechanicalbehaviour of Kevlar-49 reinforcedcomposites with three different thermoplasticmaterials asmatrix intension, compression, and bending; (ii)toapplysimplemicromechanicalmodels to evaluate the mechanical properties so as to compare with the experimentallyobtainedones,and(iii)to establish cost per unit performance of mechanical properties of the developed composites so as to develop selection guidelines for designers and manufacturers.

2. MICROMECHANICS MODELS FOR PREDICTION OF MECHANICAL PROPERTIES

Micromechanical composite models have been derived, based on the properties of the individual components of the composite and their arrangement. Properties such as elastic modulus (E), Poisson’s ratio (v) and volume fractionofbothfibre(Vf)andmatrix(Vm) are the fundamental quantitiesneeded to predict the properties of the composite. Simple models such as RuleofMixture (ROM)and Inverse

RuleofMixture(IROM)9, models by Caruso and Chamis10 and Knight11 are inusetopredictthetensileandflexuralmodulus under the assumptions of constant stress or strain through the composite.Budiansky12 formulated a complexmodelbasedonself-consistentmethodsandAboudi13 used method of cell to predict the tensile properties. OtherattemptsincludethosebyZhaoand Weng14, Eshelby15 and Mori and Tanaka16 to predict flexural moduli.Halpin and Tsai17developedexpressionsfor modulus considering the effect ofcontiguity,fibreandfibrepackinggeometry, loading conditions, etc.

2.1 Classical Lamination TheoryROMequationassumesthatthefibresare oriented and fully strained along their length. The IROM equationassumesthatthefibresandmatrixareequallystressed.ROMequation18 for apparentYoung’smodulusinthefibredirection is:

E1 = Ef1Vf +EmVm (1)

where Ef, Em, Vf and Vm are the moduli andvolumefractionsofthefibreandmatrixmaterialsrespectively.

Usually, the modulus of fibrous composites is well predicted by the ModifiedRuleofMixtures(ModifiedROM) as follows:

E1 = κEf1Vf + EmVm (2)

where κ = is the geometrical factor with consideration of the interfacial bonding behaviour. For composites underaxialtension,κ=0.919,andforchoppedfibrouscomposites,κ=0.27or0.375havebeenusedpreviously19 to account for the random orientation and variationofaspectratioofthefibresin the composite.

The elastic modulus of the composite in the transverse direction (E2) is determined by an inverse rule of mixturesequation18:

E2 =Ef2Em

Ef2Vm + EmVf (3)

The elastic modulus of the composite inthetransversedirection(E2) can also be determined when s2isunequalinboththefibreandthematrix,followingHalpin-Tsai semi-empirical relation17 andReuss’sassumption,andisknownasmodifiedIROM.

E2 = Em(1+Vf )Ef2 +VmEmVmEf2 + (1+Vf )Em (4)

2.2 Halpin-Tsai EquationsThe Halpin-Tsai17,20 equationsare simple approximate forms ofthe generalized self-consistent micromechanics solutions developed by Hill21. The modulus values based ontheseequationsagreedreasonablywellwiththeexperimentalvaluesforavariety of reinforcement geometries22. The following form of the Halpin and Tsaiequation17,20 can be used to predict the tensile modulus and strength of composites,neglectingthefibrecrimpsand pattern effects:

E1 = Em1+ ξηVf1−ηVf

(5)

In equation (5) the parameter η is given as23:

η =Ef1 / Em( ) −1Ef1 / Em( ) + ξ (6)

where ξinEqns.(5)and(6)isashapefittingparametertofittheHalpin-Tsaiequationtotheexperimentaldata.Thesignificanceoftheparameterξ is that ittakesintoconsiderationthepackingarrangement and the geometry of the reinforcingfibres20,26.

Avarietyofempiricalequationsforξ are available in the literature, and they depend on the shape of the particle/fibreandonthemodulusthatisbeingpredicted25. If the tensile modulus in the principalfibredirectionisdesired,and



thefibresarerectangularorcircularinshape, then ξ is given by the following equation25:

ξ = 2 LT

or ξ = 2

LD

(7)

where Lreferstothelengthofafibrein the longitudinal direction and T or D isthethicknessordiameterofthefibreinthethree-direction.Fromequations(5)to(7),wefinallyget:

E1 = EmEf1(1+ 2sVf )+ 2sEmVmEf1Vm +Em (2s+Vf ) (8)

where s =l2r

=ld =fibreaspectratio.

As the fibre aspect ratio increases(s→∞), the material becomes a continuous-fibre composite, and theaboveequation reduces to theROMequation(Eqn.(1)).Incontrast,asthefibresgetshorter(i.e.,(s→0),Eqn.(8)isreduced to the series model prediction ofIROM(Eqn.(3)).Themodelisidealwhenallfibresandmatrixarelinearlyelasticandthefibresareaxisymmetric,identical in shape and size, and can be characterized by an aspect ratio l / d. In addition,thefibreandmatrixarewellbonded at their interface, and remain that way during deformation.

2.3 Rosen and Xu-Reifsnider Models25

Compressive stresses may be generated in a structural element either due to direct compressive loading and/or due to bending or impact loading. Further, the compressive strength is generally lower than the tensile strength. Thus failure may initiate due to compressive stresses.

Fibres under compression generally do not fail in pure compression mode; rather, they fail by local buckling.Micro-buckling of fibres is nowaccepted to be the mechanism by which unidirectional composites fail undercompressioneitherinextensionmode or shearmode.The extension

modeinvolvesstretchandcompressionofmatrix inanout-of-phasemanner.Thecompressionstrengthofcompositeinanextensionorout-of-phasebucklingmode is23:

σCIC = 2VfVfEmEf13(1−Vf ) (9)

Intheshearorin-phasebucklingmodethefibresbuckleandthematrixissheared,andtheresultingbucklingstressis:

σCIC =Gm

1−Vf (10)

The simple two-dimensional model using traditional energy method to estimate forfibreinstabilityproposedbyRosen24 is based on the above two modes of elasticbucklingoffibresembeddedinmatrix.Ingeneral,compressivestrengthpredictedbyextensionmodeismorethanthatbyshearmode.Also,Eqn.(10)shows the strength is independent of the type of reinforcement. It was found28 that strengthsestimatedinbothmodesarehigherthanthoseobtainedexperimentally.

Xu andReifsnider25 predicted the compressive strength based on the use of micro-bucklingmodelof a representativevolumeelementusing aBeam-on-Elastic Foundation Model26.Theeffectofmatrixslippageandfibre-matrixbondcondition was included by two factors, namely, ξ and η.Thefinalexpressioninterms of the constituent properties and micro-geometrical parameters is given bythefollowingexpression:

σCIC = Gm Vf +EmEf1

(1−Vf )

×

2(1+ vm )π πηrf

3 EmEf1

Vf

EmEf1

+1−Vf

1+Vfvf + vm (1−Vf )( )

+1− ξ − sin πξ2π

(11)

where ξ=2s/L isthematrixslippageinpercentage,ηisthefibre-matrixbondconditionanditvariesfrom1to2(η=1ifthefibreiscompletelyseparatedfrommatrixononeside,andη=2ifthefibreiswellbondedonbothsides).Thatis,avalueof1≤ η≤2satisfiestherequirementforactualcomposites.

2.4 Estimation of Manufacturing Cost Comparative cost analysis is necessary to compare and assess the potential deployment of thermosetting and thermoplastic composites in an economical way. The total manufacturing cost of a part is obtained by summing the costs incurredduringeachoperationofthemanufacturingsequence.Thus,combiningtheaboveequations:



CMANUF

= QhP

h(1+ F

scrap ,hh=1

m

∑ )+ (1+ Fovh

)i=1

n

∑

×S

w, j

Zi

tsetup ,ij=1

oi

∑ + Fpres,k Sw,ktrun,i (1+ Frew,i )k=1

pi

∑

+

Sw,l

Mil=1

qi

∑ tmove,i

+IRVi−1

tqueue, i

HY

+ IRVi

twait , i

HY

+

2N

is+1

(E0,is − Fsal E0,is )+ FmtnE0,is

s=1

ri

∑ +IR E0,is +U E ,is

HEQ,is

trun, is

trun,is

HEQ,is

1+ Frew,i( )

(12)

It should be stressed that if any of the n manufacturing operations is not performed inhouse,theexpressioninsquarebracketsinEqn.(12)issimplyreplacedbythe cost of subcontracting this operation.

3. PREPARATION OF COMPOSITES

The mechanical properties of thermoplastic composite materials are dependent toalargerextentonthereinforcingfibre.Highspecificstrengthandmodulusareprimaryconsiderationsbutthefibresurfacetextureandfunctionalityarealsoveryimportantinordertopromoteadherencetothematrix.

3.1 Materials SelectedKevlar-49fibre(notatedasfibreF)fromDupontTM deNemoursisselectedforthisstudy.Theglasstransitiontemperatureofthismaterialis212°C.Materialsusedas thermoplasticmatrixare: (i)P1 - thermoplasticDOWTMSAN110(StyreneAcrylonitrile)fromDowPlastics,(ii)P2-ASTALAC®ABS(AcrylonitrileButadieneStyrene) 2029-2 fromMarplexAustralia Pty. Ltd., and (iii)P3 - DOWLEX® Polyethylene Resins 2045G from Dow Chemical Company, USA. The basicphysicalandmechanicalpropertiesoffibreandmatrixmaterialsusedinthisstudyare listed in Table 1. The notation used for the prepared composites is as follows:

Composite1:FP1-Kevlar-49fibrewiththermoplasticP1(SAN)

Composite2:FP2-Kevlar-49fibrewiththermoplasticP2(ABS)

Composite3:FP3-Kevlar-49fibrewiththermoplasticP3(Polyethylene)

3.2 Composite Preparation MethodChemicaltreatmentofKevlar-49fibreswasdoneusing10wt.%phosphoricacid(H3PO4) solution in a laboratory scale apparatus. The residual chemicals onfibresweresubsequentlyremovedbyboilingwithacetoneat80°Cfor2h.Thefibreswerethenwashedseveraltimes with distilled water and dried in a vacuumovenat80°Cfor24hinorderto improve fibre-matrix interfacialstrength.Alltreatmentswerebasedonthe recommendations made by Turner and Cogswell27. The architecture, manufacturing,andqualitycontrolinpreparing the specimens of the three types of composites followed the established recommendations28. Three types of composite panels (labelledFP1,FP2andFP3)werefabricated.Thevolumefractionoffibresusedinallthethree sets of prepared composites was maintainedat61%of thecompositevolume. The composites were prepared by pultrusion on a RDXLL-5000(ShanghaiD&GInstrumentsCo.,Ltd)injectionmouldingmachineaccordingtospecificationGB1040-79(China).Meltandmouldtemperatureof260°C

Table1a.BasicpropertiesofKevlar-49fibreFibre Notation

FibreDia., df, (μm)

Density, rf

(kg/m3)Longitudinal Modulus, Ef1

(GPa)

Transverse Modulus, Ef2

(GPa)

Longitudinal Shear

Modulus, Gf1 (GPa)

Longitudinal Poisson’s Ratio vf1

Longitudinal Tensile

Strength, sfT (MPa)

Longitudinal Compressive Strength, sfC

(MPa)F 12 1467.0 151.7 4.14 2.90 0.35 2757.9 517.1*as provided by the manufacturer, Dupont Inc. USA

Table 1b. Basic properties of the thermoplastics used as matrixMatrix Notation

MatrixMaterial

Density, rm (kg/m3)

Modulus, Em (GPa)

Shear Modulus, Gm (GPa)

Tensile Strength, smT (MPa)

Compressive Strength, smC

(MPa)

Shear Strength, tm,

(MPa)P1 SAN 1080.0 3.45 1.29 69.0 106.9 93.8P2 ABS 1050.0 2.41 1.06 45.5 66.5 62.1P3 Polyethylene 920.0 1.10 0.73 10.8 27.6 21.7



and80°C,respectively,havebeenused.A schematic of the matrix injectionpultrusion process used is shown in Figure 1. The pultrusion machine includes the following components: filtercreelfibre,pre-formplate,matrix-injection/wet-out chamber, formingdie, heater die, puller mechanism and cut-offsaw.A“closedbath”techniqueofmatrixinjectionpultrusionwasused,inwhich the fibre is pulled through

thematrixinjectionchamberandthethermoplastic matrix is melted andinjectedunderpressure.

4. MECHANICAL TESTING

4.1 Uniaxial Tensile TestFive dog-bone shaped specimens were testedfollowingBS2792Part3Method321:1994foreachtypeofcomposite

laminate at the ambient conditions using an extension rate of 1 mm/minonacomputercontrolled30kNMTSAllianceRT/30testingmachineequipped with a digital controllerand computer data acquisition,as per the schematic arrangement shown in Figure 2a. Hydraulic grips with surfalloy faces were used for specimen loading. A biaxialextensometerwasusedtomonitorthe

Figure 1. Schematic of the matrix injection pultrusion process for thermoplastic composites

Figure2.Schematicofthevarioustestconfigurations(dimensionsinmm):(a)UniaxialTensileTest,(b)CompressionTest,and(c)Three-pointflexuraltest(allunitsareinmillimetres)



longitudinal and transverse strains in the specimen. Instantaneous loads P and displacements were recorded at a rate of one set per second. The elastic modulus(E)andthePoisson’sratio(v) were calculated using data regression, and the tensile strength was calculated fromthemaximumloadandtheactualspecimen cross-sectional area.

4.2 Compression TestThe compression tests were also carriedoutusing30kNMTStestingmachine (Figure 2b). Specimen of 5mmnominal thicknesswithgauge

length(betweengrips)of10mmwasusedasdescribedinBS2792Part3Method345A:1993.Five specimenswere tested for each sample. The displacement during the test was measured using a clip gauge transducer. The compressive modulus and the compressive strength corresponding to themaximumloadatfailurecouldbedetermined from the load-displacement curve obtained for each specimen.

4.3 Three-point Flexural TestFive specimens for each composite type were tested in bending using

the same testing machine, as per the configuration shown in Figure 2c. The radius of the loading roller tip was5mm.ThespanlengthLwaskeptas40mmtomaintainaspan lengthto thickness ratio 8. Instantaneousload P and crosshead displacement δ measured by a linear variable differential transformer (LVDT)were recorded by a computerized dataacquisitionsystematonesecondintervals.Theflexuralmodulus(ECIB) and strength (sC1B) were calculated following BS 2782 Part 3 Method335A:1993.

5. RESULTS AND DISCUSSION

5.1 Density of the Prepared CompositesThe theoretical and experimentaldensities of the prepared composites are shown in Table 2.Theexperimentalvalues obtained following ASTMD3171areapproximately90%ofthe

Table 2. Theoretical and experimentally obtained densities of the prepared compositeswith61%volumefractionofKevlar-49fibresDensity Composite type

FP1 FP2 FP3Theoreticaldensity(kg/m3)-(x) 1316.1 1304.4 1253.7Experimentaldensity(kg/m3)-(y) 1180.0 1130.0 1235.0Ratio(y/x) 0.900 0.905 0.901

Table 3. Theoretically predicted values of mechanical properties of the compositesLoading Type Model Property (notation), unit Composite type

FP1 FP2 FP3Longitudinal – Tension mode

ROM TensileStrength(sC1T), MPa 1709.0 1700.0 1693.0TensileModulus(EC1T) GPa 93.9 93.5 93.0

Halpin-Tsai TensileStrength(sC1T) MPa 1649.0 1641 1634.0TensileModulus(EC1T) GPa 90.6 90.2 89.7

ModifiedROM TensileStrength(sC1T) MPa 1573.0 1564 1557.0TensileModulus(EC1T) GPa 86.4 86.0 85.5

Longitudinal – Compression mode

Rosen–ExtensionalMode CompressiveStrength(sC1C) MPa 20152.0 16843.0 11379.0Rosen – Shear Mode CompressiveStrength(sC1C) MPa 3329.0 2729.0 1880.0Xu–ReifsniderModel CompressiveStrength(sC1C) MPa 132.5 102.6 42.6

Transverse IROM TransverseStrength(sC1B) MPa 162.80 137.20 86.60TransverseModulus(EC1B) GPa 3.84 3.23 2.04

ModifiedIROM TransverseStrength(sC1B) MPa 163.40 141.40 99.60TransverseModulus(EC1B) GPa 3.86 3.34 2.35

Table 4. Experimental values of various mechanical properties of the prepared compositesComposite Tensile properties Compressive properties Flexural properties

sC1T EC1T εC1T P(p)CIT sC1C EC1C P(p)C1C sC2B EC2B εC2B P(p)C2B

MPa GPa % kN MPa GPa kN MPa GPa % kNFP1 171.5 9.38 2.20 10.63 120.4 8.68 6.33 138.4 3.26 0.77 8.22FP2 169.6 9.30 2.22 10.55 92.7 6.65 4.89 116.7 2.76 0.74 6.97FP3 167.2 9.18 2.26 10.18 38.3 2.76 2.00 72.9 1.73 0.75 4.47



theoretical values. The differences can be attributed to generation of voids amongpackinglayers.Theseresultsarewithin the range of densities obtained in such types of composites as observed by Cervera et al.29

5.2 Tensile PropertiesThe theoretical and experimentaltensile properties obtained for the three thermoplastic composites are given in Tables 3 and 4. Figure 3 illustrates typical load-displacement (P-δ) responses of the three fibrereinforced thermoplastic samples. FP1andFP2samplesshowedslightlyhigherpeakloadsorstrengthsthanthatofFP3.Thetrendissimilarforelasticmodulus. Though it is of less practical significance,allthethreecompositesexhibited some nonlinearity in theirtensile response. Apparently, theincreasingloadstraightensthefibres,resulting in a higher tensile modulus as thestrainisincreased.Zweben30 also examined unidirectional compositeswith Kevlar-49 fibres and foundthat the waviness of fibres led tonon-linear stress–strain behaviour. Typically, fabrication methods introduceunintendedfibrecurvatureduetounevenresinflowthatresults

in a non-linear tension stress-strain curve having a slope that increases with increasing stress.

Figure 4 shows the experimentaltensile strength and modulus values with those predicted by the classical lamination theories and Halpin-Tsai equation. Theoretical prediction

for the three reinforced composites estimatedbythemodelequationsareapproximatelyanorderofmagnitudehigher than those experimentallymeasured values. The reason for the deviation is likely due to the rapidgrowth of fibre packing defects athigher reinforcement levels, leading to lowexperimentalstrengthandmodulus

Figure 3. Typical engineering load-displacement curves from the uniaxial tensile tests for composites FP1, FP2 and FP

Figure 4. Comparison of theoretical and experimental values of (a) tensile strength and (b) tensile modulus of the tested composite



values.Prematurefibrebreakageandmatrixfailuresarethepossiblereasonsfor much large reduction in strength values. The model of composite tensile behaviourcalledACK31 showed one component (matrix in this case) canbeexpectedtobreakbeforetheother.Onceacrackisformedinthephaseand,if there is an interfacial bond between thefibreandmatrix,forthematrixtosupportadditionalloadfromthefibrethis load transfer will occur across the break through local intact shearof thematrix.Then theeffectof thefibrebreakwillbeisolated,permittingit to be reloaded at a distance from thebreak.Whenthestressgraduallyapplied on the composite reaches sCIT, the composite can be reloaded and the additional load will be supported only bythematrix.Thecompositewillfailcompletely when the stress supported by the matrix reaches the matrixfailure stress32. Though the mechanical properties of the three chosen matrices aresignificantlydifferent(seeTable 2), the mechanical properties of the respective composites in tension are almost identical, indicating the critical and limiting roleof thefibres in theperformance of the composites.

5.3 Compressive PropertiesThe compressive load versus displacement responses of the composites loaded along the in-plane direction are given in Figure 5. The slopes of curves in Figure 5 indicate thatthecompressivemoduliofFP2and FP3 are lower than FP1. Theload-displacement behaviour of all the tested specimens showed nearly linear elasticity up to the yield point. There is a sudden drop of stress after themaximumyieldstressandfailureoccurred rapidly indicating the loss ofcompositeintegrity.AmongthemFP1showsthehigheststrengthwhileFP3has the lowestproperties.Thestrengths of composites (Table 4) aresignificantlydifferent,followingthe order of matrices strength. So the matrix-property is more dominantthan inter-laminar and interfacial bonding.

The oscillating nature of the load-displacement curve immediately after yielding and continuous dropping load up to failure is the common feature in all three composites (refto Figure 5). When a composite materialissubjectedtoacompressiveload, several mechanisms contribute to failure, and the major ones are:matrixyieldfollowedbyfibremicro-buckling, local fibremicro-buckingwithinanelasticmatrix,shearfailure,and pure fibre compressive failure.During the pultrusion process, allfibres are not perfectly aligned ina uniaxial direction. Such fibremisalignment plays an important role in affecting the actual compressive strength.Inaddition,thefibresmayhinder the specimen’s ability to bend, contributing to increase in composite strength.Themisalignedfibresbeginto buckle when the matrix startsyielding.After yielding occurs, thematrix surrounding the fibres willharden. This failure process repeats itself and this mechanism leads to the oscillating nature of the load-displacement curve.

As can be seen fromFigure 6 and Tables 3 and 4,theXuandReifsnider31 model predicts the compressive strengthquiteaccurately.Therefore,theselectedvalueof1.5fortheparameterη inEq.(11)representingthefibre-matrixcondition seems reasonable. On the

other hand, the predictions including shearandextensionmodeproposedbyRosen25 significantly over-predictedthe properties.

5.4 Flexural PropertiesThe three common modes of material failure under flexural transversetensionare:(i)failureoffibre-matrixinterface(adhesionfailure),(ii)failureof thematrix(cohesionfailure),and(iii)fibrefailure.Figure 7 compares the stress-strain curves from the three-point bending test for the three thermoplastic composites. They show that the tested composites failed gradually and strains at maximumstress remained nearly the same. FP1exhibitsthehigheststrengthandflexuralmodulus,followedbyFP2andFP3.TheKevlar-49fibrecompositesgenerally fail gradually at larger strainswhencomparedtothatoffibreglass reinforced plastics, indicating increasing energy absorption and better damage tolerance.

Figure 8 shows the comparison of predictions of IROM and modifiedIROMmodelswiththeexperimentalresults of strength and flexuralmodulus for the three composites. The figuresindicatethattheexperimentalvalues are somewhat closer to that of theoretical values obtained using classicallaminatetheory(CLT).

Figure 5. Typical load-displacement curves obtained in compression tests for composites FP1, FP2 and FP3



5.5 Comparison of Predicted Models and Their ResponsesTables 5a, 5b and 5c show comparison of experimentally obtained valuesof tensile, compressive and flexuralstrengths with respect to the properties of their respective constituents, namely, fibreandmatrix.Table 5a shows that the theoretically calculated tensile strengths are an order of magnitude higher compared to those obtained experimentally. The ratio betweenexperimentally determined tensilestrengthvaluestothefibrestrengthisnearlyconstant(approximately0.10)for the three thermoplastic composites. This clearly indicates that the tensile strengths obtained are controlled by fibrealoneandthematrixcontributionis negligible.

As can be seen in Table 5b , experimentallyobtainedcompressivestrengths are about 10% lower thanthoseestimatedusingXu-Reifsnidermodel26 for all three composites. This small difference can be attributed to the expected deficiencies in themanufacturing of composites. Fibre bundling, interfacial debonding, waviness, misalignments, material nonlinearity, void formation etc. reduce the strength of composites. The relative strengths of the three composites with respect to the properties of constituents indicate thatmatrix seems to be thecontrolling contributor.

In Table 5c,experimentdatashowsthattheobtainedflexuralstrengthsareabout15% lower than the correspondingtheoretical values using IROM model for all three composites. Such deviations, similar to those observed in compression, may be attributable to the limitations of manufacturing of these composites.

The consolidated results presented in Figure 9 indicate that values of tensile strength and modulus are similar since they are fibre dominated. Thecompressive strengths are lower and appear to bematrix dependent. The

Figure 6. Comparison of theoretical and experimental values of compressive strength for composites FP1, FP2 and FP3

Figure7.Typicalstress-straincurvesobtainedfromthreepointflexuraltestforcomposites FP1, FP2 and FP3

flexuralstrengths,asseeninFigure 9, are between the tensile and compressive values,whereastheflexuralmodulusvalues are the lowest among the three types of loading.

Theselectionoffibreisveryimportantwhen the application of thermoplastic composites is dominated by their tensile behaviour. On the other hand, the selection of suitable thermoplastic



matrix assumes a controlling rolewhen the composites are to be used in compressive stress environment.Ascan be seen from the results presented in Tables 3 and 4 and Figure 6, it may be concluded that micro-bucklingapproach has had little success in predicting the compressive strength in fibrecomposites.Rosen’smodel25of micro-bucklingusingthematrixshearmodulus as the controlling property has been found to grossly overestimate the failure stress of composite materials. In general,matrixnonlinearityandfibrewaviness or misalignments operate intandemtoproducemicro-bucklingloads that are lower than Rosen’s original elastic results for straight fibres.

5.6 Cost AnalysisThe manufacturing cost can be estimated with a rigorous consideration of the process-performance-cost interrelations based on the fundamental data. The composites produced for this studyhadaC-channelconfigurationwithagirthof320mmandinlengthsof 6000 mm, and 6 pieces wereproducedatatime.Quantitiesoffibresandmatrixusedfortheestimationoftotal manufacturing cost are given in

Table 5a. Comparison of experimentally obtained tensile strengths of the composites with respect to model prediction and the respective properties of the constituentsComposite Exp. (σCIT )

ROM (σCIT )Exp. (σCIT )

σ fT

Exp. (σCIT )σmT

FP1 0.10 0.062 2.49FP2 0.099 0.062 3.77FP3 0.099 0.061 15.88

Table 5b. Comparison of experimentally obtained compressive strengths of the composites with respect to model prediction and the respective properties of the constituentsComposite Exp. (σC1C )

Xu-Reifsnider Model (σC1C )Exp. (σC1C )

σ fC

Exp. (σC1C )σmC

FP1 0.91 0.23 1.13FP2 0.90 0.18 1.39FP3 0.90 0.074 1.39

Table5c.Comparisonofexperimentallyobtainedflexuralstrengthsofthecomposites with respect to model prediction and the respective properties of the constituentsComposite Exp. (σC2B)

IROM (σC1B )Exp. (σC2B )σ fT +σ fC

2

Exp. (σC2B)σmT +σmC

2

FP1 0.85 0.085 1.57FP2 0.85 0.071 2.08FP3 0.84 0.045 3.80

Figure8.ComparisonofIROMandmodifiedIROMmodelestimationswiththeexperimentallyobtainedvaluesof(a)flexuralstrengthand(b)flexuralmodulus



Table 6.Allinputparametersforthecalculation of total manufacturing cost for Kevlar-49 thermoplasticcompositesusingEq.(12)areshownin Table 7.

Thequantityofmaterial is thetotalweight of constituents per cubic millimetre while material purchasing price shows the total material cost of constituents including additives in USD per kilogram. This costevaluation procedure is relatively simple and applicable to a wide range of manufacturing processes. The model considers the manufacturing cost as the sum of the material cost, the labour cost and the overhead cost, and the estimated values are shown in Table 8. The economic potential of the

Table6.QuantitiesoffibresandmatrixusedbypultrusioninthecasestudyfortotalmanufacturingcostevaluationCross

Sectional Area

L Total Volume

Total Weight for 6 m

Measured Density

No. of 6 m specimens produced

Total Fibre

Weight

Fibre Weight / Piece

Fibre Volume / Specimen

Total Matrix Volume

(6 pieces)

Matrix Volume / piece

Vf Vm

m2 m m3 kg kg/m3 kg kg m3 litres litres % %FP1 0.0016 6 0.0096 11.37 1184.46 6.00 54.00 9.00 0.0061 23.52 3.92 61.01 38.99FP2 0.0016 6 0.0096 11.27 1173.93 6.00 54.00 9.00 0.0061 23.52 3.92 61.01 38.99FP3 0.0016 6 0.0096 10.83 1128.30 6.00 54.00 9.00 0.0061 23.52 3.92 61.01 38.99

Figure9.Comparisonofexperimentallymeasuredvaluesoftensile,compressiveandflexural(a)strengthand(b)modulusforcomposites FP1, FP2 and FP3

Table 7. Model input data for the manufacturing cost estimation of Kevlar-49 thermoplastic composites by pultrusionModel Input Data Symbol Unit FP1 FP2 FP3Quantity of material Q kg 11.37 11.27 10.83Material Purchasing Price P USD/kg 8.63 8.05 7.95Scrap factor Fscrap % 3 3 3Equipmentpurchaseprice E0 USD 120k 120k 120kEquipmentlifetime N yr 15 15 15Equipmentsalvagefactor Fsal % 20 20 20Equipmentmaintenancefactor Fmtn % 1 1 1Equipmentutilities Ug USD 0 0 0No.ofworkers o,p Manday 1.5 1.5 1.5Worker’swages Sw USD/hr 15 15 15Worker’spresencefactor Fpres % 100 100 100Run Time trun Min 0.67 0.67 0.67Reworkfactor Frew % 0 0 0Interest rate IR % 8 8 8Overhead Factor Fovh % 75 75 75



composites can be derived on the basis ofcostperunitproperty(strengthormodulus), and the results are shown in Table 9.Acomparisonoftheresultsobtained in this study indicates that the best results are obtained with FP1(followedbyFP2andFP3)fortensileproperties,FP3(followedbyFP1andFP2)forflexuralpropertiesand compressive properties. The results can assist a designer to choose the most suitable composites during preliminary engineering design stage itself.

6. CONCLUSIONS

The mechanical properties of three Kevlar-49 based thermoplasticcomposites with fibre fraction of 61% were obtained experimentallyand predicted using micromechanical models. Cost estimation of the

manufacturing of these composites was made, and the following conclusions are drawn from this study:

1. Th e t e n s i l e s t r e ng t h o ft he rmop la s t i c compos i t e s predicted using ROM, modifiedROM and Halpin-Tsai models were approximately an orderof magnitude higher than the experimentallymeasuredvalues.

2. The compressive strengthspredictedbytheXuandReifsnidermodel were in very good agreement with the experimental values forall composites whereas the Rosen modelpredictionswereextremelyhigh.

3. The flexural properties obtainedusing IROM model were about 15%higherthantheexperimentallyobtained values.

4. The compression and flexuralproperties, i.e., strength and modulus, of all three thermoplastic composites were significantly lower than their tensile properties.

5. Thestrengthsofcompositesusinga SAN matrix were higher thanthatofABSpolyester,followedbypolyethylene.

6. From the cost analysis, FP1provided the best performance in terms of cost per unit tensile strengthandmodulus,whereasFP3providedthebestresultsforflexuraland compressive properties.

7. NOMENCLATURE

d Fibre diameterε1 Strain in longitudinal

directionεf Fibre strainεm Matrixstrain

Table 8. Results of cost for manufacturing of the Kevlar-49 thermoplastic compositesComposite Cost in USD Notation FP1 FP2 FP3Total manufacturing cost per 6 m length x 250.70 229.03 211.42Unitcostforthechosensection(permeter) y=x/6 41.78 38.17 35.24Unitcostfor1mx1mx5mmthicknesssheet z=y / 0.32 130.58 119.29 110.11Cost per m3 z/0.005 26,115 23,858 22,022Costperkg - 22.04 20.22 19.49Referencepriceofconstituents(USD):Cost per m3offibre - 13,467 13,467 13,467Costperkgoffibre - 9.18 9.18 9.18Cost per m3ofmatrix - 7,630 4,953 3,421Costperkgofmatrix - 7.06 4.72 3.60

Table9.Costcomparisononthebasisoftensile,compressiveandflexuralpropertiesofthetestedcompositesProperty Composite Costperkg

(USD)Strength(MPa)

Cost to Strength Ratio

(USD/MPa)

Modulus(GPa)

Cost to Modulus Ratio

(USD/GPa)

Order of Preference

(per unit cost)Tensile FP1 22.04 171.5 0.129 9.38 2.350 3

FP2 20.22 169.6 0.119 9.3 2.174 2FP3 19.49 167.2 0.117 9.18 2.123 1

Compressive FP1 22.04 120.4 0.183 8.68 2.539 1FP2 20.22 92.7 0.218 6.65 3.041 2FP3 19.49 38.3 0.509 2.76 7.062 3

Flexural FP1 22.04 138.4 0.159 3.26 6.76 1FP2 20.22 116.7 0.173 2.76 7.33 2FP3 19.49 72.9 0.267 1.73 11.27 3



εCIT Elongation under tensile loadεC2B Elongationunderflexural

loadEC1T Tensile modulusEC2B FlexuralmodulusEf Young’smodulusoffibreEf1 Young’s modulus in

longitudinal directionEf2 Young’s modulus in

transverse directionEm Young’smodulusoffibreξ ShapefittingparameterG12 Shearmodulusin1-2

directionGf ShearmodulusoffibreGm Shearmodulusofmatrixκ Geometrical factorm materialsn operationsoi workersforsetupactivity

included in operation ipi workersforrunactivity

included in operation irf Densityoffibrerm Densityofmatrixqi, workersformoveactivity

included in operation iri equipmentcomponents

included in operation iS Fibre aspect ratios1 Stress in longitudinal

directions2 Stress in transverse directionsCIT Tensile strengthsC2B FlexuralstrengthsmC Compressive strengthP(p) C2B PeakloadinflexuraltestP(p) CIC PeakloadincompressivetestP(p) CIT Peakloadintensiletesttm ShearmodulusofmatrixVf Fibre volume fractionVm Matrixvolumefraction

REFERENCES1. VedulaM.andKoczakM.J.,

Journal of Thermoplastic Composite Materials, 2(1989)154-163.

2. SchinnerG.,BrandtJ.andRichterH., Journal of Thermoplastic Composite Materials, 9(1996)239-245.

3. HouM.,YeL.andMaiY.W.,Plastics Rubber and Composites Processing and Applications, 23 (1995)279-293.

4. Lin T.K., Wu S.J., Lai J.G. and Shyu S.S., Composite Science and Technology,60(2000)1873–1878.

5. ParkS.K.,KimM.H.,Journal of Materials Science, 35(2000)1901-1905.

6. NardinM.,ElMalikiaA.andSchultzJ., The Journal of Adhesion, 40 (1993)93–106.

7. Wu S.R., Sheu G.S. and Shyu S.S., Journal of Applied Polymer Science, 62(1996)1347-1360.

8. KimJ.H.,LeeM.G.,RyouH.,LeeM.H., Chung K., Kang T.J., and YounJ.R.,Proceedingsofthe8thEsaform Conference on Material Forming,Cluj-Napoca,Romania,pp.955-958(2005).

9. ChristensenR.M.,Mechanics of Composite Materials, John Wiley, NewYork,USA(1984).

10.CarusoJ.J.andChamisC.C., Journal of Composites Technology & Research, 8(1986)77-83.

11.KnightM.,Journal of Composite Materials, 16(1982)153-159.

12.BudianskyB.,Journal of the Mechanics and Physics of Solids, 13 (1965)223-227.

13.AboudiJ.,Mechanicsofcompositematerials:aunifiedmicromechanicalapproach,Elsevier,NewYork,USA(1991).

14.ZhaoY.H.andWengG.J.,Journal of Applied Mechanics, 57(1990)158-167.

15.EshelbyJ.D.,Proceedings of The Royal Society of London, A241 (1957)376-396.

16.MoriT.andTanakaK.,Acta Metallurgica, 21(1973)571-574.

17.HalpinJ.C.andTsaiS.W.,In:Effect of environmental factors

on composite materials. primer of compositematerials:analysis(2nded.),AirForceTechnicalReportAFML-TR-67-423,Technomic,Westport,CT,p.77(1969).

18.TuckerC.L.andLiangE.,Composites Science and Technology, 59(1999)655-671.

19.FukudaH.andChouT.W.,Journal of Materials Science, 16(1981)1088-1096.

20.HalpinJ.C.andKardosJ.L.,Polymer Engineering and Science, 16(1976)344-352.

21.HillR.,Journal of the Mechanics and Physics of Solids, 12(1964)213-218.

22.MinusM.L.andKumarS.,JOM Journal of the Minerals, Metals and Materials Society, 57(2005)52-58.

23.KellyA.andDaviesG.,The Metallurgical Reviews, 10(1965)1-77.

24.RosenB.W.,American Society for Metals,MetalsPark,Ohio,pp.37-75(1965).

25.XuY.L.andReifsniderK.L.,Journal of Composite Materials, 27(1993)572-588.

26.WangR.andRavi-ChandarK.,Journal of Applied Mechanics, 71 (2004)706-712.

27.TurnerR.M.andCogswellF.N.,SAMPE Journal, 23(1987)40-44.

28.GentzM.,ArmentroutD.,RupnowskiP.,KumosaL.,ShinE., Sutter J.K. and Kumos M., Composites Science and Technology, 64(2004)203-220.

29.CerveraF.,SanchisL.,Sanchez-PerezJ.V.,Martınez-SalaR.,RubioC., Meseguer F., Lopez C., Caballero D.andSanchez-DehesaJ.,Physical Review Letters, 88(2002)23902.

30.ZwebenC.,AIAA Journal, 6(1968)2325-2331.

31.AvestonJ.,CooperG.A.andKellyA.,In:The Properties of Fibre Composites, IPC Science and TechnologyPress,Surrey,pp.15-26(1971).

32.BunsellA.R.andRenardJ.,Fundamental of Fibre Reinforced Composite Materials, CRC Press, ISBN: 9780750306898.



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