IntroductiontoCompositeMaterials复合材料概论.docx

IntroductiontoCompositeMaterials复合材料概论.docx

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IntroductiontoCompositeMaterials复合材料概论

IntroductiontoCompositeMaterials

References

∙AERO304notesandIntroductiontoAerospaceStructuralAnalysis,AllenandHaisler

∙PrinciplesofCompositeMaterialMechanics,R.F.Gibson,McGraw-Hill,1994

∙MechanicsofCompositeMaterials,RobertM.Jones,McGraw-Hill,1975

∙IntroductiontoCompositeMaterials,S.W.TsaiandH.T.Hahn,TechnomicPublishingCo.,1980

IntroductionandTerminology

Structuralmaterialscanbedividedinto4basiccategories:

∙Metals

∙Polymers

∙Ceramics

∙Composites

Composites,whichconsistoftwoormoreseparatematerialscombinedinamacroscopicstructuralunit,aremadefromvariouscombinationsoftheotherthreematerials.

TherelativeimportanceofthefourbasicmaterialsinahistoricalcontexthasbeenpresentedbyAshby（Technologyofthe1990s:

AdvancedMaterialsandPredictiveDesign,M.F.Ashby,PhilosophicalTransactionsoftheRoyalSocietyofLondon,A322,393-407,1987）andisshownschematicallybelow（figuretakenfromGibson）:

Mankindhasusedcompositessinceearlytime;forexample,straw-reinforcedclaybricksusedbyIsraelites（thebookofExodusintheOldTestament）,plantfiber-reinforcedpottery,etc.Theyknewfromdailyusethatfiberreinforcementofamaterialisveryeffectivebecausemanymaterials（butnotall）aremuchstrongerandstifferinfiberformthantheyareinbulkform.

Forexample,Griffithfoundthatasglassrodsandfibersgotthinner,theygotstronger.Hefoundthatthatforverysmalldiametersthefiberstrengthapproachedthetheoreticalcohesivestrengthbetweenadjacentlayersofatoms,whereasforlargediametersthefiberstrengthdroppedtonearthestrengthofbulkglass.

Fibersallowonetoobtainthemaximumtensilestrengthandstiffnessofamaterial,buttherearedisadvantages.Fibersalonecannotsupportlongitudinalcompressiveloadsandtheirtransversemechanicalpropertiesaregenerallynotasgoodasthecorrespondinglongitudinal（fiberdirection）properties.Thus,thereisoftentheneedtoplacefibersindifferentdirectionsdependingupontheparticularloadingapplication.

TypesofFiber-ReinforcedComposites

Onegenerallyfindsfourtypesoffiber-reinforcedcompositesasshownbelow（fromGibson）.Theydifferinhowthefibersareutilizedtomakethecomposite（orientationandlengthoffibers）.

Continuousfibercompositesaregenerally"laid-up"inplies（orlaminae）witheachplyhavingfibersorientedinthesamedirection.Alayeroffibersallorientedinthesamedirectionisimbeddedinahomogeneousmaterial（calledthematrix）tomakeasingleplyorlaminae.Forexample,glass-epoxyhasalayerofglassfibersrunningmore-or-lessparallelwithinanepoxyresinmatrixmaterial.

Individualpliescanbestackedorlayeredandbondedtogetherwithindividualplyfiberdirectionsbeingselectedsoastotailorthelay-up（orlaminate）tohavedesiredoverallstructuralcharacteristicsofthelaminate.Underloading,thepotentialfordelamination（orseparationofthelaminae）isamajorproblembecausetheinterlaminarstrengthismatrixdominated（i.e.,ifthematrixisweak,plydelaminationcanoccur）.

Wovenfibercompositesaresimilartoordinaryclothusedinthetextileindustry.Thewovenfibermaybe2-D（fibersinterwovenin2directions）or3-D（fibersinterwovenin3directions）.Wovenfibercompositesdonotgenerallyhavedistinctlaminaeandarenotnearlyassusceptibletodelamination;however,strengthandstiffnessaresacrificedduetothefactthatthefibersarenotasstraight（becauseoftheweaving）asinthecontinuousfiberlaminate.

Choppedfibercompositeshavefibersthatarerelativelyshortandhavearandomorientationanddistributionofthefibers.Choppedfibercompositesgenerallyhavemechanicalpropertiesthatareconsiderablepoorerthanthoseofcontinuousfibercomposites.Howevertheyarecheapertomanufactureandareusedinhigh-volumeapplications.

Hybridcompositesgenerallyconsistofmixedchoppedandcontinuousfibers;ormixedfibertypessuchasglass/graphite.

Sandwichcompositesarealsocommon.Theyconsistofhighstrengthcompositefacingsheets（whichmaybeanyofthefourfibercompositesdiscussedabove）bondedtoalightweightfoamorhoneycombcore（fromGibson）.

Sandwichstructureshaveextremelyhighflexuralstiffness-to-weightratiosandarewidelyusedinaerospacestructures.Thedesignflexibilityofferedbytheseandothercompositeconfigurationsisobviouslyquiteattractivetodesigners,andthepotentialnowexisttodesignnotonlythestructure,butalsothestructuralmaterialitself.

Almostallofthefiber-reinforcedcompositetypesdiscussedabovecanbeutilizedincomplexcurvedgeometriesalthoughthemanufacturingprocessmaybemuchmorecostlyanddifficult.Forexample,woundfiber-reinforcedpressurevesselsarecommonandaremanufacturedbywindingeitherindividualfiberfilamentsonamandrel（havingtheshapeofthevessel）orindividualpliesarewoundonthemandrel.Curvedcompositematerialpanelsonaircraftwings,fuselageandnacellesarecommon.

Allofthecompositetypeshavevariousmanufacturingprocessesrequiredtobondindividualplies.Commonglass-epoxycompositesmustbecuredusingahightemperatureandvacuum（orpressure）processdesignednotonlytobondindividualpliesbutalsotominimizeresiduallaminaeandinterlaminarstresses.

FiberMaterials

Glassfibersconsistprimarilyofsilica（silicondioxide）andmetallic-oxide-modifyingelementsaregenerallyproducedbymechanicaldrawingofmoltenglassthroughasmallorifice.E-glassaccountsformostoftheglassfiberproductionandisthemostwidelyusedreinforcementforcomposites.Thesecondmostpopularglassfiber,S-glass,hasroughly30percentgreatertensilestrengthand20percentgreatermodulusofelasticitythanE-glassbutisnotaswidelyusedbecauseofitshighercost.

Graphiteorcarbonfibersarethemostwidelyusedadvancedfiber,andgraphite/epoxyorcarbon/epoxycompositesarenowusedroutinelyinaerospacestructures.Theactualfibersareusuallyproducedbysubjectingorganicprecursorfiberssuchaspolyacrylonitrile（PAN）orrayontoasequenceofheattreatments,sothattheprecursorisconvertedtocarbonbypyrolysis.Graphitefibersaretypicallysubjectedtohigherheattreatmentsthanarecarbonfibers.Carbonfibersaretypically90-95%carbon,whereasgraphitefibersareatleast99%carbon.

Aramidpolymerfibers,producedprimarilybyE.I.duPontdeNemours&Companyunderthetradename"Kevlar®,"wereoriginallydevelopedforuseinradialtires.ThedensityofKevlarisabouthalfthatofglassanditsspecificstrengthisamongthehighestofcurrentlyavailablefibers.Kevlaralsohasexcellenttoughness,ductility,andimpactresistance;unlikebrittleglassorgraphitefibers.

Boronfibersareactuallycompositesconsistingofaboroncoatingonasubstrateoftungstenorcarbon.Thediameterofboronfibersisamongthelargestofalltheadvancedfibers,typically0.002-0.008in.Boronfibershavemuchhigherstrengthandstiffnessthangraphite,buttheyalsohavehigherdensity.Boron/epoxyandboron/aluminumcompositesarewidelyusedinaerospacestructures,buthighcostpreventsmorewidespreaduse.

Siliconcarbide（SiC）fibersareusedprimarilyinhigh-temperaturemetalandceramicmatrixcompositesbecauseoftheirexcellentoxidationresistanceandhigh-temperaturestrengthretention.SiCwhisker-reinforcedmetalsareincreasinglybeingusedasalternativetoun-reinforcedmetalsandcontinuousfiber-reinforcedmetals.SiCwhiskersarequitesmall,typically8-20μin.diameterandabout0.0012in.longsothatstandardmetal-formingprocessessuchasextrusion,rollingandforgingcanbeeasilyused.

Thelistoffibersgoeson…Onthefollowingpagesarea）Selectedpropertiesoffibersandbulkmetals,b）Specificstrengthvs.specificmodulusforvariousfibersandc）Specificstrengthvs.specificmodulus（stiffness）forvariouscomposites（fromGibson）.Specificvalueisthevalueofthepropertydividedbyitsdensity.

MatrixandFillerMaterials

Polymers,metalsandceramicsareallusedasmatrixmaterialsincomposites.Thematrix

∙holdsthefiberstogetherinastructuralunit,

∙protectsthemfromexternaldamage,

∙transfersanddistributestheappliedloadstothefibers,and

∙inmanycases,contributessomeneededpropertysuchasductility,toughness,orelectricalinsulation.

Becausethematrixmusttransferloadtothefibers,astronginterfacebondbetweenthefiberandmatrixisextremelyimportant;eitherthroughamechanicalorchemicalbondbetweenfibersandmatrix.Fibersandmatrixmustobviouslybechemicallycompatibletopreventundesirablereactionsattheinterface;thisisespeciallyimportantathightemperaturewherechemicalreactionscanbeaccelerated.

Servicetemperatureisquiteoftenacontrollingfactorinconsiderationofamatrixmaterial.Listedinorderofincreasingtemperaturecapability,wehave:

Polymersarethemostwidelyusedmatrixmaterials.Theymaybeeitherthermosets（e.g.,epoxy,polyester,phenolics）orthermoplastics（e.g.,polyimide（PI）,polyetheretherketone（PEEK）,polyphenylenesulfide（PPS））.Uponcuring,thermosetsformahighlycross-linked,three-dimensionalmolecularnetworkwhichdoesnotmeltathightemperature.Thermoplastics,however,arebasedonpolymerchainsthatdonotcross-link.Asaresult,thermoplasticswillsoftenandmeltathightemperature,thenhardenagainuponcooling.

Epoxiesandpolyestersarealsowidelyused.Highgradeepoxiesaretypicallycuredatabout350Fandaregenerallynotusedattemperaturesabout300︒F.Theadvancedthermoplastics（PEEK,PIandPPS）havemelti