IntroductiontoCompositeMaterials复合材料概论.docx
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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
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