Performance of long fiber reinforced thermoplastic

PerformanceoflongfiberreinforcedthermoplasticssubjectedtotransverseintermediatevelocitybluntobjectimpactS.D.Bartus,U.K.Vaidya*DepartmentofMaterialsScienceandEngineering,TheUniversityofAlabamaatBirmingham,Birmingham,AL35294-4461,USAAvailableonline27August2004AbstractAnunderstandingoflongfiberreinforcedthermoplastic(LFT)behaviorwhileundergoinghighenergy,highstrainrateimpactisessentialinpromotingitsuseinautomotiveandothermarkets.Inthiswork,damagetoleranceofpolypropylene(PP)/E-glassLFTssubjectedtotransversebluntobjectimpact(BOI)wasinvestigated,treatedfromanexperimentalstandpoint,inordertocharacterizeenergydissipationanddamagemodes.LFTsubjectedtoBOIexhibitedhighimpactenergydissipation,whichincreasedlinearlywithincreasingarealdensity.Theaverageimpactenergydissipationatthecriticalvelocity(ballisticlimit)was167Jand121Jfora4.61gcm
2specimenimpactedbyflatandconicallyshapedprojectiles,respectively.Thefiberorientationalsoplayedalargeroleinenergydissipation;failureappearedtooccuralongplanesofpreferentialfiberorientation.Thecriticalenergyforspecimenper-forationdidnotvarywiththemassoftheimpactor,i.e.strainrate,intherangeinvestigated.Thisoverallworkadvancesthestate-of-the-artinLFTswithanautomotivefocus.
2004ElsevierLtd.Allrightsreserved.Keywords:Automotivecomposites;Impact;Thermoplasticcomposites1.Introduction1.1.ThermoplasticcompositesTheuseofthermoplasticcompositeshasgainedstea-dyfavorovertraditionalmaterialssuchassteelinstruc-turalandsemi-structuralapplicationsduetotheprominentphysicalandmechanicalbehaviorofthesematerialssuchasspecificstrength,damping,corrosionresistance,andimpactproperties.Moreover,closedmoldeddiscontinuousLFTcompositessharetheattrac-tivefeaturesofpossessinggreaterstrength,stiffnessandimpactpropertiesincontrasttoshortfiberreinforcedthermoplastics,inadditiontohighvolumeprocessabil-ity,abilitytofillcomplexgeometries,intrinsicrecyclabil-ity,andthecapacityforpartintegration.ThecurrentUnitedStatesmarketforsuchmaterialsisinexcessof4.54·108kgperannum,halfofwhichisconsumedbytheautomotiveindustry[1].LongFiberreinforcedThermoplastic(LFT)compositeshaveoneofthehigh-estgrowthratesinthepolymermaterialareas,sustain-ingaprojected30%growthfrom2000to2004[2].ThermoplasticcompositestypicallycompriseacosteffectivecommoditymatrixsuchasPP,polyethylene(PE),nylon,etc.;reinforcedwithglass,carbon,orara-midfibers.E-glassisthemostcommonreinforcementsincetheautomotivemarketnicheisdrivenmorebycost/performanceratiothanweight/performanceratioasdemandedbytheaerospaceindustry.Thermoplasticcompositesusedintheseapplicationscanbeshortfiberreinforcedthermoplastic(SFRT),glassmatthermoplastic(GMT),orLFT.InjectionmoldedSFRTcompositeswithstartingfiberlengthslessthan4mm(beforefiberlengthdegradationduringprocessing)arecurrentlythemostprevalentoftheafore-mentionedcomposites.However,thefulladvantageof0263-8223/$-seefrontmatter
2004ElsevierLtd.Allrightsreserved.doi:10.1016/j.compstruct.2004.07.023*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](U.K.Vaidya).CompositeStructures67(2005)263–277fiberisnotrealizedduetothelowfiberaspectratio.InjectionmoldedLFTsalsosufferfromexcessivefiberlengthdegradationintheplasticationandinjectionstages.Inaddition,difficultiesariseinprocessingcomponentswithhighfibercontentandstartingfiberlengthsinexcessof13mmduetothehighmeltviscosity.ThelargefiberaspectratioofLFTsfullytakesadvantageofthefiberstrengthincontrasttoitsshortfibercounterpart.Injection,injection-compressionandextrusion-compressionmoldingtechniquesareem-ployedforLFTprocessing.StartingLFTfiberlengthsaretypicallygreaterthan13mmanddependonthede-siredproperties,fiberconcentration,andprocessingtechnique.LFTsofferseveraladvantagesincontrasttoGMTssuchas:thepossibilitytoworkwithoutsemi-fin-ishedmats(e.g.,in-lineextrusion)makingitlesslaborintensive,andlowercompressionforcesduetoade-creaseinmeltviscosity,whichresultsincapitalcostsav-ingsintoolingandmachinery.Moreover,LFTsofferhighersurfacequality,lesspartrejectionduetoanin-creaseintheabilitytofillcomplexfeaturesandinte-gratedrecyclability.Anotheradvantageisgreaterfreedominchoosingfiberandmatrixmaterials.Inthecompressionmoldingprocess,fibersdevelopinplaneorientationsduringflow,whichcanplaguetheconsolidatedcomponent.Preferentialorientationduringcompressionmoldingcanreducestrengthandstiffnessincriticalareasandwillinducewarpingthroughaniso-tropiccontractionuponcooling[3].Althoughlargefiberaspectratiosandhighfiberloadingdecreasefibermobil-ityinthemelttherebydecreasingthedegreeofpreferen-tialorientation,ahighdegreeofpreferentialorientationmaystilloccurdependingonmoldgeometry,chargelocation,andpreorientationofthefibersinthecharge.ThisisdescribedindetailinpreviousworkusingfiniteelementmethodstosimulatethecompressionmoldingprocesswithCADpress-Thermoplastic[4].AstheuseofLFTsgrowsinautomotiveandotherindustries,theneedtodetermineimpactresponseofthesematerialsincreasestoensurethesafetyandstabil-ityofdesignedstructures[5].LFTwasrecentlyem-ployedfortheunderside‘‘bellypan’’ofDaimlerChrysler
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