DAMAGE-TOLERANCE CHARACTERISTICS OF COMPOSITE FUSE

NASATechnicalMemorandum110303Damage-ToleranceCharacteristicsofCompositeFuselageSandwichStructuresWithThickFacesheetsDavidM.McGowanandDamodarR.AmburLangleyResearchCenter,Hampton,VirginiaFebruary1997NationalAeronauticsandSpaceAdministrationLangleyResearchCenterHampton,Virginia23681-00011DAMAGE-TOLERANCECHARACTERISTICSOFCOMPOSITEFUSELAGESANDWICHSTRUCTURESWITHTHICKFACESHEETSDavidM.McGowanandDamodarR.AmburNASALangleyResearchCenterHampton,VA23681-0001ABSTRACTDamagetolerancecharacteristicsandresultsfromexperimentalandanalyticalstudiesofacompositefuselagekeelsandwichstructuresubjectedtolow-speedimpactdamageanddiscrete-sourcedamagearepresented.Thetestspecimensareconstructedfromgraphite-epoxyskinsbondedtoahoneycombcore,andtheyarerepresentativeofahighlyloadedfuselagekeelstructure.Resultsofcompression-after-impact(CAI)andnotch-lengthsensitivitystudiesof5-in.-wideby10-in-longspecimensarepresented.Acorrelationbetweenlow-speed-impactdentdepth,theassociateddamagearea,andresidualstrengthfordifferentimpact-energylevelsisdescribed;andacomparisonofthestrengthforundamagedanddamagedspecimenswithdifferentnotch-length-to-specimen-widthratiosispresented.Surfacestrainsinthefacesheetsoftheundamagedspecimensaswellassurfacestrainsthatillustratetheloadredistributionaroundthenotchsitesinthenotchedspecimensarepresentedandcomparedwithresultsfromfiniteelementanalyses.Reductionsinstrengthofasmuchas53.1percentfortheimpactedspecimensand64.7percentforthenotchedspecimensareobserved.INTRODUCTIONOneoftheprimarygoalsoftheAdvancedCompositesTechnology(ACT)programistodeveloptheenablingtechnologythatwillallowcompositematerialstobeusedintheprimarywingandfuselagestructuresofthenextgenerationadvancedsubsonictransportaircraft.AspartoftheACTprogram,theBoeingCommercialAircraftGroup(BCAG)hasbeenworkingtodevelopcost-effectiveandstructurallyefficientcompositefuselagestructure.1Thefocusofthisworkhasbeenonthefuselagesectionjustaftofthemainlandinggearwheelwellofamodernwide-bodytransportwhichisdesignatedassection46onaBoeingaircraft.Thisfuselagesectionis33-feetlongand20-feetindiameter,andcontainscrown,sideandkeelquadrantsectionsasshowninFig.1.2AsillustratedinFig.1,thecurrentfuselagedesignconceptutilizesskin-stringerconstructioninthecrownquadrantsectionandsandwichconstructioninthesideandkeelquadrantsections.2Sandwichstructureshavebeenusedextensivelyinstiffness-criticalaircraftsecondarystructures.Theapplicationofsandwichstructuresinaircrafthasbeenrestrictedinthepastduetoitsundesirablemoistureabsorptionandmoistureretentioncharacteristics,andduetoaninsufficientunderstandingoflow-speedimpact-damagemechanismsandtheeffectofsuchdamage,aswellasdiscrete-sourcedamage,onthestructuralperformanceofsuchstructures.Sandwichstructuresstillhavethepotentialforimprovedstructuralefficiencyandreducedmanufacturingcost.Nowthatsandwichconceptsarebeingconsideredforapplicationtoaircraftprimarystructures,understandingtheeffectsoflow-speedimpactdamageandpenetrationdamageonsandwichstructureshasbecomemoreimportant.AjointNASA/BoeingstudyhasbeenconductedaspartoftheACTprogramtoaddressofthetechnologyissuesassociatedwithusingcomposite-facesheetsandwichstructureinsubsonictransportfuselagesideandkeelpanelapplications.Thisstudyconsistedofmanufacturing,testingandanalyzingfull-scalekeel-andside-quadrant-sectionsandwichpanels.Aspartofanongoingefforttodevelopkeelpanelstructureforsection46ofthefuselage,akeelpanelrepresentativeoftheforwardpartofthesection46keelwasdesignedandfabricatedbytheBoeingCommercialAirplaneGroup.Thepanelisconstructedfromgraphite-epoxyfacesheetsvaryinginthicknessalongthepanellengthfrom50to36pliesbondedtoafiberglasshoneycombcore.AnexperimentalandanalyticalstudyofthiskeelpanelwiththickfacesheetswasconductedatNASALangleyResearchCentertounderstandtheloaddistributionandfailuremechanismsofthepanelwithandwithoutimpactanddiscrete-sourcedamage.3Theimpact-damageconditionssimulatedintheexperimentalstudyofRef.3correspondtobarelyvisibleimpactdamage(BVID).Forthetestpanelstudied,BVIDwasconsideredtohaveoccurredwhentheresidualdentdepthonthefacesheetmeasuredattheimpactsitewasgreaterthanorequalto0.05in.oriftheimpact-energylevelwasequalto100ft-lb.Thedamageconditionsatdifferentlocationsofthepanelcorrespondingtodifferentimpact-energylevelsweredeterminedusingimpact-damagescreening-testresultsfromanotherpanelofthesamedesign.Theimpact-damagescreening-testresultsindicatedthatasignificantamountofthrough-the-thicknessdamageoccurredinthefacesheetatmuchlowerimpact-energylevelsthanformonolithiclaminateswiththesameconstructionfeaturesasthefacesheets.Also,forimpact-energylevelsashighas100ft-lbs,theresidualdent-depthvaluesweremuchlessthan0.05in.forthickfacesheetsandwichplates.Theseimpact-damagecharacteristicsalsosuggestthat,forlow-levelimpact-energylevels,considerablereductionincompression-after-impactstrengthoccursforthesandwichstructures,andthereductioninstrengthcouldbemuchgreaterifadent-depthvalueof0.05in.wasusedasaBVIDcriterion.Thedent-depthcriterionassumedinthestudyofRef.3doesnotappeartobeappropriateforthedamage-toler