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ExperimentalanalysisofacompositeautomotivesuspensionarmM.PINFOLDandG.CALVERTUniversityofWarwick/RoverGroupGaydon,UKReceived11November1992revised26March1993Inapplicationswhereweightsavingandpartsintegrationcanbeachieved,theRoverGrouphasbeeninvestigatingthedesignandmanufactureofcomponentsfromcompositematerials.Themethodsusedinthedifferentstepsinthedesigntomanufacturecycleinthehighvolumeautomotiveindustryarerelativelywellknownforasteelcomponent,butarenotsowellestablishedforacompositecomponent.Adesignmethodologyforcompositeshasbeenemerginginwhichaprincipalprocedureisdesignanalysis.Oneofthemostestablishedmethodsofanalysisisthatusingthefiniteelementtechnique,andthisisbeingsupplementedwithexperimentaltestsonprototypesusingphotoelasticanalysisandstresspatternanalysisbythermalemission,coupledwithconventionalstraingaugemonitoring.Littleworkhasbeenundertakentocorrelatetheresultsobtainedfromthesedifferenttestmethodsandtocomparetheresultswithmeasurementsmadeonanactualcomponent.Thispaperpresentssomeoftheworkundertakenconcerningtheanalysisandtestingofacompositeautomotivesuspensionarm.Theresultsobtainedfromthethreedifferentanalysistechniquesarecomparedwithexperimentaltestresults,andtheiraccuracyisdiscussed.KeywordsautmotivesuspensionarmstressanalysisfiniteelementmethodphotoelasticanalysisSPATEstraingaugessheetmouldingcompoundSolanddeWildestatethatcompositematerialshavebeenusedincreasinglyasstructuralmaterials.Areasonforthis..,isthatcompositematerialshavehighstrengthtoweightandhighstiffnesstoweightratioswhichcansignificantlyreducetheweightofastructure...Perhapsthemostimportantfeatureofcompositematerialsisthattheirmechanicalpopertiescanbetailoredtomeetaspecificcriterion.However,Johnsonetalsuggestthatcompositedesign,analysisandfabricationtechnologymustundergomajordevelopmentsandsuccessfuldemonstrationsbeforesignificantstructuralcomponentswillbeincorporatedinproductionautomobilesandtrucks.Compositematerialshavetocompetewithsteelwithintheengineeringenvironment.WithintheautomotiveindustrythisrequiresacertainamountoftechnologytransferfromplacessuchastheAdvancedTechnologyCentreattheUniversityofWarwick,whichworkwithmaterialmanufacturersandautomotiveengineerstoenableunderstandingaboutthesematerialsasanalternativetothetraditionalmaterialssuchassteel.Ifcompositesaretocompetewithtraditionalmaterialsinarealsense,thenautomotivedesignersneedtobefullyaware00104361/94/01005905©oftheirstrengthsandlimitationssothattheycanbeoneofperhapsmanyoptionsconsideredattheconceptstageofthedesign.Forthistohappenautomotiveengineersneedtocatchuponthetechniquesofdesigning,testingandmanufacturingcomponentsfromcomposites.ThiswillincludeunderstandinghowvariousmethodssuchasfiniteelementFEanalysis,stresspatternanalysisbythermalemissionSPATEandphotoelasticanalysiscanbeappliedtocompositecomponentsintheirdesignanddevelopment.Thusfarlittleworkappearstohavebeenundertakentostudywhethertheresultsobtainedfromthesedifferentanalysismethodscorrelatewithoneanotherorwithactualexperimentalresultsobtainedfromtestingarealcomponent.Inordertostudytheapplicationandcorrelationofthedifferentanalysismethodstocompositematerials,acompositecomponentanautomotivelowersuspensionarmwasmanufactured.Thiscompositecomponentwasanalysedbythethreemethodsdescribedaboveandalsotestedunderrealisticloadingconditions,withexperimentalresultsbeingobtainedfromstraingauges.1994ButterworthHeinemannktdCOMPOSITES.VOLUME25.NUMBER1.199459,BallJoint°HousingFig.1ThecompositesuspensionarmDESIGNTheexistingsteellowersuspensionarmconsistsofninepiecesweldedtogetherwhilsttheredesignedcompositecomponentwhichcanbeseeninFig.1isasinglemouldedpart.ThematerialusedtomanufacturethesuspensionarmwasasheetmouldingcompoundSMC,comprisingapolyesterresinbondingagentwitha30contentofrandomlyarrangedshortglassfibresandcalciumcarbonatefiIler.Theweightofthesteelsuspensionarmis2.53kgwhilsttheredesignedSMCsuspensionarmcompletewithbushesandballjointweighs1.5kg.Thematerialpropertiesusedforthecompositesuspensionarmintheseanalyses,obtainedfromtestscarriedoutatRoversmaterialslaboratory,wereYoungsmodulus10.5GPa,Poissonsratio0.26anddensity1.8x106kgmm3.EXPERIMENTALTECHNIQUESPriortoundertakingexperimentalanalysisofanactualengineeringcomponent,someinitialvalidationworkwasrequiredtogainconfidenceinthetechniqueswhenappliedtosheetmouldingcompound.Therefore,fiatplates,beamsanddiscsconstructedfromSMCwereanalysedundervariousloadingconditionsbeforeprogressingontothedesignedcomponent.Mostvalidationtestswerecarriedoutusingstraingaugedspecimenstocorrelatewiththefiniteelementanalysisresults.AlthoughitisrecognizedthatSMCisnotanisotropiematerialduetosomefibreorientationduringprocessing,forthepurposesofanalysisthematerialwasassumedtobeisotropic.Also,whentheactualSMCsuspensionarmwascutupandexamined,significantfibredistributionwasobservedintheribs.Itisfeltthatthecorrelationbetweentheexperimentalandanalysisresultsvalidatedthisassumptioninthecaseofthisparticularcomponent.StraingaugetestsBeforeundertakingtheexperimentaltestwork,thecompositecomponentwasmountedviaitsrubbermountingbushesontoarelativelyinfinitelystiffstructure.Itisverydifficulttocoveralloftheloadingconditionswhenconductingexperimentaltestsandthusaworstcasescenarioisusuallyassumed.Theworstcaseloadingconditiononsuspensioncomponentsisknownaspotholebrake.Thisattemptstosimulatethevehiclefallingintoadeeppotholeat30mphwiththebrakesfullyappliedatthepointofimpact.Theresultantfore/aftandlateralloadsarethencalculatedbasedontheweightandvelocityofthevehicle.Duetothelimitationsofthetestrigthefullpotholeloadscouldnotbeappliedtothecomponent,andthusreducedloadswiththesameresultantdirectionasthepotholeloadswereappliedandtheresultsscaled.Theloadsappliedforthefullpotholebrakecasewere24.2kNinXand8.2kNinY,andforthereducedloadcasewere5.9kNinXand2.02kNinYseeFig.1.Thestraingaugesusedconsistedofsixthreeaxisrosettegaugesand13singlegridgauges,with2.5mmgridlengths,chosentofitintotheradiiofthecomponentinanattempttomeasurethemaximumstrain,Gaugesweresituatedneartheballjointhousing,wheretheloadswereapplied,andaroundtheradiiofthebodymountingbushes,wherethecomponentwouldbemountedtothecarsubframe.Additionalstraingaugesweresituatedonsomeofthestrengtheningribsandclosetotheantirollbarmountingposition.SPATEanalysisStresspatternanalysisbythermalemissionSPATEcanbeusedtodeterminethesurfacestressesofcomponentsbystudyingthesmallchangesintemperatureduetocyclicloadingconditions.SPATEequipmentcomprisesadetectorunitwithscanninghead,ananaloguesignalprocessingunitandadigitalelectronicdataunit.Thesystemworksbydetectingtheminutetemperaturechangeswhichoccurwhenastructureiscyclicallyloaded.Theinfrareddetectorscansthestructureandcorrelatesthemeasuredoutputwithareferencesignalfromtheloadingsystem.Anelectronicdataprocessingsystemcorrelatesthedetectedstressinducedthermalfluctuationswiththeloadingreferencesignal.Acolourcontourmapofthesumoftheprincipalstressescr4isthenplotted,togetherwithabarchartgivingactualvalues.Thiscorrelationofsignalseffectivelyeliminatesallsignalfrequenciesotherthanthosecausedbytheloadingsystem,i.e.,allambienttemperaturefluctuations.TheSPATEsystemhasatemperatureresolutionof0.001°C,andaspatialresolutionoflessthanImm.ThistypeofanalysishasbeenshownbyanumberofauthorsTMtoalsobeapplicabletononisotropicmaterialssuchascomposites,andthesmallerrors6demonstratedfromsuchstudieswhencomparedwiththeoreticalorFEresultsarefelttobeduetoinaccuraciesinthematerialdataused4.Itisapparentfromthestudiesundertakenthattheuseofthermoelasticstressanalysistoevaluatestressesandstrainsinanisotropiccompositematerialsismorecomplexthanforisotropicmaterials.However,ithasbeenshownthatthetechniquecanprovidevaluablequalitativeinformationonstressdistribution,effectsofsurfacedefectsandcrackgrowthpredictions.Ithasalsobeendemonstratedthat,givenaccuratedetailsofmaterialpropertiesincludingexpansioncoefficients,quantitativeresultscanbeobtaineddependinguponthedegreeofanisotropyofthematerial.PriortoundertakingafullSPATEanalysisofthesuspensionarmitwasnecessarytodetermineacalibrationfactorforthematerialused.Thiscanbeachievedintwoways,eitherbyloadingadiscofthematerialincompressionandcomparingtheSPATEoutputwiththetheoreti60COMPOSITES.NUMBER1.1994calsolution,orbystraingaugingdirectlyontothecomponentinanareaofevenstressdistribution,therebyobtainingadirectcomparisonwiththeSPATEoutput.Bothmethodswereusedinthiscase,butdirectcalibrationwithstraingaugescanovercomealotoftheproblems,thusallowingsignificantinformationtobeobtainedfromtheSPATEoutput.PhotoelasticanalysisThemajorityofphotoelasticworkinvestigatingthemacromechanicalbehaviourofcompositematerialshasbeenundertakenusingphotoelasticcoatingtechniques.Thisisdonetoavoidthecomplexitiesofconstructingaphotoelasticmodelwithanisotropicpropertiesandthusconstructingacompositeliketheoriginalwhichwouldloseitstransparencyandcouldnotbeanalysed.However,forcomplexfibrelayupsthiswouldbetheonlymethodofconductingphotoelasticanalysis,andthussomeresearchhasbeenundertakeninvestigatingtheuseoftheactualcompositesj730.Reasonableresultshavebeenobtainedfromsuchanalyses,butwithlimitationsduetothenecessityfortransparencywithinthecomposite.However,thecompositecomponentconsideredinthisstudywasmanufacturedfromSMCandthematerialwasassumedtobeisotropic,thussimplifyingthecreationofaphotoelasticmodel.Athreedimensionalepoxyresinmodelofthesuspensionarmwasconstructedforthephotoelasticanalysis.Themodelwasthenloadedinarepresentativemanner,withscaleddownloads,andsubjectedtoastressfreezingcycle.Thisinvolvesheatingthemodeluptothematerialsglasstransitiontemperature,atwhichpointtheYoungsmoduluschanges,andthemodeldeformsundertheappliedloads.Themodelisthenslowlycooled,avoidinganyuneventemperaturedistributionwhichcouldresultinunwantedthermalstresses.Duringthecoolingcyclethedeformationsandstressesarelockedintothemodel.Whenviewedunderpolarizedlightthethreedimensionalmodelisajumbleofinterferencefringes.Inordertodeterminebothmagnitudeanddirectionoftheprincipalstressesatanypoint,asliceisremovedandobservedunderpolarizedlight.Bycountingthefringesthestressesinthemodelcanbecalculatedandconvertedintoactualstressinthecomponent.Thisisdonebymeansofproportionality,betweenthemodelandcomponentmaterials,andtheloadinganddimensionalparameters.Thelowersuspensionarmismountedtotherestofthecarviarubbermountingbushes.Investigationswerecarriedoutastothepossibilityofmodellingthesemountingbushes.However,experimentswithsiliconandfoamrubbersshowedthattherequiredscaleddownstiffnessofthebushesduringstressfreezingatelevatedtemperaturescouldnotbemaintained.Thephotoelasticanalysisthusassumedthatthesuspensionarmwassolidlymounted.FINITEELEMENTANALYSISThecompositesuspensionarmwasmodelledusingapproximately1300oftheSTIF45ANSYSsolidelements.Thesuspensionarmismountedtothesubframeviarubbermountingbushestheseweremodelledwithspringelementstorepresentthestiffnessofthebushesandtocreatearealisticloaddistributionthroughoutthecomponent.LoadswereappliedtotheFEmodelviabeamelementsattheballjoint.ThreeloadcaseswereanalysedusingtheANSYSFEsoftware.Thefirstloadcasesimulatedthefullpotholebrakeloads.Thesecondsimulatedthereducedloadusedinthetestsduetothelimitationsofthetestrig,toenablecomparisonswiththeresultsfromtheexperimentalstraingaugeanalysis.Thesetwoloadcasesusedspringelementstosimulatethestiffnessoftherubbermountingbushes.Thethirdloadcaseagainusedthereducedloadsbutthistimeomittedthespringelementsi.e.,thesuspensionarmwasmodelledasbeingsolidlymounted.ThisthirdloadcasewasrequiredtocorrelatewiththeSPATEandphotoelasticanalyses.RESULTSFiniteelementanalysisAnalysisofthesuspensionarmshowedthatthemaximumequivalentstressinthecomponentfortheloadcaseconsideredisveryclosetotheultimatetensilestrengthoftheproposedmaterialforthepotholeloadingcondition,whichistheworstloadingcondition.Thismeansthatthecomponentmayneedtobemanufacturedfromadifferentmaterial,orthatothermaterialsneedtobepositionedinareasofhighstresstostrengthenthecomponentlocally.Duetoconstraintsupontheamountofcomputerdiscspaceavailable,thenumberofelementsusedwithintheFEmodelwasrelativelylowandthusthesizeoftheelementswithintheareaoftheradiiaroundthebodymountingbusheswastoolargetodetectanylargestressconcentrations.Also,thetypesofelementusedaroundtheseareas,duetothegeometryofthecomponent,wereamixtureofbrick,wedgeandtetrahedral.Thelattershapetendstobetoostifftogivegoodresultsandisnotrecommended.Ifmoredetailedresultswererequiredintheseareas,thentheseradiiwouldhavetobemodelledingreaterdetailwithmoreandsmallerelementsintheareasofhighstressgradient.PhotoelasticanalysisTheanalysisofthephotoelasticmodelofthesuspensionarmwasundertakenassumingthatthedirectionsofthemaximumprincipalstresseslayinahorizontalplanethroughthemodelinthedirectionofthefore/aftload.Whilstthisisnotstrictlytrueinpracticeduetolocalgeometryeffectsincertainareas,theassumptiongavesufficientlyaccurateresults.Ifobviousdiscrepancieswerefoundinparticularareasthenitwaspossibletotakeslicesfromdifferentplanes.Maximumstresseswereseentooccurinthevicinityoftheballjointhousingandthebodymounts.Duetotheabilityofphotoelasticanalysistopinpointverysmallareasofhighstress,themaximumstressvaluesgivenbyphotoelasticitytendedtobehigherthanthestraingaugeresults.Forexample,maximumstresslevelsintheinternalradiusoftheleadingbodymountwerefoundtobe43MPacomparedwithaSPATEvalueof26MPa.ThisdifferencecanbeexplainedbyexaminingtheslicetakenthroughthephotoelasticmodelwhichshowsthatthemaximumstressonlyoccursatapositionCOMPOSITES.NUMBER1.199461Table1.StressresultsMPaforfullloadconditionsPositionStraingaugesFEPhotoelasticBalljointhousing176165176spanning3mmandthatthestressvalueseithersideofthemaximumarearound25MPa.SPATEanalysisTheinitialSPATEscanshowedlargebandsofstressrunningacrossthemountingareasandsomeconfusionastowhethertheseareaswereintensionorcompression.Theproblemwasidentifiedasexcessivemovementinthesuspensionarmbodymountingpositionsduetodistortionoftherubberbushesasexperiencedinthestraingaugetests.SPATEisequippedwithamotioncompensatordeviceifrequired,whichdeflectsthescanningmirrorsinsidethedetectorintimewiththeoscillationsofthetestpiece,therebyeliminatingthemovement.However,inthisparticularcase,thegeometryanddirectionofmovementcouldnotbeeliminatedovertheentireareaatthesametime,andthusitwasnecessarytoremovetherubberbushesandtoreplacethemwithaluminiumones.TheSPATEanalysiswasrepeatedwiththesolidbushesandshowedareasofhightensilestress26MPaalongtheleadingedgeandaroundtheinnerradiusoftheleadingbodymountingposition.Unfortunately,noSPATEanalysiscouldbeundertakenattheballjointendofthecomponentasitwasobscuredbythelargeloadingadaptorrequiredtofitthehydraulicactuatorsupplyingthecyclicloading.COMPARISONOFRESULTSItshouldbeclarifiedthatthestressvaluesquotedinthetablesfromthestraingaugeresultswerecalculatedfromtherosettegaugestogiveavalueofmaximumprincipalstress.Thephotoelasticanalysisalsogivesmaximumprincipalstressesunlessthevaluesaretakeninboardofafreeedgeinwhichcasetheyaredifferencesinprincipalstresseso.o,,.SPATEanalysisgivesanoutputintheformofthesummationoftheprincipalstressesor.a2whereastheFEoutputcanbeinanyformrequiredinthiscaseyonMises.Duetothegeometryofthecomponentandthewayinwhichtheloadswereapplied,thevaluesofor2andcr3werealwayssmall,andthusdirectcomparisonscouldbemadebetweenthedifferentanalysismethodswithoutfurtherconversion.Tablelcomparestheresultsobtainedforthemaximumpotholeloadconditions.Themaximumstressvaluesalloccurattheballjointareaandcorrelateverywell.Theseresultantstressesforthestraingaugesandphotoelasticitywerecalculatedfromtheresultsobtainedforthereducedload.Themodelstresswasmultipliedbyaloadingfactorastheratiobetweenthefore/aftandlateralloadingremainedconstantandinthesameproportionasthefullpotholebrakeloadappliedtothesuspensionarlTI.TheresultsoftheanalysesundertakenwithreducedTable2.StressresultsMPaforredTJcedloadswithmountingbushesPositionStraingaugesFEInnerradiusofbody2520mountBalljointhousing4940Table3.StressresultsMPaforreducedloadswithoutmountingbushesPositionFESPATEPhotoelasticInnerradiusofbody22mountBalljointhousing302643254225loadingbutwiththemountingbushesincludedcanbeseeninTable2.Table3presentstheresultsoftheanalysesundertakenwithreducedloadingandwithoutthemountingbushesbeingused.ThestressgivenbythephotoelasticanalysisisconcentratedataverysmallpointwhereasthestressgivenbyFEanalysisisaveragedoverarelativelylargearea.Inthecaseofthephotoelasticresults,anaverageofthenominalstressesonbothsidesoftheconcentrationpointisalsoquotedinbracketstogiveafairercomparison.Comparedwiththestraingaugeresults,thevaluesgivenbySPATEareverysimilarforthemaximumstress.IntheorySPATEshouldbemoreeffectivethanstraingaugeswheninvestigatingstressconcentrationeffects,asitismeasuringvaluesoverasmallerareadependinguponitsdistancefromtheobjectduringscanning.InthiscasethemeasurementpointofSPATEwassetatImmdiametercomparedwitha2.5mmgridlengthonthestraingauges.However,inthisinstancethedifferencesweresmallandmovementofthecomponentduringloadcyclinginevitablyblurredtheimagetosomeextent,thusthedifferenceinresolutionwasprobablynegligible.CONCLUSIONSAlltheanalysistechniquesusedi.e.,SPATE,photoelastic,finiteelementandstraingaugeanalysesshowedthattheareaofhigheststresswasinthevicinityoftheballjointhousing.Allofthemethodsalsoshowedsignificantstressesintheareasofthebodymountingbushes.However,theFEanalysisdidnotalwaysaccuratelyidentifythesehighstressesduetothesizeoftheelementswithintheseareasbeingtoolarge.IfmoredetailedresultswererequiredfortheseareasfromtheFEanalysis,thentheywouldhavetobemodelledinmoredetailwithagreaternumberofelementsintheareasofhighstressgradient.Theoverallpatternofstressdistributionwasthesameforeachanalysistechnique.Thedifferences62COMPOSITES.NUMBER1.1994
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