1JurnalMekanikalDecember2008,No.26,76-8576STRESSANALYSISOFHEAVYDUTYTRUCKCHASSISASAPRELIMINARYDATAFORITSFATIGUELIFEPREDICTIONUSINGFEMRoslanAbdRahman,MohdNasirTamin,OjoKurdi*FacultyofMechanicalEngineering,UniversitiTeknologiMalaysia,ABSTRACTThispaperpresentsthestressanalysisofheavydutytruckchassis.Thestressanalysisisimportantinfatiguestudyandlifepredictionofcomponentstodeterminethecriticalpointwhichhasthehigheststress.TheanalysiswasdoneforatruckmodelbyutilizingacommercialfiniteelementpackagedABAQUS.Themodelhasalengthof12.35mandwidthof2.45m.ThematerialofchassisisASTMLowAlloySteelA710C(Class3)with552MPaofyieldstrengthand620MPaoftensilestrength.Theresultshowsthatthecriticalpointofstressoccurredattheopeningofchassiswhichisincontactwiththebolt.Thestressmagnitudeofcriticalpointis386.9MPa.Thiscriticalpointisaninitialtoprobablefailuresincefatiguefailurestartedfromthehigheststresspoint.Keyword:Stressanalysis,fatiguelifeprediction,truckchassis1.0INTRODUCTIONTheageofmanytruckchassisinMalaysiaareofmorethan20yearsandthereisalwaysaquestionarisingwhetherthechassisisstillsafetouse.Thus,fatiguestudyandlifepredictiononthechassisisnecessaryinordertoverifythesafetyofthischassisduringitsoperation.StressanalysisusingFiniteElementMethod(FEM)canbeusedtolocatethecriticalpointwhichhasthehigheststress.Thiscriticalpointisoneofthefactorsthatmaycausethefatiguefailure.Themagnitudeofthestresscanbeusedtopredictthelifespanofthetruckchassis.Theaccuracyofpredictionlifeoftruckchassisisdependingontheresultofitsstressanalysis.Themoreaccurateresultofstressanalysisthemorevalidthepredictedlifeofobject.Inthisstudy,thestressanalysisisaccomplishedbythecommercialfiniteelementpackagedABAQUS.Theautomotiveindustry(vehiclesandcomponents)representsastrategicandimportantbusinesssectorinMalaysia.WiththeeventualtradeliberalizationofASEANFreeTradeArea(AFTA),localautomotivemanufacturersandvendorsshallrequirecarsandcomponentsofworldclassstandard.Noiseandvibrationarekeyelementsinsuchstandard.TheautomotiveindustryinMalaysiaismuchrelyingonforeigntechnology.Truckchassis,whichisimportantstructureoflightweightcommercialvehicle,ismostlydesignedandimportedfromforeigncountry.Inordertochangethistrend,itisnecessarytodevelopandbuiltMalaysianownchassisdesign.Studyandresearchontruckchassisisthusrequiredtoachievethisgoal.Thechassisoftrucksisthebackboneofvehiclesandintegratesthemaintruck2componentsystemssuchastheaxles,suspension,powertrain,cabandtrailer.Thetruckchassisisusuallyloadedbystatic,dynamicandalsocyclicloading.Staticloadingcomesfromtheweightofcabin,itscontentandpassengers.Themovementoftruckaffectsadynamicloadingtothechassis.Thevibrationofenginesandtheroughnessofroadgiveacyclicloading.Theexistingtruckchassisdesignisnormallydesignedbasedonstaticanalysis.Theemphasisofdesignisonthestrengthofstructuretosupporttheloadingplaceduponit.However,thetruckchassishasbeenloadedbycomplextypeofloads,includingstatic,dynamicsandfatigueaspects.Itisestimatedthatfatigueisresponsiblefor85%to90%ofallstructuralfailures.Theknowledgeofdynamicandfatiguebehavioroftruckchassisinsuchenvironmentisthusimportantsothatthemountingpointofthecomponentslikeengine,suspension,transmissionandmorecanbedeterminedandoptimized.Manyresearcherscarriedoutstudyontruckchassis.KaraogluandKuralayinvestigatedstressanalysisofatruckchassiswithrivetedjointsusingFEM.Numericalresultsshowedthatstressesonthesidemembercanbereducedbyincreasingthesidememberthicknesslocally.Ifthethicknesschangeisnotpossible,increasingtheconnectionplatelengthmaybeagoodalternative.FermeretalinvestigatedthefatiguelifeofVolvoS80Bi-FuelusingMSC/Fatigue.ConleandChudidresearchaboutfatigueanalysisandthelocalstress-strainapproachincomplexvehicularstructures.StructuraloptimizationofautomotivecomponentsappliedtodurabilityproblemshasbeeninvestigatedbyFerreiraetal.FermrandSvenssonstudiedonindustrialexperiencesofFE-basedfatiguelifepredictionsofweldedautomotivestructures.Filhoet.al.haveinvestigatedandoptimizedachassisdesignforanoffroadvehiclewiththeappropriatedynamicandstructuralbehavior,takingintoaccounttheaspectsrelativetotheeconomicalviabilityofaninitialsmallscaleproduction.Thedesignofanoff-roadvehiclechassisisoptimizedbyincreasingthetorsionalstiffness,maintenanceofcenterofgravity,totalweightofstructureandsimplergeometryforreductionofproductioncost.Theintegrationofcomputeraideddesignandengineeringsoftwarecodes(Pro/Engineer,ADAMS,andANSYS)tosimulatetheeffectofdesignchangestothetruckframehasbeenstudiedbyCosmeetal.Chiewanichakornetalinvestigatedthebehaviorofatrussbridge,whereanFRPdeckreplacedanolddeterioratedconcretedeck,usingexperimentallyvalidatedfiniteelement(FE)models.Numericalresultsshowthatthefatiguelifeofthebridgeafterrehabilitationwouldbedoubledcomparedtopre-rehabilitatedreinforcedconcretedecksystem.Basedontheestimatedtrucktrafficthatthebridgecarries,stressrangesoftheFRPdecksystemlieinaninfinitefatigueliferegime,whichimpliesthatnofatiguefailureoftrussesandfloorsystemwouldbeexpectedanytimeduringitsservicelife.YeandMoanhaveinvestigatedthestaticandfatiguebehaviorofaluminiumbox-stiffener/webframeconnectionsusingFiniteElementAnalysis(FEA)toprovideaconnectionsolutionthatcanreducethefabricationcostsby3changingthecuttingshapesonthewebframeandcorrespondinglytheweldprocessmeanwhilesufficientfatiguestrengthcanbeachieved.FEbasedfatiguewasusedtolocatethecriticalpointofprobablecrackinitiationandtopredictthelifeinadoorhingesystem.Inthisstudy,stressanalysisofheavydutytruckchassisloadedbystaticforcewillbeinvestigatedtodeterminethelocationofcriticalpointofcrackinitiationasapreliminarydataforfatiguelifepredictionofthistruckchassis.2.0FINITEELEMENTANALYSISOFTRUCKCHASSIS2.1BasicConceptofFEMThefiniteelementmethod(FEM)isacomputationaltechniqueusedtoobtainapproximatesolutionsofboundaryvalueproblemsinengineering.Simplystated,aboundaryvalueproblemisamathematicalprobleminwhichoneormoredependentvariablesmustsatisfyadifferentialequationeverywherewithinaknowndomainofindependentvariablesandsatisfyspecificconditionsontheboundaryofthedomain.AnunsophisticateddescriptionoftheFEmethodisthatitinvolvescuttingastructureintoseveralelements(piecesofstructure),describingthebehaviorofeachelementinasimpleway,thenreconnectingelementsatnodesasifnodeswerepinsordropsofgluethatholdelementstogether.Thisprocessresultsinasetofsimultaneousalgebraicequations.Instressanalysistheseequationareequilibriumequationsofthenodes.Theremaybeseveralhundredorseveralthousandsuchequations,whichmeanthatcomputerimplementationismandatory.2.2AGeneralProcedureforFEATherearecertaincommonstepsinformulatingafiniteelementanalysisofaphysicalproblem,whetherstructural,fluidflow,heattransferandsomeothersproblem.Thesestepsareusuallyembodiedincommercialfiniteelementsoftwarepackages.Therearethreemainsteps,namely:preprocessing,solutionandpostprocessing.Thepreprocessing(modeldefinition)stepiscritical.Aperfectlycomputedfiniteelementsolutionisofabsolutelynovalueifitcorrespondstothewrongproblem.Thisstepincludes:definethegeometricdomainoftheproblem,theelementtype(s)tobeused,thematerialpropertiesoftheelements,thegeometricpropertiesoftheelements(length,area,andthelike),theelementconnectivity(meshthemodel),thephysicalconstraints(boundaryconditions)andtheloadings.Thenextstepissolution,inthisstepthegoverningalgebraicequationsinmatrixformandcomputestheunknownvaluesoftheprimaryfieldvariable(s)areassembled.Thecomputedresultsarethenusedbybacksubstitutiontodetermineadditional,derivedvariables,suchasreactionforces,elementstressesandheatflow.Actuallythefeaturesinthisstepsuchasmatrixmanipulation,numericalintegrationandequationsolvingarecarriedoutautomaticallybycommercialsoftware.Thefinalstepispostprocessing,theanalysisandevaluationoftheresultisconductedinthisstep.Examplesofoperationsthatcanbeaccomplishedinclude4sortelementstressesinorderofmagnitude,checkequilibrium,calculatefactorsofsafety,plotdeformedstructuralshape,animatedynamicmodelbehaviorandproducecolor-codedtemperatureplots.Thelargesoftwarehasapreprocessorandpostprocessortoaccompanytheanalysisportionandthebothprocessorcancommunicatewiththeotherlargeprograms.Specificproceduresofpreandpostaredifferentdependentupontheprogram.2.3TruckdefinitionandclassificationGenerally,truckisanyofvariousheavymotorvehiclesdesignedforcarryingorpullingloads.Otherdefinitionofthetruckisanautomotivevehiclesuitableforhauling.Someotherdefinitionarevarieddependingonthetypeoftruck,suchasDumpTruckisatruckwhosecontentscanbeemptiedwithouthandling；thefrontendoftheplatformcanbepneumaticallyraisedsothattheloadisdischargedbygravity.TherearetwoclassificationsmostapplicabletoRecreationalVehicletowtrucks.Thefirstoneistheweightclasses,asdefinedbytheUSgovernment,rangingfromClass1toClass8aslistedinTable1andTable2.Thesecondisclassifiedintoabroadercategory:?LightDutyTruck?MediumDutyTruck?HeavyDutyTruck2.4ModelofTruckChassisThemodelisdepictedinFigure2.Themodelhaslengthof12.35mandwidthof2.45m.ThematerialofchassisisASTMLowAlloySteelA710C(Class3)with552MPaofyieldstrengthand620MPaoftensilestrength.Theotherpropertiesofchassismaterialaretabulated.2.5LoadingThetruckchassismodelisloadedbystaticforcesfromthetruckbodyandcargo.Forthismodel,themaximumloadedweightoftruckpluscargois36.000kg.The