JurnalMekanikalDecember2008,No.26,76-8576STRESSANALYSISOFHEAVYDUTYTRUCKCHASSISASAPRELIMINARYDATAFORITSFATIGUELIFEPREDICTIONUSINGFEMRoslanAbdRahman,MohdNasirTamin,OjoKurdi*FacultyofMechanicalEngineering,UniversitiTeknologiMalaysia,81310UTM,Skudai,JohorBahruABSTRACTThispaperpresentsthestressanalysisofheavydutytruckchassis.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.Noiseandvibrationare*Correspondingauthor:E-mail:JurnalMekanikal,December200877keyelementsinsuchstandard.TheautomotiveindustryinMalaysiaismuchrelyingonforeigntechnology.Truckchassis,whichisimportantstructureoflightweightcommercialvehicle,ismostlydesignedandimportedfromforeigncountry.Inordertochangethistrend,itisnecessarytodevelopandbuiltMalaysianownchassisdesign.Studyandresearchontruckchassisisthusrequiredtoachievethisgoal.Thechassisoftrucksisthebackboneofvehiclesandintegratesthemaintruckcomponentsystemssuchastheaxles,suspension,powertrain,cabandtrailer.Thetruckchassisisusuallyloadedbystatic,dynamicandalsocyclicloading.Staticloadingcomesfromtheweightofcabin,itscontentandpassengers.Themovementoftruckaffectsadynamicloadingtothechassis.Thevibrationofenginesandtheroughnessofroadgiveacyclicloading.Theexistingtruckchassisdesignisnormallydesignedbasedonstaticanalysis.Theemphasisofdesignisonthestrengthofstructuretosupporttheloadingplaceduponit.However,thetruckchassishasbeenloadedbycomplextypeofloads,includingstatic,dynamicsandfatigueaspects.Itisestimatedthatfatigueisresponsiblefor85%to90%ofallstructuralfailures1.Theknowledgeofdynamicandfatiguebehavioroftruckchassisinsuchenvironmentisthusimportantsothatthemountingpointofthecomponentslikeengine,suspension,transmissionandmorecanbedeterminedandoptimized.Manyresearcherscarriedoutstudyontruckchassis.KaraogluandKuralay2investigatedstressanalysisofatruckchassiswithrivetedjointsusingFEM.Numericalresultsshowedthatstressesonthesidemembercanbereducedbyincreasingthesidememberthicknesslocally.Ifthethicknesschangeisnotpossible,increasingtheconnectionplatelengthmaybeagoodalternative.Fermeretal3investigatedthefatiguelifeofVolvoS80Bi-FuelusingMSC/Fatigue.ConleandChu4didresearchaboutfatigueanalysisandthelocalstress-strainapproachincomplexvehicularstructures.StructuraloptimizationofautomotivecomponentsappliedtodurabilityproblemshasbeeninvestigatedbyFerreiraetal5.FermrandSvensson6studiedonindustrialexperiencesofFE-basedfatiguelifepredictionsofweldedautomotivestructures.Filhoet.al.7haveinvestigatedandoptimizedachassisdesignforanoffroadvehiclewiththeappropriatedynamicandstructuralbehavior,takingintoaccounttheaspectsrelativetotheeconomicalviabilityofaninitialsmallscaleproduction.Thedesignofanoff-roadvehiclechassisisoptimizedbyincreasingthetorsionalstiffness,maintenanceofcenterofgravity,totalweightofstructureandsimplergeometryforreductionofproductioncost.Theintegrationofcomputeraideddesignandengineeringsoftwarecodes(Pro/Engineer,ADAMS,andANSYS)tosimulatetheeffectofdesignchangestothetruckframehasbeenstudiedbyCosmeetal8.Chiewanichakornetal9investigatedthebehaviorofatrussbridge,whereanFRPdeckreplacedanolddeterioratedconcretedeck,usingexperimentallyvalidatedfiniteelement(FE)models.Numericalresultsshowthatthefatiguelifeofthebridgeafterrehabilitationwouldbedoubledcomparedtopre-rehabilitatedreinforcedconcretedecksystem.Basedontheestimatedtrucktrafficthatthebridgecarries,stressrangesoftheFRPdecksystemlieinaninfinitefatiguelifeJurnalMekanikal,December200878regime,whichimpliesthatnofatiguefailureoftrussesandfloorsystemwouldbeexpectedanytimeduringitsservicelife.YeandMoan10haveinvestigatedthestaticandfatiguebehaviorofaluminiumbox-stiffener/webframeconnectionsusingFiniteElementAnalysis(FEA)toprovideaconnectionsolutionthatcanreducethefabricationcostsbychangingthecuttingshapesonthewebframeandcorrespondinglytheweldprocessmeanwhilesufficientfatiguestrengthcanbeachieved.FEbasedfatiguewasusedtolocatethecriticalpointofprobablecrackinitiationandtopredictthelifeinadoorhingesystem11.Inthisstudy,stressanalysisofheavydutytruckchassisloadedbystaticforcewillbeinvestigatedtodeterminethelocationofcriticalpointofcrackinitiationasapreliminarydataforfatiguelifepredictionofthistruckchassis.2.0FINITEELEMENTANALYSISOFTRUCKCHASSIS2.1BasicConceptofFEMThefiniteelementmethod(FEM)isacomputationaltechniqueusedtoobtainapproximatesolutionsofboundaryvalueproblemsinengineering.Simplystated,aboundaryvalueproblemisamathematicalprobleminwhichoneormoredependentvariablesmustsatisfyadifferentialequationeverywherewithinaknowndomainofindependentvariablesandsatisfyspecificconditionsontheboundaryofthedomain12.AnunsophisticateddescriptionoftheFEmethodisthatitinvolvescuttingastructureintoseveralelements(piecesofstructure),describingthebehaviorofeachelementinasimpleway,thenreconnectingelementsatnodesasifnodeswerepinsordropsofgluethatholdelementstogether(Figure1).Thisprocessresultsinasetofsimultaneousalgebraicequations.Instressanalysistheseequationareequilibriumequationsofthenodes.Theremaybeseveralhundredorseveralthousandsuchequations,whichmeanthatcomputerimplementationismandatory13.Figure1:Acoarsemesh,two-dimensionalmodelofgeartooth.Allnodesandelementslieinplaneofthepaper13JurnalMekanikal,December2008792.2AGeneralProcedureforFEATherearecertaincommonstepsinformulatingafiniteelementanalysisofaphysicalproblem,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)andtheloadings12.Thenextstepissolution,inthisstepthegoverningalgebraicequationsinmatrixformandcomputestheunknownvaluesoftheprimaryfieldvariable(s)areassembled.Thecomputedresultsarethenusedbybacksubstitutiontodetermineadditional,derivedvariables,suchasreactionforces,elementstressesandheatflow.Actuallythefeaturesinthisstepsuchasmatrixmanipulation,numericalintegrationandequationsolvingarecarriedoutautomaticallybycommercialsoftware13.Thefinalstepispostprocessing,theanalysisandevaluationoftheresultisconductedinthisstep.Examplesofoperationsthatcanbeaccomplishedincludesortelementstressesinorderofmagnitude,checkequilibrium,calculatefactorsofsafety,plotdeformedstructuralshape,animatedynamicm