某汽车操纵系统的转向横拉杆故障研究外文文献翻译中英文翻译外文翻译.docx
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某汽车操纵系统的转向横拉杆故障研究外文文献翻译中英文翻译外文翻译.docx
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某汽车操纵系统的转向横拉杆故障研究外文文献翻译中英文翻译外文翻译
EngineeringFailureAnalysis14(2007)895–902
Failureinvestigationofatierodendofanautomobile
steeringsystem
A.H.Falah*,M.A.Alfares,A.H.Elkholy
MechanicalEngineeringDepartment,KuwaitUniversity,
P.O.Box5969,Safat13060,Kuwait
Received30May2006;accepted19November2006
Abstract
Afailureanalysisofatierodendofasportsutilityvehicle(SUV)steeringmechanismhasbeencarriedoutinthisstudy.Thetierodendiscomposedoftwopartsfittedtogether:
athreadedpartandanembracingpart.FailuretookplaceinthethreadedpartwhichismadeofAISI8620steel.Thevehiclehadbeeninserviceforapproximatelytwoyearsandaccumulatedlessthan30,000km.Anevaluationofthefailedpartwasundertakentodeterminethecauseoffailureandassessitsintegrity.Visualexamination,photodocumentation,chemicalanalysis,hardnessmeasurement,andmetallographicexaminationwereallconducted.Thefailuresurfacewasexaminedwiththehelpofascanningelectronmicroscope(SEM)equippedwithEDAXfacilitythatdetermineschemicalcompositionatdesiredlocationswithinthepart.Resultsindicatedthatthetierodendhadfailedbyfatiguewithacrackinitiationatthethroat(minimum)areaofthethreadedpartduetomaterialdeficiencyandimproperheattreatment.
Keywords:
Fatigue;Crackinitiation/propagation;Materialproperty
1.Introduction
Tierodsconnectthecenterlinktothesteeringknuckleonautomobileswithconventionalsuspensionsystemsandrecirculatingballsteeringgears,Fig.1.OnautomobileswithMacPhersonstrutsuspensionandrack-and-pinionsteeringgears,tierodsconnecttheendoftheracktothesteeringknuckle,Fig.2.Atierodconsistsofaninnerandanouterendasshowninbothpreviousfigures.
Tierodstransmitforcefromthesteeringcenterlinkortherackgeartothesteeringknuckle,causingthewheelstoturn.Theoutertierodendconnectswithanadjustingsleeve,whichallowsthelengthofthetierodtobeadjust-able.Thisadjustmentisusedtosetavehicle’stoes,acriticalalignmentangle,sometimesreferredtoasthecasterandcamberangles.
Fig.1.Conventionalsuspension.
Fig.2.McPhersonsuspensionwithrackandpinion.
Avehicle’ssteeringandsuspensionsystemsshouldbecheckedregularly,atleastonceayearalongwithacompletewheelalignment.Aworntierodend,duetorubbingandwearing,cancausewandering,erraticsteeringandexcessivetirewear.Iftierodreplacementisnecessary,awheelalignmentisalsorequiredbecausetierodreplacementdisturbsthetoesetting.
Tierodsmayfailinmanydifferentways,andexceptforaslightincreaseinnoiselevelandvibration,thereisoftennoindicationofdifficultyuntiltotalfailureoccurs.Ingeneral,eachtypeoffailureleavescharacteristic
clues,anddetailedexaminationoftenyieldsenoughinformationtoestablishthecauseoffailure.Thegeneraltypesoftierodfailuremodesinclude
fatigue,impactfracture,wearandstressrupture[1].Severalcausesoftie
rodendfailurehavebeenidentified.Theseincludepoordesign,incorrectassembly,overloads,inadvertentstressraisersorsubsurfacedefectsincriticalareas,useofincorrectmaterialsand/ormanufactureprocess,
andimproperheattreatment[2].Tierodsinautomobilesuspensionaregenerallyrobustandreliablecomponents.However,problemsdooccurparticularlyduetomanufactureerrorordrivermisuse[3].
Thecaseunderinvestigationinvolvesfailureoftheouterpartofanautomobiletierod.ItwasbroughtforanalysisbytheinvestigationbureauoftheMinistryofInterioroveralegaldisputebetweenthedriverofanSUVandalocalcardealerwhosoldhimthevehicle.Thevehiclewasdrivenfornearlytwoyearsandhadregisteredlessthan30,000km.Thedriverclaimedthatwhilehewasdrivingthevehicle,asuddenbangwasheardandhelostcontrolofthevehicleandhitthemedianrailguardofthehighway.Thevehiclewasdamagedandthedriver,thoughstillconscious,wasslightlyinjured.Hebelievedthattherewassomethingwentsuddenlywrongwithamechanicalcomponentofthevehicleandthatcausedtheaccident.Thelocalcardealer,ontheotherhand,disagreedwiththedriver’sscenarioongroundsthatthemanufacturerproducedthousandsofsuchvehicleseveryyearandtheywere,andstillare,runningfineallovertheworldwithoutanyreportedseriousfailure.Thedealerattributedtheaccidenttocarelessdrivingbehaviorthatresultedinalossofcontroloverthevehicle,whichinturnhittheguardrailandledtovehicledamage.Tosettlethedispute,itwasdecidedtoundertakeathoroughfailureanalysisinvestigationofallcomponentsofthesteeringmechanismtodeterminethecauseoffailure.Allsteeringcomponentswerefoundintactthoughbadlybent,exceptfortheoutertierodendwhichwasfracturedatthethroatedareaofitsthreadedpart.Theembracingpartoftheoutertierodend,however,wasintact,exceptfortwoscarsatitsrimthatcouldhavehappenedwhenthethreadedpartbrokeintotwopieces.ThegeneralappearanceofthepartsofthefailedtierodendisshowninFig.3a,wherethetwofragmentedpiecesofthethreadedpartwerebroughttogethertoshowhowthetierodendappearedbeforefailure.Fig.3bshowsthetwofracturedpartsseparated.Fig.4givesthevisualappearanceoftheembracingpartandonefragmentofthethreadedpart,bothfacingup.Itisclearthatfracturetookplaceatthethroatareaofthethreadedpartwherestressisexpectedtobehighduetoreducedcrosssectionalareaandstressconcentration.Furtherexaminationofthethreadedpartwasconductedtodeterminetheexactcauseoffailure.
Fig.3.Partsoffracturedtierodend(a)assembledand(b)separated.
Fig.4.Threadedpartfracturesurfaceandrimscarsonembracingpart.
2.Experimentalprocedure
Thefailedthreadedpartofthetierodendwasinspectedvisuallyandmacroscopicallytakingcaretoavoiddamageoffracturedsurface.Thefailedthreadedpartofthetierodendwasultrasonicallycleanedpriortomicroscopicexamination,photodocumentation,chemicalanalysisandhardnessmeasurementatthefracturesurfaceandawayfromit.Scanningelectronmicroscope(SEM)equippedwithEDAXfacilityandanoptical
microscopewerebothusedintheinvestigation.
3.Resultsanddiscussion
ChemicalanalysisusingatomicabsorptionspectrophotometrywascarriedoutatseverallocationsofthefailedthreadedpartofthetierodendandtheaveragevaluesofthetestresultsaregiveninTable1alongwiththespecifiedchemicalcomposition.SpectrumanalysisrevealedthatthethreadedpartmaterialwasAISI8620steelwhichisusuallyusedformainautomobilesteeringcomponents.Thelowpercentageofmanganeseandof
chromiuminthetestedsamplesuggeststhatthefinalhardnessofthepartwouldbesubstantiallyreduced.Ontheotherhand,thehighpercentageofnickelinthetestedsamplewouldresultinlowertoughnessthereby
compromisingthemechanicalpropertythatisrequiredtowithstandimpactloadsresultingfrombumpyroads.Thesurfacehardnessofthefracturedtierodendwasmeasuredtobe45.6HRC.Thissuggeststhatthetierodwasnothardenedproperly,sincehardnessoftierods,ingeneral,isexpectedintherangeoffiftiesforsuchapplications.
Table1
ChemicalcompositionoffailedthreadedpartoftierodendandAISI8620steel
Fig.5.SEMmicrographshowingcrackpropagationregion.
ItisevidentfromFig.4thatthereexisttwodistinctareasonthefracturesurface;oneissmoothwhiletheotherisrelativelyrough.Thisisatypicalfatiguefracturewherecrackoriginatesattheedgeofthesmootharea
andpropagatestowardstherougharea,whichrepresentsfinalfailure.Ontheotherhand,thesmoothareaofthefracturesurfaceisdominantasseeninFig.4.Thisindicatesthatthetierodendtooksometimetobreakfromtheinstantofcrackinitiationtillcompletefracture;i.e.ahighcyclefatiguefailurecase.ThisprovesthatthecauseoftheSUVaccidentwasalackofstrengthandlowresistancetoimpactloadsinthematerialofthethreadedpartofthetierodendthatinitiatedacrackandthentooksometimetoreachcompleteseparation.
Aspecimenfromthefracturedsurfacewasmetallographicallypreparedandobservedinascanningelectronmicroscope(SEM).Significantfatiguecrackswereobservedatthesmoothareaofthefracturesurface.The
originofcrackswasattheedgeofthesmoothareaofthethreadedpartthroat,suggestingthatthestresseswerehighestatthisregion.Fig.5showscrackpropagationonthefracturesurface.Beachmarkscanbeobservedclearlywhichisatypicalfeatureoffatiguefailure[4].
Theoriginofthecrackwassurroundedbybeachmarks.Also,thefracturesurfaceatthefatigueregionhadasmoothappearancewitharippledbeachmarkpatternwhichindicatesthatfatiguehadinitiatedatonepointofthecircumferenceandthengrownacrossthefracturearea.Asmallareahasarough,jaggedlookwherethelastportionofthethroatbrokeaway.Nocorrosionmediawerefoundonfracturesurfaces.
Fig.6.SEMmicrographshowingtypicalbrittlefractureobservedinfinalstageofcrackpropagationzone.
BrittlefracturewasobservedinthefinalstageatcrackpropagationasseeninFig.6.Fig.7showsthemicrographyoftheroughareaofthefracturesurfacewherevariationofgrainsizecombinedwithshallowdimplesisevident.Thelightlinessurroundingthegrainsinthefigureindicateintergranularcrackingthatisusuallyobservedwithbrittlefracture.Aclose-upofsuchgrainsrevealedbothintergranularandtransgranularcrackingatsomelocationsa
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