穩(wěn)態(tài)和瞬態(tài)工況下柴油機(jī)氣缸蓋應(yīng)力分布的實(shí)際測(cè)量與理論預(yù)測(cè)-外文文獻(xiàn)及翻譯

PAGExx建筑大學(xué)畢業(yè)論文外文文獻(xiàn)及譯文PAGEPAGE1本科畢業(yè)論文外文文獻(xiàn)及譯文文獻(xiàn)、資料題目：MeasurementsandPredictionsofSteady-StateandTransientStressDistributionsinaDieselEngineCylinderHead文獻(xiàn)、資料來(lái)源：SAE文獻(xiàn)、資料發(fā)表日期：1999.4院（部）：機(jī)電工程學(xué)院專(zhuān)業(yè)：班級(jí)：姓名：學(xué)號(hào)：指導(dǎo)教師：完成日期：PAGEPAGE20外文文獻(xiàn)：MeasurementsandPredictionsofSteady-StateandTransientStressDistributionsinaDieselEngineCylinderHeadABSTRACTAcombinedexperimentalandanalyticalapproachwasfollowedinthisworktostudystressdistributionsandcausesoffailureindieselcylinderheadsundersteady-stateandtransientoperation.Experimentalstudieswereconductedfirsttomeasuretemperatures,heatfluxesandstressesunderaseriesofsteady-stateoperatingconditions.Furthermore,byplacinghightemperaturestraingageswithinthethermalpenetrationdepthofthecylinderhead,theeffectofthermalshockloadingunderrapidtransientswasstudied.Acomparisonofoursteady-stateandtransientmeasurementssuggeststhatthesteady-statetemperaturegradientsandtheleveloftemperaturesaretheprimarycausesofthermalfatigueincast-ironcylinderheads.Subsequently,afiniteelementanalysiswasconductedtopredictthedetailedsteady-statetemperatureandstressdistributionswithinthecylinderhead.Acomparisonofthepredictedsteady-statetemperaturesandstressescomparedwellwithourmeasurements.Furthermore,thepredictedlocationofthecrackinitiationpointcorrelatedwellwithexperimentalobservations.Thissuggeststhatavalidatedsteady-stateFEMstressanalysiscanplayaveryeffectiveroleintherapidprototypingofcast-ironcylinderheads.INTRODUCTIONHeavy-dutydieselenginecylinderheadsexperienceseverethermalandmechanicalloading,underbothsteady-stateandtransientengineoperation.Consequently,cylinderheaddesignisverysophisticatedasitneedstohousecomplexcoolingpassagesforensuringcompliancewiththermalstresses,whileprovidingsufficientmechanicalstrengthtowithstandcombustionpressures,andyetaccommodatingintakeandexhaustvalvesandports,andthefuelinjector.Asaresultofdesign,weightandmanufacturingcompromises,cylinderheadsoftenfailinoperationduetocracksthatareinitiatedduetothermalfatigueinregionswherecoolingislimited,suchasinthenarrowbridgebetweenvalves,oraroundtheexhaustvalveseat.Anumberofstudieshavesofarbeenconductedtodevelopanalyticalmethodologiessuitableforrapiddesignandvirtualprototypingofcylinderheads.Thefiniteelementmethodhasbeenthefoundationofmanyoftheanalysesthatpredictthethermalandstressfieldswithinthecylinderhead.However,theaccuracyofsuchanalysescriticallydependsonourunderstandingoftheproblem,andtheaccuracyoftheboundaryconditionsusedintheformulation.Thermalstressesareinducedbyanyofthefollowingcauses：Temperaturegradientsundersteady-stateoperation,includingtheeffectsofcyclictemperaturechangesinthecombustionchamberwallAnincreaseinthemeantemperatureofacomponent,whichaffectstheexpansionanddistortioncharacteristics,thusinducingstressesThermalshockloadingresultingfromasuddenchangeinspeedorloadduringtransients,whichchangetherateofheatfluxfromthegastothecylinderhead.Duetotheinherentdifficultiesinmeasuringstressfieldsnearthecriticalregionsonthefiredecksurface,especiallyundertransientconditions,limitedsetsofmeasurementsthatcanshedlightontheproblemhavebeenreported.AnumericalstudyofthermalshockcalculationsbyKeribarandMorelhasshownthatthermalwavespropagateintocomponentsduringenginetransients,withthesteepnessofthefrontdependingonmaterialthermalproperties.Whileforaceramiccomponentsevereshockloadscancausesurfacecompressivestressestoovershootfinalsteady-statevalues,theeffectwasnotpronouncedinhigherconductivitymaterials.Inordertovalidatethisanalyticalfinding,andattributeappropriatelycausesoffailureincast-ironcylinderheads,acombinedexperimentalandanalyticalapproachisfollowedheretostudystressdistributionsundersteady-stateandtransientoperation.Experimentalstudiesareconductedfirsttomeasuretemperatures,heatfluxesandstressesunderaseriesofsteady-stateandtransientoperatingconditions.Bothbiaxialanduni-axialhightemperaturestraingageshavebeeninsertedwithinthethermalpenetrationdepthofadieselenginecylinderhead.Thestraingageinsertionbeneaththesurfaceofthefiredeckensuresthedurabilityandreliabilityofthesensor.Atthesametime,theplacementwithinthethermalpenetrationdepthallowsforstudyingtheeffectofthermalshockloadingunderrapidtransients,andforcontrastingthosemeasurementswithcorrespondingsteady-statemagnitudes.Subsequently,afiniteelementanalysisisconductedtopredictthesteady-statetemperatureandstressdistributionswithinthecylinderhead.Predictionsarecomparedwithmeasurements,andthepotentialofthemethodtopredicthighstressregionsthatcouldleadtocrackinitiationisexplored.EXPERIMENTALMEASUREMENTSEXPERIMENTALSETUP–Temperature,heatfluxandstressmeasurementswereacquiredonasix-cylinder,naturally-aspirated,direct-injection,Hyundaidieselengine,primarilyusedinbusapplications.TheprimaryspecificationsoftheenginearereportedinTable1.TEMPERATUREANDHEATFLUXSENSORS–Atotalof8steady-statetemperatureandheatfluxprobeswereinstalledaroundtheintakeandexhaustvalveseatsofcylinders#2and#6.Theprobetipsweremountedatadepthof1.0mmbeneaththefiredecksurface,atthelocationsshowninFigs.1and2.AschematicdiagramshowingtheconstructionofthetemperatureandheatfluxprobeisshowninFig.3.TheprobewasmadeofKtypethermocouples.Anear-surface(1.0mmbeneaththefiredeck)andanin-depthjunction(4.0mmbeneaththesurface)makeitpossibletocalculateheatflux.Toenhancethesensitivityofthejunctions,athin(1mmthickness),circularcopperplatewasweldedatthetipofthesensor.Temperatureandheatfluxdatawereacquiredevery1second,underfullload,overaspeedrangefrom1000rpmto2500rpm,every500rpm.Table1.SpecificationofthetestengineDisplacementVolume7545ccBoreXStroke118X115mmCompressionRatio17.5IgnitionOrder1-5-3-6-2-4MaximumTorque475N·m@1500rpmMaximumPower123kW@2200rpmFigure1.TemperatureandheatfluxmeasuringpointsonthefiredeckHIGHTEMPERATURESTRAINGAGES–Formeasuringstresswithinenginecylinderheads,especiallynearthegas-sidesurface,straingageswithhightemperaturedurabilityareneeded.Aspecialprocedurehasbeendevelopedinthisworkforconstructingastraingagesensorplug(seeFig.4)thatissuitableforsuchmeasurements.Thedetailsofthesensorselection,attachmentintheinstrumentationplug,andverificationofitsoperationaredescribednext.Figure2.LocationofsensoronthefiredeckFigure3.SchematicdiagramoftemperatureandheatfluxsensorFigure4.SchematicdiagramillustratingstraingagesensorandcriticaldimensionsTwotypesofhightemperaturestraingages(120and350)wereused.ThespecificationsofthesensorsmadebyMicromeasurementCo.aredescribedinTable2.Accordingtothemanufacturer,theresponsetimeofthestraingageswas300kHz(3.33s).Incaseofthe120straingage,thestraingagewascoatedwithhightemperatureresistancebondafterattachmenttotheinsidesurfaceofacup-shapedplug.Then,thestraingagewasheatedinamicrowaveovenfor3hours.Aftercoolingtoambienttemperature,thestraingagewasrecoatedandre-heatedat150°Cfor3morehours.Incaseofthe350straingage,heatingwasappliedforagrandtotalof4hoursatatemperatureof175°C.Table2.SpecificationsofhightemperaturestraingagesGageTypeWA-06-062TT-120WA-06-60WT-350Resistancein120.0±0.4%350.0±0.4%LotNumberD-A38AD73K44FD121GageFactorAt75°F2.01±0.5%2.07±1.0%RangeCont.Use-75to205°C-269to290°CShortUse-195to260°C370°CFigure5showsapictureofthefinishedhightemperature,straingageplugassembly.Followingconstructionoftheinstrumentationplug,itssensingbehaviorwasexplored.Theplugtemperaturewasvariedbyexposingittoatorch,andrecordedviaanattachedthermocouple.Correspondingstrainreadingswerealsorecorded.Theexperimentallymeasuredstrainversustemperaturecharacteristicwascomparedtotheonepublishedbythemanufacturer,andusedasthebasisforvalidatingthesensorplugbehavior.Figure5.AphotographofhightemperaturestraingageThehighestcomponenttemperatures,andhencethermally-inducedstressesareexperiencedatthecombustionchambersurface.Whileitisdesirabletomeasurestressesonthesurface,sensorsmountedflushwiththesurfacehaveaveryshortlife.Inordertoensurethedurabilityandreliabilityofthestraingagesensorplug,itwasinserted1.5mmbeneaththesurface.Thislocationwasstillwithinthepenetrationdepthofthermaltransientsoriginatingatthegas-sidesurface.Thus,itallowedstudyingtheeffectofthermalshockloadingunderrapidtransients.Atotalof4straingagesensorplugswereinsertedneartheintakeandexhaustvalveseatsofcylinder#2and#4(seeFig.6).Figure6.SchematicdiagramofstraingagepositionThestaingagesinsertedincylinder#4wereofthebiaxialtype,measuringstraininthexandydirections,asdefinedinFig.7.Thestraingagesinsertedincylinder#2wereoftheuni-axialtype,measuringstrainina45°axis.Sincestraingagesignalscanbehighlyaffectedbyevenminuteleadwiremovement,carewasexercisedtoattachthemfirmlytotheenginehead.Steady-statestressesweremeasuredasspeedwasvariedfrom1000rpmto2000rpm,inincrementsof250rpm,underfullload.Transientstressmeasurementswerealsoacquiredevery0.01seconds,whileloadwascycledbetween0and100%forseveralenginespeeds.Figure7.StressmeasurementdirectionsTEMPERATUREANDHEATFLUXMEASUREMENTS–Figures8and9showthesteady-statetemperaturesmeasuredatthefourlocationswithinthefiredeckofcylinders#2and#6,respectively.Inallcases,themeasuredtemperaturesincreaselinearlywithrespecttoenginespeed.Increasingspeedallowslesstimeforheattransfertothecoolantbetweencombustionevents.ThehighesttemperaturevaluesarerecordedatlocationB,followedbythoseatA,C,andD.ItshouldbenotedthatlocationBisbetweentheinjectornozzleholeandtheexhaustvalve.Sincethereisnocoolantpassagenearthatregion,thisexplainswhylocationBreachesthehighesttemperatureofthefourlocationsinvestigated.Ontheotherhand,locationDexperiencesthelowesttemperatureasitisexposedtosignificantforcedcoolingfromtheadjacentcoolantpassageandfromtheinducedfreshair.Figure8.Steady-statewalltemperaturesincylinder#2overarangeofspeedsFigure9.Steady-statewalltemperaturesincylinder#6overarangeofspeedsFigures10and11showthecorrespondingsteady-stateheatfluxescomputedatthesamelocationswithincylinders#2and#6,respectively.Again,heatfluxincreaseslinearlywithenginespeed.TheheatfluxmagnitudesarehigherforpositionsBandC,locatedaroundtheexhaustvalveseat,comparedtothoseatAandD,locatedaroundtheintakevalveseat.Notethatasspeedisincreasing,differentlocationsexperiencedifferentratesofincreaseofheatflux,afactthatisattributedtodifferencesinturbulentgasmotionandcoolantflowpatterns.Whentheheatfluxratesincylinders#2and#6arecompared(seeFigs.10and11),itcanbenoticedthattheformerexperienceshigherheatfluxratesthanthelatter.Thisisattributedtothefactthatthecoolantflowsfirstaroundcylinder#2;bythetimeitreachescylinder#6,thecoolanthaspickedupsomeheatanditstemperaturegraduallyrises,thusreducingthepotentialforheattransferfromcylinder#6.Asaresultofthehigherheatfluxes,thefiredecktemperaturesaroundcylinder#2arelower(byabout10°C)thanthosearoundcylinder#6thatislocatedonthecoolantoutletside.STRESSMEASUREMENTS–Inordertobeabletoisolatetheeffectofpre-loadingonthetotalstressmeasurementsrecordedinafiredenginebythevariousstraingages,stressmeasurementsweretakenduringtheengineassemblyprocess.Measurementsweretakenatthefourstraingagesfollowingthetighteningofeachheadbolt.TheinitialstressvariationisshowninFig.12.Whilethetighteningofdifferentboltsproduceddifferentamountsoftensionandcompressionatthemeasurementlocation,noconsistentpatternwasrevealedbythemeasurements.However,itisimportanttonoticethatpre-loadingproducedanegligiblestress(within±5MPa)atthemeasurementlocations,irrespectiveofdirections.Figure10.Steady-stateheatfluxesincylinder#2overarangeofspeedsFigure11.Steady-stateheatfluxesincylinder#6overarangeofspeedsFigure12.Effectofbolttighteningonpre-loadingstressFigure13showsthesteady-statestressesrecordedbythebi-axialstraingagesatintakeandexhaustvalvelocationsofcylinder#4,aswellasthestressesrecordedbytheuni-axialstraingagesatintakeandexhaustvalvelocationsofcylinder#2.Themeasurementsweretakenoverarangeofenginespeedsandatfullload,i.e.conditionsthatwouldproducethemaximumstressateachspeed.Itmustbenotedthatasenginespeedisincreasedtowardsthemaximumtorquespeed(1500rpm),themeasuredstressesincrease(ordecrease)atafasterrate.Beyondthemaximumtorquespeed,thestressvariationisslight.Bothtensileandcompressivestressesappearsimultaneouslyatthedifferentlocations,asshowninFig.13,thusindicatingthecomplexcharacterofthestressfield.Figure13.Steady-statestressvariationwithrespecttoenginespeedatfullloadThemeasuredstressneartheexhaustvalveseathasnegativevalues(x,ydirections),indicatingacompressioneffect.Thiscouldhavebeenproducedfromatendencyofhightemperatureregions(suchnon-adequatelycooledregionsaroundtheinjectorandthevalvebridge)toexpand,whilesubjectedtomechanicalconstraints.Asaresult,therestofthefiredeckexpandsmoreinrelativeterms,assuggestedbythetensilestressesexperiencedattheothermeasurementlocations.Itmustalsobenotedthat,atthevariousspeeds,theabsolutemagnitudeofthestressesneartheexhaustvalvearetwotothreetimeslargerthanthoseattheintakevalve,foreithercylinder#2or#4.Thiscorrelateswellwiththetwotothreetimeshigherheatfluxmeasuredontheexhaustvalveseatcomparedtothecorrespondingheatfluxontheintakevalveseat(refertoresultsforlocationsBandAinFigs.10and11).Figure14.Transientstressvariationinxandydirectionsatexhaustvalveofcylinder#4Figure15.Transientstressvariationinxandydirectionsatintakevalveofcylinder#4Atransienttestschedulehasalsobeendevelopedinordertoassesstheeffectofthermalshockloadingonthestressesmeasuredatthesamelocationswheresteady-statemeasurementswerereported.Theschedulefollowedengineoperationfromcoldstarttofiringunderaseriesofspeedsandloads.Figures14to16showthetransientstressesrecordedbythebi-axialanduni-axialstraingagesattheinstrumentedlocationsneartheexhaustandintakevalvesofcylinders#4and#2.Allfiguresindicatethesamegeneralcharacteristics.TheperiodfromAtoBindicatestheequilibriumstatewithcoolantatroomtemperatureandnothermallyinducedstresses.Uponturningontheengine(stateB),astepchangeinstresslevelwasobservedatallmeasurementlocations,exceptforthe45°directionaroundtheintakevalveseatofcylinder#2(seefig.16).FromBtoC,temperaturesandthermalstresseswerestabilizedwhiletheenginewasidlingataspeedof700rpm.Thestressescontinuedtoincrease(ordecrease)smoothly,asspeedwasgraduallyincreasedfrom700rpmto1000rpm(CtoD).Figure16.Transientstressvariationin45°directionsatexhaustandintakevalvesofcylinder#2Followingenginewarm-up(fromstateDon),acyclictestpatternwasimposed.Speedwasincreasedfrom1000rpmto2000rpminincrementsof250rpm,whiletheloadwascycledbetweenfullloadandnoloadateachtestspeed.Themeasuredstressesfollowedacyclicpatternasaresultoftheimposedlargeswingsingastemperaturesandgas-sidesurfacetemperatures.Thethermalshockloadingexperiencedbythecylinderheadduringthisseveretransientisevident.Itshouldbenoted,however,thattheabsolutemagnitudesofthestressesrecordedatanyinstantduringthetransientexceedonlymarginallythelevelsthatwouldcorrespondtosteady-stateoperationundereachofthoseconditions.Thisisattributedtothefactthatthermalshockwavespenetratefastintothecast-ironcylinderhead.Followingthecyclicoperation,theenginewasturned-off(stateF),andanabruptchangeinstresslevelswasrecorded.However,someresidualstressesremainedaftershut-off,whichrequiredmorethan7hourstoberelaxed.COMPUTATIONALPREDICTIONSAthree-dimensionalnumericalanalysisbasedonthefiniteelementmethod(FEM)canbeusedtopredictthedetailedsteady-statetemperatureandstressdistributionswithinthecylinderhead.AsynopsisoftheFEMmodel,itsvalidationagainstourmeasurements,andpredictionsusingthemodelarereportedbelow.FEMANALYSIS–Athree-dimensionalfiniteelementmodelofthecylinderheadandblockwascomposed,asshowninFig.17.Mostofthegridelementsareisoparametricsolidbrickwiththerestofthembeingprismelements.Agasketmodel,representedasonerowofelementshasbeeninsertedbetweentheheadandblockmodels.Atotalof12,156nodespointsand7,803elementswereemployedtodescribedtheFEMmodel.Thecylinderheadandblockweremadeofcast-iron,whilethegasketwasassumedtobeanindiumcompositematerial.MaterialpropertiesaresummarizedinTables3and4.Steady-state,heattransferandstressanalyseswereconductedusingthecommercialcodesNISAII(solver)andDISPLAYIII(preandpost-processor).Theheattransferanalysiswasconductedfirst.Subsequently,theheattransferresultswereusedtoperformthestressanalysis.Figure17.3-DFEMmodelofheadandblockTable3.Propertiesofcast-ironTable4.PropertiesofindiumcompositematerialThermalconductivity,k[W/m·K]50.2Thermalconductivity,k[W/m·K]0.17Young’sModulus,E[GPa]120.0Young’sModulus,E[GPa]19.15Poisson’sRatio,0.29Poisson’sRatio,0.4ThermalExpansionCoefficient,[1/K]12.0x10-6ThermalExpansionCoefficient,[1/K]2.7x10-7Fortheheattransferanalysis,theboundaryconditionsshowninTable5werespecified.Thecyclic-meanvaluesofthein-cylindergas-to-wallheattransfercoefficientandbulkgastemperaturewereobtainedfromthecomprehensivethermodynamiccyclesimulationdevelopedbyAssanisandHeywood.TheboundaryconditionsattheintakeandexhaustportwereobtainedbasedonexperimentalcorrelationsreportedbyAnnandandHires.Coolantsideboundaryconditionswerebasedonvaluesreportedintheliterature.Thelowervaluesforthecoolanttemperatureandheattransfercoefficientwereusedinregionsoflowercoolantvelocity,suchasinbetweencylinderbores.Forthestressanalysis,onlythermalstresseswereconsidered.Asmechanicalboundaryconditions,twopointswerefixed(x=y=z=0)insidetheheadbolthole,andtherigidlinkmethodwasapplied.Thegasketcontactsurfacewasassumedtobeunconstrained.Table5.ThermalboundaryconditionsHeatTransferBoundaryVariable1000rpm1500rpm2000rpm2500rpmIn-cylinderh[W/m2·K]404502631753T[K]738762786806Intakeporth[W/m2·K]272350426482T[K]60606060Exhaustporth[W/m2·K]778845893960T[K]749810855894Coolanth[W/m2·K]4000to45004000to45004000to45004000to4500T[K]353to358353to358353to358353to358FEMMODELVALIDATION–InordertovalidatetheFEMmodel,steady-statepredictionsatselectedpointsofthethermalandstressfieldswerecomparedwithmeasurementsrecordedatthesamepoints.AsshowninFig.18,measuredandpredictedsteady-statewalltemperaturescomparefavorablyoverarangeofenginespeeds,bothinmagnitudeandtrend.ComparisonsbetweenmeasuredandpredictedsteadystatestressesareshowninFigs.19and20.Whiletheagreementintrendissatisfactory,somedifferencesinmagnitudeareobserved.Thediscrepanciescanbeattributedtothefollowingreasons.First,experimentaldataincludetheeffectsofboththermalloadingandcombustionpressure.Ontheotherhand,onlythethermaleffectisconsideredinFEManalysis.Nevertheless,therelativelysmalldifferenceinmagnitudesconfirmsthattheeffectofmechanicalstressesisrelativelysmall.Second,thestiffnessofthefiredeckmayhavebeenalteredduetothestraingageinsertionprocess.Third,theFEManalysishasnotaccountedforcylinder-by-cylindervariationinboundaryconditionsduetofactorssuchasmanifoldgasdynamics,coolantmaldistribution,etc.Overall,itisconcludedthattheFEMmodelwiththeappliedboundaryconditionshasthepotentialtocapturethethermalandstressfiledwithinthecylinderheadwithacceptableaccuracy.Figure18.Comparisonbetweenmeasuredandpredictedsteady-statewalltemperaturesoverarangeofenginespeedsFigure19.Comparisonbetweenmeasuredandpredictedsteady-statestressesneartheintakevalve,overarangeofenginespeedsFigure20.Comparisonbetweenmeasuredandpredictedsteady-statestressesneartheexhaustvalve,overarangeofenginespeedsSTEADY-STATEPREDICTIONSOFTHERMALANDSTRESSDISTRIBUTIONS–FEMpredictionsofthesteady-statetemperatureandstressdistributionsinthefiredeckareshowninFig.21forarangeofenginespeedsatfullload.Theisothermplotsa-d,onthelefthandsideFig.21,indicatethattheregionofthefiredeckinthevicinityoftheexhaustvalveseatexperiencesconsiderablyhighertemperatures(80°Cto100°C)thantherestofthefiredeck.Themaximumtemperaturevariesfrom232°Cat1000rpmto312°Cat2500rpmasaresultofincreasingmeangastemperatureandheattransfercoefficientwithincreasedenginespeed.Noticealsothatasspeedisincreasing,thehotregionpropagatesfromtheexhaustvalvesidetotheintakevalveside.Outsidethefiredeck,thecylinderheadfaceexperiencestemperaturesclosetothecoolanttemperature(around80°C),independentofspeed.Itisimportanttoobservethatthelargesttemperaturegradientsoccurbetweentheexhaustvalveseatandtheinjectornozzlehole,andthevalvebridgebetweenthetwovalves.Atthetwolowerenginespeeds,theinjectornozzleholeandthevalvebridgearesurroundedbyuniformlylowertemperatures.However,thepropagationofthehotfrontwithincreasingspeedresultsinanasymmetricexposureofthosecriticalregionstohotandcoldtemperatures.Itisthereforeanticipatedthatthehighestthermalstressesshouldbeconcentratedoneithertheinjectorholeorthevalvebridgeregion,asthermalstresslinearlydependsontemperaturegradient.(a)Predictedisothermsat1000rpm;min=80°C,max=232°C,increment=10.1°C(e)Predictediso-stresscontoursat1000rpm;min=3MPa,max=251MPa,increment=16.5MPa(b)Predictedisothermsat1500rpm;min=80°C,max=263°C,increment=12.2°C(f)Predictediso-stresscontoursat1500rpm;min=3MPa,max=299MPa,increment=19.7MPa(c)Predictedisothermsat2000rpm;min=80°C,max=287°C,increment=13.8°C(g)Predictediso-stresscontoursat2000rpm;min=5MPa,max=354MPa,increment=23.3MPa(d)Predictedisothermsat2500rpm;min=80°C,max=312°C,increment=15.5°C(h)Predictediso-stresscontoursat2500rpm;min=5MPa,max=405MPa,increment=26.7MPaFigure21.Steady-statetemperature[°C]andthermalstress[MPa]distributionsofthefiredeck.Plotse-h,ontherighthandsideofFig.21,showthepredictedthermalstressdistributionswithinthefiredeckasVonMisesstress.Ingeneral,higherlevelsofstressesareexperiencedasspeed,gasandwalltemperaturesincrease,consistentwithcorrespondingpredictionsofthethermalfieldinthefiredeck.Noticehoweverthatateveryspeed,themaximumstressvaluesoccurwherethemaximumtemperaturegradients(andnotwherethemaximumtemperatures)arefound.Hence,thelocationofthemaximumthermalstressesisindeedaroundtheinjectornozzleholeandthevalvebridgeregion,assuggestedfromourpredictionsofthetemperaturegradients.Themaximumstressatthosecriticallocationsincreasesfrom251MPato405MPa,asspeedincreasesfrom1000rpmto2500rpm.Giventhattheyieldstressofheattreatedcast-ironisaround350MPathissuggeststhatregionsofthecylinderheadmaybepronetoplasticdeformationatconditionsover2,000rpmandfullload.Underseverethermalloading,thisplasticdeformationwouldfirstleadtocrackinitiationaroundthenozzleholeandvalvebridge.Indeed,Fig.22showsthatanengineoperatedundersimilarconditionsdevelopedacrackinthecylinderheadfiredeckintheregionbetweentheinjectornozzleholeandtheexhaustvalveseat,thusvalidatingthenumericalpredictions.CONCLUSIONSAcombinedexperimentalandanalyticalapproachwasfollowedinthisworktostudystressdistributionsandcausesoffailureindieselcylinderheadsundersteady-stateandtransientoperation.Experimentalstudieswereconductedfirsttomeasuretemperatures,heatfluxesandstressesunderaseriesofsteady-stateandtransientoperatingconditions.Subsequently,afiniteelementanalysiswasconductedtopredictthedetailedsteady-statetemperatureandstressdistributionswithinthecylinderhead.Thefollowingconclusionscanbedrawnfromourstudy:Figure22.Photographofatypicalcrackinitiationpoint1.Thermalshockloadingplaysaroleinthermalfatigue,alongwithsteady-statetemperaturegradientsandtheleveloftemperatures.Whentheengineisturnedonoroff，andduringperiodsofchangingloadatagivenspeed,thestresslevelchangessignificantly.However,acomparisonofoursteady-stateandtransientmeasurementsindicatesthatstressesrecordedatanyinstantduringaseveretran