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KeywordsFailureanalysisFrettingwearsimulatorVibratingshaftHightemperatureandhighpressurewaterFracturesurfaceminethefailureinitiationandfailuremode.Itwasfoundthatthefailurehadinitiatedataofthesenuclearfuelrods,whichhavethefollowingdimensions9.5mmouterdiameter,0.6mmwallthicknessandabouta4mlength.BecauseoftheFIVphenomenonbyaprimarycoolant,theseslenderfuelrodscouldeasilyvibrate,thustheyaresupportedbyseveralspacergridstructuresasshowninFig.1.Eachcellofaspacergridstructureconsistsoftwospringsandfourdimples.Therefore,anuclearfuelrodissupportedbyacertainamountofspringforce,whichdependsonthespringshapeanditsstiffness.UnderahightemperatureinanoperatingNPPcondition,however,thecontactforcebetweenthefuel13506307/seefrontmatterC2112008ElsevierLtd.Allrightsreserved.Correspondingauthor.Tel.8242688761fax82428630565.Emailaddressleeyhkaeri.re.krY.H.Lee.EngineeringFailureAnalysis1620091238–1244ContentslistsavailableatScienceDirectEngineeringFailureAnalysisdoi10.1016/j.engfailanal.2008.08.013temperatureC24320C176C,pressurizedwaterC2415MPaandradioactiveenvironment.Thisconditioninevitablyresultsinmaterialdegradationssuchasacorrosion,fatigue,irradiationembrittlement,creep,etc.duringtheiroperation.AnothersevereconditionfordegradingstructuralmaterialsisknownasaflowinducedvibrationFIVofrelativelyslenderstructuresduetotherapidflowvelocityofprimaryandsecondarycoolants,whichresultsinfrettingdamagesbetweentheseslenderstructuresandtheirsupportingstructures.ThesekindsoffrettingdamageshavebeenexperiencedinthemajorcomponentsofNPPssuchasthenuclearfuelrods,steamgeneratorSGtubesandcontrolrods1.Amongthesecomponents,inourlaboratory,thefrettingwearphenomenonofnuclearfuelrodshasbeeninvestigatedinordertoexamineanddevelopafrettingwearmechanismandadefectfreefuelassemblyFA,respectively.Generally,aFAconsistsof16C216or17C2171.IntroductionThefailurephenomenaofmechanicalfrequentlyinvariousindustries.Onecontactregionbetweenthevibratingshaftandthefuelrodholderjigandafatiguecrackwaspropagatedalthoughitwasdifficulttoproveitconclusivelyduetotheheavilyoxidizedfracturesurface.Nearthefailurelocations,however,thethreadholewassubjectedtoarepeatedloadingduetothefactthatthespecimenholderjighadacircularmotionforsimulatingavibrationmotion.ThissuggeststhatthevibratingshaftfailureresultedfromcorrosionfatiguephenomenonbecausethefrettingtesthadbeenperformedathightemperatureC24320C176CandpressurizeddistilledwaterC2415MPaconditions.Inthispaper,thereasonsforthisfailureandthefracturemechanismsarerevealedanddiscussedbyusingtheOMandSEMresultsofthefailuresurfaceandthestressanalysisofthecontactregionsbetweenthevibratingshaftandthespecimenholderjig.Finally,theaboveresultswereappliedtoadesignchangeofthevibratingshaft.C2112008ElsevierLtd.Allrightsreserved.structuresbymaterialdegradationsinextremeenvironmentshavebeenreportedexamples,isinnuclearpowerplantsNPPs.Generally,NPPsoperateinahighFailureanalysisofavibratingshaftinafrettingwearsimulatorYoungHoLee,JaeYongKim,HyungKyuKimKoreaAtomicEnergyResearchInstitute,150Dukjindong,Yuseonggu,Daejeon305353,RepublicofKoreaarticleinfoArticlehistoryReceived11July2008Accepted1August2008Availableonline15August2008abstractRecently,afrettingwearsimulatorwasdevelopedinordertoevaluatethefrettingwearbehaviorofnuclearcomponentsinhightemperatureandhighpressureHTHPwaterconditions.After500htests,however,twovibratingshaftsamongfourwerefractured,whichwereconnectedtoaspecimenholderjigbyusingabolt.ThefracturesurfacewasexaminedusingbothanopticalmicroscopeOMandascanningelectronmicroscopeSEMtodeterjournalhomepagewww.elsevier.com/locate/engfailanalY.H.Leeetal./EngineeringFailureAnalysis1620091238–12441239rodandspring/dimpleisgraduallydecreasedduetoaspringrelaxation.Inaddition,thespringstiffnessisalsograduallyincreasedbecauseofanirradiationembrittlementofaspacergridspring/dimple.Eventhoughacertainamountofspringforceisexertedonthefuelrodatitsinitialoperation,arelativelysmallslipamplitudeonthecontactsurfaceisunavoidableunderasevereFIVcondition.Consequently,thecontactforceisgraduallydecreasedandfinally,agapisopenedup.ThisFig.1.Theschematicviewofanuclearfuelassemblythatconsistsof16C216or17C217fuelrodsandseveralspacergridassembly.meansthatthefrettingwearmodecouldbechangedfromaslidingweartoanimpactingwearwithincreasingoperatingtime.Anotherimportantpointisthatthevibrationcharacteristicsofanuclearfuelrodalsochangedduetoacontactconditionchangeofafuelrod.Therefore,itisquitedifficulttoexaminethefrettingwearmechanismofanuclearfuelrodinNPPoperatingconditions.AverticaltypeofafrettingweartesterFRETONUS,FRTttingTesterOfNUclearSystemsforaHTHPwaterconditionwasdevelopedinordertoevaluatethefrettingwearbehaviorofnuclearcomponents2.After500htests,however,twovibratingshaftsamongfourwerefractured,whichwereconnectedtoaspecimenholderjigbyusingaboltasshowninFig.2.ItisFig.2.Schematicviewsoftheassemblingmethodbetweenthevibratingshaftandrodspecimenholderjigandthefracturedvibratingshaftafter500htestsathightemperatureandpressurizedwatercondition.essentialtoevaluatethereasonfortheirfailureandtoimprovethevibratingshaftdesigntoconfirmthereliabilityofthefrettingweartesterforalongexperiment.Inthisstudy,thereasonsforthefailureandthefracturemechanismsarerevealedanddiscussedbyusingtheOMandSEMresultsofthefracturesurface,measurementofthemicrovickershardnessandthestressanalysisofthevibratingshaft.Finally,theaboveresultswereappliedtoadesignchangeofthevibratingshaft.2.Fracturesurfaceanalysis2.1.OMobservationThevibratingshaftismadeofaconventionalhighcarbonmartensiticstainlesssteelSUS440Candthismaterialiswidelyusedastoolsorbladesforuseinrelativelycorrosiveatmospheres3.ThechemicalcompositionandmechanicalpropertiesarelistedinTable.1.Fig.3showstheOMresultofthefracturesurfaceforthevibratingshaft.DuetotheHTHPwatercondition,thefracturesurfacewasheavilyoxidizedanditwasdifficulttoobtainmetallographicproofbyusingtheOMobservation.However,itisexpectedthatthefailurewasinitiatedatacontactregionbetweenthevibratingshaftandthefuelrodholderjigbecausethesetwopartswasassembledbyusingaboltasshowninFig.2.Duringthefrettingweartests,fourvibratingshaftswerereciprocatedintheiraxialdirectionwitharangeof±200lm,afrequencyof30C176Hzandasinusoidalmotionbytwoelectromagneticactuatorsthatwerearrayedatanangleof90o.TogenerateacircularmotionofthefuelrodspecimenwhichisregardedasaconservativesimulationofanactualfuelrodvibrationinoperatingNPPs,thephasedifferenceofthesinusoidalwaveformfortheshakesignalsoftwoactuatorsissetto90oasshowninFig.4.Whenthefuelrodspecimenisvibratedwithacircularmotion,thecontactregionofthevibratingshaftswiththefuelrodholderjigisexpectedtoexperiencefatigueloads.Generally,striationtracesthataredistinctproofforafatiguefailurecouldbefoundeasilyonafracturesurface,ifthestructuralmaterialsarefracturedunderthefatigueloadingconditions.However,theoxidizedfracturesurfaceofthevibratingshaftwasexaminedbyusingSEMwithoutanycleaningsbecausethestriationtracescouldbeexpectedtoremovebytheoxideremovalprocessi.e.acidcleaning.2.2.HardnessvariationBeforetheSEMobservation,itisnecessarytoexaminethemechanicalpropertychangesofthevibratingshaft,becauseitwasexposedtotheHTHPwaterconditionduring500htests.So,amicrovickershardnessofthefracturedvibratingshaft1240Y.H.Leeetal./EngineeringFailureAnalysis1620091238–1244wasmeasuredanditsresultisshowninFig.5.Itisapparentthatthemicrovickershardnessvalueincreasedaccordingtotheexposedtemperatureforthevibratingshaft.ThisresultcorrespondswellwiththetemperingtemperatureeffectTable1ChemicalcompositionsandmechanicalpropertiesroomtemperatureofaconventionalhighcarbonmartensiticstainlesssteelSUS440C4CMnSiPSCrMoFeChemicalcompositionswt1.01.01.00.040.03170.75Bal.YieldstrengthTensilestrengthElongationElasticmodulusDensityPoissonsratioMechanicalproperties1280MPa1750MPa4200GPa7.8g/cm30.3Fig.3.OMresultofthefracturesurfaceforthevibratingshaftarrowsindicatetheexpectedcrackinitiationregion.Y.H.Leeetal./EngineeringFailureAnalysis1620091238–12441241Fig.4.ActuatingmechanismofthefuelrodbyusingtwoelectromagneticactuatorsinFRETONUSnotethatthefuelrodmotionsarechangeablebyadjustingvibrationamplitudesandphasedifferencebetweentwoactuators.onthemechanicalpropertiesofSUS440C4.Inthisstudy,thehighercarbonmartensiticstainlesssteelislikelytoretainalargeamountofuntransformedaustenite.Itisthoughtthatadelayedtransformationmayoccurastemperaturefluctuationsinseveralfrettingweartestsbecausethe500htestsatabout320C176Cdidnothaveanegligibleeffectonthestressrelievingofthevibratingshaftmaterial.Inaddition,itispossiblethatsomelossinductilitymayresultfromahydrogenembrittlementthatisanimportantconcerninmartensiticstainlesssteelsinaheattreatingatmospherecontaininghydrogenintheformofdistilledwater.2.3.SEMresultsFig.6showsthemorphologyofthefracturesurfacebyusingSEM.Itisapparentthatthefracturesurfacewasalmostcoveredwithoxidesanditisdifficulttodetectevidenceofafatiguefailuresuchasstriationtraces.Fromacarefulinspectionoftheresults,however,weakstriationtracesappearednearalateraloutersurface.Also,acrackpropagationtracewasfoundatthecontactregionbetweenthevibratingshaftandthefuelrodholderjigeventhoughitscharacteristiccouldnotbedefinedduetothesevereoxidationintheHTHPwatercondition.Oneoftheinterestingresultsisthatanimpactingwearscarappearedinthefracturesurfaceofthevibratingshaft.Thisresultmeansthattheimpactingwearbetweenthetwofracturedsurfacesisgeneratedafterafracturebecausethefracturedvibratingshaftiscontinuouslyreciprocatedbyanelectromagneticactuatorregardlessoftheshaftfailure.So,itisexpectedthatthefatiguestriationtraceshaddisappearedandwerecoveredbytheimpactingwearmotionandbytheweardebriswithasevereoxidation,respectively.Consequently,thevibratingshaftfailureisexpectedtobeinitiatedbyacontactfatigueatthecontactregionandthenthecontactforceexertedbyaboltFig.5.Measurementresultofthemicrovickershardnessaccordingtotheexposedtemperatureforthevibratingshaft.
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