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1、PublishedinReviewofProgressinQuantitativeNondestructiveEvaluation,Vol.28,editedbyD.O.ThompsonandD.E.Chimenti(AIP,Melville,2009),pp.19-32.NONDESTRUCTIVEEVALUATIONOFMETALFATIGUEUSINGNONLINEARACOUSTICSJohnH.CantrellNASALangleyResearchCenter,MailStop231,Hampton,VA23681,USAAB
2、STRACT.Safe-lifeanddamage-tolerantdesignphilosophiesofhighperformancestructureshavedriventhedevelopmentofvariousmethodstoevaluatenondestructivelytheaccumulationofdamageinsuchstructuresresultingfromcyclicloading.Althoughmanytechniqueshaveprovenuseful,nonehasbeenabletoprov
3、ideanunambiguous,quantitativeassessmentofdamageaccumulationateachstageoffatiguefromthevirginstatetofracture.Amethodbasedonnonlinearacousticsisshowntoprovidesuchameanstoassessthestateofmetalfatigue.Thesalientfeaturesofananalyticalmodelarepresentedofthemicroelastic-plastic
4、nonlinearitiesresultingfromtheinteractionofanacousticwavewithfatigue-generateddislocationsubstructuresandcracksthatpredictablyevolveduringthemetalfatigueprocess.Theinteractionisquantifiedbythematerial(acoustic)nonlinearityparameterβextractedfromacousticharmonicgeneration
5、measurements.Theβparameterstypicallyincreasemonotonicallybyseveralhundredpercentoverthefatiguelifeofthemetal,thusprovidingauniquemeasureofthestateoffatigue.Applicationofthemodeltoaluminumalloy2024-T4,410Cbstainlesssteel,andIN100nickel-basesuperalloyspecimensfatiguedusing
6、differentloadingconditionsyieldsgoodagreementbetweentheoryandexperiment.Applicationofthemodelandmeasurementtechniquetotheon-siteinspectionofsteamturbinebladesisdiscussed.Keywords:NonlinearAcoustics,MetalFatigue,Microelastic-PlasticNonlinearity,DislocationSubstructuresPAC
7、S:62.20.Mk,61.72.Hh,62.65.+k,43.25.BaINTRODUCTIONOneofthelongstandinggoalsofnondestructiveevaluationhasbeentodevelopaquantitativemeansofassessingthestateofmetalfatigueand,hence,theremaininglifeofthematerial.Overthelastseveraldecadesanumberoftechniqueshavebeendeveloped,bu
8、tnoneofthemhasbeenabletoprovideaquantitative,unambiguousassessmentofthestateoffatiguefromthevirginstate
