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1、QuantitativeassessmentoffatiguedamageaccumulationinwavyslipmetalsfromacousticharmonicgenerationByJohnH.CantrellNASALangleyResearchCenter,MailStop231,Hampton,VA23681USA(ReceivedABSTRACTAcomprehensive,analyticaltreatmentispresentedofthemicroelastic-plasticnonlinearitiesresultingfro
2、mtheinteractionofastressperturbationwithdislocationsubstructures(veinsandpersistentslipbands)andcracksthatevolveduringhigh-cyclefatigueofwavyslipmetals.Thenonlinearinteractionisquantifiedbyamaterial(acoustic)nonlinearityparameterβextractedfromacousticharmonicgenerationmeasurement
3、s.ThecontributiontoβfromthesubstructuresisobtainedfromtheanalysisofCantrell[Cantrell,J.H.,2004,Proc.R.Soc.LondonA,460,757].ThecontributiontoβfromcracksisobtainedbyapplyingtheParislawforcrackpropagationtotheNazarov-Sutincracknonlinearityequation[Nazarov,V.E.,andSutin,A.M.,1997,J.A
4、coust.Soc.Am.102,3349].Thenonlinearityparameterresultingfromthetwocontributionsispredictedtoincreasemonotonicallybyhundredsofpercentduringfatiguefromthevirginstatetofracture.Theincreaseinβduringthefirst80-90percentoffatiguelifeisdominatedbytheevolutionofdislocationsubstructures,w
5、hilethelast10-20percentisdominatedbycrackgrowth.Themodelisappliedtothefatigueofaluminiumalloy2024-T4instress-controlled1loadingat276MPaforwhichexperimentaldataarereported.Theagreementbetweentheoryandexperimentisexcellent.PACSnumbers:62.20.Mk;61.72.Hh;62.65.+k;43.25.Ba1.INTRODUCTI
6、ONVarioustechniquesandmethodologieshaveevolvedoverthelastseveraldecadesthatattempttocharacterizenondestructivelythestateofmetalfatigue.Whilemanyofthesetechniqueshaveprovenuseful,nomethodhasbeenabletoprovideanunambiguousassessmentofaccumulateddamageatagivenleveloffatigueovertheent
7、irerangefromthevirginstatetofracture.Cantrell(2004)hasrecentlyshownthatself-organizedsubstructuralarrangementsofdislocationsformedinwavyslipmetalsduringcyclicstress-inducedfatigueproducesubstantialchangesinthemicroelastic-plasticnonlinearityofthematerialthatcanbequantifiedbyamate
8、rialnonlinearityparameterβexperimentally
