论文On_some_basic_problems_of_fatigue_research_in_en

InternationalJournalofFatigue23(2001)751–766’an,Shaanxi710072,People’sRepublicofChinaReceived1March2001;receivedinrevisedform19April2001;accepted26April2001AbstractResearchonthebasicfatigueformula,thecriterionforomittingsmallloads,theruleforcumulativefatiguedamage,andmodelsforpredictingthefatiguecrackinitiation(FCI)lifeandprobabilitydistributionundervariable-amplitudeloadingarecomprehensivelystudiedandsummarizedinthepresentpaper.First,theformulaforFCIlifeisgivenasafunctionoftheequivalentstressamplitude,andtheproceduresforinvestigatingoverloadeffectsareintroduced.UsingtheaboveformulatoanalyzetestresultsoftheoverloadeffectonFCIlifeyieldstheformulaforFCIlifecontainingtheoverloadeffectfactor,z,whichcanbeadoptedtocharacterizetheloadinteractioneffectwhenMiner’sruleisusedtocomputethecumulativefatiguedamage.FatiguetestresultsandanalysisoftheunderloadeffectshowthattheFCIthresholdintheFCIlifeformulacanbetakenasthecriterionforomittingsmallloadsinlifepredictionandincompilationoftheloadspectrum.Then,thefatiguedamagefunctionisdefined,thecumulativefatiguedamagecanbecomputedandtheFCIlifeofstructuremembersundervariable-amplitudeloadingcanbeobtainedbyusingMiner’srule.ProceduresforpredictingtheprobabilitydistributionofFCIlifeorfatiguelifearedescribedandexamplesareintroduced.Further-more,factorsaffectingthevalueofzandtheapplicabilityofMiner’srulearebrieflydiscussed.Finally,furtherareasforfatigueresearcharetentativelyproposed.Mostinterestshouldbefocusedonresearchintogenerallyapplicablelifepredictionmodelsandtheapproachforfatiguereliabilityassessmentofstructuredetailsmadeofthemetalswithcontinuousstrain-hardeningcharacteristics,whichhavebeenwidelyadoptedinaeronauticalengineering,underserviceloadandenvironments.2001ElsevierScienceLtd.Allrightsreserved.Keywords:Fatigueformula;Fatiguecrackinitiation;Overloadeffect;Criterionforomittingsmallload;Strain-hardeningcharacteristics;Fatiguedamagefunction;Miner’srule;Lifeprediction;Joints1.IntroductionFromtheviewpointofengineeringapplications,thepurposeoffatigueresearchconsistsof:(1)predictingthefatiguelifeofstructures,(2)increasingfatiguelifeand(3)simplifyingfatiguetests,especiallyfatiguetestsoffull-scalestructuresunderarandomloadspectrum[1].Thefatiguelifeofanengineeringstructureprincipallydependsuponthatofitscriticalstructuremembers.Thefatiguelifeofanaircraftstructuremembercanbedividedintotwophases,thefatiguecrackinitiation(FCI)lifeandthefatiguecrackpropagation(FCP)life,tobeexperimentallyinvestigatedandanalyzed[1–10].Evenforbuttweldscontaininguncrack-likedefects,theFCIlifeoccupiesagreatportionofthetotalfatiguelifeandcannotbeneglected[11].FCIlifehasbeenmeas-E-mailaddress:[e-mail](X.Zheng).0142-1123/01/$-seefrontmatter2001ElsevierScienceLtd.Allrightsreserved.PII:S0142-1123(01)00040-8uredbyusingnotchedspecimenstosimulatethegeometrydiscontinuityinstructuralmembers,wherethestressconcentrationoccurs[1,3–5,9],andthetestresultsanalyzedbyoneoftheexistingformulae,eitherempiri-caloranalytical[1,3,4,12–14].TheFCPlifeofstructuralmembersisanalyzedbyusingthefracturemechanicsapproach[3–5,8],andwillnotbediscussedinthepresentpaper.Aircraftstructuresusuallyoperateunderrandomlyvariableloading[15].Todate,formulatingmodelsforpredictingthefatiguelifeofelementsundervariable-amplitudeloading(VAL)isstilltheactivefieldinfatigueresearch[7,9,10,16–18].ItiswellknownthattherearethreegoverningfactorsinthelifepredictionofstructuralmembersunderVAL:thefatiguelifecurveandthecorrespondingexpressionunderconstant-ampli-tudeloading(CAL),theloadspectrumsustainedbythestructureelements,andtheruleforcomputingcumulat-ivefatiguedamage.Actually,thecumulativefatigue752X.Zheng/InternationalJournalofFatigue23(2001)751–766damageruleisusedtoconvertthefatiguedamageunderVALintothatunderCAL.Untilnow,onlyfatiguetestdataor/andthefatiguecurvesunderCALhavebeengiveninmanyhandbooksofthefatigueandmechanicalpropertiesofmetals[19–21],andempiricalfatiguefor-mulaswereappliedinthefatiguedesignofmetallicstructures[22–25].Intherealserviceloadsofengineeringstructures,smallloadsoccupythegreatestportionoftheloadspec-trum[15,24,25].Therefore,howtodistinguishthesmallloadcyclesthatdonotresultinfatiguedamageinmetalsandthenomitthemisveryimportantinloadspectrumcompilationandlifepredictionofstructuremembers,andinfatiguetestsoffull-scalestructuresunderrandomVAL,whicharethemostexpensivetypeoftest[26].However,thecriterionforomittingsmallloadsintheloadspectrumhasnotbeenclearlyexpounded[2],oranempiricalcriterionwasadopted[15,22–25].Thereexistmorethan50modelsforcumulativefatiguedamageandlifeprediction,whichwerereviewedrecently[27].However,noneofabove-mentionedmod-elscanbegenerallyaccepted,andmoreeffortsareneededinordertoprovidegeneralandreliablecumulat-ivefatiguedamageanalysisandlifepredictionmodelstostructuredesigners[27].Inpastfatigueresearch,mostattentionwasfocusedonsearchingfordamagefun