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外文翻译--铝合金随机荷载作用下的疲劳性能 英文版【优秀】.pdf

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外文翻译--铝合金随机荷载作用下的疲劳性能 英文版【优秀】.pdf

TheFatigueofAluminumAlloysSubjectedtoRandomLoadingExperimentalinvestigationisundertakenbytheauthorstodeterminethefatiguelifeof2024T3and6061T6aluminumalloysG.W.BrownandR.IkegamiABSTRACTThispaperdescribesanexperimentalinvestigationwhichwascarriedouttodeterminethefatiguelifeoftwoaluminumalloys2024T3and6061T6.Theyweresubjectedtobothconstantstrainamplitudesinusoidalandnarrowbandrandomstrainamplitudefatigueloadings.ThefatiguelifevaluesobtainedfromthenarrowbandrandomtestingwerecomparedwiththeoreticalpredictionsbasedonMinerslinearaccumulationofdamagehypothesis.Cantileverbeamtestspecimensfabricatedfromthealuminumalloysweresubjectedtoeitheraconstantstrainamplitudesinusoidaloranarrowbandrandombaseexcitationbymeansofanelectromagneticvibrationsexciter.ItwasfoundthattheeNcurvesforbothalloyscouldbeapproximatedbythreestraightlinesegmentsinthelow,intermediateandhighcyclefatigueliferanges.MinershypothesiswasusedtopredictthenarrowbandrandomfatiguelivesofmaterialswiththistypeofNbehavior.Thesefatiguelifepredictionswerefoundtoconsistentlyoverestimatetheactualfatiguelivesbyafactorof2or3.However,theshapeofthepredictedfatiguelifecurvesandthehighcyclefatiguebehaviorofbothmaterialswerefoundtobeingoodagreementwiththeexperimentalresults.Symbolsbl,bconstantskl,k2constantsNnumberofcyclestofailureSstressSyieldstress1,N1referencepointoneNdiagramestraineyieldstraineppeakstraineendurancelimitstrainstrainhardeningexponentG.W.BrownisProfessorofMechanicalEngineering,UniversityofCalifornia,Berkeley,Calif.R.IkegamiisResearchEngineer,StructuralDynamicsGroup,BoeingAircraft,Seattle,Wash.Paperwaspresentedat1970SESASpringMeetingheldinHuntsville,Ala.onMay1922.rstrainvarianceSymbolsnotshownherearedefinedinthetext.IntroductionTheproblemofpredictingthefatiguelivesofmetalstructureswhicharesubjectedtorandomloadingsisgenerallysolvedbyfirstformulatinganaccumulationofdamagecriteria,thenapplyingthiscriteriatothespecifiedconditionsofvaryingcyclicloadamplitude.ThefirstandstillthemostcommonlyusedcriterionforpredictingtheaccumulationofdamageinfatiguewasproposedbyA.PalmgrenandappliedbyM.A.Miner.1ThiscriterionassumesthattheproblemofaccumulationofdamagemaybetreatedasoneinwhichthefractionsoffatiguelifeusedupatdifferentloadlevelsasdeterminedfromtheconstantamplitudeeNcurvemaybesimplyaddedtogiveanindexofthefatiguedamageandisgenerallyknownasMinerslinearaccumulationofdamagecriteria.Thispaperdescribesaportionoftheresultsofastudy2whichwasconductedtodetermineamethodofpredictingthefatiguelivesofalumimumalloystructureswhichweresubjectedtonarrowbandrandomloadings.Anexperimentalprogramwascarriedouttodeterminethelivesofcantileverbeamtestspecimenswhichweresubjectedtoeitherconstantstrainamplitudesinusoidalornarrowbandrandomstrainamplitudefatigueloadings.ThefatiguelivesofthetestspecimenssubjectedtotherandomloadingswerethencomparedtopredictionsbasedontheapplicationofMinerscriteria.ExperimentalProgramThefatiguetestswereperformedontwocommonlyusedaluminumalloys,2024T3and6061T6.ThemechanicalpropertiesofthesetwoalloysareExperimentalMechanics\321u10.310.2TRUESTRAIN\Lin/in101Fig.1Truestressvs.truestrainfor2024T3aluminum0.5a.BeamConfigurationb.NormalizedModeShape0.5O0.5STRAINGAGEACCEL.III10003_§iirTr10210ITRUESTRAINin/inIFig.2Truestressvs.truestrainfor6061T6aluminum0.5NormalizedBendingStress1/stMode0.51Fig.3VibrationcharacteristicsoffatiguespecimengiveninTable1,andthetruestressvs.truestraincurvesareshowninFigs.1and2.Thestrainhardeningexponent,,characterizesthestressstrainrelationshipintheplasticrange.Thesepropertiesweredeterminedfromuniaxialtensiletestsusingtensionspecimensmadefromthetwoalloys.Inbothcases,thetensionspecimensandthefatiguespecimensweremachinedfromthesamesheetsofaluminum,withthelongitudinalaxesofthespecimensparalleltothedirectionofrolling.Thiswasdonetoinsureuniformitybetweenthesetwotypesoftests.Themajorityofthefatiguetestswasdoneonanelectromagneticvibrationsexciter,withasmallportionofthelowcycleconstantstrainamplitudetestsbeingperformedonanInstrontester.Forthetestingwhichwasperformedonthevibrationsexciter,thefatiguetestmodelwasacantileverTABLE1MATERIALPROPERTIESMaterialTrueTrueStrainElasticYieldfracturefracturehardenmodulus,stress,stress,strain,ingexpsipsipsiin./in,ponent2024T3i0.6X10651,00090,7000.2400.1476061T610.6X10840,50062,0000.4400.0875beamsubjectedtoabaseexcitation,asshownschematicallyinFig.3a.Forthenarrowbandrandomfatiguetests,theexcitationwasanarrowbandsignalwithaGaussianbaseaccelerationamplitudeanduniformspectrumoveraspecifiedfrequencybandwidth.Theexcitationbandwascenteredatthefundamentalbeamresonance.Thesinusoidalfatiguetestswereperformedwiththeexcitationfrequencyslightlyabovethefundamentalbeamresonance.Thetypeofcyclicloadingwasthereforecompletelyreversedbending.Thecantileverspecimenswereprofiledalongthelengthofthebeamtomovethemaximumbendingstressinthefirstmodeofvibrationawayfromthefixedend.AsketchofthetestspecimenconfigurationisshowninFig.4.Thetestspecimenswereclampedinthemiddlebyamountingfixturewhichwasattacheddirectlytothearmatureofthevibrationsexciter.Ascanbeseenfromthefigure,eachfatiguetestspecimencontainedtwocantileverbeamspecimenswhichwereexcitedsimultaneously.Anendmass,intheformofanEndevcoModel2216crystalaccelerometer,wasattachedatthefreeendofthecantileverbeamspecimens.ThemodeshapesandcorrespondingbendingstressdistributionsforthefirsttwomodesofvibrationareshowninFigs.3band3c.Atthefundamentalbeamresonance,themaximumbendingstressoccurredat322IAugust1970iCLAMPEDoej1IztrFig.4Fatiguetestspecimenp,fc._EzrFig.5Fatiguespecimenonvibrationexciteradistanceof7/8in.fromthefixedendofthebeam.Thefrequencyofthefirstbeamresonancewasapproximately115cps.Thespecimenswerecarefullyhandpolishedpriortotestingtoremoveanysharpcornersandtoeliminateallvisiblesurfacescratchesintheregioninwhichthemaximumstressoccurred.Tomeasurethestrainlevelduringthefatiguetests,straingagesweremountedoneveryspecimenatthepointwherethemaximumbendingstressoccurred.Itwasfoundthatthefatiguelifeofthestraingageinstallationwasgenerallymuchsmallerthanthefatiguelifeofthespecimen.Forthisreason,thesignalfromtheaccelerometermountedatthefreeendofthebeamwasusedtodeterminethetimetofailureofthefatiguespecimens.Thesignalfromtheaccelerometerwasusedtotriggerarelaywhichdeactivatedatimerwhentheaccelerationleveldroppedto50percentofthenominalRMSaccelerationlevel.Itwasobservedthat,atfailure,theaccelerationleveldroppedveryrapidlysothatthetimersindicatedverycloselythetotaltimetofailureofthespecimen.ThepictureinFig.5showsafatiguetestspecimenmountedonthevibrationsexciterjustpriortotesting.Ascanbeseenfromthisfigure,specialaccelerometercableswereconstructedbysplicingstandardMicrodotaccelerometercableswithtwosmaller,moreflexible,leadwires.Thiswasdonetominimizetheeffectofthevibrationsoftheaccelerometercableonthespecimen.Althoughthespliceincreasedthenoisepickup,thesignallevelwassolargethatthisincreaseinnoisewasnotnoticeable.6080100FREQUENCYHzFig.6Strainspectraldensity120140160Fig.7FatiguespecimeninInstrontesterThesignalsfromthestraingagesandtheaccelerometersweremonitoredduringthetestsandrecordedonmagnetictape.Aftereachtest,therecordedsignalswereplayedbackintoawaveExperimentalMechanicsI323analyzersystemtodeterminetheRMSlevels.Adigitalcomputerwasusedtoperformatimeseriesanalysisoftherandomsignalsobtainedfromthenarrowbandrandomfatiguetests.AstrainspectraldensityplotofthestraingageresponseduringatypicalnarrowbandrandomtestisshowninFig.6.Asexpected,thisplotindicatesthatthefatiguespecimencanbeconsideredtobeaverylightlydamped,singledegreeoffreedomsystem.Themostprobablefrequencyofvibrationofthisnarrowbandresponsecanbeshowntobetheresonancefrequencyofthesystem.Therefore,thetotalnumberofcyclestofailurei.e.,thetotalnumberofzerocrossingswithpositiveslopewasassumedtobethetotaltimetofailureinsecondsmultipliedbytheresonancefrequencyincyclespersecond.Intheconstantamplitude,sinusoidalteststhetotalnumberofcyclestofailurewasmerelythetotaltimetofailureinsecondsmultipliedbytheexcitationfrequencyincyclespersecond.Forthetestsperformedonthevibrationsexciter,therangeofthetotalcyclestofailurewasfrom210ato510cycles.Thecorrespondingstrainlevelsrangedfrom600to3500microstrainRMSforthenarrowbandrandomfatiguetests,andfrom1700to7000microstrainfortheconstantamplitudetesting.Duetothefastrateofcyclingonthevibrationsexciter,itwasnotpossibletoobtainfatiguedataforlifevaluesbelowapproximately2103cyclesusingthetestsetupdescribedabove.Forthisreason,somelowcycle,constantstrainamplitudefatiguetestingwasperformedonanInstrontester.Aknowledgeofthelowcycle,constantamplitude.....__,987104I0sCYCLESTOFAILUREConstantAmplitudeResultsNarrowBandResultsSNDiagramMinersPrediction_L_i..lOsID71oFig.3Fatiguetestresultsfor2024T3aluminumoE..Jn9\.\___9ConstantAmplitudeResults.....SNDiagram104105l0s1071DsCYCLESTOFAILUREFig.9Fatiguetestresultsfor6061T6aluminum324\August1970

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