Fundamentals of Acoustic Emission method声发射技术的基本原理

Dr.BorisMuravinAcousticEmissionMethod

History.Fundamentals.Applications.AshortpresentationforstudentsAcousticEmissionphenomena.HistoryofAcousticEmissionfromStoneAgetothesedays.AEinstrumentation:Sensors,preamplifiers,cables(types,specificapplications).DataAcquisitionsystems(analoganddigital,signaldigitation,filtration).PrincipalsofAEdatameasurementandanalysis.Sourcelocation.Attenuation,dispersion,diffractionandscatteringphenomena.AEinmetals.RelationshipbetweenAEandfracturemechanicsparametersandeffectsofAE.AEapplications.InternationalAEstandards.Conclusions.OutlineDefinitionofAcousticEmissionPhenomenonAcousticEmissionisaphenomenonofsoundandultrasoundwaveradiationinmaterialsundergodeformationandfractureprocesses.WhowastheFirst?TheyweretheFirstwhousedAEasanalarmsystemHewastheFirstwhousedAEasaforecastingtoolEarlyHistoryofAEקולזעקהמבבלושברגדולמארץכשדיםירמיהונא,נד“ThesoundofacryfromBabylonandthesoundofgreatfracture<comes>fromthelandoftheChaldeans.”Jeremiah51:54

OneofthefirstsourcesthatassociatessoundwithfracturecanbefoundintheBible.ProbablythefirstpracticaluseofAEwasbypotterymakers,thousandsofyearsbeforerecordedhistory,toassesthequalityofthereproducts.ProbablythefirstobservationofAEinmetalwasduringtwinningofpuretinasearlyas3700B.C.ThefirstdocumentedobservationofAEinMiddleAgeswasmadebyanArabianalchemist,Geber,intheeighthcentury.Geberdescribedthe“harshsoundorcrashingnoise”emittedfromtin.Healsodescribesironas“soundingmuch”duringforging.HistoryofFirstAEExperimentsIn1920,AbramJoffe(Russia)observedthenoisegeneratedbydeformationprocessofSaltandZinccrystals.“ThePhysicsofCrystals”,1928.In1936,FriedrichForsterandErichScheil(Germany)conductedexperimentsthatmeasuredsmallvoltageandresistancevariationscausedbysuddenstrainmovementscausedbymartensitictransformations.In1948,WarrenP.Mason,HerbertJ.McSkiminandWilliamShockley(UnitedStates)suggestedmeasuringAEtoobservethemovingdislocationsbymeansofthestresswavestheygenerated.In1950,D.JMillard(UnitedKingdom)performedtwinningexperimentsonsinglecrystalwiresofcadmium.Thetwinningwasdetectedusingarochellesalttransducer.HistoryofFirstAEExperimentsIn1950,JosefKaiser(Germany)usedtensileteststodeterminethecharacteristicsofAEinengineeringmaterials.Theresultfromhisinvestigationwastheobservationoftheirreversibilityphenomenonthatnowbearshisname,theKaiserEffect.ThefirstextensiveresearchafterKaiserwasdoneintheUnitedStatesbyBradfordH.Schofieldin1954.SchofieldinvestigatedtheapplicationofAEinthefieldofmaterialsengineeringandthesourceofAE.HeconcludedthatAEismainlyavolumeeffectandnotasurfaceeffect.In1957,ClementA.Tatro,afterperformingextensivelaboratorystudies,suggestedtouseAEasamethodtostudytheproblemsofbehaviorofengineeringmetals.HealsoforesawtheuseofAEasanNDTmethod.StartofIndustrialApplicationofAEThefirstAEtestinUSAwasconductedintheAerospaceindustrytoverifytheintegrityofthePolarisrocketmotorfortheU.SNavy(1961).Afternoticingaudiblesoundsduringhydrostatictestingitwasdecidedtotesttherocketusingcontactmicrophones,ataperecorderandsoundlevelanalysisequipment.In1963,DunegansuggestedtheuseofAEforexaminationofhighpressurevessels.Inearly1965,attheNationalReactorTestingStation,researcherswerelookingforaNDTmethodfordetectingthelossofcoolantinanuclearreactor.AcousticEmissionwasappliedsuccessfully.In1969,DuneganfoundedthefirstcompanythatspecializesintheproductionofAEequipment.Today,AENon-DestructiveTestingusedpracticallyinallindustriesaroundtheworldfordifferenttypesofstructuresandmaterials.AcousticEmissionInstrumentationTypicalAEapparatusconsistofthefollowingcomponents:SensorsusedtodetectAEevents.Preamplifiersamplifiesinitialsignal.Typicalamplificationgainis40or60dB.Cablestransfersignalsondistancesupto200mtoAEdevices.Cablesaretypicallyofcoaxialtype.Dataacquisitiondeviceperformsfiltration,signals’parametersevaluation,dataanalysisandcharting.SensorsMainamplifierswithfiltersMeasurementCircuitryPreamplifierswithfiltersComputerAcquisitionsoftwareDatastorageDatapresentationAESensorsPurposeofAEsensorsistodetectstresswavesmotionthatcausealocaldynamicmaterialdisplacementandconvertthisdisplacementtoanelectricalsignal.AEsensorsaretypicallypiezoelectricsensorswithelementsmaidofspecialceramicelementslikeleadzirconatetitanate(PZT).Mechanicalstrainofapiezoelementgeneratesanelectricsignals.Sensorsmayhaveinternallyinstalledpreamplifier(integralsensors).Othertypesofsensorsincludecapacitivetransducers,laserinterferometers.RegularpiezoelectricsensorIntegralpiezoelectricsensorPreamplifier60dBSensorsCharacteristicsTypicalfrequencyrangeinAEapplicationsvariesbetween20kHzand1MHz.Selectionofaspecificsensordependsontheapplicationandtypeofflawstoberevealed.Therearetwoqualitativetypeofsensoraccordingtotheirfrequencyresponds:resonantandwidebandsensors.Thicknessofpiezoelectricelementdefinestheresonancefrequencyofsensor.Diameterdefinestheareaoverwhichthesensoraveragessurfacemotion.AnotherimportantpropertyofAEsensorsincludesCuriePoint,thetemperatureunderwhichpiezoelectricelementlosespermanentlyitspiezoelectricproperties.Curietemperaturevariesfordifferentceramicsfrom120to400C0.Thereareceramicswithover1200C0Curietemperature.AEsignalofleadbreakandcorrespondingPowerspectrum.InstallationofSensorsonStructureTypeofinstallationandchoiceofcouplantmaterialisdefinedbyaspecificsofapplication.Glue(supergluetype)iscommonlyusedforpipinginspections.Magnetsusuallyusedtoholdsensorsonmetalpressurevessels.Greaseandoilthenusedasacouplant.Bandsusedformechanicalattachmentofsensorsinlongtermapplications.Waveguides(weldedormechanicallyattached)usedinhightemperatureapplications.Rollingsensorsareusedforinspectionrotatingstructures.SpecialPbblanketsusedtoprotectsensorsinnuclearindustry.SensorattachedwithmagnetPbblanketinnuclearapplicationsWaveguideRollingsensorproducesbyPACMethodsofAESensorsCalibrationThecalibrationofasensoristhemeasurementofitsvoltageoutputintoanestablishedelectricalloadforagivenmechanicalinput.Calibrationresultscanbeexpressedeitherasfrequencyresponseorasanimpulseresponse.SurfacecalibrationorRayleighcalibration:Thesensorandthesourcearelocatedonthesameplanesurfaceofthetestblock.TheenergyatthesensortravelsattheRayleighspeedandthecalibrationisinfluencedbytheapertureeffect.ApertureEffect:Throughpulsecalibration:Thesensorandthesourcearecoaxiallylocatedonoppositeparallelsurfaces.Allwavemotionisfreeofanyapertureeffect.AEDataAcquisitionDevicesExampleofAEdeviceparameters:16bit,10MHzA/Dconverter.Maximumsignalamplitude100dBAE.4HighPassfiltersforeachchannelwitharangefrom10KHzto200KHz(undersoftwarecontrol).4LowPassfiltersforeachchannelwitharangefrom100KHzto2.1MHz(undersoftwarecontrol).32bitDigitalSignalProcessor.1MbyteDSPandWaveformbuffer.PrincipalsofAEDataMeasurementandAnalysisThresholdandHitDefinitionTime(HDT)Hit1Hit1Hit2ShortHDTLongHDTThresholdLongHDTShortHDTThresholdandHDTareparametersthatusedfordetectionAEsignalsintraditionalAEdevices.HDT:Enablesthesystemtodeterminetheendofahit,closeoutthe

measurementprocessandstorethemeasuredattributesofthesignal.BurstandContinuousAESignalsBurstAEisaqualitativedescriptionofthediscretesignal'srelatedtoindividualemissioneventsoccurringwithinthematerial.ContinuousAEisaqualitativedescriptionofthesustainedsignalproducedbytime-overlappingsignals.“AETestingFundamentals,Equipment,Applications”,H.VallenAEParametersPeakamplitude-ThemaximumofAEsignal.dB=20log10(Vmax/1µvolt)-preamlifiergain

Energy–IntegraloftherectifiedvoltagesignaloverthedurationoftheAEhit.Duration–Thetimefromthefirstthresholdcrossingtotheendofthelastthresholdcrossing.Counts–ThenumberofAEsignalexceedsthreshold.AverageFrequency–DeterminestheaveragefrequencyinkHzovertheentireAEhit.

Risetime-Thetimefromthefirstthresholdcrossingtothemaximumamplitude.Countrate-Numberofcountspertimeunit.BackgroundNoise:SignalsproducedbycausesotherthanacousticemissionandarenotrelevanttothepurposeofthetestTypesofnoise:Hydraulicnoise–Cavitations,turbulentflows,boilingoffluidsandleaks.Mechanicalnoise–Movementofmechanicalpartsincontactwiththestructuree.g.frettingofpressurevesselsagainsttheirsupportscausedbyelasticexpansionunderpressure.Cyclicnoise–Repetitivenoisesuchasthatfromreciprocatingorrotatingmachinery.Electro-magneticnoise.Controlofnoisesources:RiseTimeDiscriminator–Thereissignificantdifferencebetweenrisetimeofmechanicalnoiseandacousticemission.FrequencyDiscriminator–Thefrequencyofmechanicalnoiseisusuallylowerthananacousticemissionburstfromcracks.FloatingThresholdorSmartThreshold–Varieswithtimeasafunctionofnoiseoutput.Usedtodistinguishbetweenthebackgroundnoiseandacousticemissioneventsunderconditionsofhigh,varyingbackgroundnoise.BackgroundNoiseTimeAmplitudeFloatingthresholdMaster–SlaveTechnique–Mastersensoraremountedneartheareaofinterestandaresurroundedbyslaveorguardsensors.Theguardsensorseliminatenoisethataregeneratedfromoutsidetheareaofinterest.

Attenuation,Dispersion,DiffractionandScatteringPhenomenaThefollowingphenomenatakeplaceasAEwavepropagatealongthestructure:Attenuation:ThedecreaseinAEamplitudeasastresswavepropagatealongastructureduetoEnergylossmechanisms,fromdispersion,diffractionorscattering.Dispersion:Aphenomenoncausedbythefrequencydependenceofspeedforwaves.Soundwavesarecomposedofdifferentfrequencieshencethespeedofthewavediffersfordifferentfrequencyspectrums.Diffraction:Thespreadingorbendingofwavespassingthroughanapertureoraroundtheedgeofabarrier.Scattering:Thedispersion,deflectionofwavesencounteringadiscontinuityinthematerialsuchasholes,sharpedges,cracksinclusionsetc….

Attenuationtestshavetobeperformedontheactualstructuresduringtheirinspection.Theattenuationcurvesallowstoestimateamplitudeorenergyofasignalattheatthegiventhedistancefromthesensor.SourceLocationSourceLocationConceptsTimedifferencebasedonthresholdcrossing.Cross-correlationtimedifference.Zonelocation.LinearLocationLinearlocationisatimedifferencemethodcommonlyusedtolocateAEsourceonlinearstructuressuchaspipes.Itisbasedonthearrivaltimedifferencebetweentwosensorsforknownvelocity.Soundvelocityevaluatedbygeneratingsignalsatknowdistances.MaterialEffectivevelocityinathinrod[m/s]Shear[m/s]

Longitudinal[m/s]Brass348020294280Steel347500030895739Aluminum500031296319TwoDimensionalSourceLocationForlocationofanAEsourceonaplanetwosensorsareused.Thesourceissituatedonahyperbola.Sensor1Sensor2Sensor1ZDR3R2R1R1R2R3Sensor2Sensor3Threesensorsareusedtolocateasourcetoapointbyintersectingtwohyperbolaeusingthesametechniqueastwosensors.Ch1Ch2ΔtNormalizedcross-correlationfunctionΔtCross-correlationfunctionCross-correlationbasedLocationCross-correlationmethodistypicallyappliedforlocationofcontinuousAEsignals.ZoneLocationZonelocationisbasedontheprinciplethatthesensorwiththehighestamplitudeorenergyoutputwillbeclosesttothesource.Zonallocationaimstotracethewavestoaspecificzoneorregionaroundasensor.Zonescanbelengths,areasorvolumesdependingonthedimensionsofthearray.Withadditionalsensorsadded,asequenceofsignalscanbedetectedgivingamoreaccurateresultusingtimedifferencesandattenuationcharacteristicsofthewave.AcousticEmissioninMetalsSourcesofAEinMetalsMajormacroscopicsourcesofAEinmetalsare:crackjumps,plasticdeformationdevelopment,fracturingandde-bondingofhardinclusions.Microscopicsourcesincludesdislocationmovement,interaction,annihilation,slipformation,voidsnucleation,growthandinteractionandmanyother.Morethen80%ofenergyexpendedonfractureincommonindustrialmetalsgoestodevelopmentofplasticdeformation.PlasticDeformationPlasticdeformationdevelopmentisaccompaniedbythemotionofalargenumbersofdislocations.Theprocessbywhichplasticdeformationisproducedbydislocationmotionsiscalledslip.Thecrystallographicplanealongwhichthedislocationlinemovesiscalledtheslipplaneandthedirectionofmovementiscalledtheslipdirection.Thecombinationofthetwoistermedtheslipsystem.(1)Themotionofasinglevacancyandasingledislocationemitsasignalofabout0.01-0.05eV.ThebestsensitivityofmodernAEdevicesequals50-100eV.EdgedislocationScrewdislocationMixeddislocationEdgedislocationmotion12345Edgeandscrewarethetwofundamentaltypesofdislocation.(1)MaterialsScienceandEngineeringanIntroduction,WilliamD.Callister,Jr.PhysicalProcessActivationEnergy(eV)Dislocationglide1.2Formationofdislocation8-10PlasticZoneattheCrackTipFlawsinmetalscanberevealedbydetectionofindicationsofplasticdeformationdevelopmentaroundthem.Cracks,inclusions,andotherdiscontinuitiesinmaterialsconcentratestresses.Atthecracktipstressescanexceedyieldstresslevelcausingplasticdeformationdevelopment.ThesizeofaplasticzonecanbeevaluatedusingthestressintensityfactorK,whichisthemeasureofstressmagnitudeatthecracktip.Thecriticalvalueofstressintensityfactor,KICisthematerialpropertycalledfracturetoughness.FractureMechanicsFundamentalsandApplications,SecondEdition,T.LAnderson.FactorsthatTendtoIncreaseorDecreasetheAmplitudeofAENondestructiveTestingHandbook,volume6“AcousticEmissionTesting”,ThirdEdition,ASNT.RelationshipbetweenAEandFractureMechanicsParametersandAEEffectsModelsofAEinMetalsPlasticDeformationModelPlasticdeformationmodelrelatesAEandthestressintensityfactor().AEisproportionaltothesizeoftheplasticdeformationzone.Severalassumptionsaremadeinthismodel:(1)ThematerialgivesthehighestrateofAEwhenitisloadedtotheyieldstrain.(2)Thesizeandshapeoftheplasticzoneaheadofthecrackaredeterminedfromlinearelasticfracturemechanicsconcepts.(3)Strainsatthecracktipvaryatwhereristheradialdistancefromthecracktip.(4)Theassumptionsleadtodevelopmentofthefollowingequationsforthemodel(

)FatigueCrackModelSeveralmodelsweredevelopedtorelateAEcountratewithcrackpropagationrate.Thecombinedcontributionofbothplasticdeformationandfracturemechanismisasfollowsforplasticyielding:AEEffectsKaisereffectistheabsenceofdetectableAEatafixedsensitivitylevel,untilpreviouslyappliedstresslevelsareexceeded.DunegancorollarystatesthatifAEisobservedpriortoapreviousmaximumload,sometypeofnewdamagehasoccurred.Thedunegancorollaryisusedinprooftestingofpressurevessels.

FelicityeffectisthepresenceofAE,detectableatafixedpredeterminedsensitivitylevelatstresslevelsbelowthosepreviouslyapplied.Thefelicityeffectisusedinthetestingoffiberglassvesselsandstoragetanks.Kaisereffect(BCB)

Felicityeffect(DEF)ApplicationsAEInspectionofPressureVesselsAEInspectionofPressureVesselsAETestingofPressureVessels(1)NondestructiveTestingHandbook,volume6“AcousticEmissionTesting”,ThirdEdition,ASNT.PressurePolicyforaNewVessel(1)ExampleofTransducersDistributiononVessel'sSurface(1)TypicalResultsRepresentationofAcousticEmissionTesting(1)

ExampleofPressureVesselEvaluationHistoricindexisaratioofaveragesignalstrengthofthelast20%or200,whicheverisless,ofeventstoaveragesignalstrengthofallevents.(1)NondestructiveTestingHandbook,volume6“AcousticEmissionTesting”,ThirdEdition,ASNT.Thenumbersonplotcorrespondtosensorsnumbers.(1)N–numberofhits,S0i–thesignalstrengthofthei-thevent,J–specificnumberofeventsK=0.8JforJ≤N≤1000andK=N-200forN>1000Severityistheaverageofteneventshavingthelargestnumericalvalueofsignalstrength.AEStandardsAEStandardsASME-AmericanSocietyofMechanicalEngineersAcousticEmissionExaminationofFiber-ReinforcedPlasticVessels,Article11,SubsectionA,SectionV,BoilerandPressureVesselCodeAcousticEmissionExaminationofMetallicVesselsDuringPressureTesting,Article12,SubsectionA,SectionV,BoilerandPressureVesselCodeContinuousAcousticEmissionMonitoring,Article13SectionVASTM-AmericanSocietyforTestingandMaterialsE569-97StandardPracticeforAcousticEmissionMonitoringofStructuresDuringControlledStimulationE650-97StandardGuideforMountingPiezoelectricAcousticEmissionSensorsE749-96StandardPracticeforAcousticEmissionMonitoringDuringContinuousWeldingE750-98StandardPracticeforCharacterizingAcousticEmissionInstrumentationE976-00StandardGuideforDeterminingtheReproducibilityofAcousticEmissionSensorResponseE1067-96StandardPracticeforAcousticEmissionExaminationofFiberglassReinforcedPlasticResin(FRP)Tanks/VesselsE1106-86(1997)StandardMethodforPrimaryCalibrationofAcousticEmissionSensorsE1118-95StandardPracticeforAcousticEmissionExaminationofReinforcedThermosettingResinPipe(RTRP)E1139-97StandardPracticeforContinuousMonitoringofAcousticEmissionfromMetalPressureBoundariesE1211-97StandardPracticeforLeakDetectionandLocationUsingSurface-MountedAcousticEmissionSensorsE1316-00StandardTerminologyforNondestructiveExaminationsE1419-00StandardTestMethodforExaminationofSeamless,Gas-Filled,PressureVesselsUsingAcousticEmissionE1781-98StandardPracticeforSecondaryCalibrationofAcousticEmissionSensorsE1932-97StandardGuideforAcousticEmissionExaminationofSmallPartsE1930-97StandardTestMethodforExaminationofLiquidFilledAtmosphericandLowPressureMetalStorageTanksUsingAcousticEmissionE2075-00StandardPracticeforVerifyingtheConsistencyofAE-SensorResponseUsinganAcrylicRodE2076-00StandardTestMethodforExaminationofFiberglassReinforcedPlasticFanBladesUsingAcousticEmissionAEStandardsASNT-AmericanSocietyforNondestructiveTestingANSI/ASNTCP-189,ASNTStandardforQualificationandCertificationofNondestructiveTestingPersonnel.CARPRecommendedPracticeforAcousticEmissionTestingofPressurizedHighwayTankersMadeofFiberglassreinforcedwithBalsaCores.RecommendedPracticeNo.SNT-TC-1A.AssociationofAmericanRailroadsProcedureforAc