《无损检测专业英语》-ITEM7

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resources.B.Todoagoodjobofdetectionofsafetyprotection.【7.1】IntroductionofEddyCurrentTesting

〖Point1〗EddyCurrentTestingEddycurrenttestingisusedtodetectsurfaceandnear-surfaceflawsinconductivematerials.Thisnon-destructivetestingtechniqueisnon-hazardousandcommonlyusedinindustriessuchasaerospace,rail,automotive,marineandmanufacturing.Oneofthemajoradvantagesofeddycurrenttestingisthatinspectionrequiresminimumpreparationasthereisnoneedtoremovesurfacepaintorcoating.Thismakesitsuitableforinspectingpaintedstructures,partsandcomponents.Eddycurrenttestingequipmentishighlyportable,reliableandcandetectverysmallcracks.Resultsareinstant,idealforon-sitetestingon-siteandplantinspections.Flawscanbereportedimmediatelytositeandoperationmanagers,allowingforquickerdecisionmaking.Inaddition,theportabilityofequipmentmeansthatwecaninspectequipmentorassetsthataredifficulttoaccess,andtestcomplexshapesandsizes.

Eddycurrenttestinguseselectromagneticinductiontodetectdefectsinbothferrousandnon-ferrousmaterialsbyinducinganeddycurrentfieldinthespecimenundertest.Avarietyofinspectionscanbeperformedwitheddycurrenttestinganditistypicallyusedforsurfaceandnear-surfaceflawdetection,metalandcoatingthicknessmeasurement,andmetalsortingbygradeandhardness.

Aspeciallydesignedcoilenergisedwithanalternating-currentisplacedinproximitytothetestsurface,generatingachangingmagneticfieldthatinteractswiththetest-partandproduceseddycurrentsinthevicinity.Variationsinthechangingphasesandmagnitudeoftheseeddycurrentsarethenmonitoredthroughtheuseofareceiver-coilorbymeasuringchangestothealternatecurrentflowingintheprimaryexcitation-coil.Theelectricalconductivityvariations,themagneticpermeabilityofthetest-part,orthepresenceofanydiscontinuities,willcauseachangeintheeddycurrentandacorrespondingchangeinphasesandamplitudeofthemeasuredcurrent.Thechangesareshownonascreenandareinterpretedtoidentifydefects.〖Point2〗Workprocess

Theprocessreliesuponamaterialcharacteristicknownaselectromagneticinduction.Whenanalternatingcurrentispassedthroughaconductor–acoppercoilforexample–analternatingmagneticfieldisdevelopedaroundthecoilandthefieldexpandsandcontractsasthealternatingcurrentrisesandfalls.Ifthecoilisthenbroughtclosetoanotherelectricalconductor,thefluctuatingmagneticfieldsurroundingthecoilpermeatesthematerialand,byLenz’sLaw,inducesaneddycurrenttoflowintheconductor.Thiseddycurrent,inturn,developsitsownmagneticfield.This‘secondary’magneticfieldopposesthe‘primary’magneticfieldandthusaffectsthecurrentandvoltageflowinginthecoil.Anychangesintheconductivityofthematerialbeingexamined,suchasnear-surfacedefectsordifferencesinthickness,willaffectthemagnitudeoftheeddycurrent.Thischangeisdetectedusingeithertheprimarycoilorthesecondarydetectorcoil,formingthebasisoftheeddycurrenttestinginspectiontechnique.Permeabilityistheeaseinwhichamaterialcanbemagnetised.Thegreaterthepermeabilitythesmallerthedepthofpenetration.Non-magneticmetalssuchasausteniticstainlesssteels,aluminiumandcopperhaveverylowpermeability,whereasferriticsteelshaveamagneticpermeabilityseveralhundredtimesgreater.Eddycurrentdensityishigher,anddefectsensitivityisgreatest,atthesurfaceandthisdecreaseswithdepth.Therateofthedecreasedependsonthe“conductivity”and“permeability”ofthemetal.Theconductivityofthematerialaffectsthedepthofpenetration.Thereisagreaterflowofeddycurrentatthesurfaceinhighconductivitymetalsandadecreaseinpenetrationinmetalssuchascopperandaluminium.Thedepthofpenetrationmaybevariedbychangingthefrequencyofthealternationcurrent–thelowerthefrequency,thegreaterdepthofpenetration.Therefore,highfrequenciescanbeusedtodetectnear-surfacedefectsandlow-frequenciestodetectdeeperdefects.Unfortunately,asthefrequencyisdecreasedtogivegreaterpenetration,thedefectdetectionsensitivityisalsoreduced.Thereistherefore,foreachtest,anoptimumfrequencytogivetherequireddepthofpenetrationandsensitivity.〖Point3〗HistoryofEddyCurrentTesting

EddycurrenttestinghasitsoriginswithMichaelFaraday'sdiscoveryofelectromagneticinductionin1831.FaradaywasachemistinEnglandduringtheearly1800'sandiscreditedwiththediscoveryofelectromagneticinduction,electromagneticrotations,themagneto-opticaleffect,diamagnetism,andotherphenomena.In1879,anotherscientistnamedHughesrecordedchangesinthepropertiesofacoilwhenplacedincontactwithmetalsofdifferentconductivityandpermeability.However,itwasnotuntiltheSecondWorldWarthattheseeffectswereputtopracticalusefortestingmaterials.Muchworkwasdoneinthe1950'sand60's,particularlyintheaircraftandnuclearindustries.Eddycurrenttestingisnowawidelyusedandwell-understoodinspectiontechnique.AdvantagesAbletodetectsurfaceandnear-surfacecracksassmallas0.5mmAbletodetectdefectsthroughseverallayers,includingnon-conductivesurfacecoatings,withoutinterferencefromplanardefectsNon-contactmethodmakingitpossibletoinspecthigh-temperaturesurfacesandunderwatersurfacesEffectiveontestobjectswithphysicallycomplexgeometriesProvidesimmediatefeedback〖Point4〗AdvantagesandLimitationsPortableandlightequipmentQuickpreparationtime–surfacesrequirelittlepre-cleaningandcouplantisnotrequiredAbletothemeasureelectricalconductivityoftestobjectsCanbeautomatedforinspectinguniformpartssuchaswheels,boilertubes,oraero-enginedisksLimitationsCanonlybeusedonconductivematerialsThedepthofpenetrationisvariableVerysusceptibletomagneticpermeabilitychanges–makingtestingofweldsinferromagneticmaterialsdifficult–butwithmoderndigitalflawdetectorsandprobedesign,notimpossibleUnabletodetectdefectsthatareparalleltothetestobject’ssurfaceCarefulsignalinterpretationisrequiredtodifferentiatebetweenrelevantandnon-relevantindications〖Point5〗ApplicationsInspectionofpartsorcomponentsincluding:

Weldedjoints

Boresofin-servicetubes

Boresofboltholes

Metaltubes

Frictionstirwelds

Gasturbineblades

Nozzleweldsinnuclearreactors

Hurricanepropellerhubs

Castironbridges

GasturbinebladesDetectionofdefectsincluding:

Surface-breakingdefects

Lineardefects(assmallas0.5mmdeepand5mmlong)

Cracks

Lackoffusion

Generalisedcorrosion(particularlyintheaircraftindustryfortheexaminationofaircraftskins)Otherapplications

Identificationofbothferrousandnon-ferrousmetalsandwithcertainalloys–inparticularthealuminiumalloys

Establishingtheheattreatmentcondition

Determiningwhetheracoatingisnon-conductive

Heattreatverificationofmetals【7.2】Instrumentation

〖Point1〗EddyCurrentInstruments

Fig.7-2-1EddycurrentinstrumentsEddycurrentinstrumentscanbepurchasedinalargevarietyofconfigurations.Bothanaloganddigitalinstrumentsareavailable.Instrumentsarecommonlyclassifiedbythetypeofdisplayusedtopresentthedata.Thecommondisplaytypesareanalogmeter,digitalreadout,impedanceplaneandtimeversussignalamplitude.Someinstrumentsarecapableofpresentingdatainseveraldisplayformats.Themostbasiceddycurrenttestinginstrumentconsistsofanalternatingcurrentsource,acoilofwireconnectedtothissource,andavoltmetertomeasurethevoltagechangeacrossthecoil.Anammetercouldalsobeusedtomeasurethecurrentchangeinthecircuitinsteadofusingthevoltmeter.

Whileitmightactuallybepossibletodetectsometypesofdefectswiththistypeofequipment,mosteddycurrentinstrumentsareabitmoresophisticated.Inthefollowingpages,afewofthemoreimportantaspectsofeddycurrentinstrumentationwillbediscussed.〖Point3〗ResonantCircuitsEddycurrentprobestypicallyhaveafrequencyorarangeoffrequenciesthattheyaredesignedtooperated.Whentheprobeisoperatedoutsideofthisrange,problemswiththedatacanoccur.Whenaprobeisoperatedattoohighofafrequency,resonancecanoccursinthecircuit.Inaparallelcircuitwithresistance(R),inductance(XL)andcapacitance(XC),asthefrequencyincreasesXLdecreasesandXCincrease.ResonanceoccurswhenXLandXCareequalbutoppositeinstrength.Attheresonantfrequency,thetotalimpedanceofthecircuitappearstocomeonlyfromresistancesinceXLandXCcancelout.Everycircuitcontainingcapacitanceandinductancehasaresonantfrequencythatisinverselyproportionaltothesquarerootoftheproductofthecapacitanceandinductance.Ineddycurrentprobesandcables,itiscommonlystatedthatcapacitanceisnegligible.However,evencircuitsnotcontainingdiscreetcomponentsforresistance,capacitance,andinductancecanstillexhibittheireffects.Whentwoconductorsareplacedsidebyside,thereisalwayssomecapacitancebetweenthem.Thus,whenmanyturnsofwireareplacedclosetogetherinacoil,acertainamountofstraycapacitanceisproduced.Additionally,thecableusedtointerconnectpiecesofelectronicequipmentorequipmenttoprobes,oftenhassomecapacitance,aswellas,inductance.Thisstraycapacitanceisusuallyverysmallandinmostcaseshasnosignificanteffect.However,theyarenotnegligibleinsensitivecircuitsandathighfrequenciestheybecomequiteimportant.Theappletbelowrepresentsaneddycurrentprobewithadefaultresonantfrequencyofabout1.0kHz.Anidealprobemightcontainjusttheinductance,butarealisticprobehassomeresistanceandsomecapacitance.Theappletinitiallyshowsasinglecycleofthe1.0kHzcurrentpassingthroughtheinductor.〖Point4〗BridgesThebridgecircuitshownintheappletbelowisknownastheMaxwell-Wienbridge(oftencalledtheMaxwellbridge),andisusedtomeasureunknowninductancesintermsofcalibratedresistanceandcapacitance.Calibration-gradeinductorsaremoredifficulttomanufacturethancapacitorsofsimilarprecision,andsotheuseofasimple"symmetrical"inductancebridgeisnotalwayspractical.Becausethephaseshiftsofinductorsandcapacitorsareexactlyoppositeeachother,acapacitiveimpedancecanbalanceoutaninductiveimpedanceiftheyarelocatedinoppositelegsofabridge,astheyarehere.UnlikethisstraightWienbridge,thebalanceoftheMaxwell-Wienbridgeisindependentofthesourcefrequency.Insomecases,thisbridgecanbemadetobalanceinthepresenceofmixedfrequenciesfromtheACvoltagesource,thelimitingfactorbeingtheinductor'sstabilityoverawidefrequencyrange.〖Point5〗ComplexImpedancePlane:EddyCurrentScopePhaseAngle:Thedifferenceinphasebetweentwosinusoidallyvaryingquantities.CapacitiveReactance:Apropertyofacircuitcontainingcapacitancethattogetherwithanyresistancemakesupitsimpedance.InductiveReactance:Apropertyofacircuitcontaininginductancethattogetherwithanyresistancemakesupitsimpedance.〖Point6〗AnalogMeterFig.7-2-4AnalogMeterAnaloginstrumentsarethesimplestoftheinstrumentsavailableforeddycurrentinspections.Theyareusedforcrackdetection,corrosioninspection,orconductivitytesting.Thesetypesofinstrumentscontainasimplebridgecircuit,whichcomparesabalancingloadtothatmeasuredonthetestspecimen.Ifanychangesinthetestspecimenoccurwhichdeviatefromnormalyouwillseeamovementontheinstrumentsmeter.AnalogmeterssuchastheD'Arsonvaldesignpicturedintheappletbelow,must"rectify"theACintoDC.Thisismosteasilyaccomplishedthroughtheuseofdevicescalleddiodes.Withoutgoingintoelaboratedetailoverhowandwhydiodesworkastheydo,rememberthattheyeachactlikeaone-wayvalveforelectronstoflow.Theyactasaconductorforonepolarityandaninsulatorforanother.Arrangedinabridge,fourdiodeswillservetosteerACthroughthemetermovementinaconstantdirection.Ananalogmetercaneasilymeasurejustafewmicroamperesofcurrentandiswellsuitedforuseinbalancingbridges.【7.3】

Probe/CoilDesign

〖Point1〗ModeofOperationEddycurrentprobesareavailableinalargevarietyofshapesandsizes.Infact,oneofthemajoradvantagesofeddycurrentinspectionisthatprobescanbecustomdesignedforawidevarietyofapplications.Eddycurrentprobesareclassifiedbytheconfigurationandmodeofoperationofthetestcoils.Theconfigurationoftheprobegenerallyreferstothewaythecoilorcoilsarepackagedtobest"couple"tothetestareaofinterest.Anexampleofdifferentconfigurationsofprobeswouldbebobbinprobes,whichareinsertedintoapieceofpipetoinspectfromtheinsideout,versusencirclingprobes,inwhichthecoilorcoilsencirclethepipetoinspectfromtheoutsidein.Themodeofoperationreferstothewaythecoilorcoilsarewiredandinterfacewiththetestequipment.Themodeofoperationofaprobegenerallyfallsintooneoffourcategories:absolute,differential,reflectionandhybrid.Eachoftheseclassificationswillbediscussedinmoredetailbelow.〖Point2〗ConfigurationsAsmentionedonthepreviouspage,eddycurrentprobesareclassifiedbytheconfigurationandmodeofoperationofthetestcoils.Theconfigurationoftheprobegenerallyreferstothewaythecoilorcoilsarepackagedtobest"couple"tothetestareaofinterest.Someofthecommonclassificationsofprobesbasedontheirconfigurationincludesurfaceprobes,boltholeprobes,insidediameter(ID)probes,andoutsidediameter(OD)probes.BoltHoleProbesBoltholeprobesareaspecialtypeofsurfaceprobethatisdesignedtobeusedwithaboltholescanner.Theyhaveasurfacecoilthatismountedinsideahousingthatmatchesthediameteroftheholebeinginspected.Theprobeisinsertedintheholeandthescannerrotatestheprobewithinthehole.〖Point3〗ShieldingandLoading

Oneofthechallengesofperforminganeddycurrentinspectionisgettingsufficienteddycurrentfieldstrengthintheregionofinterestwithinthematerial.Anotherchallengeiskeepingthefieldawayfromnonrelevantfeaturesofthetestcomponent.Theimpedancechangecausedbynonrelevantfeaturescancomplicatetheinterpretationofthesignal.Probeshieldingandloadingaresometimesusedtolimitthespreadandconcentratethemagneticfieldofthecoil.Ofcourse,ifthemagneticfieldisconcentratednearthecoil,theeddycurrentswillalsobeconcentratedinthisarea.〖Point4〗CoilDesignThemostimportantfeatureineddycurrenttestingisthewayinwhichtheeddycurrentsareinducedanddetectedinthematerialundertest.Thisdependsonthedesignoftheprobe.Asdiscussedinthepreviouspages,probescancontainoneormorecoils,acoreandshielding.Allhaveanimportanteffectontheprobe,butthecoilrequiresthemostdesignconsideration.Acoilconsistsofalengthofwirewoundinahelicalmanneraroundthelengthofaformer.Themainpurposeoftheformeristoprovideasufficientamountofrigidityinthecoiltopreventdistortion.Formersusedforcoilswithdiametersgreaterthanafewmillimeters(i.e.encirclingandpancakecoils),generallytaketheformoftubesorringsmadefromdielectricmaterials.Small-diametercoilsareusuallywounddirectlyontoasolidformer.Theregioninsidetheformeriscalledthecore,whichcanconsistofeitherasolidmaterialorjustair.Whenthecoreisairoranonconductivematerial,theprobeisoftenreferredtoasanair-coreprobe.Somecoilsarewoundaroundaferritecorewhichconcentratesthethecoil'smagneticfieldintoasmallerarea.Thesecoilsarereferredtoas"loaded"coils.Thewireusedinaneddycurrentprobeistypicallymadefromcopperorothernonferrousmetaltoavoidmagnetichysteresiseffects.Thewindingusuallyhasmorethanonelayersoastoincreasethevalueofinductanceforagivenlengthofcoil.Thehighertheinductance(L)ofacoil,atagivenfrequency,thegreaterthesensitivityofeddycurrenttesting.Itisessentialthatthecurrentthroughthecoilisaslowaspossible.Toohighacurrentmayproduce: ariseintemperature,henceanexpansionofthecoil,whichincreasesthevalueofL. magnetichysteresis,whichissmallbutdetectablewhenaferritecoreisused.〖Point5〗ImpedanceMatchingEddycurrenttestingrequiresustodeterminethecomponentsoftheimpedanceofthedetectingcoilorthepotentialdifferenceacrossit.Mostapplicationsrequirethedeterminationonlyofchangesinimpedance,whichcanbemeasuredwithahighdegreeofsensitivityusinganACbridge.TheprinciplesofoperationofthemostcommonlyusededdycurrentinstrumentsarebasedonMaxwell'sinductancebridge,inwhichthecomponentsoftheimpedanceofthedetectingcoil,commonlycalledaprobe,arecomparedwithknownvariableimpedancesconnectedinseriesandformingthebalancingarmofthebridge.ReferbacktoBridges.TheinputtothebridgeisanACoscillator,oftenvariableinbothfrequencyandamplitude.Thedetectorarmtakestheformofeitherameterorastoragecathode-rayoscilloscope,aphase-sensitivedetector,arectifiertoprovideasteadyindication,andusuallyanattenuatortoconfinetheoutputindicationwithinaconvenientrange.Storagefacilitiesarenecessaryintheoscilloscopeinordertoretainthesignalfromthedetectorforreferenceduringscanningwiththeprobe.Thehighestsensitivityofdetectionisachievedbyproperlymatchingtheimpedanceoftheprobetotheimpedanceofthemeasuringinstrument.Thus,withabridgecircuitthatisinitiallybalanced,asubsequentbutusuallysmallvariationintheimpedanceoftheprobeupsetsthebalance,andapotentialdifferenceappearsacrossthedetectorarmofthebridge.【7.4】

ProcedureIssues

Ineddycurrenttesting,theuseofreferencestandardsinsettinguptheequipmentisparticularlyimportantsincesignalsareaffectedbymanydifferentvariablesandslightchangesinequipmentsetupcandrasticallyaltertheappearanceofasignal.AswithmostotherNDTmethods,themostusefulinformationisobtainedwhencomparingtheresultsfromanunknownobjecttoresultsfromasimilarobjectwithwellcharacterizedfeaturesanddefects.Inalmostallcases,eddycurrentinspectionproceduresrequiretheequipmenttobeconfiguredusingreferencestandards.Forcrackdetection,corrosionthinningandothermaterialdamage,referencestandardsareusedtosetuptheequipmenttoproducearecognizablesignalorsetofsignalsfromadefectorsetofdefects.Inmanycases,theappearanceofatestsignalcanberelatedtotheappearanceofasignalfromaknowndefectonthereferencestandardtoestimatethesizeofadefectinthetestcomponent.Signalsthatvarysignificantlyfromtheresponsesproducedbythereferencestandardmustbefurtherinvestigatedtothedeterminethesourceofthesignal.Thereferencestandardshouldbeofthesamematerialasthetestarticle.Ifthisisnotpossibleorpractical,itshouldbeofmaterialthathasthesameelectricalconductivityandmagneticpermeability.Componentfeatures(materialthickness,geometry,etc.)shouldbethesameinthereferencestandardasthoseinthetestregionofinterest.Ifthereferencestandardisthetypewithintentionaldefects,thesedefectsshouldbeasrepresentativeofactualdefectsinthetestcomponentaspossible.Thecloserthereferencestandardistotheactualtestcomponent,thebetter.However,sincecracksandcorrosiondamageareoftendifficultandcostlytoproduce,artificialdefectsarecommonlyused.Narrownotchesproducedwithelectrondischargemachining(EDM)andsawcutsarecommonlyusedtorepresentcracks,anddrilledholesareoftenusedtosimulatecorrosionpitting.〖Point2〗SignalFilteringSignalfilteringisoftenusedineddycurrenttestingtoeliminateunwantedfrequenciesfromthereceiversignal.Whilethecorrectfiltersettingscansignificantlyimprovethevisibilityofadefectsignal,incorrectsettingscandistortthesignalpresentationandeveneliminatethedefectsignalcompletely.Therefore,itisimportanttounderstandtheconceptofsignalfiltering.〖Point3〗PhaseLagPhaselagisaparameteroftheeddycurrentsignalthatmakesitpossibletoobtaininformationaboutthedepthofadefectwithinamaterial.Phaselagistheshiftintimebetweentheeddycurrentresponsefromadisruptiononthesurfaceandadisruptionatsomedistancebelowthesurface.Thegenerationofeddycurrentscanbethoughtofasatimedependentprocess,meaningthattheeddycurrentsbelowthesurfacetakealittlelongertoformthanthoseatthesurface.Disruptionsintheeddycurrentsawayfromthesurfacewillproducemorephaselagthandisruptionsnearthesurface.Boththesignalvoltageandcurrentwillhavethisphaseshiftorlagwithdepth,whichisdifferentfromthephaseanglediscussedearlier.(Withthephaseangle,thecurrentshiftedwithrespecttothevoltage.)【7.5】

Applications

〖Point1〗SurfaceBreakingCrac