Steel billet inspection using laserEMAT systemWord文档下载推荐.docx

Steel billet inspection using laserEMAT systemWord文档下载推荐.docx

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generateboththesewavemodessimultaneously.

1.Introduction

Electromagneticacoustictransducers（EMATs）canbeusedto

generateanddetectultrasonicwavesinmetalsviaaLorentz

forceormagnetoelasticmechanism[1–4].Whenusedasa

detector,anEMATcanbedesignedtobesensitivetoin-plane

orout-of-planeparticlevelocitybythechoiceoftherelative

orientationofastaticmagneticfieldandtheEMATdetection

coil[2,5].Inordertodetectboththein-planeandout-ofplane

particlevelocityofultrasonicwavesofthesurfaceofa

sample,thecoilispositionedneartheedgeofamagnetwhere

thein-planeandout-of-planecomponentsofmagneticfield

arecomparabletoeachother.AnEMATdetectorcanoperate

withoutcontacttothesampleandcanbeusedinahazardous

environment.

Alaserbeamcanbeusedtogenerateultrasonicwaves

inthermo-elasticregimeforlowopticalenergydensityandin

ablativeregimeforhighopticalenergydensity[6–15].When

alaserworksinthethermo-elasticregime,thedamagecaused

isverylight.Whenalaserworksintheablativeregime,aburnt

spotisgenerallyfoundonthesamplesurface.However,when

theinspectionofcastingsteelbilletisdonebeforetherolling

process,theburntspotwilldisappearafterthesteelbilletis

rolled.Thereisnoworryoftheburntspotremainingonthe

billetsurface.

Alaser-EMATsystemhasbeenreportedforultrasonic

inspections[12–15],whereapermanentmagnetisusedinthe

EMAT.Thetemperatureofthemagnetmustbemaintained

belowtheCuriepointofthemagnet（typicallylessthan300◦C）

andthetemperatureofthecoilmustbeavalue（typicallyinthe

range120–180◦C）topreventdamagetothecoil’sinsulation.

Alaser-EMATthatisappliedintheinspectionofsteelbillet

attemperaturesinexcessof800◦Cthusrequirescooling.

Ultrasonicinspectionofsurfaceandinternaldefectshasthe

advantagesofdefectsizingandlocalization[16–18].

Thereareothernon-ultrasonicmethodsfortheinspection

ofsteelproducts[19–22].Theeddycurrentapproachisbased

onthemeasurementoftheeddycurrentthatisinducedin

theskindepthofametal[19].Itiseffectiveandrelatively

simplebutissubjecttotheskindepth.Inordertodetect

deeperdefect,anexcitationcurrentoflowerfrequencyhas

tobeused,whichlimitstherepeatrateofpulseeddycurrent

andtheinspectionspeed.Anopticalapproachisbasedonthe

temperaturedifferencethatdevelopsacrossthedefectusing

infrareddetectors[20–22].Thedetectionwavelengthwas

optimizedtoimprovetheratioofsignaltonoise.Itisfast

buthaslowresolution.

Thispaperinvestigatesonlineinspectionofsteelbillets

usingthelaser-EMATsystem.Unlikeothersampleswhere

theirsurfacesaresmooth,asteelbillethasreal‘ascast’rough

surfaces.WedisclosethenoveldesignofthedetectionEMAT

thatmakesthisinspectionpossible.Theexactexplanationand

simulationofthepatternofthemode-convertedsignalfromthe

crack-likedefecthasbeengiven.Thispaperisorganizedas

follows.Section2presentslaserultrasonicgenerationandthe

developmentofwater-cooledEMATsasultrasonicdetectors.

Section3reportsresultsoftheonlinemeasurementsoncast

steelbillet,andtheresultsoffiniteelementmethod（FEM）

0022-3727/07/051501+06$30.00©

2007IOPPublishingLtdPrintedintheUK1501

XJianetal

90o

60o

30o

0o

-30o

-60o

-90o

Cwave

Swave

Figure1.Directivityofthebulkshearandcompressionwaves

generatedbyapointlaser.Maximumshearwaveisatanangleof

51◦Ctothesurfacenormal.

simulations.Section4presentsconclusionsrelatedtotheuse

ofthelaser-EMATsystemforsteelbilletinspection.

2.Methods

2.1.Laser-generatedultrasonicfield

Thelaserbeamusedintheseexperimentshadadiameterof

1mmatapulsedenergyof800mJ,whichinteractswithasteel

billetinanablativeregime.Theforcethatisproducedbythe

ablationofthesurfaceofthesteelbilletilluminatedbythe

laserbeamisnormaltothebilletsurfacewithapproximately

Gaussiandistributionintheamplitudeoftheforce.

UsingtheFEM,theultrasonicfieldthatisgeneratedby

suchaforcecanbecalculated[6],whichincludesasurface

Rayleighwaveandbulkcompressionandshearwaves.Fora

steelbilletwithfiniteboundariesvariousreflectionsofthese

ultrasonicwavesmayarriveatthedetectoratapproximatelythe

sametimeandinterferewitheachotherinthedetectedsignal

waveform.Itisofinteresttoknowthedistributionofthese

generatedwavessothatwecanoptimizethemeasurement

setuptoavoidorminimizetheoverlapofthesewaves.

Figure1showsthedirectivityofthebulkcompression

wave（Cwave）andshearwave（Swave）calculatedbyFEM

foranormalablativeforce.Themaximumamplitudeofthe

bulkshearwaveoccursatanangleof51◦tothecentreaxisof

thelaserbeam,andislargerthanthatofthebulkcompression

wave;

thusitisconsistentwithdatapresentedbyprevious

researchers[6].Infigure1,themaximumamplitudeofthebulk

shearwavehasbeennormalizedtounityforbettercomparison.

2.2.DetectionEMAT

Whenanultrasonicwavearrivesatthesurfaceofthesample,

thesurfacevibrates.Suchparticlevibrationscaninducean

eddycurrentintheskindepthofthesamplewhereastatic

magneticfieldB0isheld,whichisgivenby

Jeddy=σv×

B0,

（1）

whereσistheconductivityofthesampleandvistheparticle

velocity.Theinducedtimevaryingeddycurrentwillinturn

produceatimevaryingmagneticfluxthroughdetectionEMAT

coil.Thechangesinmagneticfluxthroughthedetection

EMATcoilwillgenerateavoltageinthecoilwhichisamplified

andmeasured.Whenapermanentmagnetisarrangedto

Watercavity

Coilcavity

CeramicdiscWaterinlet

Wateroutlet

Cable

Fibredisc

Stainless

steelsheet

Figure2.Schematicstructureofthewater-cooledEMATusedin

thispaper.FrontsurfaceoftheEMATiscoveredbyathinmetal

platetoradiatetheincomingheat.

produceanin-planeonlymagneticfield,onlyanout-of-plane

particlevelocitycanbedetected.Whenapermanentmagnet

isarrangedtoproduceanout-of-planemagneticfield,onlyan

in-planeparticlevelocitycanbedetected.Inordertoincrease

thedefectdetectioncapability,theEMATcoilispositioned

closetotheedgeofapermanentmagnetwherethemagnetic

fieldhassignificantmagnitudesofboththein-planeandthe

out-of-planecomponents,andisthereforeabletodetectboth

in-planeandout-of-planeparticlevelocity,whichisassociated

within-planeandout-of-planesurfacedisplacement.

Figure2isaschematicdiagramofawater-cooledEMAT

thathasbeenusedinthemeasurementsdescribedinthispaper.

HeretheEMATholdercontainsamachinedchannelwithin

itswallsforwatercooling.WaterflowsthroughtheEMAT

holdertomaintainthetemperatureofthepermanentmagnet

andcoilaroundthemagnet.Aceramicdiscisbondedtothe

frontsurfaceoftheEMAT,andahightemperaturefibredisc

isplacedontheceramicdisc.Thenthefrontandsideofthe

EMATholderiscoveredbyathinstainlesssteelplatetoreflect

theradiatedheatfromthesteelbilletwhichisverysignificant

atthesetemperatures.Inpracticalapplications,theEMAT

togetherwiththeconnectingcableiscoveredbythermally

insulatingmaterial.

Thelinearcoilisapproximately1mmwideand25mm

long,whichcanbeusedtodetecttheRayleighwaveof

frequencyupto1.5MHzinsteelwherethesensitiveareaof

thedetectorisstilleffectivelypoint-like.

2.3.Experimentalsetup

ANd:

YaglaserisusedforultrasonicgenerationandawatercooledEMAT

isusedforultrasonicdetection.Thepulsedlaser

beamgenerateswidebandultrasonicwavesinthesteelbillet

remotely.Aseparationof135mmbetweenthelaserbeam

anddetectionEMATismaintainedconsistentlythroughout

theexperimentalmeasurement.Thelaserbeamisnormal

incidentonthesurfaceofthebilletandhasabeamdiameter

ofapproximately1mm.ThedetectionEMATisconnectedto

apreamplifierwithagainof40dB.Theairgapbetweenthe

detectionEMATandthesteelbilletisaround1–3mm.

Thesteelbillethasacrosssectionof110mmsquare.In

production,itistypicallycutinto2200mmlengths.Thelaser

anddetectionEMATarefixedrelativetoeachotherwhilethe

steelbilletmovesonrollersasshowninfigure3（a）.Theorigin

ofthecoordinatesystemisatthecrackandthex-axisisinthe

oppositedirectionofthebilletmotion.Figure3（b）showsthe

1502

Laser

Lens

Mirror

EMAT

holder

Billet

Front

view

Billetsidevi