TOFD厚壁压力容器焊接检测超声成像计算机技术.docx

TOFD厚壁压力容器焊接检测超声成像计算机技术.docx

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TOFD厚壁压力容器焊接检测超声成像计算机技术

TOFD-theemergingultrasoniccomputerizedtechnique,forheavywallPressureVesselweldsexamination

TOFD–厚壁压力容器焊接检测超声成像计算机技术

F.Betti,A.Guidi,B.Raffarta-NUMPIGNONE-Massa（Italy）

G.Nardoni,P.Nardoni,D.Nardoni-I&TBrescia（Italy）

L.Nottingham-StructuralIntegrity-U.S.A

Index

▪Abstract

▪FundamentalsoftheTOFDtechnique

▪TOFDnorms

▪TOFDApplications

▪PhysicalprincipleofTOFD

▪Configurationofsystem

▪Scanningplan

▪Scanningdirection

▪Realtimeacquisitionandrecords

▪Imageevaluation

▪Experiencesonexaminationofwelds

▪Conclusions

▪Bibliography

Abstract

ThepaperpresentsTOFD,theTimeofFlightDiffractionTechnique,currentlythemostpromisingultrasonictechniqueforexaminationofheavywallthicknessweldsonPressureVesselsTOFD,isacomputerizedultrasonicsystem,abletoscan,store,andevaluateindicationsintermsofheight（throughthicknessweld）,length,position,withadegreeofaccuracyneverachievedwithotherultrasonictechniques.ArecordofexaminedsectionsisavailableinunprocessedformonfloppydiskorCDandreadableinWordforWindows95.TheASMECode,withCodeCaseno.2235hasacceptedthismethodinlieuofradiographyexaminationforthicknessesover4"

（1）.Experienceson75mmupto300mmthickweldsof2.5/3%Q1Mo,0.25VandCarbonsteelarediscussed.Thetechniquehasbeenappliedonlongitudinal,circumferentialandnozzle-shell/beadwelds.ScanningPlanDataanditsvalidationarereportedwithusedtestblocks.CladdedlayersinTOFDimagesarepresented.Basedonexperiencetodate,prospectsforthisnewtechniqueareofgreatinterest.

FundamentalsoftheTOFDtechnique

TheTOFDtechniquewasfirstappliedin1985attheHarwellCenter（UK）inresponsetoinsistentrequeststosizecracksinnuclearreactorwelds

（2）.

TheTOFDtechniqueisafullycomputerizedsystemabletoscan,store,andevaluateindicationsintermsofheight,length,andpositionwithagradeofaccuracyneverachievedbyotherultrasonictechniques.

TheTOFDtechniqueisbasedondiffractionofultrasonicwavesontipsofdiscontinuities,insteadofgeometricalreflectionontheinterfaceofthediscontinuities.

ThisphenomenamakesTOFDidealforidentifyingcracks,lackoffusionlocatedalongtheverticalaxisoftheweld（inparticularfornarrowgappreparation）orwithanyotherorientations,becausedetectionisnotaffectedbythenegativeconsequenceofultrasonicbeamdeviationfromthereceiverduetounfavourableorientationofthediscontinuity.

ThesefeatureshaveextendedtheuseofTOFDtoreplaceRadiographyandcomplexUltrasonicinspectionbytandemtechniquewhereverplanardefects（cracks,lackoffusion）arethemainobjectofexamination.

ThemaindisadvantagestoboththeaforementionedtechniquesareexpensivemanipulationforRadiographyonbigpartsandtheamountoftimerequiredtoperformthetandemtechniqueduetotheaccuracynecessaryforreliabletesting.

TOFDovercomesbothtechniquesintermsofspeedofexaminationandhigheraccuracy.

TOFDnorms

ExperienceacquiredinapplicationofTOFDhasfullydemonstrateditsreliabilityanddetectioncapacitywithrespecttoothermethods（3）.ThishasinducedpotentialuserstoestablishproceduresforcodingTOFDinrecognizedstandardizednorms.

TheBritishStandardInstitutehasissuedthefirstdraft:

BSNo7706（4）

ASMECodehasincludedinSectionV,art.4,AppendixE,thecomputerizedultrasonicsystemandTOFDtechniqueforultrasonicexaminationofwelds（5）.

An"adhoc"WorkgrouphasbeennominatedfromwithintheASMETechnicalCommitteetoprepareadocumentestablishingtheconditionsforusingcomputerizedsystems,suchasTOFDinreplacingRadiographyexaminationsofwelds,intheover4"thicknessrange（seeCodeCase）.

CEN（comiteEuropeendeNormalisation）hasissuedthedocumentCEN/138/WGonpracticalapplicationofTOFDforultrasonicexaminationofwelds（6）.

TOFDApplications

OneofthemajorapplicationsofTOFDistheultrasonicexaminationofweldsafterfinalheattreatmentand/orhydraulictesting,toverifytheabsenceofcracksnotdetectablebyRadiographyandtoproveconformitywithpriorultrasonicmanualexaminationcarriedoutduringconstruction.

Inadditiontohighersensitivity,theTOFDtechniqueallowsthefullexaminationdatatoberecordedonharddisk,displayingalldiscontinuitiesinC/scanimages.Thisenables"offline"evaluationofindicationsbycomputerusingdedicatedsoftware.Veryaccuratesizingofdefectscanbeachievedandprintedfordocumentation.

AnotheradvantageandcurrentapplicationofTOFDisitsuseinmonitoringweldsduringtheservicelifeofcomponents.Storeddataacquiredfrominitialexaminations,madeduringthefinalstageofconstruction,canbecomparedwithnewdataobtainedfromin-serviceinspection.Thisallowsthestabilityofexistingindicationstobedeterminedwithhighaccuracyandreliability.

ThemoreaccuracyguaranteedbyTOFDinsizingthruwallextentionofflawsallowsmorereliablefracturemechaniccalculationfortheresiduallifeevaluation.

CompanieslikeExxon,Shell,FluorDaniel,Texaco,Chevronetc.areusingTOFDtoreplaceradiographyinexaminationofweldsafterPVMTorhydraulictesting.ThemaininspectionagencieshaveapprovedproceduresoftheTOFDTechniqueanditsvalidation.

PhysicalprincipleofTOFD

Fig1:

TOFDwavespatternandcorrespondingA-Scanpresentation

Fig2:

Compressionalwaves（left）crossingthetipofadiscontinuitygenerateawidebeamofsphericalwaves（right）

Fig.3:

TOFDmapofthecalibrationonASMEtestblockof185mm（7.28inch）withthreeholesof7.5nun（1/3inch.）diameter.TheTOFDmapshowsthethreeholeswithsymmetricalmodeconversionindication.ThestraightlinesatleftandrightoftheTOFDmaparerespectivelythelaterwave（scanningsurface）andtheback-wallwaves（oppositesurface）.Thespacebetweenthesetwolinerepresentsthethicknessoftheweld.

TypicalpatternofwavesinvolvedinTOFDtechnique

FourdifferenttypesofwavesareinvolvedintheconstructionofaTOFDimage:

▪longitudinalwavegeneratedbythetransmitterandpartiallytransformedinsphericalwavewhenthebeamcrossthetipofdefect

▪thelateralwavethatflowsonthesurfacebetweenthetwotransducers

▪thelongitudinalwavereflectedbythebackwall

▪theshearwavesgeneratedbythemodeconversionL/Tontheinterfaceofdiscontinuities

InFigure1TOFDwavespatternandthecorrespondingA-SCANimagearerepresented.

TheFigure2showshowsphericalwavesaregeneratedonthetipsofthecrackswhenincidentlongitudinalwavescrossthetip.

TheFigure3showsatypicalTOFDimageobtainedduringcalibrationofequipmentontwocylindricalholes.

TheFigure4representsthemathematicalandgeometricalmodelonwhichTOFDsoftwareisbased.

MATHEMATICMODELOFTOFD

Fig.4-OnPitagoraprincipleisbasedthesoftwareto

measurethetroughthicknessdimensioninTOFDTechnique.

Configurationofsystem

Fig5:

BlockdiagramofTOFDultrasoniccomputerizedsystem.

Fig5a:

TheTOFDsysteminitspracticalaspect.

TheFigure5representstheblockdiagramofTOFDsystemforexaminationofweld.Thesystemiscomposedby:

▪computerwithdedicatedboards（P/R,A/D）andTOFDsoftware

▪probesassembledonaspecialholdertoallowscanningparallelandtransversaltotheweldsaxis

▪Printertoprinttheresult

ProbesneedspecialcharacteristicstoemitshortpulsestoimproveaccuracymTimeOfFlightmeasurement.Incaseofsmallthicknesseshighfrequencycanbeused,4-8MHz.

Computer'sfeatures

Portablecomputer,Pentium,withminimum3slotfreeforinstalledtheboards,colorscreenactivematrixTFT,Window's95.

Scanningplan

ParametersaffectingtheapplicationofTOFDtechniquearethefollowing:

▪PCS（ProbeCenterSpacing）

▪DZ（DepthZone）foreachrangeofthicknessanappropriatevalueshallbeestablishedandvalidatedontestblock

▪Probecenterline

▪Probefrequency

▪Probeangle

▪Crystaldiameter

▪Testblock

Allthementionedparameters,aftervalidation,arereportedintherelevantSCANNINGPLAN,seeanexampleonFigure6.

SCANPLANTAB.1

Fig.6-ScanPlanindicatingthemainparametersvaluetouseinrelationofthethicknessexamined.

Block

Ident

Thickness

ofweld

Thickness

calibration

block

Calibration

hole

ø（mm）

Calibration

grove

Angle

Beam

DZ

DepthZone

PCS-ProbeCenter

Spacing

BC:

Beam

Centerline

mm

mm

0

Location

depth

degree

mm

1

85

90

2.5

1/4T

1/2T

3/4T

3,28mm

52°

0-90

BC=57mm

PCS=184mm

2

164

170

2.5

1/4T

1/2T

3/4T

3,28mm

52°

35°

0-90

90-170

BC=57mm

PCS=184mm

BC=125mm

PCS=175mm

Scanningdirection

Fig7a:

-Scanningdirection-paralleltotheweldaxis.

Fig:

7b-Scanningdirection-perpendiculartotheweldaxistoimprovesizingaccuracyofindication.

Thescanningdirectionsforexaminationofweldedjointhascarriedoutmovingtheholderparalleltotheweldaxis,seeFigure7a.Incaseofflaws,inordertobettercharacterizetheshapeanditsdimensionanadditionalscanningperpendiculartotheweldaxisshallbeperformed（Figure7b）.

Whentransversalcrackshavetobeinvestigatedascanin90°orientationwithrespecttotheprimaryscanningshallbeapplied

Realtimeacquisitionandrecords

Fig8:

B-ScanimageinrealtimecreateandvisualizeonPCduringthescanningof1meterlengthweldand135mm（5.3inch.）ofthickness.

Alldataofexaminationarecollectedinrealtimebythe