Download - Ultrasonic Testing and Image Processing for in-progress Weld Inspection 123

NDTnetApril1996,Vol.1No.04

Ultrasonictestingandimageprocessingforinprogressweldinspection*

byShaunLawson

Download:ut_tofd.exe88kbyte

Abstract

Ultrasonicmethodsarewidelyusedinthenondestructivetestingofallmannerofweldedjoints.However,duetothepracticaldifficultiesinvolvedintheNDTofincompleteorhotwelds,muchofthistestingisundertakenafterajointiscompleted.Alternatively,ifdefectscouldbedetectedbetweenweldrunsthenpotentiallylargecostsavingswouldresultfromreducedwastageofmaterialsandgouging/repairtimes.Inaddition,iftheNDTresultscouldbeinterpretedfastenough(e.g.byanautomatedsystem)thenweldprocesscontrolmayalsobepossible.Thisshortartcicledescribessomeoftheproblemstobeencountredwhenusingpulseechomethodsforflawdetectioninincompletewelds.Asanalternative,TimeofFlightDiffractionmethodsareproposedandimageprocessingtechniquesforautomaticdefectdetectionaredescribed.

TableofcontentsIntroductionaresumeofultrasonicweldinspectiononthispageSemiautomatedpulseechoscanningand'inprogress'inspectionTimeofflightdiffraction(TOFD)techniqueSummaryReferences

Introductionaresumeofultrasonicweldinspection

Ultrasonictestingisroutinelyusedforimagingofsubjectsinboththemedicalandindustrialfields.Thehistoryofthedevelopmentofthetechniqueshowsthatindustrialusageprecedesmedicalpractice(Webb,1988):thefirstNDTpulseechoexaminationsweremadepossibleduetotheelectronicadvancesmadeduringtheSecondWorldWar,whilsttheresultsofthefirstexaminationsoflivingtissuewerenotpublisheduntil1950(Wild,1950).

Ultrasoundundergoesrefractionandreflectionatinterfacesbetweenmediaofdifferentacousticrefractiveindices.Henceacrack,whichhasalowerdensity,andhencealowerrefractiveindex,fromthesurroundingweldmaterialwillrefractandreflectincidentultrasound,whilsttheinterfacebetweentheweldandtheparentmetal,ifcorrectlyfused,shouldallowwavepropagationwithminimalinterference.NearlyallultrasonicNDTusesthe'pulseecho'methodofdetectingflawsinspecimens,ignoringtheloweramplituderefractedsignalsandconcentratingononlythereflectedenergy.Thebasicprincipleistomoveasmallultrasonicprobeortransduceracrossthesurfaceoftheparentplateadjacenttotheweld.

Probeswithdifferentbeamanglescanbeusedtodetectplanardefectsatvaryinganglesoforientation.Adefectorientednormaltoabeamanglewillproduceasignificantdetectorsignal,whereaswithabeamangle20or30degreesawayfromthenormal,thereflectedsignalmaybeverysmall,ornonexistent.ThemaximumamplitudeofthedeflectedsignalisonlyreachedwhenthedefectsurfaceareainthebeamequalsthebeamareaHencethesizeofthereflectedsignalcanonlygiveanpoorindicationoftheactualsizeofthedefect.

Aswellaspossiblereflectionsfromdefectsignalsintheweldregion,largeechoesarereceivedfromthedifferentfacesoftheweldedcomponentsgeometryparticularlyfromthesurface(pluscouplant)andthebackwall.ThisisillustratedinFig.1whichshowstheexpectedseriesofAscansasa60probeismovedoverasteelblockcontainingacracklikedefect.Inposition(a)onlythesurfaceandbackwallechoesarevisible.Astheprobemovesovertheedgeofthedefectthenthedefectsignalbecomesvisiblehoweverastheprobebeammovesentirelyoverthedefectthebackwallechomaybeblockedcompletely,asshowninposition(c).Inpractice,reflectionsoffthebackwallmayusedinasuchawayastoprovideenhancedinspectioncoverage.

MostultrasonictestingisundertakenmanuallywithanNDTinspectormovingaprobeoverthesurfaceofthecomponentinarasterfashionbetweenthefullandhalfskipdistancesalongthelengthoftheweld,whilstobservingthereflectedsignalonaflawdetectoroscilloscope.Clearlyforlargecomponents,thisproceduremaybelengthy,tediousandsubjecttomanualerrorhence,wherepossible,automaticscanningtechniques,arebeingusedwithincreasingregularity.

Author

ShaunLawson

ShaunLawsonhasworkedinthefieldofcomputervisioninautomaticinspectionfor5years.Duringthistimehehasworkedbothinindustry(forRoyalOrdnanceplc,investigatingtheautomationofindustrialradiographicinspectioncycles)andinacademia.HeiscurrentlyemployedasaresearchofficerattheUniversityofSurreyintheUKwhereheisabouttosubmithisPhDonimagesegmentationandneuralnetworksappliedtonondestructivetesting.Email:[email protected]:http://robots.surrey.ac.uk/People/mes3sl/mes3sl.html*ThisworkisfundedbytheEuropeanCommissionundertheBRITEEURAMIIprogramme(projectno.5907).TheBRITEEURAMprojectNDTMethodsforFlawDetectionDuringWelding

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RolfDiederichs1.April.1996,[email protected]/DB:Article/AU:Lawson_S/IN:SurreyUni/CN:UK/CT:UT/CT:TOFD/CT:weld/ED:199604