Ultrasonic Testing and Image Processing for in-progress Weld Inspection 123
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NDTnet - April 1996, Vol.1 No.04 Ultrasonic testing and image processing for in-progress weld inspection * by Shaun Lawson © Download: ut_tofd.exe 88 kbyte Abstract Ultrasonic methods are widely used in the non-destructive testing of all manner of welded joints. However, due to the practical difficulties involved in the NDT of incomplete or hot welds, much of this testing is undertaken after a joint is completed. Alternatively, if defects could be detected between weld runs then potentially large cost savings would result from reduced wastage of materials and gouging/repair times. In addition, if the NDT results could be interpreted fast enough (e.g. by an automated system) then weld process control may also be possible. This short artcicle describes some of the problems to be encountred when using pulse echo methods for flaw de tection in incomplete welds. As an alternative, Time of Flight Diffraction methods are proposed and image processing techniques for automatic defect detection are described. Table of contents Introduction - a resume of ultrasonic weld inspection on this page Semi-automated pulse echo scanning and 'in-progress' inspection Time-of-flight diffraction (TOFD) technique Summary References Introduction - a resume of ultrasonic weld inspection Ultrasonic testing is routinely used for imaging of subjects in both the medical and industrial fields. The history of the development of the technique shows that industrial usage precedes medical practice (Webb, 1988): the first NDT pulse echo examinations were made possible due to the electronic advances made during the Second World War, whilst the results of the first examinations of living tissue were not published until 1950 (Wild, 1950). Ultrasound undergoes refraction and reflection at interfaces between media of different acoustic refractive indices. Hence a crack, which has a lower density, and hence a lower refractive index, from the surrounding weld material will refract and reflect incident ultrasound, whilst the interface between the weld and the parent metal, if correctly fused, should allow wave propagation with minimal interference. Nearly all ultrasonic NDT uses the 'pulse-echo' method of detecting flaws in specimens, ignoring the lower amplitude refracted signals and concentrating on only the reflected energy. The basic principle is to move a small ultrasonic probe or transducer across the surface of the parent plate adjacent to the weld.
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Transcript of Ultrasonic Testing and Image Processing for in-progress Weld Inspection 123
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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.
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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
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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
