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Radiographic and ultrasonic weld inspectionEstablishing weld integrity without destroying the component

By Tony Anderson

December 13, 2001

This article outlines the differences in radiographic and ultrasonic weld inspection, thetwo most common methods if nondestructive testing. It gives an overview of bothmethods, including how they are used.

Radiographicand ultrasonic weldinspectionare the two most commonnondestructive testing(NDT) methods used todetect discontinuities within the internal structure ofwelds. The obviousadvantage ofbothofthese testing methodsis theirability tohelp establish the weld'sinternalintegrity without destroying the weldedcomponent.

Radiographic Testing

Radiographic testing (RT) usually issuitable for testing welded joints that canbe accessedfrom

bothsides, with the exceptionofdouble-wall signal image techniques usedonsome pipe.Although thisisaslow and expensive NDT method,it isadependable way todetect porosity,inclusions,cracks,andvoidsin weldinteriors.

RT makes use ofX-raysorgammarays. X-raysare producedby an X-ray tube,andgammaraysare producedby aradioactive isotope. The basicprinciple ofradiographic weldinspectionis the same as that ofmedical radiography. Penetratingradiationispassed throughasolidobject (in thiscase,a weldratherthanpart ofthe humanbody) ontophotographicfilm,creatinganimage ofthe object'sinternal structure on the film.

The amount ofenergy absorbedby the object dependsonits thicknessanddensity. Energy notabsorbedby the object causes exposure ofthe radiographicfilm. These areas will be dark whenthe film isdeveloped. Areasofthe film exposed to less energy remain lighter. Therefore,areasof the object where the thicknesshasbeenchangedby discontinuities,suchasporosity orcracks, will appearasdarkoutlineson the film. Inclusionsof low density,suchasslag, willappearasdarkareason the film, while inclusionsofhighdensity,suchas tungsten, will appearas light areas.

All discontinuitiesare detectedby viewing the weldshape andvariationsin the density oftheprocessedfilm. Thispermanent film recordofweldquality isrelatively easy tointerpret ifpersonnel are properly trained. Only qualifiedpersonnel shouldconduct radiography andradiographicinterpretationbecause false readingscanbe expensive andcaninterfere seriouslywithproductivity,andbecause invisible X-ray andgammaradiationcanbe hazardous.

Ultrasonic Testing

Ultrasonic testing (UT) canbe usedonferrousandnonferrous materialsandoftenissuitedfortesting thickersectionsaccessible from one side only. Ingeneral,it candetect finer linearorplanardefects thancanRT.

UT makes use ofmechanical vibrationssimilartosound wavesbut ofhigherfrequency. A beamofultrasonic energy isdirectedinto the object tobe tested. Thisbeam travels through the objectwithinsignificant energy loss, except whenit isinterceptedandreflectedby adiscontinuity.

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The ultrasoniccontact pulse reflection technique is usedin UT. Thissystem usesa transducer,whichconverts electrical energy into mechanical energy. The transduceris excitedby ahigh-frequency voltage that causesacrystal tovibrate mechanically. The crystal probe becomes thesource ofultrasonic mechanical vibration. These vibrationsare transmittedinto the test piecethroughacouplingfluid, usually afilm ofoil,calledacouplant.

When the ultrasonic wavespulse strikesadiscontinuity in the test piece,it isreflectedback toitspoint oforigin. Thus, the energy returns to the transducer. The transducernow servesasareceiverfor the reflected energy.

The initial signal,ormainbang; the returned echoesfrom the discontinuities;and the echoofthe rearsurface ofthe test piece all are displayedby a trace on the screenofacathode-rayoscilloscope. The detection, location,and evaluationofdiscontinuitiesbecome possiblebecause the velocity ofsound througha material isnearly constant, makingdistancemeasurement possible,and the relative amplitude ofareflectedpulse is more or lessproportional to the size ofthe reflector.

One ofthe most useful characteristicsofUT isitsability todetermine the exact positionofadiscontinuity ina weld. This testing methodrequiresahigh level ofoperatortrainingand

competence anddependson establishingandapplyingsuitable testingprocedures.