2024年4月22日发(作者：省略号的作用)

ResNondestrEval(1996)8:83–100

©1996Springer-VerlagNewYorkInc.

EffectofStressConcentrationonMagneticFluxLeakage

SignalsfromBlind-HoleDefectsinStressedPipelineSteel

,,,son,,on

DepartmentofPhysics,Queen’sUniversity,Kingston,Ontario,K7L3N6,Canada

-dependentmagneticfluxleakage(MFL)signalsofthenormalsurfacecompo-

nent(radial)ifferent

stressrigswithuniaxialstressandfieldconfie-peakfeatureinthe

MFLsignalwasdefinedquantitativelybyasaddleamplitude,whichwastakenasthedifference

sindicated

thatthesaddleamplitudeincreasedlinearlywithincreasingtensilesurfacestressanddecreased,

ordidnotexist,ess-dependentsaddleamplitude

-elementcalculationsindicatedthat

surementsandanaly-

sisdemonstratethatthestress-dependentsaddleamplitudebehaviorintheradialMFLsignalis

associatedwithsurface-stressconcentrationsneartheblind-holedefects.

Introduction

Magneticfluxleakage(MFL)techniquesarecommonlyusedforthein-lineinspection

ofpipelinesformetallossdefectssuchascorrosionpits[1].Thein-serviceoperating

pressuresofgaspipelinesgeneratelargecircumferentialstressesthatmayreach70%of

n-servicestressesaffectthefluxleakagepatterns

andhavebeenstudiedpreviously[2]–[7].Inthepresenceofstress,defectsactas“stress

raisers”[8].Dependentuponthedefectdepth[9],thedefectsmaygeneratestresscon-

raisingarounddefects

alsomayleadtoenhancedstresscorrosioncracking[10].

Therearetwoeffectsthatmaycontributetothegenerationofthestress-dependent

MFLsignal:1)thebulkeffectofstressonthemagneticproperties[11]–[16]and2)the

effectofthedefectasastressraiserthatisalsodependentonthedepthofthedefect

[9].Metallossresultingfromincreasingdefectdepthincreasesthelevelofmagnetic

saturationinthevicinityofthedefectand,therefore,rly,

byaffectingthestress-dependentmagneticpropertiesofthesteelinthevicinityofthe

defect,theapplicationofabulkstressalsoaffectsthepeak-to-peakMFL(MFL

pp

)

apreviousconsideration[17],underabendingstressthetwo-dimensionalsolutionfor

a100%through-walldefectorholegeneratesapeakstresslevelattheedgeofthe

84Krauseetal.

holethatis2.4timesthatofthenominalbackgroundstress[8,17].Finite-element

calculationsandstressmeasurements[17]indicatethat,forthesamebendingstress,the

stressconcentrationforaround-bottomedpitthatis50%ofthethrough-wallthickness

ateunderuniformtensilestress,themaximum

stressattheedgeofafullcylindricalthrough-holeisthreetimesthatofthenominalstress

[8,18].Stressconcentrationsoccuratthetwoedgesofthedefectthataretangentialto

theappliedstressdirection.

AnincreaseinthepipewallfluxdensitytypicallyresultsinanincreaseintheMFL

ectofstresson

theMFL

pp

signalhasbeenshowntoincreaseforincreasingfluxdensitiesintherangeof

0.65to1.24T[9,13]–[16].Itisexpected,therefore,thatstressconcentrationcombined

withincreasingfluxdensitymaysimilarlyaffecttheMFLsignal.

Observationsofadouble-peakfeaturethatincreasesinamplitudewithincreasing

appliedtensilestresshavebeenmadefornormal-surfacecomponent(radial)MFLsignals

forvariousuniaxialorientationsofstressandfieldappliedtopipelinesteel[5,11].In

particular,theamplitudeofthedouble-peakfeature(hereafterreferredtoasthesaddle

amplitude)hasbeenobservedtoincreaselinearlywithincreasinglevelsofappliedstress

andhasbeenassociatedwithstresspatternsaroundthedefectitself[12,14].Inthis

r,itisdemonstrated

thatthedouble-peakfeatureintheMFLsignalmaybeassociatedprimarilywithstress

concentrationsthatappearinthevicinityofthedefectnearthesurfaceofthesteelpipeline

sample,andalsothatthestressconcentrationandresultantsaddleamplitudeintheMFL

signalincreasewithincreasingdefectdepth.

ExperimentalApparatus

Theexperimentalapparatusisdescribedindetailelsewhere[11,12].Theapparatusused

tomeasuretheradialcomponentofthefluxleakagefieldfromadefectonthesame

sideofthesampleasthemeasuringapparatus(nearside)consistedofaHallprobe,an

amplifiertoamplifytheHallsignal,ialflux

leakagesignalwasmeasuredatscannedpositionssetat1-mmintervals(0.5mmforthe

semicircularpipesection)ialfluxleakagesignal

wastakenastheaverageof100measurementstakenateachposition.

PipelineSampleandStressingApparatus

Samplesofpipelinesteelusedinthisstudywerecutfroma610-mm(24-in.)diameter

firstsampleusedwasa102-mm(4-in.)wide

amplesusedwere4.27-m

longaxialstripsthatwerealso102mm(4in.)elinesteelcompositionis

givenelsewhere[17].

firstapparatusisthesemi-

ondandthirdapparatususethe

single-stripbeam-bendingarrangementandthecompositebeam-bendingarrangement,