Air-coupledUltrasonicInfraredThermographyforInspectingImpactDamages ChenDapeng1*,ZhiZeng2,CunlinZhang3,NingTao3,Zheng1Schoolofmaterialscienceandengineering,BeihangUniversity,Beijing,No.37XueyuanRoadHaidianDistrict,Beijing100191,2InstituteofPhysicsandElectronicEngineering,ChongqingNormalUniversity,No.12TianchenRoad,ShabaDistrict,Chongqing400047,3BeijingKeyLaboratoryfor hertzSpectroscopyandImaging，KeyLaboratoryof No.105,XisanhuanNorthRoad,HaidianDistrict,Beijing100048,*Correspondingauthor::UltrasonicthermographyorthermosonicshasbeenprovedaneffectiveNDTmethodforinspectingCFRPcomposites;however,thepotentialdamagesforthestructurecan’tbeignored,becauseofthecontactvibrationbetweentheultrasonichornandthespecimen.Thisworkaimsatdeveloanewexcitationmethodforultrasonicthermography—air-coupledultrasonicexcitation.CFRminateswithimpactdamagesaretestedbyair-coupledultrasonicthermography,andthetheoreticalmodelofheatconductionisgiven.ResultsdemonstrategoodexcitationperformanceforimpactdamagesdetectioninCFRPcomposites.Moreover,theconventionalultrasonicthermographyresultsareshownascomparison,andtheprospectofair-coupledultrasonicthermographyisdiscussed.OCIScodes:040.6808,100.2960,Keywords:air-coupled,ultrasonicthermograhy,impactdamage, 12號400047；驗室，市海淀區(qū)西三環(huán)北路105號，100048OCIScodes:040.6808,100.2960,Inrecentdecade,ultrasonicthermographyorthermosonicshasbeenprovedaneffectiveNDT/Emethod.Itutilizesapulseof20kHz–40kHzultrasoundtocausethedefectinterfacestoclaporrub,theheatingisthenobservedbyaninfraredcamera.Subsurfacedefectsbecomevisiblewithtimedelaysthataredeterminedbydiffusionofheatfromthedefectstosurface.Earlystudiesaremoreconcernedonmetaldefectsdetection.InWayneStateUniversity,L.D.Fayro,XiaoyanHanandZhiZengetal.didagoodjobaboutthedetectionforverticalmicrometalcracks[1,2],andproposedtheacousticchaostheoryinthermosonics[3].In ,StateKeyLaboratoryofModernAcoustics,NanjingUniversity,appliedultrasonicthermographytodetectfatiguecracksinaluminumalloy,anddidnumericalsimulationstudyabouttheheatgenerationinRecentyears,thistechniqueisgraduallyappliedforcompositestesting.InBritain,UniversityofBath,longpulseandlowpowerultrasoundisusedtodetectimpactdamageFRPcomposites,trytowhilsteliminatingdamageattheexciterattaentpoint[5].InGermany,UniversityofStuttgart,frequencymodulatedultrasonicexcitationisusedtodetectcorrosiondefectsinsandwichstructuresandimpactdamagesinlaminates[6,7].AndinAmerica,XiaoyanHanetal.usethethermosonicstodetectsub-surfacefatiguecracksingraphite/epoxycomposites,anddevelopedfiniteelementmodelfortransientheattransferysisofthefrictionalheatingintheHowever,thepotentialdamagesforcompositescannotbeignored.Thereasonisthatultrasonichornhastobemechanicallyontactwiththestructurebypressingthespecimenwitharelativelylargepressure,andthecontacthighfrequencyvibrationcouldcausedamagetothespecimeninsomedegrees.Anotherproblemisthatthesignalofdefectswouldbeconfusedbythefrictionalheatingattheexcitationlocationandthemovementofthespecimencausedbytheimpactingofultrasonichorn,maybebringdifficultiesaboutthethermalsignalreconstruction.Inthisstudy,anewexcitationmethod,air-coupledultrasonicexcitation,forinfraredthermographyispresented.Thistechniqueutilizedanon-contactultrasonicexcitation(20KHz)asaheatsource,withaninfrareddetectorlocatedinthevicinityofthespecimentomonitorthesurfacetemperaturevariationanddistribution.CFRminateswithimpactdamagesaretestedbyair-coupledultrasonicthermography,andtheapproximatetheoreticalmodelofheatconductionisgiven.Atpresent,thepoweroftheultrasonicenergyisrelativelylowinthefollowingexperiment,however,theimpactdamageFRPcompositeisastonishinglyrevealedinthethermalimages.Moreover,theconventionalultrasonicthermographyresultsareshownascomparison,andtheprospectofair-coupledultrasonicthermographyisdiscussed.Itisshownthatair-coupledultrasonicexcitationthermographyprovidedapotentialtechniqueforNDT/E.Fig.1Sketapofair-coupledultrasonicThe ipleandexperimentalsetupofair-coupledultrasonicthermographyisshowninFig.1,speciallymanufacturedultrasonichornconnectedwithapiezoelectrictransducerisusedasaheatgenerator.Typically,theultrasonichornispositionedacertaindistance(Integralmultiplesofultrasoundhalf-wavelengthintheair)fromthesamplesothatthecouplingofultrasoundtothesamplecanbeenhanced.Ifthedistancefromtheultrasonichorntothesampleischanged,thesignalofdefectsmaybevariedsignificantly.Theultrasoundvibrationforseveralsecondscausedtheinterfacesofthedefectstoclaporrub,thesurfacetemperaturerisingisdelayedbythermaldiffusionfromsubsurfacedefects.Theimpactdamagedefectcanbeseenasacontinuousheatsource.Thegeneralheatdiffusionequationiswrittenas[9,10,11]:[T(r,t)]cT(r,t)f(r,

whereT(r,t)(K)isthetemperatureatpositionrandtimet,f(r,t)istheheatsourcefunctionwhichgiveshowtheheatingisappliedtoamedium,(W/mK)isthethermal(kg/m3)ismaterialdensityandc(J/kgK)isspecificheat.Theratioofthethermalconductivitytothevolumetricheatcapacityisdefinedasthethermaldiffusivity(m2/s)

Tosolvetheequation(1)acrosstheinterfacweentwomaterialsZ=d,twogeneralboundaryconditionsmustbesatisfiedatalltime.Theyaretemperaturecontinuityboundary zTheassumptionisthatthematerialisisotropicandhomogeneous,andacontinuousheatsourceisappliedatt=0,z=0,ofconstantstrengthqandradiusr0.SothesolutionforEq.(1)atr=0,z=d,cangivenT

q/(r2)

)]exp( d 2 4(tt'2Fig.2showstheexperimentalsetupofair-coupledultrasonicthermography,amanufacturedultrasonichorn(23cm×16cm×11cm)whichismadeofaluminumalloysisapplied.Inthefollowingexperiment,distancweenthehornandspecimenisapproximay8.5cm.Thepositionofthespecimenisnotcritical,foritcanbecedunderorbesidethehorn. Fig.2Experimentalsetupforair-coupledultrasonicoutputpoweroftheultrasonicgeneratorisafewhundredsofwatt.Theelasticwaveisgeneratedbyultrasonictransducerandpropagatesintheairwheremechanicalvibrationiscoupledintothespecimen.TheIRcamerahasasensitivewaverangeof3.7m-4.8m,andatemperatureresolutionof0.02℃.ResultsandSpecimenFig.3showsthephotoofCFRPboards;size89mm×55mm×1.5mm.ThecenteroftheFig.3photoofCFRPCFRPboardsareimpactedbydifferentenergy(2J、3J、4J、5J)tocausedifferentdegreesofFig.4showsthethermalimagesobtainedbyseveralsecondsofair-coupledultrasonicexcitaion.Impactdamagesofdifferentdegreesareheatedbytheair-coupledvibration,andrevealedinthethermalimages.Itisfoundthatspecimenwithhigherdamagedegreeshasalargerheatingarea.Fig.4Resultsofair-coupledultrasonicFig.5showsthetemperaturevs.timecurveforthecenterofthedefects,itisfoundthatFig.5Temperaturevs.timecurveofaircoupledultrasonicspecimenwithhigherimpactdamagedegreehasahigherpeaktemperatureriseundertheeffectofair-coupledultrasonicvibration.Ascomparison,Fig.6showstheresultsofconventionalultrasonicpulseexcitation.Theimpactdamagesarerevealedbytheultrasonicpulseexcitation,butmeanwhile,agreatheatinggeneratedbetweenthecontactpointofultrasonichornandthespecimen.Thoughthesignalvs.noiseismuchhigherthanair-coupledexcitation,itmaycausenewdamagesonthesurfaceattheexcitationpoint.Theresultsalsoshowthatspecimenwithhigherdamagedegreeshasalargerheatingarea.Fig.6ResultsofconventionalultrasonicThetemperaturevs.timecurveforconventionalultrasonicthermographyatthecenterofthedefectsisalsoshowninFig.7.AscomparedwithFig.5,itisfoundthatthepeaktemperatureriseofcontactultrasonicexcitationisaboutseveraltimeshigherthanthatofair-coupledultrasonicFig.7Temperaturevs.timecurveofconventionalultrasonicthermograpghyAsthecomparisonshown,theconventionalultrasonicthermographyseemstobterthanair-coupledultrasonicthermography,asthesignalvs.noiseofthethermalimagesandthepeaktemperatureriseareconsidered.Themainreasonisthattheexcitationpowerismuchlowerinair-coupledultrasonicthermographyduetoinefficiencycouplingofultrasoundbetweenthehornandair.Furtherworkshouldconsiderimprovingthepoweroftheultrasoundexcitation,andoptimizingthestructureoftheultrasonichorn.Theultrasoundimpedancematchingbetweenthehornandairshouldalsobeconsidered,maybeaddsomecouplingmaterialstoofthehorn.Inthisstudy,anewNDTmethod,air-coupledultrasonicthermographyisintroduced.FourCFRPlaminateswithimpactdamagesaretestedbythistechnique,withthetheoreticalmodelofheatconductiondescribed.ResultsshowefficientexcitationforimpactdamageFRP.Ascomparedwiththeconventionalultrasonicexcitationresults,itisfoundthatconventionalultrasonicthermographyhasabettersignalvs.noiselevelandapeaktemperaturerise,butmaycausenewdamagesonthesurfaceattheexcitationpoint.Furtherworkaboutair-coupledultrasonicthermographycanfocusonthestructureoptimizingoftheultrasonichornandimprovingtheimpedancematchingbetweenthehornandairbyaddingsomecouplingmaterials.ProjectsupportedbytheJointFundsoftheNationalNaturalScienceFoundationof(GrantNo.61079020).L.D.Favro,X.Y.Han,andO.Y.Zhong,Sonicinfraredimagingoffatiguecracks,Int.J.Fatigue.23,4(2001).X.Y.Han,Z.Zeng,W.Li,J.P.Lu,andV.Loggins,Acousticchaosforenhanceddetectabilityofcracksbysonicinfraredimaging,J.Appl.Phy.95,7(2004).X.Y.Han,W.Li,Z.Zeng,L.D.Favro,andR.L.Thomas,Acousticchaosandsonicinfraredimaging,Appl.Phys.Lett.81,3(2002).L.Pengcheng,M.Xiaobing,andZ.Shuyi,FEMysisofTransientTemperatureFieldsSpecimenswithDefectsduringUltrasonicPulseExcitation,JournalofNanjingUniversity,41,9(2005).T.J.Barden,D.P.Alomd,S.G.Pickering,M.Morbidini,andP.Cawly,DetectionofimpactdamageFRPcompositesbythermosonics,NDT&E,22,2(2007). A.Dillenz,“Ultrasoundlockinthermography:feasibilitiesandlimitations”,inProceedingsofSPIE,theInternationalSocietyforOpticalEngineering,SocietyofPhoto-OpticalInstrumentationEngineers,ed.(Academic,Bellingham,WA,USA,1999),p.10-15.G..Riegert,“AcousticLock-inThermographyofImperf