在这一区域,α与f2.成正比。
17.这些数据主要从单晶材料获得,同时它对多晶合金材料还是有效的,由于混乱,有一个额外的衰减影响,这最终有可能涉及超声衰减测量疲劳强度的属性。
至少,超声衰减可用于检测材料性能,但目前似乎有超声波应力波相互作用,材料微观结构和金属机械失效方面的理论不足,。
18.已经有超声衰减和某些钢的抗冲击强度,衰减和断裂韧性,以及纵波速度与抗拉强度的铸铁相关的经验。
使用单个不锈钢样本的断裂韧性会随着温度的变化而变化,这听上去超声衰减和断裂韧性
的相关性是不好的,断裂韧性
的变化远远大于衰减的变化。
原因被认为是在其他材料性能的变化,如屈服应力,也会影响断裂韧性
,而超声衰减,主要取决于晶粒的大小。
3.11应力测量
19.超声速度测量可用于测量金属应力。
这项技术应用并不容易,如超声波速度随压力变化很小,并且不同的材料也不一样,但超声脉冲准确上升时间的能力使得该技术技术可行,通过几个实际应用,例如钢螺栓和残余应力测量中的轴向载荷的测量。
超声速度也随温度变化,所以试样温度必须准确控制。
然后技术的真正的限制是,在很多材料超声脉冲变成了扭曲的,所以测量的精度降低。
20,避免此问题的技术是通过改变频率相位差的常数来测量双颜色之间的相位差。
例如使用小试样,用水浴,然后接收脉冲前后表面的重叠。
21.应力的存在旋转极化平面的极化剪切波,并且旋转角度和压力的大小有相关性。
因此这种旋转测量可用于测量平均量超声波束在固体材料内部的应力。
22.测量残余应力的更先进的技术是使用剪切波在两个相互垂直的方向上极化。
这些波速度相近,所以发生干涉,因此随着传感器旋转,当偏振平面平行、垂直轴压力,干扰消失了。
一旦这个轴是已知的,就可以从速度计算实际的压力。
23.瑞利表面波声弹效应也被用于测量在不锈钢管道的残余应力。
使用急救医疗助理传感器,以消除耦合问题和提高可重复性。
使用1.5兆赫的频率和校准曲线上开发的测试样本,然后管道是使用应变计来测量。
在零压力的瑞利波速度是未知的,是微观结构变化的影响,所以主要的使用则相对而非绝对测量技术。
3.12临界角反射率
24.如果从液/固界面的反射率来衡量,有一个非常尖锐的最小反射率在瑞利临界角(图3.6)。
从这个角度的知识,瑞利和Lamb波的速度可以确定,最低的形状是受固体表面层的衰减特性,或固体的表面层的接口属性,或接口之间两种不同材料的属性的影响。
超声波测向仪进行测量和大部分的金属临界角的值可以确定±1分钟,从而使表面波速度可以测量到±1ms-1。
因此,可将该技术用于测量灰铸铁抗拉强度,措施对奥氏体钢冷作模具的条件,研究热处理或纹理,并衡量然后厚度薄表层,如胶结厚度。
25.最小反射的超声波频率和超声衰减,晶粒尺寸有关,但关系极为复杂,
至今还没有发现哪种实用的应用来进行该测量。
图3.6临界角反射率水不锈钢界面显示一个尖锐的最低反射
原文:
3.7Acousticholography
Opticalholographyisnowwellknownasamethodofstoringthree-dimensionaldataonatwo-dimensionalrecordingsurface,andasanultrasonicpulseisaformofcoherentradiation,itispossibletoapplysimilartechniquestoultrasound.Itmust,however,berememberedthatultrasoundasusedinNDThasalongwavelength,ofthesamemagnitudeasthesizeofthedefectstobedetected.Theultrasonicaperturesizeisalsoonlyafewwavelengths,solongitudinalimageresolutionislikelytobepoor.。
Theprincipleoflinearacousticholographyisthattheultrasonictransducerisscannedalongalineandproducesawidesoundfieldintheplaneofthescanline.Thereceivedechoesaremultipliedwithtworeferencesignals,one
and
shiftedagainstthetransmittedpulse,soproducingarealandimaginarypartofthereceivedechoatanumberofsamplingpointsalongtheaperture.Thedataisstoredinacomputerandbyusingquadraticphasefunctions;thecomputerdefinesareconstructionplaneandanobservationangle,andcalculatestheultrasoundfieldintensityattheregionofadefect.Fromtheprofileofthisultrasoundintensityfield,thedefectsizecanbederived.Themaindisadvantageofacousticholographyisthepoorresolutioninthedirectionoftheultrasoundfield,butthiscanbeovercometosomeextentwithafocusingprobe,orbymulti-frequencyholography,orbysyntheticaperturefocusingtechniques.--
Earlyworkonacousticholographyusedanimmersionsystemwithtwoultrasonictransducerswithoverlappingultrasoundfieldsatthespecimen,onetransducerproducingthereferencebeam;theinterferingfieldsproducedastaticsurfacedisplacementontheliquidsurface,whichwasopticallyilluminatedwithalaserbeamtoproduceahologramofthespecimen.Later,anelectronicreferencebeamwasused,butboththeresystemsappeartohavebeensupersededbycomputerreconstructiontechniqueswithsyntheticaperturescanning.
3.8Syntheticaperturefocusingtechnique(SAFT)
Variousmethodshavebeenproposedforcollectingandprocessingdatafromascannedtransducer,soastosimulateamuchlargertransducerhavingmuchbetterresolutionandsignal-to-noiseratio.SAFT,appliedtoanultrasonictransducer,isderivedinprinciplefromsyntheticapertureradarsystems.Intheory,itispossibletotakeintoaccountirregularrefractiveinterfacesinthespecimenandcomplexangularscatteringbydefects.
SAFTusespulsedultrasound,recordsbothwaveamplitudeandphase,anddoesnotneedareferencefunction.Thedatafromaseriesofprobepositionsisstored,afterdigitization(Fig.3.5).Thetimeofflightfromtheprobetothedefect,fordifferentprobepositions,iscorrectedbyatime-basedalgorithm;depthoffieldeffectscanbeovercomebyafocusingalgorithmandtheunevennatureofthematerialinterfacesbyasurface—phase-correctionalgorithm;noiseisreducerbyaveragingaseriesofpulsesfromeachprobeposition.Thepracticalaperturesize;whichisthesizeofthetheoreticallargetransducer,cannotofcoursebeinfinitelylarge,astheultrasonicbeamfromtheprobepositionedattheextremitiesoftheaperturemuststillreachtheflaw.
ThemainadvantageofSAFToveracousticholographyappearstobetheimprovementwhichispossibleinlongitudinalresolution,althoughthisisobtainedonlybycomplexcomputerprograms.
3.9Ultrasonicimagingsystem
7.Anultrasonic′image′ofaspecimencanbeobtainedbytheC-scandisplayalreadydescribed,byscanningaprobeoverthesurface,usuallyinanimmersiontank,onaregularraster.However,suchmechanicalscanningisslow,andthetransverseresolutionisseverelylimitedbythebeamdiameter.Ifafocusedultrasonicprobeisused,thefieldwidthissmaller,andtransverseresolutionisimproved,butmechanicalscanningtimeswillbestilllonger.Electronicscanningisneeded,togetherwithamulti-elementlineararrayoftransducers,tospeeduptheprocess.
8.Sometime-delaysmustbebuiltintothetransducerelementssothatasignalfromonepointinthespecimenreachesalltheelementsatthesamemoment.Suchmulti-arraysystemshavefoundwidespreaduseforultrasonicimaginginthemedicalfield,butnotyetforindustrialapplications.Thescanningcanbemechanicalorelectronicusingasteeredarray(phasedarray).
9.Inaphasedarray,theelementspacingmustbelessthanhalfthewavelengthtoavoidspurious
Fig.3.705Syntheticaperturefocusingtechnique(SAFT):
onlythreeprobepositionsareshownforsimplicityafterdigitizationandtime-of–flightcorrection,thesignalsareintegrated
diffractionlobes,sotherearedifficultfabricationproblems;however,thereisalsoausefulflexibilitybetweenframerate,linedensity,andfieldofview.Systemsworkingbetween3and15MHzarenowinuse.
10.Awholerangeofequipmenthasbeenproposedfordirectultrasonicimaginginmetals,fromtelevision-typecameratubeswithafaceplatesensitivetoultrasound,throughtheuseofultrasoniclenses,tocomputer-reconstructedimages.Ultrasoniclensescanbemadeofsolidmaterials,orwithliquidsinsideathin,shapedskin.Lensdesignisanalogoustoopticallensdesign,withappropriatevaluesofrefractiveindices,basedontheultrasonicvelocitiesinthedifferentmaterials.
11.Severalmethodshavebeenusedtomakeaspatialultrasonicimagevisible,asfollows.
(1)liquidsurfacelevitation(asmentionedbrieflyforacousticholographysystems).
(2)Elasto-opticaleffects:
insuitabletransparentsolids,polarizedlightcanmakeultrasonicwavevisible,bybecomingopticallydoublerefractingunderstress.
(3)Schlierenmethods:
thechangeinopticalrefractiveindex,whenatransparentmaterialissubjectedtoapressurechange,canbeusedtomakevisibleanultrasonicfield.
(4)Nematicliquidcrystalscanbestimulatedbyanelectricfieldtobecometurbidandscatterlight,andultrasonicwavescanproducethesameeffect.
(5)Diffractioneffectsfromtheinteractionofalightbeamandanultrasonicbeaminaliquidthelattercanberegardedasanopticalgratingwithchangesinrefractiveindexalongthegrating.IftheBrag