ASTM E1774-1996(2007) Standard Guide for Electromagnetic Acoustic Transducers (EMATs)《电磁声学传感器用标准指南》.pdf

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1、Designation: E 1774 96 (Reapproved 2007)Standard Guide forElectromagnetic Acoustic Transducers (EMATs)1This standard is issued under the fixed designation E 1774; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last rev

2、ision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONGeneralThe usefulness of ultrasonic techniques is well established in the literature of nonde-structive examination. The gene

3、ration of ultrasonic waves is achieved primarily by means of someform of electromechanical conversion, usually the piezoelectric effect. This highly efficient method ofgenerating ultrasonic waves has a disadvantage in that a fluid is generally required for mechanicalcoupling of the sound into the ma

4、terial being examined. The use of a couplant generally requires thatthe material being examined be either immersed in a fluid or covered with a thin layer of fluid.PrincipleAn electromagnetic acoustic transducer (EMAT) generates and receives ultrasonicwaves without the need to contact the material i

5、n which the acoustic waves are traveling. The use ofan EMAT requires that the material to be examined be electrically conductive or ferromagnetic, orboth. The EMAT as a generator of ultrasonic waves is basically a coil of wire, excited by an alternatingelectric current, placed in a uniform magnetic

6、field near the surface of an electrically conductive orferromagnetic material.Asurface current is induced in the material by transformer action. This surfacecurrent in the presence of a magnetic field experiences Lorentz forces that produce oscillating stresswaves. Upon reception of an ultrasonic wa

7、ve, the surface of the conductor oscillates in the presenceof a magnetic field, thus inducing a voltage in the coil. The transduction process occurs within anelectromagnetic skin depth. An EMAT forms the basis for a very reproducible noncontact system forgenerating and detecting ultrasonic waves.1.

8、Scope1.1 This guide is intended primarily for tutorial purposes. Itprovides an overview of the general principles governing theoperation and use of electromagnetic acoustic transducers(EMATs) for ultrasonic examination.1.2 This guide describes a non-contact technique for cou-pling ultrasonic energy

9、into an electrically conductive orferromagnetic material, or both, through the use of electromag-netic fields. This guide describes the theory of operation andbasic design considerations as well as the advantages andlimitations of the technique.1.3 This guide is intended to serve as a general refere

10、nce toassist in determining the usefulness of EMATs for a givenapplication as well as provide fundamental information regard-ing their design and operation. This guide provides guidancefor the generation of longitudinal, shear, Rayleigh, and Lambwave modes using EMATs.1.4 This guide does not contain

11、 detailed procedures for theuse of EMATs in any specific applications; nor does it promotethe use of EMATs without thorough testing prior to their usefor examination purposes. Some applications in which EMATshave been applied successfully are outlined in Section 9.1.5 This standard does not purport

12、to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 127 P

13、ractice for Fabricating and Checking AluminumAlloy Ultrasonic Standard Reference BlocksE 428 Practice for Fabrication and Control of Metal, Otherthan Aluminum Reference, Blocks Used in UltrasonicExaminationE 1065 Guide for Evaluating Characteristics of UltrasonicSearch UnitsE 1316 Terminology for No

14、ndestructive Examinations2.2 ASNT Document:1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.06 on UltrasonicMethod.Current edition approved July 1, 2007. Published July 2007. Originally approvedin 1995. Last p

15、revious edition approved in 2002 as E 1774 - 96(2002).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyri

16、ght ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.SNT-TC-1A Recommended Practice for Personnel Qualifi-cations and Certification in Nondestructive Testing33. Terminology3.1 DefinitionsRelated terminology is defined in Termi-nology E 1316.3.2

17、Definitions of Terms Specific to This Standard:3.2.1 electromagnetic acoustic transducer (EMAT)anelectromagnetic device for converting electrical energy intoacoustical energy in the presence of a magnetic field.3.2.2 Lorentz forcesforces applied to electric currentswhen placed in a magnetic field. L

18、orentz forces are perpen-dicular to the direction of both the magnetic field and thecurrent direction. Lorentz forces are the forces behind theprinciple of electric motors.3.2.3 magnetostrictive forcesforces arising from magneticdomain wall movements within a magnetic material duringmagnetization.3.

19、2.4 meander coilan EMAT coil consisting of periodic,winding, non-intersecting, and usually evenly-spaced conduc-tors.3.2.5 pancake coil (spiral)an EMAT coil consisting ofspirally-wound, usually evenly-spaced conductors.3.2.6 bulk wavean ultrasonic wave, either longitudinal orshear mode, used in nond

20、estructive testing to interrogate thevolume of a material.4. Significance and Use4.1 GeneralUltrasonic testing is a widely used nonde-structive method for the examination of a material. Themajority of ultrasonic examinations are performed using trans-ducers that directly convert electrical energy in

21、to acousticenergy through the use of piezoelectric crystals. This guidedescribes an alternate technique in which electromagneticenergy is used to produce acoustic energy inside an electricallyconductive or ferromagnetic material. EMATs have uniquecharacteristics when compared to conventional piezoel

22、ectricultrasonic search units, making them a significant tool for someultrasonic examination applications.4.2 Specific AdvantagesSince the EMAT technique isnoncontacting, it requires no fluid couplant. Important conse-quences of this include applications to moving objects, inremote or hazardous loca

23、tions, to objects at elevated tempera-tures, or to objects with rough surfaces. The technique isenvironmentally safe since it does not use potentially pollutingor hazardous chemicals. The technique facilitates the rapidscanning of components having complex geometries. EMATsignals are highly reproduc

24、ible as a consequence of the mannerin which the acoustic waves are generated. EMATs canproduce horizontally polarized shear (SH) waves without modeconversion and can accommodate scanning while using SHwaves. (Note that in order to produce this wave mode byconventional ultrasonic techniques, either a

25、n epoxy or a highlyviscous couplant is required. Thus, conventional ultrasonictechniques do not lend themselves easily to scanning whenusing SH wave modes.) Also, EMATs provide for the capabil-ity to steer shear waves electronically.4.3 Specific LimitationsEMATs have very low efficiency.The insertio

26、n loss of EMATs can be as much as 40 dB or morewhen compared to conventional ultrasonic methods. TheEMAT technique can be used only on materials that areelectrical conductors or ferromagnetic. The design of EMATprobes is usually more complex than comparable piezoelectricsearch units. Due to their lo

27、w efficiency, EMATs usuallyrequire more specialized instrumentation for the generation anddetection of ultrasonic signals. High transmitting currents,low-noise receivers, and careful electrical matching is impera-tive in system design. In general, EMAT probes areapplication-specific, in the same way

28、 as piezoelectric transduc-ers.5. Calibration and Standardization5.1 Reference StandardsAs with conventional piezoelec-tric ultrasonic examinations, it is imperative that a set ofreference samples exhibiting the full range of expected mate-rial defect states be acquired or fabricated and consequentl

29、yexamined by the technique to establish sensitivity (see Prac-tices E 127 and E 428).5.2 Transducer CharacterizationMany of the conven-tional contact piezoelectric search unit characterization proce-dures are generally adaptable to EMAT transducers withappropriate modifications, or variations thereo

30、f (see GuideE 1065). Specific characterization procedures for EMATs arenot available and are beyond the scope of this document.6. Theory (1-3)46.1 Nonmagnetic Conducting MaterialsThe mechanismsresponsible for the generation of elastic waves in a conductingmaterial are dependent on the characteristic

31、s of that material.The generation of acoustic waves in a nonmagnetic conductivematerial is a result of the Lorentz force acting on the lattice ofthe material. In an effort to understand the action of the Lorentzforce, one can use the free electron model of solids. Accordingto the free electron model

32、 of conductors, the outer valenceelectrons have been stripped from the atomic lattice, leaving alattice of positively charged ions in a sea of free electrons. Inorder to generate elastic waves in a material, a net force mustbe transmitted to the lattice of the material. If only anelectromagnetic fie

33、ld is generated in a conductor (via an eddycurrent-type coil), the net force on the lattice is zero becausethe forces on the electrons and ions are equal and opposite. Forexample:force on electrons 52qEforce on ions 51qEwhere:q = electron charge, andE = electric field vector of EMAT wave.However, if

34、 the same electromagnetic field is generated inthe presence of an applied static magnetic field, a net force is3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.4The boldface numbers in parentheses refer

35、to the list of references at the end ofthis guide.E 1774 96 (2007)2transmitted to the lattice and results in the generation of elasticwaves. The reason for this net force is the Lorentz force actingon the electrons and ions.Lorentz force 5 FL5 qv 3 B (1)where:v = velocity of electrons, andB = static

36、 magnetic inductor vector.Since the electrons are free to move and the ions are boundto the lattice, the Lorentz force on the electrons is much greaterdue to its velocity dependence, and this force is transmitted tothe ions in the lattice via the collision process.6.2 Magnetic Conducting MaterialsFo

37、r magnetic conduc-tors, other forces such as magnetostrictive forces, in addition tothe Lorentz force, influence ion motion. In magnetic materials,the electromagnetic field can modulate the magnetization in thematerial to produce periodic magnetostrictive stresses thatmust be added to the stresses c

38、aused by the Lorentz force. Themagnetostrictive stresses are complicated and depend on themagnetic domain distribution, which also depends on thestrength and direction of the applied static magnetic field.Although the magnetostrictive forces present in magneticconductors may complicate the theoretic

39、al analysis, this addi-tional coupling can be an asset because it can significantlyincrease the signal strength compared to that obtained by theLorentz force alone. At high applied magnetic field strengthsabove the magnetic saturation of the material, the Lorentz forceis the only source of acoustic

40、wave generation. The magneto-strictive force dominates at low field strengths, however, andthe acoustic energy can be much greater than for correspondingfield strengths with only the Lorentz mechanism. Therefore, acareful examination of the relationship at low applied fieldstrengths should be made i

41、n order to take full advantage of themagnetostrictive effort in magnetic materials.6.3 Wave ModesWith the proper combination of magnetand coil design, EMATs can produce longitudinal, shear,Rayleigh, and Lamb wave modes (2-4). The direction of theapplied magnetic field, geometry of the coil, and freq

42、uency ofthe electromagnetic field will determine the type of wave modegenerated with EMATs.6.3.1 Longitudinal Wave ModeFig. 1 illustrates how thedirection of the applied static magnetic field in a conductor andthe resultant direction of the Lorentz force can producelongitudinal elastic waves. For lo

43、ngitudinal wave generation,the Lorentz force and thus ion displacement is perpendicular tothe surface of the conductor. The efficiency of longitudinalwave generation, as compared with other modes excited inferromagnetic materials, is very low, and has no practicalrelevance.6.3.2 Shear Wave ModesFig.

44、 2 shows how the directionof the applied static magnetic field in a conductor and theresultant direction of the Lorentz force can produce shearelastic waves. For shear wave generation, the Lorentz force andthus ion displacement is parallel to the surface of the conduc-tor. EMATs are also capable of

45、producing shear wave modeswith both vertical and horizontal polarizations. The distinctionbetween these two shear wave polarization modes is illustratedin Fig. 3.6.3.3 Rayleigh Wave ModeIn general, for Rayleigh orsurface wave generation, the applied static magnetic field willbe oriented perpendicula

46、r to the surface of the conductor in thesame manner used for shear wave propagation. A meander lineor serpentine-type coil is used to provide a tuned frequencyEMAT. The frequency of the EMAT is determined by thegeometry (that is, line spacing) of the meander lines in the coil.By proper selection of

47、frequency, it is possible to propagateonly Rayleigh or surface waves. If the thickness of the materialis at least five times the acoustic wavelength that is determinedby the frequency and wave velocity, then Rayleigh wavegeneration is essentially ensured.6.3.4 Lamb Wave ModesThe various Lamb wave mo

48、des(symmetric and antisymmetric) can be generated in a mannersimilar to Rayleigh wave propagation. For Lamb wave produc-tion, the tuned frequency of the meander line coil is chosen togive the desired Lamb wave mode and is dependent on thematerial thickness.7. System Configuration7.1 TransducersAs in

49、 conventional piezoelectric-type ul-trasonic examination, there are basically two types of EMATsFIG. 1 EMAT Generation of Longitudinal WavesE 1774 96 (2007)3with respect to beam direction. EMATs can be designed foreither straight or angle beam examination. Examples of thesetwo types of transducers are presented in the followingsections.7.1.1 Straight BeamThe spiral or pancake coil design isone of the most efficient EMATs for producing a straightultrasonic beam. The direction of the applied magnetic field isperpendicular to the plane of the spiral coil, as show