1、Designation: E1774 12Standard Guide forElectromagnetic Acoustic Transducers (EMATs)1This standard is issued under the fixed designation E1774; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in p
2、arentheses indicates the year of last reapproval. Asuperscript epsilon () 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 generation of ultrasonic
3、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 material being examined
4、. 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 in which the acoustic
5、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 field near the surfac
6、e 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 wave, the surface of th
7、e 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. Scope*1.1 This guide
8、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 into an electrically
9、 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 reference toassist in dete
10、rmining 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 detailed procedures
11、 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 UnitsThe value stated in inch-pound units are to be
12、regarded as the standard. The values given in parentheses arefor information only.1.6 This standard does not purport 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 d
13、etermine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E127 Practice for Fabricating and Checking AluminumAlloy Ultrasonic Standard Reference Blocks1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the
14、 direct responsibility of Subcommittee E07.06 on UltrasonicMethod.Current edition approved June 15, 2012. Published August 2012. Originallyapproved in 1995. Last previous edition approved in 2007 as E1774 - 96 (2007).DOI: 10.1520/E1774-12.2For referenced ASTM standards, visit the ASTM website, www.a
15、stm.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.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO
16、 Box C700, West Conshohocken, PA 19428-2959, United States.E428 Practice for Fabrication and Control of Metal, Otherthan Aluminum, Reference Blocks Used in UltrasonicTestingE1065 Guide for Evaluating Characteristics of UltrasonicSearch UnitsE1316 Terminology for Nondestructive ExaminationsE543 Speci
17、fication for Agencies Performing Nondestruc-tive Testing2.2 ASNT Document:SNT-TC-1A Recommended Practice for Personnel Qualifi-cations and Certification in Nondestructive Testing32.3 Aerospace Industries Association Standard:NAS-410 Certification and Qualification of NondestructiveTest Personnel43.
18、Terminology3.1 DefinitionsRelated terminology is defined in Termi-nology E1316.3.2 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
19、Lorentz forcesforces applied to electric currentswhen placed in a magnetic field. Lorentz 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 f
20、rom magneticdomain wall movements within a magnetic material duringmagnetization.3.2.4 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 conductor
21、s.3.2.6 bulk wavean ultrasonic wave, either longitudinal orshear mode, used in nondestructive 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 exami
22、nations are performed using trans-ducers that directly convert electrical energy into 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 ferromagnet
23、ic material. EMATs have uniquecharacteristics when compared to conventional piezoelectricultrasonic 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
24、-quences of this include applications to moving objects, inremote or hazardous locations, 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 rap
25、idscanning of components having complex geometries. EMATsignals are highly reproducible 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 tha
26、t in order to produce this wave mode byconventional ultrasonic techniques, either an 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
27、 electronically.4.3 Specific LimitationsEMATs have very low efficiency.The insertion 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
28、 is usually more complex than comparable piezoelectricsearch units. Due to their low 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
29、 in system design. In general, EMAT probes areapplication-specific, in the same way as piezoelectric transduc-ers.5. Basis of Application5.1 The following items are subject to contractual agree-ment between the parties using or referencing this guide.5.2 If specified in the contractual agreement, pe
30、rsonnelperforming examinations to this standard shall be qualified inaccordance with a nationally or internationally recognizedNDT personnel qualification practice or standard such asANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, or a similardocument and certified by the employer or certifying agency,as appl
31、icable. The practice or standard used and its applicablerevision shall be identified in the contractual agreement be-tween the using parties.5.3 Qualification of Nondestructive AgenciesIf specifiedin the contractual agreement, NDT agencies shall be qualifiedand evaluated as describes in Practice E54
32、3. The applicableedition of Practice E543 shall be specified in the contractualagreement.6. Standardization6.1 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
33、 be acquired or fabricated and consequentlyexamined by the technique to establish sensitivity (see Prac-tices E127 and E428).6.2 Transducer CharacterizationMany of the conven-tional contact piezoelectric search unit characterization proce-dures are generally adaptable to EMAT transducers withappropr
34、iate modifications, or variations thereof (see Guide3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.4Available from Aerospace Industries Association of America, Inc. (AIA), 1000Wilson Blvd., Suite 1700,
35、Arlington, VA22209-3928, http:/www.aia-aerospace.org.E1774 122E1065). Specific characterization procedures for EMATs arenot available and are beyond the scope of this document.7. Theory (1-3)57.1 Nonmagnetic Conducting MaterialsThe mechanismsresponsible for the generation of elastic waves in a condu
36、ctingmaterial are dependent on the characteristics 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 mod
37、el of solids. Accordingto the free electron model 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 latti
38、ce of the material. If only anelectromagnetic field 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
39、 = electric field vector of EMAT wave.However, if the same electromagnetic field is generated inthe presence of an applied static magnetic field, a net force istransmitted to the lattice and results in the generation of elasticwaves. The reason for this net force is the Lorentz force actingon the el
40、ectrons and ions.Lorentz force 5 FL5 qv 3 B (1)where:v = velocity of electrons, andB = static 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 transm
41、itted tothe ions in the lattice via the collision process.7.2 Magnetic Conducting MaterialsFor 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
42、thematerial to produce periodic magnetostrictive stresses thatmust be added to the stresses caused 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.Al
43、though the magnetostrictive forces present in magneticconductors may complicate the theoretical 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 strengthsab
44、ove the magnetic saturation of the material, the Lorentz forceis the only source of acoustic 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. There
45、fore, acareful examination of the relationship at low applied fieldstrengths should be made in order to take full advantage of themagnetostrictive effort in magnetic materials.7.3 Wave ModesWith the proper combination of magnetand coil design, EMATs can produce longitudinal, shear,Rayleigh, and Lamb
46、 wave modes (2-4). The direction of theapplied magnetic field, geometry of the coil, and frequency ofthe electromagnetic field will determine the type of wave modegenerated with EMATs.7.3.1 Longitudinal Wave ModeFig. 1 illustrates how thedirection of the applied static magnetic field in a conductor
47、andthe resultant direction of the Lorentz force can producelongitudinal elastic waves. For longitudinal wave generation,the Lorentz force and thus ion displacement is perpendicular tothe surface of the conductor. The efficiency of longitudinal5The boldface numbers in parentheses refer to the list of
48、 references at the end ofthis guide.FIG. 1 EMAT Generation of Longitudinal WavesE1774 123wave generation, as compared with other modes excited inferromagnetic materials, is very low, and has no practicalrelevance.7.3.2 Shear Wave ModesFig. 2 shows how the directionof the applied static magnetic fiel
49、d 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 producing shear wave modeswith both vertical and horizontal polarizations. The distinctionbetween these two shear wave polarization modes is illustratedin Fig. 3.7.3.3 Rayleigh Wave ModeIn general, for Rayleigh orsurface wave generation, the applied static magnetic field willbe oriented perpendicular to the surface of the co
