1、Designation: E1774 12E1774 17Standard 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 num
2、ber in parentheses 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 ult
3、rasonic 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
4、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 in which the a
5、coustic waves are traveling. The use ofan EMAT requires that the material to be examined be electrically conductive or ferromagnetic, orboth.The EMATas a generator of ultrasonic waves is basically a coil of wire, excited by an alternatingelectric current, placed in a uniform magnetic field near the
6、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 wave, the surface
7、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. Scope*1.1 This g
8、uide is intended primarily for tutorial purposes. It provides 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 coupling ultrasonic energy into an electr
9、ically conductive or ferromagneticmaterial, or both, through the use of electromagnetic fields. This guide describes the theory of operation and basic designconsiderations as well as the advantages and limitations of the technique.1.3 This guide is intended to serve as a general reference to assist
10、in determining the usefulness of EMATs for a givenapplication as well as provide fundamental information regarding their design and operation. This guide provides guidance for thegeneration of longitudinal, shear, Rayleigh, and Lamb wave modes using EMATs.1.4 This guide does not contain detailed pro
11、cedures for the use of EMATs in any specific applications; nor does it promote theuse of EMATs without thorough testing prior to their use for examination purposes. Some applications in which EMATs have beenapplied successfully are outlined in Section 9.1.5 UnitsThe valuevalues stated in inch-pound
12、units are to be regarded as the standard. The SI values given in parenthesesare for information only.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and hea
13、lth practices and determine the applicability of regulatorylimitations prior to use.1.7 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guide
14、s and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.1 This guide is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.06 on Ultrasonic Method.Current edition approved June 15
15、, 2012June 1, 2017. Published August 2012June 2017. Originally approved in 1995. Last previous edition approved in 20072012 asE1774 - 96 (2007).E1774 - 12. DOI: 10.1520/E1774-12.10.1520/E1774-17.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an ind
16、ication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be
17、considered the official document.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States12. Referenced Documents2.1 ASTM Standards:2E127 Practice for Fabrication and Control of
18、Aluminum Alloy Ultrasonic Standard Reference BlocksE428 Practice for Fabrication and Control of Metal, Other than Aluminum, Reference Blocks Used in Ultrasonic TestingE1065 Practice for Evaluating Characteristics of Ultrasonic Search UnitsE1316 Terminology for Nondestructive ExaminationsE543 Specifi
19、cation for Agencies Performing Nondestructive Testing2.2 ASNT Document:Documents:3SNT-TC-1A Recommended Practice for Personnel Qualifications and Certification in Nondestructive TestingANSI/ASNT CP-189 Standard for Qualification and Certification for Nondestructive Testing Personnel2.3 Aerospace Ind
20、ustries Association Standard:4NAS-410 Certification and Qualification of Nondestructive Test Personnel2.4 ISO Standard:5ISO 9712 Non-Destructive Testing: Qualification and Certification of NDT Personnel3. Terminology3.1 DefinitionsRelated terminology is defined in Terminology E1316.3.2 Definitions o
21、f Terms Specific to This Standard:3.2.1 bulk wavean ultrasonic wave, either longitudinal or shear mode, used in nondestructive testing to interrogate the volumeof a material.3.2.2 electromagnetic acoustic transducer (EMAT)an electromagnetic device for converting electrical energy into acousticalener
22、gy in the presence of a magnetic field.3.2.3 Lorentz forcesforces applied to electric currents when placed in a magnetic field. Lorentz forces are perpendicular to thedirection of both the magnetic field and the current direction. Lorentz forces are the forces behind the principle of electric motors
23、.3.2.4 magnetostrictive forcesforces arising from magnetic domain wall movements within a magnetic material duringmagnetization.magnetization, where magnetostrictive materials will undergo a strain in the presence of a magnetic field.3.2.5 meander coilan EMAT coil consisting of periodic, winding, no
24、n-intersecting, and usually evenly-spaced conductors.3.2.6 pancake coil (spiral)an EMAT coil consisting of spirally-wound, usually evenly-spaced conductors.3.2.6 bulk wavean ultrasonic wave, either longitudinal or shear mode, used in nondestructive testing to interrogate the volumeof a material.4. S
25、ignificance and Use4.1 GeneralUltrasonic testing is a widely used nondestructive method for the examination of a material. The majority ofultrasonic examinations are performed using transducers that directly convert electrical energy into acoustic energy through the useof piezoelectric crystals. Thi
26、s guide describes an alternate technique in which electromagnetic energy is used to produce acousticenergy inside an electrically conductive or ferromagnetic material. EMATs have unique characteristics when compared toconventional piezoelectric ultrasonic search units, making them a significant tool
27、 for some ultrasonic examination applications.4.2 PrincipleAn electromagnetic acoustic transducer (EMAT) generates and receives ultrasonic waves without the need tocontact the material in which the acoustic waves are traveling. The use of an EMAT requires that the material to be examined beelectrica
28、lly conductive or ferromagnetic, or both. There are two basic components of an EMAT system, a magnet and a coil. Themagnet may be an electromagnet or a permanent magnet, which is used to produce a magnetic field in the material under test. Thecoil is driven using alternating current at the desired u
29、ltrasonic frequency. The coil andAC current also induce a surface magneticfield in the material under test. In the presence of the static magnetic field, the surface current experiences Lorentz forces thatproduce the desired ultrasonic waves. Upon reception of an ultrasonic wave, the surface of the
30、conductor oscillates in the presenceof a magnetic field, thus inducing a voltage in the coil. The transduction process occurs within an electromagnetic skin depth. TheEMAT forms the basis for a very reproducible noncontact system for generating and detecting ultrasonic waves.4.3 Specific AdvantagesS
31、ince the EMAT technique is noncontacting, it requires no fluid couplant. an EMAT technique doesnot have to be in contact with the material under examination, no fluid couplant is required. Important consequences of this includeapplications to moving objects, in remote or hazardous locations, to obje
32、cts at elevated temperatures, or to objects with roughsurfaces. The EMAT technique is environmentally safe since it does not use potentially polluting or hazardous chemicals. The2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. F
33、or Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 Available from American Society for Nondestructive Testing (ASNT), P.O. Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.4 Available from Aerospace Industr
34、ies Association of America, Inc. (AIA), 1000 Wilson Blvd., Suite 1700, Arlington, VA 22209-3928, http:/www.aia-aerospace.org.5 Available from International Organization for Standardization (ISO), ISO Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,Switzerland, http
35、:/www.iso.org.E1774 172technique facilitates the rapid scanning of components having complex geometries. EMAT signals are highly reproducible as aconsequence of the manner in which the acoustic waves are generated. EMATs can also produce horizontally polarized shear (SH)waves without mode conversion
36、 and can accommodate scanning while using SH waves. (Note that in order to produce this wavemode by conventional ultrasonic techniques, either an epoxy or a highly viscous couplant is required.Thus, conventional ultrasonictechniques do not lend themselves easily to scanning when using SH wave modes.
37、) Also,Additionally, EMATs provide forcanallow the capabilityuser to electronically steer shear waves electronically.waves.4.4 Specific LimitationsEMATs have very low efficiency. The insertion loss of EMATs can be as much asefficiency ascompared with conventional ultrasonic methods, with insertion l
38、osses of 40 dB or more when compared to conventional ultrasonicmethods. more. The EMAT technique can be used only on materials that are electrical conductors or ferromagnetic. areferromagnetic. Highly corroded surfaces, especially inner surfaces, may render EMAT unsuitable for use if the surface dis
39、turbs thegeneration of the Lorentz forces. The design of EMAT probes is usually more complex than comparable piezoelectric search units.units, and are usually relatively large in size. Due to their low efficiency, EMATs usually require more specialized instrumentationfor the generation and detection
40、 of ultrasonic signals. High transmitting currents, low-noise receivers, and careful electricalmatching isare imperative in system design. In general, EMAT probes are application-specific, in the same way as are piezoelectrictransducers.5. Basis of Application5.1 The following items are subject to c
41、ontractual agreement between the parties using or referencing this guide.5.2 If specified in the contractual agreement, personnel performing examinations to this standard shall be qualified inaccordance with a nationally or internationally recognized NDT personnel qualification practice or standard
42、such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, or a similar document and certified by the employer or certifying agency, as applicable. Thepractice or standard used and its applicable revision shall be identified in the contractual agreement between the usingparties.Personnel Qualification:5.2.1 If s
43、pecified in the contractual agreement, personnel performing examinations to this standard shall be qualified inaccordance with a nationally or internationally recognized NDT personnel qualification practice or standard such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, ISO 9712, or a similar document and
44、 certified by the employer or certifying agency, as applicable.The practice or standard used and its applicable revision shall be identified in the contractual agreement between the using parties.5.3 Qualification of Nondestructive AgenciesIf specified in the contractual agreement, NDT agencies shal
45、l be qualified andevaluated as describesdescribed in Practice E543. The applicable edition of Practice E543 shall be specified in the contractualagreement.5.4 Procedures and TechniquesThe procedures and techniques to be utilized shall be as specified in the contractual agreement.5.5 Surface Preparat
46、ionThe pre-examination surface preparation criteria shall be as specified in the contractual agreement.5.6 Timing of ExaminationThe timing of examination shall be as specified in the contractual agreement.5.7 Extent of ExaminationThe extent of the examination shall be as specified in the contractual
47、 agreement.5.8 Reporting Criteria/Acceptance CriteriaReporting criteria for the examination results shall be in accordance with thecontractual agreement. Since acceptance criteria (e.g. for reference radiographs) are not specified in this guide, they shall be statedin the contractual agreement.5.9 R
48、eexamination of Repaired/Reworked ItemsReexamination of repaired/reworked items is not addressed in this guide andif required shall be specified in the contractual agreement.6. Standardization6.1 Reference StandardsAs with conventional piezoelectric ultrasonic examinations, it is imperative that a s
49、et of referencesamples exhibiting the full range of expected material defect states be acquired or fabricated and consequently examined by thetechnique to establish sensitivity (see Practices E127 and E428). for descriptions of standard configuration and fabrication).6.2 Transducer CharacterizationMany of the conventional contact piezoelectric search unit characterization procedures aregenerally adaptable to EMAT transducers with appropriate modifications, or variations thereof modifications (see Guide E1065).for such transducer characterization procedures
