ASTM E1774-17 Standard Guide for Electromagnetic Acoustic Transducers (EMATs).pdf

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1、Designation: E1774 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 number in p

2、arentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This guide is intended primarily for tutorial purposes. Itprovides an overview of the general principles governing theoperation and use of electr

3、omagnetic acoustic transducers(EMATs) for ultrasonic examination.1.2 This guide describes a non-contact technique for cou-pling ultrasonic energy into an electrically conductive orferromagnetic material, or both, through the use of electromag-netic fields. This guide describes the theory of operatio

4、n 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 determining the usefulness of EMATs for a givenapplication as well as provide fundamental information regard-ing their design and operatio

5、n. This guide provides guidancefor the generation of longitudinal, shear, Rayleigh, and Lambwave modes using EMATs.1.4 This guide does not contain detailed procedures for theuse of EMATs in any specific applications; nor does it promotethe use of EMATs without thorough testing prior to their usefor

6、examination purposes. Some applications in which EMATshave been applied successfully are outlined in Section 9.1.5 UnitsThe values stated in inch-pound units are to beregarded as the standard. The SI values given in parenthesesare for information only.1.6 This standard does not purport to address al

7、l 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.1.7 This international standard was developed in accor-dance wi

8、th internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standard

9、s:2E127 Practice for Fabrication and Control of AluminumAlloy Ultrasonic Standard Reference BlocksE428 Practice for Fabrication and Control of Metal, Otherthan Aluminum, Reference Blocks Used in UltrasonicTestingE1065 Practice for Evaluating Characteristics of UltrasonicSearch UnitsE1316 Terminology

10、 for Nondestructive ExaminationsE543 Specification for Agencies Performing NondestructiveTesting2.2 ASNT Documents:3SNT-TC-1A Recommended Practice for Personnel Qualifi-cations and Certification in Nondestructive TestingANSI/ASNT CP-189 Standard for Qualification and Certifi-cation for Nondestructiv

11、e Testing Personnel2.3 Aerospace Industries Association Standard:4NAS-410 Certification and Qualification of NondestructiveTest Personnel2.4 ISO Standard:5ISO 9712 Non-Destructive Testing: Qualification and Certi-fication of NDT Personnel3. Terminology3.1 DefinitionsRelated terminology is defined in

12、 Termi-nology E1316.3.2 Definitions of Terms Specific to This Standard:3.2.1 bulk wavean ultrasonic wave, either longitudinal orshear mode, used in nondestructive testing to interrogate thevolume of a material.3.2.2 electromagnetic acoustic transducer (EMAT)anelectromagnetic device for converting el

13、ectrical energy intoacoustical energy in the presence of a magnetic field.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 June 1, 2017. Published June 2017. Ori

14、ginallyapproved in 1995. Last previous edition approved in 2012 as E1774 - 12. DOI:10.1520/E1774-17.2For 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 Do

15、cument Summary page onthe ASTM website.3Available 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,Arlington, VA

16、22209-3928, http:/www.aia-aerospace.org.5Available from International Organization for Standardization (ISO), ISOCentral Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,Geneva, Switzerland, http:/www.iso.org.*A Summary of Changes section appears at the end of this standardCopyrigh

17、t ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International

18、 Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1Mon Apr 30 41 3.2.3 Lorentz forcesforces applied to electric currentswhen placed in a magnetic field. Lorentz forces are perpen-dicular to the direction of both the magnetic fie

19、ld and thecurrent direction.3.2.4 magnetostrictive forcesforces arising from magneticdomain wall movements within a magnetic material duringmagnetization, where magnetostrictive materials will undergoa strain in the presence of a magnetic field.3.2.5 meander coilan EMAT coil consisting of periodic,w

20、inding, non-intersecting, and usually evenly-spaced conduc-tors.3.2.6 pancake coil (spiral)an EMAT coil consisting ofspirally-wound, usually evenly-spaced conductors.4. Significance and Use4.1 GeneralUltrasonic testing is a widely used nonde-structive method for the examination of a material. Themaj

21、ority of ultrasonic examinations 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 electrically

22、conductive or ferromagnetic material. EMATs have uniquecharacteristics when compared to conventional piezoelectricultrasonic search units, making them a significant tool for someultrasonic examination applications.4.2 PrincipleAn electromagnetic acoustic transducer(EMAT) generates and receives ultra

23、sonic waves without theneed to contact the material in which the acoustic waves aretraveling. The use of an EMAT requires that the material to beexamined be electrically conductive or ferromagnetic, or both.There are two basic components of an EMAT system, a magnetand a coil. The magnet may be an el

24、ectromagnet or apermanent magnet, which is used to produce a magnetic fieldin the material under test. The coil is driven using alternatingcurrent at the desired ultrasonic frequency. The coil and ACcurrent also induce a surface magnetic field in the materialunder test. In the presence of the static

25、 magnetic field, thesurface current experiences Lorentz forces that produce thedesired ultrasonic waves. Upon reception of an ultrasonicwave, the surface of the conductor oscillates in the presence ofa magnetic field, thus inducing a voltage in the coil. Thetransduction process occurs within an elec

26、tromagnetic skindepth. The EMAT forms the basis for a very reproduciblenoncontact system for generating and detecting ultrasonicwaves.4.3 Specific AdvantagesSince an EMAT technique doesnot have to be in contact with the material under examination,no fluid couplant is required. Important consequences

27、 of thisinclude applications to moving objects, in remote or hazardouslocations, to objects at elevated temperatures, or to objects withrough surfaces. The EMAT technique is environmentally safesince it does not use potentially polluting or hazardous chemi-cals. The technique facilitates the rapid s

28、canning of compo-nents having complex geometries. EMAT signals are highlyreproducible as a consequence of the manner in which theacoustic waves are generated. EMATs can also produce hori-zontally polarized shear (SH) waves without mode conversionand can accommodate scanning while using SH waves. (No

29、tethat in order to produce this wave mode by conventionalultrasonic techniques, either an epoxy or a highly viscouscouplant is required. Thus, conventional ultrasonic techniquesdo not lend themselves easily to scanning when using SH wavemodes.) Additionally, EMATs can allow the user to electroni-cal

30、ly steer shear waves.4.4 Specific LimitationsEMATs have very low efficiencyas compared with conventional ultrasonic methods, with inser-tion losses of 40 dB or more. The EMAT technique can be usedonly on materials that are electrical conductors or are ferro-magnetic. Highly corroded surfaces, especi

31、ally inner surfaces,may render EMAT unsuitable for use if the surface disturbs thegeneration of the Lorentz forces. The design of EMAT probesis usually more complex than comparable piezoelectric searchunits, and are usually relatively large in size. Due to their lowefficiency, EMATs usually require

32、more specialized instrumen-tation for the generation and detection of ultrasonic signals.High transmitting currents, low-noise receivers, and carefulelectrical matching are imperative in system design. In general,EMAT probes are application-specific, in the same way as arepiezoelectric transducers.5

33、. Basis of Application5.1 The following items are subject to contractual agree-ment between the parties using or referencing this guide.5.2 Personnel Qualification:5.2.1 If specified in the contractual agreement, personnelperforming examinations to this standard shall be qualified inaccordance with

34、a nationally or internationally recognizedNDT personnel qualification practice or standard such asANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, ISO 9712, or asimilar document and certified by the employer or certifyingagency, as applicable. The practice or standard used and itsapplicable revision shall be i

35、dentified in the contractual agree-ment between the using parties.5.3 Qualification of Nondestructive AgenciesIf specifiedin the contractual agreement, NDT agencies shall be qualifiedand evaluated as described in Practice E543. The applicableedition of Practice E543 shall be specified in the contrac

36、tualagreement.5.4 Procedures and TechniquesThe procedures and tech-niques to be utilized shall be as specified in the contractualagreement.5.5 Surface PreparationThe pre-examination surfacepreparation criteria shall be as specified in the contractualagreement.5.6 Timing of ExaminationThe timing of e

37、xaminationshall be as specified in the contractual agreement.5.7 Extent of ExaminationThe extent of the examinationshall be as specified in the contractual agreement.5.8 Reporting Criteria/Acceptance CriteriaReporting cri-teria for the examination results shall be in accordance with thecontractual a

38、greement. Since acceptance criteria (e.g. forE1774 172Mon Apr 30 41 reference radiographs) are not specified in this guide, they shallbe stated in the contractual agreement.5.9 Reexamination of Repaired/Reworked ItemsReexamination of repaired/reworked items is not addressed inthis guide and if requi

39、red 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 be acquired or fabricated and consequ

40、entlyexamined by the technique to establish sensitivity (see Prac-tices E127 and E428 for descriptions of standard configurationand fabrication).6.2 Transducer CharacterizationMany of the conven-tional contact piezoelectric search unit characterization proce-dures are generally adaptable to EMAT tra

41、nsducers withappropriate modifications (see Guide E1065 for such trans-ducer characterization procedures). Specific characterizationprocedures for EMATs are not available and are beyond thescope of this document.7. Theory (1-3)67.1 Nonmagnetic Conducting MaterialsThe mechanismsresponsible for the ge

42、neration of elastic waves in a conductingmaterial 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 Lorentzforc

43、e, one can use the free electron model 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 f

44、orce mustbe transmitted to the lattice 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

45、 51qEwhere:q = electron charge, andE = 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

46、is the Lorentz force actingon the electrons 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

47、dependence, and this force is transmitted tothe ions in the lattice via the collision process.7.2 Magnetic Conducting MaterialsFor magneticconductors, other forces such as magnetostrictive forces, inaddition to the Lorentz force, influence ion motion. In magneticmaterials, the electromagnetic field

48、can modulate the magne-tization in the material to produce periodic magnetostrictivestresses that must be added to the stresses caused by theLorentz force. The magnetostrictive stresses are complicatedand depend on the magnetic domain distribution, which alsodepends on the strength and direction of

49、the applied staticmagnetic field. Although the magnetostrictive forces present inmagnetic conductors may complicate the theoretical analysis,this additional coupling can be an asset because it cansignificantly increase the signal strength compared to thatobtained by the Lorentz force alone. At high applied magneticfield strengths above the magnetic saturation of the material,the Lorentz force is the only source of acoustic wave genera-tion. The magnetostrictive force dominates at low fieldstrengths, however, and the acoustic energy can be muchgreate