ASTM_E_1774_-_12.pdf

Designation:E177412Standard 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 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 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 material 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 in 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 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 wave,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.Scope*1.1 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 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 reference 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 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 UnitsThe value stated in inch-pound units are to beregarded 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 determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2E127 Practice for Fabricating and Checking Aluminum Al-loy Ultrasonic Standard Reference Blocks1This 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 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.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.*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 States1 E428 Practice for Fabrication and Control of Metal,Otherthan Aluminum,Reference Blocks Used in UltrasonicTestingE1065 Practice for Evaluating Characteristics of UltrasonicSearch UnitsE1316 Terminology for Nondestructive ExaminationsE543 Specification for Agencies Performing NondestructiveTesting2.2 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 Qualificati