1、Designation: E1816 12Standard Practice forUltrasonic Testing Using Electromagnetic AcousticTransducer (EMAT) Techniques1This standard is issued under the fixed designation E1816; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、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.1. Scope*1.1 This practice covers procedures for the use of electro-magnetic acoustic transducers (EMATs) for specific ult
3、rasonicexamination applications. Recommendations are given forspecific applications for using EMAT techniques to detectflaws through both surface and volumetric examinations aswell as to measure thickness.1.2 These procedures recommend technical details andguidelines for the reliable and reproducibl
4、e ultrasonic detectionof flaws and thickness measurements using electromagneticacoustic transducers for both the pulsing and receiving ofultrasonic waves. The EMAT techniques described herein canbe used as a basis for assessing the serviceability of variouscomponents nondestructively, as well as for
5、 process control inmanufacturing.1.3 These procedures cover noncontact techniques for cou-pling ultrasonic energy into materials through the use ofelectromagnetic fields. Surface, Lamb, longitudinal, and shearwave modes are discussed.1.4 These procedures are intended to describe specificEMAT applica
6、tions. These procedures are intended for appli-cations in which the user has determined that the use of EMATtechniques can offer substantial benefits over conventionalpiezoelectric search units. It is not intended that EMATtechniques should be used in applications in which conven-tional techniques a
7、nd applications offer superior benefits (referto Guide E1774).1.5 These procedures are applicable to any material inwhich acoustic waves can be introduced electromagnetically.This includes any material that is either electrically conductiveor ferromagnetic.1.6 The procedures outlined in this practic
8、e address provenEMAT techniques for specific applications; they do not purportto address the only variation or all variations of EMATtechniques to address the given applications. Latitude inapplication techniques is offered where options are consideredappropriate.1.7 The values stated in inch-pound
9、units are to be regardedas the standard. The values given in parentheses are forinformation only.1.8 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
10、practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E114 Practice for Ultrasonic Pulse-Echo Straight-BeamContact TestingE494 Practice for Measuring Ultrasonic Velocity in Materi-alsE543 Specification for Agencies Performing Non
11、destructiveTestingE587 Practice for Ultrasonic Angle-Beam Contact TestingE797 Practice for Measuring Thickness by Manual Ultra-sonic Pulse-Echo Contact MethodE1316 Terminology for Nondestructive ExaminationsE1774 Guide for Electromagnetic Acoustic Transducers(EMATs)2.2 ASNT Standards:3SNT-TC-1A Reco
12、mmended Practice for Personnel Qualifi-cations and Certification in Nondestructive TestingANSI/ASNT CP-189 Standard for Qualification and Certifi-cation of Nondestructive Testing Personnel2.3 Aerospace Industries Association Standard:NAS 410 Certification and Qualification of NondestructiveTest Pers
13、onnel41This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.06 onUltrasonic Method.Current edition approved Nov. 1, 2012. Published November 2012. Originallyapproved in 1996. Last previous edition approved in 20
14、07 as E1816 - 07. DOI:10.1520/E1816-12.2For 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.3Available fromAmerica
15、n 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, VA22209-3928, http:/www.aia-aerospace.org.*A Summary of Changes
16、 section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13. Terminology3.1 Definitions: Related terminology is defined in Terminol-ogy E1316.3.2 Definitions of Terms Specific to This Standard:3.2.1 b
17、ulk 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 electrical energy intoacoustical energy in the presence of a magnetic field.3.2
18、.3 lift-off effectrefer to Terminology E1316, Section C.3.2.4 Lorentz forcesforces applied to electric currentswhen placed in a magnetic field. Lorentz forces are perpen-dicular to both the direction of the magnetic field and thecurrent direction. Lorentz forces are the forces responsiblebehind the
19、principle of electric motors.3.2.5 magnetostrictive forcesforces arising from magneticdomain wall movements within a magnetic material duringmagnetization.3.2.6 meander coilan EMAT coil consisting of periodic,winding, nonintersecting, and usually evenly spaced conduc-tors.3.2.7 pancake (spiral) coil
20、an EMAT coil consisting ofspirally wound, usually evenly spaced conductors.4. Summary of Practice4.1 Surface Examination:4.1.1 The generation of Rayleigh or surface waves in thematerial to be examined allows for sensitivity to surface flawsand discontinuities. Flaws can be detected by reflections of
21、acoustic waves from the discontinuity interfaces or by acousticwave attenuation in traversing across the surface of thecomponent. Either pulse-echo or pitch-catch ultrasonic tech-niques can be used.4.1.2 Fig. 1 shows a typical EMAT setup for the transduc-tion of Rayleigh or Lamb waves. As shown, an
22、externalmagnetic induction B is applied parallel to the surface of anelectrically conductive or ferromagnetic material. A meandercoil is used. The coil is oriented in the same plane as thesurface of the material and is excited by an electrical radiofrequency (RF) pulse. A surface current is produced
23、 in thematerial by transformer action. The surface current, in thepresence of the magnetic field, experiences Lorentz forces thatproduce oscillating stress waves perpendicular to the surface ofthe material to produce surface acoustic waves. Basic EMATdesigns generate bidirectional surface waves. Spe
24、cialized de-signs can be used to generate unidirectional waves, as withconventional ultrasonic examination.4.1.3 Surface flaws or discontinuities lead to reflections orattenuation of the surface waves. Upon approach to thereceiver EMAT, the reflected or attenuated ultrasonic wavesproduce oscillation
25、s within the conductor in the presence of themagnetic field and thus produce a voltage induction in the coil,allowing for detection.4.2 Volumetric Examination:4.2.1 Sensitivity to flaws or discontinuities within a partrequires the use of bulk acoustic wave modes to interrogate thevolume of the mater
26、ial. As with surface examinations, relianceon the reflection or attenuation of acoustic waves from discon-tinuity interfaces forms the basis for the detection of flaws.4.2.2 Depending on the particular application, either longi-tudinal or shear wave modes may be desirable. While straightbeam applica
27、tions using pulse-echo techniques are the moststraightforward, angle beam pitch-catch techniques could bedesirable, depending on such factors as expected flaw locationand orientation.4.2.3 Fig. 2 shows one typical EMAT setup for the trans-duction of bulk waves. As shown, an external magneticinductio
28、n B is applied normal to the surface of an electricallyconductive or ferromagnetic material. A spiral EMAT coil isused for this example. The coil is positioned in a plane parallelto the surface of the material and is excited by an electricalcurrent pulse. A surface current is produced in the materia
29、l bytransformer action. The surface current, in the presence of themagnetic field, experiences Lorentz forces that produce oscil-lating stress waves originating in the surface of the material.Radially polarized shear waves are generated for this example.Depending on the design characteristics of the
30、 magnetic field,the excitation of either radially polarized or planar-polarizedshear waves, propagating normal to the surface, can beintroduced. Longitudinal wave modes can also be generatedand used effectively in non-ferromagnetic materials. Longitu-dinal wave generation in ferromagnetic materials
31、is impracticaldue to unacceptably low coupling efficiency. Mode-convertedlongitudinal waves can be used effectively. Paragraph 7.2 andFIG. 1 Typical EMAT Configuration for Rayleigh or Lamb Wave GenerationE1816 122the subparagraphs of 7.2 give a more in-depth discussion of thevarious EMAT/magnet conf
32、igurations for producing variousbulk wave modes.4.3 Thickness Gaging:4.3.1 Determining the thickness of a material by ultrasonicmeans is a matter of coupling an ultrasonic wave into thematerial, allowing the sound wave to propagate through thematerial, reflect from the backwall boundary interface of
33、 thematerial, and propagate back to the front surface. The thicknessof the material can be calculated by measuring the transit timeof the ultrasonic wave and the knowledge of the ultrasonicwave velocity. Thickness measurements can also be extrapo-lated for a given material through standardizations o
34、f transittime as a function of thickness as derived from a referenceblock (see Practice E797 and 7.3.1).4.3.2 The ultrasonic velocity of the material under exami-nation is a function of the physical properties of the material,namely, stiffness and density. It is usually assumed to beconstant for a g
35、iven class of materials. Approximate velocityvalues are available in tabular format from numerous sources,including the ASNT Nondestructive Testing Handbook.5Velocity values can also be determined empirically (see Prac-tice E494).4.3.3 Determination of the transit time of an acoustic wavethrough a m
36、aterial requires the use of bulk acoustic wavemodes. While longitudinal waves can be used, it is oftendesirable to use shear waves since their slower propagationvelocities lend themselves to more accurate measurements ofthin materials. While straight beam applications using pulse-echo techniques are
37、 the most straightforward and popular,angle beam pitch-catch techniques could be desirable, espe-cially in applications in which fast scan rates are needed orhigh resolution is desired for thin material. The generation ofbulk waves by means of the EMAT technique is discussed in4.2.3 and depicted in
38、Fig. 2.5. Significance and Use5.1 Since EMAT techniques are noncontacting, they shouldbe considered for ultrasonic examinations in which applica-tions involve automation, high-speed examinations, movingobjects, applications in remote or hazardous locations, andapplications to objects at elevated tem
39、peratures or objects withrough surfaces. This practice describes procedures for usingEMAT techniques as associated with the ultrasonic method todetect flaws for both surface and volumetric examinations aswell as to measure thickness.5.2 The uniqueness of the electromagnetic acoustic trans-ducer tech
40、nique for ultrasonic examination basically lies in thegeneration and reception of the ultrasonic waves. Otherwise,conventional ultrasonic techniques and methodologies gener-ally apply.5.3 An EMAT generates and receives acoustic waves in amaterial by electromagnetic means; electrically conductive orf
41、erromagnetic materials can be examined. In its simplest form,an EMAT as a generator of ultrasonic waves is basically a coilof wire, excited by an alternating current, and placed in auniform magnetic field near the surface of a material. Forconductive materials, eddy currents are induced as a result
42、ofthe alternating current. Due to the magnetic field, these eddycurrents experience Lorentz forces that in turn are transmittedto the solid by collisions with the lattice or other microscopicprocesses. These forces are alternating at the frequency of thedriving current and act as a source of ultraso
43、nic waves. If thematerial is ferromagnetic, additional coupling mechanismsplay a part in the generation of ultrasonic waves. Interactionsbetween the dynamic magnetic field generated by the alternat-ing currents and the magnetization associated with the materialoffer a source of coupling, as do the a
44、ssociated magnetostric-tive influences. Reciprocal processes exist whereby all of thesemechanisms lead to detection. Fig. 3 depicts the mechanisms(forces), along with associated direction, for electromagneticultrasound generation.5.4 The EMAT can be used to generate all ultrasonic modesof vibration.
45、 As with conventional ultrasonic techniques, ma-terial types, probable flaw locations, and flaw orientationsdetermine the selection of beam directions and modes ofvibration. The use of EMATs and selection of the proper wavemode presuppose a knowledge of the geometry of the object;the probable locati
46、on, size, orientation, and reflectivity of theexpected flaws; the allowable range of EMAT lift-off; and thelaws of physics governing the propagation of ultrasonic waves.5Nondestructive Testing Handbook, 2nd ed., Vol 7, Ultrasonic Testing,A.S.Birks, R. E. Green, and P. McIntire, eds., American Societ
47、y for NondestructiveTesting, Columbus, OH, 1991.FIG. 2 Typical EMAT Configuration for Bulk Wave GenerationE1816 1235.5 The EMAT techniques show benefits and advantagesover conventional piezoelectric ultrasonic techniques in specialapplications in which flexibility in the type of wave modegeneration
48、is desired. The EMATs are highly efficient ingenerating surface waves. The EMATs lend themselves tohorizontally polarized shear wave (SH) generation more easilythan do conventional ultrasonic search units. This is importantsince SH shear waves produce no mode conversions atinterfaces and their angle
49、 of introduction can be varied from 0to 90 simply by sweeping through various frequency RFgeneration. The EMATs can also be configured to produceLamb wave modes whose use can provide the full circumfer-ential examination of tubular products or volumetric examina-tion of thin plate material. The EMATs also lend themselveseasily to the repeatability of sensor fabrication, and hence theassociated sensor response is highly reproducible.6. Basis of Application6.1 The following items are subject to contractual agree-ment between the parties using or referencing this standard.6.2
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