ASTM E1774-96(2007)
(Guide)Standard Guide for Electromagnetic Acoustic Transducers (EMATs)
Standard Guide for Electromagnetic Acoustic Transducers (EMATs)
SCOPE
1.1 This guide is intended primarily for tutorial purposes. It provides an overview of the general principles governing the operation 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 electrically conductive or ferromagnetic material, or both, through the use of electromagnetic fields. This guide describes the theory of operation and basic design considerations as well as the advantages and limitations of the technique.
1.3 This guide is intended to serve as a general reference to assist in determining the usefulness of EMATs for a given application as well as provide fundamental information regarding their design and operation. This guide provides guidance for the generation of longitudinal, shear, Rayleigh, and Lamb wave modes using EMATs.
1.4 This guide does not contain detailed procedures for the use of EMATs in any specific applications; nor does it promote the use of EMATs without thorough testing prior to their use for examination purposes. Some applications in which EMATs have been applied successfully are outlined in Section 9.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E1774–96 (Reapproved 2007)
Standard Guide for
Electromagnetic Acoustic Transducers (EMATs)
This standard is issued under the fixed designation E1774; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
General—The 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 some
form of electromechanical conversion, usually the piezoelectric effect.This highly efficient method of
generating ultrasonic waves has a disadvantage in that a fluid is generally required for mechanical
coupling of the sound into the material being examined. The use of a couplant generally requires that
the material being examined be either immersed in a fluid or covered with a thin layer of fluid.
Principle—An electromagnetic acoustic transducer (EMAT) generates and receives ultrasonic
waves without the need to contact the material in which the acoustic waves are traveling. The use of
an EMAT requires that the material to be examined be electrically conductive or ferromagnetic, or
both.TheEMATasageneratorofultrasonicwavesisbasicallyacoilofwire,excitedbyanalternating
electric current, placed in a uniform magnetic field near the surface of an electrically conductive or
ferromagneticmaterial.Asurfacecurrentisinducedinthematerialbytransformeraction.Thissurface
current in the presence of a magnetic field experiences Lorentz forces that produce oscillating stress
waves. Upon reception of an ultrasonic wave, the surface of the conductor oscillates in the presence
of a magnetic field, thus inducing a voltage in the coil. The transduction process occurs within an
electromagnetic skin depth.An EMAT forms the basis for a very reproducible noncontact system for
generating and detecting ultrasonic waves.
1. Scope 1.4 This guide does not contain detailed procedures for the
use of EMATs in any specific applications; nor does it promote
1.1 This guide is intended primarily for tutorial purposes. It
the use of EMATs without thorough testing prior to their use
provides an overview of the general principles governing the
for examination purposes. Some applications in which EMATs
operation and use of electromagnetic acoustic transducers
have been applied successfully are outlined in Section 9.
(EMATs) for ultrasonic examination.
1.5 This standard does not purport to address all of the
1.2 This guide describes a non-contact technique for cou-
safety concerns, if any, associated with its use. It is the
pling ultrasonic energy into an electrically conductive or
responsibility of the user of this standard to establish appro-
ferromagneticmaterial,orboth,throughtheuseofelectromag-
priate safety and health practices and determine the applica-
netic fields. This guide describes the theory of operation and
bility of regulatory limitations prior to use.
basic design considerations as well as the advantages and
limitations of the technique.
2. Referenced Documents
1.3 This guide is intended to serve as a general reference to
2.1 ASTM Standards:
assist in determining the usefulness of EMATs for a given
E127 Practice for Fabricating and Checking Aluminum
application as well as provide fundamental information regard-
Alloy Ultrasonic Standard Reference Blocks
ing their design and operation. This guide provides guidance
E428 Practice for Fabrication and Control of Metal, Other
for the generation of longitudinal, shear, Rayleigh, and Lamb
than Aluminum, Reference Blocks Used in Ultrasonic
wave modes using EMATs.
Testing
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.06 on Ultrasonic
Method. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJuly1,2007.PublishedJuly2007.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1995. Last previous edition approved in 2002 as E1774 - 96(2002). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E1774-96R07. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1774–96 (2007)
E1065 Guide for Evaluating Characteristics of Ultrasonic waves. (Note that in order to produce this wave mode by
Search Units conventional ultrasonic techniques, either an epoxy or a highly
E1316 Terminology for Nondestructive Examinations viscous couplant is required. Thus, conventional ultrasonic
2.2 ASNT Document: techniques do not lend themselves easily to scanning when
SNT-TC-1A Recommended Practice for Personnel Qualifi- using SH wave modes.)Also, EMATs provide for the capabil-
cations and Certification in Nondestructive Testing ity to steer shear waves electronically.
4.3 Specific Limitations—EMATs have very low efficiency.
3. Terminology
The insertion loss of EMATs can be as much as 40 dB or more
when compared to conventional ultrasonic methods. The
3.1 Definitions—Related terminology is defined in Termi-
EMAT technique can be used only on materials that are
nology E1316.
electrical conductors or ferromagnetic. The design of EMAT
3.2 Definitions of Terms Specific to This Standard:
3.2.1 electromagnetic acoustic transducer (EMAT)—an probes is usually more complex than comparable piezoelectric
search units. Due to their low efficiency, EMATs usually
electromagnetic device for converting electrical energy into
acoustical energy in the presence of a magnetic field. requiremorespecializedinstrumentationforthegenerationand
detection of ultrasonic signals. High transmitting currents,
3.2.2 Lorentz forces—forces applied to electric currents
low-noise receivers, and careful electrical matching is impera-
when placed in a magnetic field. Lorentz forces are perpen-
tive in system design. In general, EMAT probes are
dicular to the direction of both the magnetic field and the
application-specific, in the same way as piezoelectric transduc-
current direction. Lorentz forces are the forces behind the
ers.
principle of electric motors.
3.2.3 magnetostrictiveforces—forcesarisingfrommagnetic
5. Calibration and Standardization
domain wall movements within a magnetic material during
5.1 Reference Standards—As with conventional piezoelec-
magnetization.
tric ultrasonic examinations, it is imperative that a set of
3.2.4 meander coil—an EMAT coil consisting of periodic,
reference samples exhibiting the full range of expected mate-
winding, non-intersecting, and usually evenly-spaced conduc-
rial defect states be acquired or fabricated and consequently
tors.
examined by the technique to establish sensitivity (see Prac-
3.2.5 pancake coil (spiral)—an EMAT coil consisting of
tices E127 and E428).
spirally-wound, usually evenly-spaced conductors.
5.2 Transducer Characterization—Many of the conven-
3.2.6 bulk wave—an ultrasonic wave, either longitudinal or
tional contact piezoelectric search unit characterization proce-
shear mode, used in nondestructive testing to interrogate the
dures are generally adaptable to EMAT transducers with
volume of a material.
appropriate modifications, or variations thereof (see Guide
4. Significance and Use
E1065). Specific characterization procedures for EMATs are
not available and are beyond the scope of this document.
4.1 General—Ultrasonic testing is a widely used nonde-
structive method for the examination of a material. The
6. Theory (1-3)
majority of ultrasonic examinations are performed using trans-
6.1 Nonmagnetic Conducting Materials—The mechanisms
ducers that directly convert electrical energy into acoustic
responsible for the generation of elastic waves in a conducting
energy through the use of piezoelectric crystals. This guide
material are dependent on the characteristics of that material.
describes an alternate technique in which electromagnetic
The generation of acoustic waves in a nonmagnetic conductive
energy is used to produce acoustic energy inside an electrically
material is a result of the Lorentz force acting on the lattice of
conductive or ferromagnetic material. EMATs have unique
thematerial.InanefforttounderstandtheactionoftheLorentz
characteristics when compared to conventional piezoelectric
force, one can use the free electron model of solids.According
ultrasonicsearchunits,makingthemasignificanttoolforsome
to the free electron model of conductors, the outer valence
ultrasonic examination applications.
electrons have been stripped from the atomic lattice, leaving a
4.2 Specific Advantages—Since the EMAT technique is
lattice of positively charged ions in a sea of free electrons. In
noncontacting, it requires no fluid couplant. Important conse-
order to generate elastic waves in a material, a net force must
quences of this include applications to moving objects, in
be transmitted to the lattice of the material. If only an
remote or hazardous locations, to objects at elevated tempera-
electromagnetic field is generated in a conductor (via an eddy
tures, or to objects with rough surfaces. The technique is
current-type coil), the net force on the lattice is zero because
environmentally safe since it does not use potentially polluting
the forces on the electrons and ions are equal and opposite. For
or hazardous chemicals. The technique facilitates the rapid
example:
scanning of components having complex geometries. EMAT
signalsarehighlyreproducibleasaconsequenceofthemanner
force on electrons52qE
in which the acoustic waves are generated. EMATs can
force on ions51qE
producehorizontallypolarizedshear(SH)waveswithoutmode
where:
conversion and can accommodate scanning while using SH
3 4
AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box The boldface numbers in parentheses refer to the list of references at the end of
28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org. this guide.
E1774–96 (2007)
strengths should be made in order to take full advantage of the
q = electron charge, and
magnetostrictive effort in magnetic materials.
E = electric field vector of EMAT wave.
However, if the same electromagnetic field is generated in 6.3 Wave Modes—With the proper combination of magnet
the presence of an applied static magnetic field, a net force is
and coil design, EMATs can produce longitudinal, shear,
transmitted to the lattice and results in the generation of elastic
Rayleigh, and Lamb wave modes (2-4). The direction of the
waves. The reason for this net force is the Lorentz force acting
applied magnetic field, geometry of the coil, and frequency of
on the electrons and ions.
theelectromagneticfieldwilldeterminethetypeofwavemode
generated with EMATs.
Lorentz force 5 F 5 qv 3 B (1)
L
6.3.1 Longitudinal Wave Mode—Fig. 1 illustrates how the
where:
direction of the applied static magnetic field in a conductor and
v = velocity of electrons, and
the resultant direction of the Lorentz force can produce
B = static magnetic inductor vector.
longitudinal elastic waves. For longitudinal wave generation,
Since the electrons are free to move and the ions are bound
the Lorentz force and thus ion displacement is perpendicular to
tothelattice,theLorentzforceontheelectronsismuchgreater
the surface of the conductor. The efficiency of longitudinal
due to its velocity dependence, and this force is transmitted to
wave generation, as compared with other modes excited in
the ions in the lattice via the collision process.
ferromagnetic materials, is very low, and has no practical
6.2 MagneticConductingMaterials—Formagneticconduc-
relevance.
tors,otherforcessuchasmagnetostrictiveforces,inadditionto
6.3.2 Shear Wave Modes—Fig. 2 shows how the direction
the Lorentz force, influence ion motion. In magnetic materials,
of the applied static magnetic field in a conductor and the
theelectromagneticfieldcanmodulatethemagnetizationinthe
resultant direction of the Lorentz force can produce shear
material to produce periodic magnetostrictive stresses that
elasticwaves.Forshearwavegeneration,theLorentzforceand
must be added to the stresses caused by the Lorentz force. The
thus ion displacement is parallel to the surface of the conduc-
magnetostrictive stresses are complicated and depend on the
tor. EMATs are also capable of producing shear wave modes
magnetic domain distribution, which also depends on the
with both vertical and horizontal polarizations. The distinction
strength and direction of the applied static magnetic field.
between these two shear wave polarization modes is illustrated
Although the magnetostrictive forces present in magnetic
in Fig. 3.
conductors may complicate the theoretical analysis, this addi-
tional coupling can be an asset because it can significantly 6.3.3 Rayleigh Wave Mode—In general, for Rayleigh or
increase the signal strength compared to that obtained by the surface wave generation, the applied static magnetic field will
Lorentz force alone. At high applied magnetic field strengths be oriented perpendicular to the surface of the conductor in the
abovethemagneticsaturationofthematerial,theLorentzforce same manner used for shear wave propagation.Ameander line
is the only source of acoustic wave generation. The magneto- or serpentine-type coil is used to provide a tuned frequency
strictive force dominates at low field strengths, however, and EMAT. The frequency of the EMAT is determined by the
theacousticenergycanbemuchgreaterthanforcorresponding geometry(thatis,linespacing)ofthemeanderlinesinthecoil.
field strengths with only the Lorentz mechanism. Therefore, a By proper selection of frequency, it is possible to propagate
careful examination of the relationship at low applied field onlyRayleighorsurfacewaves.Ifthethicknessofthematerial
FIG. 1 EMAT Generation of Longitudinal Waves
E1774–96 (2007)
FIG. 2 EMAT Generation of Shear Waves
FIG. 3 Illustration of Horizontal and Vertical Polarizations for Shear Waves
is at least five times the acoustic wavelength that is determined 7.1.1 Straight Beam—The spiral or pancake coil design is
by the frequency and wave velocity, then Rayleigh wave one of the most efficient EMATs for producing a straight
generation is essentially ensured. ultrasonic beam. The direction of the applied magnetic field is
6.3.4 Lamb Wave Modes—The various Lamb wave modes perpendicular to the plane of the spiral coil, as shown in Fig. 4.
(symmetric and antisymmetric) can be generated in a manner Themagne
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