ASTM E1774-96(2002)
(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.
1.5 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 2002)
Standard Guide for
Electromagnetic Acoustic Transducers (EMATs)
This standard is issued under the fixed designation E 1774; 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 (e) 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
E 127 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
E 428 Practice for Fabrication and Control of Steel Refer-
for the generation of longitudinal, shear, Rayleigh, and Lamb
ence Blocks Used in Ultrasonic Examination
wave modes using EMATs.
E 1065 Guide for Evaluating Characteristics of Ultrasonic
Search Units
1 E 1316 Terminology for Nondestructive Examinations
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 2.2 ASNT Document:
Method.
Current edition approved December 10, 1996. Published February 1997. Origi-
nally published as E 1774 – 95. Last previous edition E 1774 – 95. Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1774–96 (2002)
Recommended Practice SNT-TC-1A Personnel Qualifica- using SH wave modes.)Also, EMATs provide for the capabil-
tions 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-
nology E 1316. EMAT technique can be used only on materials that are
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
when placed in a magnetic field. Lorentz forces are perpen- low-noise receivers, and careful electrical matching is impera-
tive in system design. In general, EMAT probes are
dicular to the direction of both the magnetic field and the
current direction. Lorentz forces are the forces behind the application-specific, in the same way as piezoelectric transduc-
principle of electric motors. ers.
3.2.3 magnetostrictiveforces—forcesarisingfrommagnetic
5. Calibration and Standardization
domain wall movements within a magnetic material during
magnetization. 5.1 Reference Standards—As with conventional piezoelec-
3.2.4 meander coil—an EMAT coil consisting of periodic, tric ultrasonic examinations, it is imperative that a set of
winding, non-intersecting, and usually evenly-spaced conduc- reference samples exhibiting the full range of expected mate-
tors. rial defect states be acquired or fabricated and consequently
3.2.5 pancake coil (spiral)—an EMAT coil consisting of examined by the technique to establish sensitivity (see Prac-
spirally-wound, usually evenly-spaced conductors. tices E 127 and E 428).
3.2.6 bulk wave—an ultrasonic wave, either longitudinal or 5.2 Transducer Characterization—Many of the conven-
shear mode, used in nondestructive testing to interrogate the tional contact piezoelectric search unit characterization proce-
volume of a material. dures are generally adaptable to EMAT transducers with
appropriate modifications, or variations thereof (see Guide
4. Significance and Use
E 1065). 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-
ducers that directly convert electrical energy into acoustic 6.1 Nonmagnetic Conducting Materials—The mechanisms
energy through the use of piezoelectric crystals. This guide responsible for the generation of elastic waves in a conducting
describes an alternate technique in which electromagnetic material are dependent on the characteristics of that material.
energy is used to produce acoustic energy inside an electrically The generation of acoustic waves in a nonmagnetic conductive
conductive or ferromagnetic material. EMATs have unique material is a result of the Lorentz force acting on the lattice of
characteristics when compared to conventional piezoelectric thematerial.InanefforttounderstandtheactionoftheLorentz
ultrasonicsearchunits,makingthemasignificanttoolforsome force, one can use the free electron model of solids.According
ultrasonic examination applications. to the free electron model of conductors, the outer valence
4.2 Specific Advantages—Since the EMAT technique is electrons have been stripped from the atomic lattice, leaving a
noncontacting, it requires no fluid couplant. Important conse- lattice of positively charged ions in a sea of free electrons. In
quences of this include applications to moving objects, in order to generate elastic waves in a material, a net force must
remote or hazardous locations, to objects at elevated tempera- be transmitted to the lattice of the material. If only an
tures, or to objects with rough surfaces. The technique is electromagnetic field is generated in a conductor (via an eddy
environmentally safe since it does not use potentially polluting current-type coil), the net force on the lattice is zero because
or hazardous chemicals. The technique facilitates the rapid the forces on the electrons and ions are equal and opposite. For
scanning of components having complex geometries. EMAT example:
signalsarehighlyreproducibleasaconsequenceofthemanner
force on electrons52qE
in which the acoustic waves are generated. EMATs can
force on ions51qE
producehorizontallypolarizedshear(SH)waveswithoutmode
conversion and can accommodate scanning while using SH
where:
waves. (Note that in order to produce this wave mode by q = electron charge, and
conventional ultrasonic techniques, either an epoxy or a highly E = electric field vector of EMAT wave.
viscous couplant is required. Thus, conventional ultrasonic
However, if the same electromagnetic field is generated in
techniques do not lend themselves easily to scanning when the presence of an applied static magnetic field, a net force is
3 4
Available from American Society for Nondestructive Testing, 1711 Arlingate The boldface numbers in parentheses refer to the list of references at the end of
Plaza, Columbus, OH 43228. this guide.
E1774–96 (2002)
transmitted to the lattice and results in the generation of elastic 6.3.1 Longitudinal Wave Mode—Fig. 1 illustrates how the
waves. The reason for this net force is the Lorentz force acting direction of the applied static magnetic field in a conductor and
on the electrons and ions. the resultant direction of the Lorentz force can produce
longitudinal elastic waves. For longitudinal wave generation,
Lorentz force 5 F 5 qv 3 B (1)
L
the Lorentz force and thus ion displacement is perpendicular to
where:
the surface of the conductor. The efficiency of longitudinal
v = velocity of electrons, and
wave generation, as compared with other modes excited in
B = static magnetic inductor vector.
ferromagnetic materials, is very low, and has no practical
Since the electrons are free to move and the ions are bound
relevance.
tothelattice,theLorentzforceontheelectronsismuchgreater
6.3.2 Shear Wave Modes—Fig. 2 shows how the direction
due to its velocity dependence, and this force is transmitted to
of the applied static magnetic field in a conductor and the
the ions in the lattice via the collision process.
resultant direction of the Lorentz force can produce shear
6.2 MagneticConductingMaterials—Formagneticconduc-
elasticwaves.Forshearwavegeneration,theLorentzforceand
tors,otherforcessuchasmagnetostrictiveforces,inadditionto
thus ion displacement is parallel to the surface of the conduc-
the Lorentz force, influence ion motion. In magnetic materials,
tor. EMATs are also capable of producing shear wave modes
theelectromagneticfieldcanmodulatethemagnetizationinthe
with both vertical and horizontal polarizations. The distinction
material to produce periodic magnetostrictive stresses that
between these two shear wave polarization modes is illustrated
must be added to the stresses caused by the Lorentz force. The
in Fig. 3.
magnetostrictive stresses are complicated and depend on the
6.3.3 Rayleigh Wave Mode—In general, for Rayleigh or
magnetic domain distribution, which also depends on the
surface wave generation, the applied static magnetic field will
strength and direction of the applied static magnetic field.
be oriented perpendicular to the surface of the conductor in the
Although the magnetostrictive forces present in magnetic
same manner used for shear wave propagation.Ameander line
conductors may complicate the theoretical analysis, this addi-
or serpentine-type coil is used to provide a tuned frequency
tional coupling can be an asset because it can significantly
EMAT. The frequency of the EMAT is determined by the
increase the signal strength compared to that obtained by the
geometry(thatis,linespacing)ofthemeanderlinesinthecoil.
Lorentz force alone. At high applied magnetic field strengths
By proper selection of frequency, it is possible to propagate
abovethemagneticsaturationofthematerial,theLorentzforce
onlyRayleighorsurfacewaves.Ifthethicknessofthematerial
is the only source of acoustic wave generation. The magneto-
is at least five times the acoustic wavelength that is determined
strictive force dominates at low field strengths, however, and
by the frequency and wave velocity, then Rayleigh wave
theacousticenergycanbemuchgreaterthanforcorresponding
generation is essentially ensured.
field strengths with only the Lorentz mechanism. Therefore, a
6.3.4 Lamb Wave Modes—The various Lamb wave modes
careful examination of the relationship at low applied field
(symmetric and antisymmetric) can be generated in a manner
strengths should be made in order to take full advantage of the
similar to Rayleigh wave propagation. For Lamb wave produc-
magnetostrictive effort in magnetic materials.
tion, the tuned frequency of the meander line coil is chosen to
6.3 Wave Modes—With the proper combination of magnet
give the desired Lamb wave mode and is dependent on the
and coil design, EMATs can produce longitudinal, shear,
material thickness.
Rayleigh, and Lamb wave modes (2-4). The direction of the
7. System Configuration
applied magnetic field, geometry of the coil, and frequency of
theelectromagneticfieldwilldeterminethetypeofwavemode 7.1 Transducers—As in conventional piezoelectric-type ul-
generated with EMATs. trasonic examination, there are basically two types of EMATs
FIG. 1 EMAT Generation of Longitudinal Waves
E1774–96 (2002)
FIG. 2 EMAT Generation of Shear Waves
FIG. 3 Illustration of Horizontal and Vertical Polarizations for Shear Waves
with respect to beam direction. EMATs can be designed for design of the EMAT.The same holds for butterfly coils, placed
either straight or angle beam examination. Examples of the
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