ASTM E1774-96

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.

General Information

• Status: Historical
• Publication Date: 09-Dec-1996
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.06 - Ultrasonic Method

Relations

• Replaced By: ASTM E1774-96(2002) - Standard Guide for Electromagnetic Acoustic Transducers (EMATs)
• Effective Date: 10-Dec-1996
• Referred By: ASTM E1816-18(2022) - Standard Practice for Measuring thickness by Pulse-Echo Electromagnetic Acoustic Transducer (EMAT) Methods
• Effective Date: 10-Dec-1996
• Referred By: ASTM E543-21 - Standard Specification for Agencies Performing Nondestructive Testing
• Effective Date: 10-Dec-1996
• Referred By: ASTM E213-22 - Standard Practice for Ultrasonic Testing of Metal Pipe and Tubing
• Effective Date: 10-Dec-1996

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 1774 – 96 An American National Standard
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. The EMAT as a generator of ultrasonic waves is basically a coil of wire, excited by an alternating
electric current, placed in a uniform magnetic field near the surface of an electrically conductive or
ferromagnetic material. A surface current is induced in the material by transformer action. This surface
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 the use of EMATs without thorough testing prior to their use
for examination purposes. Some applications in which EMATs
1.1 This guide is intended primarily for tutorial purposes. It
have been applied successfully are outlined in Section 9.
provides an overview of the general principles governing the
1.5 This standard does not purport to address all of the
operation and use of electromagnetic acoustic transducers
safety concerns, if any, associated with its use. It is the
(EMATs) for ultrasonic examination.
responsibility of the user of this standard to establish appro-
1.2 This guide describes a non-contact technique for cou-
priate safety and health practices and determine the applica-
pling ultrasonic energy into an electrically conductive or
bility of regulatory limitations prior to use.
ferromagnetic material, or both, through the use of electromag-
netic fields. This guide describes the theory of operation and
2. Referenced Documents
basic design considerations as well as the advantages and
2.1 ASTM Standards:
limitations of the technique.
E 127 Practice for Fabricating and Checking Aluminum
1.3 This guide is intended to serve as a general reference to
Alloy Ultrasonic Standard Reference Blocks
assist in determining the usefulness of EMATs for a given
E 428 Practice for Fabrication and Control of Steel Refer-
application as well as provide fundamental information regard-
ence Blocks Used in Ultrasonic Inspection
ing their design and operation. This guide provides guidance
E 1065 Guide for Evaluating Characteristics of Ultrasonic
for the generation of longitudinal, shear, Rayleigh, and Lamb
Search Units
wave modes using EMATs.
E 1316 Terminology for Nondestructive Examinations
1.4 This guide does not contain detailed procedures for the
2.2 ASNT Document:
use of EMATs in any specific applications; nor does it promote
Recommended Practice SNT-TC-1A Personnel Qualifica-
tions and Certification in Nondestructive Testing
This guide is under the jurisdiction of ASTM Committee E-7 on Nondestructive
Testing and is the direct responsibility of Subcommittee E07.06 on Ultrasonic
Method. Annual Book of ASTM Standards, Vol 03.03.
Current edition approved Dec. 10, 1996. Published February 1997. Originally Available from American Society for Nondestructive Testing, 1711 Arlingate
published as E 1774 – 95. Last previous edition E 1774 – 95. Plaza, Columbus, OH 43228.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1774
3. Terminology electrical conductors or ferromagnetic. The design of EMAT
probes is usually more complex than comparable piezoelectric
3.1 Definitions—Related terminology is defined in Termi-
search units. Due to their low efficiency, EMATs usually
nology E 1316.
require more specialized instrumentation for the generation and
3.2 Definitions of Terms Specific to This Standard:
detection of ultrasonic signals. High transmitting currents,
3.2.1 electromagnetic acoustic transducer (EMAT)—an
low-noise receivers, and careful electrical matching is impera-
electromagnetic device for converting electrical energy into
tive in system design. In general, EMAT probes are
acoustical energy in the presence of a magnetic field.
application-specific, in the same way as piezoelectric transduc-
3.2.2 Lorentz forces—forces applied to electric currents
ers.
when placed in a magnetic field. Lorentz forces are perpen-
dicular to the direction of both the magnetic field and the
5. Calibration and Standardization
current direction. Lorentz forces are the forces behind the
5.1 Reference Standards—As with conventional piezoelec-
principle of electric motors.
tric ultrasonic examinations, it is imperative that a set of
3.2.3 magnetostrictive forces—forces arising from magnetic
reference samples exhibiting the full range of expected mate-
domain wall movements within a magnetic material during
rial defect states be acquired or fabricated and consequently
magnetization.
examined by the technique to establish sensitivity (see Prac-
3.2.4 meander coil—an EMAT coil consisting of periodic,
tices E 127 and E 428).
winding, non-intersecting, and usually evenly-spaced conduc-
5.2 Transducer Characterization—Many of the conven-
tors.
tional contact piezoelectric search unit characterization proce-
3.2.5 pancake coil (spiral)—an EMAT coil consisting of
dures are generally adaptable to EMAT transducers with
spirally-wound, usually evenly-spaced conductors.
appropriate modifications, or variations thereof (see Guide
3.2.6 bulk wave—an ultrasonic wave, either longitudinal or
E 1065). Specific characterization procedures for EMATs are
shear mode, used in nondestructive testing to interrogate the
not available and are beyond the scope of this document.
volume of a material.
6. Theory (1-3)
4. Significance and Use
6.1 Nonmagnetic Conducting Materials—The mechanisms
4.1 General—Ultrasonic testing is a widely used nonde-
responsible for the generation of elastic waves in a conducting
structive method for the examination of a material. The
material are dependent on the characteristics of that material.
majority of ultrasonic tests are performed using transducers
The generation of acoustic waves in a nonmagnetic conductive
that directly convert electrical energy into acoustic energy
material is a result of the Lorentz force acting on the lattice of
through the use of piezoelectric crystals. This guide describes
the material. In an effort to understand the action of the Lorentz
an alternate technique in which electromagnetic energy is used
force, one can use the free electron model of solids. According
to produce acoustic energy inside an electrically conductive or
to the free electron model of conductors, the outer valence
ferromagnetic material. EMATs have unique characteristics
electrons have been stripped from the atomic lattice, leaving a
when compared to conventional piezoelectric ultrasonic search
lattice of positively charged ions in a sea of free electrons. In
units, making them a significant tool for some ultrasonic
order to generate elastic waves in a material, a net force must
testing applications.
be transmitted to the lattice of the material. If only an
4.2 Specific Advantages—Since the EMAT technique is
electromagnetic field is generated in a conductor (via an eddy
noncontacting, it requires no fluid couplant. Important conse-
current-type coil), the net force on the lattice is zero because
quences of this include applications to moving objects, in
the forces on the electrons and ions are equal and opposite. For
remote or hazardous locations, to objects at elevated tempera-
example:
tures, or to objects with rough surfaces. The technique is
force on electrons52qE
environmentally safe since it does not use potentially polluting
force on ions51qE
or hazardous chemicals. The technique facilitates the rapid
scanning of components having complex geometries. EMAT
where:
signals are highly reproducible as a consequence of the manner
q 5 electron charge, and
in which the acoustic waves are generated. EMATs can
E 5 electric field vector of EMAT wave.
produce horizontally polarized shear (SH) waves without mode
However, if the same electromagnetic field is generated in
conversion and can accommodate scanning while using SH
the presence of an applied static magnetic field, a net force is
waves. (Note that in order to produce this wave mode by
transmitted to the lattice and results in the generation of elastic
conventional ultrasonic techniques, either an epoxy or a highly
waves. The reason for this net force is the Lorentz force acting
viscous couplant is required. Thus, conventional ultrasonic
on the electrons and ions.
techniques do not lend themselves easily to scanning when
Lorentz force 5 F 5 qv 3 B (1)
L
using SH wave modes.) Also, EMATs provide for the capabil-
ity to steer shear waves electronically.
where:
4.3 Specific Limitations—EMATs have very low efficiency.
The insertion loss of EMATs can be as much as 40 dB or more
when compared to conventional ultrasonic methods. The
The boldface numbers in parentheses refer to the list of references at the end of
EMAT technique can be used only on materials that are this guide.
E 1774
wave generation, as compared with other modes excited in
v 5 velocity of electrons, and
ferromagnetic materials, is very low, and has no practical
B 5 static magnetic inductor vector.
relevance.
Since the electrons are free to move and the ions are bound
6.3.2 Shear Wave Modes—Fig. 2 shows how the direction
to the lattice, the Lorentz force on the electrons is much greater
of the applied static magnetic field in a conductor and the
due to its velocity dependence, and this force is transmitted to
resultant direction of the Lorentz force can produce shear
the ions in the lattice via the collision process.
elastic waves. For shear wave generation, the Lorentz force and
6.2 Magnetic Conducting Materials—For magnetic conduc-
thus ion displacement is parallel to the surface of the conduc-
tors, other forces such as magnetostrictive forces, in addition to
tor. EMATs are also capable of producing shear wave modes
the Lorentz force, influence ion motion. In magnetic materials,
with both vertical and horizontal polarizations. The distinction
the electromagnetic field can modulate the magnetization in the
between these two shear wave polarization modes is illustrated
material to produce periodic magnetostrictive stresses that
in Fig. 3.
must be added to the stresses caused by the Lorentz force. The
6.3.3 Rayleigh Wave Mode—In general, for Rayleigh or
magnetostrictive stresses are complicated and depend on the
surface wave generation, the applied static magnetic field will
magnetic domain distribution, which also depends on the
be oriented perpendicular to the surface of the conductor in the
strength and direction of the applied static magnetic field.
same manner used for shear wave propagation. A meander line
Although the magnetostrictive forces present in magnetic
or serpentine-type coil is used to provide a tuned frequency
conductors may complicate the theoretical analysis, this addi-
EMAT. The frequency of the EMAT is determined by the
tional coupling can be an asset because it can significantly
geometry (that is, line spacing) of the meander lines in the coil.
increase the signal strength compared to that obtained by the
By proper selection of frequency, it is possible to propagate
Lorentz force alone. At high applied magnetic field strengths
only Rayleigh or surface waves. If the thickness of the material
above the magnetic saturation of the material, the Lorentz force
is at least five times the acoustic wavelength that is determined
is the only source of acoustic wave generation. The magneto-
by the frequency and wave velocity, then Rayleigh wave
strictive force dominates at low field strengths, however, and
generation is essentially ensured.
the acoustic energy can be much greater than for corresponding
6.3.4 Lamb Wave Modes—The various Lamb wave modes
field strengths with only the Lorentz mechanism. Therefore, a
(symmetric and antisymmetric) can be generated in a manner
careful examination of the relationship at low applied field
similar to Rayleigh wave propagation. For Lamb wave produc-
strengths should be made in order to take full advantage of the
tion, the tuned frequency of the meander line coil is chosen to
magnetostrictive effort in magnetic materials.
give the desired Lamb wave mode and is dependent on the
6.3 Wave Modes—With the proper combination of magnet
material thickness.
and coil design, EMATs can produce longitudinal, shear,
Rayleigh, and Lamb wave modes (2-4). The direction of the
7. System Configuration
applied magnetic field, geometry of the coil, and frequency of
the electromagnetic field will determine the type of wave mode 7.1 Transducers—As in conventional piezoelectric-type ul-
generated with EMATs. trasonic testing, there are basically two types of EMATs with
6.3.1 Longitudinal Wave Mode—Fig. 1 illustrates how the respect to beam direction. EMATs can be designed for either
direction of the applied static magnetic field in a conductor and straight or angle beam inspection. Examples of these two types
the resultant direction of the Lorentz force can produce of transducers are presented in the following sections.
longitudinal elastic waves. For longitudinal wave generation, 7.1.1 Straight Beam—The spiral or pancake coil design is
the Lorentz force and thus ion displacement is perpendicular to one of the most efficient EMATs for producing a straight
the surface of the conductor. The efficiency
...