ASTM E1962-98

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Designation: E 1962 – 98
Standard Test Method for
Ultrasonic Surface Examinations Using Electromagnetic
Acoustic Transducer (EMAT) Techniques
This standard is issued under the fixed designation E 1962; 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.
1. Scope E 587 Practice for Ultrasonic Angle-Beam Examination by
the Contact Method
1.1 This test method covers guidelines for utilizing EMAT
E 1316 Terminology for Nondestructive Examinations
techniques for detecting material discontinuities that are pri-
E 1774 Guide to Electromagnetic Acoustic Transducers
marily open to the surface (for example, cracks, seams, laps,
(EMATs)
cold shuts, laminations, through leaks, lack of fusion). This
E 1816 Practice for Ultrasonic Examinations Using Electro-
technique can also be sensitive to flaws and discontinuities that
magnetic Acoustic Transducer (EMAT) Techniques
are not surface-breaking, provided their proximity to the
2.2 ANSI/ASNT Standards:
surface is less than or equal to the Rayleigh wave length.
Recommended Practice SNT-TC-1A Personnel Qualifica-
1.2 This test method covers procedures for the non-contact
tion and Certification in Nondestructive Testing
coupling of surface waves into a material via electromagnetic
ANSI/ASNT CP-189 Standard for Qualification and Certi-
fields.
fication of Nondestructive Testing Personnel
1.3 The procedures of this test method are applicable to any
2.3 Military Standard:
material in which acoustic waves can be introduced electro-
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
magnetically. This includes any material that is either electri-
tion and Certification
cally conductive or ferromagnetic, or both.
1.4 This test method is intended to provide examination
3. Terminology
capabilities for in-process, final, and maintenance applications.
3.1 Definitions:
1.5 This test method does not provide standards for the
3.1.1 Additional related terminology is defined in Terminol-
evaluation of derived indications. Interpretation, classification,
ogy E 1316.
and ultimate evaluation of indications, albeit necessary, are
3.2 Definitions of Terms Specific to This Standard:
beyond the scope of this document. Separate specifications or
3.2.1 electromagnetic acoustic transducer (EMAT)—an
agreement will be necessary to define the type, size, location,
electromagnetic device for converting electrical energy into
and direction of indications considered acceptable or non-
acoustical energy in the presence of a magnetic field.
acceptable.
3.2.2 Lorentz forces—applied to electric currents when
1.6 The values stated in inch-pound units are to be regarded
placed in a magnetic field. Lorentz forces are perpendicular to
as the standards. The SI units given in parentheses are for
both the direction of the magnetic field and the current
information only.
direction. Lorentz forces are the forces responsible behind the
1.7 This standard does not purport to address all of the
principle of electric motors.
safety concerns, if any, associated with its use. It is the
3.2.3 magnetostrictive forces—forces arising from magnetic
responsibility of the user of this standard to establish appro-
domain wall movements within a magnetic material during
priate safety and health practices and determine the applica-
magnetization.
bility of regulatory limitations prior to use.
3.2.4 meander coil—an EMAT coil consisting of periodic,
2. Referenced Documents winding, non-intersecting, and usually evenly-spaced conduc-
tors.
2.1 ASTM Standards:
Annual Book of ASTM Standards, Vol 03.03.
1 3
This test method is under the jurisdiction of ASTM Committee E-7 on Available from American National Standards Institute, 11 W. 42nd St. 13th
Nondestructive Testing and is the direct responsibility of Subcommittee E07.06 on Floor, New York, NY 10036.
Ultrasonic Method. Available from the Superintendent of Documents, U.S. Government Printing
Current edition approved July 10, 1998. Published December 1998. Office, Washington, DC 20402.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1962
4. Summary of Test Method
4.1 The techniques outlined in this test method address the
electromagnetic generation of Rayleigh acoustic wave modes
for sensitivity to surface or near-surface flaws or discontinui-
ties. Flaws are detected by reflection or attenuation of acoustic
FIG. 2 Typical EMAT Meander Coil for Generation of Surface
waves from interactions at discontinuity interfaces. Waves
4.2 Fig. 1 shows one typical EMAT setup for the generation
5. Significance and Use
of Rayleigh waves. An external magnetic induction Bo parallel
to the surface is applicable on ferromagnetic material. The
5.1 EMAT techniques show benefits and advantages over
magnetic field may be generated by a permanent magnet, a conventional piezoelectric ultrasonic techniques in special
pulsed magnet, or a DC electromagnet. A meander RF coil is applications where flexibility in the type of wave mode
generation is desired. EMATs are highly efficient in the
oriented in the plane of and near the surface of the object to be
generation of surface waves.
tested. The magnetic field lines are tangential to the coil and
5.2 Since EMATs are highly efficient in the generation of
perpendicular to the conductor. The coil is excited by an RF
surface waves, and since acoustic techniques utilizing surface
toneburst pulse produced by a specialized EMAT pulser. A
waves are proven effective for detecting surface and near-
surface current is induced in the test sample by transformer
surface discontinuities, they should be considered for any
action. The surface current interacts with the external magnetic
applications where conventional penetrant testing and mag-
field by means of the Lorentz force. This disturbance is
netic particle NDT techniques are effective but undesirable.
transferred to the lattice of the solid and is thus the ultrasonic
5.3 Since EMAT techniques are non-contacting, they should
source responsible for producing the surface acoustic waves.
be considered for ultrasonic examinations where applications
As shown in Fig. 1, an ordinary meander coil produces
involve automation, high-speed inspections, moving objects,
bidirectional waves. In practice, specially designed meander
applications in remote or hazardous locations, applications to
coils can be designed to produce unidirectional waves.
objects at elevated temperatures, or objects with rough sur-
4.3 Fig. 2 illustrates a typical meander coil for generation of
faces.
surface waves. The following relationship must be valid for
5.4 The purpose of this test method is to promote the EMAT
Rayleigh wave generation with a meander coil:
technique of the ultrasonic method as a viable alternative to
V 5 2Df (1) conventional PT and MPT methods for detecting the presence
R
of surface and near-surface material discontinuities.
where:
5.5 The use of EMATs and the selection of appropriate
V 5 Rayleigh wave velocity,
R
operating parameters presuppose a knowledge of the geometry
D 5 separation of adjacent conductors, and
of the component; the probable location, size, orientation, and
f 5 frequency.
reflectivity of the expected flaws; the allowable range of EMAT
4.4 Surface flaws or discontinuities lead to reflection or
lift-off; and the laws of physics governing the propagation of
attenuation of the surface waves. Either pulse-echo or pitch-
ultrasonic waves. This procedure pertains to a specific EMAT
catch modes can be used. Upon approaching the receiver
surface inspection application.
EMAT, the reflected or attenuated ultrasonic waves produce
6. Basis of Application
oscillations within the conductor in the presence of the
magnetic field and thus induce a voltage in the receiver coil 6.1 Personnel Qualification—NDT Personnel should be
(similar to an electric generator) allowing detection. qualified in accordance with a nationally recognized NDT
FIG. 1 Typical EMAT Configuration for Rayleigh Wave Generation
E 1962
personnel qualification practice or standard such as ANSI/
ASNT-CP-189, SNT-TC-1A, MIL STD-410, or a similar docu-
ment. The practice or standard used and its applicable revision
shall be specified in the contractual agreement between the
using parties.
6.2 Procedures and Techniques—The procedures and tech-
niques to be utilized shall be as described in this document
unless otherwise specified. Specific techniques may be speci-
fied in the contractual agreement.
6.3 Reporting Criteria/Acceptance Criteria—Reporting cri-
teria for the examination results shall be in accordance with
Section 12 unless otherwise specified. Acceptance criteria shall
be specified in the contractual agreement.
6.4 Reexamination of Repaired/Re-Worked Items—
Reexamination of repaired/re-worked items is not addressed in
FIG. 3 Attenuation Technique
this test method and, if required, shall be specified in the
contractual agreement.
to the beam width, which must be narrow or focused in order
to achieve a minimum of 6 dB of attenuation.
7. Techniques
7.4 Diffraction Technique—The basis of the diffraction
7.1 This test method describes three separate techniques for
technique is illustrated in Fig. 4. Two collinear focused EMATs
EMAT surface wave examination. The first involves pulse-
(transmitter and receiver) or one pulse-echo EMAT are posi-
echo or pitch-catch techniques for the detection of reflected
tioned at an angle (the diffraction angle) with respect to the
surface waves. The second technique involves a pitch-catch
normal to the weld centerline. The weld root and crown act as
method sensitive to the attenuation of surface waves. The third
a specular reflector whose signals are reflected away from the
technique involves surface wave diffraction with focused
EMAT receiver. The flaw is, however, detected over a wide
meander coils.
angular range by means of diffraction. Depending upon the size
7.2 Pulse-Echo or Pitch-Catch Reflected Surface Wave
of the flaw relative to the ultrasonic wavelength, it acts as
Technique—These techniques are analogous to conventional
either a point diffractor or a series of point diffractors. A natural
ultrasonic techniques. The techniques use either one (pulse-
flaw such as a fatigue crack has a series of facets and branches
echo) or two (pitch-catch)EMAT sensors and rely upon the
that act as point diffractors. Therefore, it is typically possible to
reception of reflected surface waves from the flaw. The
detect the presence of a natural flaw several inches long with
advantage of these techniques is simplicity. One disadvantage
this technique. Surface waves can be focused to a region
is the difficulty in detecting all flaw orientations without
approximating a point focus (a region approaching a wave-
elaborate scanning routines. Also, when these techniques are
length as a limit). It is frequently advantageous to have a
used for weld applications, a problem arises in that the root and
reasonable focal depth to increase the area covered with each
crown of the weld can produce reflections that are prominent
linear scan. For many welds, it is possible to scan one half of
enough to interfere with and even obscure flaw signals. When
the weld crown from each side of the weld. A single EMAT
such interferences are apparent, it is recommended that one of
sensor is sensitive to all orientations except possibly a crack
the other two techniques described herein be utilized to avoid
parallel to the incident beam. Therefore, two sensors with
these problems.
positive and negative diffraction angles will be sensitive to all
7.3 Pitch-Catch Attenuation Technique—The attenuation flaw orientations.
technique is most effectively applied using the arrangement of
8. Apparatus
sensors illustrated in Fig. 3. The technique indicates the
presence of a flaw by noting attenuation of the UT signal. The 8.1 The apparatus may be considered to consist of the
sensors use small permanent magnets to generate narrow EMAT sensor and the EMAT instrumentation. The sensor
surface wave beams that cross at right angles. The preferred consists of an RF coil and a source of magnetic field. The
technique requires two channels of EMAT instrumentation instrumentation consists of a tone burst pulser/receiver, a data
although it may be modified for one-channel operation. To acquisition system or display device such as an oscilloscope,
implement one-channel operation, the distance between one an impedance matching network, a preamplifier (preferred),
transmitter receiver pair is increased slightly to displace the and a well shielded cable from the pulser/receiver to the sensor.
two received signals in time. The transmitter coils are then 8.2 Coil Design:
wired in series and the receiver coils are wired in series. This 8.2.1 A meander coil is used to produce surface waves. Coil
arrangement allows both pairs of EMAT coils to be used with design and conductor spacing required to produce a given
one channel of EMAT instrumentation. One advantage to the frequency has been discussed in 4.3. The coil may be focused
attenuation technique is sensitivity to all flaw orientations. or nonfocused depending upon the technique being used and
Another advantage to using the attenuation technique is the the desired resolution and sensitivity. Fig. 5 gives an example
ability to scan both sides of a weld simultaneously. It also scans of typical design parameters for a focused coil, similar to that
large areas of the material in one scan. A disadvantage relates used for the diffraction technique. Many EMAT coils are
E 1962
FIG. 4 The Diffraction Technique
FIG. 5 Design Parameters for Typical Focused EMAT Coil
flexible printed circuits produced by photoetching on a polya- The purpose of the wearplate is to maintain a constant liftoff
mide substrate. The surface of the coil can be backed with a for the EMAT coil. The wearplate should be pliable and
thin layer of foam and covered with a thin (0.001 to 0.005 in.) conform to irregular or curved surfaces. The polyamide sub-
wearplate of high molecular weight polyethylene or titanium. strate usually has a thickness of 0.001 or 0.002 in. For “as
E 1962
welded” crowns, the selected frequency for any of the tech- than those reference reflectors exemplifying the necessary
niques should be no greater than necessary in order to sensitivity. Flaw dimensions of length, depth, and width, must
minimize the respon
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