ASTM E1816-96
(Practice)Standard Practice for Ultrasonic Examinations Using Electromagnetic Acoustic Transducer (EMAT) Techniques
Standard Practice for Ultrasonic Examinations Using Electromagnetic Acoustic Transducer (EMAT) Techniques
SCOPE
1.1 This practice covers procedures for the use of electromagnetic acoustic transducers (EMATs) for specific ultrasonic examination applications. Recommendations are given for specific applications for using EMAT techniques to detect flaws through both surface and volumetric inspections as well as to measure thickness.
1.2 These procedures recommend technical details and guidelines for the reliable and reproductible ultrasonic detection of flaws and thickness measurements using electromagnetic acoustic transducers for both the pulsing and receiving of ultrasonic waves. The EMAT techniques described herein can be used as a basis for assessing the serviceability of various components nondestructively, as well as for process control in manufacturing.
1.3 These procedures cover noncontact techniques for coupling ultrasonic energy into materials through the use of electromagnetic fields. Surface, Lamb, longitudinal, and shear wave modes are discussed.
1.4 These procedures are intended to describe specific EMAT applications. These procedures are intended for applications in which the user has determined that the use of EMAT techniques can offer substantial benefits over conventional piezoelectric search units. It is not intended that EMAT techniques should be used in applications in which conventional techniques and applications offer superior benefits (refer to Guide E 1774).
1.5 These procedures are applicable to any material in which acoustic waves can be introduced electromagnetically. This includes any material that is either electrically conductive or ferromagnetic.
1.6 The procedures outlined in this practice address proven EMAT techniques for specific applications; they do nor purport to address the only variation or all variations of EMAT techniques to address the given applications. Latitude in application techniques is offered where options are considered appropriate.
1.7 The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are for information only.
1.8 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: E 1816 – 96
Standard Practice for
Ultrasonic Examinations Using Electromagnetic Acoustic
Transducer (EMAT) Techniques
This standard is issued under the fixed designation E 1816; 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 as the standard. The values given in parentheses are for
information only.
1.1 This practice covers procedures for the use of electro-
1.8 This standard does not purport to address all of the
magnetic acoustic transducers (EMATs) for specific ultrasonic
safety concerns, if any, associated with its use. It is the
examination applications. Recommendations are given for
responsibility of the user of this standard to establish appro-
specific applications for using EMAT techniques to detect
priate safety and health practices and determine the applica-
flaws through both surface and volumetric inspections as well
bility of regulatory limitations prior to use.
as to measure thickness.
1.2 These procedures recommend technical details and
2. Referenced Documents
guidelines for the reliable and reproducible ultrasonic detection
2.1 ASTM Standards:
of flaws and thickness measurements using electromagnetic
E 114 Practice for Ultrasonic Pulse-Echo Straight-Beam
acoustic transducers for both the pulsing and receiving of
Examination by the Contact Method
ultrasonic waves. The EMAT techniques described herein can
E 494 Practice for Measuring Ultrasonic Velocity in Mate-
be used as a basis for assessing the serviceability of various
rials
components nondestructively, as well as for process control in
E 587 Practice for Ultrasonic Angle-Beam Examination by
manufacturing.
the Contact Method
1.3 These procedures cover noncontact techniques for cou-
E 797 Practice for Measuring Thickness by Manual Ultra-
pling ultrasonic energy into materials through the use of
sonic Pulse-Echo Contact Method
electromagnetic fields. Surface, Lamb, longitudinal, and shear
E 1316 Terminology for Nondestructive Examinations
wave modes are discussed.
E 1774 Guide to Electromagnetic Acoustic Transducers
1.4 These procedures are intended to describe specific
(EMATs)
EMAT applications. These procedures are intended for appli-
2.2 ASNT Standards:
cations in which the user has determined that the use of EMAT
SNT-TC-1A Recommended Practice for Personnel Qualifi-
techniques can offer substantial benefits over conventional
cations and Certification in Nondestructive Testing
piezoelectric search units. It is not intended that EMAT
ANSI/ASNT CP-189 Standard for Qualification and Certi-
techniques should be used in applications in which conven-
fication of Nondestructive Testing Personnel
tional techniques and applications offer superior benefits (refer
2.3 Military Standard:
to Guide E 1774).
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
1.5 These procedures are applicable to any material in
tion and Certification
which acoustic waves can be introduced electromagnetically.
This includes any material that is either electrically conductive
3. Terminology
or ferromagnetic.
3.1 Definitions—Related terminology is defined in Termi-
1.6 The procedures outlined in this practice address proven
nology E 1316.
EMAT techniques for specific applications; they do not purport
3.2 Definitions of Terms Specific to This Standard:
to address the only variation or all variations of EMAT
3.2.1 bulk wave—an ultrasonic wave, either longitudinal or
techniques to address the given applications. Latitude in
shear mode, used in nondestructive testing to interrogate the
application techniques is offered where options are considered
volume of a material.
appropriate.
3.2.2 electromagnetic acoustic transducer (EMAT)—an
1.7 The values stated in inch-pound units are to be regarded
electromagnetic device for converting electrical energy into
Annual Book of ASTM Standards, Vol 03.03.
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde-
Available from American Society for Nondestructive Testing, 1711 Arlingate
structive Testing and is the direct responsibility of Subcommittee E07.06 on
Plaza, P.O. Box 28518, Columbus, OH 43228-0518.
Ultrasonic Method.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
Current edition approved May 10, 1996. Published July 1996.
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1816
acoustical energy in the presence of a magnetic field. 4.2 Volumetric Examination:
3.2.3 lift-off effect—refer to Terminology E 1316, Section C. 4.2.1 Sensitivity to flaws or discontinuities within a part
3.2.4 Lorentz forces—forces applied to electric currents
requires the use of bulk acoustic wave modes to interrogate the
when placed in a magnetic field. Lorentz forces are perpen- volume of the material. As with surface examinations, reliance
dicular to both the direction of the magnetic field and the on the reflection or attenuation of acoustic waves from discon-
current direction. Lorentz forces are the forces responsible tinuity interfaces forms the basis for the detection of flaws.
behind the principle of electric motors.
4.2.2 Depending on the particular application, either longi-
3.2.5 magnetostrictive forces—forces arising from magnetic tudinal or shear wave modes may be desirable. While straight
domain wall movements within a magnetic material during beam applications using pulse-echo techniques are the most
magnetization. straightforward, angle beam pitch-catch techniques could be
3.2.6 meander coil—an EMAT coil consisting of periodic, desirable, depending on such factors as expected flaw location
winding, nonintersecting, and usually evenly spaced conduc- and orientation.
tors. 4.2.3 Fig. 2 shows one typical EMAT setup for the trans-
3.2.7 pancake (spiral) coil—an EMAT coil consisting of duction of bulk waves. As shown, an external magnetic
spirally wound, usually evenly spaced conductors. induction B is applied normal to the surface of an electrically
conductive or ferromagnetic material. A spiral EMAT coil is
4. Summary of Practice
used for this example. The coil is positioned in a plane parallel
4.1 Surface Examination: to the surface of the material and is excited by an electrical
current pulse. A surface current is produced in the material by
4.1.1 The generation of Rayleigh or surface waves in the
material to be examined allows for sensitivity to surface flaws transformer action. The surface current, in the presence of the
magnetic field, experiences Lorentz forces that produce oscil-
and discontinuities. Flaws can be detected by reflections of
acoustic waves from the discontinuity interfaces or by acoustic lating stress waves originating in the surface of the material.
Radially polarized shear waves are generated for this example.
wave attenuation in traversing across the surface of the
component. Either pulse-echo or pitch-catch ultrasonic tech- Depending on the design characteristics of the magnetic field,
niques can be used. the excitation of either radially polarized or planar-polarized
4.1.2 Fig. 1 shows a typical EMAT setup for the transduc- shear waves, propagating normal to the surface, can be
tion of Rayleigh or Lamb waves. As shown, an external introduced. Longitudinal wave modes can also be generated
magnetic induction B is applied parallel to the surface of an and used effectively in non-ferromagnetic materials. Longitu-
electrically conductive or ferromagnetic material. A meander dinal wave generation in ferromagnetic materials is impractical
coil is used. The coil is oriented in the same plane as the due to unacceptably low coupling efficiency. Mode-converted
surface of the material and is excited by an electrical radio longitudinal waves can be used effectively. Paragraph 7.2 and
the subparagraphs of 7.2 give a more in-depth discussion of the
frequency (RF) pulse. A surface current is produced in the
material by transformer action. The surface current, in the various EMAT/magnet configurations for producing various
bulk wave modes.
presence of the magnetic field, experiences Lorentz forces that
produce oscillating stress waves perpendicular to the surface of 4.3 Thickness Gaging:
the material to produce surface acoustic waves. Basic EMAT 4.3.1 Determining the thickness of a material by ultrasonic
designs generate bidirectional surface waves. Specialized de- means is a matter of coupling an ultrasonic wave into the
signs can be used to generate unidirectional waves, as with material, allowing the sound wave to propagate through the
conventional ultrasonic testing. material, reflect from the backwall boundary interface of the
4.1.3 Surface flaws or discontinuities lead to reflections or material, and propagate back to the front surface. The thickness
attenuation of the surface waves. Upon approach to the of the material can be calculated by measuring the transit time
receiver EMAT, the reflected or attenuated ultrasonic waves of the ultrasonic wave, and through knowledge of the ultra-
produce oscillations within the conductor in the presence of the sonic wave velocity. Thickness measurements can also be
magnetic field and thus produce a voltage induction in the coil, extrapolated for a given material through standardizations of
allowing for detection. transit time as a function of thickness as derived from a
FIG. 1 Typical EMAT Configuration for Rayleigh or Lamb Wave Generation
E 1816
be considered for ultrasonic examinations in which applica-
tions involve automation, high-speed inspections, moving
objects, applications in remote or hazardous locations, and
applications to objects at elevated temperatures or objects with
rough surfaces. This practice describes procedures for using
EMAT techniques as associated with the ultrasonic method to
detect flaws for both surface and volumetric examinations as
well as to measure thickness.
5.2 The uniqueness of the electromagnetic acoustic trans-
ducer technique for ultrasonic examination basically lies in the
generation 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 a
material by electromagnetic means; electrically conductive or
ferromagnetic materials can be examined. In its simplest form,
an EMAT as a generator of ultrasonic waves is basically a coil
of wire, excited by an alternating current, and placed in a
uniform magnetic field near the surface of a material. For
conductive materials, eddy currents are induced as a result of
the alternating current. Due to the magnetic field, these eddy
currents experience Lorentz forces that in turn are transmitted
to the solid by collisions with the lattice or other microscopic
processes. These forces are alternating at the frequency of the
driving current and act as a source of ultrasonic waves. If the
material is ferromagnetic, additional coupling mechanisms
play a part in the generation of ultrasonic waves. Interactions
between the dynamic magnetic field generated by the alternat-
ing currents and the magnetization associated with the material
offer a source of coupling, as do the associated magnetostric-
FIG. 2 Typical EMAT Configuration for Bulk Wave Generation
tive influences. Reciprocal processes exist whereby all of these
mechanisms lead to detection. Fig. 3 depicts the mechanisms
standardization block (see Practice E 797 and 7.3.1).
4.3.2 The ultrasonic velocity of the material under exami- (forces), along with associated direction, for electromagnetic
ultrasound generation.
nation is a function of the physical properties of the material,
namely, stiffness and density. It is usually assumed to be 5.4 The EMAT can be used to generate all ultrasonic modes
of vibration. As with conventional ultrasonic techniques, ma-
constant for a given class of materials. Approximate velocity
values are available in tabular format from numerous sources, terial types, probable flaw locations, and flaw orientations
determine the selection of beam directions and modes of
including the ASNT Nondestructive Testing Handbook. Ve-
locity values can also be determined empirically (see Practice vibration. The use of EMATs and selection of the proper wave
mode presuppose a knowledge of the geometry of the object;
E 494).
the probable location, size, orientation, and reflectivity of the
4.3.3 Determination of the transit time of an acoustic wave
expected flaws; the allowable range of EMAT lift-off; and the
through a material requires the use of bulk acoustic wave
laws of physics governing the propagation of ultrasonic waves.
modes. While longitudinal waves can be used, it is often
5.5 The EMAT techniques show benefits and advantages
desirable to use shear waves since their slower propagation
velocities lend themselves to more accurate measurements of
thin materials. While straight beam applications using pulse-
echo techniques are the most straightforward and popular,
angle beam pitch-catch techniques could be desirable, espe-
cially in applications in which fast scan rates are needed or
high resolution is desired for thin material. The generation of
bulk waves by means of the EMAT technique is discussed in
4.2.3 and depicted in Fig. 2.
5. Significance and Use
5.1 Since EMAT techniques are noncontacting, they should
NOTE 1—j 5 current in a single conductor, Bo 5 magnetization from
external magnet, Fm 5 magnetic force (ferromagnetic material),
Fms 5 magnetostrictive force (ferromagnetic material), and
Nondestructive Testing Handbook, 2nd ed., Vol 7, Ultrasonic Testing,A. S.
FL 5 Lorentz force (conductive material).
Birks, R. E. Green, and P. McIntire, eds., American Society for Nondestructive
Testing, Columbus, OH, 1991. FIG. 3 Mechanisms of Electromagnetic Ultrasound Generation
E 1816
over conventional piezoelectric ultrasonic techniques in special
applications in which flexibility in the type of wave mode
generation is desired. The EMATs are highly efficient in
generating surface waves. The EMATs lend themselves to
horizontally polarized shear wave (SH) generation more easily
than do conventional ultrasonic search units. This is important
since SH shear waves produce no mode conversions at
interfaces and their angle of introduction can be varied from 0
to 90° simply by sweeping through various frequency RF
generation. The EMATs can also be configured to produce
Lamb wave modes whose use can provide the ful
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