ASTM E1816-96(2002)
(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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Standards Content (Sample)
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Designation: E 1816 – 96 (Reapproved 2002)
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 1.7 The values stated in inch-pound units are to be regarded
as the standard. The values given in parentheses are for
1.1 This practice covers procedures for the use of electro-
information only.
magnetic acoustic transducers (EMATs) for specific ultrasonic
1.8 This standard does not purport to address all of the
examination applications. Recommendations are given for
safety concerns, if any, associated with its use. It is the
specific applications for using EMAT techniques to detect
responsibility of the user of this standard to establish appro-
flaws through both surface and volumetric examinations as
priate safety and health practices and determine the applica-
well as to measure thickness.
bility of regulatory limitations prior to use.
1.2 These procedures recommend technical details and
guidelinesforthereliableandreproducibleultrasonicdetection
2. Referenced Documents
of flaws and thickness measurements using electromagnetic
2.1 ASTM Standards:
acoustic transducers for both the pulsing and receiving of
E 114 Practice for Ultrasonic Pulse-Echo Straight-Beam
ultrasonic waves. The EMAT techniques described herein can
Examination by the Contact Method
be used as a basis for assessing the serviceability of various
E 494 Practice for Measuring Ultrasonic Velocity in Mate-
components nondestructively, as well as for process control in
rials
manufacturing.
E 587 Practice for Ultrasonic Angle-Beam Examination by
1.3 These procedures cover noncontact techniques for cou-
the Contact Method
pling ultrasonic energy into materials through the use of
E 797 Practice for Measuring Thickness by Manual Ultra-
electromagnetic fields. Surface, Lamb, longitudinal, and shear
sonic Pulse-Echo Contact Method
wave modes are discussed.
E 1316 Terminology for Nondestructive Examinations
1.4 These procedures are intended to describe specific
E 1774 Guide to Electromagnetic Acoustic Transducers
EMAT applications. These procedures are intended for appli-
(EMATs)
cations in which the user has determined that the use of EMAT
2.2 ASNT Standards:
techniques can offer substantial benefits over conventional
SNT-TC-1A Recommended Practice for Personnel Qualifi-
piezoelectric search units. It is not intended that EMAT
cations and Certification in Nondestructive Testing
techniques should be used in applications in which conven-
ANSI/ASNT CP-189 Standard for Qualification and Certi-
tional techniques and applications offer superior benefits (refer
fication of Nondestructive Testing Personnel
to Guide E 1774).
2.3 Military Standard:
1.5 These procedures are applicable to any material in
MIL-STD-410 Nondestructive Testing Personnel Qualifica-
which acoustic waves can be introduced electromagnetically.
tion and Certification
This includes any material that is either electrically conductive
or ferromagnetic.
3. Terminology
1.6 The procedures outlined in this practice address proven
3.1 Definitions—Related terminology is defined in Termi-
EMATtechniques for specific applications; they do not purport
nology E 1316.
to address the only variation or all variations of EMAT
3.2 Definitions of Terms Specific to This Standard:
techniques to address the given applications. Latitude in
application techniques is offered where options are considered
appropriate.
Annual Book of ASTM Standards, Vol 03.03.
1 3
This practice is under the jurisdiction of ASTM Committee E07 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. AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
Current edition approved May 10, 1996. Published July 1996. Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1816 – 96 (2002)
3.2.1 bulk wave—an ultrasonic wave, either longitudinal or 4.1.3 Surface flaws or discontinuities lead to reflections or
shear mode, used in nondestructive testing to interrogate the attenuation of the surface waves. Upon approach to the
volume of a material. receiver EMAT, the reflected or attenuated ultrasonic waves
3.2.2 electromagnetic acoustic transducer (EMAT)—an produceoscillationswithintheconductorinthepresenceofthe
electromagnetic device for converting electrical energy into magnetic field and thus produce a voltage induction in the coil,
acoustical energy in the presence of a magnetic field. allowing for detection.
3.2.3 lift-off effect—refertoTerminologyE 1316,SectionC.
4.2 Volumetric Examination:
3.2.4 Lorentz forces—forces applied to electric currents
4.2.1 Sensitivity to flaws or discontinuities within a part
when placed in a magnetic field. Lorentz forces are perpen-
requires the use of bulk acoustic wave modes to interrogate the
dicular to both the direction of the magnetic field and the
volume of the material.As with surface examinations, reliance
current direction. Lorentz forces are the forces responsible
on the reflection or attenuation of acoustic waves from discon-
behind the principle of electric motors.
tinuity interfaces forms the basis for the detection of flaws.
3.2.5 magnetostrictive forces—forcesarisingfrommagnetic
4.2.2 Depending on the particular application, either longi-
domain wall movements within a magnetic material during
tudinal or shear wave modes may be desirable. While straight
magnetization.
beam applications using pulse-echo techniques are the most
3.2.6 meander coil—an EMAT coil consisting of periodic,
straightforward, angle beam pitch-catch techniques could be
winding, nonintersecting, and usually evenly spaced conduc-
desirable, depending on such factors as expected flaw location
tors.
and orientation.
3.2.7 pancake (spiral) coil—an EMAT coil consisting of
4.2.3 Fig. 2 shows one typical EMAT setup for the trans-
spirally wound, usually evenly spaced conductors.
duction of bulk waves. As shown, an external magnetic
induction B is applied normal to the surface of an electrically
4. Summary of Practice
conductive or ferromagnetic material. A spiral EMAT coil is
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
4.1.1 The generation of Rayleigh or surface waves in the
current pulse. A surface current is produced in the material by
material to be examined allows for sensitivity to surface flaws
transformer action. The surface current, in the presence of the
and discontinuities. Flaws can be detected by reflections of
magnetic field, experiences Lorentz forces that produce oscil-
acoustic waves from the discontinuity interfaces or by acoustic
lating stress waves originating in the surface of the material.
wave attenuation in traversing across the surface of the
Radially polarized shear waves are generated for this example.
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
dinalwavegenerationinferromagneticmaterialsisimpractical
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
frequency (RF) pulse. A surface current is produced in the
thesubparagraphsof7.2giveamorein-depthdiscussionofthe
material by transformer action. The surface current, in the
various EMAT/magnet configurations for producing various
presence of the magnetic field, experiences Lorentz forces that
bulk wave modes.
produceoscillatingstresswavesperpendiculartothesurfaceof
4.3 Thickness Gaging:
the material to produce surface acoustic waves. Basic EMAT
designs generate bidirectional surface waves. Specialized de- 4.3.1 Determining the thickness of a material by ultrasonic
signs can be used to generate unidirectional waves, as with means is a matter of coupling an ultrasonic wave into the
conventional ultrasonic examination. material, allowing the sound wave to propagate through the
FIG. 1 Typical EMAT Configuration for Rayleigh or Lamb Wave Generation
E 1816 – 96 (2002)
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
be considered for ultrasonic examinations in which applica-
tions involve automation, high-speed examinations, 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
FIG. 2 Typical EMAT Configuration for Bulk Wave Generation
currents experience Lorentz forces that in turn are transmitted
to the solid by collisions with the lattice or other microscopic
material, reflect from the backwall boundary interface of the
processes. These forces are alternating at the frequency of the
material,andpropagatebacktothefrontsurface.Thethickness
driving current and act as a source of ultrasonic waves. If the
of the material can be calculated by measuring the transit time
material is ferromagnetic, additional coupling mechanisms
of the ultrasonic wave, and through knowledge of the ultra-
play a part in the generation of ultrasonic waves. Interactions
sonic wave velocity. Thickness measurements can also be
between the dynamic magnetic field generated by the alternat-
extrapolated for a given material through standardizations of
ing currents and the magnetization associated with the material
transit time as a function of thickness as derived from a
offer a source of coupling, as do the associated magnetostric-
standardization block (see Practice E 797 and 7.3.1).
tive influences. Reciprocal processes exist whereby all of these
4.3.2 The ultrasonic velocity of the material under exami-
mechanisms lead to detection. Fig. 3 depicts the mechanisms
nation is a function of the physical properties of the material,
(forces), along with associated direction, for electromagnetic
namely, stiffness and density. It is usually assumed to be
ultrasound generation.
constant for a given class of materials. Approximate velocity
values are available in tabular format from numerous sources,
including the ASNT Nondestructive Testing Handbook. Ve-
locity values can also be determined empirically (see Practice
E 494).
4.3.3 Determination of the transit time of an acoustic wave
through a material requires the use of bulk acoustic wave
modes. While longitudinal waves can be used, it is often
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,
NOTE 1—j = current in a single conductor, Bo = magnetization from
external magnet, Fm = magnetic force (ferromagnetic material),
Fms = magnetostrictive force (ferromagnetic material), and FL = Lorentz
Nondestructive Testing Handbook, 2nd ed., Vol 7, Ultrasonic Testing,A. S.
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 – 96 (2002)
5.4 The EMAT can be used to generate all ultrasonic modes
of vibration. As with conventional ultrasonic techniques, ma-
terial types, probable flaw locations, and flaw orientations
determine the selection of beam directions and modes of
vibration. The use of EMATs and selection of the proper wave
mode presuppose a knowledge of the geometry of the object;
the probable location, size, orientation, and reflectivity of the
expected flaws; the allowable range of EMAT lift-off; and the
laws of physics governing the propagation of ultrasonic waves.
5.5 The EMAT techniques show benefits and advantages
overconventionalpiezoelectricultrasonictechniquesinspecial
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
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