ASTM E2884-13
(Guide)Standard Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays
Standard Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays
SIGNIFICANCE AND USE
5.1 Eddy current methods are used for nondestructively locating and characterizing discontinuities in magnetic or nonmagnetic electrically conducting materials. Conformable eddy current sensor arrays permit examination of planar and non-planar materials but usually require suitable fixtures to hold the sensor array near the surface of the material of interest, such as a layer of foam behind the sensor array along with a rigid support structure.
5.2 In operation, the sensor arrays are standardized with measurements in air and/or a reference part. Responses measured from the sensor array may be converted into physical property values, such as lift-off, electrical conductivity, and/or magnetic permeability. Proper instrument operation is verified by ensuring that these measurement responses or property values are within a prescribed range. Performance verification on reference standards with known discontinuities is performed periodically.
5.3 The sensor array dimensions, including the size and number of sense elements, and the operating frequency are selected based on the type of examination being performed. The depth of penetration of eddy currents into the material under examination depends upon the frequency of the signal, the electrical conductivity and magnetic permeability of the material, and some dimensions of the sensor array. The depth of penetration is equal to the conventional skin depth at high frequencies but is also related to the sensor array dimensions at low frequencies, such as the size of the drive winding and the gap distance between the drive winding and sense element array. For surface-breaking discontinuities on the surface adjacent to the sensor array, high frequencies should be used where the penetration depth is less than the thickness of the material under examination. For subsurface discontinuities or wall thickness measurements, lower frequencies and larger sensor dimensions should be used so that the depth of penetration is compara...
SCOPE
1.1 This guide covers the use of conformable eddy current sensor arrays for nondestructive examination of electrically conducting materials for discontinuities and material quality. The discontinuities include surface breaking and subsurface cracks and pitting as well as near-surface and hidden-surface material loss. The material quality includes coating thickness, electrical conductivity, magnetic permeability, surface roughness and other properties that vary with the electrical conductivity or magnetic permeability.
1.2 This guide is intended for use on nonmagnetic and magnetic metals as well as composite materials with an electrically conducting component, such as reinforced carbon-carbon composite or polymer matrix composites with carbon fibers.
1.3 This guide applies to planar as well as non-planar materials with and without insulating coating layers.
1.4 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.
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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Standards Content (Sample)
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Designation: E2884 − 13
StandardGuide for
Eddy Current Testing of Electrically Conducting Materials
Using Conformable Sensor Arrays
This standard is issued under the fixed designation E2884; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope E2238 Guide for Evacuation Route Diagrams
1.1 This guide covers the use of conformable eddy current 2.2 ASNT Documents:
sensor arrays for nondestructive examination of electrically SNT-TC-1A Recommended Practice for Personnel Qualifi-
conducting materials for discontinuities and material quality. cation and Certification in Nondestructive Testing
The discontinuities include surface breaking and subsurface ANSI/ASNT-CP-189 Standard for Qualification and Certifi-
cracks and pitting as well as near-surface and hidden-surface cation of NDT Personnel
material loss. The material quality includes coating thickness,
2.3 AIA Standard:
electrical conductivity, magnetic permeability, surface rough-
NAS 410 Certification and Qualification of Nondestructive
ness and other properties that vary with the electrical conduc-
Testing Personnel
tivity or magnetic permeability.
2.4 Department of Defense Handbook:
1.2 This guide is intended for use on nonmagnetic and
MIL-HDBK–1823A Nondestructive Evaluation System Re-
magnetic metals as well as composite materials with an
liability Assessment
electrically conducting component, such as reinforced carbon-
carbon composite or polymer matrix composites with carbon
3. Terminology
fibers.
3.1 Definitions—For definitions of terms relating to this
1.3 This guide applies to planar as well as non-planar
guide refer to Terminology E1316.
materials with and without insulating coating layers.
3.2 Definitions of Terms Specific to This Standard:
1.4 Units—The values stated in SI units are to be regarded
3.2.1 B-Scan—a method of data presentation utilizing a
as standard. The values given in parentheses are mathematical
horizontal base line that indicates distance along the surface of
conversions to inch-pound units that are provided for informa-
a material and a vertical deflection that represents a measure-
tion only and are not considered standard.
ment response for the material being examined.
1.5 This standard does not purport to address all of the
3.2.2 C-Scan—a method of data presentation which pro-
safety concerns, if any, associated with its use. It is the
vides measurement responses for the material being examined
responsibility of the user of this standard to establish appro-
in two-dimensions over the surface of the material.
priate safety and health practices and determine the applica-
3.2.3 conformable—refers to an ability of sensors or sensor
bility of regulatory limitations prior to use.
arrays to conform to non-planar surfaces without significant
effects on the measurement results, or with effects that are
2. Referenced Documents
limited to a quantifiable bound.
2.1 ASTM Standards:
3.2.4 depth of sensitivity—depth to which the sensor re-
E543 Specification for Agencies Performing Nondestructive
sponse to features or properties of interest exceeds a noise
Testing
threshold.
E1316 Terminology for Nondestructive Examinations
3.2.4.1 Discussion—The depth of sensitivity is generally
smaller than the depth of penetration since it incorporates a
1 comparison between the signal obtained from a feature as well
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-
tive Testing and is the direct responsibility of Subcommittee E07.07 on Electro-
magnetic Method.
Current edition approved June 1, 2013. Published June 2013. DOI: 10.1520/
E2884-13. AvailablefromAmericanSocietyforNondestructiveTesting(ASNT),P.O.Box
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http://www.asnt.org.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Available fromAerospace IndustriesAssociation ofAmerica, Inc. (AIA), 1000
Standards volume information, refer to the standard’s Document Summary page on WilsonBlvd.,Suite1700,Arlington,VA22209-3928,http://www.aia-aerospace.org.
the ASTM website. (Replacement standard for MIL-STD-410.)
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2884 − 13
as measurement noise, whereas the depth of penetration refers 4. Summary of Guide
to the decrease in field intensity with distance away from a test
4.1 The examination is performed by scanning a conform-
coil.
able eddy current sensor array over the surface of the material
3.2.5 discontinuity-containing reference standard—a region of interest, with the sensor array energized with alternating
current of one or more frequencies. The electrical response
of the material under examination or a material having elec-
tromagnetic properties similar to the material under examina- from each sensing element of the eddy current sensor array is
tion for which a discontinuity having known characteristics is modified by the proximity and local condition of the material
present. being examined. The extent of this modification is determined
by the distance between the eddy current sensor array and the
3.2.6 discontinuity-free reference standard—a region of the
material being examined, as well as the dimensions and
material under examination or a material having electromag-
electrical properties (electrical conductivity and magnetic per-
netic properties similar to the material under examination for
meability) of the material. The presence of metallurgical or
which no discontinuities are present.
mechanical discontinuities in the material alters the measured
3.2.7 drive winding—a conductor pattern or coil that pro-
impedance of the eddy current sense elements. While scanning
duces a magnetic field that couples to the material being
over the material, the position at each measurement location
examined.
should be recorded along with the response of each sensing
3.2.7.1 Discussion—The drive winding can have various
element in the sensor array. The measured responses and
geometries, including: 1) a simple linear conductor that is
location information can then be used, typically in the form of
placed adjacent to a one-dimensional array of sensing ele-
a displayed image (C-scan (3.2.2)) or in the form of a plot
ments; 2) one or multiple conducting loops driven to create a
(B-scan (3.2.1)), to determine the presence and characteristics
complex field pattern; and 3) multiple conducting loops with a
of material property variations or discontinuities.
separate loop for each sensing element.
4.2 The eddy current sensor arrays used for the examination
3.2.8 insulating shims—conformable and substantially non-
are flexible and, with a suitable backing layer, can conform to
conducting or insulating foils that are used to measure effects
both flat and curved surfaces, including fillets, cylindrical
of small lift-off excursions on sensor response.
surfaces, etc. The sensor array can have a variety of configu-
rations. These include: 1) a linear drive conductor that is
3.2.9 lift off—normal distance from the plane of the con-
energized by the instrument alternating current and a linear
formable sensor winding conductors to the surface of the
arrayofabsolutesenseelementspositionedparalleltothedrive
conducting material under examination.
conductor; 2) a complex drive conductor that produces a
3.2.10 model for sensor response—a relation between the
desired field pattern at each sensing element; and 3) individual
response of the sensor (for example, impedance magnitude and
drive conductors associated with each sensing element. Asso-
phase or real and imaginary parts) and properties of interest
ciated with each sense element are one or more measurement
(for example, electrical conductivity, magnetic permeability,
responses that reflect the local material condition at each
lift-off, and material thickness) for at least one sensing element
location over the surface. The sensor arrays may be used with
and at least one drive winding.
models for the sensor response and appropriate algorithms to
3.2.10.1 Discussion—These model responses may be ob-
convert measured responses for each sensing element into
tained from database tables and may be analysis-based or
physical properties, such as lift-off, electrical conductivity,
empirical.
magnetic permeability, coating thickness, and/or substrate
thickness. Baseline values for these measurement responses or
3.2.11 sensing element—a means for measuring the mag-
physical properties are used to ensure proper operation during
netic field intensity or rate of change of magnetic field
the examination while local variations in one or more of these
intensity, such as an inductive coil or a solid-state device.
properties can be used to detect and characterize the disconti-
3.2.11.1 Discussion—The sensing elements can be arranged
nuity.Forexample,although,animpedancemagnitudeorother
in one or two-dimensional arrays. They can provide either an
sensing element response can be used without a model to
absolute signal related to the magnetic field in the vicinity of
determine the presence of a flaw, a measurement of the lift-off
the sense element or a differential signal.
at each sensing element location ensures that the sensor is
3.2.12 spatial half-wavelength—spacing between the con-
conforming properly to the surface. Also, a position measure-
ductors of a linear drive winding with current flow in opposite
ment capability, such as a rolling position encoder, can be used
directions.
to measure location in the scan direction and ensure that
3.2.12.1 Discussion—This spacing affects the depth of sen-
sufficient data resolution is achieved. Visual or audio signaling
sitivity. The spatial wavelength equals two times this spacing.
devices may be used to indicate the position of the disconti-
For a circular drive winding, the effective spatial half-
nuity.
wavelength is equal to the drive winding diameter.
5. Significance and Use
3.2.13 system performance verification—the use of a mea-
surement of one or more response values, typically physical 5.1 Eddy current methods are used for nondestructively
property values, for a reference part to confirm that the locating and characterizing discontinuities in magnetic or
response values are within specified tolerances to validate the nonmagnetic electrically conducting materials. Conformable
system standardization and verify proper instrument operation. eddy current sensor arrays permit examination of planar and
E2884 − 13
non-planar materials but usually require suitable fixtures to used; these different depths of penetration can be achieved by
hold the sensor array near the surface of the material of using multiple operational frequencies or multiple spatial
wavelengths.
interest, such as a layer of foam behind the sensor array along
with a rigid support structure.
5.6 Processing of the measurement response or property
value data may be performed to highlight the presence of
5.2 In operation, the sensor arrays are standardized with
discontinuities,toreducebackgroundnoise,andtocharacterize
measurements in air and/or a reference part. Responses mea-
detected discontinuities.As an example, a correlation filter can
sured from the sensor array may be converted into physical
be applied in which a reference signature response for a
property values, such as lift-off, electrical conductivity, and/or
discontinuity is compared to the measured responses for each
magnetic permeability. Proper instrument operation is verified
sensor array element to highlight discontinuity-like defects.
by ensuring that these measurement responses or property
Care must be taken to properly account for the effect of
values are within a prescribed range. Performance verification
interferences such as edges and coatings on such signatures.
onreferencestandardswithknowndiscontinuitiesisperformed
periodically.
6. Basis of Application
5.3 The sensor array dimensions, including the size and
6.1 The following items are subject to contractual agree-
number of sense elements, and the operating frequency are
ment between the parties using or referencing this standard.
selected based on the type of examination being performed.
6.2 Personnel Qualification—If specified in the contractual
The depth of penetration of eddy currents into the material
agreement, personnel performing examinations to this standard
under examination depends upon the frequency of the signal,
shall be qualified in accordance with a nationally or interna-
the electrical conductivity and magnetic permeability of the
tionally recognized NDT personnel qualification practice or
material, and some dimensions of the sensor array. The depth
standard such asANSI/ASNT-CP-189, SNT-TC-1A, NAS 410
of penetration is equal to the conventional skin depth at high
or a similar document and certified by the employer or
frequencies but is also related to the sensor array dimensions at
certifying agency, as applicable. The practice or standard used
low frequencies, such as the size of the drive winding and the
and its applicable revision shall be identified in the contractual
gap distance between the drive winding and sense element
agreement between the using parties.
array. For surface-breaking discontinuities on the surface
6.3 Qualification of Nondestructive Testing Agencies—If
adjacent to the sensor array, high frequencies should be used
specified in the contractual agreement, NDT agencies shall be
where the penetration depth is less than the thickness of the
qualified and evaluated as specified in E543. The applicable
material under examination. For subsurface discontinuities or
edition of E543 shall be specified in the contractual agreement.
wall thickness measurements, lower frequencies and larger
sensor dimensions should be used so that the depth of
7. Interferences
penetration is comparable to the material thickness.
7.1 Base Material Property Variations—Local variations in
5.4 Insulatinglayersorcoatingsmaybepresentbetweenthe
the magnetic permeability and electrical conductivity of the
sensor array and the surface of the electrically conducting
material under examination, possibly due to microstructural
material under examination. The sensitivity of a measurement
variations, can contribute to measurement noise that limits the
to a discontinuity generally decreases as the coating thickness
capability of detecting small discontinuities. Shape filtering to
and/or lift-off incre
...
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