ASTM E2884-22

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 and geometric property variations 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 or a reference part, or both. Responses measured from the sensor array may be converted into physical property values, such as lift-off, electrical conductivity, or magnetic permeability, or a combination thereof. Proper instrument operation is verified by ensuring that these measurement responses or property values are within a prescribed range. Performance verification is performed periodically. Performance verification on a discontinuity-free reference standard or regions of the material being examined that do not contain discontinuities ensures that the electrical and geometric properties, such as electrical conductivity, layer thickness, or lift-off, or a combination thereof, are appropriate for the sensor array. Performance verification on a discontinuity-containing reference standard ensures that the sensor array response to the discontinuity is appropriate.  
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 frequen...
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 or layer 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-May-2022
• ICS: 17.220.20 - Measurement of electrical and magnetic quantities
• ICS: 19.100 - Non-destructive testing
• Technical Committee: E07 - Nondestructive Testing
• Drafting Committee: E07.07 - Electromagnetic Method

Relations

• Refers: ASTM E1316-24 - Standard Terminology for <?Pub Dtl?>Nondestructive Examinations
• Effective Date: 01-Feb-2024
• Refers: ASTM E1316-19b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2019
• Refers: ASTM E1316-19 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Mar-2019
• Refers: ASTM E1316-18 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jan-2018
• Refers: ASTM E1316-17a - Standard Terminology for Nondestructive Examinations
• Effective Date: 15-Jun-2017
• Refers: ASTM E1316-17 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2017
• Refers: ASTM E1316-16a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Aug-2016
• Refers: ASTM E1316-16 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Feb-2016
• Refers: ASTM E1316-15a - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2015
• Refers: ASTM E1316-15 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Sep-2015
• Refers: ASTM E1316-14e1 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-14 - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2014
• Refers: ASTM E1316-13d - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Dec-2013
• Refers: ASTM E1316-13c - Standard Terminology for Nondestructive Examinations
• Effective Date: 15-Jun-2013
• Refers: ASTM E1316-13b - Standard Terminology for Nondestructive Examinations
• Effective Date: 01-Jun-2013

Overview

ASTM E2884-22, published by ASTM International, provides a standardized guide for the application of eddy current testing (ECT) using conformable sensor arrays to nondestructively examine electrically conducting materials. This method allows for the detection and characterization of surface-breaking and subsurface discontinuities, such as cracks or corrosion, as well as assessment of material quality metrics, including coating or layer thickness, electrical conductivity, magnetic permeability, and surface roughness. The standard is applicable to both magnetic and nonmagnetic metals, as well as composite materials containing electrically conducting components, and is suitable for use on planar and non-planar surfaces.

Key Topics

• Nondestructive Examination: Utilizes the principles of eddy current testing to assess materials without causing damage.
• Conformable Sensor Arrays: Flexible sensor arrays are able to conform to both flat and curved surfaces, enabling the inspection of complex geometries.
• Detection of Discontinuities:
  • Surface and subsurface cracks
  • Near-surface and hidden material loss
• Material Property Evaluation:
  • Coating and layer thickness
  • Electrical conductivity and magnetic permeability
  • Surface roughness
• Standardization and Calibration:
  • Involves air or reference part standardization to justify measurement accuracy.
  • Regular performance verification against reference standards to ensure reliability.
• Instrument Requirements:
  • Energizes sensor arrays with one or more frequencies.
  • Must capture and analyze impedance changes corresponding to local material properties.
• Impact of External Factors:
  • Addresses effects such as material composition, surface curvature, coatings, instrument stability, and operational temperature.
• Reporting and Quality Control:
  • Specifies necessary data to document, including operator, equipment used, calibration, frequencies, and examination outcomes.

Applications

ASTM E2884-22 is valuable across multiple industries where the integrity of electrically conducting materials is critical:

• Aerospace and Automotive: Inspecting aircraft and vehicle components for fatigue cracks, corrosion, or manufacturing defects.
• Energy and Utilities: Monitoring power generation equipment, pipelines, and pressure vessels for material loss or damage without service interruption.
• Manufacturing and Quality Assurance: Assessing incoming material quality or performing in-process inspection of complex assemblies, including those with layered materials.
• Infrastructure: Evaluating structural components in bridges, railways, and buildings where access to only one side of the material is possible, or where surfaces are non-planar.
• Composite Materials: Inspection of advanced composites with conducting elements, such as carbon-fiber-reinforced structures in aerospace or wind energy applications.

Related Standards

• ASTM E543 - Specification for Agencies Performing Nondestructive Testing
• ASTM E1316 - Terminology for Nondestructive Examinations
• ASNT SNT-TC-1A - Personnel Qualification and Certification in Nondestructive Testing
• ANSI/ASNT-CP-189 - Qualification and Certification of NDT Personnel
• AIA NAS 410 - Certification and Qualification of Nondestructive Testing Personnel
• MIL-HDBK-1823A - Nondestructive Evaluation System Reliability Assessment
• ISO 9712 - Qualification and Certification of NDT Personnel

Practical Value

The application of ASTM E2884-22 ensures consistent, reliable nondestructive evaluations of electrically conducting materials with complex geometries and compositions. By following the procedures outlined in this standard, organizations can improve the safety, operational efficiency, and lifespan of critical infrastructure and components. The use of conformable eddy current sensor arrays allows for efficient large-area inspections, minimizes downtime, and enables early detection of flaws, contributing to cost-effective maintenance and quality assurance programs.

Keywords: ASTM E2884-22, eddy current testing, conformable sensor arrays, nondestructive examination, material discontinuity detection, electrical conductivity measurement, coating thickness, composite materials inspection.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E2884 − 22
Standard Guide 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* 2. Referenced Documents
2.1 ASTM Standards:
1.1 This guide covers the use of conformable eddy current
E543 Specification for Agencies Performing Nondestructive
sensor arrays for nondestructive examination of electrically
Testing
conducting materials for discontinuities and material quality.
E1316 Terminology for Nondestructive Examinations
The discontinuities include surface breaking and subsurface
cracks and pitting as well as near-surface and hidden-surface
3. Terminology
material loss. The material quality includes coating or layer
3.1 Definitions: For definitions of terms relating to this
thickness, electrical conductivity, magnetic permeability, sur-
guide, refer to Terminology E1316, including Section C on
face roughness, and other properties that vary with the electri-
Electromagnetic Testing.
cal conductivity or magnetic permeability.
3.1.1 B-Scan, n—a method of data presentation utilizing a
horizontal base line that indicates distance along the surface of
1.2 This guide is intended for use on nonmagnetic and
a material and a vertical deflection that represents a measure-
magnetic metals as well as composite materials with an
ment response for the material being examined.
electrically conducting component, such as reinforced carbon-
carbon composite or polymer matrix composites with carbon
3.1.2 C-Scan, n—a method of data presentation which
fibers.
provides measurement responses for the material being exam-
ined in two-dimensions over the surface of the material.
1.3 This guide applies to planar as well as non-planar
3.1.3 discontinuity-containing reference standard, n—a re-
materials with and without insulating coating layers.
gion of the material under examination or a material having
1.4 Units—The values stated in SI units are to be regarded
electromagnetic properties similar to the material under exami-
as standard. The values given in parentheses are mathematical
nation for which a discontinuity having known characteristics
conversions to inch-pound units that are provided for informa-
is present.
tion only and are not considered standard.
3.1.4 discontinuity-free reference standard, n—a region of
the material under examination or a material having electro-
1.5 This standard does not purport to address all of the
magnetic properties similar to the material under examination
safety concerns, if any, associated with its use. It is the
for which no discontinuities are present.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.1.5 sensing element, n—a means for measuring the mag-
mine the applicability of regulatory limitations prior to use.
netic field intensity or rate of change of magnetic field
intensity, such as an inductive coil or a solid-state device.
1.6 This international standard was developed in accor-
3.1.5.1 Discussion—The sensing elements can be arranged
dance with internationally recognized principles on standard-
in one or two-dimensional arrays. They can provide either an
ization established in the Decision on Principles for the
absolute signal related to the magnetic field in the vicinity of
Development of International Standards, Guides and Recom-
the sense element or a differential signal.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
4. Summary of Guide
4.1 The examination is performed by scanning a conform-
able eddy current sensor array over the surface of the material
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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2022. Published June 2022. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2013. Last previous edition approved in 2017 as E2884 – 17. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E2884-22. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2884 − 22
of interest, with the sensor array energized with alternating usually require suitable fixtures to hold the sensor array near
current of one or more frequencies. The electrical response the surface of the material of interest, such as a layer of foam
from each sensing element of the eddy current sensor array is behind the sensor array along with a rigid support structure.
modified by the proximity and local condition of the material
5.2 In operation, the sensor arrays are standardized with
being examined. The extent of this modification is determined
measurements in air or a reference part, or both. Responses
by the distance between the eddy current sensor array and the
measured from the sensor array may be converted into physical
material being examined, as well as the dimensions and
property values, such as lift-off, electrical conductivity, or
electrical properties (electrical conductivity and magnetic per-
magnetic permeability, or a combination thereof. Proper instru-
meability) of the material. The presence of metallurgical or
ment operation is verified by ensuring that these measurement
mechanical discontinuities in the material and the presence of
responses or property values are within a prescribed range.
geometric changes in the material, such as coating thickness,
Performance verification is performed periodically. Perfor-
layer thickness, or gap thickness between layers, alter the
manceverificationonadiscontinuity-freereferencestandardor
measuredimpedanceoftheeddycurrentsenseelements.While
regions of the material being examined that do not contain
scanning over the material, the position at each measurement
discontinuities ensures that the electrical and geometric
location should be recorded along with the response of each
properties, such as electrical conductivity, layer thickness, or
sensing element in the sensor array. The measured responses
lift-off, or a combination thereof, are appropriate for the sensor
andlocationinformationcanthenbeused,typicallyintheform
array. Performance verification on a discontinuity-containing
of a displayed image (C-scan (3.1.2)) or in the form of a plot
reference standard ensures that the sensor array response to the
(B-scan (3.1.1)), to determine the presence and characteristics
discontinuity is appropriate.
of material property variations or discontinuities.
5.3 The sensor array dimensions, including the size and
4.2 The eddy current sensor arrays used for the examination
number of sense elements, and the operating frequency are
are flexible and, with a suitable backing layer, can conform to
selected based on the type of examination being performed.
both flat and curved surfaces, including fillets, cylindrical
The depth of penetration of eddy currents into the material
surfaces, etc. The sensor array can have a variety of configu-
under examination depends upon the frequency of the signal,
rations. These include: (1) a linear drive conductor that is
the electrical conductivity and magnetic permeability of the
energized by the instrument alternating current and a linear
material, and some dimensions of the sensor array. The depth
arrayofabsolutesenseelementspositionedparalleltothedrive
of penetration is equal to the conventional skin depth at high
conductor; (2) a complex drive conductor that produces a
frequencies but is also related to the sensor array dimensions at
desired field pattern at each sensing element; and (3) individual
low frequencies, such as the size of the drive winding and the
drive conductors associated with each sensing element. Asso-
gap distance between the drive winding and sense element
ciated with each sense element are one or more measurement
array. For surface-breaking discontinuities on the surface
responses that reflect the local material condition at each
adjacent to the sensor array, high frequencies should be used
location over the surface. The sensor arrays may be used with
where the penetration depth is less than the thickness of the
models for the sensor response and appropriate algorithms to
material under examination. For subsurface discontinuities or
convert measured responses for each sensing element into
wall thickness measurements, lower frequencies and larger
physical properties, such as lift-off, electrical conductivity,
sensor dimensions should be used so that the depth of
magnetic permeability, coating thickness, or substrate
penetration is comparable to the material thickness.
thickness, or a combination thereof. Baseline values for these
measurement responses or physical properties are used to 5.4 Insulatinglayersorcoatingsmaybepresentbetweenthe
ensure proper operation during the examination while local sensor array and the surface of the electrically conducting
variations in one or more of these properties can be used to material under examination. The sensitivity of a measurement
detect and characterize the discontinuity. For example, to a discontinuity generally decreases as the coating thickness
although, an impedance magnitude or other sensing element or lift-off, or both, increases. For eddy current sensor arrays
response can be used without a model to determine the having a linear drive conductor and a linear array of sense
presenceofaflaw,ameasurementofthelift-offateachsensing elements, the spacing between the drive conductor and the
elementlocationensuresthatthesensorisconformingproperly array of sense elements should be smaller than or comparable
to the surface.Also, a position measurement capability, such as to the thickness of the insulating coating. For other array
a rolling position encoder, can be used to measure location in formats the depth of sensitivity should be verified empirically.
the scan direction and ensure that sufficient data resolution is
5.5 Models for the sensor response may be used to convert
achieved. Visual or audio signaling devices may be used to
responses measured from the sensor array into physical prop-
indicate the position of the discontinuity.
erty values, such as lift-off, electrical conductivity, magnetic
permeability, coating thickness, or substrate thickness, or a
5. Significance and Use
combination thereof. For determining two property values, one
5.1 Eddy current methods are used for nondestructively operational frequency can be used. For nonmagnetic materials
locating and characterizing discontinuities and geometric prop- and examination for crack-like discontinuities, the lift-off and
erty variations in magnetic or nonmagnetic electrically con- electrical conductivity should be determined. For magnetic
ducting materials. Conformable eddy current sensor arrays materials, when the electrical conductivity can be measured or
permit examination of planar and non-planar materials but assumed constant, then the lift-off and magnetic permeability
E2884 − 22
should be determined. The thickness can only be determined if thickness can be determined as a property value using the
a sufficiently low excitation frequency is used where the depth model for the sensor response.
of sensitivity is greater than the material thickness of interest.
7.3 Residual Magnetism—In magnetic materials, residual
For determining more than two property values, measurements
magnetism may affect the measurement and appear as a local
at operating conditions having at least two depths of penetra-
response change. In some cases, it may be necessary to
tionshouldbeused;thesedifferentdepthsofpenetrationcanbe
demagnetize the specimen or part to get valid results.
achieved by using multiple operational frequencies or multiple
spatial wavelengths.
7.4 Residual Stress—Directional stress variations for mag-
netizable materials may affect results. To verify results of the
5.6 Processing of the measurement response or property
measurements, directional sensitivity should be determined
value data may be performed to highlight the presence of
and performance standards may be required for careful valida-
discontinuities,toreducebackgroundnoise,andtocharacterize
tion.
detected discontinuities.As an example, a correlation filter can
be applied in which a reference signature response for a
7.5 Curvature of Examination Surface—For surfaces with a
discontinuity is compared to the measured responses for each
single radius of curvature (for example, cylindrical or conical),
sensor array element to highlight discontinuity-like defects.
the radius of curvature should be large compared to the sensor
Care must be taken to properly account for the effect of
half-wavelength. In the case of a double curvature, at least one
interferences such as edges and coatings on such signatures.
of the radii should significantly exceed the sensor footprint and
5.7 The measurement and analysis methods described in
the other radius should be at least comparable to the sensor
this guide can also be applied to applications where the sensor
footprint, unless customized sensors are designed to match the
array is mounted against a surface or embedded within the
double curvature. System performance verification tests should
material being examined. In that situation the sensor array
be run to verify lift-off sensitivity using insulating shims.
response is monitored over a period of time instead of the
7.6 Conductive Coatings—The presence of electrically con-
scanning the sensor array over a specific location.This leads to
ductive coatings at the surface of the material under examina-
the horizontal axes for the B-scans and C-scans to correspond
tion can influence the measurement response. A reference
to time or some other input associated with the test such as the
number of loading cycles. standardization performed with a nominal conductive coating
thickness can help to account for the presence of this type of
6. Basis of Application
coating, but it will not necessarily account for conductive
6.1 The following items are subject to contractual agree- coating thickness variations over the material surface.
ment between the parties using or referencing this guide. Preferably, the models for the sensor response should account
for the presence of this type of coating with a physical property
6.2 Personnel Qualification—If specified in the contractual
that is determined, such as the coating thickness or coating
agreement, personnel performing examinations to this guide
electrical conductivity, or both.
should be qualified in accordance with a nationally or interna-
tionally recognized NDT personnel qualification practice or
7.7 Insulating Coatings—The thickness of insulating coat-
standard and certified by the employer or certifying agency, as
ings at the surface of the material under examination will affect
applicable. The practice or standard used and its applicable
the measurement response. The sensitivity to discrete features
revision should be identified in the contractual agreement
is generally reduced as the insulating coating thickness in-
between the using parties.
creases. If models for the sensor response are used, the models
6.3 Qualification of Nondestructive Testing Agencies—If
should account for the presence of this type of coating. Coating
specified in the contractual agreement, NDT agencies should
thickness variations over the material surface can be absorbed
be qualified and evaluated as specified in Specification E543.
into the lift-off property value.
The applicable edition of Specification E543 should be speci-
7.8 Edge Effect—Examination methods may be sensitive to
fied in the contractual agreement.
abrupt surface changes near material edges. Therefore, mea-
7. Interferences surements made too near an edge or inside corner may not be
valid or may be insufficiently accurate unless the instrument is
7.1 Base Material Property Variations—Local variations in
used with a procedure that specifically addresses such a
the magnetic permeability and electrical conductivity of the
measurement. Edge-effect correction procedures should either
material under examination, possibly due to microstructural
account for edge effects in the property estimation algorithm
variations, can contribute to measurement noise that limits the
(for example, in the sensor response model) or incorporate
capability of detecting small discontinuities. Shape filtering to
careful standardization on reference parts with fixtures to
candidate signature responses can help to reduce this effect.
control sensor position relative to the edge.
This also includes the presence and size of surface breaking
and subsurface discrete features such as welds, fasteners, and
7.9 Instrument Stability—Drift and noise in the instrumen-
cooling holes.
tation can cause inaccuracies in the measurement. Restandard-
7.2 Base Material Thickness—The thickness of the material ization and system performance verification should be per-
formed whenever the baseline response values exceed the
under examination can affect the measurement if it is smaller
than or comparable to the depth of sensitivity. If necessary, the threshold range.
E2884 − 22
7.10 Pressure of the Sensor Array against Surface under 8.2.1 Alinear drive conductor and one or more linear arrays
Examination—Insulating coating thickness readings can be of absolute sense elements positioned parallel to the drive
sensitive to the pressure used to hold the sensor array against conductor, where the second linear array is aligned with the
the surface. first row to add redundancy or offset to improve image
resolution in the direction transverse to the scan direction,
7.11 Temperature—Eddy current measurements are gener-
8.2
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2884 − 17 E2884 − 22
Standard Guide 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*
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 or layer 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E543 Specification for Agencies Performing Nondestructive Testing
E1316 Terminology for Nondestructive Examinations
2.2 ASNT Documents:
SNT-TC-1A Recommended Practice for Personnel Qualification and Certification in Nondestructive Testing
ANSI/ASNT-CP-189 Standard for Qualification and Certification of NDT Personnel
This guide is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.07 on Electromagnetic
Method.
Current edition approved Nov. 1, 2017June 1, 2022. Published December 2017June 2022. Originally approved in 2013. Last previous edition approved in 20132017 as
ɛ1
E2884E2884 – 17.–13 . DOI: 10.1520/E2884-17.10.1520/E2884-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2884 − 22
2.3 AIA Standard:
NAS 410 Certification and Qualification of Nondestructive Testing Personnel
2.4 Department of Defense Handbook:
MIL-HDBK–1823A Nondestructive Evaluation System Reliability Assessment
2.5 ISO Standards:
ISO 9712 Non-destructive Testing—Qualification and Certification of NDT Personnel
3. Terminology
3.1 Definitions—For definitions of terms relating to this guide refer to Terminology E1316.
3.1 Definitions of Terms Specific to This Standard:Definitions: For definitions of terms relating to this guide, refer to
Terminology E1316, including Section C on Electromagnetic Testing.
3.1.1 B-Scan—B-Scan, n—a method of data presentation utilizing a horizontal base line that indicates distance along the surface
of a material and a vertical deflection that represents a measurement response for the material being examined.
3.1.2 C-Scan—C-Scan, n—a method of data presentation which provides measurement responses for the material being examined
in two-dimensions over the surface of the material.
3.2.3 conformable—refers to an ability of sensors or sensor arrays to conform to non-planar surfaces without significant effects
on the measurement results, or with effects that are limited to a quantifiable bound.
3.2.4 depth of sensitivity—depth to which the sensor response to features or properties of interest exceeds a noise threshold.
3.2.4.1 Discussion—
The depth of sensitivity can be larger or smaller than the depth of penetration since it incorporates a comparison between the signal
obtained from a feature as well as measurement noise, whereas the depth of penetration refers to the decrease in field intensity with
distance away from a test coil.
3.1.3 discontinuity-containing reference standard—standard, n—a region of the material under examination or a material having
electromagnetic properties similar to the material under examination for which a discontinuity having known characteristics is
present.
3.1.4 discontinuity-free reference standard—standard, n—a region of the material under examination or a material having
electromagnetic properties similar to the material under examination for which no discontinuities are present.
3.2.7 drive winding—a conductor pattern or coil that produces a magnetic field that couples to the material being examined.
3.2.7.1 Discussion—
The drive winding can have various geometries, including: 1) a simple linear conductor that is placed adjacent to a one-dimensional
array of sensing elements; 2) one or multiple conducting loops driven to create a complex field pattern; and 3) multiple conducting
loops with a separate loop for each sensing element.
3.2.8 insulating shims—conformable and substantially non-conducting or insulating foils that are used to measure effects of small
lift-off excursions on sensor response.
3.2.9 lift off—normal distance from the plane of the conformable sensor winding conductors to the surface of the conducting
material under examination.
3.2.10 model for sensor response—a relation between the response of the sensor (for example, impedance magnitude and phase
or real and imaginary parts) and properties of interest (for example, electrical conductivity, magnetic permeability, lift-off, and
material thickness) for at least one sensing element and at least one drive winding.
3.2.10.1 Discussion—
These model responses may be obtained from database tables and may be analysis-based or empirical.
3.1.5 sensing element—element, n—a means for measuring the magnetic field intensity or rate of change of magnetic field
intensity, such as an inductive coil or a solid-state device.
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3.1.5.1 Discussion—
The sensing elements can be arranged in one or two-dimensional arrays. They can provide either an absolute signal related to the
magnetic field in the vicinity of the sense element or a differential signal.
3.2.12 spatial half-wavelength—spacing between the conductors of a linear drive winding with current flow in opposite directions.
3.2.12.1 Discussion—
This spacing affects the depth of sensitivity. The spatial wavelength equals two times this spacing. For a circular drive winding,
the effective spatial half-wavelength is equal to the drive winding diameter.
3.2.13 system performance verification—the use of a measurement of one or more response values, typically physical property
values, for a reference part to confirm that the response values are within specified tolerances to validate the system standardization
and verify proper instrument operation.
4. Summary of Guide
4.1 The examination is performed by scanning a conformable eddy current sensor array over the surface of the material of interest,
with the sensor array energized with alternating current of one or more frequencies. The electrical response from each sensing
element of the eddy current sensor array is modified by the proximity and local condition of the material being examined. The
extent of this modification is determined by the distance between the eddy current sensor array and the material being examined,
as well as the dimensions and electrical properties (electrical conductivity and magnetic permeability) of the material. The presence
of metallurgical or mechanical discontinuities in the material alters the and the presence of geometric changes in the material, such
as coating thickness, layer thickness, or gap thickness between layers, alter the measured impedance of the eddy current sense
elements. While scanning over the material, the position at each measurement location should be recorded along with the response
of each sensing element in the sensor array. The measured responses and location information can then be used, typically in the
form of a displayed image (C-scan (3.2.23.1.2)) or in the form of a plot (B-scan (3.2.13.1.1)), to determine the presence and
characteristics of material property variations or discontinuities.
4.2 The eddy current sensor arrays used for the examination are flexible and, with a suitable backing layer, can conform to both
flat and curved surfaces, including fillets, cylindrical surfaces, etc. The sensor array can have a variety of configurations. These
include: 1)(1) a linear drive conductor that is energized by the instrument alternating current and a linear array of absolute sense
elements positioned parallel to the drive conductor; 2)(2) a complex drive conductor that produces a desired field pattern at each
sensing element; and 3)(3) individual drive conductors associated with each sensing element. Associated with each sense element
are one or more measurement responses that reflect the local material condition at each location over the surface. The sensor arrays
may be used with models for the sensor response and appropriate algorithms to convert measured responses for each sensing
element into physical properties, such as lift-off, electrical conductivity, magnetic permeability, coating thickness, and/or substrate
thickness. or substrate thickness, or a combination thereof. Baseline values for these measurement responses or physical properties
are used to ensure proper operation during the examination while local variations in one or more of these properties can be used
to detect and characterize the discontinuity. For example, although, an impedance magnitude or other sensing element response
can be used without a model to determine the presence of a flaw, a measurement of the lift-off at each sensing element location
ensures that the sensor is conforming properly to the surface. Also, a position measurement capability, such as a rolling position
encoder, can be used to measure location in the scan direction and ensure that sufficient data resolution is achieved. Visual or audio
signaling devices may be used to indicate the position of the discontinuity.
5. Significance and Use
5.1 Eddy current methods are used for nondestructively locating and characterizing discontinuities and geometric property
variations 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/oror a reference part. part, or both. Responses
measured from the sensor array may be converted into physical property values, such as lift-off, electrical conductivity, and/or
magnetic permeability. or magnetic permeability, or a combination thereof. 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.is performed periodically. Performance verification on a discontinuity-free
reference standard or regions of the material being examined that do not contain discontinuities ensures that the electrical and
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geometric properties, such as electrical conductivity, layer thickness, or lift-off, or a combination thereof, are appropriate for the
sensor array. Performance verification on a discontinuity-containing reference standard ensures that the sensor array response to
the discontinuity is appropriate.
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
comparable to the material thickness.
5.4 Insulating layers or coatings may be present between the sensor array and the surface of the electrically conducting material
under examination. The sensitivity of a measurement to a discontinuity generally decreases as the coating thickness and/or lift-off
or lift-off, or both, increases. For eddy current sensor arrays having a linear drive conductor and a linear array of sense elements,
the spacing between the drive conductor and the array of sense elements should be smaller than or comparable to the thickness
of the insulating coating. For other array formats the depth of sensitivity should be verified empirically.
5.5 Models for the sensor response may be used to convert responses measured from the sensor array into physical property
values, such as lift-off, electrical conductivity, magnetic permeability, coating thickness, and/or substrate thickness. or substrate
thickness, or a combination thereof. For determining two property values, one operational frequency can be used. For nonmagnetic
materials and examination for crack-like discontinuities, the lift-off and electrical conductivity should be determined. For magnetic
materials, when the electrical conductivity can be measured or assumed constant, then the lift-off and magnetic permeability should
be determined. The thickness can only be determined if a sufficiently low excitation frequency is used where the depth of
sensitivity is greater than the material thickness of interest. For determining more than two property values, measurements at
operating conditions having at least two depths of penetration should be used; these different depths of penetration can be achieved
by using multiple operational frequencies or multiple spatial wavelengths.
5.6 Processing of the measurement response or property value data may be performed to highlight the presence of discontinuities,
to reduce background noise, and to characterize detected discontinuities. As an example, a correlation filter can be applied in which
a reference signature response for a discontinuity is compared to the measured responses for each sensor array element to highlight
discontinuity-like defects. Care must be taken to properly account for the effect of interferences such as edges and coatings on such
signatures.
5.7 The measurement and analysis methods described in this guide can also be applied to applications where the sensor array is
mounted against a surface or embedded within the material being examined. In that situation the sensor array response is monitored
over a period of time instead of the scanning the sensor array over a specific location. This leads to the horizontal axes for the
B-scans and C-scans to correspond to time or some other input associated with the test such as the number of loading cycles.
6. Basis of Application
6.1 The following items are subject to contractual agreement between the parties using or referencing this standard.guide.
6.2 Personnel Qualification—If specified in the contractual agreement, personnel performing examinations to this standardguide
should be qualified in accordance with a nationally or internationally recognized NDT personnel qualification practice or standard
such as ANSI/ASNT-CP-189, SNT-TC-1A, NAS 410, ISO 9712, or a similar document and certified by the employer or certifying
agency, as applicable. The practice or standard used and its applicable revision should be identified in the contractual agreement
between the using parties.
6.3 Qualification of Nondestructive Testing Agencies—If specified in the contractual agreement, NDT agencies should be qualified
and evaluated as specified in Specification E543. The applicable edition of Specification E543 should be specified in the contractual
agreement.
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7. Interferences
7.1 Base Material Property Variations—Local variations in the magnetic permeability and electrical conductivity of the material
under examination, possibly due to microstructural variations, can contribute to measurement noise that limits the capability of
detecting small discontinuities. Shape filtering to candidate signature responses can help to reduce this effect. This also includes
the presence and size of surface breaking and subsurface discrete features such as welds, fasteners, and cooling holes.
7.2 Base Material Thickness—The thickness of the material under examination can affect the measurement if it is smaller than or
comparable to the depth of sensitivity. If necessary, the thickness can be determined as a property value using the model for the
sensor response.
7.3 Residual Magnetism—In magnetic materials, residual magnetism may affect the measurement and appear as a local response
change. In some cases, it may be necessary to demagnetize the specimen or part to get valid results.
7.4 Residual Stress—Directional stress variations for magnetizable materials may affect results. To verify results of the
measurements, directional sensitivity should be determined and performance standards may be required for careful validation.
7.5 Curvature of Examination Surface—For surfaces with a single radius of curvature (for example, cylindrical or conical), the
radius of curvature should be large compared to the sensor half-wavelength. In the case of a double curvature, at least one of the
radii should significantly exceed the sensor footprint and the other radius should be at least comparable to the sensor footprint,
unless customized sensors are designed to match the double curvature. System performance verification tests should be run to
verify lift-off sensitivity using insulating shims.
7.6 Conductive Coatings—The presence of electrically conductive coatings at the surface of the material under examination can
influence the measurement response. A reference standardization performed with a nominal conductive coating thickness can help
to account for the presence of this type of coating, but it will not necessarily account for conductive coating thickness variations
over the material surface. Preferably, the models for the sensor response should account for the presence of this type of coating
with a physical property that is determined, such as the coating thickness or coating electrical conductivity, or both, a physical
property that is determined.both.
7.7 Insulating Coatings—The thickness of insulating coatings at the surface of the material under examination will affect the
measurement response. The sensitivity to discrete features is generally reduced as the insulating coating thickness increases. If
models for the sensor response are used, the models should account for the presence of this type of coating. Coating thick
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