ASTM E1629-94(2001)
(Practice)Standard Practice for Determining the Impedance of Absolute Eddy-Current Probes
Standard Practice for Determining the Impedance of Absolute Eddy-Current Probes
SCOPE
1.1 This practice covers a procedure for determining the impedance of absolute eddy-current probes (bridge-type, air or ferrite core, wire wound, shielded, or unshielded) used for finding material defects in electrically conducting material. This practice is intended to establish a uniform test methodology to measure the impedance of eddy-current probes prior to receipt of these probes by the purchaser or the specifier.
1.2 Limitations -This practice does not address the characterization or measurement of the impedance of differential, a-c coupled, or transmit/receive types of probes. This practice does not address the use of magnetic materials in testing probes. This practice shall not be used as a basis for selection of the best probe for a particular application or as a means by which to calibrate a probe for a specific examination.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 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:E1629–94 (Reapproved 2001)
Standard Practice for
Determining the Impedance of Absolute Eddy-Current
Probes
This standard is issued under the fixed designation E1629; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2 Definitions of Terms Specific to This Standard:
3.2.1 eddy-current test block— for the purposes of the
1.1 This practice covers a procedure for determining the
method described in this practice, a rectangular block made of
impedance of absolute eddy-current probes (bridge-type, air or
an aluminum alloy (see 6.1.2) to which an active eddy-current
ferrite core, wire wound, shielded, or unshielded) used for
probe is applied.
finding material defects in electrically conducting material.
3.3 Mathematical Symbols:
This practice is intended to establish a uniform test methodol-
3.3.1 j—asymbolusedinelectricalengineeringtorepresent
ogy to measure the impedance of eddy-current probes prior to
21 . It is associated with the restriction to the flow of
=
receipt of these probes by the purchaser or the specifier.
electrical current caused by capacitors and coils.
1.2 Limitations—This practice does not address the charac-
3.3.2 N— any number.
terization or measurement of the impedance of differential, a-c
3.3.3 |N|—the magnitude of N, regardless whether N is
coupled,ortransmit/receivetypesofprobes.Thispracticedoes
positive, negative, or a vector quantity.
not address the use of magnetic materials in testing probes.
3.3.4 N —the square root of N.
=
This practice shall not be used as a basis for selection of the
3.3.5 (N) —N squared, that is, N 3 N.
best probe for a particular application or as a means by which
3.3.6 DN— delta N, the change or difference in N.
to calibrate a probe for a specific examination.
3.4 Abbreviations:Abbreviation:
1.3 The values stated in SI units are to be regarded as the
−1
3.4.1 tan—used for the tangent function. The tan , arctan-
standard. The values given in parentheses are for information
gent or inverse tangent function, returns a value that is a
only.
measure of an angle and can be in either degrees or radians.
1.4 This standard does not purport to address all of the
−1
When using a calculator to determine the tan , care should be
safety concerns, if any, associated with its use. It is the
taken to determine whether the answer is in degrees or radians
responsibility of the user of this standard to establish appro-
since the numerical values that represent the same angle are
priate safety and health practices and determine the applica-
different.
bility of regulatory limitations prior to use.
4. Significance and Use
2. Referenced Documents
4.1 Eddy-currentprobesmaybeusedforthenondestructive
2.1 The following document forms a part of this practice to
examinationofpartsorstructuresmadeofelectricallyconduct-
the extent specified herein:
ing materials. Many of these examinations are intended to
2.2 ASTM Standards:
discover material defects, such as fatigue cracks, that may
E1316 Terminology for Nondestructive Examination
cause the part or structure to be unsafe or unfit for further
3. Terminology
service. Eddy-current probes that fail to meet the performance
level requirements of this practice shall not be used for the
3.1 Definitions—The terminology relating to eddy-current
examination of material or hardware unless the probe is
examination that appears inTerminology E1316 shall apply to
qualified by some other system or an agreement has been
the terms used in this practice.
reached by the probe manufacturer and the purchaser, or both.
5. General Practice
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.07 on
5.1 Use of Test Blocks—The test blocks described in this
Electromagnetic Method.
practiceshallnotbeusedforpurposesotherthanmeasuringthe
Current edition approved Sept. 15, 1994. Published November 1994.
impedance of eddy-current probes as specified in this practice.
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1629–94 (2001)
They shall not be used for standardizing an examination or for
determining flaw sensitivity.
5.2 Responsibility—Initial determination of the impedance
values of eddy-current probes shall be performed by the probe
manufacturerinaccordancewiththispractice.Theresultsshall
be delivered with the probe and maintained by the using
organization. While the retesting of probes may be performed
usingthispractice,onlytheresultsobtainedbeforetheprobeis
inuseshouldbecomparedtotheinitialimpedancevalues.The
impedance results should not be compared to the initial values
after a probe has been used.
6. Specific Practice
FIG. 1 Representative Test Block
6.1 Test Method—Impedance measurements shall be made
on an aluminum alloy test block with a machined slot that
conforms to the requirements of this practice. The operating 6.2 Measurement Procedure:
frequency (as specified by the probe manufacturer) may vary 6.2.1 Measurements—Impedance values can be expressed
for each probe examined, depending on the specific probe in different ways. Impedances are most commonly given in
geometry, skin depth, matching impedance, desired signal either rectangular or polar form. In polar form, the impedance
strength, and application.Acommercial impedance measuring is expressed as a magnitude, |Z|, with a corresponding phase
instrument that conforms to 6.1.1 shall be used to make the angle, u, and often appears as |Z|/ u. The rectangular form
measurements. The measurements will be recorded on the expresses the impedance as a combination of a resistive
worksheet (Appendix X1) to calculate the probe impedance. component, R, and a reactive or imaginary (denoted by j)
The calculated values shall be compared to the acceptable
component, X.Thisformoftenappearsas R 6 jX.Fig.2shows
criteria (6.3) to determine probe acceptability. twopointsmeasuredinbothformsandtheresultingimpedance
6.1.1 Test Equipment—The test instrument shall be either a
change (DZ) calculation. This is typical of the way in which
commercialimpedancemeasuringinstrumentoranLCRmeter impedance changes are measured on actual eddy-current
with an oscillator capable of driving a current in the probe at
probes using the test method specified in this practice. The
the probe’s operating frequency. The output shall display the impedance of a probe is measured first off the slot and then on
probe impedance in either polar form, providing a magnitude
the slot in this method, and the difference between these two
and a phase angle, or rectangular form, providing resistive and measurements is calculated. An error may occur in the calcu-
reactive components of the impedance, or both. This instru-
lations if appropriate coordinate conversions are not made
ment shall be calibrated in accordance with the manufacturer’s (addition and subtraction are performed on rectangular coor-
specifications at the required interval. dinates and multiplication and division on polar coordinates).
6.1.2 Test Blocks—The test block shall be fabricated from 6.2.1.1 All performance tests shall be conducted within the
7075-T6 aluminum alloy 1.9-cm (0.75-in.) thick, with slotted temperaturerangefrom15to27°C(60to80°F).Theprobetest
holes for testing bolt hole probes and a slotted flat section for frequency shall depend on the rated operating frequency of the
testing surface probes.All test surfaces shall be polished to an particular probe under test (see X1.4.2.3).
average finish # 15 µm (591 µin.). The conductivity of the 6.2.2 Probe Impedance in Air—Attach the probe to the
aluminum alloy should be between 30 and 35% IACS. impedance measuring instrument, and position the probe at
6.1.2.1 Theslottedflatsectionshallhavesidemeasurements least 50.8 mm (2 in.) away from any electrically conducting
of at least six times the coil diameter or 5.1 by 5.1 cm (2 by 2 material or hardware, or both. Measure the impedance and
in.),whicheverislarger.Theslotdimensionsshallbemachined
record the impedance values on the worksheet. Compare the
across the block’s surface and shall measure at least 5.0 cm measurement to the values listed in 6.3.1.
(2-in.) long. The slot cross section shall measure 0.1 6 0.01
6.2.3 Average Off-Slot Probe Impedance— Place the probe
mm (0.004 6 0.004 in.) wide and 0.5 6 0.025 mm (0.02 6 onthesurfaceoforintheholeinthetestblock,asappropriate.
0.001 in.) deep.
For a surface probe, place the probe on four different positions
6.1.2.2 The test holes shall be made for all of the nominal on the face of the block.The center of each position shall be at
sizesofboltholeprobestobeexamined.Theedgesoftheholes
shall be spaced 1.9-cm (0.75-in.) apart from each other and
fromtheblockedges.Theslotshallruntheentirelengthofthe
holdandwillbe0.1 60.01-mm(0.004 60.0004-in.)wideand
0.5 6 0.025-mm (0.02 6 0.001-in.) deep.
6.1.2.3 Fig. 1 shows a representative test block with the
holes used for testing two different sizes of bolt hole probes.
The length of the block (x+5.1 cm) depends on the number
and size of the test holes required by the user, as well as the
amount of clearance required between each hole and the
block’s edges. FIG. 2 Rectangular and Polar Coordinates and ResultingDZ
E1629–94 (2001)
leastfourcoildiametersfromanyedge,slot,orhole.Forabolt 6.3.1 Probe Impedance in Air—Unless otherwise specified,
hole probe, rotate the probe face in the hole to four different the magnitude of the probe impedance in air shall be between
positions that are away from the slot and the top and bottom of
20 and 1000 V, and the phase shall be between 70 and 90 deg.
the hole. Measure the impedance and record the four imped-
An impedance value below 20 V indicates the possibility of a
ance values on the worksheet in either polar or rectangular
short circuit in the probe coil, and a value above 1000 V
coordinates. If necessary, convert the polar values to R and
off indicates a possible open circuit.
X (resistive and reactive components). Calculate the magni-
off
6.3.1.1 The magnitude of the impedance in air shall be
tude of each of the measurements and record them on the
within 10% of the value specified for that type of probe by the
worksheet. Determine the variation (scatter) in the magnitudes
probe manufacturer and be within the input impedance range
of the measurements and compare it to the requirements given
specified for the measuring instrument.
in6.3.2.Whenacceptablevaluesareobtained,averagethefour
6.3.2 Measurement Scatter—A variation greater than 4%
values to calculate R and X . Record the averages on
off avg off avg
among the off-slot impedance measurements indicates that the
the worksheet.
values are too scattered. The measurements must be repeated
6.2.4 Maximum On-Slot Impedance—Position the probe
using greater care in holding the surface probe more securely
face on the slot to obtain a maximum impedance reading.
Performthisprocedurefourtimes,andrecordthefourresulting orfittingtheboltholeprobemoresnuglyinthehole.Repeated
impedances in rectangular coordinates, R and X , on the highvariationindicatesthatananalysisofthesystemshouldbe
on on
worksheet.Averagethefourvaluestocalculate R and X
performed using different examiners or known acceptable
on avg on
avg. Record the averages on the worksheet.
probes. The probe is unacceptable if the measurement scatter
6.2.5 Probe Performance Criterion—The probe impedance
cannot be reduced to the acceptable value.
changecausedbytheslot, DZ,isthemagnitudeof R − R
on avg off
6.3.3 Probe Impedance Ratio—These ratios will be deter-
avg and X − X .The fractional change is the absolute
on avg off avg
mined by agreement between the eddy-current probe manufac-
value of D Z divided by |Z |. Appendix X1 contains the
off avg
turer and the probe purchaser.
formulasforcalculatingthevaluesof|DZ|and| Z |.Record
off avg
the calculated values on the worksheet and compare them to
7. Keywords
the requirements of 6.3.3 to determine acceptance.
6.3 Acceptance Criteria—Acceptance of a probe being 7.1 absolute eddy-current probes; eddy-current probes; im-
characterized by this practice requires that it meet all of the
pedance; nondestructive testing
following criteria:
APPENDIXES
(Nonmandatory Information)
X1. WORKSHEET FOR CALCULATION OF RESULTS
R 5|Z| cos u X 5|Z| sin u (X1.1)
X1.1 General
The use of a calculator with trigonometric functions is
X1.1.1 Scope—This appendix provides mathematical for-
recommended. The abbreviation “cos” is used for the cosine
mulas and a worksheet for recording measurements and
function, and the abbreviation “sin” is used for the sine
calculating results. It is recommended that the worksheet in
function.
this practice be left blank and photocopied for each probe
X1.2.1.2 Rectangular to Polar Conversion—The conver-
tested. Appendix X2 contains an example of a completed
sions from rectangular (R6 jX) to polar (|Z| / u) can be
worksheet.
performed using the following formulas:
2 2 –1
X1.2 General Practice
|Z| 5 =R 1 X u5 tan ~X/R! (X1.2)
X1.2.1 These measurements should be performed using a
X1.3 Specific Practice
test instrument that displays impedance in polar or rectangular
coordinates, or both. To perform the necessary calculations,
X1.3.1 It will be necessary to perform the following calcu-
both forms of complex impedance (polar and rectangular) are
lations after recording the measured impedances:
necessary. Mathematical conversion will be necessary if the
X1.3.1.1 Average Value— This involves adding the four
test instrument does not display the impedance in both forms.
measurement values (R + R + R + R and X +
1 2 3 4 1
X1.2.1.1 Polar to Rectangular Conversion—The conver- X + X + X ), dividing the totals by four, and recording those
2 3 4
sions from polar (|Z| / u) to rectangular (R 6 jX) can be results in the appropriate space on the worksheet (see X1.4.6.1
performed using the following formulas: or X1.4.6.2).
E1629–94 (2001)
2 2
Add each column: Total=____ ____ (V)
X1.3.1.2 Impedance Magnitude—|Z| 5 = R 1 X .
Divide by 4: Z =____ ____ (V)
X1.3.1.3 Measurement Scatter—A percentage value ob- off avg
convert R + jX to |Z | using: |Z |
tained by choosing the highest and lowest values from the four off avg off avg off avg off avg
off-flaw impedance magnitudes and applying the following
2 2
5 ~R ! 1 ~X !
= off avg off avg
formula: 2 2
(R ) = ________, (X ) = _____
...
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