ASTM E1629-94

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Designation: E 1629 – 94 An American National Standard
Standard Practice for
Determining the Impedance of Absolute Eddy-Current
Probes
This standard is issued under the fixed designation E 1629; 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 an aluminum alloy (see 6.1.2) to which an active eddy-current
probe is applied.
1.1 This practice covers a procedure for determining the
3.3 Mathematical Symbols:
impedance of absolute eddy-current probes (bridge-type, air or
3.3.1 j—a symbol used in electrical engineering to represent
ferrite core, wire wound, shielded, or unshielded) used for
21 . It is associated with the restriction to the flow of
=
finding material defects in electrically conducting material.
electrical current caused by capacitors and coils.
This practice is intended to establish a uniform test methodol-
3.3.2 N—any number.
ogy to measure the impedance of eddy-current probes prior to
3.3.3 |N|—the magnitude of N, regardless whether N is
receipt of these probes by the purchaser or the specifier.
positive, negative, or a vector quantity.
1.2 Limitations—This practice does not address the charac-
3.3.4 N —the square root of N.
=
terization or measurement of the impedance of differential, a-c
3.3.5 (N) —N squared, that is, N 3 N.
coupled, or transmit/receive types of probes. This practice does
3.3.6 DN—delta N, the change or difference in N.
not address the use of magnetic materials in testing probes.
3.4 Abbreviation:
This practice shall not be used as a basis for selection of the
−1
3.4.1 tan—used for the tangent function. The tan , arctan-
best probe for a particular application or as a means by which
gent or inverse tangent function, returns a value that is a
to calibrate a probe for a specific examination.
measure of an angle and can be in either degrees or radians.
1.3 The values stated in SI units are to be regarded as the
−1
When using a calculator to determine the tan , care should be
standard. The values given in parentheses are for information
taken to determine whether the answer is in degrees or radians
only.
since the numerical values that represent the same angle are
1.4 This standard does not purport to address all of the
different.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 Eddy-current probes may be used for the nondestructive
bility of regulatory limitations prior to use.
examination of parts or structures made of electrically conduct-
2. Referenced Documents ing materials. Many of these examinations are intended to
discover material defects, such as fatigue cracks, that may
2.1 The following document forms a part of this practice to
cause the part or structure to be unsafe or unfit for further
the extent specified herein:
service. Eddy-current probes that fail to meet the performance
2.2 ASTM Standards:
level requirements of this practice shall not be used for the
E 1316 Terminology for Nondestructive Examination
examination of material or hardware unless the probe is
3. Terminology
qualified by some other system or an agreement has been
reached by the probe manufacturer and the purchaser, or both.
3.1 Definitions—The terminology relating to eddy-current
examination that appears in Terminology E 1316 shall apply to
5. General Practice
the terms used in this practice.
5.1 Use of Test Blocks—The test blocks described in this
3.2 Definitions of Terms Specific to This Standard:
practice shall not be used for purposes other than measuring the
3.2.1 eddy-current test block—for the purposes of the
impedance of eddy-current probes as specified in this practice.
method described in this practice, a rectangular block made of
They shall not be used for calibrating 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
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.07 on
manufacturer in accordance with this practice. The results shall
Electromagnetic Method.
be delivered with the probe and maintained by the using
Current edition approved Sept. 15, 1994. Published November 1994.
organization. While the retesting of probes may be performed
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1629
using this practice, only the results obtained before the probe is
in use should be compared to the initial impedance values. The
impedance results should not be compared to the initial values
after a probe has been used.
6. Specific Practice
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
frequency (as specified by the probe manufacturer) may vary
for each probe examined, depending on the specific probe
geometry, skin depth, matching impedance, desired signal
strength, and application. A commercial impedance measuring
FIG. 1 Representative Test Block
instrument that conforms to 6.1.1 shall be used to make the
measurements. The measurements will be recorded on the
component, R, and a reactive or imaginary (denoted by j)
worksheet (Appendix X1) to calculate the probe impedance.
component, X. This form often appears as R 6 jX. Fig. 2 shows
The calculated values shall be compared to the acceptable
two points measured in both forms and the resulting impedance
criteria (6.3) to determine probe acceptability.
change (DZ) calculation. This is typical of the way in which
6.1.1 Test Equipment—The test instrument shall be either a
impedance changes are measured on actual eddy-current
commercial impedance measuring instrument or an LCR meter
probes using the test method specified in this practice. The
with an oscillator capable of driving a current in the probe at
impedance of a probe is measured first off the slot and then on
the probe’s operating frequency. The output shall display the
the slot in this method, and the difference between these two
probe impedance in either polar form, providing a magnitude
measurements is calculated. An error may occur in the calcu-
and a phase angle, or rectangular form, providing resistive and
lations if appropriate coordinate conversions are not made
reactive components of the impedance, or both. This instru-
(addition and subtraction are performed on rectangular coor-
ment shall be calibrated in accordance with the manufacturer’s
dinates and multiplication and division on polar coordinates).
specifications at the required interval.
6.2.1.1 All performance tests shall be conducted within the
6.1.2 Test Blocks—The test block shall be fabricated from
temperature range from 15 to 27°C (60 to 80°F). The probe test
7075-T6 aluminum alloy 1.9-cm (0.75-in.) thick, with slotted
frequency shall depend on the rated operating frequency of the
holes for testing bolt hole probes and a slotted flat section for
particular probe under test (see X1.4.2.3).
testing surface probes. All test surfaces shall be polished to an
6.2.2 Probe Impedance in Air—Attach the probe to the
average finish # 15 μm (591 μin.). The conductivity of the
impedance measuring instrument, and position the probe at
aluminum alloy should be between 30 and 35 % IACS.
least 50.8 mm (2 in.) away from any electrically conducting
6.1.2.1 The slotted flat section shall have side measurements
material or hardware, or both. Measure the impedance and
of at least six times the coil diameter or 5.1 by 5.1 cm (2 by 2
record the impedance values on the worksheet. Compare the
in.), whichever is larger. The slot dimensions shall be machined
measurement to the values listed in 6.3.1.
across the block’s surface and shall measure at least 5.0 cm
6.2.3 Average Off-Slot Probe Impedance—Place the probe
(2-in.) long. The slot cross section shall measure 0.1 6 0.01
on the surface of or in the hole in the test block, as appropriate.
mm (0.004 6 0.004 in.) wide and 0.5 6 0.025 mm (0.02 6
For a surface probe, place the probe on four different positions
0.001 in.) deep.
on the face of the block. The center of each position shall be at
6.1.2.2 The test holes shall be made for all of the nominal
least four coil diameters from any edge, slot, or hole. For a bolt
sizes of bolt hole probes to be examined. The edges of the holes
hole probe, rotate the probe face in the hole to four different
shall be spaced 1.9-cm (0.75-in.) apart from each other and
positions that are away from the slot and the top and bottom of
from the block edges. The slot shall run the entire length of the
the hole. Measure the impedance and record the four imped-
hold and will be 0.1 6 0.01-mm (0.004 6 0.0004-in.) wide and
ance values on the worksheet in either polar or rectangular
0.5 6 0.025-mm (0.02 6 0.001-in.) deep.
coordinates. If necessary, convert the polar values to R and
off
6.1.2.3 Fig. 1 shows a representative test block with the
X (resistive and reactive components). Calculate the magni-
off
holes used for testing two different sizes of bolt hole probes.
tude of each of the measurements and record them on the
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.
6.2 Measurement Procedure:
6.2.1 Measurements—Impedance values can be expressed
in different ways. Impedances are most commonly given in
either rectangular or polar form. In polar form, the impedance
is expressed as a magnitude, |Z|, with a corresponding phase
angle, u, and often appears as |Z| / u. The rectangular form
expresses the impedance as a combination of a resistive FIG. 2 Rectangular and Polar Coordinates and Resulting DZ
E 1629
worksheet. Determine the variation (scatter) in the magnitudes An impedance value below 20 V indicates the possibility of a
of the measurements and compare it to the requirements given short circuit in the probe coil, and a value above 1000 V
in 6.3.2. When acceptable values are obtained, average the four indicates a possible open circuit.
values to calculate R and X . Record the averages on
off avg off avg 6.3.1.1 The magnitude of the impedance in air shall be
the worksheet.
within 10 % of the value specified for that type of probe by the
6.2.4 Maximum On-Slot Impedance—Position the probe
probe manufacturer and be within the input impedance range
face on the slot to obtain a maximum impedance reading.
specified for the measuring instrument.
Perform this procedure four times, and record the four resulting
6.3.2 Measurement Scatter—A variation greater than 4 %
impedances in rectangular coordinates, R and X ,onthe
on on
among the off-slot impedance measurements indicates that the
worksheet. Average the four values to calculate R and X
on avg on
values are too scattered. The measurements must be repeated
avg. Record the averages on the worksheet.
using greater care in holding the surface probe more securely
6.2.5 Probe Performance Criterion—The probe impedance
or fitting the bolt hole probe more snugly in the hole. Repeated
change caused by the slot, DZ, is the magnitude of R − R
on avg off
high variation indicates an analysis of the system using
avg and X − X . The fractional change is the absolute
on avg off avg
different examiners or known acceptable probes. The probe is
value of DZ divided by |Z |. Appendix X1 contains the
off avg
unacceptable if the measurement scatter cannot be reduced to
formulas for calculating the values of |DZ| and| Z |. Record
off avg
the acceptable value.
the calculated values on the worksheet and compare them to
6.3.3 Probe Impedance Ratio—These ratios will be deter-
the requirements of 6.3.3 to determine acceptance.
mined by agreement between the eddy-current probe manufac-
6.3 Acceptance Criteria—Acceptance of a probe being
turer and the probe purchaser.
characterized by this practice requires that it meet all of the
following criteria:
7. Keywords
6.3.1 Probe Impedance in Air—Unless otherwise specified,
the magnitude of the probe impedance in air shall be between 7.1 absolute eddy-current probes; eddy-current probes; im-
20 and 1000 V, and the phase shall be between 70 and 90 deg. pedance; nondestructive testing
APPENDIXES
(Nonmandatory Information)
X1. WORKSHEET FOR CALCULATION OF RESULTS
X1.1 General performed using the following formulas:
2 2 –1
X1.1.1 Scope—This appendix provides mathematical for-
|Z| 5 =R 1 X u5 tan ~X/R! (X1.2)
mulas and a worksheet for recording measurements and
X1.3 Specific Practice
calculating results. It is recommended that the worksheet in
this practice be left blank and photocopied for each probe
X1.3.1 It will be necessary to perform the following calcu-
tested. Appendix X2 contains an example of a completed
lations after recording the measured impedances:
worksheet.
X1.3.1.1 Average Value—This involves adding the four
measurement values (R + R + R + R and
1 2 3 4
X1.2 General Practice
X + X + X + X ), dividing the totals by four, and recording
1 2 3 4
X1.2.1 These measurements should be performed using a those results in the appropriate space on the worksheet (see
test instrument that displays impedance in polar or rectangular X1.4.6.1 or X1.4.6.2).
2 2
coordinates, or both. To perform the necessary calculations,
X1.3.1.2 Impedance Magnitude—|Z| 5 R 1 X .
=
both forms of complex impedance (polar and rectangular) are
X1.3.1.3 Measurement Scatter—A percentage value ob-
necessary. Mathematical conversion will be necessary if the
tained by choosing the highest and lowest values from the four
test instrument does not display the impedance in both forms.
off-flaw impedance magnitudes and applying the following
X1.2.1.1 Polar to Rectangular Conversion—The conver-
formula:
sions from polar (|Z| / u) to rectangular (R 6 jX) can be
@~highest value 2 lowest value!/highest value#3 100 (X1.3)
performed using the following formulas:
X1.3.1.4 Probe Impedance Change—The average probe
R 5 |Z| cos u X 5 |Z| sin u (X1.1)
impedance calculated for the off-slot impedance is subtracted
The use of a calculator with trigonometric functions is
from the average maximum on-slot impedance measurement.
recommended. The abbreviation “cos” is used for the cosine The impedance change in rectangular form is converted
...