IEC TR 62581:2010

Electrical steel - Methods of measurement of the magnetostriction characteristics by means of single sheet and Epstein test specimens

IEC/TR 62581:2010(E) describes the general principles and technical details of the measurement of the magnetostriction of single sheet specimens preferably 500 mm long and 100 mm wide and Epstein strip specimens, specified in IEC 60404-2, of electrical steel by means of optical sensors and accelerometers. These methods are applicable to test specimens obtained from electrical steel sheets and strips of any grade. The characteristics of magnetostriction are determined for a sinusoidal induced voltage, for specified peak values of magnetic polarization and for a specified frequency. The measurements are made at an ambient temperature of 23 °C +/- 5 °C on test specimens which have first been demagnetized.

General Information

• Status: Published
• Publication Date: 05-Aug-2010
• ICS: 29.030 - Magnetic materials
• Technical Committee: TC 68 - Magnetic alloys and steels
• Drafting Committee: WG 2 - TC 68/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 06-Aug-2010
• Completion Date: 31-Oct-2010

Standards Content (Sample)

IEC/TR 62581 ®
Edition 1.0 2010-08
TECHNICAL
REPORT
Electrical steel – Methods of measurement of the magnetostriction
characteristics by means of single sheet and Epstein test specimens

IEC/TR 62581:2010(E)
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IEC/TR 62581 ®
Edition 1.0 2010-08
TECHNICAL
REPORT
Electrical steel – Methods of measurement of the magnetostriction
characteristics by means of single sheet and Epstein test specimens

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 29.030 ISBN 978-2-88912-101-4
– 2 – TR 62581 © IEC:2010(E)
CONTENTS
FOREWORD.5
INTRODUCTION.7
1 Scope.8
2 Normative references .8
3 Terms and definitions .8
4 Method of measurement of the magnetostriction characteristics of electrical steel
sheets under applied stress by means of a single sheet tester.9
4.1 Principle of the method.9
4.2 Test specimen.11
4.3 Yokes.12
4.4 Windings .13
4.5 Air flux compensation .14
4.6 Power supply.14
4.7 Optical sensor .14
4.8 Stressing device.15
4.9 Data acquisitions.15
4.10 Data processing .16
4.11 Preparation for measurement .16
4.12 Adjustment of power supply.17
4.13 Measurement .17
4.14 Determination of the butterfly loop.20
4.15 Determinations of the zero-to-peak and peak-to-peak values.20
4.16 Reproducibility .20
5 Examples of the measurement systems.20
5.1 Single sheet tester .20
5.2 Epstein strip tester .25
6 Examples of measurement .26
6.1 Magnetostriction without external stress .26
6.2 Magnetostriction under applied stress .27
6.3 Variation of magnetostriction with coating tension .30
6.4 Factors affecting precision and reproducibility .34
6.4.1 General .34
6.4.2 Overlap length between test specimen and yoke .34
6.4.3 The averaging effect on environmental noise.34
6.4.4 Gap between test specimen and yoke.34
6.4.5 Resetting the test specimen.35
7 Methods of evaluation of the magnetostriction behaviour.36
7.1 Relationship between magnetostriction and magnetic domain structure.36
7.2 A simple model of magnetostriction behaviour.37
Annex A (informative) Requirements concerning the prevention of out-of-plane
deformations.40
Annex B (informative) Application of retained stress model to measured stress shifts .42
Annex C (informative) A-weighted magnetostriction characteristics .45
Bibliography.48

TR 62581 © IEC:2010(E) – 3 –
Figure 1 – Measurement systems for magnetostriction.9
Figure 2 – Section of the test frame; A-A’ in Figure 1 .10
Figure 3 – Block diagram of the measurement system .10
Figure 4 – Frames with various types of yoke .13
Figure 5 – Base length l for various types of frame (see Figure 4) .18
Figure 6 – Butterfly loop and determinations of zero-to-peak and peak-to-peak values of
magnetostriction .20
Figure 7 – Measurement system using a Michelson interferometer; differential
measurement [1].21
Figure 8 – Measurement system using a laser Doppler vibrometer; differential
measurement [2], [3], [17] .21
Figure 9 – Measurement system using a laser Doppler vibrometer; differential
measurement [4],[5].23
Figure 10 – Measurement system using a laser displacement meter; single point
measurement [7].23
Figure 11 – Measurement system using a laser displacement meter; single point
measurement [6].24
Figure 12 – Measurement system using a laser Doppler vibrometer; single point
measurement [8].24
Figure 13 – Schematic diagram of an automated system using accelerometer sensors [12] .25
Figure 14 – Example of measured results for high permeability grain-oriented electrical
steel of 0,3 mm thick sheet; at 1,3 T, 1,5 T, 1,7 T, 1,8 T and 1,9 T, 50 Hz [2].29
Figure 15 – Increase in magnetostriction with compressive stress in the rolling direction;
at 1,5 T, 1,7 T and 1,9 T, 50 Hz [2] .29
Figure 16 – Typical zero-to-peak magnetostriction versus applied stress for high
permeability grain-oriented electrical steel sheet at 1,5 T, 50 Hz [12].29
Figure 17 – Stress sensitivity of magnetostriction and permeability in a typical fully
processed sample [12].30
Figure 18 – Typical harmonics of magnetostriction versus applied stress for
conventional grain-oriented electrical steel at 1,5 T, 50 Hz [12] .30
Figure 19 – Variation of maximum magnetostriction under compressive stress in high
permeability grain-oriented electrical steel at 1,5 T, 50 Hz [20] .31
Figure 20 – Variation of maximum magnetostriction under compressive stress in
conventional grain-oriented electrical steel at 1,5 T, 50 Hz [20] .31
Figure 21 – Magnetostriction versus stress characteristics in the rolling direction of
conventional grain-oriented electrical steel before and after coating removal at 1,5 T,
50 Hz [20] .31
Figure 22 – Magnetostriction versus stress characteristics in the transverse direction of
conventional grain-oriented electrical steel before and after coating removal at 1,5 T,
50 Hz [20] .31
Figure 23 – Magnetostriction versus peak value of magnetic polarization for high
permeability 0,30 mm grain-oriented electrical steel sheets with three different coatings;
external stress was not applied [17] .33
Figure 24 – Magnetostriction versus peak value of magnetic polarization for high
permeability 0,30 mm grain-oriented electrical steel sheets with three different coatings;
external compressive stress of 3 MPa was applied in the rolling direction [17] .33
Figure 25 – Effects of overlap length on the reproducibility of measurement [4] .34
Figure 26 – Effect of averaging number on reduction of the error caused by the
environmental noise [5].34

– 4 – TR 62581 © IEC:2010(E)
Figure 27 – Effect of gap between the test specimen and the yoke on the reproducibility
of measurement; the test specimen was reset at every measurement [5] .35
Figure 28 – Effect of reset of the test specimen on the reproducibility of measurement;
the gap distance was 1,2 mm [5] .35
Figure 29 – Magnetic domain patterns on a grain-oriented electrical steel sheet [2] .36
Figure 30 – Schematic diagrams for explanation of magnetic domains and
magnetostriction [2],[17] .36
Figure 31 – Separation of the different features of peak-to-peak magnetostriction
according to the proposed model [27] .38
Figure 32 – Measured peak-to-peak and zero-to-peak magnetostriction of a grain-oriented
electrical steel sheet with fitted curves according to the proposed model [27] .38
Figure 33 – Effect of coating tension on J − λ curves; λ is the normalized value
m sp sp
of zero-to-peak magnetostriction to the value at saturation polarization [17] .39
Figure 34 – Effect of laser irradiation on J − λ curves; λ is the normalized value
sp
m sp
of zero-to-peak magnetostriction to the value at saturation polarization [17] .39
Figure A.1 – Schematic diagram of out-of-plane deformation of test specimen (length
l ) with radius r .41
m
Figure A.2 – Errors in length change of the test specimen Δl / l versus out-of-plane
m
deformation distance Δd .41
Figure B.1 – Variation of coating stress with coating thickness for forsterite and
phosphate coating [20].44
Figure C.1 – Frequency response of the acoustic A-weighting filter, specified in
IEC 61672-1 .45
Figure C.2 – A-weighted magnetostriction acceleration levels of CGO-0,30 mm and
HGO-0,30 mm materials .47

Table B.1 – Measured stress shifts for two stage coating removal .43

TR 62581 © IEC:2010(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL STEEL –
METHODS OF MEASUREMENT OF
THE MAGNETOSTRICTION CHARACTERISTICS
BY MEANS OF SINGLE SHEET AND EPSTEIN TEST SPECIMENS

FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 62581, which is a technical report, has been prepared by IEC technical committee 68:
Magnetic alloys and steels.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
68/411/DTR 68/414/RVC
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.

– 6 – TR 62581 © IEC:2010(E)
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

TR 62581 © IEC:2010(E) – 7 –
INTRODUCTION
Magnetostriction is one of the magnetic properties that accompany ferromagnetism. It causes
reversible deformations of a material body due to magnetization arising from an applied
magnetic field.
Nowadays, the environmental problem of acoustic noise pollution caused by transformers and
other applications of electrical steels (e.g. ballast, motors, etc.) is a concern of industry [31] .
Magnetostriction of electrical steels is recognized as one of the causes of the problem and a
standardization of methods of measurement of the magnetostriction is required to advance
developments in materials to address this problem.
Historically, several methods have been used to measure magnetostriction including strain
gauge, capacitance, differential transformer, piezoelectric pick-up and piezoelectric
accelerometer methods. However, these methods require skill to set up the sensor accurately
and to avoid vibrational noise that accompanies these contact methods. To solve these
problems, optical methods that adopt optical vibrometers and optical displacement meters have
been developed [1]-[8].
The optical method satisfies the following requirements for the measurement: non-contact, high
resolution, high reproducibility and ease of operation without any special skill on the part of the
operator. Several optical sensors can be used: laser Doppler vibrometers, heterodyne
displacement meters and laser displacement meters with high resolution.
Magnetostriction is a magneto-mechanical phenomenon which accompanies the change of the
volume fraction of magnetic domains which have a certain magnetic orientation with respect to
the direction of the applied magnetic field, and which is intrinsically sensitive to stress [14],[15].
The stress sensitivity is dependent on material conditions such as grain orientation, residual
stress and coating tension. The magnetostriction of electrical steel is increased by compressive
stresses in the magnetizing direction rather than tensile stresses [9],[16]-[23]. Magnetic cores of
electrical machines such as transformers often contain areas of increased stress. Therefore the
stress sensitivity should be evaluated under a specified stress.
The acoustic noise emission from transformers and other machines is usually evaluated in terms
of the A-weighted sound pressure level specified in IEC 61672-1. Vibration velocities caused by
magnetostriction are transformed into sound pressure on the surface of the materials. Therefore,
A-weighted characteristics of magnetostriction, such as A-weighted magnetostriction velocity
level or A-weighted magnetostriction acceleration level, are necessary for the assessment of
electrical steel sheets with respect to the acoustic noise [24]-[26].
This technical report is comprised of articles which review the optical and accelerometer
methods of measurement of magnetostriction with the aim of producing a standard method of
measurement of magnetostriction.
Two methods, by a single sheet tester and by a single strip tester, are described. The former
should be applied to single sheet specimens with width of not less than 100 mm which have not
been stress relief annealed. The latter method should be applied to Epstein test specimens,
which may have been stress relief annealed to remove stresses imparted to the specimens
during preparation.
___________
The figures in square brackets refer to the Bibliography.

– 8 – TR 62581 © IEC:2010(E)
ELECTRICAL STEEL –
METHODS OF MEASUREMENT OF
THE MAGNETOSTRICTION CHARACTERISTICS
BY MEANS OF SINGLE SHEET AND EPSTEIN TEST SPECIMENS

1 Scope
This technical report describes the general principles and technical details of the measurement
of the magnetostriction of single sheet specimens preferably 500 mm long and 100 mm wide
and Epstein strip specimens, specified in IEC 60404-2, of electrical steel by means of optical
sensors and accelerometers.
These methods are applicable to test specimens obtained from electrical steel sheets and strips
of any grade. The characteristics of magnetostriction are determined for a sinusoidal induced
voltage, for specified peak values of magnetic polarization and for a specified frequency.
The measurements are made at an ambient temperature of 23 °C ± 5 °C on test specimens
which have first been demagnetized.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
IEC 60050-121, International Electrotechnical Vocabulary – Part 121: Electromagnetism
IEC 60050-221, International Electrotechnical Vocabulary – Chapter 221: Magnetic materials
and components
IEC 60404-2, Magnetic materials – Part 2: Methods of measurement of the magnetic properties
of electrical steel sheet and strip by means of an Epstein frame
IEC 60404-3:1992, Magnetic materials – Part 3: Methods of measurement of the magnetic
properties of electrical steel strip and sheet by means of a single sheet tester
Amendment 1 (2002)
Amendment 2 (2009)
IEC 61672-1, Electroacoustics – Sound level meters – Part 1: Specifications
3 Terms and definitions
For the purpose of this document, the definitions of the principal terms relating to magnetic
properties given in IEC 60050-121 and IEC 60050-221 apply, as well as the following terms and
definitions:
3.1
butterfly loop
hysteresis loop of the strain measured in the direction of applied field versus the magnetic
polarization for a period of an alternating magnetization

TR 62581 © IEC:2010(E) – 9 –
3.2
zero-to-peak magnetostriction
λ
o−p
net strain measured in the direction of applied field from the zero magnetic polarization to a
given magnetic polarization
3.3
peak-to-peak magnetostriction
λ
p−p
amplitude of the strain measured in the direction of the applied field under alternating
magnetization
4 Method of measurement of the magnetostriction characteristics of electrical
steel sheets under applied stress by means of a single sheet tester
4.1 Principle of the method
Measurement systems for magnetostriction are shown in Figure 1 and Figure 2. A block diagram
of the measurement system is shown in Figure 3.

Mutual inductor
Clamp Mutual inductor Clamp
A'
for air flux compensation for air flux compensation

Windings Windings
Optical target Optical target
(pasted on the (pasted on the
test specimen) test specimen)

Test specimen Test specimen
Stressing device
Stressing device
(air cylinder drive)
(air cylinder drive)
Yoke
Yoke
Base plate Base plate
Anti-vibration table
Anti-vibration table
A
Reflector
Optical sensor head Optical sensor heads
(single point measurement) (differential measurement)

Figure 1a − Single point measurement Figure 1b − Differential measurement
Figure 1 – Measurement systems for magnetostriction

– 10 – TR 62581 © IEC:2010(E)
The distance between the clamp Clamp
and the optical target (l )
Optical sensor head Optical target 0
Test specimen
Spacer sheet
(under the test specimen)
Yoke
Positioner
Base plate
Air cylinder
Anti-vibration table
Load cell Stressing device Windings

Figure 2 – Section of the test frame; A-A’ in Figure 1

Computer
sampling clock
S test specimen
M
C
T optical target
Arbitrary
Power
ch. X
C clamp
waveform
amplifier
N N M mutual inductor for air flux compensation
generator 1 2
N primary winding
2-channel
N secondary winding
digitizer
S T
Optical
ch. Y
sensor
Figure 3 – Block diagram of the measurement system

TR 62581 © IEC:2010(E) – 11 –
The test specimen comprises a sample of electrical steel sheet and is placed inside two
windings:
– an external primary winding;
– an interior secondary winding.
The flux closure is made by a magnetic circuit consisting of a yoke, the cross-section of which is
large compared with that of the test specimen. There are narrow and homogeneous gaps
between the test specimen and the pole faces of the yoke to weaken the electromagnetic force
between them. The test frame that consists of the yoke, the windings and a clamp should be
permanently fixed to a rigid base during the measurement.
The test specimen is fixed to the base at one end of the windings using the clamp shown in
Figure 1. An optical target is pasted on the centerline of the surface of the test specimen at the
other end of the windings. Changes in length between the clamping point and the optical target
are measured using an optical sensor.
In order to reduce the effect of stray fields between the test specimen and the pole faces, the
optical sensor should be at a sufficient working distance from the test frame.
Two measurement types of optical sensor can be used: a single point measurement and a
differential measurement. The single point measurement uses a sensor head fixed on the base
and measures vibration or displacement between the optical target and the sensor head. The
differential measurement uses two sensor heads and measures vibration and displacement
between the optical target and a reflector fixed on the base. The latter system has advantages of
(a) cancellation of external noise and in (b) setting the sensors separately from the base.
Care shall be taken to minimize noise vibrations caused by resonances of the test frame and
external vibrations. The test frame shall be placed on an anti-vibration table in order to isolate
the test frame from external vibration.
Care shall be taken to prevent out-of-plane deformations of the test specimen. The test
specimen shall be placed on a flat and smooth surface in the test frame and kept flat during the
measurement (see Annex A).
4.2 Test specimen
The length of the test specimen should be not less than 500 mm. The part of the specimen
situated outside the pole faces should not be longer than is necessary to facilitate insertion and
removal of the test specimen and to apply an external stress to the test specimen in the
longitudinal direction.
The width of the test specimen should be not less than 100 mm.
NOTE Since the average grain diameter for the high permeability grain-oriented electrical steel sheet is about 10-20
mm, a comparatively large sample size is required. The test specimen should be wide enough to take into account the
affected region close to the cut edges. However, it may be difficult to produce a flux closure yoke with flat and
coplanar faces for wider test specimens.
The test specimen should be cut without forming large burrs or mechanical distortion. The test
specimen shall be flat. When a test specimen is cut, the edge of the parent strip is taken as the
reference direction. The following tolerances are allowed for in the angle between the direction
of rolling and that of cutting:
– ±1° for grain-oriented electrical steel sheet;
– ±5° for non-oriented electrical steel sheet.

– 12 – TR 62581 © IEC:2010(E)
4.3 Yokes
Several types of yoke can be used (see Figure 4):
– a horizontal single or double yoke;
– a vertical single or double yoke.
The horizontal yokes make a horizontal flux closure and the vertical yokes make a vertical flux
closure. Each pole face is horizontal in both types of yoke.
Each yoke is made up of insulated sheets of grain-oriented electrical steel or nickel iron alloy. It
should have a low reluctance and therefore stress relief annealing of the cut strips is required.
The two pole faces of each yoke shall be coplanar within 0,03 mm (see Annex A). The yokes
shall be constructed in accordance with the requirements of Annex A of IEC 60404-3.
In order to reduce the effect of eddy currents and give a more homogeneous distribution of the
flux over the inside of the yokes, the yokes are made of a glued stack of laminations or C-cores.
In the former case the corners have staggered butt joints. The overlap length of the test
specimen and the pole faces shall be long enough and not less than 25 mm.
Before use, the yokes should be carefully demagnetized.
The electromagnetic force between the test specimen and the pole faces should be reduced by
inserting narrow and uniform gaps between them. This can be achieved by inserting a sheet
under the test specimen. The sheet shall be made of a non-compressible, non-conducting and
non-magnetic material with flat and smooth surfaces. The thickness of the sheet should be
uniform and between 0,1 mm and 1,0 mm.
NOTE 1 The electromagnetic force may increase out-of-plane vibrations of the test specimen and friction between
the test specimen and the pole faces. These may reduce the accuracy and reproducibility of the measurement.
Each yoke is glued to the base plate. Resonances in each yoke that may affect the
measurement shall be avoided.
In the case of the vertical double yoke, care should be taken to maintain a constant gap between
the test specimen and the pole faces of the upper yoke.
NOTE 2 Because the upper yoke may block the optical beam from the sensor heads, the optical target may be
outside the upper yoke in this case. Over the pole face region there will be perpendicular components of the magnetic
flux in the test specimen and therefore over this region the magnetostriction behaviour may change. Verification of
test results with the other test frame may be necessary.

TR 62581 © IEC:2010(E) – 13 –
Clamp Clamp
Windings
Yoke Yoke
Windings
Optical target
Optical target
Test specimen
Test specimen
Figure 4a – Horizontal single yoke Figure 4b – Horizontal double yoke

Clamp
Clamp
Windings
Windings
Optical target
Optical target
Yoke
Yoke
Test specimen
Test specimen
Figure 4c – Vertical single yoke Figure 4d – Vertical double yoke
Figure 4 – Frames with various types of yoke
4.4 Windings
The primary and secondary windings are wound on a rectangular former made of
non-conducting, non-magnetic material. The length of the former is shorter than the distance
between the pole faces of the yokes to avoid the effect of stray fields between the test specimen
and the pole faces.
The number of turns of the primary winding will depend on the characteristics of the power
supply.
The number of turns of the secondary winding will depend on the characteristics of the
measuring instruments.
The test specimen is inserted through the inside hollow of the former and supported on a plate
of non-conducting and non-magnetic material. The surface of the plate in contact with the test
specimen shall be flat and smooth with its surface coplanar with the pole faces within 0,03 mm
(see Annex A).
– 14 – TR 62581 © IEC:2010(E)
4.5 Air flux compensation
Compensation should be made for the effect of air flux, for example, by means of a mutual
inductor.
The primary winding of the mutual inductor is connected in series with the primary winding of the
test frame, while the secondary winding of the mutual inductor is connected to the secondary
winding of the test frame in series opposition.
The adjustment of the mutual inductance shall be made so that, when passing an alternating
current through the primary winding in the absence of the specimen in the test frame, the voltage
measured between the non-common terminals of the secondary windings should be no more
than 0,1 % of the voltage appearing across the secondary winding of the test frame alone.
Thus the average value of the rectified voltage induced in the combined secondary windings is
proportional to the peak value of the magnetic polarization in the test specimen.
Compensation can be achieved by a digital method without the mutual inductor. The contribution
of air flux in the secondary induced voltage can be calculated from the primary current and then
by subtracting it from the measured secondary induced voltage.
4.6 Power supply
The power supply should be of low internal impedance and should be highly stable in terms of
voltage and frequency. During the measurement, the voltage and the frequency should be
maintained constant within ± 0,2 %.
The power supply comprises an arbitrary signal generator consisting of a waveform synthesizer
and voltage and frequency controller and a power amplifier. The arbitrary signal generator shall
have two outputs: one for the magnetizing signal supplied to the power amplifier and the other
for the synchronized sampling clock supplied to a 2-channel digitizer.
In addition, the waveform of the secondary induced voltage should be maintained as near
sinusoidal as possible. It is preferable to maintain the form factor of the secondary voltage to
within ± 1 % of 1,111. This can be achieved by various means, for example, by using an
electronic feedback amplifier or by a digital feedback procedure through a computer.
4.7 Optical sensor
The optical sensor can detect changes in displacement of the optical target fixed on the test
specimen with a high resolution of better than 10 nm in the displacement of the optical target at
the frequency of 100 Hz. However, a resolution of better than 3 nm is recommended.
–8
NOTE The resolution of the length of 10 nm corresponds to a resolution of 3,3 × 10 in magnetostriction for a
–8
distance of 300 mm between the clamp and the optical target. The higher resolution of 1 × 10 in magnetostriction is
required for high permeability grain-oriented electrical steel sheet used for low noise transformers in which the
–6
magnetostriction is less than 1 × 10 .
Either optical vibrometer or optical displacement meters can be used. The optical vibrometer,
e.g. a laser Doppler vibrometer, detects changes in displacement of the optical target and is
suitable for a.c. measurements. The optical displacement meter, e.g. a heterodyne
displacement meter, detects the displacement of the optical target from the sensor head and is
suitable for d.c. measurements and a.c. measurements at low frequency.
A laser Doppler vibrometer is recommended for the optical vibrometer. This sensor has
adequate performance for measurements of the magnetostriction: high spatial resolution, high
stability, a wide range of frequency and velocity, and is unaffected by magnetic field and
temperature. It may be used remotely to enable flexibility in sensor positioning. It is readily
available and requires little operator skill.

TR 62581 © IEC:2010(E) – 15 –
Two measurement methods are available for optical sensors: a single point measurement
method and a differential measurement method. The single point measurement method adopts
a sensor head and measures vibration or displacement between the optical target and the
sensor head. The differential measurement method adopts two sensor heads and measures the
difference of vibration or displacement between the optical target on the test specimen and the
reflector fixed on the test frame base. The differential measurement method has the advantage
that the sensor heads are divorced from the test frame base and any external noise is cancelled
and therefore is not detected.
In the case of optical sensors, beams of light are focused on the optical target or the reflector.
The optical target and the reflector reflect back the beams of light to the optical sensor. The
optical target should be of low profile, low mass (less than 0,05 g) and made of a non-conducting
and non-magnetic material in order to undertake measurements of magnetostriction: for
example, 4 mm in height and 3 mm to 5 mm in width and depth. The optical target can be pasted
at a fixed position on the test specimen before inserting the test specimen into the test frame.
Reflecting thin films can be pasted on the surfaces of the optical target and the reflector facing
the optical sensors.
4.8 Stressing device
The stressing device should apply the stress along the axis of the test specimen. An air cylinder,
which is fastened to the base plate, holds the stressing device and drives the stressing device in
the direction of the axis of the test specimen. Another device can be used instead of the air
cylinder if it is flexible to length change of the test specimen caused by magnetostriction. A load
cell is installed between the stressing device and the air cylinder to detect stresses applied to
the test specimen. These devices shall be prepared so as not to prevent the magnetostriction
measurement by those resonances.
The stressing device applies compressive stress to the test specimen when the
magnetostriction under compressive stress is measured, otherwise the stressing device shall be
retracted and removed from the test specimen. Stresses of up to 5 MPa can be applied between
the clamp and the stressing device in the longitudinal direction of the test specimen. A low
friction, non-rotating air cylinder can be used for this purpose (see Figure 2).
NOTE The stress sensitivity of magnetostriction may be measured at a specified compressive stress of, for example,
3 MPa.
The pressure of the air cylinder can be controlled easily by air through an electro-pneumatic
valve controlled by a d.c. voltage. The load cell installed between the test specimen and the air
cylinder detects the stress.
4.9 Data acquisitions
The output voltage of the optical sensor and the secondary induced voltage shall be digitized
simultaneously and recorded as a set of signal data by a 2-channel digitizer.
NOTE A 2-channel digital sampling oscilloscope can be used as the 2-channel digitizer.
The 2-channel digitizer has two independent channels composed of calibrated amplifiers,
sample-and-hold circuits and calibrated analogue to digital converters (ADC). The gain
selectable amplifiers should enable a wide range of input voltage and frequency. The
2-channels are sampled simultaneously with the sampling clock generated by the arbitrary
signal generator and then digitized. The resolution of ADC shall be a minimum of 12 bits.
To avoid loading the secondary winding, the 2-channel digitizer should have sufficiently high
input impedance and low capacitance (typically 1 MΩ in parallel with about 100 pF).
The output voltage of the laser Doppler vibrometer is proportional to the time derivative of
magnetostriction dλ(t)/ dt . On the other hand, the output of the heterodyne displacement meter
is proportional to the magnetostriction .
λ(t)
– 16 – TR 62581 © IEC:2010(E)
The secondary induced voltage is proportion to the time derivative of magnetic polarization
. Two approaches can be used for the integration of derivative signals to obtain
dJ(t)/ dt λ(t)
from dλ(t)/ dt and J (t) from dJ(t)/ dt :
– analogue integration and digitizing;
– digitizing and numerical integration.
The former approach requires analogue integrators with wide input voltage and frequency
ranges. The latter approach requires ADC with the wide input ranges and higher resolutions.
It is recommended that the sampling frequency is a multiple of the magnetizing frequency
(Nyquist condition) and sufficiently larg
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