ASTM A977/A977M-07(2020)
(Test Method)Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs
Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs
SIGNIFICANCE AND USE
4.1 This test method is suitable for magnet specification, acceptance, service evaluation, quality control in magnet production, research and development, and design.
4.2 When a test specimen is cut or fabricated from a larger magnet, the magnetic properties measured on it are not necessarily exactly those of the original sample, even if the material is in the same condition. In such instances, the test results must be viewed in context of part performance history.
4.3 Tests performed in general conformity to this test method and even on the same specimen, but using different test systems, may not yield identical results. The main source of discrepancies are variations between the different test systems in the geometry of the region surrounding the sample, such as, size and shape of the electromagnet pole caps (see Annex A1 and Appendix X1), air gaps at the specimen end faces, and especially the size and location of the measuring devices for H and B or for their corresponding flux values (Hall-effect probes, inductive sensing coils). Also important is the method of B calibration, for example, a volt-second calibration of the fluxmeter alone versus an overall system calibration using a physical reference sample. The method of B and H sensing should be indicated in test reports (see Section 9).
SCOPE
1.1 This test method covers how to determine the magnetic characteristics of magnetically hard materials (permanent magnets), particularly their initial magnetization, demagnetization, and recoil curves, and such quantities as the residual induction, coercive field strength, knee field, energy product, and recoil permeability. This test method is suitable for all materials processed into bulk magnets by any common fabrication technique (casting, sintering, rolling, molding, and so forth), but not for thin films or for magnets that are very small or of unusual shape. Uniformity of composition, structure, and properties throughout the magnet volume is necessary to obtain repeatable results. Particular attention is paid to the problems posed by modern materials combining very high coercivity with high saturation induction, such as the rare-earth magnets, for which older test methods (see Test Method A341/A341M) are unsuitable. An applicable international standard is IEC Publication 60404-5.
1.2 The magnetic system (circuit) in a device or machine generally comprises flux-conducting and nonmagnetic structural members with air gaps in addition to the permanent magnet. The system behavior depends on properties and geometry of all these components and on the operating temperature. This test method describes only how to measure the properties of the permanent magnet material. The basic test method incorporates the magnetic specimen in a magnetic circuit with a closed flux path. Test methods using ring samples or frames composed entirely of the magnetic material to be characterized, as commonly used for magnetically soft materials, are not applicable to permanent magnets.
1.3 This test method shall be used in conjunction with Practice A34/A34M.
1.4 The values and equations stated in customary (cgs-emu or inch-pound) or SI units are to be regarded separately as standard. Within this test method, SI units are shown in brackets except for the sections concerning calculations where there are separate sections for the respective unit systems. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with this test method.
1.5 The names and symbols of magnetic quantities used in this test method, summarized in Table 1, are those generally accepted by the industry.
1.6 This test method is useful for magnet materials having Hci values between about 100 Oe and 35 kOe [8 kA/m and 2.8 MA/m], and Br values in the approximate range from 500 G to 20 kG [50 mT to...
General Information
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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: A977/A977M − 07 (Reapproved 2020)
Standard Test Method for
Magnetic Properties of High-Coercivity Permanent Magnet
Materials Using Hysteresigraphs
This standard is issued under the fixed designationA977/A977M; 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 brackets except for the sections concerning calculations where
there are separate sections for the respective unit systems. The
1.1 This test method covers how to determine the magnetic
values stated in each system may not be exact equivalents;
characteristics of magnetically hard materials (permanent
therefore,eachsystemshallbeusedindependentlyoftheother.
magnets), particularly their initial magnetization,
Combining values from the two systems may result in noncon-
demagnetization, and recoil curves, and such quantities as the
formance with this test method.
residual induction, coercive field strength, knee field, energy
product, and recoil permeability. This test method is suitable 1.5 The names and symbols of magnetic quantities used in
for all materials processed into bulk magnets by any common this test method, summarized in Table 1, are those generally
fabrication technique (casting, sintering, rolling, molding, and accepted by the industry.
so forth), but not for thin films or for magnets that are very
1.6 This test method is useful for magnet materials having
small or of unusual shape. Uniformity of composition,
H values between about 100 Oe and 35 kOe [8 kA/m and 2.8
ci
structure, and properties throughout the magnet volume is
MA/m], and B values in the approximate range from 500 G to
r
necessary to obtain repeatable results. Particular attention is
20 kG [50 mT to 2 T]. High-coercivity rare-earth magnet test
paid to the problems posed by modern materials combining
specimens may require much higher magnetizing fields than
very high coercivity with high saturation induction, such as the
iron-core electromagnets can produce. Such samples must be
rare-earth magnets, for which older test methods (see Test
premagnetized externally and transferred into the measuring
Method A341/A341M) are unsuitable. An applicable interna-
yoke. Typical values of the magnetizing fields, H , required
mag
tional standard is IEC Publication 60404-5.
for saturating magnet materials are shown in Table A2.1.
1.2 The magnetic system (circuit) in a device or machine
1.7 This standard does not purport to address all of the
generally comprises flux-conducting and nonmagnetic struc-
safety concerns, if any, associated with its use. It is the
tural members with air gaps in addition to the permanent
responsibility of the user of this standard to establish appro-
magnet. The system behavior depends on properties and
priate safety, health, and environmental practices and deter-
geometry of all these components and on the operating
mine the applicability of regulatory limitations prior to use.
temperature. This test method describes only how to measure
1.8 This international standard was developed in accor-
the properties of the permanent magnet material.The basic test
dance with internationally recognized principles on standard-
method incorporates the magnetic specimen in a magnetic
ization established in the Decision on Principles for the
circuitwithaclosedfluxpath.Testmethodsusingringsamples
Development of International Standards, Guides and Recom-
or frames composed entirely of the magnetic material to be
mendations issued by the World Trade Organization Technical
characterized, as commonly used for magnetically soft
Barriers to Trade (TBT) Committee.
materials, are not applicable to permanent magnets.
2. Referenced Documents
1.3 This test method shall be used in conjunction with
Practice A34/A34M.
2.1 ASTM Standards:
A34/A34M Practice for Sampling and Procurement Testing
1.4 The values and equations stated in customary (cgs-emu
of Magnetic Materials
or inch-pound) or SI units are to be regarded separately as
A340 Terminology of Symbols and Definitions Relating to
standard. Within this test method, SI units are shown in
Magnetic Testing
This test method is under the jurisdiction of ASTM Committee A06 on
MagneticPropertiesandisthedirectresponsibilityofSubcommitteeA06.01onTest
Methods. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2020.PublishedJuly2020.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1997. Last previous edition approved in 2013 as A977/A977M – 07 (2013). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/A0977_A0977M-07R20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
A977/A977M − 07 (2020)
TABLE 1 Symbols, Quantities, and Units
4.3 Tests performed in general conformity to this test
methodandevenonthesamespecimen,butusingdifferenttest
NOTE 1—IEC nomenclature calls B “remanence,” when B represents
r r
systems, may not yield identical results. The main source of
the B at H = 0 of the outermost hysteresis loop, and it calls B “remanent
r
magnetic induction” for B at H = 0 at smaller loops.
discrepancies are variations between the different test systems
Customary in the geometry of the region surrounding the sample, such as,
Symbol Quantity SI Unit
cgs-emu
size and shape of the electromagnet pole caps (see Annex A1
2 2
A Cross section of search coil [m]cm
t
and Appendix X1), air gaps at the specimen end faces, and
B Magnetic induction at BH [T] G
d max
especially the size and location of the measuring devices for H
B Magnetic induction at low point of [T] G
rec
recoil loop and B or for their corresponding flux values (Hall-effect
B Magnetic induction at remanence [T] G
r
probes, inductive sensing coils). Also important is the method
d Diameter of pole piece [m] cm
l
of B calibration, for example, a volt-second calibration of the
d Diameter of homogeneous field [m] cm
H Magnetic field strength at BH [A/m] Oe
fluxmeter alone versus an overall system calibration using a
d max
H Magnetic field strength at low [A/m] Oe
p
physical reference sample. The method of B and H sensing
point of
should be indicated in test reports (see Section 9).
recoil loop
l Distance between pole faces [m] cm
l Length of test sample [m] cm
r
5. Measuring Methods and Apparatus
N Number of turns of test coil
e Voltage induced in test coil V V
5.1 Measuring Flux and Induction (Flux Density):
d Total air gap between test [m] cm
5.1.1 In the preferred B-measuring method, the total flux is
sample and
pole faces measured with a sensing coil (search coil) that surrounds the
µ A constant with value µ =4π
0 0
test specimen and is wound as closely as possible to the
-7
specimen surface. Its winding length should be no more than a
H/m
µ Recoil permability
third of the specimen length, preferably less than one fifth, and
rec
must be centered on the specimen. The leads shall be twisted
tightly.Asthefluxchangesinresponsetosweepingtheapplied
field, H, the total flux is measured by taking the time integral
A341/A341M Test Method for Direct Current Magnetic of the voltage induced in this coil. This measurement is taken
Properties of Soft Magnetic Materials Using D-C Per- with a fluxmeter. Modern hysteresigraphs use electronic inte-
meameters and the Point by Point (Ballistic)Test Methods grating fluxmeters that allow convenient continuous integra-
E177 Practice for Use of the Terms Precision and Bias in tion and direct graphic recording of magnetization curves. If
ASTM Test Methods the signal is large enough, high-speed voltage sampling at the
coil and digital integration is also possible.
2.2 Magnetic Materials Procedure Association Standard:
5.1.2 The magnetic induction B is determined by dividing
MMPANo. 0100–00 Standard Specifications for Permanent
the total flux by the area-turns product NA of the B-sensing
Magnet Materials
coil. For permanent magnets in general, and especially for
2.3 International Electrotechnical Commission Document:
high-coercivitymaterials,anair-fluxcorrectionisrequired(see
Publication 60404-5 Magnetic Materials– Part 5: Permanent
5.1.5).
Magnet (Magnetically Hard) Materials – Methods of
5.1.3 Thetotalerrorofmeasuring Bshallbenotgreaterthan
Measurement of Magnetic Properties
62%.
5.1.4 The change of magnetic induction, ∆ B = B – B,in
2 1
3. Terminology
thetimeintervalbetweenthetimes t and t isgivenasfollows:
1 2
3.1 Basic magnetic units are defined in Terminology A340
t
and MMPA No. 0100–00. Additional definitions with symbols ∆ B 5 10 /AN edt customary units (1)
~ ! * ~ !
t
and units are given in Table 1 and Figs. 1-3 of this test method.
t
∆ B 5 1/AN edt SI units (2)
~ ! * ~ !
t
4. Significance and Use
where:
4.1 This test method is suitable for magnet specification,
B = magnetic induction, G [T];
acceptance, service evaluation, quality control in magnet
2 2
A = cross-sectional area of the test specimen, cm [m ];
production, research and development, and design.
N = number of turns on the B-sensing coil;
e = voltage induced in the coil, V;
4.2 When a test specimen is cut or fabricated from a larger
t = time, s; and
magnet, the magnetic properties measured on it are not
t
*
edt = voltage integral = flux, V-s [Weber].
necessarily exactly those of the original sample, even if the t
material is in the same condition. In such instances, the test
5.1.5 The change in the magnetic induction shall be cor-
results must be viewed in context of part performance history.
rectedtotakeintoaccounttheairfluxoutsidethetestspecimen
that is linked by the sensing coil. The corrected change, B ,
corr
is given as follows:
Available from Magnetic Materials ProducersAssociation, 8 S. MichiganAve.,
t
Suite 1000, Chicago, IL 60603.
∆ B 5 ~10 /AN! edt 2 ∆ H ~A 2 A!/A ~customary units!
*
corr t
4 t
Available from International Electrotechnical Commission (IEC), 3 rue de
Varembé, P.O. Box 131, CH-1211, Geneva 20, Switzerland. (3)
A977/A977M − 07 (2020)
FIG. 1 Normal and Intrinsic Hysteresis Loops and Initial Magnetization Curves for Permanent Magnet Materials Illustrating Two Ex-
tremes of Virgin Sample Behavior
t
5.2 Determining Intrinsic Induction :
∆B 5 ~1/AN! edt 2 µ ∆H ~A 2 A!/A ~SI units! (4)
*
corr 0 t
t
5.2.1 For high-coercivity magnets, it is more convenient to
where:
sense directly an electrical signal proportional to the intrinsic
A = average cross-sectional area of the sensing coil, induction, derive the average B by dividing this flux by the
i
2 2
cm [m ];
area-turns product of the surrounding B coil, and to plot B
i
∆ H = change in field from t until t , Oe [A/m]; and
1 2 versus H. B then is obtained by mathematical or electronic
-7
µ = magnetic constant [4π 10 H/m].
addition of H to B.
A977/A977M − 07 (2020)
t
∆ B 5 ~10 / AN! * edt ~customary units! (5)
i
t
t
∆B 5 ~1/AN! * edt ~SI units! (6)
i
t
where:
B = intrinsic induction, G [T];
i
2 2
A = cross section of the test specimen, cm [m ]; and
N = number of turns on Coil 1 containing the test specimen.
5.2.3 The two-sensing-coil device shall lie totally within the
homogeneous field defined by Eq A1.1 and Eq A1.2. Test
specimens of lower-coercivity magnets having a range of
cross-sectional areas and shapes can then be measured with the
samecoildevice.Anarrangementofside-by-sidecoilsofequal
size is useful. Serious errors, however, are incurred when
measuring B this way on high-B or high/coercivity magnets,
i r
or both, at applied fields of about 10 kOe or more. The errors
are most severe for test specimens of short pole-to-pole length.
Local pole-piece saturation causes strong field inhomogene-
ities. The specimen then must fill the cross section of Coil 1,
FIG. 2 Normal and Intrinsic Demagnetization Curves with Sym-
bols for Special Points of Interest and Definition of Salient Prop-
andCoil2mustbeathinandflatcoil,oracoaxialannularcoil,
erties. Illustration of Maximum Energy Product, Coercive Fields,
either centered on the specimen or in close proximity to its
and Definition of Knee Field
surface (see 5.3).
5.2.4 The total error of measuring B shall be not greater
i
than 62%.
5.3 Measuring the Magnetic Field Strength:
5.3.1 For correct magnetization curves, one should know
the magnetic field strength, H, inside the test specimen,
averaged over the specimen volume if H is not uniform. But
this inner field cannot be measured. At the surface of the test
specimen, H is equal to the local field strength just inside the
specimen in those locations (and only there) where the H
vectorisparalleltothesidesurfaceofthespecimen.Therefore,
a magnetic field strength sensor of small dimensions relative to
the specimen is placed near the specimen surface and sym-
metrical with respect to the end faces, covering the shortest
possible center portion of the specimen length. It shall be so
oriented that it correctly measures the tangential field compo-
nent.
5.3.2 To determine the magnetic field strength, a flat surface
coil, a tightly fitted annular coil, a magnetic potentiometer, or
FIG. 3 Normal and Intrinsic Demagnetization Curves with Sym-
a Hall probe is used together with suitable instruments. The
bols for Special Points of Interest and Definition of Salient Prop-
dimensions of the magnetic field sensor and its location shall
erties. Illustration of Recoil Loop. Recoil Permeability is Defined
be such that it is within an area of limited diameter around the
as µ = ∆B/∆H
rec
test specimen (see Annex A1).
5.3.3 The provisions of 5.3.2 are adequate for measure-
ments on magnets having low-to-moderate intrinsic coercivity,
such as Alnico and bonded ferrites. For high-coercivity, dense
5.2.2 The change of intrinsic induction in the test specimen
ferrites and especially for most rare earth-transition metal
can be determined by integrating the voltage induced in a
materials, it is essential for accurate measurement to use thin
device comprising two sensing coils, both subject to the same
flat or radially thin annular H-sensing coils of short length
applied field H, where the test specimen is contained in only
(<1/5 to 1/3 of the specimen length), centered on the specimen
one of the coils (Coil 1). If each individual coil has the same
and placed as close as possible to the specimen surfac
...
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