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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.

TABLE 1 Symbols, Quantities, and Units
Note 1-IEC nomenclature calls Br “remanence,” when Br represents the B at H = 0 of the outermost hysteresis loop, and it calls Br “remanent magnetic induction” for B at H = 0 at smaller loops.

Symbol

Quantity

SI Unit

Customary
cgs-emu

At

Cross section of search coil

[m²]

cm²

Bd

Magnetic induction at BHmax

[T]

G

Brec

Magnetic induction at low point of
 recoil loop

[T]

G

Br

Magnetic induction at remanence

[T]

G

dl

Diameter of pole piece

[m]

cm

d2

Diameter of homogeneous field

[m]

cm

Hd

Magnetic field strength at BHmax

[A/m]

Oe

Hp

Magnetic field strength at low point of
 recoil loop

[A/m]

Oe

l

Distance between pole faces

[m]

cm

lr

Length of test sample

[m]

cm

N

Number of turns of test coil

 

 

e

Voltage induced in test coil

V

V

d

Total air gap between test sample and
 pole faces

[m]

cm

μ0

A constant with value μ0 = 4π 10-7
 H/m

 

 

μrec

Recoil permability

 

 


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 2 T]. High-coercivity rare-earth magnet test specimens may require much higher magnetizing fields than iron-core electromagnets can produce. Such samples must be premagnetized externally and transferred into the measuring yoke. Typical values of the magnetizing fields, Hmag, required for saturating magnet materials are shown in Table A2.1.

1.7 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.

 

Document History

  1. ASTM A977/A977M-07(2013)

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    Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs

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  2. ASTM A977/A977M-07


    Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs

    • Historical Version
  3. ASTM A977/A977M-02


    Standard Test Method for Magnetic Properties of High-Coercivity Permanent Magnet Materials Using Hysteresigraphs

    • Historical Version