ASTM D7972-14(2020)

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: D7972 −14 (Reapproved 2020)
Standard Test Method for
Flexural Strength of Manufactured Carbon and Graphite
Articles Using Three-Point Loading at Room Temperature
This standard is issued under the fixed designation D7972; 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 Fracture Origins in Advanced Ceramics
D7775 Guide for Measurements on Small Graphite Speci-
1.1 This test method covers determination of the flexural
mens
strength of manufactured carbon and graphite articles using a
E4 Practices for Force Verification of Testing Machines
square, rectangular or cylindrical beam in three-point loading
E691 Practice for Conducting an Interlaboratory Study to
at room temperature.
Determine the Precision of a Test Method
1.2 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
3. Terminology
standard.
3.1 Definitions:
1.3 This standard does not purport to address all of the
3.1.1 flexural strength, n—a measure of the ultimate load
safety concerns, if any, associated with its use. It is the
carrying capacity of a specified beam in bending.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 3.1.2 grain, n—in manufactured (synthetic) carbon and
graphite, particle of filler material (usually coke or graphite) in
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor- the starting mix formulation. Also referred to as granular
material, filler particle, or aggregate material.
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- 4. Significance and Use
mendations issued by the World Trade Organization Technical
4.1 This test method provides a framework for material
Barriers to Trade (TBT) Committee.
development, quality control, characterization, and design data
generation purposes. The user needs to assess the applicability
2. Referenced Documents
of the method on the specific material and for the intended use,
2.1 ASTM Standards:
as shown by the interlaboratory study.
C78 Test Method for Flexural Strength of Concrete (Using
4.2 This test method determines the maximum loading on a
Simple Beam with Third-Point Loading)
graphite specimen with simple beam geometry in three–point
C559 Test Method for Bulk Density by Physical Measure-
bending, and it provides a means for the calculation of flexural
ments of Manufactured Carbon and Graphite Articles
strength at ambient temperature and environmental conditions.
C1161 Test Method for Flexural Strength of Advanced
Ceramics at Ambient Temperature
4.3 The flexure stress is computed based on simple beam
C1239 Practice for Reporting Uniaxial Strength Data and
theory with assumptions that the material is isotropic and
Estimating Weibull Distribution Parameters forAdvanced
homogeneous, the moduli of elasticity in tension and compres-
Ceramics
sion are identical, and the material is linearly elastic. For
C1322 Practice for Fractography and Characterization of
materials with large grains, the minimum specimen dimension
shouldbesignificantlylargerthanthemaximumgrainsize(see
Guide D7775).
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
4.4 Flexural strength of a group of test specimens is
Subcommittee D02.F0 on Manufactured Carbon and Graphite Products.
influenced by several parameters associated with the test
Current edition approved May 1, 2020. Published June 2020. Originally
procedure. Such factors include the loading rate, test
approved in 2014. Last previous edition approved in 2014 as D7972 – 14. DOI:
10.1520/D7972-14R20.
environment, specimen size, specimen preparation, and test
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
fixtures. Specimen sizes and fixtures should be chosen to
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
reduce errors due to material variability or testing parameters,
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. such as friction and non-parallelism of specimen surfaces.
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959. United States
D7972 − 14 (2020)
TABLE 1 Specimen Sizes and Testing Configurations in the
4.5 The flexural strength of a manufactured graphite or
Interlaboratory Study
carbon material is dependent on both its inherent resistance to
Nominal Crosshead
fracture and the size and severity of flaws. Variations in these
Specimen Support
Specimen Speed,
Configuration Thickness, d Span, L
cause a natural scatter in test results for a sample of test
Size mm/s
(mm) (mm)
specimens. Fractographic analysis of fracture surfaces, al-
(mm) (mm/m)
though beyond the scope of this standard, is highly recom- I 10×10× 10 50.00 0.0042 (0.25)
mended for all purposes, especially if the data will be used for
II 9.5 × 4.8 × 4.8 50.00 0.0087 (0.52)
design as discussed in Practices C1239 and C1322.
III Ø10 × 64 10 50.00 0.0042 (0.25)
4.6 The three-point test configuration exposes only a very
IV 25×25× 25 100.00 0.0067 (0.40)
small portion of the specimen to the maximum stress.
V Ø25 × 150 25 100.00 0.0067 (0.40)
Therefore, three-point flexural strengths are likely to be much
greater than four-point flexural strengths. Three-point flexure
has some advantages. It uses simpler test fixtures, allowing
small specimen testing and fracture toughness measurements.
5.3.2 Fully-articulated Three-point Fixture—Specimens
However, four-point flexure is preferred and recommended for
that do not meet the parallelism requirements of 6.1 shall be
most characterization purposes.
tested in a fully-articulated fixture. Well-machined specimens
may also be tested in a fully-articulating fixture. The two
5. Apparatus
support (outer) bearings shall be free to roll outwards. The
middle bearing shall not roll.Any two of the bearings shall be
5.1 Loading—Specimens may be loaded in any suitable
capable of articulating to match the specimen surface.All three
testing machine provided that uniform rates of loading can be
bearings shall rest uniformly and evenly across the specimen
maintained. The testing machine shall be equipped with a
surface.The fixture shall be designed to apply equal load to the
means for retaining read-out of the maximum force applied to
two outer bearings.
the specimen. The accuracy of the testing machine shall be in
accordance with Practice E4.
6. Test Specimen
5.2 Fixture—The three-point loading fixture shall consist of
6.1 Specimen Size—The size and geometry of the test
bearing blocks or cylindrical bearings spaced in a three-point
specimensusedinthisinterlaboratorystudyareshowninTable
loading configuration (see Test Method C1161). A hardened
1. It is recommended that the size of the test specimen is
steel bearing block or its equivalent is necessary to prevent
selected such that the minimum dimension of the specimen is
distortion of the loading member.
greater than 5 times the largest particle dimension. It is
5.2.1 Thefixtureshallensurethatforcesappliedtothebeam
recommended that the test specimen has a length to thickness/
are normal only and without eccentricity through the use of
diameter ratio of at least 6, and a width to thickness ratio not
spherical bearing blocks (see Test Method C78) or articulating
greater than 2.
roller bearing assemblies (see 5.3 and Test Method C1161).
6.1.1 For test specimens that do not meet this ratio for
5.2.2 The cylindrical bearing length shall be such that the 3
strength testing, see Ref (1) and Guide D7775.
test specimen width is fully supported, and the cylindrical
6.2 Preparation—The test specimen shall be prepared to
bearing diameter shall be 0.75 to 1.5 times the specimen
yield a parallelepiped of square or rectangular cross section or
thickness/diameter.
a cylinder. The faces of the parallelepiped specimens shall be
5.2.3 The lower support bearings shall be free to rotate in
parallelandflatwithin0.025 mm⁄mm.Inaddition,thesamples
order to relieve frictional constraints. The middle load bearing
having a maximum particle size less than 0.15 mm in diameter
of the three-point fixture need not rotate. The three bearings
must be finished so that the surface roughness is less than 3 µm
shall be parallel over their length. The load application bearing
Ra. Sample edges should be free from visible flaws and chips.
(upper bearing) shall be centered with respect to the two lower
NOTE 1—For ease of machining to conventional standards, 3 µm Ra is
support bearings within 60.10 mm.
equivalent to 125 µin. AA. For finishing of specimens with maximum
particle sizes of greater than 0.150 mm, grain structure and porosity can
5.3 Thedirectionsofloadsandreactionsmaybemaintained
limit the accurate measurement of roughness. In these cases, the surface
parallel by judicious use of linkages, rocker bearings, and
roughness should be visually equivalent to 3 µm Ra as estimated based on
flexure plates. Eccentricity of loading can be avoided by the
the visible surface of the graphite.
use of spherical bearing blocks or articulating roller bearings.
NOTE 2—Surface preparation of test specimens can introduce machin-
ing microcracks which may have a pronounced effect on flexural strength.
5.3.1 Semi-articulated Three-point Fixture—Specimenspre-
Machining damage imposed during specimen preparation can be either a
pared in accordance with the parallelism requirements of 6.1
random interfering factor, or an inherent part of the strength characteristic
may be tested in a semi-articulated fixture. The middle bearing
to be measured. With proper care and good machining practice, it is
shall be fixed and not free to roll. The two outer bearings shall
possible to obtain fractures during strength testing from the material’s
be parallel to each other over their length. The two outer natural flaws. Surface preparation can also lead to residual stresses.
Universal or standardized test methods of surface preparation do not exist.
bearings shall articulate together as a pair to match the
It should be understood that final machining steps may or may not negate
specimen surface, or the middle bearing shall articulate to
match the specimen surface. All three bearings shall rest
uniformly and evenly across the specimen surface. The fixture
The boldface numbers in parentheses refer to the list of references at the end of
shall be designed to apply equal load to the two outer bearings. this standard.
D7972 − 14 (2020)
FIG. 1 The Three-Point Fixture Configuration
machining damage introduced during the early course or intermediate
9. Calculation
machining.
9.1 If the fracture occurs directly underneath the load
6.3 Measurements—All dimensions shall be measured to an
bearing, calculate the flexural strength as follows:
accuracy of 0.5 % (see Test Method C559).
9.1.1 For square cross-section specimens:
6.4 Orientation—The specimen shall be marked or other- 3
σ 5 3PL ⁄ 2d (1)
~ ! ~ !
wise identified to denote its orientation with respect to the
where:
parent stock.
P = break force,
6.5 Drying—Each specimen must be dried in a vented oven
L = support span, and
at 110 °C for a period of 2 h (see Test Method C559). The
d =
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