ASTM C1684-18(2023)
(Test Method)Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature—Cylindrical Rod Strength
Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature—Cylindrical Rod Strength
SIGNIFICANCE AND USE
4.1 This test method may be used for material development, quality control, characterization, and design data generation purposes. This test method is intended to be used with ceramics whose strength is 50 MPa (~7 ksi) or greater. The test method may also be used with glass test specimens, although Test Methods C158 is specifically designed to be used for glasses. This test method may be used with machined, drawn, extruded, and as-fired round specimens. This test method may be used with specimens that have elliptical cross section geometries.
4.2 The flexure strength is computed based on simple beam theory with assumptions that the material is isotropic and homogeneous, the moduli of elasticity in tension and compression are identical, and the material is linearly elastic. The average grain size should be no greater than one-fiftieth of the rod diameter. The homogeneity and isotropy assumptions in the standard rule out the use of this test for continuous fiber-reinforced ceramics.
4.3 Flexural strength of a group of test specimens is influenced by several parameters associated with the test procedure. Such factors include the loading rate, test environment, specimen size, specimen preparation, and test fixtures (1-3).3 This method includes specific specimen-fixture size combinations, but permits alternative configurations within specified limits. These combinations were chosen to be practical, to minimize experimental error, and permit easy comparison of cylindrical rod strengths with data for other configurations. Equations for the Weibull effective volume and Weibull effective surface are included.
4.4 The flexural strength of a ceramic material is dependent on both its inherent resistance to fracture and the size and severity of flaws in the material. Flaws in rods may be intrinsically volume-distributed throughout the bulk. Some of these flaws by chance may be located at or near the outer surface. Flaws may alternatively be intrinsically surface-distrib...
SCOPE
1.1 This test method is for the determination of flexural strength of rod-shaped specimens of advanced ceramic materials at ambient temperature. In many instances it is preferable to test round specimens rather than rectangular bend specimens, especially if the material is fabricated in rod form. This method permits testing of machined, drawn, or as-fired rod-shaped specimens. It allows some latitude in the rod sizes and cross section shape uniformity. Rod diameters between 1.5 and 8 mm and lengths from 25 to 85 mm are recommended, but other sizes are permitted. Four-point-1/4-point as shown in Fig. 1 is the preferred testing configuration. Three-point loading is permitted. This method describes the apparatus, specimen requirements, test procedure, calculations, and reporting requirements. The method is applicable to monolithic or particulate- or whisker-reinforced ceramics. It may also be used for glasses. It is not applicable to continuous fiber-reinforced ceramic composites.
FIG. 1 Four-Point-1/4-Point Flexure Loading Configuration
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 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.
General Information
Relations
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: C1684 − 18 (Reapproved 2023)
Standard Test Method for
Flexural Strength of Advanced Ceramics at Ambient
Temperature—Cylindrical Rod Strength
This standard is issued under the fixed designation C1684; 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 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method is for the determination of flexural
C158 Test Methods for Strength of Glass by Flexure (De-
strength of rod-shaped specimens of advanced ceramic mate-
termination of Modulus of Rupture)
rials at ambient temperature. In many instances it is preferable
C1145 Terminology of Advanced Ceramics
to test round specimens rather than rectangular bend
C1161 Test Method for Flexural Strength of Advanced
specimens, especially if the material is fabricated in rod form.
Ceramics at Ambient Temperature
This method permits testing of machined, drawn, or as-fired
C1239 Practice for Reporting Uniaxial Strength Data and
rod-shaped specimens. It allows some latitude in the rod sizes
Estimating Weibull Distribution Parameters for Advanced
and cross section shape uniformity. Rod diameters between 1.5
Ceramics
and 8 mm and lengths from 25 to 85 mm are recommended, but
other sizes are permitted. Four-point- ⁄4-point as shown in Fig. C1322 Practice for Fractography and Characterization of
Fracture Origins in Advanced Ceramics
1 is the preferred testing configuration. Three-point loading is
permitted. This method describes the apparatus, specimen C1368 Test Method for Determination of Slow Crack
Growth Parameters of Advanced Ceramics by Constant
requirements, test procedure, calculations, and reporting re-
quirements. The method is applicable to monolithic or Stress Rate Strength Testing at Ambient Temperature
E4 Practices for Force Calibration and Verification of Test-
particulate- or whisker-reinforced ceramics. It may also be
used for glasses. It is not applicable to continuous fiber- ing Machines
E337 Test Method for Measuring Humidity with a Psy-
reinforced ceramic composites.
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
1.2 The values stated in SI units are to be regarded as the
peratures)
standard. The values given in parentheses are for information
only.
3. Terminology
1.3 This standard does not purport to address all of the
3.1 Definitions:
safety concerns, if any, associated with its use. It is the
3.1.1 complete gage section, n—the portion of the specimen
responsibility of the user of this standard to establish appro-
between the two outer loading points in four-point flexure and
priate safety, health, and environmental practices and deter-
three-point flexure fixtures. C1161
mine the applicability of regulatory limitations prior to use.
3.1.2 flaw, n—a structural discontinuity in an advanced
1.4 This international standard was developed in accor-
ceramic body that acts as a highly localized stress raiser.
dance with internationally recognized principles on standard-
3.1.2.1 Discussion—The presence of such discontinuities
ization established in the Decision on Principles for the
does not necessarily imply that the ceramic has been prepared
Development of International Standards, Guides and Recom-
C1322
improperly or is faulty.
mendations issued by the World Trade Organization Technical
–2
3.1.3 flexural strength, [FL ], n—a measure of the ultimate
Barriers to Trade (TBT) Committee.
strength of a specified beam in bending. C1145, C1161
3.1.4 four-point- ⁄4-point flexure, n—configuration of flex-
ural strength testing where a specimen is symmetrically loaded
This test method is under the jurisdiction of ASTM Committee C28 on
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
Mechanical Properties and Performance. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Jan. 1, 2023. Published February 2023. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2008. Last previous edition approved in 2018 as C1684 – 18. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1684-18R23. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1684 − 18 (2023)
FIG. 1 Four-Point- ⁄4-Point Flexure Loading Configuration
at two locations that are situated one-quarter of the overall span 3.1.6.1 Discussion—An inert condition may be obtained by
away from the outer two support loading points (see Fig. 1). using vacuum, low temperatures, very fast test rates, or any
C1145, C1161
inert media. C1161
–2
3.1.5 fracture origin, n—the source from which brittle
3.1.7 inherent flexural strength, [FL ], n—the flexural
fracture commences. C1145, C1322
strength of a material in the absence of any effect of surface
–2
3.1.6 inert flexural strength, [FL ], n—a measure of the grinding or other surface finishing process, or of extraneous
strength of specified beam in bending as determined in an damage that may be present. The measured inherent strength is
appropriate inert condition whereby no slow crack growth in general a function of the flexure test method, test conditions,
occurs. and test specimen size. C1161
FIG. 2 Three-Point Flexure Loading Configuration
C1684 − 18 (2023)
3.1.8 inner gage section, n—the portion of the specimen mended for all purposes, especially if the data will be used for
between the inner two loading points in a four-point flexure design as discussed in Refs (3-5) and Practices C1322 and
fixture. C1161 C1239.
3.1.9 slow crack growth (SCG), n—subcritical crack growth 4.5 The three-point test configuration exposes only a very
small portion of the specimen to the maximum stress.
(extension) which may result from, but is not restricted to, such
mechanisms as environmentally assisted stress corrosion or Therefore, three-point flexural strengths are likely to be greater
diffusive crack growth. C1145, C1161 than four-point flexural strengths. Three-point flexure has some
advantages. It uses simpler test fixtures, it is easier to adapt to
3.1.10 three-point flexure, n—configuration of flexural
high temperature and fracture toughness testing, and it is
strength testing where a specimen is loaded at a location
sometimes helpful in Weibull statistical studies. It also uses
midway between two support loading points (see Fig. 2).
smaller force to break a specimen. It is also convenient for very
C1145, C1161
short, stubby specimens which would be difficult to test in
four-point loading. Nevertheless, four-point flexure is preferred
4. Significance and Use
and recommended for most characterization purposes.
4.1 This test method may be used for material development,
5. Interferences
quality control, characterization, and design data generation
purposes. This test method is intended to be used with ceramics 5.1 The effects of time-dependent phenomena, such as stress
whose strength is 50 MPa (~7 ksi) or greater. The test method corrosion or slow crack growth on strength tests conducted at
may also be used with glass test specimens, although Test ambient temperature, can be meaningful even for the relatively
Methods C158 is specifically designed to be used for glasses. short times involved during testing. Such influences must be
This test method may be used with machined, drawn, extruded, considered if flexure tests are to be used to generate design
and as-fired round specimens. This test method may be used data. Slow crack growth can lead to a rate dependency of
with specimens that have elliptical cross section geometries. flexural strength. The testing rate specified in this standard may
or may not produce the inert flexural strength whereby negli-
4.2 The flexure strength is computed based on simple beam
gible slow crack growth occurs. See Test Method C1368.
theory with assumptions that the material is isotropic and
5.2 Surface preparation of test specimens can introduce
homogeneous, the moduli of elasticity in tension and compres-
machining microcracks which may have a pronounced effect
sion are identical, and the material is linearly elastic. The
on flexural strength (6). Machining damage imposed during
average grain size should be no greater than one-fiftieth of the
specimen preparation can be either a random interfering factor,
rod diameter. The homogeneity and isotropy assumptions in
or an inherent part of the strength characteristic to be mea-
the standard rule out the use of this test for continuous
sured. With proper care and good machining practice, it is
fiber-reinforced ceramics.
possible to obtain fractures from the material’s natural flaws.
4.3 Flexural strength of a group of test specimens is
Surface preparation can also lead to residual stresses. It should
influenced by several parameters associated with the test
be understood that final machining steps may or may not
procedure. Such factors include the loading rate, test
negate machining damage introduced during the early coarse or
environment, specimen size, specimen preparation, and test
intermediate machining.
fixtures (1-3). This method includes specific specimen-fixture
5.3 This test method allows several options for the prepa-
size combinations, but permits alternative configurations
ration of specimens. The method allows testing of as-fabricated
within specified limits. These combinations were chosen to be
(for example, as-fired or as-drawn), application-matched
practical, to minimize experimental error, and permit easy
machining, customary, or one of three specific grinding proce-
comparison of cylindrical rod strengths with data for other
dures. The latter “standard procedures” (see 7.2.4) are satis-
configurations. Equations for the Weibull effective volume and
factory for many (but certainly not all) ceramics. Centerless or
Weibull effective surface are included.
transverse grinding aligns the severest machining microcracks
4.4 The flexural strength of a ceramic material is dependent
perpendicular to the rod tension stress axis. The specimen may
on both its inherent resistance to fracture and the size and
fracture from the machining microcracks. Transverse-ground
severity of flaws in the material. Flaws in rods may be
specimens in many instances may provide a more “practical
intrinsically volume-distributed throughout the bulk. Some of
strength” that is relevant to machined ceramic components
these flaws by chance may be located at or near the outer
whereby it may not be possible to favorably align the machin-
surface. Flaws may alternatively be intrinsically surface-
ing direction. Therefore, this test method allows transverse
distributed with all flaws located on the outer specimen
grinding for normal specimen preparation purposes. Longitu-
surface. Grinding cracks fit the latter category. Variations in the
dinal grinding, which is commonly used to orient grinding
flaws cause a natural scatter in strengths for a set of test
damage cracks in rectangular bend bars, is less commonly used
specimens. Fractographic analysis of fracture surfaces, al-
for rod specimens, but is also permitted by this test method.
though beyond the scope of this standard, is highly recom-
6. Apparatus
6.1 Loading—Specimens may be loaded in any suitable
testing machine provided that uniform rates of direct loading
The boldface numbers in parentheses refer to the list of references at the end of
this standard. can be maintained. The force measuring system shall be free of
C1684 − 18 (2023)
initial lag at the loading rates used and shall be equipped with configuration. The lower the material’s fracture toughness and
a means for retaining read-out of the maximum force applied to the higher the elastic modulus, the more likely that contact
the specimen. The accuracy of the testing machine shall be in cracks will cause premature fracture. The larger the test
accordance with Practices E4. specimen diameter for a given test span, the more likely that
contact fracture will occur since larger forces are applied to
6.2 Four-Point Flexure—Four-point- ⁄4-point fixtures are
break them. In other words, short, stubby rod specimens are
the preferred configuration. When possible, use one of the
more likely to have problems than long, slender rods. This test
outer support and inner loading span combinations listed in
method allows considerable latitude in the selection of speci-
Table 1. Other span sizes may be used if these sizes are not
men sizes and testing geometries. If specimens break prema-
suitable for a specific round part. The ratio of the fixture outer
turely from contact cracks, the user shall either reduce the test
span length to the specimen diameter shall not be less than 3.0.
specimen diameter, use longer rod specimens with longer span
6.3 Three-Point Flexure—Three-point flexure may be used
test fixtures, use fixtures with cradles (see 6.5), or shift to
if four-point is not satisfactory, such as if the specimens are
three-point loading.
very short and stubby and consequently require very large
6.4.4 The rollers shall be free to rotate or roll to minimize
breaking forces in four-point loading. When possible, use one
frictional constraint as the specimen stretches or contracts
of the support spans listed in Table 1 for three-point loading.
during loading. The sole exception is the middle load roller in
Other span sizes may be used if these sizes are not suitable for
three-point flexure which need not rotate. Note that the outer
a specific round part. The outer fixture span length to specimen
support rollers roll outward and the inner loading rollers roll
diameter ratio shall not be less than 3.0.
inward. The rollers may roll on a fixture base as shown in Fig.
6.4 Loading Rollers—Force shall be applied to the test 3 or alternatively, they may be mounted in roller assemblies
pieces directly by rollers as described in this section (6.4) or
that allow them to rotate. Cradle inserts such as shown in Fig.
alternatively by rollers with cradles as described in 6.5. 4 may be used in conjunction with loading rollers if necessary
6.4.1 This test method permits direct contact of rod speci-
to eliminate fractures at the loading points induced by severe
mens with loading and support rollers. Direct contact may contact loading stresses associated with a round specim
...
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