ASTM C1684-08
(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
SCOPE
1.1 This test method is for the determination of flexural strength of rod shape 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.
1.2 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.
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Designation: C1684 − 08
StandardTest 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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope C1322Practice for Fractography and Characterization of
Fracture Origins in Advanced Ceramics
1.1 This test method is for the determination of flexural
C1368 Test Method for Determination of Slow Crack
strengthofrodshapespecimensofadvancedceramicmaterials
Growth Parameters of Advanced Ceramics by Constant
at ambient temperature. In many instances it is preferable to
Stress-Rate Strength Testing at Ambient Temperature
test round specimens rather than rectangular bend specimens,
E4Practices for Force Verification of Testing Machines
especiallyifthematerialisfabricatedinrodform.Thismethod
E337Test Method for Measuring Humidity with a Psy-
permits testing of machined, drawn, or as-fired rod shaped
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
specimens. It allows some latitude in the rod sizes and cross
peratures)
sectionshapeuniformity.Roddiametersbetween1.5and8mm
and lengths from 25 to 85 mm are recommended, but other
3. Terminology
sizes are permitted. Four-point- ⁄4 point as shown in Fig. 1 is
the preferred testing configuration. Three-point loading is 3.1 Definitions:
permitted. This method describes the apparatus, specimen 3.1.1 complete gage section, n—theportionofthespecimen
requirements, test procedure, calculations, and reporting re- between the two outer loading points in four-point flexure and
quirements. The method is applicable to monolithic or three-point flexure fixtures. C1161
particulate- or whisker-reinforced ceramics. It may also be
3.1.2 flaw, n—a structural discontinuity in an advanced
used for glasses. It is not applicable to continuous fiber-
ceramic body that acts as a highly localized stress raiser.
reinforced ceramic composites.
3.1.2.1 Discussion—The presence of such discontinuities
1.2 This standard does not purport to address all of the does not necessarily imply that the ceramic has been prepared
safety concerns, if any, associated with its use. It is the improperly or is faulty. C1322
responsibility of the user of this standard to establish appro-
3.1.3 flexural strength, n—a measure of the ultimate
priate safety and health practices and determine the applica-
strength of a specified beam in bending. C1145, C1161
bility of regulatory limitations prior to use.
3.1.4 four-point- ⁄4 point flexure, n—configuration of flex-
uralstrengthtestingwhereaspecimenissymmetricallyloaded
2. Referenced Documents
2 attwolocationsthataresituatedonequarteroftheoverallspan
2.1 ASTM Standards:
away from the outer two support loading points (see Fig. 1).
C158Test Methods for Strength of Glass by Flexure (De-
C1145, C1161
termination of Modulus of Rupture)
3.1.5 fracture origin, n—the source from which brittle
C1145Terminology of Advanced Ceramics
fracture commences. C1145, C1322
C1161Test Method for Flexural Strength of Advanced
Ceramics at Ambient Temperature
3.1.6 inert flexural strength, n—ameasureofthestrengthof
C1239Practice for Reporting Uniaxial Strength Data and
specifiedbeaminbendingasdeterminedinanappropriateinert
Estimating Weibull Distribution Parameters forAdvanced
condition whereby no slow crack growth occurs.
Ceramics
3.1.6.1 Discussion—An inert condition may be obtained by
using vacuum, low temperatures, very fast test rates, or any
This test method is under the jurisdiction of ASTM Committee C28 on inert media. C1161
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
3.1.7 inherent flexural strength, n—theflexuralstrengthofa
Mechanical Properties and Performance.
material in the absence of any effect of surface grinding or
Current edition approved Jan. 1, 2008. Published January 2008. DOI: 10.1520/
C1684-08.
other surface finishing process, or of extraneous damage that
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
may be present.The measured inherent strength is in general a
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
function of the flexure test method, test conditions, and
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. specimen size. C1161
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1684 − 08
FIG. 1 Four-Point- ⁄4 Point Flexure Loading Configuration
3.1.8 inner gage section, n—the portion of the specimen may also be used with glass test specimens, although Test
between the inner two loading points in a four-point flexure Methods C158 is specifically designed to be used for glasses.
fixture. C1161 Thistestmethodmaybeusedwithmachined,drawn,extruded,
and as-fired round specimens. This test method may be used
3.1.9 slow crack growth (SCG), n—subcriticalcrackgrowth
with specimens that have elliptical cross section geometries.
(extension)whichmayresultfrom,butisnotrestrictedto,such
mechanisms as environmentally-assisted stress corrosion or
4.2 The flexure strength is computed based on simple beam
diffusive crack growth. C1145, C1161 theory with assumptions that the material is isotropic and
homogeneous, the moduli of elasticity in tension and compres-
3.1.10 three-point flexure, n—configuration of flexural
sion are identical, and the material is linearly elastic. The
strength testing where a specimen is loaded at a location
average grain size should be no greater than one fiftieth of the
midway between two support loading points (see Fig. 2).
rod diameter. The homogeneity and isotropy assumptions in
C1145, C1161
the standard rule out the use of this test for continuous
4. Significance and Use
fiber-reinforced ceramics.
4.1 Thistestmethodmaybeusedformaterialdevelopment, 4.3 Flexural strength of a group of test specimens is
quality control, characterization, and design data generation
influenced by several parameters associated with the test
purposes.Thistestmethodisintendedtobeusedwithceramics procedure. Such factors include the loading rate, test environ-
whose strength is 50 MPa (~7 ksi) or greater. The test method ment, specimen size, specimen preparation, and test fixtures
FIG. 2 Three-Point Flexure Loading Configuration
C1684 − 08
(1-3). This method includes specific specimen-fixture size 5.3 This test method allows several options for the prepa-
combinations, but permits alternative configurations within rationofspecimens.Themethodallowstestingofas-fabricated
specified limits. These combinations were chosen to be prac- (e.g., as-fired or as-drawn), application-matched machining,
tical, to minimize experimental error, and permit easy com- customary, or one of three specific grinding procedures. The
parison of cylindrical rod strengths with data for other con- latter “standard procedures” (see 7.2.4) are satisfactory for
figurations. Equations for the Weibull effective volume and many (but certainly not all) ceramics. Centerless or transverse
Weibull effective surface are included. grinding aligns the severest machining microcracks perpen-
dicular to the rod tension stress axis. The specimen may
4.4 The flexural strength of a ceramic material is dependent
fracture from the machining microcracks. Transverse-ground
on both its inherent resistance to fracture and the size and
specimens in many instances may provide a more “practical
severity of flaws in the material. Flaws in rods may be
strength” that is relevant to machined ceramic components
intrinsically volume-distributed throughout the bulk. Some of
whereby it may not be possible to favorably align the machin-
these flaws by chance may be located at or near the outer
ing direction. Therefore, this test method allows transverse
surface. Flaws may alternatively be intrinsically surface-
grinding for normal specimen preparation purposes. Longitu-
distributed with all flaws located on the outer specimen
dinal grinding, which is commonly used to orient grinding
surface.Grindingcracksfitthelattercategory.Variationsinthe
damagecracksinrectangularbendbars,islesscommonlyused
flaws cause a natural scatter in strengths for a set of test
for rod specimens, but is also permitted by this test method.
specimens. Fractographic analysis of fracture surfaces, al-
though beyond the scope of this standard, is highly recom-
6. Apparatus
mended for all purposes, especially if the data will be used for
6.1 Loading—Specimens may be loaded in any suitable
design as discussed in Refs (3-5) and Practices C1322 and
testing machine provided that uniform rates of direct loading
C1239.
canbemaintained.Theforcemeasuringsystemshallbefreeof
4.5 The three-point test configuration exposes only a very
initial lag at the loading rates used and shall be equipped with
small portion of the specimen to the maximum stress. There-
ameansforretainingread-outofthemaximumforceappliedto
fore, three-point flexural strengths are likely to be greater than
the specimen. The accuracy of the testing machine shall be in
four-point flexural strengths. Three-point flexure has some
accordance with Practices E4.
advantages. It uses simpler test fixtures, it is easier to adapt to
6.2 Four-Point Flexure—Four-point- ⁄4pointfixturesarethe
high temperature and fracture toughness testing, and it is
preferred configuration. When possible, use one of the outer
sometimes helpful in Weibull statistical studies. It also uses
support and inner loading span combinations listed in Table 1.
smallerforcetobreakaspecimen.Itisalsoconvenientforvery
Other span sizes may be used if these sizes are not suitable for
short, stubby specimens which would be difficult to test in
a specific round part. The ratio of the fixture outer span length
four-pointloading.Nevertheless,four-pointflexureispreferred
to the specimen diameter shall not be less than 3.0.
and recommended for most characterization purposes.
6.3 Three-Point Flexure—Three-point flexure may be used
5. Interferences
if four-point is not satisfactory, such as if the specimens are
very short and stubby and consequently require very large
5.1 Theeffectsoftime-dependentphenomena,suchasstress
breaking forces in four-point loading. When possible, use one
corrosion or slow crack growth on strength tests conducted at
of the support spans listed in Table 1 for three-point loading.
ambienttemperature,canbemeaningfulevenfortherelatively
Other span sizes may be used if these sizes are not suitable for
short times involved during testing. Such influences must be
aspecificroundpart.Theouterfixturespanlengthtospecimen
considered if flexure tests are to be used to generate design
diameter ratio shall not be less than 3.0.
data. Slow crack growth can lead to a rate dependency of
flexuralstrength.Thetestingratespecifiedinthisstandardmay
6.4 Loading Rollers—Force shall be applied to the test
or may not produce the inert flexural strength whereby negli-
pieces directly by rollers as described in this section (6.4)or
gible slow crack growth occurs. See Test Method C1368. alternatively by rollers with cradles as described in 6.5.
6.4.1 This test method permits direct contact of rod speci-
5.2 Surface preparation of test specimens can introduce
mens with loading and support rollers. Direct contact may
machining microcracks which may have a pronounced effect
cause two problems, however. The crossed cylinder arrange-
on flexural strength (6). Machining damage imposed during
ment creates intense contact stresses in both the loading roller
specimenpreparationcanbeeitherarandominterferingfactor,
and the test specimen due to the very small contact footprint.
or an inherent part of the strength characteristic to be mea-
Themagnitudeofthecontactstressesdependsupontheapplied
sured. With proper care and good machining practice, it is
forces, the roller and test specimen diameters, and their elastic
possible to obtain fractures from the material’s natural flaws.
properties.
Surface preparation can also lead to residual stresses. It should
be understood that final machining steps may or may not
TABLE 1 Preferred Fixture Spans
negatemachiningdamageintroducedduringtheearlycoarseor
Support Outer Span Loading Inner Span
Configuration
intermediate machining.
(L ), mm (L), mm
o i
A20 10
B40 20
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof C80 40
this standard.
C1684 − 08
6.4.2 Section 6.4.5 provides guidance on how to minimize more likely to have problems than long slender rods. This
or eliminate permanent deformation that may occur in the standard allows considerable latitude in the selection of speci-
loading rollers due to contact stresses.
men sizes and testing geometries. If specimens break prema-
6.4.3 Direct loading by rollers onto the rod test specimens
turely from contact cracks, the user shall either: reduce the test
may cause premature test specimen fracture invalidating the
specimen diameter, or use longer rod specimens with longer
test. Examples are shown in Annex A1. Contact stresses may
span test fixtures, or use fixtures with cradles (see 6.5), or shift
generate shallow Hertzian cone cracks in the test specimen.
to three-point loading.
Minor cracking at an inner loading point (on the compression-
6.4.4 The rollers shall be free to rotate or roll to minimize
loadedsideofthetestrod)usuallyisharmlesssinceitdoesnot
frictional constraint as the specimen stretches or contracts
cause specimen breakage and forces are transmitted through
during loading. The sole exception is the middle-load roller in
the crack faces. In extreme conditions, however, such as
three-point flexure which need not rotate. Note that the
loading of short stubby specimens in 3-point or 4-point
outer-support rollers roll outward and the inner-loading rollers
loading, the magnitude of the forces and contact stresses may
roll inward. The rollers may roll on a fixture base as shown in
be great enough to drive a Hertzian crack deep into the test
Fig. 3 or alternatively, they may be mounted in roller assem-
specimen cross section. Contact cracks at the outer support
blies that allow them to rotate. Cradle inserts such as shown in
rollers may be deleterious and cause an undesirable fracture of
Fig. 4 may be used in conjunction with loading rollers if
the specimen, even though these locations are far away from
the inner span in 4-point loading or the middle in 3-point necessary to eliminate fractures at the loading points ind
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