ASTM C1161-02c(2008)e1
(Test Method)Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
SIGNIFICANCE AND USE
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 flexure stress 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 beam thickness. The homogeneity and isotropy assumption in the standard rule out the use of this test for continuous fiber-reinforced ceramics.
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. Specimen sizes and fixtures were chosen to provide a balance between practical configurations and resulting errors, as discussed in MIL-STD 1942 (MR) and Refs (1) and (2). Specific fixture and specimen configurations were designated in order to permit ready comparison of data without the need for Weibull-size scaling.
The flexural strength of a ceramic material is dependent on both its inherent resistance to fracture and the size and severity of flaws. Variations in these cause a natural scatter in test results for a sample of test specimens. Fractographic analysis of fracture surfaces, although beyond the scope of this standard, is highly recommended for all purposes, especially if the data will be used for design as discussed in MIL-STD-1942 (MR) and Refs (2–5) and Practices C 1322 and C 1239.
The three-point test configuration exposes only a very small portion of the specimen to the maximum stress. 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 fixtu...
SCOPE
1.1 This test method covers the determination of flexural strength of advanced ceramic materials at ambient temperature. Four-point– ¼ point and three-point loadings with prescribed spans are the standard. Rectangular specimens of prescribed cross-section sizes are used with specified features in prescribed specimen-fixture combinations.
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 and health practices and determine the applicability of regulatory limitations prior to use.
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´1
Designation: C1161 − 02c(Reapproved 2008)
Standard Test Method for
Flexural Strength of Advanced Ceramics at Ambient
Temperature
This standard is issued under the fixed designation C1161; 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.
This standard has been approved for use by agencies of the Department of Defense.
´ NOTE—Added research report footnote to Sections 11.3, 11.4, and 11.5 editorially in September 2008.
1. Scope 2.2 Military Standard:
MIL-STD-1942 (MR) Flexural Strength of High Perfor-
1.1 This test method covers the determination of flexural
mance Ceramics at Ambient Temperature
strengthofadvancedceramicmaterialsatambienttemperature.
Four-point– ⁄4 point and three-point loadings with prescribed
3. Terminology
spans are the standard. Rectangular specimens of prescribed
3.1 Definitions:
cross-section sizes are used with specified features in pre-
3.1.1 complete gage section, n—theportionofthespecimen
scribed specimen-fixture combinations.
between the two outer bearings in four-point flexure and
1.2 The values stated in SI units are to be regarded as the
three-point flexure fixtures.
standard. The values given in parentheses are for information
only. NOTE 1—In this standard, the complete four-point flexure gage section
istwicethesizeoftheinnergagesection.Weibullstatisticalanalysisonly
1.3 This standard does not purport to address all of the
includes portions of the specimen volume or surface which experience
safety concerns, if any, associated with its use. It is the
tensile stresses.
responsibility of the user of this standard to establish appro-
3.1.2 flexural strength—a measure of the ultimate strength
priate safety and health practices and determine the applica-
of a specified beam in bending.
bility of regulatory limitations prior to use.
3.1.3 four-point– ⁄4 point flexure—configuration of flexural
2. Referenced Documents
strength testing where a specimen is symmetrically loaded at
two locations that are situated one quarter of the overall span,
2.1 ASTM Standards:
away from the outer two support bearings (see Fig. 1).
E4Practices for Force Verification of Testing Machines
C1239Practice for Reporting Uniaxial Strength Data and 3.1.4 Fully-articulating fixture, n—a flexure fixture de-
Estimating Weibull Distribution Parameters forAdvanced signedtobeusedeitherwithflatandparallelspecimensorwith
Ceramics
uneven or nonparallel specimens. The fixture allows full
C1322Practice for Fractography and Characterization of independent articulation, or pivoting, of all rollers about the
Fracture Origins in Advanced Ceramics
specimen long axis to match the specimen surface. In addition,
C1368 Test Method for Determination of Slow Crack the upper or lower pairs are free to pivot to distribute force
Growth Parameters of Advanced Ceramics by Constant
evenly to the bearing cylinders on either side.
Stress-Rate Strength Testing at Ambient Temperature
NOTE 2—See Annex A1 for schematic illustrations of the required
E337Test Method for Measuring Humidity with a Psy-
pivoting movements.
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
NOTE 3—A three-point fixture has the inner pair of bearing cylinders
replaced by a single bearing cylinder.
peratures)
3.1.5 inert flexural strength, n—ameasureofthestrengthof
This test method is under the jurisdiction of ASTM Committee C28 on
specifiedbeaminbendingasdeterminedinanappropriateinert
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
condition whereby no slow crack growth occurs.
Mechanical Properties and Performance.
Current edition approved Jan. 1, 2008. Published January 2008. Originally
NOTE 4—An inert condition may be obtained by using vacuum, low
´1
approved in 1990. Last previous edition approved in 2002 as C1161–02c . DOI:
10.1520/C1161-02CR08E01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
Standards volume information, refer to the standard’s Document Summary page on Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
the ASTM website. www.dodssp.daps.mil.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
C1161 − 02c(Reapproved 2008)
temperatures, very fast test rates, or any inert media.
´1
C1161 − 02c (2008)
purposes.Thistestmethodisintendedtobeusedwithceramics
whose strength is 50 MPa (~7 ksi) or greater.
4.2 The flexure stress is computed based on simple beam
theory with assumptions that the material is isotropic and
homogeneous, the moduli of elasticity in tension and compres-
sion are identical, and the material is linearly elastic. The
average grain size should be no greater than one fiftieth of the
beam thickness. The homogeneity and isotropy assumption 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 environ-
ment, specimen size, specimen preparation, and test fixtures.
Specimen sizes and fixtures were chosen to provide a balance
between practical configurations and resulting errors, as dis-
cussed in MIL-STD1942(MR) and Refs (1) and (2). Specific
fixtureandspecimenconfigurationsweredesignatedinorderto
permit ready comparison of data without the need forWeibull-
size scaling.
NOTE 1—Configuration:
4.4 The flexural strength of a ceramic material is dependent
A: L = 20 mm
on both its inherent resistance to fracture and the size and
B: L = 40 mm
C: L = 80 mm severity of flaws. Variations in these cause a natural scatter in
FIG. 1 The Four-Point– ⁄4 Point and Three-Point Fixture Configu-
test results for a sample of test specimens. Fractographic
ration
analysisoffracturesurfaces,althoughbeyondthescopeofthis
standard, is highly recommended for all purposes, especially if
3.1.6 inherent flexural strength, n—theflexuralstrengthofa
thedatawillbeusedfordesignasdiscussedinMIL-STD-1942
material in the absence of any effect of surface grinding or
(MR) and Refs (2–5) and Practices C1322 and C1239.
other surface finishing process, or of extraneous damage that
4.5 The three-point test configuration exposes only a very
may be present.The measured inherent strength is in general a
small portion of the specimen to the maximum stress. There-
function of the flexure test method, test conditions, and
fore,three-pointflexuralstrengthsarelikelytobemuchgreater
specimen size.
thanfour-pointflexuralstrengths.Three-pointflexurehassome
3.1.7 inner gage section, n—the portion of the specimen
advantages. It uses simpler test fixtures, it is easier to adapt to
between the inner two bearings in a four-point flexure fixture.
high temperature and fracture toughness testing, and it is
sometimes helpful in Weibull statistical studies. However,
3.1.8 Semi-articulating fixture, n—aflexurefixturedesigned
four-point flexure is preferred and recommended for most
to be used with flat and parallel specimens. The fixture allows
characterization purposes.
somearticulation,orpivoting,toensurethetoppair(orbottom
pair) of bearing cylinders pivot together about an axis parallel
4.6 This method determines the flexural strength at ambient
to the specimen long axis, in order to match the specimen
temperature and environmental conditions. The flexural
surfaces. In addition, the upper or lower pairs are free to pivot
strength under ambient conditions may or may not necessarily
to distribute force evenly to the bearing cylinders on either
be the inert flexural strength.
side.
NOTE 7—time dependent effects may be minimized through the use of
NOTE 5—See Annex A1 for schematic illustrations of the required inert testing atmosphere such as dry nitrogen gas, oil, or vacuum.
pivoting movements. Alternatively, testing rates faster than specified in this standard may be
NOTE 6—A three-point fixture has the inner pair of bearing cylinders used. Oxide ceramics, glasses, and ceramics containing boundary phase
replaced by a single bearing cylinder. glass are susceptible to slow crack growth even at room temperature.
Water, either in the form of liquid or as humidity in air, can have a
3.1.9 slow crack growth (SCG), n—subcritical crack growth
significant effect, even at the rates specified in this standard. On the other
(extension)whichmayresultfrom,butisnotrestrictedto,such
hand, many ceramics such as boron carbide, silicon carbide, aluminum
mechanisms as environmentally-assisted stress corrosion or
nitride and many silicon nitrides have no sensitivity to slow crack growth
diffusive crack growth. at room temperature and the flexural strength in laboratory ambient
conditions is the inert flexural strength.
3.1.10 three-point flexure—configuration of flexural
strength testing where a specimen is loaded at a location 5. Interferences
midway between two support bearings (see Fig. 1).
5.1 Theeffectsoftime-dependentphenomena,suchasstress
corrosion or slow crack growth on strength tests conducted at
4. Significance and Use
4.1 Thistestmethodmaybeusedformaterialdevelopment,
The boldface numbers in parentheses refer to the references at the end of this
quality control, characterization, and design data generation test method.
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C1161 − 02c (2008)
ambienttemperature,canbemeaningfulevenfortherelatively 6.3 Three-Point Flexure—Three-point fixtures (Fig. 1) shall
short times involved during testing. Such influences must be have a support span as shown in Table 1.
considered if flexure tests are to be used to generate design
6.4 Bearings—Three- and four-point flexure:
data.Slowcrackgrowthcanleadaratedependencyofflexural
6.4.1 Cylindricalbearingedgesshallbeusedforthesupport
strength. The testing rate specifed in this standard may or may
of the test specimen and for the application of load. The
notproducetheinertflexuralstrengthwherebynegligibleslow
cylindersshallbemadeofhardenedsteelwhichhasahardness
crack growth occurs. See Test Method C1368.
no less than HRC 40 or which has a yield strength no less than
5.2 Surface preparation of test specimens can introduce 1240 MPa (;180 ksi). Alternatively, the cylinders may be
machining microcracks which may have a pronounced effect madeofaceramicwithanelasticmodulusbetween2.0and4.0
5 6
on flexural strength. Machining damage imposed during speci- ×10 MPa (30–60 × 10 psi) and a flexural strength no less
menpreparationcanbeeitherarandominterferingfactor,oran than 275 MPa (;40 ksi). The portions of the test fixture that
inherentpartofthestrengthcharacteristictobemeasured.With support the bearings may need to be hardened to prevent
proper care and good machining practice, it is possible to permanentdeformation.Thecylindricalbearinglengthshallbe
obtain fractures from the material’s natural flaws. Surface at least three times the specimen width. The above require-
preparation can also lead to residual stresses. Universal or mentsareintendedtoensurethatceramicswithstrengthsupto
standardizedtestmethodsofsurfacepreparationdonotexist.It 1400 MPa (;200 ksi) and elastic moduli as high as 4.8 × 10
should be understood that final machining steps may or may MPa (70 × 10 psi) can be tested without fixture damage.
not negate machining damage introduced during the early Higher strength and stiffer ceramic specimens may require
course or intermediate machining. harder bearings.
6.4.2 The bearing cylinder diameter shall be approximately
5.3 This test method allows several options for the machin-
1.5 times the beam depth of the test specimen size employed.
ing of specimens, and includes a general procedure (“Stan-
See Table 2.
dard” procedure, 7.2.4), which is satisfactory for many (but
6.4.3 The bearing cylinders shall be carefully positioned
certainly not all) ceramics. The general procedure used pro-
such that the spans are accurate within 60.10 mm. The load
gressivelyfinerlongitudinalgrindingstepsthataredesignedto
application bearing for the three-point configurations shall be
minimize subsurface microcracking. Longitudinal grinding
positioned midway between the support bearing within 60.10
aligns the most severe subsurface microcracks parallel to the
mm. The load application (inner) bearings for the four-point
specimen tension stress axis.This allows a greater opportunity
configurations shall be centered with respect to the support
tomeasuretheinherentflexuralstrengthor“potentialstrength”
(outer) bearings within 60.10 mm.
of the material as controlled by the material’s natural flaws. In
6.4.4 Thebearingcylindersshallbefreetorotateinorderto
contrast, transverse grinding aligns the severest subsurface
relieve frictional constraints (with the exception of the middle-
machining microcracks perpendicular to the tension stress axis
loadbearinginthree-pointflexurewhichneednotrotate).This
andthespecimenismorelikelytofracturefromthemachining
can be accomplished by mounting the cylinders in needle
microcracks. Transverse-ground specimens in many instances
bearing assemblies, or more simply by mounting the cylinders
may provide a more “practical strength” that is relevant to
as shown in Fig. 2 and Fig. 3. AnnexA1 illustrates the action
machined ceramic components whereby it may not be possible
required of the bearing cylinders. Note that the outer-support
to favorably align the machining direction. Transverse-ground
bearings roll outward and the inner-loading bearings roll
specimens may be tested in accordance with 7.2.2. Data from
inward.
transverse-ground specimens may correlate better with data
from biaxial disk or plate strength tests, wherein machining 6.5 Semiarticulating–Four-Point Fixture—Specimens pre-
direction cannot be aligned. pared in accordance with the parallelism requirements of 7.1
may be tested in a semiarticulating fixture as illustrated in Fig.
6. Apparatus
2 and in Fig.A1.1a.All four bearings shall be free to roll. The
two inner bearings shall be parallel to each other to within
6.1 Loading—Specimens may be loaded in any suitable
0.015mmovertheirlengthandtheyshallarticulatetogetheras
testing machine provided that uniform rates of direct loading
a pair.The two outer bearings shall be parallel to each other to
canbemaintained.Theforce-measuringsystemshallbefreeof
within 0.015 mm over their length and they shall articulate
initial lag at the loading rates used and shall be equipped with
togeth
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation:C 1161–02b Designation: C 1161 – 02c (Reapproved 2008)
Standard Test Method for
Flexural Strength of Advanced Ceramics at Ambient
Temperature
This standard is issued under the fixed designation C 1161; 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.
This standard has been approved for use by agencies of the Department of Defense.
´ NOTE—Added research report footnote to Sections 11.3, 11.4, and 11.5 editorially in September 2008.
1. Scope
1.1 This test method covers the determination of flexural strength of advanced ceramic materials at ambient temperature.
Four-point– ⁄4 point and three-point loadings with prescribed spans are the standard. Rectangular specimens of prescribed
cross-section sizes are used with specified features in prescribed specimen-fixture combinations.
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 and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E4 Practices for Force Verification of Testing Machines
C1239 Practice for Reporting Uniaxial Strength Data and EstimatingWeibull Distribution Parameters forAdvanced Ceramics
C1322 Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
C1368 Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate
Flexural Testing at Ambient Temperature
E337 Test Method for MeasuredMeasuring Humidity with a Psychrometer (The(the Measurement of Wet- and Dry-Bulb
Temperatures)
2.2 Military Standard:
MIL-STD-1942 (MR) Flexural Strength of High Performance Ceramics at Ambient Temperature
3. Terminology
3.1 Definitions:
3.1.1 complete gage section, n—theportionofthespecimenbetweenthetwoouterbearingsinfour-pointflexureandthree-point
flexure fixtures.
NOTE 1—In this standard, the complete four-point flexure gage section is twice the size of the inner gage section. Weibull statistical analysis only
includes portions of the specimen volume or surface which experience tensile stresses.
3.1.2 flexural strength—a measure of the ultimate strength of a specified beam in bending.
3.1.3 four-point– ⁄4 point flexure—configuration of flexural strength testing where a specimen is symmetrically loaded at two
locations that are situated one quarter of the overall span, away from the outer two support bearings (see Fig. 1).
3.1.4 Fully-articulating fixture, n—a flexure fixture designed to be used either with flat and parallel specimens or with uneven
or nonparallel specimens. The fixture allows full independent articulation, or pivoting, of all rollers about the specimen long axis
This test method is under the jurisdiction ofASTM Committee C28 onAdvanced Ceramics and is the direct responsibility of Subcommittee C28.01 on Properties and
Performance.
Current edition approved June 10, 2002. Published August 2002. Originally published as C1161–90. Last previous edition C1161–02a.
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.
e1
Current edition approved Jan. 1, 2008. Published January 2008. Originally approved in 1990. Last previous edition approved in 2002 as C1161–02c .
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.For Annual Book of ASTM Standards
, Vol 03.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Annual Book of ASTM Standards, Vol 15.01.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://www.dodssp.daps.mil.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
´1
C 1161 – 02c (2008)
NOTE 1—Configuration:
A: L = 20 mm
B: L = 40 mm
C: L = 80 mm
FIG. 1 1The Four-Point– ⁄4 Point and Three-Point Fixture
Configuration
to match the specimen surface. In addition, the upper or lower pairs are free to pivot to distribute force evenly to the bearing
cylinders on either side.
NOTE 2—See Annex A1 for schematic illustrations of the required pivoting movements.
NOTE 3—A three-point fixture has the inner pair of bearing cylinders replaced by a single bearing cylinder.
3.1.5 inert flexural strength, n—a measure of the strength of specified beam in bending as determined in an appropriate inert
condition whereby no slow crack growth occurs.
NOTE 4—An inert condition may be obtained by using vacuum, low temperatures, very fast test rates, or any inert media.
3.1.6 inherent flexural strength, n—the flexural strength of a material in the absence of any effect of surface grinding or other
surface finishing process, or of extraneous damage that may be present. The measured inherent strength is in general a function
of the flexure test method, test conditions, and specimen size.
3.1.7 inner gage section, n—the portion of the specimen between the inner two bearings in a four-point flexure fixture.
3.1.8 Semi-articulating fixture, n—aflexurefixturedesignedtobeusedwithflatandparallelspecimens.Thefixtureallowssome
articulation, or pivoting, to ensure the top pair (or bottom pair) of bearing cylinders pivot together about an axis parallel to the
specimen long axis, in order to match the specimen surfaces. In addition, the upper or lower pairs are free to pivot to distribute
force evenly to the bearing cylinders on either side.
NOTE 5—See Annex A1 for schematic illustrations of the required pivoting movements.
NOTE 6—A three-point fixture has the inner pair of bearing cylinders replaced by a single bearing cylinder.
3.1.9 slow crack growth (SCG), n—subcritical crack growth (extension) which may result from, but is not restricted to, such
mechanisms as environmentally-assisted stress corrosion or diffusive crack growth.
3.1.10 three-point flexure—configuration of flexural strength testing where a specimen is loaded at a location midway between
two support bearings (see Fig. 1).
4. 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.
4.2 The flexure stress 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 beam thickness. The homogeneity and isotropy assumption in the standard
rule out the use of this test for continuous fiber-reinforced ceramics.
´1
C 1161 – 02c (2008)
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. Specimen sizes and
fixtures were chosen to provide a balance between practical configurations and resulting errors, as discussed in MIL-
STD1942(MR) and Refs (1) and (2) . Specific fixture and specimen configurations were designated in order to permit ready
comparison of data without the need for Weibull-size scaling.
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.Variations in these cause a natural scatter in test results for a sample of test specimens. Fractographic analysis of fracture
surfaces, although beyond the scope of this standard, is highly recommended for all purposes, especially if the data will be used
for design as discussed in MIL-STD-1942(MR) and Refs (2–5) and Practices C 1322 and C 1239. and Practices C1322 and
C1239.
4.5 The three-point test configuration exposes only a very small portion of the specimen to the maximum stress. Therefore,
three-point flexural strengths are likely to be much greater than four-point flexural strengths. Three-point flexure has some
advantages.Itusessimplertestfixtures,itiseasiertoadapttohightemperatureandfracturetoughnesstesting,anditissometimes
helpful in Weibull statistical studies. However, four-point flexure is preferred and recommended for most characterization
purposes.
4.6 This method determines the flexural strength at ambient temperature and environmental conditions. The flexural strength
under ambient conditions may or may not necessarily be the inert flexural strength.
NOTE 7—time dependent effects may be minimized through the use of inert testing atmosphere such as dry nitrogen gas, oil, or vacuum.Alternatively,
testing rates faster than specified in this standard may be used. Oxide ceramics, glasses, and ceramics containing boundary phase glass are susceptible
to slow crack growth even at room temperature. Water, either in the form of liquid or as humidity in air, can have a significant effect, even at the rates
specified in this standard. On the other hand, many ceramics such as boron carbide, silicon carbide, aluminum nitride and many silicon nitrides have no
sensitivity to slow crack growth at room temperature and the flexural strength in laboratory ambient conditions is the inert flexural strength.
5. Interferences
5.1 The effects of time-dependent phenomena, such as stress corrosion or slow crack growth on strength tests conducted at
ambient temperature, can be meaningful even for the relatively short times involved during testing. Such influences must be
consideredifflexuretestsaretobeusedtogeneratedesigndata.Slowcrackgrowthcanleadaratedependencyofflexuralstrength.
Thetestingratespecifedinthisstandardmayormaynotproducetheinertflexuralstrengthwherebynegligibleslowcrackgrowth
occurs. See Test Method C1368.
5.2 Surfacepreparationoftestspecimenscanintroducemachiningmicrocrackswhichmayhaveapronouncedeffectonflexural
strength. Machining damage imposed during specimen preparation can be either a random interfering factor, or an inherent part
of the strength characteristic to be measured.With proper care and good machining practice, it is possible to obtain fractures from
thematerial’snaturalflaws.Surfacepreparationcanalsoleadtoresidualstresses.Universalorstandardizedtestmethodsofsurface
preparation do not exist. It should be understood that final machining steps may or may not negate machining damage introduced
during the early course or intermediate machining.
5.3 This test method allows several options for the machining of specimens, and includes a general procedure (“Standard”
procedure, 7.2.4), which is satisfactory for many (but certainly not all) ceramics. The general procedure used progressively finer
longitudinal grinding steps that are designed to minimize subsurface microcracking. Longitudinal grinding aligns the most severe
subsurface microcracks parallel to the specimen tension stress axis. This allows a greater opportunity to measure the inherent
flexuralstrengthor“potentialstrength”ofthematerialascontrolledbythematerial’snaturalflaws.Incontrast,transversegrinding
aligns the severest subsurface machining microcracks perpendicular to the tension stress axis and the specimen is more likely to
fracturefromthemachiningmicrocracks.Transverse-groundspecimensinmanyinstancesmayprovideamore“practicalstrength”
that is relevant to machined ceramic components whereby it may not be possible to favorably align the machining direction.
Transverse-groundspecimensmaybetestedinaccordancewith7.2.2.Datafromtransverse-groundspecimensmaycorrelatebetter
with data from biaxial disk or plate strength tests, wherein machining direction cannot be aligned.
6. Apparatus
6.1 Loading—Specimens may be loaded in any suitable testing machine provided that uniform rates of direct loading can be
maintained. The force-measuring system shall be free of initial lag at the loading rates used and shall be equipped with a means
for retaining read-out of the maximum force applied to the specimen. The accuracy of the testing machine shall be in accordance
with Practices E4 but within 0.5%.
6.2 Four-Point Flexure—Four-point– ⁄4 point fixtures (Fig. 1) shall have support and loading spans as shown in Table 1.
6.3 Three-Point Flexure—Three-point fixtures (Fig. 1) shall have a support span as shown in Table 1.
6.4 Bearings—Three- and four-point flexure:
6.4.1 Cylindrical bearing edges shall be used for the support of the test specimen and for the application of load.The cylinders
shall be made of hardened steel which has a hardness no less than HRC 40 or which has a yield strength no less than 1240 MPa
Annual Book of ASTM Standards, Vol 11.03.
The boldface numbers in parentheses refer to the references at the end of this test method.
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C 1161 – 02c (2008)
TABLE 1 Fixture Spans
Configuration Support Span (L), mm Loading Span, mm
A20 10
B40 20
C80 40
(;180ksi).Alternatively,thecylindersmaybemadeofaceramicwithanelasticmodulusbetween2.0and4.0 310 MPa(30–60
3 10 psi) and a flexural strength no less than 275 MPa (;40 ksi). The portions of the test fixture that support the bearings may
need to be hardened to prevent permanent deformation. The cylindrical bearing length shall be at least three times the specimen
width.The above requirements are intended to ensure that ceramics with strengths up to 1400 MPa (;200 ksi) and elastic moduli
5 6
ashighas4.8 310 MPa(70 310 psi)canbetestedwithoutfixturedamage.Higherstrengthandstifferceramicspecimensmay
require harder bearings.
6.4.2 The bearing cylinder diameter shall be approximately 1.5 times the beam depth of the test specimen size employed. See
Table 2.
6.4.3 Thebearingcylindersshallbecarefullypositionedsuchthatthespansareaccuratewithin 60.10mm.Theloadapplication
bearing for the three-point configurations shall be positioned midway between the support bearing within 60.10 mm. The load
application (inne
...
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