ASTM C1161-02c(2008)
(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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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 (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope MIL-STD-1942 (MR) Flexural Strength of High Perfor-
mance Ceramics at Ambient Temperature
1.1 This test method covers the determination of flexural
strengthofadvancedceramicmaterialsatambienttemperature.
3. Terminology
Four-point– ⁄4 point and three-point loadings with prescribed
3.1 Definitions:
spans are the standard. Rectangular specimens of prescribed
3.1.1 complete gage section, n—theportionofthespecimen
cross-section sizes are used with specified features in pre-
between the two outer bearings in four-point flexure and
scribed specimen-fixture combinations.
three-point flexure fixtures.
1.2 The values stated in SI units are to be regarded as the
standard. The values given in parentheses are for information
NOTE 1—In this standard, the complete four-point flexure gage section
only. istwicethesizeoftheinnergagesection.Weibullstatisticalanalysisonly
includes portions of the specimen volume or surface which experience
1.3 This standard does not purport to address all of the
tensile stresses.
safety concerns, if any, associated with its use. It is the
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
strength testing where a specimen is symmetrically loaded at
2. Referenced Documents
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).
E4 Practices for Force Verification of Testing Machines
3.1.4 Fully-articulating fixture, n—a flexure fixture de-
C1239 Practice for Reporting Uniaxial Strength Data and
signedtobeusedeitherwithflatandparallelspecimensorwith
Estimating Weibull Distribution Parameters for Advanced
uneven or nonparallel specimens. The fixture allows full
Ceramics
independent articulation, or pivoting, of all rollers about the
C1322 Practice for Fractography and Characterization of
specimenlongaxistomatchthespecimensurface.Inaddition,
Fracture Origins in Advanced Ceramics
the upper or lower pairs are free to pivot to distribute force
C1368 Test Method for Determination of Slow Crack
evenly to the bearing cylinders on either side.
Growth Parameters of Advanced Ceramics by Constant
NOTE 2—See Annex A1 for schematic illustrations of the required
Stress-Rate Flexural Testing at Ambient Temperature
pivoting movements.
E337 Test Method for Measuring Humidity with a Psy-
NOTE 3—A three-point fixture has the inner pair of bearing cylinders
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
replaced by a single bearing cylinder.
peratures)
3.1.5 inert flexural strength, n—ameasureofthestrengthof
2.2 Military Standard:
specifiedbeaminbendingasdeterminedinanappropriateinert
condition whereby no slow crack growth occurs.
This test method is under the jurisdiction of ASTM Committee C28 on
NOTE 4—An inert condition may be obtained by using vacuum, low
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
temperatures, very fast test rates, or any inert media.
Mechanical Properties and Performance.
Current edition approved Jan. 1, 2008. Published January 2008. Originally
e1
approved in 1990. Last previous edition approved in 2002 as C1161–02c .
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.
C 1161 – 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,themoduliofelasticityintensionandcompres-
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.
4.4 The flexural strength of a ceramic material is dependent
on both its inherent resistance to fracture and the size and
NOTE 1—Configuration:
A: L = 20 mm
severity of flaws. Variations in these cause a natural scatter in
B: L = 40 mm
test results for a sample of test specimens. Fractographic
C: L = 80 mm
analysisoffracturesurfaces,althoughbeyondthescopeofthis
FIG. 1 The Four-Point– ⁄4 Point and Three-Point Fixture
standard, is highly recommended for all purposes, especially if
Configuration
thedatawillbeusedfordesignasdiscussedinMIL-STD-1942
(MR) and Refs (2–5) and Practices C1322 and C1239.
3.1.6 inherent flexural strength, n—theflexuralstrengthofa
4.5 The three-point test configuration exposes only a very
material in the absence of any effect of surface grinding or
small portion of the specimen to the maximum stress. There-
other surface finishing process, or of extraneous damage that
fore,three-pointflexuralstrengthsarelikelytobemuchgreater
may be present.The measured inherent strength is in general a
thanfour-pointflexuralstrengths.Three-pointflexurehassome
function of the flexure test method, test conditions, and
advantages. It uses simpler test fixtures, it is easier to adapt to
specimen size.
high temperature and fracture toughness testing, and it is
3.1.7 inner gage section, n—the portion of the specimen
sometimes helpful in Weibull statistical studies. However,
between the inner two bearings in a four-point flexure fixture.
four-point flexure is preferred and recommended for most
3.1.8 Semi-articulating fixture, n—aflexurefixturedesigned
characterization purposes.
to be used with flat and parallel specimens. The fixture allows
4.6 This method determines the flexural strength at ambient
somearticulation,orpivoting,toensurethetoppair(orbottom
temperature and environmental conditions. The flexural
pair) of bearing cylinders pivot together about an axis parallel
strength under ambient conditions may or may not necessarily
to the specimen long axis, in order to match the specimen
be the inert flexural strength.
surfaces. In addition, the upper or lower pairs are free to pivot
NOTE 7—time dependent effects may be minimized through the use of
to distribute force evenly to the bearing cylinders on either
inert testing atmosphere such as dry nitrogen gas, oil, or vacuum.
side.
Alternatively, testing rates faster than specified in this standard may be
NOTE 5—See Annex A1 for schematic illustrations of the required
used. Oxide ceramics, glasses, and ceramics containing boundary phase
pivoting movements.
glass are susceptible to slow crack growth even at room temperature.
NOTE 6—A three-point fixture has the inner pair of bearing cylinders
Water, either in the form of liquid or as humidity in air, can have a
replaced by a single bearing cylinder.
significant effect, even at the rates specified in this standard. On the other
hand, many ceramics such as boron carbide, silicon carbide, aluminum
3.1.9 slow crack growth (SCG), n—subcriticalcrackgrowth
nitride and many silicon nitrides have no sensitivity to slow crack growth
(extension)whichmayresultfrom,butisnotrestrictedto,such
at room temperature and the flexural strength in laboratory ambient
mechanisms as environmentally-assisted stress corrosion or
conditions is the inert flexural strength.
diffusive crack growth.
5. Interferences
3.1.10 three-point flexure—configuration of flexural
strength testing where a specimen is loaded at a location
5.1 Theeffectsoftime-dependentphenomena,suchasstress
midway between two support bearings (see Fig. 1).
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.
C 1161 – 02c (2008)
ambienttemperature,canbemeaningfulevenfortherelatively 6.3 Three-Point Flexure—Three-pointfixtures(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- 3 10 MPa (30–60 3 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 3 10
should be understood that final machining steps may or may MPa (70 3 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.
5.3 This test method allows several options for the machin- 6.4.2 The bearing cylinder diameter shall be approximately
ing of specimens, and includes a general procedure (“Stan- 1.5 times the beam depth of the test specimen size employed.
dard” procedure, 7.2.4), which is satisfactory for many (but See Table 2.
certainly not all) ceramics. The general procedure used pro- 6.4.3 The bearing cylinders shall be carefully positioned
gressivelyfinerlongitudinalgrindingstepsthataredesignedto such that the spans are accurate within 60.10 mm. The load
minimize subsurface microcracking. Longitudinal grinding application bearing for the three-point configurations shall be
aligns the most severe subsurface microcracks parallel to the positioned midway between the support bearing within 60.10
specimen tension stress axis.This allows a greater opportunity mm. The load application (inner) bearings for the four-point
tomeasuretheinherentflexuralstrengthor“potentialstrength” configurations shall be centered with respect to the support
of the material as controlled by the material’s natural flaws. In (outer) bearings within 60.10 mm.
contrast, transverse grinding aligns the severest subsurface 6.4.4 Thebearingcylindersshallbefreetorotateinorderto
machining microcracks perpendicular to the tension stress axis relieve frictional constraints (with the exception of the middle-
andthespecimenismorelikelytofracturefromthemachining loadbearinginthree-pointflexurewhichneednotrotate).This
microcracks. Transverse-ground specimens in many instances can be accomplished by mounting the cylinders in needle
may provide a more “practical strength” that is relevant to bearing assemblies, or more simply by mounting the cylinders
machined ceramic components whereby it may not be possible as shown in Fig. 2 and Fig. 3. Annex A1 illustrates the action
to favorably align the machining direction. Transverse-ground required of the bearing cylinders. Note that the outer-support
specimens may be tested in accordance with 7.2.2. Data from bearings roll outward and the inner-loading bearings roll
transverse-ground specimens may correlate better with data inward.
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
together as a pair. The inner bearings shall be supported
ameansforretainingread-outofthemaximumforceappliedto
independentlyoftheouterbearings.Allfourbearingsshallrest
the specimen. The accuracy of the testing machine shall be in
uniformlyandevenlyacros
...
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.
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 (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
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.
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.
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.
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 (inner) bearings for the four-point configurations shall be centered with respect to the support (outer) bearings within
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Questions, Comments and Discussion
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