ASTM C1465-00(2006)
(Test Method)Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Elevated Temperatures
Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Elevated Temperatures
SCOPE
1.1 This test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant stress-rate flexural testing in which flexural strength is determined as a function of applied stress rate in a given environment at elevated temperatures. The strength degradation exhibited with decreasing applied stress rate in a specified environment is the basis of this test method which enables the evaluation of slow crack growth parameters of a material.Note 1
This test method is frequently referred to as "dynamic fatigue" testing (Refs ()) in which the term "fatigue" is used interchangeably with the term "slow crack growth." To avoid possible confusion with the "fatigue" phenomenon of a material which occurs exclusively under cyclic loading, as defined in Terminology E 1823, this test method uses the term "constant stress-rate testing" rather than "dynamic fatigue" testing.Note 2
In glass and ceramics technology, static tests of considerable duration are called "static fatigue" tests, a type of test designated as stress-rupture (Terminology E 1823).
1.2 This test method is intended primarily to be used for negligible creep of test specimens, with specific limits on creep imposed in this test method.
1.3 This test method applies primarily to advanced ceramics that are macroscopically homogeneous and isotropic. This test method may also be applied to certain whisker- or particle-reinforced ceramics that exhibit macroscopically homogeneous behavior.
1.4 This test method is intended for use with various test environments such as air, vacuum, inert, and any other gaseous environments.
1.5 Values expressed in this standard test are in accordance with the International System of Units (SI) and Practice E 380.
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 1465 – 00 (Reapproved 2006)
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress-Rate Flexural
Testing at Elevated Temperatures
This standard is issued under the fixed designation C1465; 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.
1. Scope responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 Thistestmethodcoversthedeterminationofslowcrack
bility of regulatory limitations prior to use.
growth (SCG) parameters of advanced ceramics by using
constantstress-rateflexuraltestinginwhichflexuralstrengthis
2. Referenced Documents
determined as a function of applied stress rate in a given
2.1 ASTM Standards:
environment at elevated temperatures. The strength degrada-
C1145 Terminology of Advanced Ceramics
tion exhibited with decreasing applied stress rate in a specified
C1211 Test Method for Flexural Strength of Advanced
environment is the basis of this test method which enables the
Ceramics at Elevated Temperatures
evaluation of slow crack growth parameters of a material.
C1239 Practice for Reporting Uniaxial Strength Data and
NOTE 1—This test method is frequently referred to as “dynamic
Estimating Weibull Distribution Parameters for Advanced
fatigue” testing (Refs (1-3)) in which the term “fatigue” is used
Ceramics
interchangeably with the term “slow crack growth.” To avoid possible
C1322 Practice for Fractography and Characterization of
confusion with the “fatigue” phenomenon of a material which occurs
Fracture Origins in Advanced Ceramics
exclusively under cyclic loading, as defined in Terminology E1823, this
C1368 Test Method for Determination of Slow Crack
test method uses the term “constant stress-rate testing” rather than
“dynamic fatigue” testing.
Growth Parameters of Advanced Ceramics by Constant
NOTE 2—In glass and ceramics technology, static tests of considerable
Stress-Rate Flexural Testing at Ambient Temperature
duration are called “static fatigue” tests, a type of test designated as
E4 Practices for Force Verification of Testing Machines
stress-rupture (Terminology E1823).
E6 Terminology Relating to Methods of Mechanical Test-
1.2 This test method is intended primarily to be used for
ing
negligiblecreepoftestspecimens,withspecificlimitsoncreep
E220 Test Method for Calibration of Thermocouples By
imposed in this test method.
Comparison Techniques
1.3 Thistestmethodappliesprimarilytoadvancedceramics
E230 Specification and Temperature-Electromotive Force
that are macroscopically homogeneous and isotropic. This test
(EMF) Tables for Standardized Thermocouples
method may also be applied to certain whisker- or particle-
E337 Test Method for Measuring Humidity with a Psy-
reinforcedceramicsthatexhibitmacroscopicallyhomogeneous
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
behavior.
peratures)
1.4 This test method is intended for use with various test
IEEE/ASTMSI10 American National Standard for Use of
environmentssuchasair,vacuum,inert,andanyothergaseous
the International System of Units (SI): The Modern Metric
environments.
System
1.5 Values expressed in this standard test are in accordance
E1823 Terminology Relating to Fatigue and Fracture Test-
with the International System of Units (SI) and IEEE/
ing
ASTMSI10.
3. Terminology
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.1 Definitions—The terms described in Terminologies
C1145, E6, and E1823 are applicable to this test method.
Specific terms relevant to this test method are as follows:
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.
Current edition approved Jan. 1, 2006. Published January 2006. Originally For referenced ASTM standards, visit the ASTM website, www.astm.org, or
approved in 2000. Last previous edition approved in 2000 as C1465–00. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 1465 – 00 (2006)
3.1.1 advanced ceramic, n—a highly engineered, high- 4. Significance and Use
performance, predominately, nonmetallic, inorganic, ceramic
4.1 For many structural ceramic components in service,
material having specific functional attributes. (C 1145)
their use is often limited by lifetimes that are controlled by a
−2 −1
3.1.2 constant stress rate, s˙ [FL t ], n—aconstantrateof
process of slow crack growth. This test method provides the
increase of maximum flexural stress applied to a specified
empirical parameters for appraising the relative slow crack
beam by using either a constant load or constant displacement
growth susceptibility of ceramic materials under specified
rate of a testing machine.
environments at elevated temperatures. This test method is
3.1.3 environment, n—the aggregate of chemical species
similar to Test Method C1368 with the exception that provi-
and energy that surrounds a test specimen. (E 1150)
sions for testing at elevated temperatures are given. Further-
3.1.4 environmental chamber, n—a container surrounding
more, this test method may establish the influences of process-
the test specimen and capable of providing controlled local
ing variables and composition on slow crack growth as well as
environmental condition.
on strength behavior of newly developed or existing materials,
−2
3.1.5 flexural strength, s [FL ], n—a measure of the
thus allowing tailoring and optimizing material processing for
f
ultimate strength of a specified beam specimen in bending
furthermodification.Insummary,thistestmethodmaybeused
determined at a given stress rate in a particular environment.
formaterialdevelopment,qualitycontrol,characterization,and
3.1.6 flexural strength-stress rate diagram—a plot of flex-
limited design data generation purposes.
ural strength as a function of stress rate. Flexural strength and
NOTE 3—Data generated by this test method do not necessarily corre-
stress rate are both plotted on logarithmic scales.
spond to crack velocities that may be encountered in service conditions.
3.1.7 flexural strength-stress rate curve—a curve fitted to
The use of data generated by this test method for design purposes may
the values of flexural strength at each of several stress rates,
entail considerable extrapolation and loss of accuracy.
based on the relationship between flexural strength and stress
4.2 In this test method, the flexural stress computation is
rate:
based on simple beam theory, with the assumptions that the
log s = [1/(n + 1)] log s˙ + log D (see Appendix X1)
f
materialisisotropicandhomogeneous,themoduliofelasticity
3.1.7.1 Discussion—In the ceramics literature, this is often
in tension and compression are identical, and the material is
called a “dynamic fatigue” curve.
linearly elastic. The average grain size should be no greater
−3/2
3.1.8 fracturetoughness,K [FL ],n—agenerictermfor
IC
than one fiftieth (1/50) of the beam thickness.
measures of resistance to extension of a crack. (E 616)
−2 4.3 In this test method, the test specimen sizes and test
3.1.9 inert flexural strength [FL ], n—a measure of the
fixtures were chosen in accordance with Test Method C1211,
strength of a specified beam specimen in bending as deter-
which provides a balance between practical configurations and
minedinanappropriateinertconditionwherebynoslowcrack
resultingerrors,asdiscussedinRefs(5,6).Onlythefour-point
growth occurs.
test configuration is used in this test method.
3.1.9.1 Discussion—An inert condition at near room tem-
4.4 Inthistestmethod,theslowcrackgrowthparameters(n
perature may be obtained by using vacuum, low temperatures,
and D) are determined based on the mathematical relationship
very fast test rates, or any inert media. However, at elevated
between flexural strength and applied stress rate, log s = [1/(n
f
temperatures, the definition or concept of an inert condition is
+ 1)] log s˙ + log D, together with the measured experimental
unclear since temperature itself acts as a degrading environ-
data.Thebasicunderlyingassumptiononthederivationofthis
ment. It has been shown that for some ceramics one approach
relationship is that slow crack growth is governed by an
to obtain an inert condition (thus, inert strength) at elevated
n
4 empirical power-law crack velocity,v=A[K/K ] (see Ap-
I IC
temperaturesistouseveryfast(ultra-fast)testrates$3 310
pendix X1).
MPa/s, where the time for slow crack growth would be
minimized or eliminated (4).
NOTE 4—There are various other forms of crack velocity laws which
are usually more complex or less convenient mathematically, or both, but
3.1.10 slow crack growth (SCG), n—subcritical crack
maybephysicallymorerealistic(7).Themathematicalanalysisinthistest
growth(extension)which may result from, but is notrestricted
method does not cover such alternative crack velocity formulations.
to, such mechanisms as environmentally assisted stress corro-
sion or diffusive crack growth.
4.5 In this test method, the mathematical relationship be-
−3/2
3.1.11 stress intensity factor, K [FL ], n—the magnitude
tweenflexuralstrengthandstressratewasderivedbasedonthe
I
of the ideal-crack-tip stress field (stress-field singularly) sub-
assumption that the slow crack growth parameter is at least n
jectedtoModeIloadinginahomogeneous,linearelasticbody.
$ 5 (1, 8). Therefore, if a material exhibits a very high
(E 616)
susceptibility to slow crack growth, that is, n < 5, special care
3.1.12 R-curve, n—a plot of crack-extension resistance as a should be taken when interpreting the results.
function of stable crack extension. (E 616)
4.6 The mathematical analysis of test results according to
3.2 Definitions of Terms Specific to This Standard: the method in 4.4 assumes that the material displays no rising
3.2.1 slow crack growth parameters, n and D, n—the R-curve behavior, that is, no increasing fracture resistance (or
parameters estimated as constants in the flexural strength (in crack-extension resistance) with increasing crack length. It
megapascals)-stressrate(inmegapascalspersecond)equation, should be noted that the existence of such behavior cannot be
which represent a measure of susceptibility to slow crack determinedfromthistestmethod.Theanalysisfurtherassumes
growthofamaterial(seeAppendixX1).Fortheunitsof D,see that the same flaw types control strength over the entire test
9.3.1. range. That is, no new flaws are created, and the flaws that
C 1465 – 00 (2006)
control the strength at the highest stress rate control the MPa/s) may remain unchanged so that a plateau is observed in
strength at the lowest stress rate. the plot of strength versus stress rate, see Fig. 1a (4).Ifthe
4.7 Slow crack growth behavior of ceramic materials can strength data determined in this plateau region are included in
vary as a function of mechanical, material, thermal, and the analysis, a misleading estimate of the SCG parameters will
environmental variables. Therefore, it is essential that test beobtained.Therefore,thestrengthdataintheplateaushallbe
results accurately reflect the effects of specific variables under excluded as data points in estimating the SCG parameters of
study. Only then can data be compared from one investigation the material. This test method addresses this issue by recom-
to another on a valid basis, or serve as a valid basis for mending that the highest stress rate be#1000 MPa/s.
characterizing materials and assessing structural behavior. 5.4 Aconsiderable strength degradation may be observed at
4.8 The strength of advanced ceramics is probabilistic in lower stress rates and higher temperatures for some materials.
In these cases, excessive creep damage in the form of creep
nature. Therefore, slow crack growth that is determined from
the flexural strengths of a ceramic material is also a probabi- cavities,micro-ormacro-cracks,orboth,developinthetensile
surface (10-13). This results in a nonlinearity in the relation-
listic phenomenon. Hence, a proper range and number of test
rates in conjunction with an appropriate number of specimens ship between log (flexural strength) and log (applied stress
at each test rate are required for statistical reproducibility and
design (2). Guidance is provided in this test method.
NOTE 5—For a given ceramic material/environment system, the SCG
parameter nisindependentofspecimensizealthoughitsreproducibilityis
dependent on the variables previously mentioned. By contrast, the SCG
parameter D depends significantly on strength, and thus on specimen size
(see Eq X1.7).
4.9 The elevated-temperature strength of a ceramic material
for a given test specimen and test fixture configuration is
dependentonitsinherentresistancetofracture,thepresenceof
flaws, test rate, and environmental effects. Analysis of a
fracture surface, fractography, though beyond the scope of this
test method, is highly recommended for all purposes, espe-
cially to verify the mechanism(s) associated with failure (refer
to Practice C1322).
5. Interferences
5.1 Slowcrackgrowthmaybetheproductofbothmechani-
calandchemicaldrivingforces.Thechemicaldrivingforcefor
a given material can strongly vary with the composition and
temperatureofatestenvironment.Notethatslowcrackgrowth
testing is time-consuming. It may take several weeks to
complete testing of a typical, advanced ceramic. Because of
this long test time, the chemical variables of the test environ-
ment must be prevented from changing throughout the tests.
Inadequate control of these chemical variables may result in
inaccurate strength data and SCG parameters, especially for
materials that are sensitive to the environment.
5.2 Significant creep at both higher temperatures and lower
testratesresultsinnonlinearityinstress-strainrelationsaswell
as accumulated tensile damage in flexure (9). This, depending
on the degree of nonlinearity, may limit the applicability of
linear elastic fracture mechanics (LEFM), since the resulting
relationship between strength and stress rate derived under
constant stress-rate testing condition is based on an LEFM
approach with negligible creep (creep strain less than 0.1%).
Therefore, creep shou
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