ASTM C1465-08(2013)
(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
SIGNIFICANCE AND USE
4.1 For many structural ceramic components in service, their use is often limited by lifetimes that are controlled by a process of slow crack growth. This test method provides the empirical parameters for appraising the relative slow crack growth susceptibility of ceramic materials under specified environments at elevated temperatures. This test method is similar to Test Method C1368 with the exception that provisions for testing at elevated temperatures are given. Furthermore, this test method may establish the influences of processing variables and composition on slow crack growth as well as on strength behavior of newly developed or existing materials, thus allowing tailoring and optimizing material processing for further modification. In summary, this test method may be used for material development, quality control, characterization, and limited design data generation purposes.Note 3—Data generated by this test method do not necessarily correspond to crack velocities that may be encountered in service conditions. The use of data generated by this test method for design purposes may entail considerable extrapolation and loss of accuracy.
4.2 In this test method, the flexural stress computation is based on simple beam theory, with the 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 (1/50) of the beam thickness.
4.3 In this test method, the test specimen sizes and test fixtures were chosen in accordance with Test Method C1211, which provides a balance between practical configurations and resulting errors, as discussed in Refs (7, 8). Only the four-point test configuration is used in this test method.
4.4 In this test method, the slow crack growth parameters (n and D) are determined based on the mathematical relationship between flexural strength and applied stress rate, log σf =...
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 (3-5))2 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 E1823, 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 E1823).
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 IEEE/ASTM SI 10.
1.6 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 ap...
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Designation: C1465 − 08(Reapproved 2013)
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 Thistestmethodcoversthedeterminationofslowcrack
responsibility of the user of this standard to establish appro-
growth (SCG) parameters of advanced ceramics by using
priate safety and health practices and determine the applica-
constantstress-rateflexuraltestinginwhichflexuralstrengthis
bility of regulatory limitations prior to use.
determined as a function of applied stress rate in a given
environment at elevated temperatures. The strength degrada-
2. Referenced Documents
tion exhibited with decreasing applied stress rate in a specified
environment is the basis of this test method which enables the 2.1 ASTM Standards:
evaluation of slow crack growth parameters of a material. C1145Terminology of Advanced Ceramics
C1211Test Method for Flexural Strength of Advanced
NOTE 1—This test method is frequently referred to as “dynamic
Ceramics at Elevated Temperatures
fatigue” testing (Refs (1-3)) in which the term “fatigue” is used
C1239Practice for Reporting Uniaxial Strength Data and
interchangeably with the term “slow crack growth.” To avoid possible
confusion with the “fatigue” phenomenon of a material which occurs
Estimating Weibull Distribution Parameters forAdvanced
exclusively under cyclic loading, as defined in Terminology E1823, this
Ceramics
test method uses the term “constant stress-rate testing” rather than
C1322Practice for Fractography and Characterization of
“dynamic fatigue” testing.
Fracture Origins in Advanced Ceramics
NOTE 2—In glass and ceramics technology, static tests of considerable
duration are called “static fatigue” tests, a type of test designated as C1368 Test Method for Determination of Slow Crack
stress-rupture (Terminology E1823).
Growth Parameters of Advanced Ceramics by Constant
Stress-Rate Strength Testing at Ambient Temperature
1.2 This test method is intended primarily to be used for
D1239Test Method for Resistance of Plastic Films to
negligiblecreepoftestspecimens,withspecificlimitsoncreep
Extraction by Chemicals
imposed in this test method.
E4Practices for Force Verification of Testing Machines
1.3 Thistestmethodappliesprimarilytoadvancedceramics
E6Terminology Relating to Methods of Mechanical Testing
that are macroscopically homogeneous and isotropic. This test
E220Test Method for Calibration of Thermocouples By
method may also be applied to certain whisker- or particle-
Comparison Techniques
reinforcedceramicsthatexhibitmacroscopicallyhomogeneous
E230Specification and Temperature-Electromotive Force
behavior.
(EMF) Tables for Standardized Thermocouples
1.4 This test method is intended for use with various test
E337Test Method for Measuring Humidity with a Psy-
environmentssuchasair,vacuum,inert,andanyothergaseous
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
environments.
peratures)
E616Terminology Relating to Fracture Testing (Discontin-
1.5 Values expressed in this standard test are in accordance
ued 1996) (Withdrawn 1996)
with the International System of Units (SI) and IEEE/
E1150Definitions of Terms Relating to Fatigue (Withdrawn
ASTMSI10.
1996)
This test method is under the jurisdiction of ASTM Committee C28 on
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
Mechanical Properties and Performance. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Aug. 1, 2013. Published September 2013. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2000. Last previous edition approved in 2008 as C1465–08. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1465-08R13. the ASTM website.
2 4
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof The last approved version of this historical standard is referenced on
this standard. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1465 − 08 (2013)
−3/2
IEEE/ASTMSI10American National Standard for Use of 3.1.12 stress intensity factor, K [FL ],n—the magnitude
I
theInternationalSystemofUnits(SI):TheModernMetric of the ideal-crack-tip stress field (stress-field singularly) sub-
System jectedtoModeIloadinginahomogeneous,linearelasticbody.
E1823TerminologyRelatingtoFatigueandFractureTesting (E616)
3.1.13 R-curve, n—a plot of crack-extension resistance as a
3. Terminology
function of stable crack extension. (E616)
3.1 Definitions:
3.2 Definitions of Terms Specific to This Standard:
3.1.1 The terms described inTerminologies C1145, E6, and
3.2.1 slow crack growth parameters, n and D, n—the
E1823 are applicable to this test method. Specific terms
parameters estimated as constants in the flexural strength (in
relevant to this test method are as follows:
megapascals)-stressrate(inmegapascalspersecond)equation,
3.1.2 advanced ceramic, n—a highly engineered, high-
which represent a measure of susceptibility to slow crack
performance, predominately, nonmetallic, inorganic, ceramic
growthofamaterial(seeAppendixX1).Fortheunitsof D,see
material having specific functional attributes. (C1145)
9.3.1.
−2 −1
3.1.3 constant stress rate, σ˙[FL t ],n—aconstantrateof
4. Significance and Use
increase of maximum flexural stress applied to a specified
beam by using either a constant load or constant displacement
4.1 For many structural ceramic components in service,
rate of a testing machine.
their use is often limited by lifetimes that are controlled by a
process of slow crack growth. This test method provides the
3.1.4 environment, n—the aggregate of chemical species
empirical parameters for appraising the relative slow crack
and energy that surrounds a test specimen. (E1150)
growth susceptibility of ceramic materials under specified
3.1.5 environmental chamber, n—a container surrounding
environments at elevated temperatures. This test method is
the test specimen and capable of providing controlled local
similar to Test Method C1368 with the exception that provi-
environmental condition.
sions for testing at elevated temperatures are given.
−2
3.1.6 flexural strength, σ [FL ],n—a measure of the
Furthermore, this test method may establish the influences of
f
ultimate strength of a specified beam specimen in bending
processing variables and composition on slow crack growth as
determined at a given stress rate in a particular environment.
well as on strength behavior of newly developed or existing
materials, thus allowing tailoring and optimizing material
3.1.7 flexural strength-stress rate diagram—a plot of flex-
processing for further modification. In summary, this test
ural strength as a function of stress rate. Flexural strength and
methodmaybeusedformaterialdevelopment,qualitycontrol,
stress rate are both plotted on logarithmic scales.
characterization, and limited design data generation purposes.
3.1.8 flexural strength-stress rate curve—a curve fitted to
NOTE 3—Data generated by this test method do not necessarily
the values of flexural strength at each of several stress rates,
correspond to crack velocities that may be encountered in service
based on the relationship between flexural strength and stress
conditions. The use of data generated by this test method for design
rate:
purposes may entail considerable extrapolation and loss of accuracy.
log σ = [1/(n + 1)] log σ˙ + log D (see Appendix X1)
f
4.2 In this test method, the flexural stress computation is
3.1.8.1 Discussion—In the ceramics literature, this is often
based on simple beam theory, with the assumptions that the
called a “dynamic fatigue” curve.
materialisisotropicandhomogeneous,themoduliofelasticity
−3/2
3.1.9 fracturetoughness,K [FL ],n—agenerictermfor
IC in tension and compression are identical, and the material is
measures of resistance to extension of a crack. (E616)
linearly elastic. The average grain size should be no greater
−2
than one fiftieth (1/50) of the beam thickness.
3.1.10 inert flexural strength [FL ],n—a measure of the
strength of a specified beam specimen in bending as deter-
4.3 In this test method, the test specimen sizes and test
minedinanappropriateinertconditionwherebynoslowcrack
fixtures were chosen in accordance with Test Method C1211,
growth occurs.
which provides a balance between practical configurations and
3.1.10.1 Discussion—An inert condition at near room tem-
resultingerrors,asdiscussedinRefs (5, 6).Onlythefour-point
perature may be obtained by using vacuum, low temperatures,
test configuration is used in this test method.
very fast test rates, or any inert media. However, at elevated
4.4 Inthistestmethod,theslowcrackgrowthparameters(n
temperatures, the definition or concept of an inert condition is
and D) are determined based on the mathematical relationship
unclear since temperature itself acts as a degrading environ-
between flexural strength and applied stress rate, log σ = [1/(n
f
ment. It has been shown that for some ceramics one approach
+ 1)] log σ˙ + log D, together with the measured experimental
to obtain an inert condition (thus, inert strength) at elevated
data.Thebasicunderlyingassumptiononthederivationofthis
temperatures is to use very fast (ultra-fast) test rates ≥3×10
relationship is that slow crack growth is governed by an
MPa/s, where the time for slow crack growth would be n
empirical power-law crack velocity, v= A[K /K ] (see
I IC
minimized or eliminated (4) .
Appendix X1).
3.1.11 slow crack growth (SCG),n—subcritical crack
NOTE 4—There are various other forms of crack velocity laws which
growth (extension) which may result from, but is not restricted
are usually more complex or less convenient mathematically, or both, but
to, such mechanisms as environmentally assisted stress corro-
may be physically more realistic (7) . The mathematical analysis in this
sion or diffusive crack growth. test method does not cover such alternative crack velocity formulations.
C1465 − 08 (2013)
4.5 In this test method, the mathematical relationship be- as accumulated tensile damage in flexure (9). This, depending
tweenflexuralstrengthandstressratewasderivedbasedonthe on the degree of nonlinearity, may limit the applicability of
assumption that the slow crack growth parameter is at least n
linear elastic fracture mechanics (LEFM), since the resulting
≥ 5 (1, 8). Therefore, if a material exhibits a very high relationship between strength and stress rate derived under
susceptibility to slow crack growth, that is, n < 5, special care
constant stress-rate testing condition is based on an LEFM
should be taken when interpreting the results.
approach with negligible creep (creep strain less than 0.1%).
Therefore, creep should be kept as minimal as possible, as
4.6 The mathematical analysis of test results according to
compared to the total strain at failure (see 8.11.2).
the method in 4.4 assumes that the material displays no rising
R-curve behavior, that is, no increasing fracture resistance (or
5.3 Depending on the degree of SCG susceptibility of a
crack-extension resistance) with increasing crack length. It
material, the linear relationship between log (flexural strength)
should be noted that the existence of such behavior cannot be
and log (applied stress rate) (see Appendix X1) may start to
determinedfromthistestmethod.Theanalysisfurtherassumes
deviateatacertainhighstressrate,atwhichslowcrackgrowth
that the same flaw types control strength over the entire test
diminishes or is minimized due to the extremely short test
range. That is, no new flaws are created, and the flaws that
duration. Strengths obtained at higher stress rates (>1000
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.
nature. Therefore, slow crack growth that is determined from
In these cases, excessive creep damage in the form of creep
the flexural strengths of a ceramic material is also a probabi-
cavities,micro-ormacro-cracks,orboth,developinthetensile
listic phenomenon. Hence, a proper range and number of test
surface (10-13). This results in a nonlinearity in the relation-
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
rate), see Fig. 1b. It has been reported that the strength
design (2). Guidance is provided in this test method.
degradation with respect to the expected normal strength (at
NOTE 5—For a given ceramic material/environment system, the SCG
Point N in Fig. 1b) ranged from 15 to 50% (10-12). If these
parameter nisindependentofspecimensizealthoughitsreproducibilityis
data points are used in the analysis, then an underestimate of
dependent on the variables previously mentioned. By contrast, the SCG
the SCG parameters will be obtained. Hence, the strength data
parameter D depends significantly on strength, and thus on specimen size
(see Eq X1.7).
exhibiting such a significant strength degradation occurring at
lower stress rates shall be excluded as data points in obtai
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