ASTM C1576-05(2010)
(Test Method)Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Flexural Testing (Stress Rupture) at Ambient Temperature
Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Flexural Testing (Stress Rupture) at Ambient Temperature
SIGNIFICANCE AND USE
The service life of many structural ceramic components is often limited by the subcritical growth of cracks. This test method provides an approach for appraising the relative slow crack growth susceptibility of ceramic materials under specified environments at ambient temperature. 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, design code or model verification, and limited design data generation purposes.
Note 4—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, depending on the range and magnitude of applied stresses used, may entail extrapolation and uncertainty.
This test method is related to Test Method C1368 (“constant stress-rate flexural testing”), however, C1368 uses constant stress rates to determine corresponding flexural strengths whereas this test method employs constant stress to determine corresponding times to failure. In general, the data generated by this test method may be more representative of actual service conditions as compared with those by constant stress-rate testing. However, in terms of test time, constant stress testing is inherently and significantly more time- consuming than constant stress rate testing.
The flexural stress computation in this test method is based on simple elastic 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 grain size should be no greater than one fiftieth (1/50) of the beam depth as measure...
SCOPE
1.1 This standard test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant stress flexural testing in which time to failure of flexure test specimens is determined in four-point flexure as a function of constant applied stress in a given environment at ambient temperature. In addition, test specimen fabrication methods, test stress levels, data collection and analysis, and reporting procedures are addressed. The decrease in time to failure with increasing applied stress in a specified environment is the basis of this test method that enables the evaluation of slow crack growth parameters of a material. The preferred analysis in the present method is based on a power law relationship between crack velocity and applied stress intensity; alternative analysis approaches are also discussed for situations where the power law relationship is not applicable.
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Designation: C1576 − 05 (Reapproved 2010)
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress Flexural Testing
(Stress Rupture) at Ambient Temperature
This standard is issued under the fixed designation C1576; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.3 This test method is intended for use with various test
environments such as air, other gaseous environments and
1.1 This standard test method covers the determination of
liquids.
slow crack growth (SCG) parameters of advanced ceramics by
1.4 The values stated in SI units are to be regarded as the
using constant stress flexural testing in which time to failure of
standard and in accordance with IEEE/ASTM SI 10 Standard.
flexure test specimens is determined in four-point flexure as a
1.5 This test method may involve hazardous materials,
function of constant applied stress in a given environment at
ambient temperature. In addition, test specimen fabrication operations, and equipment. This standard does not purport to
address all of the safety concerns associated with its use. It is
methods, test stress levels, data collection and analysis, and
the responsibility of the user of this standard to establish
reporting procedures are addressed. The decrease in time to
appropriate safety and health practices and determine the
failure with increasing applied stress in a specified environ-
applicability of regulatory limitations prior to use.
ment is the basis of this test method that enables the evaluation
of slow crack growth parameters of a material. The preferred
2. Referenced Documents
analysis in the present method is based on a power law
2.1 ASTM Standards:
relationship between crack velocity and applied stress inten-
C1145 Terminology of Advanced Ceramics
sity; alternative analysis approaches are also discussed for
C1161 Test Method for Flexural Strength of Advanced
situations where the power law relationship is not applicable.
Ceramics at Ambient Temperature
NOTE 1—The test method in this standard is frequently referred to as
C1322 Practice for Fractography and Characterization of
“static fatigue” or stress-rupture testing Ref (1-3) in which the term
Fracture Origins in Advanced Ceramics
“fatigue” is used interchangeably with the term “slow crack growth.” To
C1368 Test Method for Determination of Slow Crack
avoidpossibleconfusionwiththe“fatigue”phenomenonofamaterialthat
Growth Parameters of Advanced Ceramics by Constant
occursexclusivelyundercyclicloading,asdefinedinTerminologyE1823,
Stress-Rate Strength Testing at Ambient Temperature
this test method uses the term “ constant stress testing” rather than “static
fatigue” testing.
C1465 Test Method for Determination of Slow Crack
Growth Parameters of Advanced Ceramics by Constant
1.2 This test method applies primarily to monolithic ad-
Stress-Rate Flexural Testing at Elevated Temperatures
vanced ceramics that are macroscopically homogeneous and
E4 Practices for Force Verification of Testing Machines
isotropic. This test method may also be applied to certain
E6 Terminology Relating to Methods of Mechanical Testing
whisker- or particle-reinforced ceramics as well as certain
E112 Test Methods for Determining Average Grain Size
discontinuous fiber-reinforced composite ceramics that exhibit
E337 Test Method for Measuring Humidity with a Psy-
macroscopicallyhomogeneousbehavior.Generally,continuous
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
fiber ceramic composites do not exhibit macroscopically
peratures)
isotropic, homogeneous, continuous behavior, and the applica-
E399 Test Method for Linear-Elastic Plane-Strain Fracture
tion of this test method to these materials is not recommended.
Toughness K of Metallic Materials
Ic
E1823 TerminologyRelatingtoFatigueandFractureTesting
1 3. Terminology
This practice is under the jurisdiction of ASTM Committee C28 on Advanced
Ceramics and is the direct responsibility of Subcommittee C28.01 on Mechanical
3.1 Definitions:
Properties and Performance.
Current edition approved June 1, 2010. Published November 2010. Originally
approved in 2005. Last previous edition approved in 2005 as C1576 – 05. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/C1576-05R10. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
The boldface numbers in parentheses refer to a list of references at the end of 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
C1576 − 05 (2010)
3.1.1 ThetermsdescribedinTerminologyC1145,Terminol- 3.1.16 time to failure, t [t], n—total elapsed time from test
f
ogy E6, and Terminology E1823 are applicable to this test initiation to test specimen failure.
standard. Specific terms relevant to this test method are as
follows: 4. Significance and Use
3.1.2 advanced ceramic, n—a highly engineered, high
4.1 The service life of many structural ceramic components
performance, predominately non-metallic, inorganic, ceramic
is often limited by the subcritical growth of cracks. This test
material having specific functional attributes. C1145
method provides an approach for appraising the relative slow
-2
3.1.3 constant applied stress, σ[FL ], n—a constant maxi- crack growth susceptibility of ceramic materials under speci-
mum flexural stress applied to a specified beam test specimen fied environments at ambient temperature. Furthermore, this
by using a constant static force with a test machine or a test test method may establish the influences of processing vari-
fixture. ables and composition on slow crack growth as well as on
strength behavior of newly developed or existing materials,
3.1.4 ‘constant applied stress-time to failure’ diagram—a
thus allowing tailoring and optimizing material processing for
plot of constant applied stress against time to failure. Constant
furthermodification.Insummary,thistestmethodmaybeused
applied stress and time to failure are both plotted on logarith-
for material development, quality control, characterization,
mic scales.
design code or model verification, and limited design data
3.1.5 ‘constantappliedstress-timetofailure’curve—acurve
generation purposes.
fitted to the values of time to failure at each of several applied
NOTE 4—Data generated by this test method do not necessarily
stresses.
correspond to crack velocities that may be encountered in service
conditions. The use of data generated by this test method for design
NOTE 2—In the ceramics literature, this is often called a “static fatigue”
purposes, depending on the range and magnitude of applied stresses used,
curve.
may entail extrapolation and uncertainty.
3.1.6 testenvironment,n—theaggregateofchemicalspecies
4.2 This test method is related to Test Method C1368
and energy that surrounds a test specimen. E1823
(“constant stress-rate flexural testing”), however, C1368 uses
3.1.7 test environmental chamber, n—a container surround-
constant stress rates to determine corresponding flexural
ing the test specimen that is capable of providing controlled
strengths whereas this test method employs constant stress to
local environmental condition . C1368, C1465
determine corresponding times to failure. In general, the data
-2
3.1.8 flexural strength, σ [FL ], n—a measure of the
generated by this test method may be more representative of
f
ultimate strength of a specified beam test specimen in flexure actual service conditions as compared with those by constant
determined at a given stress rate in a particular environment.
stress-rate testing. However, in terms of test time, constant
stress testing is inherently and significantly more time- con-
3.1.9 fracturetoughness,(criticalstressintensityfactor)K
IC
-3/2
suming than constant stress rate testing.
[FL ], n—a generic term for measures of resistance to
extension of a crack. E1823, E399 4.3 The flexural stress computation in this test method is
-2 based on simple elastic beam theory, with the assumptions that
3.1.10 inert flexural strength [FL ], n—theflexuralstrength
the material is isotropic and homogeneous, the moduli of
of a specified beam as determined in an inert condition
elasticity in tension and compression are identical, and the
whereby no slow crack growth occurs.
material is linearly elastic. The grain size should be no greater
NOTE 3—An inert condition may be obtained by using vacuum, low
than one fiftieth (1/50) of the beam depth as measured by the
temperature,veryfasttestrate,oraninertenvironmentsuchassiliconeoil
mean linear intercept method (Test Methods E112). In cases
or high purity dry N .
where the material grain size is bimodal or the grain size
3.1.11 R-curve, n—a plot of crack-extension resistance as a
distribution is wide, the limit should apply to the larger grains.
function of stable crack extension. C1145
4.4 The test specimen sizes and test fixtures have been
3.1.12 run-out, n—a test specimen that does not fail before
selected in accordance with Test Methods C1161 and C1368,
a prescribed test time.
which provides a balance between practical configurations and
3.1.13 slow crack growth (SCG), n—subcritical crack resulting errors, as discussed in Ref (4,5).
growth (extension) which may result from, but is not restricted
4.5 The data are evaluated by regression of log applied
to, such mechanisms as environmentally-assisted stress corro-
stress vs. log time to failure to the experimental data. The
sion or diffusive crack growth. C1368, C1465
recommendation is to determine the slow crack growth param-
3.1.14 slow crack growth (SCG) parameters—the param- eters by applying the power law crack velocity function. For
derivation of this, and for alternative crack velocity functions,
eters estimated as constants in the log (time to failure) versus
log (constant applied stress), which represent a measure of see Appendix X1.
susceptibilitytoslowcrackgrowthofamaterial(seeAppendix
NOTE 5—Avariety of crack velocity functions exist in the literature.A
X1).
comparison of the functions for the prediction of long-term static fatigue
-3/2
data from short-term dynamic fatigue data [6] indicates that the exponen-
3.1.15 stress intensity factor, K [FL ,n—the magnitude
I
tial forms better predict the data than the power-law form. Further, the
of the ideal-crack-tip stress field stress field singularity) sub-
exponential form has a theoretical basis [7-10], however, the power law
jected to mode I loading in a homogeneous, linear elastic body.
form is simpler mathematically. Both have been shown to fit short-term
E1823 test data well.
C1576 − 05 (2010)
4.6 The approach used in this method assumes that the
material displays no rising R-curve behavior, that is, no
increasing fracture resistance (or crack-extension resistance)
with increasing crack length. The existence of such behavior
cannot be determined from this test method. The analysis
further assumes that the same flaw type controls all times-to-
failure.
4.7 Slow crack growth behavior of ceramic materials can
vary as a function of mechanical, material, thermal, and
environmental variables. Therefore, it is essential that test
results accurately reflect the effects of specific variables under
study. Only then can data be compared from one investigation
to another on a valid basis, or serve as a valid basis for
characterizing materials and assessing structural behavior.
4.8 Like strength, time to failure of advanced ceramics
subjected to slow crack growth is probabilistic in nature.
Therefore, slow crack growth that is determined from times to
failure under given constant applied stresses is also a probabi-
FIG. 1 Schematic Diagram Showing Unacceptable (Average) Data
listic phenomenon. The scatter in time to failure in constant
Points (With an “Open” Symbol) in the Plateau Region in Deter-
stress testing is much greater than the scatter in strength in mining Slow Crack Growth (SCG) Parameters
constant stress-rate (or any strength) testing Ref (1, 6-8), see
Appendix X2. Hence, a proper range and number of constant
the occurrence of a strength plateau observed at higher test
applied stresses, in conjunction with an appropriate number of
rates in constant stress-rate testing Ref (11). If the time-to-
test specimens, are required for statistical reproducibility and
failure data determined in this plateau region are included in
reliable design data generation Ref (1-3). This standard pro-
the analysis, a misleading estimate of the SCG parameters will
vides guidance in this regard.
beobtainedRef (12).Therefore,thestrengthdataintheplateau
4.9 The time to failure of a ceramic material for a given test
shall be excluded as data points in estimating the SCG
specimen and test fixture configuration is dependent on its
parametersofthematerial.Similarly,aplateaucanalsoexistat
inherent resistance to fracture, the presence of flaws, applied
the fatigue limit end of the curve, and these data points shall
stress, and environmental effects. Fractographic analysis to
also be excluded in estimating the SCG parameters.
verify the failure mechanisms has proven to be a valuable tool
NOTE 6—There are no simple guidelines in determining whether a
in the analysis of SCG data to verify that the same flaw type is
plateau region is reached, however with knowledge of the inert strength
dominant over the entire test range Ref (9 and 10), and it is to
and the fracture toughness of the test material, the slow crack growth rate
be used in this standard (refer to Practice C1322).
– applied stress intensity (v-K) curve may be determined. Evaluating this
will help determine where the experimental conditions fall.
5. Interferences
5.3 When testing a material exhibiting a high SCG resis-
5.1 Slow crack growth may be the product of both mechani-
tance (typically SCG parameter n > 70) an unrealistically large
calandchemicaldrivingforces.Thechemicaldrivingforcefor number of test specimens may be required in a small range of
a given material can vary strongly with the composition and
applied stresses since a significant number of test specimens
temperature of a test environment. Testing is conducted in may be expected to fail while loading. Furthermore, if lower
environments representative of service conditions so as to
stresses are to be used, unrealistically long test times are to be
evaluate material performance under use conditions. No
...
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:C1576–05 Designation:C1576–05 (Reapproved 2010)
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress Flexural Testing
(Stress Rupture) at Ambient Temperature
This standard is issued under the fixed designation C 1576; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This standard test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using
constant stress flexural testing in which time to failure of flexure test specimens is determined in four-point flexure as a function
ofconstantappliedstressinagivenenvironmentatambienttemperature.Inaddition,testspecimenfabricationmethods,teststress
levels, data collection and analysis, and reporting procedures are addressed.The decrease in time to failure with increasing applied
stress in a specified environment is the basis of this test method that enables the evaluation of slow crack growth parameters of
a material. The preferred analysis in the present method is based on a power law relationship between crack velocity and applied
stressintensity;alternativeanalysisapproachesarealsodiscussedforsituationswherethepowerlawrelationshipisnotapplicable.
NOTE 1—The test method in this test method standard is frequently referred to as “static fatigue” or stress-rupture testing (Refs (1-3))) 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 that
occurs exclusively under cyclic loading, as defined in Terminology E1823, this test method uses the term “ constant stress testing” rather than “static
fatiguetesting. ” testing.
1.2 This test method applies primarily to monolithic advanced ceramics that are macroscopically homogeneous and isotropic.
This test method may also be applied to certain whisker- or particle-reinforced ceramics as well as certain discontinuous
fiber-reinforced composite ceramics that exhibit macroscopically homogeneous behavior. Generally, continuous fiber ceramic
composites do not exhibit macroscopically isotropic, homogeneous, continuous behavior, and the application of this test method
to these materials is not recommended.
1.3 This test method is intended for use with various test environments such as air, other gaseous environments and liquids.
1.4 The values stated in SI units are to be regarded as the standard and in accordance with SI10-02 IEEE/ASTM SI 10 .
Standard.
1.5 This test method may involve hazardous materials, operations, and equipment. This standard does not purport to address
all of the safety concerns 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:
C1145 Terminology of Advanced Ceramics
C1161 Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
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
C1465 Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate
Flexural Testing at Elevated Temperatures
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing E112Test Methods for Determining Average Grain Size
This test method practice 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 June 1, 2005. Published June 2005 DOI: 10.1520/C1576-05.
Current edition approved June 1, 2010. Published November 2010. Originally approved in 2005. Last previous edition approved in 2005 as C1576 - 05. DOI:
10.1520/C1576-05R10.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C1576–05 (2010)
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness K of Metallic Materials
Ic
E1823 Terminology Relating to Fatigue and Fracture Testing SI10-02 IEEE/ASTM SI 10American National Standard for Use
of the International System of Units (SI): The Modern Metric
System
E112 Test Methods for Determining Average Grain Size
3. Terminology
3.1 Definitions: The terms described inTerminologiesTerminology C1145, ,Terminology E6, and , andTerminology E1823 are
applicable to this test method.standard. Specific terms relevant to this test method are as follows:
3.1.1 advanced ceramic, n—a highly engineered, high performance, predominately non-metallic, inorganic, ceramic material
having specific functional attributes. (C1145)
-2
3.1.2 constant applied stress, s[FL ], n—a constant maximum flexural stress applied to a specified beam test specimen by
using a constant static force with a test machine or a test fixture.
3.1.3 ‘constant applied stress-time to failure’diagram—aplotofconstantappliedstressagainsttimetofailure.Constantapplied
stress and time to failure are both plotted on logarithmic scales.
3.1.4 ‘constant applied stress-time to failure’ curve—a curve fitted to the values of time to failure at each of several applied
stresses.
NOTE 2—In the ceramics literature, this is often called a “static fatigue” curve.
3.1.5 test environment, n—the aggregate of chemical species and energy that surrounds a test specimen. specimen (E1823).
3.1.6 test environmental chamber, n—a container surrounding the test specimen that is capable of providing controlled local
environmental condition. condition (C1368, C1465).
-2
3.1.7 flexural strength, s [FL ], n—a measure of the ultimate strength of a specified beam test specimen in flexure determined
f
at a given stress rate in a particular environment.
-3/2
3.1.8 fracture toughness, (critical stress intensity factor) K [FL ], n—a generic term for measures of resistance to extension
IC
of a crack. crack (E1823, E399).
-2
3.1.9 inert flexural strength [FL ] , n—the flexural strength of a specified beam as determined in an inert condition whereby
no slow crack growth occurs.
NOTE 3—An inert condition may be obtained by using vacuum, low temperature, very fast test rate, or an inert environment such as silicone oil or high
purity dry N .
3.1.10 R-curve, n—a plot of crack-extension resistance as a function of stable crack extension. extension (C1145).
3.1.11 run-out, n—a test specimen that does not fail before a prescribed test time.
3.1.12 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. growth (C1368, C1465).
3.1.13 slow crack growth (SCG) parameters—the parameters estimated as constants in the log (time to failure) versus log
(constant applied stress), which represent a measure of susceptibility to slow crack growth of a material (see Appendix X1).
-3/2
3.1.14 stress intensity factor, K [FL ], n—the magnitude of the ideal-crack-tip stress field stress field singularity) subjected
I
to mode I loading in a homogeneous, linear elastic body. (E1823)
3.1.15 time to failure, t [t], n—total elapsed time from test initiation to test specimen failure.
f
4. Significance and Use
4.1 Theservicelifeofmanystructuralceramiccomponentsisoftenlimitedbythesubcriticalgrowthofcracks.Thistestmethod
provides an approach for appraising the relative slow crack growth susceptibility of ceramic materials under specified
environments at ambient temperature. Furthermore, this test method may establish the influences of processing variables and
compositiononslowcrackgrowthaswellasonstrengthbehaviorofnewlydevelopedorexistingmaterials,thusallowingtailoring
and optimizing material processing for further modification. In summary, this test method may be used for material development,
quality control, characterization, design code or model verification, and limited design data generation purposes.
NOTE 4—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, depending on the range and magnitude of applied stresses used, may entail extrapolation and
uncertainty.
4.2 This test method is related to Test Method C1368 (constant stress-rate flexural testing); however, Test Method (“constant
stress-rate flexural testing”), however, C1368 uses constant stress rates to determine corresponding flexural strengths whereas this
test method employs constant stress to determine corresponding times to failure. In general, the data generated by this test method
may be more representative of actual service conditions as compared with those by constant stress-rate testing. However, in terms
of test time, constant stress testing is inherently and significantly more time- consuming than constant stress rate testing.
4.3 The flexural stress computation in this test method is based on simple elastic beam theory, with the assumptions that the
materialisisotropicandhomogeneous,themoduliofelasticityintensionandcompressionareidentical,andthematerialislinearly
C1576–05 (2010)
elastic. The grain size should be no greater than one-fiftieth ( ⁄50) one fiftieth (1/50) of the beam depth as measured by the mean
linear intercept method (Test Methods (E112). In cases where the material grain size is bimodal or the grain size distribution is
wide, the limit should apply to the larger grains.
4.4 The test specimen sizes and test fixtures have been selected in accordance with Test Methods C1161 and C1368, which
provides a balance between practical configurations and resulting errors, as discussed in Refs. (4,5).
4.5 The data are evaluated by regression of log applied stress vs. log time to failure to the experimental data. The
recommendation is to determine the slow crack growth parameters by applying the power law crack velocity function. For
derivation of this, and for alternative crack velocity functions, see Appendix X1.
NOTE 5—Avariety of crack velocity functions exist in the literature.Acomparison of the functions for the prediction of long-term static fatigue data
from short-term dynamic fatigue data (6)[6] indicates that the exponential forms better predict the data than the power-law form. Further, the exponential
form has a theoretical basis (7-10);[7-10], however, the power law form is simpler mathematically. Both have been shown to fit short-term test data well.
4.6 The approach used in this test method assumes that the material displays no rising R-curve behavior, that is, no increasing
fracture resistance (or crack-extension resistance) with increasing crack length. The existence of such behavior cannot be
determined from this test method. The analysis further assumes that the same flaw type controls all times-to-failure.
4.7 Slowcrackgrowthbehaviorofceramicmaterialscanvaryasafunctionofmechanical,material,thermal,andenvironmental
variables.Therefore,itisessentialthattestresultsaccuratelyreflecttheeffectsofspecificvariablesunderstudy.Onlythencandata
be compared from one investigation to another on a valid basis, or serve as a valid basis for characterizing materials and assessing
structural behavior.
4.8 Likestrength,timetofailureofadvancedceramicssubjectedtoslowcrackgrowthisprobabilisticinnature.Therefore,slow
crackgrowththatisdeterminedfromtimestofailureundergivenconstantappliedstressesisalsoaprobabilisticphenomenon.The
scatterintimetofailureinconstantstresstestingismuchgreaterthanthescatterinstrengthinconstantstress-rate(oranystrength)
testing (Refs. (1, 11-13) (see 11-13)), see Appendix X2). Hence, a proper range and number of constant applied stresses, in
conjunction with an appropriate number of test specimens, are required for statistical reproducibility and reliable design data
generation (Ref. (1-3).)). This test method standard provides guidance in this regard.
4.9 The time to failure of a ceramic material for a given test specimen and test fixture configuration is dependent on its inherent
resistance to fracture, the presence of flaws, applied stress, and environmental effects. Fractographic analysis to verify the failure
mechanisms has proven to be a valuable tool in the analysis of SCG data to verify that the same flaw type is dominant over the
entire test range ((Refs. 14 ,and 15), and it is to be used in this test method (seestandard (refer to Practice C1322).
5. Interferences
5.1 Slow crack growth may be the product of both mechanical and chemical driving forces. The chemical driving force for a
givenmaterialcanvarystronglywiththecompositionandtemperatureofatestenvironment.Testingisconductedinenvironments
representative of service conditions so as to evaluate material performance under use conditions. Note that slow crack growth
testing, particularly constant stress testing, is very time-consuming. The overall test time is considerably greater in constant stress
testing than in constant stress-rate testing. Because of this longer test time, the chemical variables of the test environment must
be prevented from changing significantly throughout all test times. Inadequate control of these chemical variables may result in
inaccurate time-to-failure data, especially for materials that are more sensitive to the test environment.
5.2 Depending on the degree of SCG susceptibility of a material, the linear relationship between log (constant applied stress)
and log (time to failure) may start to deviate at a certain high applied stress where the crac
...
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