ASTM C1368-10(2017)
(Test Method)Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
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 SCG. This test method provides the empirical parameters for appraising the relative SCG susceptibility of ceramic materials under specified environments. Furthermore, this test method may establish the influences of processing variables and composition on SCG 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. The conventional analysis of constant stress-rate testing is based on a number of critical assumptions, the most important of which are listed in the next paragraphs.
4.2 The flexural stress computation for the rectangular beam test specimens or the equibiaxial disk flexure test specimens 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 of the beam thickness.
4.3 The test specimen sizes and fixtures for rectangular beam test specimens should be in accordance with Test Method C1161, which provides a balance between practical configurations and resulting errors, as discussed in Refs (4, 5). Only four-point test configuration is allowed in this test method for rectangular beam specimens. Three-point test configurations are not permitted. The test specimen sizes and fixtures for disk test specimens tested in ring-on-ring flexure should be chosen in accordance with Test Method C1499. The test specimens for direct tension strength testing should be chosen in accordance with Test Method C1273.
4.4 The SCG parameters (n and D) are determined by fitting the measured ...
SCOPE
1.1 This test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant stress-rate rectangular beam flexural testing, or ring-on-ring biaxial disk flexural testing, or direct tensile strength, in which strength is determined as a function of applied stress rate in a given environment at ambient temperature. 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 (1-3)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 (See Terminology E1823).
1.2 Values expressed in this test method are in accordance with the International System of Units (SI) and IEEE/ASTM SI 10.
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: C1368 − 10 (Reapproved 2017)
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress-Rate Strength
Testing at Ambient Temperature
This standard is issued under the fixed designation C1368; 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* 2. Referenced Documents
1.1 This test method covers the determination of slow crack 2.1 ASTM Standards:
growth (SCG) parameters of advanced ceramics by using C1145 Terminology of Advanced Ceramics
constant stress-rate rectangular beam flexural testing, or ring- C1161 Test Method for Flexural Strength of Advanced
on-ring biaxial disk flexural testing, or direct tensile strength, Ceramics at Ambient Temperature
in which strength is determined as a function of applied stress C1239 Practice for Reporting Uniaxial Strength Data and
rate in a given environment at ambient temperature. The Estimating Weibull Distribution Parameters forAdvanced
strength degradation exhibited with decreasing applied stress Ceramics
rate in a specified environment is the basis of this test method C1273 Test Method for Tensile Strength of Monolithic
which enables the evaluation of slow crack growth parameters Advanced Ceramics at Ambient Temperatures
of a material. C1322 Practice for Fractography and Characterization of
Fracture Origins in Advanced Ceramics
NOTE 1—This test method is frequently referred to as “dynamic
C1499 Test Method for Monotonic Equibiaxial Flexural
fatigue” testing (1-3) in which the term “fatigue” is used interchangeably
Strength of Advanced Ceramics at Ambient Temperature
with the term “slow crack growth.” To avoid possible confusion with the
“fatigue”phenomenonofamaterialwhichoccursexclusivelyundercyclic
E4 Practices for Force Verification of Testing Machines
loading, as defined in Terminology E1823, this test method uses the term
E6 Terminology Relating to Methods of Mechanical Testing
“constant stress-rate testing” rather than “dynamic fatigue” testing.
E337 Test Method for Measuring Humidity with a Psy-
NOTE 2—In glass and ceramics technology, static tests of considerable
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
duration are called “static fatigue” tests, a type of test designated as
peratures)
stress-rupture (See Terminology E1823).
E1823 TerminologyRelatingtoFatigueandFractureTesting
1.2 Values expressed in this test method are in accordance
IEEE/ASTM SI 10 American National Standard for Use of
withtheInternationalSystemofUnits(SI)andIEEE/ASTMSI
theInternationalSystemofUnits(SI):TheModernMetric
10.
System
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
3.1 Definitions:
priate safety and health practices and determine the applica-
3.1.1 The terms described in Terminologies C1145, E6, and
bility of regulatory limitations prior to use.
E1823 are applicable to this test method. Specific terms
relevant to this test method are as follows:
3.1.2 advanced ceramic, n—a highly engineered, high-
This test method is under the jurisdiction of ASTM Committee C28 on performance, predominately nonmetallic, inorganic, ceramic
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
material having specific functional attributes. (C1145)
Mechanical Properties and Performance.
Current edition approved Feb. 1, 2017. Published February 2017. Originally
approved in 1997. Last previous edition approved in 2010 as C1368 – 10. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/C1368-10R17. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
The boldface numbers in parentheses refer to the list of references at the end of Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1368 − 10 (2017)
3.1.3 constant stress rate,σ˙, n—a constant rate of maximum 4. Significance and Use
stress applied to a specified beam by using either a constant
4.1 For many structural ceramic components in service,
loading or constant displacement rate of a testing machine.
their use is often limited by lifetimes that are controlled by a
process of SCG. This test method provides the empirical
3.1.4 environment, n—the aggregate of chemical species
parameters for appraising the relative SCG susceptibility of
and energy that surrounds a test specimen. (E1823)
ceramic materials under specified environments. Furthermore,
3.1.5 environmental chamber, n—the container of bulk vol-
this test method may establish the influences of processing
ume surrounding a test specimen. (E1823)
variables and composition on SCG as well as on strength
3.1.6 equibiaxial flexural strength [F/L ], n—the maximum
behavior of newly developed or existing materials, thus allow-
stress that a material is capable of sustaining when subjected to
ing tailoring and optimizing material processing for further
flexure between two concentric rings.
modification. In summary, this test method may be used for
material development, quality control, characterization, and
3.1.6.1 Discussion—This mode of flexure is a cupping of
limited design data generation purposes. The conventional
the circular plate caused by loading at the inner load ring and
analysis of constant stress-rate testing is based on a number of
outer support ring. The equibiaxial flexural strength is calcu-
critical assumptions, the most important of which are listed in
lated from the maximum-load of a biaxial test carried to
the next paragraphs.
rupture, the original dimensions of the test specimen, and
Poisson’s ratio. (C1499)
4.2 Theflexuralstresscomputationfortherectangularbeam
test specimens or the equibiaxial disk flexure test specimens is
3.1.7 flexural strength, σ,n—a measure of the strength of a
f
based on simple beam theory, with the assumptions that the
specified beam specimen in bending determined at a given
material is isotropic and homogeneous, the moduli of elasticity
stress rate in a particular environment.
in tension and compression are identical, and the material is
3.1.8 fracture toughness, n—a generic term for measures of
linearly elastic. The average grain size should be no greater
resistance to extension of a crack. (E1823)
than one-fiftieth of the beam thickness.
3.1.9 inert strength, n—a measure of the strength of a
4.3 The test specimen sizes and fixtures for rectangular
specified strength test specimen as determined in an appropri-
beamtestspecimensshouldbeinaccordancewithTestMethod
ate inert condition whereby no slow crack growth occurs.
C1161, which provides a balance between practical configura-
3.1.9.1 Discussion—An inert condition may be obtained by
tions and resulting errors, as discussed in Refs (4, 5). Only
using vacuum, low temperatures, very fast test rates, or any
four-point test configuration is allowed in this test method for
inert mediums.
rectangular beam specimens. Three-point test configurations
are not permitted. The test specimen sizes and fixtures for disk
3.1.10 slow crack growth (SCG), n—subcritical crack
test specimens tested in ring-on-ring flexure should be chosen
growth (extension) which may result from, but is not restricted
in accordance withTest Method C1499.The test specimens for
to, such mechanisms as environmentally assisted stress corro-
direct tension strength testing should be chosen in accordance
sion or diffusive crack growth.
with Test Method C1273.
3.1.11 strength-stress rate curve, n—a curve fitted to the
4.4 The SCG parameters (n and D) are determined by fitting
values of strength at each of several stress rates, based on the
the measured experimental data to a mathematical relationship
relationship between strength and stress rate: logσ = 1/(n+1)
f
between strength and applied stress rate, logσ = 1/(n+1) logσ˙
f
log σ˙ + log D. (See Appendix X1.)
+ log D. The basic underlying assumption on the derivation of
3.1.11.1 Discussion—In the ceramics literature, this is often
this relationship is that SCG is governed by an empirical
called a dynamic fatigue curve.
n
power-law crack velocity,v=A[K /K ] (see Appendix X1).
I IC
3.1.12 strength-stress rate diagram, n—a plot of strength
NOTE 3—There are various other forms of crack velocity laws which
against stress rate. Both strength and stress rate are plotted on
are usually more complex or less convenient mathematically, or both, but
log-log scales.
may be physically more realistic (6). It is generally accepted that actual
data cannot reliably distinguish between the various formulations.
3.1.13 stress intensity factor, K,n—the magnitude of the
I
Therefore, the mathematical analysis in this test method does not cover
ideal-crack-tip stress field (stress-field singularity) subjected to
such alternative crack velocity formulations.
mode I loading in a homogeneous, linear elastic body. (E1823)
4.5 The mathematical relationship between strength and
3.1.14 tensile strength [F/L ], n—S —the maximum tensile stress rate was derived based on the assumption that the slow
2 u
stress which a material is capable of sustaining. crack growth parameter is at least n ≥ 5 (1, 7, 8). Therefore, if
a material exhibits a very high susceptibility to SCG, that is, n
3.1.14.1 Discussion—Tensile strength is calculated from the
< 5, special care should be taken when interpreting the results.
maximum force during a tension test carried to rupture and the
original cross-sectional area of the specimen. (C1273)
4.6 The mathematical analysis of test results in accordance
with the method in 4.4 assumes that the material displays no
3.2 Definitions of Terms Specific to This Standard:
rising R-curvebehavior.Itshouldbenotedthattheexistenceof
3.2.1 slow crack growth parameters, n and D, n—the
such behavior cannot be determined from this test method.
parametersestimatedasconstantsintheflexuralstrength-stress
rate equation, which represent the degree of slow crack growth 4.7 Slow crack growth behavior of ceramic materials ex-
susceptibility of a material. (See Appendix X1.) posedtostress-corrosivegasesorliquidenvironmentscanvary
C1368 − 10 (2017)
as a function of mechanical, material, and electrochemical 5.2 Depending on the degree of SCG susceptibility of a
variables. Therefore, it is essential that test results accurately material, the linear relationship between log (strength) and log
reflect the effects of specific variables under study. Only then (applied stress rate) (see Appendix X1) may start to deviate at
can data be compared from one investigation to another on a a certain high-stress rate at which slow crack growth dimin-
valid basis or serve as a valid basis for characterizing materials ishes or is minimized due to the extremely short test duration.
and assessing structural behavior. Strengths obtained at higher stress rates (>2000 MPa/s) may
remain unchanged so that a plateau is observed in the plot of
4.8 The strength of advanced ceramics is probabilistic in
strength-versus-stress rate (7). If the strength data determined
nature.Therefore,SCGthatisdeterminedfromthestrengthsof
in this plateau region are included in the analysis, a misleading
a ceramic material is also a probabilistic phenomenon. Hence,
estimate of the SCG parameters will be obtained. Therefore,
a proper range and number of applied stress rates in conjunc-
the strength data in the plateau shall be excluded as data points
tion with an appropriate number of specimens at each applied
in estimating the SCG parameters of the material. This test
stress rate are required for statistical reproducibility and design
method addresses for this factor by recommending that the
(2). Guidelines are provided in this test method.
highest stress rate be ≤2000 MPa/s.
NOTE 4—For a given ceramic material/environment system, the SCG
NOTE 5—The strength plateau of a material can be checked by
parameter n is constant regardless of specimen size although its repro-
measuring an inert strength in an appropriate inert medium.
ducibility is dependent on the variables mentioned in 4.8. By contrast, the
NOTE 6—When testing in environments with less than 100 % concen-
SCG parameter D depends significantly on strength and thus on specimen
tration of the corrosive medium (for example, air), the use of stress rates
size (see Eq X1.6 in Appendix X1).
greater than ~1 MPa/s can result in significant errors in the slow crack
4.9 The strength of a ceramic material for a given specimen growth parameters due to averaging of the regions of the slow crack
growth curve (9). Such errors can be avoided by testing in 100%
and test fixture configuration is dependent on its inherent
concentration of the corrosive medium (for example, in water instead of
resistance to fracture, the presence of flaws, and environmental
humid air). For the case of 100 % concentration of the corrosive medium,
effects. Analysis of a fracture surface, fractography, though
stress rates as large as ~2000 MPa/s may be acceptable.
beyond the scope of this test method, is highly recommended
5.3 Surface preparation of test specimens can introduce
for all purposes, especially to verify the mechanism(s) associ-
fabrication flaws which may have pronounced effects on SCG
ated with failure (refer to Practice C1322).
behavior. Machining damage imposed during specimen prepa-
4.10 The conventional analysis of constant stress-rate test-
ration can be either a random interfering factor or an inherent
ing is based on a critical assumption that stress is uniform
part of the strength characteristics to be measured. Surface
throughout the test piece.This is most easily achieved in direct
preparation can also lead to residual stress. Universal or
tension test specimens. Only test specimens that fracture in the
standardizedtestmethodsofsurfacepreparationdonotexist.It
inner gauge section in four-point testing should be used.
should be understood that the final machining steps may or
Three-point flexure shall not be used. Breakages between the
may not negate machining damage introduced during the early
outer and inner fixture contact points should be discounted.
coarse or intermediate machining steps. In some cases, speci-
The same requirement applies to biaxial disk strength testing.
mens need to be tested in the as-
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM 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: C1368 − 10 C1368 − 10 (Reapproved 2017)
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress-Rate Strength
Testing at Ambient Temperature
This standard is issued under the fixed designation C1368; 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 test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant
stress-rate rectangular beam flexural testing, or ring-on-ring biaxial disk flexural testing, or direct tensile strength, in which strength
is determined as a function of applied stress rate in a given environment at ambient temperature. 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 (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 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
(See Terminology E1823).
1.2 Values expressed in this test method are in accordance with the International System of Units (SI) and IEEE/ASTM SI 10.
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:
C1145 Terminology of Advanced Ceramics
C1161 Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
C1239 Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
C1273 Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures
C1322 Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
C1499 Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
E1823 Terminology Relating to Fatigue and Fracture Testing
IEEE/ASTM SI 10 American National Standard for Use of the International System of Units (SI): The Modern Metric System
3. Terminology
3.1 Definitions:
3.1.1 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 Dec. 1, 2010Feb. 1, 2017. Published January 2011February 2017. Originally approved in 1997. Last previous edition approved in 20062010 as
C1368 – 06.C1368 – 10. DOI: 10.1520/C1368-10.10.1520/C1368-10R17.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1368 − 10 (2017)
3.1.2 advanced ceramic, n—a highly engineered, high-performance, predominately nonmetallic, inorganic, ceramic material
having specific functional attributes. (C1145)
3.1.3 constant stress rate,σ˙, n—a constant rate of maximum stress applied to a specified beam by using either a constant loading
or constant displacement rate of a testing machine.
3.1.4 environment, n—the aggregate of chemical species and energy that surrounds a test specimen. (E1823)
3.1.5 environmental chamber, n—the container of bulk volume surrounding a test specimen. (E1823)
3.1.6 equibiaxial flexural strength [F/L ], n—the maximum stress that a material is capable of sustaining when subjected to
flexure between two concentric rings.
3.1.6.1 Discussion—
This mode of flexure is a cupping of the circular plate caused by loading at the inner load ring and outer support ring. The
equibiaxial flexural strength is calculated from the maximum-load of a biaxial test carried to rupture, the original dimensions of
the test specimen, and Poisson’s ratio. (C1499)
3.1.7 flexural strength, σ , n—a measure of the strength of a specified beam specimen in bending determined at a given stress
f
rate in a particular environment.
3.1.8 fracture toughness, n—a generic term for measures of resistance to extension of a crack. (E1823)
3.1.9 inert strength, n—a measure of the strength of a specified strength test specimen as determined in an appropriate inert
condition whereby no slow crack growth occurs.
3.1.9.1 Discussion—
An inert condition may be obtained by using vacuum, low temperatures, very fast test rates, or any inert mediums.
3.1.10 slow crack growth (SCG), n—subcritical crack growth (extension) which may result from, but is not restricted to, such
mechanisms as environmentally-assisted environmentally assisted stress corrosion or diffusive crack growth.
3.1.11 strength-stress rate curve, n—a curve fitted to the values of strength at each of several stress rates, based on the
relationship between strength and stress rate: log σ = 1/(n + 1) log σ˙ + log D. (See Appendix X1.)
f
3.1.11.1 Discussion—
In the ceramics literature, this is often called a dynamic fatigue curve.
3.1.12 strength-stress rate diagram, n—a plot of strength against stress rate. Both strength and stress rate are plotted on log-log
scales.
3.1.13 stress intensity factor, K , n—the magnitude of the ideal-crack-tip stress field (stress-field singularity) subjected to mode
I
I loading in a homogeneous, linear elastic body. (E1823)
3.1.14 tensile strength [F/L ], n—S —the maximum tensile stress which a material is capable of sustaining.
2 u
3.1.14.1 Discussion—
Tensile strength is calculated from the maximum force during a tension test carried to rupture and the original cross-sectional area
of the specimen. (C1273)
3.2 Definitions of Terms Specific to This Standard:
3.2.1 slow crack growth parameters, n and D, n—the parameters estimated as constants in the flexural strength-stress rate
equation, which represent the degree of slow crack growth susceptibility of a material. (See Appendix X1.)
4. 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 SCG. This test method provides the empirical parameters for appraising the relative SCG susceptibility of ceramic materials
under specified environments. Furthermore, this test method may establish the influences of processing variables and composition
on SCG 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. The conventional analysis of constant stress-rate testing is based on
a number of critical assumptions, the most important of which are listed in the next paragraphs.
C1368 − 10 (2017)
4.2 The flexural stress computation for the rectangular beam test specimens or the equibiaxial disk flexure test specimens 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 one-fiftieth of the beam thickness.
4.3 The test specimen sizes and fixtures for rectangular beam test specimens should be in accordance with Test Method C1161,
which provides a balance between practical configurations and resulting errors, as discussed in Refs (4, 5). Only four-point test
configuration is allowed in this test method for rectangular beam specimens. Three-point test configurations are not permitted. The
test specimen sizes and fixtures for disk test specimens tested in ring-on-ring flexure should be chosen in accordance with Test
Method C1499. The test specimens for direct tension strength testing should be chosen in accordance with Test Method C1273.
4.4 The SCG parameters (n and D) are determined by fitting the measured experimental data to a mathematical relationship
between strength and applied stress rate, log σ = 1/(n+1) log σ˙ + log D. The basic underlying assumption on the derivation of
f
n
this relationship is that SCG is governed by an empirical power-law crack velocity, v = A[K /K ] (see Appendix X1).
I IC
NOTE 3—There are various other forms of crack velocity laws which are usually more complex or less convenient mathematically, or both, but may
be physically more realistic (Ref (6)).). It is generally accepted that actual data cannot reliably distinguish between the various formulations. Therefore,
the mathematical analysis in this test method does not cover such alternative crack velocity formulations.
4.5 The mathematical relationship between strength and stress rate was derived based on the assumption that the slow crack
growth parameter is at least n ≥ 5 (Refs (1, 7, 8)).). Therefore, if a material exhibits a very high susceptibility to SCG, that is, n
< 5, special care should be taken when interpreting the results.
4.6 The mathematical analysis of test results in accordance with the method in 4.4 assumes that the material displays no rising
R-curve behavior. It should be noted that the existence of such behavior cannot be determined from this test method.
4.7 Slow crack growth behavior of ceramic materials exposed to stress-corrosive gases or liquid environments can vary as a
function of mechanical, material, and electrochemical 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 The strength of advanced ceramics is probabilistic in nature. Therefore, SCG that is determined from the strengths of a
ceramic material is also a probabilistic phenomenon. Hence, a proper range and number of applied stress rates in conjunction with
an appropriate number of specimens at each applied stress rate are required for statistical reproducibility and design (Ref (2)).).
Guidelines are provided in this test method.
NOTE 4—For a given ceramic material/environment system, the SCG parameter n is constant regardless of specimen size although its reproducibility
is dependent on the variables mentioned in 4.8. By contrast, the SCG parameter D depends significantly on strength and thus on specimen size (see Eq
X1.6 in Appendix X1).
4.9 The strength of a ceramic material for a given specimen and test fixture configuration is dependent on its inherent resistance
to fracture, the presence of flaws, and environmental effects. Analysis of a fracture surface, fractography, though beyond the scope
of this test method, is highly recommended for all purposes, especially to verify the mechanism(s) associated with failure (refer
to Practice C1322).
4.10 The conventional analysis of constant stress-rate testing is based on a critical assumption that stress is uniform throughout
the test piece. This is most easily achieved in direct tension test specimens. Only test specimens that fracture in the inner gauge
section in four-point testing should be used. Three-point flexure shall not be used. Breakages between the outer and inner fixture
contact points should be discounted. The same requirement applies to biaxial disk strength testing. Only fractures which occur in
the inner loading circle should be used. Furthermore, it is assumed that the fracture origins are near to the tensile surface and do
not grow very large relative to the thickness of rectangular beam flexure or disk strength test specimens.
4.11 The conventional analysis of constant stress-rate testing is also based on a critical assumption that the same type flaw
controls strength in all specimens at all loading rates. If the flaw distribution is multimodal, then the conventional analysis in this
standard may produce erroneous slow crack growth parameter estimates.
5. Interferences
5.1 SCG may be the product of both mechanical and chemical driving forces. The chemical driving force for a given material
with given flaw configurations can strongly vary with the composition, pH, and temperature of a test environment. Note that SCG
testing is very time-consuming: it may take several weeks to complete testing a typical, advanced ceramic. Because of this long
test time, the chemical variables of the test environment 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 Depending on the degree of SCG susceptibility of a material, the linear relationship between log (strength) and log (applied
stress rate) (see Appendix X1) may start to deviate at a certain high stress high-stress rate at which slow crack growth diminishes
or is minimized due to the extremely short test duration. Strengths obtained at higher stress rates (>2000 MPa/s)
...
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