ASTM C1368-18

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, 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Dec-2017
• ICS: 81.060.30 - Advanced ceramics
• Technical Committee: C28 - Advanced Ceramics
• Drafting Committee: C28.01 - Mechanical Properties and Performance

Relations

• Replaces: ASTM C1368-10(2017) - Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
• Effective Date: 01-Jan-2018
• Refers: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Feb-2024
• Refers: ASTM E1823-24 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2024
• Refers: ASTM E1823-20 - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Feb-2020
• Refers: ASTM C1322-15(2019) - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1499-19 - Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature
• Effective Date: 01-Jul-2019
• Refers: ASTM C1145-19 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1273-18 - Standard Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures
• Effective Date: 01-Jul-2018
• Refers: ASTM C1239-13(2018) - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Jul-2018
• Refers: ASTM C1322-15 - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2015
• Refers: ASTM C1499-15 - Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature
• Effective Date: 01-Jul-2015
• Refers: ASTM C1273-15 - Standard Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures
• Effective Date: 01-Jul-2015
• Refers: ASTM E4-14 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2014
• Refers: ASTM C1499-09(2013) - Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature
• Effective Date: 01-Aug-2013
• Refers: ASTM C1161-13 - Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
• Effective Date: 01-Aug-2013

Overview

ASTM C1368-18: Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Rate Strength Testing at Ambient Temperature establishes a methodology for assessing the slow crack growth (SCG) behavior of advanced ceramics. This test method utilizes constant stress rate strength testing-commonly referred to as “dynamic fatigue” testing-of rectangular beam flexural, ring-on-ring biaxial disk flexural, or direct tensile strength specimens at ambient temperature. The resulting data enables the empirical determination of the SCG parameters, providing valuable insights into the durability and reliability of structural ceramics in various environments.

Understanding the susceptibility of ceramics to slow crack growth is critical for optimizing materials development, quality control processes, and generating preliminary design data for engineering applications where advanced ceramics are used.

Key Topics

• Slow Crack Growth Parameters: The standard measures the parameters (n and D) by evaluating how the material’s measured strength degrades with decreasing applied stress rate in a specified environment.
• Test Configurations: Permitted test setups include four-point flexural testing for rectangular beams, ring-on-ring biaxial disk flexural testing, and direct tensile strength testing, each following relevant referenced ASTM methods.
• Specimen Requirements: Test specimens must be prepared following standards C1161, C1499, or C1273 according to the selected configuration.
• Statistical Assessments: The method emphasizes statistical reproducibility, requiring multiple stress rates and specimens (minimum of 10 per stress rate, at least 40 total), to ensure representative SCG parameter estimates.
• Test Environments: Testing can be performed in various environments, and controlling environment variables-like chemical composition, pH, humidity, and temperature-is essential for meaningful and transferable results.
• Data Analysis: Results are analyzed based on the relationship between the logarithm of measured strength and applied stress rate to determine SCG susceptibility.

Applications

• Material Development: The test method informs the development of advanced ceramic materials by revealing how changes in processing or composition affect slow crack growth and overall strength behavior.
• Quality Control: Manufacturers can use the standard to appraise the relative quality and durability of ceramic batches or production runs, ensuring products meet required performance benchmarks.
• Characterization: Research laboratories utilize this method to characterize new or existing ceramic materials, comparing their SCG parameters under controlled environments.
• Design Data Generation: While offering limited design data, results can be leveraged during the early stages of engineering analysis to compare material choices or establish performance expectations for ceramics in critical components.
• Service Life Evaluation: This test is especially relevant for structural ceramics used in applications where component lifetime is dictated by subcritical crack extension, such as medical implants, electronic substrates, turbine components, and armor systems.

Related Standards

• ASTM C1145 - Terminology of Advanced Ceramics
• ASTM C1161 - Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
• ASTM C1239 - Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• ASTM C1273 - Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures
• ASTM C1322 - Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• ASTM C1499 - Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature
• ASTM E4 - Practices for Force Verification of Testing Machines
• ASTM E6 - Terminology Relating to Methods of Mechanical Testing
• ASTM E337 - Test Method for Measuring Humidity with a Psychrometer
• ASTM 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

ASTM C1368-18 is recognized internationally and complies with principles of standardization aligned with WTO Technical Barriers to Trade (TBT), ensuring its global relevance for advanced ceramics testing and quality assurance. For manufacturers, researchers, and engineers working with advanced ceramics, this standard offers a robust method for evaluating material performance and improving component reliability.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C1368 − 18
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, ring-on- C1161 Test Method for Flexural Strength of Advanced
ring biaxial disk flexural testing, or direct tensile strength, in Ceramics at Ambient Temperature
whichstrengthisdeterminedasafunctionofappliedstressrate C1239 Practice for Reporting Uniaxial Strength Data and
in a given environment at ambient temperature. The strength Estimating Weibull Distribution Parameters forAdvanced
degradation exhibited with decreasing applied stress rate in a Ceramics
specified environment is the basis of this test method which C1273 Test Method for Tensile Strength of Monolithic
enables the evaluation of slow crack growth parameters of a Advanced Ceramics at Ambient Temperatures
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 stress
rupture (See Terminology E1823). peratures)
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, health, and environmental practices and deter-
3.1.1 The terms described in Terminologies C1145, E6, and
mine the applicability of regulatory limitations prior to use.
E1823 are applicable to this test method. Specific terms
1.4 This international standard was developed in accor-
relevant to this test method are as follows:
dance with internationally recognized principles on standard-
3.1.2 advanced ceramic, n—a highly engineered, high-
ization established in the Decision on Principles for the
performance, predominately nonmetallic, inorganic, ceramic
Development of International Standards, Guides and Recom-
material having specific functional attributes. (C1145)
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee. 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.
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, 2018. Published January 2018. Originally
approved in 1997. Last previous edition approved in 2017 as C1368 – 10 (2017). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/C1368-18. contact ASTM Customer Service at [e-mail]. 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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1368 − 18
3.1.4 environment, n—the aggregate of chemical species ceramic materials under specified environments. Furthermore,
and energy that surrounds a test specimen. (E1823) this test method may establish the influences of processing
variables and composition on SCG as well as on strength
3.1.5 environmental chamber, n—the container of bulk vol-
behavior of newly developed or existing materials, thus allow-
ume surrounding a test specimen. (E1823)
ing tailoring and optimizing material processing for further
3.1.6 equibiaxial flexural strength, [F/L ], n—themaximum
modification. In summary, this test method may be used for
stress that a material is capable of sustaining when subjected to
material development, quality control, characterization, and
flexure between two concentric rings.
limited design data generation purposes. The conventional
3.1.6.1 Discussion—This mode of flexure is a cupping of
analysis of constant stress rate testing is based on a number of
the circular plate caused by loading at the inner load ring and
critical assumptions, the most important of which are listed in
outer support ring. The equibiaxial flexural strength is calcu-
the next paragraphs.
lated from the maximum load of a biaxial test carried to
rupture, the original dimensions of the test specimen, and 4.2 Theflexuralstresscomputationfortherectangularbeam
Poisson’s ratio. (C1499)
test specimens or the equibiaxial disk flexure test specimens is
–2
based on simple beam theory, with the assumptions that the
3.1.7 flexural strength, σ, [FL ], n—a measure of the
f
material is isotropic and homogeneous, the moduli of elasticity
strengthofaspecifiedbeamspecimeninbendingdeterminedat
in tension and compression are identical, and the material is
a given stress rate in a particular environment.
linearly elastic. The average grain size should be no greater
3.1.8 fracture toughness, n—a generic term for measures of
than one-fiftieth of the beam thickness.
resistance to extension of a crack. (E1823)
–2
4.3 The test specimen sizes and fixtures for rectangular
3.1.9 inert strength, [FL ], n—a measure of the strength of
beamtestspecimensshouldbeinaccordancewithTestMethod
a specified strength test specimen as determined in an appro-
C1161, which provides a balance between practical configura-
priate inert condition whereby no slow crack growth occurs.
tions and resulting errors, as discussed in Refs (4, 5). Only
3.1.9.1 Discussion—An inert condition may be obtained by
four-point test configuration is allowed in this test method for
using vacuum, low temperatures, very fast test rates, or any
rectangular beam specimens. Three-point test configurations
inert mediums.
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˙
log σ˙ + log D. (See Appendix X1.) f
+ 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
against stress rate. Both strength and stress rate are plotted on
NOTE 3—There are various other forms of crack velocity laws which
log-log scales. are usually more complex or less convenient mathematically, or both, but
may be physically more realistic (6). It is generally accepted that actual
3.1.13 stress intensity factor, K,n—the magnitude of the
I
data cannot reliably distinguish between the various formulations.
ideal-crack-tip stress field (stress-field singularity) subjected to
Therefore, the mathematical analysis in this test method does not cover
mode I loading in a homogeneous, linear elastic body. (E1823) such alternative crack velocity formulations.
3.1.14 tensile strength, S , [F/L ], n—the maximum tensile
4.5 The mathematical relationship between strength and
u
stress which a material is capable of sustaining.
stress rate was derived based on the assumption that the slow
3.1.14.1 Discussion—Tensile strength is calculated from the
crack growth parameter is at least n ≥ 5 (1, 7, 8). Therefore, if
maximum force during a tension test carried to rupture and the
a material exhibits a very high susceptibility to SCG, that is, n
original cross-sectional area of the specimen. (C1273)
< 5, special care should be taken when interpreting the results.
3.2 Definitions of Terms Specific to This Standard:
4.6 The mathematical analysis of test results in accordance
3.2.1 slow crack growth parameters, n and D, n—the
with the method in 4.4 assumes that the material displays no
parametersestimatedasconstantsintheflexuralstrength-stress
rising R-curvebehavior.Itshouldbenotedthattheexistenceof
rate equation, which represent the degree of slow crack growth
such behavior cannot be determined from this test method.
susceptibility of a material. (See Appendix X1.)
4.7 Slow crack growth behavior of ceramic materials ex-
4. Significance and Use
posedtostress-corrosivegasesorliquidenvironmentscanvary
4.1 For many structural ceramic components in service, as a function of mechanical, material, and electrochemical
their use is often limited by lifetimes that are controlled by a variables. Therefore, it is essential that test results accurately
process of SCG. This test method provides the empirical reflect the effects of specific variables under study. Only then
parameters for appraising the relative SCG susceptibility of can data be compared from one investigation to another on a
C1368 − 18
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
and test fixture configuration is dependent on its inherent
growth curve (9). Such errors can be avoided by testing in 100 %
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-processed condition to
Only fractures which occur in the inner loading circle should
simulate a specific service condition. Therefore, specimen
be used. Furthermore, it is assumed that the fracture origins are
fabrication history may play an important role in slow crack
near to the tensile surface and do not grow very large relative
growth as well as in strength behavior.
to the thickness of rectangular beam flexure or disk strength
test specimens.
6. Apparatus
4.11 Theconventionalanalysisofconstantstressratetesting
6.1 Testing Machine—Testing machines used for this test
is also based on a critical assumption that the same type flaw
method shall conform to the requirements of Practices E4.
controls strength in all specimens at all loading rates. If the
Specimens may be loaded in any suitable testing machine
flaw distribution is multimodal, then the conventional analysis
provided that uniform test rates, either using load-controlled or
in this standard may produce erroneous slow crack growth
displacement-controlled mode, can be maintained. The loads
parameter estimates.
used in determining strength shall be accurate to within
61.0 %atanyloadwithintheselectedloadrateandloadrange
5. Interferences
of the testing machine as defined in Practices E4. The testing
5.1 SCG may be the product of both mechanical and
machine shall have a minimum capability of applying at least
chemicaldrivingforces.Thechemicaldrivingforceforagiven
four test rates with at least three orders of magnitude, ranging
–1 2 –7
material with given flaw configurations can strongly vary with
from10 to10 N/sforload-controlledmodeandfrom10 to
–4
the composition, pH, and temperature of a test environment.
10 m/s for displacement-controlled mode.
Note that SCG testing is very time consuming; it may take
6.2 Test Fixtures, Four-Point Rectangular Beam Flexure—
several weeks to complete testing a typical advanced ceramic.
The configurations and mechanical properties of test fixtures
Becauseofthislongtesttime,thechemicalvariablesofthetest
should be in accordance with Test Method C1161. The mate-
environment must be prevented from changing throughout the
rials from which the test fixtures (including bearing cylinders)
tests.Inadequatecontrolofthesechemicalvariablesmayresult
are fabricated shall be effectively inert to the test environment
in inaccurate strength data and SCG parameters, especially for
so that they do not react with or contaminate the environment.
materials that are sensitive to the environment.
NOTE 7—For testing in water, for example, it is recommended that the
5.2 Depending on the degree of SCG susceptibility of a
test fixture be fabricated from stainless steel which is effectively inert to
material, the linear relationship between log (strength) and log
water. The bearing cylinders may be machined from hardenable stainless
(applied stress rate) (see Appendix X1) may start to deviate at
steel (for example, 440C grade) or a ceramic material such as silicon
a certain high stress rate at which slow crack growth dimin- nitride, silicon carbide, or alumina.
C1368 − 18
6.2.1 Four-Point Flexure—The four-point- ⁄4-point fixture dimensions of disk-shaped flexure specimens as described in
configuration as described in 6.2 of Test Method C1161 shall 8.1ofTestMethodC1499shallbeusedinthistestmethod.The
be used in this test method. Three-point flexure is not permit- types and dimensions of tension strength specimens as de-
ted. The test fixtures shall be stiffer than the specimen, so that scribed in 8.1 of Test Method C1273 shall be used in this test
most of the crosshead or actuator travel is imposed onto the method.
specimen.
7.2 Specimen Preparation—Specimen fabrication and
6.3 Test Fixtures, Equibiaxial Disk Flexural Strength—The preparation methods as described in the appropriate sections of
configurationsandmechanicalpropertiesoftestfixturesshould Test Method C1161, C1273,or C1499 shall be used in this test
be in accordance with Test Method C1499. The materials from method.
which the test fixtures (including bearing cylinders) are fabri-
7.3 Handling, Cleaning, and Storage—Exercise care in
cated shall be effectively inert to the test environment so that
handling and storing specimens in order to avoid introducing
they do not react with or contaminate the environment. See
random and severe flaws which might occur if the specimens
Note 7. The test fixtures shall be stiffer than the specimen, so
were allowed to impact or scratch each other. Clean test
that most of the crosshead or actuator travel is imposed onto
specimens with an appropriate cleaning medium such as
the specimen.
methanol or high-purity (>99 %) isopropyl alcohol, since
surface contamination of test specimens by lubricant, residues,
6.4 Test Fixtures, Tensile Strength—The configurations and
mechanical properties of test fixtures should be in accordance rust, or dirt might affect slow crack growth behavior for certain
test environments. After cleaning and drying, store test speci-
with Test Method C1273. The materials from which the test
fixtures (including bearing cylinders) are fabricated shall be mens in vacuum or desiccators to minimize or to avoid
exposure to moisture in air. This is particularly important if
effectively inert to the test environment so that they do not
react with or contaminate the environment. See Note 7. The testing is carried out in any environment other than ambient air
test fixtures shall be stiffer than the specimen, so that most of or water. Moisture entrapped in specimen surfaces may result
the crosshead or actuator travel is imposed onto the specimen. in accelerated SCG.
7.4 Number of Test Specimens—Therequirednumberoftest
6.5 Data Acquisition—Accurate determination of both frac-
ture load and test time is important since it affects not only specimens depends on the statistical reproducibility of SCG
fracture strength but applied stress rate. At the minimum, an parameters (n and D) to be determined. The statistical repro-
autographic record of applied load versus time should be ducibility is a function of strength scatter (Weibull modulus),
determined during testing. Either analog chart recorders or numberofappliedstressrates,rangeofappliedstressrates,and
digital data acquisition systems can be used for this purpose. SCGparameter(n).Becauseofthesevariousvariables,thereis
Ideally, an analog chart recorder should be used in conjunction no single guideline as to the determination of the appropriate
with the digital data acquisition system to provide an immedi- number of test specimens. A minimum of ten specimens per
ate record of the test as a supplement to the digital record. stress rate is recommended in this test method. The total
Recordingdevicesshouldbeaccurateto1.0 %oftherecording number of test specimens shall be at least 40, with at least four
range and should have a minimum data acquisition rate of appliedstressrates.Thenumberofspecimens(andstressrates)
recommended in this test method has been established with the
1000 Hz (or 1 KHz), with a response of 5000 Hz (or 5 KHz)
deemed more than sufficient. The appropriate data acquisition intent of determining not only reasonable confidence limits on
both strength distribution and SCG parameters but also to help
rate depends on the test rate; the higher the test rate the higher
the acquisition rate, and vise versa. discern multiple-flaw populations.
6.6 Environmental Facility—If testing is conducted in any NOTE 8—Refer to Ref (2) when a specific purpose is sought for the
statistical reproducibility of SCG parameters.
environment other than ambient air, an appropriate environ-
mental chamber shall be constructed to facilitate handling and
8. Procedure
monitoring of the test environment so that constant test
conditions can be maintained.The chamber shall be effectively 8.1 Choose the appropriate fixtures for the specific testing
corrosion resistant to the test environment so that it does not
configurations in Test Method C1161, C1273,or C1499. For
react with or change the environment. The chamber should be example, for four-point flexural strength of rectangular beam
large enough to fully immerse the test specimens in the
specimens see Section 6 of Test Method C1161. Use the
environment, particularly for liquid environments. A circula- four-pointAfixture for the sizeAspecimens. Similarly, use the
tion system to replenishment the test environment may be
B fixture for B specimens and the C fixture for C specimens.A
desirable. It should provide continuous filtration of the test fully articulating fixture is required if the specimen parallelism
medium in order to remove foreign debris and corrosive
requirements cannot be met.
product.Additionally,thefacilityshallbeabletosafelycontain
8.2 Test Rates:
the test environment.
8.2.1 The choice of range and number of test rates not only
affect the statistical reproducibility of SCG parameters but
7. Test Specimen
depend on the capability of a testing machine. Since various
7.1 Specimen Size—The types and dimensions of rectangu- types of testing machines are currently available, no simple
lar beam flexure specimens as described in 7.1 of Test Method guideline regarding the range of test rates can be made.
C1161 shall be used in this test method. The types and However, when the lower limits of the test rates of most
C1368 − 18
commercial test machines are considered (often attributed to The specimen should be directly centered below the axis of the
insufficient resolution of crosshead or actuator movement applied load. Assemble the test fixture/specimen in the testing
control), it is generally recommended that the lowest test rates machine. Mark the specimen to identify the points of load
–2 –8
be ≥10 N/s and 10 m/s, respectively, for load- and application and also so that the tensile and compression faces
displacement-controlled modes. The upper limits of the test can be distinguished. Carefully drawn pencil marks will
rates of testing machines are controlled by several factors suffice.
associated with the dynamic response of the crosshead or
8.4 Slowly apply an initial preload of not more than 20 N to
actuator, the load cell, and the data acquisition system (includ-
the specimen by means of the fixture. Inspect the points of
ing the chart recorder, if used). Since these factors vary widely
contact between the test fixture and the specimen to ensure
from one test machine to another, depending on their
even loading of the rectangular beam or biaxial disk test
capability, no specific upper limit can be established. However,
specimens. If uneven loading of the specimen occurs, use fully
based on the factors common to many testing machines and in
articulating fixtures.
order to avoid data generation in a plateau region (see 5.2), it
8.5 Environment—Choose the test environment as appropri-
is generally recommended that the upper test rates be≤10 N/s
–4
ate to the test program. Fill the clean environmental chamber
and 10 m/s, respectively, for load- and displacement-
controlled modes. with the test medium so that the gauge section of the specimen
is completely immersed in or surrounded by the test environ-
8.2.2 For a testing machine equipped with load-controlled
ment. The immersion or exposure time for equilibration of the
mode, choose at least four loading rates (evenly spaced in a
test specimen in the environment should be determined by
logarithmic scale) covering three orders of magnitude (for
–1 0 1 2
agreement between the parties involved in the test program.
example, 10 ,10 ,10 , and 10 N/s). Similarly, for the testing
This is particularly important for environments which are
machine equipped with displacement-controlled mode, choose
chemically corrosive to the specimen. The environment should
at least four displacement rates (evenly spaced in a logarithmic
–7
be consistent for the test series and should be reported. If the
scale) covering three orders of magnitude (for example, 10 ,
–6 –5 –4
tests are carried out in a humid atmosphere, the relative
10 ,10 , and 10 m/s). However, for better statistical
humidity shall not vary by more than 10 % during the entire
reproducibility of SCG parameters, the use of five or more test
test series. Determine the relative humidity in accordance with
rates (evenly spaced in a logarithmic scale) covering four or
Test Method E337. It is recommended that 100 % concentra-
more orders of magnitude is recommended if the testing
tion of the corrosive medium be used in order to minimize
machineiscapableandthespecimensareavailable.Ingeneral,
averaging of the fatigue curve regions and thereby allow the
theload-controlledmodeyieldsabetteroutputwave-formthan
use of stress rates greater than 1 MPa/s (9).An example of this
the disp
...

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 (Reapproved 2017) C1368 − 18
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress-Rate 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*Scope
1.1 This test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant
stress-rate 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) 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 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
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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:
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 Feb. 1, 2017Jan. 1, 2018. Published February 2017January 2018. Originally approved in 1997. Last previous edition approved in 20102017 as
C1368 – 10.C1368 – 10 (2017). DOI: 10.1520/C1368-10R17.10.1520/C1368-18.
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 [e-mail]. 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 − 18
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:
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 maximum load of a biaxial test carried to rupture, the original
dimensions of the test specimen, and Poisson’s ratio. (C1499)
–2
3.1.7 flexural strength, σ , [FL ], n—a measure of the strength of a specified beam specimen in bending determined at a given
f
stress rate in a particular environment.
3.1.8 fracture toughness, n—a generic term for measures of resistance to extension of a crack. (E1823)
–2
3.1.9 inert strength, [FL ], 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 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 strength, S , [F/L ], n—Sthe —the maximum tensile stress which a material is capable of sustaining.
u 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,
C1368 − 18
characterization, and limited design data generation purposes. The conventional analysis of constant stress-rate 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 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 (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 (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 (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 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 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: time consuming; it may take several weeks to complete testing a typical,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.
C1368 − 18
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) may remain
unchanged so that a plateau is observed in the plot of strength-versus-stress rate (7). If the strength data determined in this plateau
region are included in the analysis, a misleading estimate of the SCG parameters will be obtained. Therefore, the strength data in
the plateau shall be excluded as data points in estimating the SCG parameters of the material. This test method addresses for this
factor by recommending that the highest stress rate be ≤2000 MPa/s.
NOTE 5—The strength plateau of a material can be checked by measuring an inert strength in an appropriate inert medium.
NOTE 6—When testing in environments with less than 100 % concentration of the corrosive medium (for example, air), the use of stress rates greater
than ~1 MPa/s can result in significant errors in the slow crack growth parameters due to averaging of the regions of the slow crack growth curve (9).
Such errors can be avoided by testing in 100%100 % concentration of the corrosive medium (for example, in water instead of humid air). For the case
of 100 % concentration of the corrosive medium, stress rates as large as ~2000 MPa/s may be acceptable.
5.3 Surface preparation of test specimens can introduce fabrication flaws which may have pronounced effects on SCG behavior.
Machining damage imposed during specimen preparation can be either a random interfering factor or an inherent part of the
strength characteristics to be measured. Surface preparation can also lead to residual stress. Universal or standardized test methods
of surface preparation do not exist. It should be understood that the final machining steps may or may not negate machining damage
introduced during the early coarse or intermediate machining steps. In some cases, specimens need to be tested in the as-processed
condition to simulate a specific service condition. Therefore, specimen fabrication history may play an important role in slow crack
growth as well as in strength behavior.
6. Apparatus
6.1 Testing Machine—Testing machines used for this test method shall conform to the requirements of Practices E4. Specimens
may be loaded in any suitable testing machine provided that uniform test rates, either using load-controlled or displacement-
controlled mode, can be maintained. The loads used in determining strength shall be accurate to within 61.0 % at any load within
the selected load rate and load range of the testing machine as defined in Practices E4. The testing machine shall have a minimum
–1 2
capability of applying at least four test rates with at least three orders of magnitude, ranging from 10 to 10 N/s for
–7 –4
load-controlled mode and from 10 to 10 m/s for displacement-controlled mode.
6.2 Test Fixtures, Four-Point Rectangular Beam Flexure—The configurations and mechanical properties of test fixtures should
be in accordance with Test Method C1161. The materials from which the test fixtures including(including bearing cylinderscyl-
inders) are fabricated shall be effectively inert to the test environment so that they do not react with or contaminate the
environment.
NOTE 7—For testing in water, for example, it is recommended that the test fixture be fabricated from stainless steel which is effectively inert to water.
The bearing cylinders may be machined from hardenable stainless steel (for example, 440C grade) or a ceramic material such as silicon nitride, silicon
carbide, or alumina.
6.2.1 Four-Point Flexure—The four-point four-point- ⁄4-point fixture configuration as described in 6.2 of Test Method C1161
shall be used in this test method. Three-point flexure is not permitted. The test fixtures shall be stiffer than the specimen, so that
most of the crosshead or actuator travel is imposed onto the specimen.
6.3 Test Fixtures, Equibiaxial Disk Flexural Strength—The configurations and mechanical properties of test fixtures should be
in accordance with Test Method C1499. The materials from which the test fixtures including(including bearing cylinderscylinders)
are fabricated shall be effectively inert to the test environment so that they do not react with or contaminate the environment. See
Note 7. The test fixtures shall be stiffer than the specimen, so that most of the crosshead or actuator travel is imposed onto the
specimen.
6.4 Test Fixtures, Tensile Strength—The configurations and mechanical properties of test fixtures should be in accordance with
Test Method C1273. The materials from which the test fixtures including(including bearing cylinderscylinders) are fabricated shall
be effectively inert to the test environment so that they do not react with or contaminate the environment. See Note 7. The test
fixtures shall be stiffer than the specimen, so that most of the crosshead or actuator travel is imposed onto the specimen.
6.5 Data Acquisition—Accurate determination of both fracture load and test time is important since it affects not only fracture
strength but applied stress rate. At the minimum, an autographic record of applied load versus time should be determined during
testing. Either analog chart recorders or digital data acquisition systems can be used for this purpose. Ideally, an analog chart
recorder should be used in conjunction with the digital data acquisition system to provide an immediate record of the test as a
supplement to the digital record. Recording devices should be accurate to 1.0 % of the recording range and should have a minimum
data acquisition rate of 1000 Hz 1000 Hz (or 1 KHz)KHz), with a response of 5000 Hz (or 5 KHz) deemed more than sufficient.
The appropriate data acquisition rate depends on the test rate; the higher the test rate the higher the acquisition rate, and vise versa.
6.6 Environmental Facility—If testing is conducted in any environment other than ambient air, an appropriate environmental
chamber shall be constructed to facilitate handling and monitoring of the test environment so that constant test conditions can be
maintained. The chamber shall be effectively corrosion resistant to the test environment so that it does not react with or change
C1368 − 18
the environment. The chamber should be large enough to fully immerse the test specimens in the environment, particularly for
liquid environments. A circulation system to replenishment the test environment may be desirable. It should provide continuous
filtration of the test medium in order to remove foreign debris and corrosive product. Additionally, the facility shall be able to safely
contain the test environment.
7. Test Specimen
7.1 Specimen Size—The types and dimensions of rectangular beam flexure specimens as described in 7.1 of Test Method C1161
shall be used in this test method. The types and dimensions of disk-shaped flexure specimens as described in 8.1 of Test Method
C1499 shall be used in this test method. The types and dimensions of tension strength specimens as described in 8.1 of Test Method
C1273 shall be used in this test method.
7.2 Specimen Preparation—Specimen fabrication and preparation methods as described in the appropriate sections of Test
MethodsMethod C1161, C1273, or C1499 shall be used in this test method.
7.3 Handling, Cleaning, and Storage—Exercise care in handling and storing specimens in order to avoid introducing random
and severe flaws which might occur if the specimens were allowed to impact or scratch each other. Clean test specimens with an
appropriate cleaning medium such as methanol or high-purity (>99 %) isopropyl alcohol, since surface contamination of test
specimens by lubricant, residues, rust, or dirt might affect slow crack growth behavior for certain test environments. After cleaning
and drying, store test specimens in vacuum or desiccators to minimize or to avoid exposure to moisture in air. This is particularly
important if testing is carried out in any environment other than ambient air or water. Moisture entrapped in specimen surfaces may
result in accelerated SCG.
7.4 Number of Test Specimens—The required number of test specimens depends on the statistical reproducibility of SCG
parameters (n and D) to be determined. The statistical reproducibility is a function of strength scatter (Weibull modulus), number
of applied stress rates, range of applied stress rates, and SCG parameter (n). Because of these various variables, there is no single
guideline as to the determination of the appropriate number of test specimens. A minimum of 10ten specimens per stress rate is
recommended in this test method. The total number of test specimens shall be at least 40, with at least four applied stress rates.
The number of specimens (and stress rates) recommended in this test method has been established with the intent of determining
not only reasonable confidence limits on both strength distribution and SCG parameters but also to help discern multiple-flaw
populations.
NOTE 8—Refer to Ref (2) when a specific purpose is sought for the statistical reproducibility of SCG parameters.
8. Procedure
8.1 Choose the appropriate fixtures for the specific testing configurations in Test MethodsMethod C1161, C1273, or C1499. For
example, for four-point flexural strength of rectangular beam specimens see Section 6 of Test Method C1161. Use the four-point
A fixture for the size A specimens. Similarly, use the B fixture for B specimens and the C fixture for C specimens. A fully
articulating fixture is required if the specimen parallelism requirements cannot be met.
8.2 Test Rates:
8.2.1 The choice of range and number of test rates not only affect the statistical reproducibility of SCG parameters but depend
on the capability of a testing machine. Since various types of testing machines are currently available, no simple guideline
regarding the range of test rates can be made. However, when the lower limits of the test rates of most commercial test machines
are considered (often attributed to insufficient resolution of crosshead or actuator movement control), it is generally recommended
–2 –8
that the lowest test rates be ≥10 N/s and 10 m/s, respectively, for load- and displacement-controlled modes. The upper limits
of the test rates of testing machines are controlled by several factors associated with the dynamic response of the crosshead or
actuator, the load cell, and the data acquisition system (including the chart recorder, if used). Since these factors vary widely from
one test machine to another, depending on their capability, no specific upper limit can be established. However, based on the factors
common to many testing machines and in order to avoid data generation in a plateau region (see 5.2), it is generally recommended
3 –4
that the upper test rates be ≤10 N/s and 10 m/s, respectively, for load- and displacement-controlled modes.
8.2.2 For a testing machine equipped with load-controlled mode, choose at least four loading rates (evenly spaced in a
–1 0 1 2
logarithmic scale) covering three orders of magnitude (for example, 10 , 10 , 10 , and 10 N/s). Similarly, for the testing machine
equipped with displacement-controlled mode, choose at least four displacement rates (evenly spaced in a logarithmic scale)
–7 –6 –5 –4
covering three orders of magnitude (for example, 10 , 10 , 10 , and 10 m/s). However, for better statistical reproducibility of
SCG parameters, the use of five or more test rates (evenly spaced in a logarithmic scale) covering four or more orders of magnitude
is recommended if the testing machine is capable and the specimens are available. In general, the load-controlled mode yields a
better output wave-form than the displacement-controlled mode, particularly at low test rates. In addition, the specified applied
loading rate can be directly related with stress rate, regardless of the system compliance of test frame, load train, fixture, and
specimen, thus simplifying data analysis. In the displacement-controlled mode, however, the loading rate to be determined is a
function of both applied displacement rate and system compliance so that the actual loading rate should always be measured and
used to calculate a corresponding stress rate, thus making data analysis complex. Therefore, a load-controlled test is the preferred
test mode.
C1368 − 18
NOTE 9—When using the faster test rates, care must be exercised particularly for the conventional, older electromechanical testing machines equipped
with slow-response load cells and chart recorders. Such machines have 100 MPa/s as an upper limit stress rate at which the chart recorder or the load
cell, or both, cannot follow load increase and hence cannot correctly monitor the fracture load (10, 11). This factor should be taken into account when
the fast crosshead speeds are selected on older testing machines. The minimum time to failure in this case should be within a few seconds (≥3 s). However,
the use of a better load cell (or piezoelectric load cell) or a fast-response chart recorder, or both, or a digital data acquisition system can improve the
existing performance so that higher test rates (up to 2000 MPa/s (10) can be achieved. It has been shown that the digitally controlled, modern testing
machine is capable of applying stress rates up to 10 MPa/s (8).
8.3 Carefully place each specimen into the test fixture to preclude possible damage and contamination and to ensure alignment
of the specimen relative to the test fixture. In particular, there should be an equal amount of overhang of the rectangul
...