ASTM C1368-00
(Test Method)Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature
Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Flexural Testing at Ambient Temperature
SCOPE
1.1 This test method covers the determination of slow crack growth (SCG) parameters of advanced ceramics by using constant stress-rate flexural testing in which flexural strength is determined as a function of applied stress rate in a given environment at 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 Definitions E1150, 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 Definitions E1150).
1.2 Values expressed in this test method are in accordance with the International System of Units (SI) and Practice E380.
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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: C 1368 – 00
Standard Test Method for
Determination of Slow Crack Growth Parameters of
Advanced Ceramics by Constant Stress-Rate Flexural
Testing at Ambient Temperature
This standard is issued under the fixed designation C 1368; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope C 1322 Practice for Fractography and Characterization of
Fracture Origins in Advanced Ceramics
1.1 This test method covers the determination of slow crack
E 4 Practices for Force Verification of Testing Machines
growth (SCG) parameters of advanced ceramics by using
E 6 Terminology Relating to Methods of Mechanical Test-
constant stress-rate flexural testing in which flexural strength is
ing
determined as a function of applied stress rate in a given
E 337 Test Method for Measuring Humidity with a Psy-
environment at ambient temperature. The strength degradation
chrometer (the Measurement of Wet-Bulb and Dry-Bulb
exhibited with decreasing applied stress rate in a specified
Temperatures)
environment is the basis of this test method which enables the
E 380 Practice for Use of the International System of Units
evaluation of slow crack growth parameters of a material.
(SI) (The Modernized Metric System)
NOTE 1—This test method is frequently referred to as “dynamic
E 1823 Terminology Relating to Fatigue and Fracture Test-
fatigue” testing (Refs (1-3) ) in which the term“ fatigue” is used
ing
interchangeably with the term “slow crack growth.” To avoid possible
2.2 Military Handbook:
confusion with the “fatigue” phenomenon of a material which occurs
MIL-HDBK-790 Fractography and Characterization of
exclusively under cyclic loading, as defined in Definitions E 1150, this test
method uses the term “constant stress-rate testing” rather than “dynamic Fracture Origins in Advanced Structural Ceramics
fatigue” testing.
3. Terminology
NOTE 2—In glass and ceramics technology, static tests of considerable
duration are called “static fatigue” tests, a type of test designated as
3.1 Definitions—The terms described in Terminology
stress-rupture (See Definitions E 1150).
C 1145, Terminology E 6, Terminology E 616, and Definitions
1.2 Values expressed in this test method are in accordance
E 1150 are applicable to this test method. Specific terms
with the International System of Units (SI) and Practice E 380.
relevant to this test method are as follows:
1.3 This standard does not purport to address all of the
3.1.1 advanced ceramic, n—a highly engineered, high-
safety concerns, if any, associated with its use. It is the
performance, predominately nonmetallic, inorganic, ceramic
responsibility of the user of this standard to establish appro-
material having specific functional attributes. (C 1145)
priate safety and health practices and determine the applica-
3.1.2 constant stress rate,s˙ , n—a constant rate of maximum
bility of regulatory limitations prior to use.
stress applied to a specified beam by using either a constant
loading or constant displacement rate of a testing machine.
2. Referenced Documents
3.1.3 environment, n—the aggregate of chemical species
2.1 ASTM Standards:
and energy that surrounds a test specimen. (E 1150)
C 1145 Terminology of Advanced Ceramics
3.1.4 environmental chamber, n—the container of bulk
C 1161 Test Method for Flexural Strength of Advanced
volume surrounding a test specimen. (E 1150)
Ceramics at Ambient Temperature
3.1.5 flexural strength, s , n—a measure of the strength of a
f
C 1239 Practice for Reporting Uniaxial Strength Data and
specified beam specimen in bending determined at a given
Estimating Weibull Distribution Parameters for Advanced
stress rate in a particular environment.
Ceramics
3.1.6 flexural strength-stress rate curve, n—a curve fitted to
the values of flexural strength at each of several stress rates,
based on the relationship between flexural strength and stress
This test method is under the jurisdiction of ASTM Committee C-28 on
rate: log s 5 1/(n +1)log s˙ + log D. (See Appendix X1.)
f
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
Properties and Performance.
Current edition approved April 10, 2000. Published July 2000. Originally
Annual Book of ASTM Standards, Vol 03.01.
published as C 1368 – 97. Last previous edition C 1368 – 97. Annual Book of ASTM Standards, Vol 11.03.
2 6
The boldface numbers in parentheses refer to the list of references at the end of Annual Book of ASTM Standards, Vol 14.02.
this standard. Available from Army Research Laboratory—Materials Directorate, Aberdeen
Annual Book of ASTM Standards, Vol 15.01. Proving Ground, MD 21005.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
C 1368
NOTE 3—In the ceramics literature, this is often called a dynamic NOTE 5—There are various other forms of crack velocity laws which
fatigue curve. are usually more complex or less convenient mathematically, or both, but
may be physically more realistic (Ref (6)). It is generally accepted that
3.1.7 flexural strength-stress rate diagram, n—a plot of
actual data cannot reliably distinguish between the various formulations.
flexural strength against stress rate. Both flexural strength and
Therefore, the mathematical analysis in this test method does not cover
stress rate are plotted on log-log scales.
such alternative crack velocity formulations.
3.1.8 fracture toughness, n—a generic term for measures of
4.5 The mathematical relationship between flexural strength
resistance to extension of a crack. (E 616)
and stress rate was derived based on the assumption that the
3.1.9 inert flexural strength, n—a measure of the strength of
slow crack growth parameter is at least n $ 5 (Refs (1, 7, 8)).
a specified beam specimen in bending as determined in an
Therefore, if a material exhibits a very high susceptibility to
appropriate inert condition whereby no slow crack growth
SCG, that is, n < 5, special care should be taken when
occurs.
interpreting the results.
NOTE 4—An inert condition may be obtained by using vacuum, low 4.6 The mathematical analysis of test results in accordance
temperatures, very fast test rates, or any inert mediums.
with the method in 4.4 assumes that the material displays no
rising R-curve behavior. It should be noted that the existence of
3.1.10 slow crack growth (SCG), n—subcritical crack
such behavior cannot be determined from this test method.
growth (extension) which may result from, but is not restricted
4.7 Slow crack growth behavior of ceramic materials ex-
to, such mechanisms as environmentally-assisted stress corro-
posed to stress-corrosive gases or liquid environments can vary
sion or diffusive crack growth.
as a function of mechanical, material, and electrochemical
3.1.11 stress intensity factor, K , n—the magnitude of the
I
variables. Therefore, it is essential that test results accurately
ideal-crack-tip stress field (stress-field singularity) subjected to
reflect the effects of specific variables under study. Only then
mode I loading in a homogeneous, linear elastic body.
can data be compared from one investigation to another on a
(E 616)
valid basis or serve as a valid basis for characterizing materials
3.2 Definition of Term Specific to This Standard:
and assessing structural behavior.
3.2.1 slow crack growth parameters, n and D, n—the
4.8 The strength of advanced ceramics is probabilistic in
parameters estimated as constants in the flexural strength-stress
nature. Therefore, SCG that is determined from the flexural
rate equation, which represent the degree of slow crack growth
strengths of a ceramic material is also a probabilistic phenom-
susceptibility of a material. (See Appendix Appendix X1.)
enon. Hence, a proper range and number of applied stress rates
4. Significance and Use in conjunction with an appropriate number of specimens at
each applied stress rate are required for statistical reproduc-
4.1 For many structural ceramic components in service,
ibility and design (Ref (2)). Guidelines are provided in this test
their use is often limited by lifetimes that are controlled by a
method.
process of SCG. This test method provides the empirical
parameters for appraising the relative SCG susceptibility of
NOTE 6—For a given ceramic material/environment system, the SCG
ceramic materials under specified environments. Furthermore,
parameter n is constant regardless of specimen size although its repro-
ducibility is dependent on the variables mentioned in 4.8. By contrast, the
this test method may establish the influences of processing
SCG parameter D depends significantly on strength and thus on specimen
variables and composition on SCG as well as on strength
size (see Eq X1.6 in Appendix X1).
behavior of newly developed or existing materials, thus allow-
ing tailoring and optimizing material processing for further 4.9 The strength of a ceramic material for a given specimen
and test fixture configuration is dependent on its inherent
modification. In summary this test method may be used for
resistance to fracture, the presence of flaws, and environmental
material development, quality control, characterization, and
effects. Analysis of a fracture surface, fractography, though
limited design data generation purposes.
beyond the scope of this test method, is highly recommended
4.2 The flexural stress computation is based on simple beam
for all purposes, especially to verify the mechanism(s) associ-
theory, with the assumptions that the material is isotropic and
ated with failure (refer to Practice C 1322 or MIL-HDBK-790,
homogeneous, the moduli of elasticity in tension and compres-
or both).
sion are identical, and the material is linearly elastic. The
average grain size should be no greater than one fiftieth of the
5. Interferences
beam thickness.
4.3 The specimen sizes and fixtures were chosen in accor- 5.1 SCG may be the product of both mechanical and
dance with Test Method C 1161, which provides a balance chemical driving forces. The chemical driving force for a given
between practical configurations and resulting errors, as dis- material with given flaw configurations can strongly vary with
cussed in Refs (4, 5). Only the four-point test configuration is the composition, pH, and temperature of a test environment.
used in this test method. Note that SCG testing is very time-consuming: it may take
4.4 The SCG parameters (n and D) are determined by fitting several weeks to complete testing a typical, advanced ceramic.
the measured experimental data to a mathematical relationship Because of this long test time, the chemical variables of the test
between flexural strength and applied stress rate, log s 5 environment must be prevented from changing throughout the
f
1/(n+1) log s˙ + log D. The basic underlying assumption on the tests. Inadequate control of these chemical variables may result
derivation of this relationship is that SCG is governed by an in inaccurate strength data and SCG parameters, especially for
n
empirical power-law crack velocity, v 5 A[K /K ] (see materials that are sensitive to the environment.
I IC
Appendix X1). 5.2 Depending on the degree of SCG susceptibility of a
C 1368
material, the linear relationship between log (flexural strength) and mechanical properties of bearing cylinders as described in
and log (applied stress rate) (see Appendix X1) may start to 6.4 of Test Method C 1161 shall be used in this test method. It
deviate at a certain high stress rate at which slow crack growth should be noted that the bearing cylinders shall be free to rotate
diminishes or is minimized due to the extremely short test in order to relieve frictional constraints, as described in 6.4.4 of
duration. Strengths obtained at higher stress rates (>2000 Test Method C 1161.
MPa/s) may remain unchanged so that a plateau is observed in 6.2.3 Semiarticulating Four-Point Fixture—The semiar-
the plot of strength versus stress rate (Ref (7)). If the strength ticulating four-point fixture as described in 6.5 of Test Method
data determined in this plateau region are included in the C 1161 may be used in this test method. This fixture shall be
analysis, a misleading estimate of the SCG parameters will be used when the parallelism requirements of test specimens are
obtained. Therefore, the strength data in the plateau shall be met in accordance with 7.1 of Test Method C 1161.
excluded as data points in estimating the SCG parameters of 6.2.4 Fully Articulating Four-Point Fixture—The fully ar-
the material. This test method addresses for this factor by ticulating four-point fixture as described in 6.6 of Test Method
recommending that the highest stress rate by #2000 MPa/s. C 1161 may be used in this test method. Specimens which do
not meet the parallelism requirements of 7.1 of Test Method
NOTE 7—The strength plateau of a material can be checked by
C 1161, due to the nature of fabrication process (as-fired,
measuring an inert flexural strength in an appropriate inert medium.
heat-treated, or oxidized), shall be tested in this fully articu-
5.3 Surface preparation of test specimens can introduce
lating fixture.
fabrication flaws which may have pronounced effects on SCG
6.2.5 Compliance of Test Fixture—The test fixtures shall be
behavior. Machining damage imposed during specimen prepa-
stiffer than the specimen, so that most of the crosshead or
ration can be either a random interfering factor or an inherent
actuator travel is imposed onto the specimen.
part of the strength characteristics to be measured. Surface
6.3 Data Acquisition—Accurate determination of both frac-
preparation can also lead to residual stress. Universal or
ture load and test time is important since it affects not only
standardized test methods of surface preparation do not exist. It
fracture strength but applied stress rate. At the minimum, an
should be understood that the final machining steps may or
autographic record of applied load versus time should be
may not negate machining damage introduced during the early
determined during testing. Either analog chart recorders or
coarse or intermediate machining steps. In some cases, speci-
digital data acquisition systems can be used for this purpose.
mens need to be tested in the as-processed condition to
Ideally, an analog chart recorder should be used in conjunction
simulate a specific service condition. Therefore, specimen
with the digital data acquisition system to provide an immedi-
fabrication history may play an important role in slow crack
ate record of the test as a supplemen
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.