ASTM C1337-10(2015)
(Test Method)Standard Test Method for Creep and Creep Rupture of Continuous Fiber-Reinforced Advanced Ceramics Under Tensile Loading at Elevated Temperatures
Standard Test Method for Creep and Creep Rupture of Continuous Fiber-Reinforced Advanced Ceramics Under Tensile Loading at Elevated Temperatures
SIGNIFICANCE AND USE
4.1 This test method may be used for material development, material comparison, quality assurance, characterization, and design data generation.
4.2 Continuous fiber-reinforced ceramic matrix composites are candidate materials for structural applications requiring high degrees of wear and corrosion resistance and toughness at high temperatures.
4.3 Creep tests measure the time-dependent deformation of a material under constant load at a given temperature. Creep rupture tests provide a measure of the life of the material when subjected to constant mechanical loading at elevated temperatures. In selecting materials and designing parts for service at elevated temperatures, the type of test data used will depend on the criteria for load-carrying capability which best defines the service usefulness of the material.
4.4 Creep and creep rupture tests provide information on the time-dependent deformation and on the time-of-failure of materials subjected to uniaxial tensile stresses at elevated temperatures. Uniform stress states are required to effectively evaluate any nonlinear stress-strain behavior which may develop as the result of cumulative damage processes (for example, matrix cracking, matrix/fiber debonding, fiber fracture, delamination, etc.) which may be influenced by testing mode, testing rate, processing or alloying effects, environmental influences, or elevated temperatures. Some of these effects may be consequences of stress corrosion or subcritical (slow) crack growth. It is noted that ceramic materials typically creep more rapidly in tension than in compression. Therefore, creep data for design and life prediction should be obtained in both tension and compression.
4.5 The results of tensile creep and tensile creep rupture tests of specimens fabricated to standardized dimensions from a particular material or selected portions of a part, or both, may not totally represent the creep deformation and creep rupture properties of the entire, full-size...
SCOPE
1.1 This test method covers the determination of the time-dependent deformation and time-to-rupture of continuous fiber-reinforced ceramic composites under constant tensile loading at elevated temperatures. This test method addresses, but is not restricted to, various suggested test specimen geometries. In addition, test specimen fabrication methods, allowable bending, temperature measurements, temperature control, data collection, and reporting procedures are addressed.
1.2 This test method is intended primarily for use with all advanced ceramic matrix composites with continuous fiber reinforcement: unidirectional (1-D), bidirectional (2-D), and tridirectional (3-D). In addition, this test method may also be used with glass matrix composites with 1-D, 2-D, and 3-D continuous fiber reinforcement. This test method does not address directly discontinuous fiber-reinforced, whisker-reinforced, or particulate-reinforced ceramics, although the test methods detailed here may be equally applicable to these composites.
1.3 Values expressed in this test method are in accordance with the International System of Units (SI) and IEEE/ASTM SI 10 .
1.4 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. Hazard statements are noted in 7.1 and 7.2.
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Designation: C1337 − 10 (Reapproved 2015)
Standard Test Method for
Creep and Creep Rupture of Continuous Fiber-Reinforced
Advanced Ceramics Under Tensile Loading at Elevated
Temperatures
This standard is issued under the fixed designation C1337; 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 the time- 2.1 ASTM Standards:
dependent deformation and time-to-rupture of continuous C1145 Terminology of Advanced Ceramics
fiber-reinforced ceramic composites under constant tensile C1275 Test Method for Monotonic Tensile Behavior of
loading at elevated temperatures. This test method addresses, Continuous Fiber-Reinforced Advanced Ceramics with
but is not restricted to, various suggested test specimen Solid Rectangular Cross-Section Test Specimens at Am-
geometries. In addition, test specimen fabrication methods, bient Temperature
allowable bending, temperature measurements, temperature D3878 Terminology for Composite Materials
control, data collection, and reporting procedures are ad- E4 Practices for Force Verification of Testing Machines
dressed. E6 Terminology Relating to Methods of Mechanical Testing
E83 Practice for Verification and Classification of Exten-
1.2 This test method is intended primarily for use with all
someter Systems
advanced ceramic matrix composites with continuous fiber
E139 Test Methods for Conducting Creep, Creep-Rupture,
reinforcement: unidirectional (1-D), bidirectional (2-D), and
and Stress-Rupture Tests of Metallic Materials
tridirectional (3-D). In addition, this test method may also be
E220 Test Method for Calibration of Thermocouples By
used with glass matrix composites with 1-D, 2-D, and 3-D
Comparison Techniques
continuous fiber reinforcement. This test method does not
E230 Specification and Temperature-Electromotive Force
address directly discontinuous fiber-reinforced, whisker-
(EMF) Tables for Standardized Thermocouples
reinforced,orparticulate-reinforcedceramics,althoughthetest
E337 Test Method for Measuring Humidity with a Psy-
methods detailed here may be equally applicable to these
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
composites.
peratures)
1.3 Values expressed in this test method are in accordance
E1012 Practice for Verification of Testing Frame and Speci-
withtheInternationalSystemofUnits(SI)andIEEE/ASTMSI
men Alignment Under Tensile and Compressive Axial
10 .
Force Application
1.4 This standard does not purport to address all of the IEEE/ASTM SI 10 American National Standard for Use of
safety concerns, if any, associated with its use. It is the
theInternationalSystemofUnits(SI):TheModernMetric
responsibility of the user of this standard to establish appro- System
priate safety and health practices and determine the applica-
3. Terminology
bility of regulatory limitations prior to use. Hazard statements
3.1 Definitions:
are noted in 7.1 and 7.2.
3.1.1 The definitions of terms relating to tensile testing
appearing in Terminology E6 apply to the terms used in this
test method. The definitions relating to advanced ceramics
This test method is under the jurisdiction of ASTM Committee C28 on
Advanced Ceramics and is the direct responsibility of Subcommittee C28.07 on
Ceramic Matrix Composites. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2015. Published September 2015. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1996. Last previous edition approved in 2010 as C1337 – 10. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C1337-10R15. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1337 − 10 (2015)
appearinginTerminologyC1145applytothetermsusedinthis fracture, delamination, etc.) which may be influenced by
test method. The definitions of terms relating to fiber rein- testing mode, testing rate, processing or alloying effects,
forced composites appearing in Terminology D3878 apply to environmental influences, or elevated temperatures. Some of
the terms used in this test method. Additional terms used in these effects may be consequences of stress corrosion or
conjunction with this test method are defined in the following: subcritical (slow) crack growth. It is noted that ceramic
3.1.2 continuous fiber-reinforced ceramic matrix composite materials typically creep more rapidly in tension than in
(CFCC)—ceramic matrix composite in which the reinforcing compression. Therefore, creep data for design and life predic-
phase consists of a continuous fiber, continuous yarn, or a tion should be obtained in both tension and compression.
woven fabric.
4.5 The results of tensile creep and tensile creep rupture
3.1.3 fracture strength (F/L )—tensile stress that the mate-
tests of specimens fabricated to standardized dimensions from
rial sustains at the instant of fracture. Fracture strength is
a particular material or selected portions of a part, or both, may
calculated from the force at fracture during a tension test
not totally represent the creep deformation and creep rupture
carried to rupture and the original cross-sectional area of the
properties of the entire, full-size end product or its in-service
test specimen.
behavior in different environments or at various elevated
3.1.3.1 Discussion—In some cases, the fracture strength
temperatures.
may be identical to the tensile strength if the load at fracture is
4.6 For quality control purposes, results derived from stan-
the maximum for the test. Factors such as load train compli-
dardizedtensiletestspecimensmaybeconsideredindicativeof
ance and fiber pull-out behavior may influence the fracture
the response of the material from which they were taken for
strength.
given primary processing conditions and post-processing heat
3.1.4 proportional limit stress—greatest stress which a ma-
treatments.
terial is capable of sustaining without any deviation from
proportionality of stress to strain (Hooke’s law).
5. Interferences
3.1.4.1 Discussion—Many experiments have shown that
5.1 Test environment (vacuum, inert gas, ambient air, etc.)
values observed for the proportional limit vary greatly with the
including moisture content (for example, relative humidity)
sensitivity and accuracy of the testing equipment, eccentricity
may have an influence on the creep and creep rupture behavior
of loading, the scale to which the stress-strain diagram is
of CFCCs. In particular, the behavior of materials susceptible
plotted, and other factors. When determination of proportional
to slow crack growth fracture and oxidation will be strongly
limit is required, the procedure and sensitivity of the test
influenced by test environment and test temperature. Testing
equipment shall be specified.
can be conducted in environments representative of service
3.1.5 slow crack growth—subcritical crack growth (exten-
conditions to evaluate material performance under these con-
sion) which may result from, but is not restricted to, such
ditions.
mechanisms as environmentally assisted stress corrosion or
5.2 Surface preparation of test specimens, although nor-
diffusive crack growth.
mally not considered a major concern with CFCCs, can
4. Significance and Use introducefabricationflawswhichmayhavepronouncedeffects
on the mechanical properties and behavior (for example, shape
4.1 This test method may be used for material development,
and level of the resulting stress-strain-time curve, etc.). Ma-
material comparison, quality assurance, characterization, and
chining damage introduced during test specimen preparation
design data generation.
can be either a random interfering factor in the ultimate
4.2 Continuous fiber-reinforced ceramic matrix composites
strength of pristine material (that is, increased frequency of
are candidate materials for structural applications requiring
surface-initiated fractures compared to volumeinitiated frac-
high degrees of wear and corrosion resistance and toughness at
tures) or an inherent part of the strength characteristics to be
high temperatures.
measured. Surface preparation can also lead to the introduction
of residual stresses. Universal or standardized test methods of
4.3 Creep tests measure the time-dependent deformation of
a material under constant load at a given temperature. Creep surface preparation do not exist. It should be understood that
rupture tests provide a measure of the life of the material when final machining steps may or may not negate machining
subjected to constant mechanical loading at elevated tempera- damage introduced during the initial machining. Thus, test
tures. In selecting materials and designing parts for service at specimen fabrication history may play an important role in the
elevatedtemperatures,thetypeoftestdatausedwilldependon measured time-to-failure or deformation, and shall be reported.
the criteria for load-carrying capability which best defines the In addition, the nature of fabrication used for certain compos-
service usefulness of the material. ites (for example, chemical vapor infiltration or hot pressing)
may require the testing of specimens in the as-processed
4.4 Creepandcreeprupturetestsprovideinformationonthe
condition (that is, it may not be possible to machine the test
time-dependent deformation and on the time-of-failure of
specimen faces without compromising the in-plane fiber archi-
materials subjected to uniaxial tensile stresses at elevated
tecture).
temperatures. Uniform stress states are required to effectively
evaluate any nonlinear stress-strain behavior which may de- 5.3 Bending in uniaxial tests does induce nonuniform stress
velop as the result of cumulative damage processes (for distributions. Bending may be introduced from several sources
example, matrix cracking, matrix/fiber debonding, fiber including misaligned load trains, eccentric or misshaped
C1337 − 10 (2015)
specimens, and nonuniformly heated specimens or grips. In used shall be accurate within 61 % at any force within the
addition, if deformations or strains are measured at surfaces selected force range of the testing machine as defined in
where maximum or minimum stresses occur, bending may
Practices E4.
introduce over or under measurement of strains depending on
6.2 Gripping Devices:
the location of the strain measuring device on the test speci-
6.2.1 General—Various types of gripping devices may be
men.Similarly,fracturefromsurfaceflawsmaybeaccentuated
used to transmit the measured force applied by the testing
or suppressed by the presence of the nonuniform stresses
machine to the test specimens. The brittle nature of the
caused by bending.
matrices of CFCCs requires that a uniform interface exists
5.4 Fractures that initiate outside the uniformly stressed
between the grip components and the gripped section of the
gage section of a specimen may be due to factors such as stress
specimen. Line or point contacts and nonuniform pressure can
concentrations or geometrical transitions, extraneous stresses
produce Hertzian-type stresses leading to crack initiation and
introduced by gripping or thermal gradients, or strength limit-
fracture of the test specimen in the gripped section. Gripping
ing features in the microstructure of the test specimen. Such
devices can be classified generally as those employing active
non-gage section fractures will normally constitute invalid
and those employing passive grip interfaces as discussed in the
tests. In addition, for face-loaded test specimen geometries,
following sections. Grips located outside the heated zone
gripping pressure is a key variable in the initiation of fracture.
surrounding the specimen may or may not employ cooling.
Insufficient pressure can shear the outer plies in laminated
Uncooled grips located outside the heated zone are termed
CFCCs, while too much pressure can cause local crushing of
warm grips and generally reduce the thermal gradient in the
the CFCC and lead to fracture in the vicinity of the grips.
test specimen but at the expense of using high-temperature
5.5 The time-dependent stress redistribution that occurs at
alloy grips and increased degradation of the grips due to
elevated temperatures among the CFCC constituents makes it
exposure to the elevated-temperature environment. Cooled
necessary that the precise loading history of a creep test
grips located outside the heated zone are termed cold grips and
specimen be specified. This is of particular importance since
generally induce a steep thermal gradient along the length of
the rate at which a creep load is initially applied can influence
the specimen.
the subsequent creep behavior and damage modes. For
example, whether matrix cracking would occur at the end of
NOTE 1—The expense of the cooling system for cold grips is balanced
loading will depend on the magnitude of the loading rate, the
against maintaining alignment that remains consistent from test to test
(stable grip temperature) and decreased degradation of the grips due to
test stress, the test temperature and the relative creep resistance
3,4
exposure to the elevated-temperature environment. When grip cooling is
of the matrix with respect to that of the fibers.
employed, provisions shall be provided to control the cooling medium to
5.6 WhenCFCCsaremechanicallyunloadedeitherpartially
maximum fluctuations of 5 K (less than 1 K preferred) about a setpoint
temperature over the course of the test to minimize thermally induced
or totally after a creep test during which the test specimen
strain changes in the test specimen. In addition, opposing grip tempera-
accumulated time-dependent deformation, the specimen may
tures should be maintained at uniform and consistent temperatures not to
exhibit creep recovery as manifested by a time-dependent
exceed a difference 65 K (less than 61 K preferred) so as to avoid
reductionofstrain.Therateofcreeprecoveryisusuallyslower
inducingunequalthermalgradientsandsubsequentnonuniaxialstressesin
than the rate of creep deformation, and both creep and creep
the specimen. Generally, the need for control of grip temperature
recovery are in most cases thermally activated processes, fluctuations or differences may be indicated if test specimen gage section
temperatures cannot be maintained within the limits prescribed in 9.2.2.
making them quite sensitive to temperature. Often it is desired
to determine the
...
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: C1337 − 10 C1337 − 10 (Reapproved 2015)
Standard Test Method for
Creep and Creep Rupture of Continuous Fiber-Reinforced
Advanced Ceramics Under Tensile Loading at Elevated
Temperatures
This standard is issued under the fixed designation C1337; 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 the time-dependent deformation and time-to-rupture of continuous
fiber-reinforced ceramic composites under constant tensile loading at elevated temperatures. This test method addresses, but is not
restricted to, various suggested test specimen geometries. In addition, test specimen fabrication methods, allowable bending,
temperature measurements, temperature control, data collection, and reporting procedures are addressed.
1.2 This test method is intended primarily for use with all advanced ceramic matrix composites with continuous fiber
reinforcement: unidirectional (1-D), bidirectional (2-D), and tridirectional (3-D). In addition, this test method may also be used
with glass matrix composites with 1-D, 2-D, and 3-D continuous fiber reinforcement. This test method does not address directly
discontinuous fiber-reinforced, whisker-reinforced, or particulate-reinforced ceramics, although the test methods detailed here may
be equally applicable to these composites.
1.3 Values expressed in this test method are in accordance with the International System of Units (SI) and IEEE/ASTM SI 10
.
1.4 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. Hazard statements are noted in 7.1 and 7.2.
2. Referenced Documents
2.1 ASTM Standards:
C1145 Terminology of Advanced Ceramics
C1275 Test Method for Monotonic Tensile Behavior of Continuous Fiber-Reinforced Advanced Ceramics with Solid
Rectangular Cross-Section Test Specimens at Ambient Temperature
D3878 Terminology for Composite Materials
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E83 Practice for Verification and Classification of Extensometer Systems
E139 Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials
E220 Test Method for Calibration of Thermocouples By Comparison Techniques
E230 Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
E1012 Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force
Application
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 C28.07 on Ceramic Matrix
Composites.
Current edition approved Dec. 1, 2010July 1, 2015. Published January 2011September 2015. Originally approved in 1996. Last previous edition approved in 20052010
as C1337 – 96C1337 – 10. (2005). DOI: 10.1520/C1337-10.10.1520/C1337-10R15.
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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1337 − 10 (2015)
3.1.1 The definitions of terms relating to tensile testing appearing in Terminology E6 apply to the terms used in this test method.
The definitions relating to advanced ceramics appearing in Terminology C1145 apply to the terms used in this test method. The
definitions of terms relating to fiber reinforced composites appearing in Terminology D3878 apply to the terms used in this test
method. Additional terms used in conjunction with this test method are defined in the following:
3.1.2 continuous fiber-reinforced ceramic matrix composite (CFCC)—ceramic matrix composite in which the reinforcing phase
consists of a continuous fiber, continuous yarn, or a woven fabric.
3.1.3 fracture strength (F/L )—tensile stress that the material sustains at the instant of fracture. Fracture strength is calculated
from the force at fracture during a tension test carried to rupture and the original cross-sectional area of the test specimen.
3.1.3.1 Discussion—
In some cases, the fracture strength may be identical to the tensile strength if the load at fracture is the maximum for the test.
Factors such as load train compliance and fiber pull-out behavior may influence the fracture strength.
3.1.4 proportional limit stress—greatest stress which a material is capable of sustaining without any deviation from
proportionality of stress to strain (Hooke’s law).
3.1.4.1 Discussion—
Many experiments have shown that values observed for the proportional limit vary greatly with the sensitivity and accuracy of the
testing equipment, eccentricity of loading, the scale to which the stress-strain diagram is plotted, and other factors. When
determination of proportional limit is required, the procedure and sensitivity of the test equipment shall be specified.
3.1.5 slow crack growth—subcritical crack growth (extension) which may result from, but is not restricted to, such mechanisms
as environmentally assisted stress corrosion or diffusive crack growth.
4. Significance and Use
4.1 This test method may be used for material development, material comparison, quality assurance, characterization, and
design data generation.
4.2 Continuous fiber-reinforced ceramic matrix composites are candidate materials for structural applications requiring high
degrees of wear and corrosion resistance and toughness at high temperatures.
4.3 Creep tests measure the time-dependent deformation of a material under constant load at a given temperature. Creep rupture
tests provide a measure of the life of the material when subjected to constant mechanical loading at elevated temperatures. In
selecting materials and designing parts for service at elevated temperatures, the type of test data used will depend on the criteria
for load-carrying capability which best defines the service usefulness of the material.
4.4 Creep and creep rupture tests provide information on the time-dependent deformation and on the time-of-failure of materials
subjected to uniaxial tensile stresses at elevated temperatures. Uniform stress states are required to effectively evaluate any
nonlinear stress-strain behavior which may develop as the result of cumulative damage processes (for example, matrix cracking,
matrix/fiber debonding, fiber fracture, delamination, etc.) which may be influenced by testing mode, testing rate, processing or
alloying effects, environmental influences, or elevated temperatures. Some of these effects may be consequences of stress corrosion
or subcritical (slow) crack growth. It is noted that ceramic materials typically creep more rapidly in tension than in compression.
Therefore, creep data for design and life prediction should be obtained in both tension and compression.
4.5 The results of tensile creep and tensile creep rupture tests of specimens fabricated to standardized dimensions from a
particular material or selected portions of a part, or both, may not totally represent the creep deformation and creep rupture
properties of the entire, full-size end product or its in-service behavior in different environments or at various elevated
temperatures.
4.6 For quality control purposes, results derived from standardized tensile test specimens may be considered indicative of the
response of the material from which they were taken for given primary processing conditions and post-processing heat treatments.
5. Interferences
5.1 Test environment (vacuum, inert gas, ambient air, etc.) including moisture content (for example, relative humidity) may
have an influence on the creep and creep rupture behavior of CFCCs. In particular, the behavior of materials susceptible to slow
crack growth fracture and oxidation will be strongly influenced by test environment and test temperature. Testing can be conducted
in environments representative of service conditions to evaluate material performance under these conditions.
5.2 Surface preparation of test specimens, although normally not considered a major concern with CFCCs, can introduce
fabrication flaws which may have pronounced effects on the mechanical properties and behavior (for example, shape and level of
the resulting stress-strain-time curve, etc.). Machining damage introduced during test specimen preparation can be either a random
C1337 − 10 (2015)
interfering factor in the ultimate strength of pristine material (that is, increased frequency of surface-initiated fractures compared
to volumeinitiated fractures) or an inherent part of the strength characteristics to be measured. Surface preparation can also lead
to the introduction of residual stresses. Universal or standardized test methods of surface preparation do not exist. It should be
understood that final machining steps may or may not negate machining damage introduced during the initial machining. Thus,
test specimen fabrication history may play an important role in the measured time-to-failure or deformation, and shall be reported.
In addition, the nature of fabrication used for certain composites (for example, chemical vapor infiltration or hot pressing) may
require the testing of specimens in the as-processed condition (that is, it may not be possible to machine the test specimen faces
without compromising the in-plane fiber architecture).
5.3 Bending in uniaxial tests does induce nonuniform stress distributions. Bending may be introduced from several sources
including misaligned load trains, eccentric or misshaped specimens, and nonuniformly heated specimens or grips. In addition, if
deformations or strains are measured at surfaces where maximum or minimum stresses occur, bending may introduce over or under
measurement of strains depending on the location of the strain measuring device on the test specimen. Similarly, fracture from
surface flaws may be accentuated or suppressed by the presence of the nonuniform stresses caused by bending.
5.4 Fractures that initiate outside the uniformly stressed gage section of a specimen may be due to factors such as stress
concentrations or geometrical transitions, extraneous stresses introduced by gripping or thermal gradients, or strength limiting
features in the microstructure of the test specimen. Such non-gage section fractures will normally constitute invalid tests. In
addition, for face-loaded test specimen geometries, gripping pressure is a key variable in the initiation of fracture. Insufficient
pressure can shear the outer plies in laminated CFCCs, while too much pressure can cause local crushing of the CFCC and lead
to fracture in the vicinity of the grips.
5.5 The time-dependent stress redistribution that occurs at elevated temperatures among the CFCC constituents makes it
necessary that the precise loading history of a creep test specimen be specified. This is of particular importance since the rate at
which a creep load is initially applied can influence the subsequent creep behavior and damage modes. For example, whether
matrix cracking would occur at the end of loading will depend on the magnitude of the loading rate, the test stress, the test
3,4
temperature and the relative creep resistance of the matrix with respect to that of the fibers.
5.6 When CFCCs are mechanically unloaded either partially or totally after a creep test during which the test specimen
accumulated time-dependent deformation, the specimen may exhibit creep recovery as manifested by a time-dependent reduction
of strain. The rate of creep recovery is usually slower than the rate of creep deformation, and both creep and creep recovery are
in most cases thermally activated processes, making them quite sensitive to temperature. Often it is desired to determine the
retained strength of a CFCC after being subjected to creep for a prescribed period of time. Therefore, it is customary to unload
the test specimen from the creep stress and then reload it monotonically until failure. Under these circumstances, the time elapsed
between the end of the creep test and the conduction of the monotonic fast fracture test to determine the retained strength as well
as the loading and unloading rates will influence the rate of internal stress redistribution among the phases and hence the CFCC
strength.
6. Apparatus
6.1 Testing Machines—Machines used for tensile testing shall conform to the requirements of Practices E4. The forces used
shall be accurate within 61 % at any force within the selected force range of the testing machine as defined in Practices E4.
6.2 Gripping Devices:
6.2.1 General—Various types of gripping devices may be used to transmit the measured force applied by the testing machine
to the test specimens. The brittle nature of the matrices of CFCCs requires that a uniform interface exists between the grip
components and the gripped section of the specimen. Line or point contacts and nonuniform pressure can produce Hertzian-type
stresses leading to crack initiation and fracture of the test specimen in the gripped section. Gripping devices can be classified
generally as those employing active and those employing passive grip interfaces as discussed in the following sections. Grips
located outside the heated zone surrounding the specimen may or may not employ cooling. Uncooled grips located outside the
heated zone are termed warm grips and generally reduce the thermal gradient in the test specimen but at the expense of using
high-temperature alloy grips and increased degradation of the grips due to exposure to the elevated-temperature environment.
Cooled grips located outside the heated zone are termed cold grips and generall
...
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