ASTM C1337-96(2000)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:C 1337–96 (Reapproved 2000)
Standard Test Method for
Creep and Creep Rupture of Continuous Fiber-Reinforced
Ceramic Composites under Tensile Loading at Elevated
Temperatures
This standard is issued under the fixed designation C 1337; 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 and Their Composites
E 4 Practices for Force Verification of Testing Machines
1.1 This test method covers the determination of the time-
E 6 Terminology Relating to Methods of Mechanical Test-
dependent deformation and time-to-rupture of continuous
ing
fiber-reinforced ceramic composites under constant tensile
E 83 Practice for Verification and Classification of Exten-
loading at elevated temperatures. This test method addresses,
someters
but is not restricted to, various suggested test specimen
E 139 Practice for Conducting Creep, Creep Rupture, and
geometries. In addition, specimen fabrication methods, allow-
Stress Rupture Tests of Metallic Materials
able bending, temperature measurements, temperature control,
E 220 Method for Calibration of Thermocouples by Com-
data collection, and reporting procedures are addressed.
parison Techniques
1.2 This test method is intended primarily for use with all
E 230 Specification for Temperature-Electromotive Force
advanced ceramic matrix composites with continuous fiber
(EMF) Tables for Standardized Thermocouples
reinforcement: unidirectional (1-D), bidirectional (2-D), and
E 337 Test Method for Measuring Humidity with a Psy-
tridirectional (3-D). In addition, this test method may also be
chrometer (The Measurement of Wet- and Dry-Bulb Tem-
used with glass matrix composites with 1-D, 2-D, and 3-D
peratures)
continuous fiber reinforcement. This test method does not
E 380 Practice for Use of the International System of Units
address directly discontinuous fiber-reinforced, whisker-
(SI) (the Modernized Metric System)
reinforced, or particulate-reinforced ceramics, although the test
E 1012 Practice for Verification of Specimen Alignment
methods detailed here may be equally applicable to these
under Tensile Loading
composites.
1.3 Values expressed in this test method are in accordance
3. Terminology
with the International System of Units (SI) and Practice E 380.
3.1 Definitions—The definitions of terms relating to tensile
1.4 This standard does not purport to address all of the
testingappearinginTerminologyE 6applytothetermsusedin
safety concerns, if any, associated with its use. It is the
this test method. The definitions relating to advanced ceramics
responsibility of the user of this standard to establish appro-
appearing in Terminology C 1145 apply to the terms used in
priate safety and health practices and determine the applica-
this test method. The definitions of terms relating to fiber
bility of regulatory limitations prior to use. Hazard statements
reinforced composites appearing in Terminology D 3878 apply
are noted in 7.1 and 7.2.
to the terms used in this test method. Additional terms used in
2. Referenced Documents conjunction with this test method are defined in the following:
3.1.1 continuous fiber-reinforced ceramic matrix composite
2.1 ASTM Standards:
(CFCC)—a ceramic matrix composite in which the reinforcing
C 1145 Terminology on Advanced Ceramics
phase consists of a continuous fiber, continuous yarn, or a
C 1275 Test Method for Monotonic Tensile Strength Test-
woven fabric.
ing of Continuous Fiber-Reinforced Advanced Ceramics
3.1.2 fracture strength—the tensile stress which the mate-
with Solid Rectangular Cross-Section Specimens at Am-
rial sustains at the instant of fracture. Fracture strength is
bient Temperatures
D 3878 Terminology of High-Modulus Reinforcing Fibers
Annual Book of ASTM Standards, Vol 15.03.
1 4
This test method is under the jurisdiction of ASTM Committee C-28 on Annual Book of ASTM Standards, Vol 03.01.
Advanced Ceramics and is the direct responsibility of Subcommittee C28.07 on Discontinued; see 1994 Annual Book of ASTM Standards, Vol 14.03.
Ceramic Matrix Composites. Annual Book of ASTM Standards, Vol 14.03.
Current edition approved June 10, 1996. Published August 1996. Annual Book of ASTM Standards, Vol 11.03.
2 8
Annual Book of ASTM Standards, Vol 15.01. Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C 1337–96 (2000)
calculatedfromtheloadatfractureduringatensiontestcarried 4.6 For quality control purposes, results derived from stan-
toruptureandtheoriginalcross-sectionalareaofthespecimen. dardizedtensiletestspecimensmaybeconsideredindicativeof
3.1.3 Discussion—In some cases, the fracture strength may the response of the material from which they were taken for
be identical to the tensile strength if the load at fracture is the given primary processing conditions and post-processing heat
maximum for the test. Factors such as load train compliance treatments.
and fiber pull-out behavior may influence the fracture strength.
3.1.4 proportional limit stress—the greatest stress which a
5. Interferences
material is capable of sustaining without any deviation from
5.1 Test environment (vacuum, inert gas, ambient air, etc.)
proportionality of stress to strain (Hooke’s law).
including moisture content (for example, relative humidity)
3.1.5 Discussion—Many experiments have shown that val-
may have an influence on the creep and creep rupture behavior
ues observed for the proportional limit vary greatly with the
of CFCCs. In particular, the behavior of materials susceptible
sensitivity and accuracy of the testing equipment, eccentricity
to slow crack growth fracture and oxidation will be strongly
of loading, the scale to which the stress-strain diagram is
influenced by test environment and test temperature. Testing
plotted, and other factors. When determination of proportional
can be conducted in environments representative of service
limit is required, the procedure and sensitivity of the test
conditions to evaluate material performance under these con-
equipment shall be specified.
ditions.
3.1.6 slow crack growth—subcritical crack growth (exten-
5.2 Surface preparation of test specimens, although nor-
sion) which may result from, but is not restricted to, such
mally not considered a major concern with CFCCs, can
mechanisms as environmentally assisted stress corrosion or
introducefabricationflawswhichmayhavepronouncedeffects
diffusive crack growth.
on the mechanical properties and behavior (for example, shape
and level of the resulting stress-strain-time curve, etc.). Ma-
4. Significance and Use
chiningdamageintroducedduringspecimenpreparationcanbe
4.1 This test method may be used for material development,
either a random interfering factor in the ultimate strength of
material comparison, quality assurance, characterization, and
pristine material (that is, increased frequency of surface-
design data generation.
initiated fractures compared to volumeinitiated fractures) or an
4.2 Continuous fiber-reinforced ceramic matrix composites
inherent part of the strength characteristics to be measured.
are candidate materials for structural applications requiring
Surfacepreparationcanalsoleadtotheintroductionofresidual
high degrees of wear and corrosion resistance and toughness at
stresses. Universal or standardized test methods of surface
high temperatures.
preparation do not exist. It should be understood that final
4.3 Creep tests measure the time-dependent deformation of
machining steps may or may not negate machining damage
a material under constant load at a given temperature. Creep
introduced during the initial machining. Thus, specimen fabri-
rupture tests provide a measure of the life of the material when
cation history may play an important role in the measured
subjected to constant mechanical loading at elevated tempera-
time-to-failure or deformation, and shall be reported. In addi-
tures. In selecting materials and designing parts for service at
tion, the nature of fabrication used for certain composites (for
elevatedtemperatures,thetypeoftestdatausedwilldependon
example, chemical vapor infiltration or hot pressing) may
the criteria for load carrying capability which best defines the
require the testing of specimens in the as-processed condition
service usefulness of the material.
(that is, it may not be possible to machine the specimen faces
4.4 Creepandcreeprupturetestsprovideinformationonthe
without compromising the in-plane fiber architecture).
time-dependent deformation and on the time-of-failure of
5.3 Bending in uniaxial tests does induce nonuniform stress
materials subjected to uniaxial tensile stresses at elevated
distributions. Bending may be introduced from several sources
temperatures. Uniform stress states are required to effectively
including misaligned load trains, eccentric or misshaped speci-
evaluate any nonlinear stress-strain behavior which may de-
mens, and nonuniformly heated specimens or grips. In addi-
velop as the result of cumulative damage processes (for
tion, if deformations or strains are measured at surfaces where
example, matrix cracking, matrix/fiber debonding, fiber frac-
maximum or minimum stresses occur, bending may introduce
ture, delamination, etc.) which may be influenced by testing
over or under measurement of strains depending on the
mode, testing rate, processing or alloying effects, environmen-
location of the strain measuring device on the specimen.
tal influences, or elevated temperatures. Some of these effects
Similarly, fracture from surface flaws may be accentuated or
may be consequences of stress corrosion or subcritical (slow)
suppressed by the presence of the nonuniform stresses caused
crack growth. It is noted that ceramic materials typically creep
by bending.
more rapidly in tension than in compression. Therefore, creep
data for design and life prediction should be obtained in both 5.4 Fractures that initiate outside the uniformly stressed
tension and compression. gage section of a specimen may be due to factors such as stress
4.5 The results of tensile creep and tensile creep rupture concentrations or geometrical transitions, extraneous stresses
tests of specimens fabricated to standardized dimensions from introduced by gripping or thermal gradients, or strength limit-
a particular material or selected portions of a part, or both, may ing features in the microstructure of the specimen. Such
not totally represent the creep deformation and creep rupture non-gage section fractures will normally constitute invalid
properties of the entire, full-size end product or its in-service tests.Inaddition,forface-loadedgeometries,grippingpressure
behavior in different environments or at various elevated is a key variable in the initiation of fracture. Insufficient
temperatures. pressure can shear the outer plies in laminated CFCCs, while
C 1337–96 (2000)
too much pressure can cause local crushing of the CFCC and grips and increased degradation of the grips due to exposure to
lead to fracture in the vicinity of the grips. the elevated-temperature environment. Cooled grips located
5.5 The time-dependent stress redistribution that occurs at
outside the heated zone are termed cold grips and generally
elevated temperatures among the CFCC constituents makes it
induce a steep thermal gradient along the length of the
necessary that the precise loading history of a creep specimen
specimen.
be specified. This is of particular importance since the rate at
NOTE 1—The expense of the cooling system for cold grips is balanced
which a creep load is initially applied can influence the
against maintaining alignment that remains consistent from test to test
subsequent creep behavior and damage modes. For example,
(stable grip temperature) and decreased degradation of the grips due to
whether matrix cracking would occur at the end of loading will
exposure to the elevated-temperature environment. When grip cooling is
depend on the magnitude of the loading rate, the test stress, the
employed, provisions shall be provided to control the cooling medium to
test temperature and the relative creep resistance of the matrix
maximum fluctuations of 5 K (less than 1 K preferred) about a setpoint
,
9 10
with respect to that of the fibers.
temperature over the course of the test to minimize thermally induced
5.6 WhenCFCCsaremechanicallyunloadedeitherpartially strain changes in the specimen. In addition, opposing grip temperatures
shouldbemaintainedatuniformandconsistenttemperaturesnottoexceed
or totally after a creep test during which the specimen
a difference 65 K (less than6 1 K preferred) so as to avoid inducing
accumulated time-dependent deformation, the specimen may
unequal thermal gradients and subsequent nonuniaxial stresses in the
exhibit creep recovery as manifested by a time-dependent
specimen. Generally, the need for control of grip temperature fluctuations
reductionofstrain.Therateofcreeprecoveryisusuallyslower
or differences may be indicated if specimen gage section temperatures
than the rate of creep deformation, and both creep and creep
cannot be maintained within the limits prescribed in 9.2.2.
recovery are in most cases thermally activated processes,
6.2.1.1 Active Grip Interfaces—Active grip interfaces re-
making them quite sensitive to temperature. Often it is desired
quire a continuous application of a mechanical, hydraulic, or
to determine the retained strength of a CFCC after being
pneumatic force to transmit the load to the test specimen.
subjected to creep for a prescribed period of time. Therefore, it
Generally, these types of grip interfaces cause a load to be
is customary to unload the specimen from the creep stress and
applied normal to the surface of the gripped section of the
then reload it monotonically until failure. Under these circum-
specimen.Transmission of the uniaxial load applied by the test
stances, the time elapsed between the end of the creep test and
the conduction of the monotonic fast fracture test to determine machine is then accomplished by friction between the speci-
men and the grip faces. Thus, important aspects of active grip
the retained strength as well as the loading and unloading rates
will influence the rate of internal stress redistribution among interfaces are: (1) uniform contact between the gripped section
the phases and hence the CFCC strength. of the specimen and the grip faces and (2) constant coefficient
of friction over the grip/specimen inter
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