ASTM C1291-18

Standard Test Method for Elevated Temperature Tensile Creep Strain, Creep Strain Rate, and Creep Time to Failure for Monolithic Advanced Ceramics

SIGNIFICANCE AND USE
4.1 Creep tests measure the time-dependent deformation under force at a given temperature, and, by implication, the force-carrying capability of the material for limited deformations. Creep rupture tests, properly interpreted, provide a measure of the force-carrying capability of the material as a function of time and temperature. The two tests complement each other in defining the force-carrying capability of a material for a given period of time. In selecting materials and designing parts for service at elevated temperatures, the type of test data used will depend on the criteria for force-carrying capability that best defines the service usefulness of the material.  
4.2 This test method may be used for material development, quality assurance, characterization, and design data generation.  
4.3 High-strength, monolithic ceramic materials, generally characterized by small grain sizes (  
4.4 Data obtained for design and predictive purposes shall be obtained using any appropriate combination of test methods that provide the most relevant information for the applications being considered. It is noted here that ceramic materials tend to creep more rapidly in tension than in compression (1-3).4 This difference results in time-dependent changes in the stress distribution and the position of the neutral axis when tests are conducted in flexure. As a consequence, deconvolution of flexural creep data to obtain the constitutive equations needed for design cannot be achieved without some degree of uncertainty concerning the form of the creep equations, and the magnitude of the creep rate in tension vis-a-vis the creep rate in compression. Therefore, creep data for design and life prediction shall be obtained in both tension and compression, as well as the expected service stress state.
SCOPE
1.1 This test method covers the determination of tensile creep strain, creep strain rate, and creep time to failure for advanced monolithic ceramics at elevated temperatures, typically between 1073 and 2073 K. A variety of test specimen geometries are included. The creep strain at a fixed temperature is evaluated from direct measurements of the gage length extension over the time of the test. The minimum creep strain rate, which may be invariant with time, is evaluated as a function of temperature and applied stress. Creep time to failure is also included in this test method.  
1.2 This test method is for use with advanced ceramics that behave as macroscopically isotropic, homogeneous, continuous materials. While this test method is intended for use on monolithic ceramics, whisker- or particle-reinforced composite ceramics as well as low-volume-fraction discontinuous fiber-reinforced composite ceramics may also meet these macroscopic behavior assumptions. Continuous fiber-reinforced ceramic composites (CFCCs) do not behave as macroscopically isotropic, homogeneous, continuous materials, and application of this test method to these materials is not recommended.  
1.3 The values in SI units are to be regarded as the standard (see IEEE/ASTM SI 10). The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 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
• ICS: 81.060.99 - Other standards related to ceramics
• Technical Committee: C28 - Advanced Ceramics
• Drafting Committee: C28.01 - Mechanical Properties and Performance

Relations

• Replaces: ASTM C1291-16 - Standard Test Method for Elevated Temperature Tensile Creep Strain, Creep Strain Rate, and Creep Time-to-Failure for Monolithic Advanced Ceramics
• Effective Date: 01-Jan-2018
• 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 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 E177-14 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2014
• Refers: ASTM E220-13 - Standard Test Method for Calibration of Thermocouples By Comparison Techniques
• Effective Date: 01-Nov-2013
• Refers: ASTM E177-13 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2013
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013
• Refers: ASTM C1145-06(2013)e1 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Feb-2013
• Refers: ASTM C1145-06(2013) - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Feb-2013
• Refers: ASTM E1012-12e1 - Standard Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• Effective Date: 01-Jun-2012
• Refers: ASTM E1012-12 - Standard Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• Effective Date: 01-Jun-2012
• Refers: ASTM E691-11 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Nov-2011
• Refers: ASTM E139-11 - Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials
• Effective Date: 01-Jun-2011

Overview

ASTM C1291-18 outlines the standard test method for determining elevated temperature tensile creep strain, creep strain rate, and creep time to failure for monolithic advanced ceramics. Developed by ASTM Committee C28, this method enables precise measurement and evaluation of the high-temperature force-carrying capability of advanced ceramics. The method is applicable for a range of advanced ceramic materials exhibiting macroscopically isotropic and homogeneous behavior, primarily in the temperature range of 1073 to 2073 K.

Creep, a time-dependent deformation under load, is a critical consideration for ceramics used in structural and high-temperature environments. ASTM C1291-18 supports activities in material development, quality assurance, and data generation for engineering design.

Key Topics

• Tensile Creep Strain Measurement: Direct measurement of gage length extension over time at controlled high temperatures.
• Creep Strain Rate Evaluation: Determination of the minimum creep strain rate as a function of temperature and applied stress.
• Creep Time to Failure: Assessment of the time required for a specimen to fail under constant load, vital for life prediction and reliability.
• Test Specimen Requirements: Covers various specimen geometries for advanced ceramics, including guidance on preparation to mitigate surface flaws and stress concentrations.
• Relevant Material Types:
  • Monolithic advanced ceramics
  • Whisker- or particle-reinforced ceramic composites (if macroscopically isotropic and continuous)
• Applicability Limitations: Not recommended for continuous fiber-reinforced ceramic composites, which do not behave as homogeneous, isotropic materials.
• Environmental and Mechanical Conditions: Includes the influence of environment (air, inert, vacuum), surface preparation, gripping technique, load-train alignment, and temperature control.
• Data for Engineering Use: Both tensile and compressive creep data are necessary for predictive modeling and design, due to differing creep rates under tension and compression.

Applications

ASTM C1291-18 is crucial for industries and research areas where ceramics are utilized under high-temperature and structural load conditions, such as:

• Turbine Engines: Evaluation of ceramic turbine blades and hot-section components for durability and performance.
• Energy Generation: Assessment of ceramic components in power plants, such as combustion liners and thermal barrier coatings.
• Automotive and Aerospace: Testing of load-bearing ceramic parts exposed to extreme thermal environments.
• Material Development: Facilitates comparison and improvement of high-performance ceramic compositions by providing reliable creep data.
• Quality Assurance: Assurance of material consistency, mechanical reliability, and prediction of in-service life through standardized testing.

Data obtained via this method informs selection, design, and maintenance schedules for advanced ceramics used in mission-critical high-temperature applications.

Related Standards

• ASTM C1273: Test method for tensile strength of monolithic advanced ceramics at ambient temperatures.
• ASTM E4: Verification of testing machine force.
• ASTM E83: Verification and classification of extensometer systems.
• ASTM E139: Methods for conducting creep, creep-rupture, and stress-rupture tests of metallic materials.
• ASTM C1145: Terminology of advanced ceramics.
• IEEE/ASTM SI 10: Usage of SI units in testing and data reporting.

These related standards help ensure test consistency, proper calibration, and accurate interpretation of results across laboratories and industries.

---

By adhering to ASTM C1291-18, organizations can achieve reliable, repeatable, and relevant data for advanced ceramics subjected to elevated temperature creep, supporting robust engineering design and material qualification in demanding applications.

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: C1291 − 18
Standard Test Method for
Elevated Temperature Tensile Creep Strain, Creep Strain
Rate, and Creep Time to Failure for Monolithic Advanced
Ceramics
This standard is issued under the fixed designation C1291; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.1 This test method covers the determination of tensile
Barriers to Trade (TBT) Committee.
creep strain, creep strain rate, and creep time to failure for
advanced monolithic ceramics at elevated temperatures, typi-
2. Referenced Documents
cally between 1073 and 2073 K. A variety of test specimen
2.1 ASTM Standards:
geometriesareincluded.Thecreepstrainatafixedtemperature
C1145Terminology of Advanced Ceramics
is evaluated from direct measurements of the gage length
C1273Test Method for Tensile Strength of Monolithic
extension over the time of the test. The minimum creep strain
Advanced Ceramics at Ambient Temperatures
rate, which may be invariant with time, is evaluated as a
E4Practices for Force Verification of Testing Machines
function of temperature and applied stress. Creep time to
E6Terminology Relating to Methods of MechanicalTesting
failure is also included in this test method.
E83Practice for Verification and Classification of Exten-
1.2 This test method is for use with advanced ceramics that
someter Systems
behave as macroscopically isotropic, homogeneous, continu-
E139Test Methods for Conducting Creep, Creep-Rupture,
ous materials. While this test method is intended for use on
and Stress-Rupture Tests of Metallic Materials
monolithicceramics,whisker-orparticle-reinforcedcomposite
E177Practice for Use of the Terms Precision and Bias in
ceramics as well as low-volume-fraction discontinuous fiber-
ASTM Test Methods
reinforced composite ceramics may also meet these macro-
E220Test Method for Calibration of Thermocouples By
scopic behavior assumptions. Continuous fiber-reinforced ce-
Comparison Techniques
ramic composites (CFCCs) do not behave as macroscopically
E230Specification and Temperature-Electromotive Force
isotropic, homogeneous, continuous materials, and application
(EMF) Tables for Standardized Thermocouples
of this test method to these materials is not recommended.
E639Test Method for Measuring Total-Radiance Tempera-
ture of Heated Surfaces Using a Radiation Pyrometer
1.3 The values in SI units are to be regarded as the standard
(Withdrawn 2011)
(see IEEE/ASTM SI 10). The values given in parentheses are
E691Practice for Conducting an Interlaboratory Study to
mathematical conversions to inch-pound units that are pro-
Determine the Precision of a Test Method
vided for information only and are not considered standard.
E1012Practice for Verification of Testing Frame and Speci-
1.4 This standard does not purport to address all of the
men Alignment Under Tensile and Compressive Axial
safety concerns, if any, associated with its use. It is the
Force Application
responsibility of the user of this standard to establish appro-
IEEE/ASTM SI 10American National Standard for Use of
priate safety, health, and environmental practices and deter-
theInternationalSystemofUnits(SI):TheModernMetric
mine the applicability of regulatory limitations prior to use.
System
1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard- 3. Terminology
ization established in the Decision on Principles for the
3.1 Definitions—The definitions of terms relating to creep
testing which appear in Section E of Terminology E6 shall
1 2
This test method is under the jurisdiction of ASTM Committee C28 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Mechanical Properties and Performance. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Jan. 1, 2018. Published January 2018. Originally the ASTM website.
approved in 1995. Last previous edition approved in 2016 as C1291– 16. DOI: The last approved version of this historical standard is referenced on
10.1520/C1291-18. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1291 − 18
apply to the terms used in this test method. For the purpose of 4. Significance and Use
this test method only, some of the more general terms are used
4.1 Creep tests measure the time-dependent deformation
with the restricted meanings given as follows.
under force at a given temperature, and, by implication, the
3.2 Definitions of Terms Specific to This Standard: force-carrying capability of the material for limited deforma-
tions. Creep rupture tests, properly interpreted, provide a
3.2.1 axial strain, ε , [L/L], n—average of the strain mea-
a
sured on diametrically opposed sides and equally distant from measure of the force-carrying capability of the material as a
function of time and temperature. The two tests complement
the test specimen axis.
each other in defining the force-carrying capability of a
3.2.2 bending strain, ε [L/L], n—difference between the
b
material for a given period of time. In selecting materials and
strain at the surface and the axial strain.
designingpartsforserviceatelevatedtemperatures,thetypeof
3.2.2.1 Discussion—In general, it varies from point to point
test data used will depend on the criteria for force-carrying
aroundandalongthegagelengthofthetestspecimen. (E1012)
capability that best defines the service usefulness of the
3.2.3 creep rupture test, n—test in which progressive test
material.
specimen deformation and the time to failure are measured. In
4.2 Thistestmethodmaybeusedformaterialdevelopment,
general, deformation is greater than that developed during a
qualityassurance,characterization,anddesigndatageneration.
creep test.
4.3 High-strength, monolithic ceramic materials, generally
3.2.4 creep strain, ε, [L/L], n—time-dependent strain that
characterized by small grain sizes (<50 µm) and bulk densities
occurs after the application of force which is thereafter
near their theoretical density, are candidates for load-bearing
maintained constant. Also known as engineering creep strain.
structural applications at elevated temperatures. These appli-
3.2.5 creep test, n—test that has as its objective the mea-
cations involve components such as turbine blades which are
surement of creep and creep rates occurring at stresses usually
subjected to stress gradients and multiaxial stresses.
well below those that would result in fast fracture.
4.4 Data obtained for design and predictive purposes shall
3.2.5.1 Discussion—Since the maximum deformation is
beobtainedusinganyappropriatecombinationoftestmethods
only a few percent, a sensitive extensometer is required.
that provide the most relevant information for the applications
3.2.6 creeptimetofailure,t,[T],n—timerequiredforatest
f beingconsidered.Itisnotedherethatceramicmaterialstendto
specimen to fracture under constant force as a result of creep.
creep more rapidly in tension than in compression (1-3). This
3.2.6.1 Discussion—This is also known as creep rupture
difference results in time-dependent changes in the stress
time.
distribution and the position of the neutral axis when tests are
conducted in flexure. As a consequence, deconvolution of
3.2.7 gage length, l, [L], n—original distance between
flexural creep data to obtain the constitutive equations needed
fiducial markers on or attached to the test specimen for
for design cannot be achieved without some degree of uncer-
determining elongation.
tainty concerning the form of the creep equations, and the
3.2.8 maximum bending strain, ε , [L/L], n—largest
bmax
magnitude of the creep rate in tension vis-a-vis the creep rate
value of bending strain along the gage length. It can be
in compression. Therefore, creep data for design and life
calculatedfrommeasurementsofstrainatthreecircumferential
prediction shall be obtained in both tension and compression,
positions at each of two different longitudinal positions.
as well as the expected service stress state.
−1
3.2.9 minimum creep strain rate, ε ,[T ], n—minimum
min
value of the strain rate prior to test specimen failure as 5. Interferences
measured from the strain-time curve. The minimum creep
5.1 Time-Dependent Phenomena—Other time-dependent
strain rate may not necessarily correspond to the steady-state
phenomena, such as stress corrosion and slow crack growth,
creep strain rate.
can interfere with determination of the creep behavior.
3.2.10 slow crack growth, (SCG), n—subcritical crack
5.2 Chemical Interactions with the Testing Environment—
growth (extension) which may result from, but is not restricted
The test environment (vacuum, inert gas, ambient air, etc.)
to, such mechanisms as environmentally assisted stress
including moisture content (for example, % relative humidity
corrosion, diffusive crack growth, or other mechanisms.
(RH))mayhaveastronginfluenceonbothcreepstrainrateand
(C1145)
creep rupture life. In particular, materials susceptible to slow
3.2.11 steady-state creep, ε , [L/L], n—stage of creep crack growth failure will be strongly influenced by the test
ss
wherein the creep rate is constant with time. environment.Surfaceoxidationmaybeeitheractiveorpassive
and thus will have a direct effect on creep behavior by
3.2.11.1 Discussion—Also known as secondary creep.
changing the material’s properties. Testing shall be conducted
3.2.12 stress corrosion, n—environmentally induced degra-
in environments that are either representative of service con-
dation that initiates from the exposed surface.
ditions or inert to the materials being tested depending on the
3.2.12.1 Discussion—Such environmental effects com-
performance being evaluated. A controlled gas environment
monlyincludetheactionofmoisture,aswellasothercorrosive
species, often with a strong temperature dependence.
3.2.13 tensile creep strain, ε, [L/L], n—creep strain that
t Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
occurs as a result of a uniaxial tensile-applied stress. this test method.
C1291 − 18
with suitable effluent controls shall be provided for any is monitored until the end-of-test condition has been reached,
material that evolves toxic vapors. there will be no record of percent bending for each test
specimen. The testing system alignment including the test
5.3 Test Specimen Surfaces—Surface preparation of test
machine, gripping devices (as described in 6.2), and load train
specimens can introduce machining flaws that may affect the
couplers (as described in 6.3), must be verified using the
test results. Machining damage imposed during test specimen
procedure detailed in the appendix such that the percent
preparation will most likely result in premature failure of the
bendingdoesnotexceedfiveatameanstressequaltoone-half
test specimen but may also introduce flaws that can grow by
the anticipated test stress. This verification shall be conducted
slow crack growth. Surface preparation can also lead to
at a minimum at the beginning and the end of each test series.
residual stresses which can be released during the test. Uni-
An additional verification of alignment is recommended, al-
versal or standardized methods of surface preparation do not
though not required, at the middle of the test series. Either a
exist. It shall be understood that final machining steps may or
dummy or actual test specimen may be used. Tensile test
may not negate machining damage introduced during earlier
specimens used for alignment verification shall be equipped
phases of machining, which tend to be rougher.
witharecommendedeightseparatelongitudinalstraingagesto
5.4 Test Specimen/Extensometer Chemical
determine bending contributions from both eccentric and
Incompatibility—Thestrainmeasurementtechniquesdescribed
angular misalignment of the grip heads. (Although it is
hereingenerallyrelyonphysicalcontactbetweenextensometer
possible to use a minimum of six separate longitudinal strain
components(contactingprobesoropticalmethodflags)andthe
gages for test specimens with circular cross sections, eight
test specimen so as to measure changes in the gage section as
strain gages are recommended here for simplicity and consis-
a function of time. Flag attachment methods and extensometer
tency in describing the technique for both circular and rectan-
contact materials shall be chosen with care to ensure that no
gular cross sections.) If dummy test specimens are used for
adversechemicalreactionsoccurduringtesting.Normally,this
alignment verification, they shall have the same geometry and
is not a problem if test specimen/probe materials that are
dimensions as the actual test specimens as well as an elastic
mutually chemically inert are employed (for example, SiC
modulusthatcloselymatchesthatofthetestmaterialtoensure
probes on Si N test specimens). The user must be aware that
3 4
similar axial and bending stiffness characteristics.
impurities or second phases in the flags or test specimens may
6.2 Gripping Devices:
be mutually chemically reactive and could influence the
6.2.1 Various types of gripping devices may be used to
results.
transmit the measured force applied by the test machine to the
5.5 Test Specimen Bending—Bending in uniaxial tensile
test specimens. The brittle nature of advanced ceramics re-
tests can cause extraneous strains or promote accelerated
quires a uniform interface between the grip components and
rupturetimes.Sincemaximumorminimumstresseswilloccur
the gripped section of the test specimen. Line or point contacts
at the surface where strain measurements are made, bending
and nonuniform pressure can produce Hertzian-type stresses
may introduce either an over or under measurement of axial
leading to crack initiation and fracture of the test specimen in
strain, if the measurement is made only on one side of the
the gripped section. Gripping devices can be classed generally
tensile test specimen. Similarly, bending stresses may accen-
as those employing active and those employing passive grip
tuate surface oxidation and may also accentuate the severity of
interfacesasdiscussedinthefollowingsections.Regardlessof
surface flaws.
the type of gripping device chosen, it shall be consistent with
5.6 Temperature Variations—Creep strain is often related to
the thermal requirements imposed on it by the elevated
temperaturethroughanexponentialfunction.Thusfluctuations
temperature nature of creep testing. This requirement may
in test temperature or change in temperature profile along the
preclude the use of some material combinations and gripping
length of the test specimen in real time can cause fluctuations
designs.
in strain measurements or changes in creep rate.
6.2.1.1 Active Grip Interfaces—Active grip interfaces re-
quire a continuous application of a mechanical, hydraulic, or
6. Apparatus
pneumatic force to transmit the force applied by the test
6.1 Force Test Machine: machine to the test specimen. Generally, these types of grip
6.1.1 Test specimens may be loaded in any suitable test interfaces cause a force to be applied normal to the surface of
machine provided that uniform, direct loading can be main- the gripped section of the test specimen. Transmission of the
tained. The test machine must maintain the desired constant uniaxialforceappliedbythetestmachineisthenaccomplished
force on the test specimen regardless of test specimen defor- by friction between the test specimen and the grip faces.Thus,
mation with time, either through dead-weight loading or important aspects of active grip interfaces are uniform contact
through active force control. The force measuring system can between the gripped section of the test specimen and the grip
be equipped with a means for retaining readout of the force, or faces, and constant coefficient of friction over the interface
the force can be recorded manually. The accuracy of the test between the test specimen and grip.
machine shall be in accordance with Practices E4. (1)For cylindrical test specimens, a one-piece split collet
6.1.2 Allowable Bending—Allowablebending,asdefinedin arrangement acts as the grip interface (4, 5). Generally, close
Practice E1012, shall not exceed 5%. This is based on the tolerances are required for concentricity of both the grip and
sameassumptionsasthosefortensilestrengthtesting(seeRef. test specimen diameters. In addition, the diameter of the
(4), for example). It shall be noted that unless percent bending gripped section of the test specimen and the unclamped, open
C1291 − 18
diameter of the grip faces shall be within similarly close couplers chosen, they shall be consistent with the thermal
tolerancestopromoteuniformcontactatthetestspecimen/grip requirements imposed on them by the elevated temperature
interface.Toleranceswillvarydependingontheexactconfigu- nature of creep testing. These requirements may preclude the
ration used. use of some material combinations and load train designs.
(2)For flat test specimens, flat-face, wedge-grip faces act 6.3.2 Fixed Load Train Couplers—Fixed couplers may
as the grip interface. Generally, close tolerances are required incorporate devices that require either a one-time, pretest
for the flatness and parallelism as well as wedge angle of the alignment adjustment of the load train which remains constant
grip faces. In addition, the thickness, flatness, and parallelism for all subsequent tests or an in situ, pretest alignment of the
of the gripped section of the test specimen shall be within loadtrainwhichisconductedseparatelyforeachtestspecimen
similarlyclosetolerancestopromoteuniformcontactatthetest and each test. Such devices (11, 12) usually employ angularity
specimen/gripinterface.Toleranceswillvarydependingonthe and concentricity adjusters to accommodate inherent load train
exact configuration used. misalignments.Regardlessofwhichmethodisused,alignment
verification shall be performed as discussed in 6.1.2.
6.2.1.2 Passive Grip Interfaces—Passive grip interfaces
6.3.3 Non-Fixed Load Train Couplers—Non-fixed couplers
transmit the force applied by the test machine to the test
may incorporate devices that promote self-alignment of the
specimen through a direct mechanical link. Generally, these
load train during the movement of the crosshead or actuator.
mechanicallinkstransmitthetestforcestothetestspecimenby
Generally, such devices rely upon freely moving linkages to
means of geometrical features of the test specimens such as
eliminate applied moments as the load train components are
button-head fillets, shank shoulders, or holes in the gripped
loaded. Knife edges, universal joints, hydraulic couplers, and
head. Thus, the important aspect of passive grip interfaces is
air bearings are examples (7, 11, 13-15) of such devices.
uniform contact between the gripped section of the test
Although non-fixed load couplers are intended to be self-
specimen and the grip faces.
aligning and thus eliminate the need to evaluate the bending in
(1)For cylindrical test specimens, a multi-piece split collet
the test specimen for each test, the operation of the couplers
arrangement acts as the grip interface at button-head fillets of
shall be verified as discussed in 6.1.2.
thetestspecimen (6).Becauseofthelimitedcontactareaatthe
test specimen/grip interface, soft, deformable metallic collets
6.4 Heating Apparatus:
maybeusedtotransfertheaxialforcetotheexactgeometryof
6.4.1 The apparatus for and method of heating the test
the test specimen. In some cases, tapered collets may be used
specimens shall provide the temperature control necessary to
to transfer the axial force to the shank of the test specimen
satisfy the requirements specified in 6.4.2 without manual
ratherthanintothebutton-headradius (6).Generally,moderate
adjustments more frequent than once in each 24-h period after
tolerances on the collet height shall be maintained to promote
force application. It shall also satisfy the requirements of the
uniform axial-loading at the test specimen/grip interface.
testing environment in 6.4.3.
Tolerances will vary depending on the exact configuration
6.4.2 Temperature—The furnace shall be capable of main-
used.
tainingthetensiletestspecimentemperatureconstantwithtime
(2)For flat test specimens, pins or pivots act as grip
to 2 K. The temperature readout device shall have a resolution
interfaces at either the shoulders of the test specimen shank (7,
of1Kor less. The furnace system shall be such that thermal
8) or at holes in the gripped test specimen head (9, 10).
gradients are minimal in the tensile test specimen so that no
Generally,closetolerancesofshoulderradiiandgripinterfaces
more than a 5-K differential exists in the test specimen gage
are required to promote uniform contact along the entire test
length at temperatures up to 1773 K.
specimen/grip interface as well as to provide for non-eccentric
6.4.3 Environment—The furnace may have an air, inert, or
loading. Generally, very close tolerances are required for
vacuum environment as required. If an inert or vacuum
longitudinal coincidence of the pin and the hole centerlines.
chamber is used, and it is necessary to direct force through
bellows, fittings, or seal, then it shall be verified that force
6.3 Load Train Couplers:
losses or errors do not exceed 1% of the applied force.
6.3.1 Various types of devices (load train couplers) may be
6.5 Temperature Measuring Devices:
used to attach the active or passive grip interface assemblies to
6.5.1 The method of temperature measurement shall be
the test machine as discussed in Test Method C1273. The load
sufficientlysensitiveandreliabletoensurethatthetemperature
train couplers, in conjunction with the type of gripping device,
of the test specimen is within the limits specified in 6.4.2.
play major roles in the alignment of the load train and thus
Depending on the temperature range being used, this can be
subsequent bending imposed on the test specimen. Load train
accomplished with either calibrated thermocouples or pyrom-
couplers can be classified generally as fixed or non-fixed as
eters.
discussed in the following sections. Note that the use of
6.5.2 Thermocouples:
well-aligned fixed or self-aligned non-fixed couplers does not
6.5.2.1 Calibration—The thermocouple(s) shall be cali-
automatically guarantee low bending in the gage section of the
brated in accordance withTest Method E220 and Specification
tensile test specimen. Generally, well-aligned fixed or self-
E230. For longer tests at higher temperatures, this shall be
aligning non-fixed couplers provide for well-aligned load
done both before the test is initiated and after the test is
trains, but the type and operation of grip interfaces as well as
the as-fabricated dimensions of the tensile test specimen can
add significantly to the final bending imposed on the gage
Thermocouples shall be periodically checked since calibration may drift with
section of the test specimen. Regardless of the type of load usage or contamination.
C1291 − 18
completed in order to determine the extent of thermocouple completeapplicationoftheforceandthetimeatwhichfracture
degradation and possible thermal drift during the test. of the test specimen occurs to within 1% of the elapsed time
6.5.2.2 Accuracy—The measurement of temperature shall shall be employed.
be accurate to within 5 K. This includes the error inherent to
the thermocouple and any error in the measuring
7. Test Specimens and Sample
6,7
instruments.
7.1 Test Specimen Size:
6.5.2.3 Extension Wire—The appropriate thermocouple ex-
7.1.1 Description—The size and shape of test specimens
tension wire shall be used to connect a thermocouple to the
shall be based on the requirements necessary to obtain repre-
furnace controller or temperature readout device, or both.
sentative samples of the material being investigated as dis-
Special attention shall be accorded to connecting the extension
cussed in Test Method C1273. The test specimen geometry
wire with the correct polarity.
shall be such that there is no more than a 5% elastic stress
6.5.2.4 Degradation—The integrity and degree of degrada-
concentration at the ends of the gage section. Typical shapes
tion of used bare thermocouples shall be verified before each
include square or rectangular cross-section dogbones and
test.At certain temperatures, oxidation and elemental diffusion
cylindrical button-head geometries, and are shown in Appen-
of the thermocouple alloys will affect the electromotive force
dix X1. It is recommended, in accordance with Test Methods
(EMF) of the thermocouple junctions. As a consequence, the
E139 and in the absence of additional information to the
EMFofabare,usedthermocouplewillnolongercorrespondto
contrary, that the grip section be at least four times larger than
the calibration values determined in the pristine condition.The
the larger dimension of either width or thickness of the gage
indicated temperature will therefore be less than the actual
section.
temperature. This is a particular problem when the same
7.1.2 Dimensions—Suggested dimensions for tensile creep
thermocouple is used for both monitoring and control of
test specimens that have been successfully used in previous
temperature. Previously used bare thermocouples shall be
investigations are given in Appendix X1. Cross-sectional
replaced (with newly welded and annealed, or cut-back,
tolerances are 0.05 mm. Parallelism tolerances on the faces of
rewelded,andannealedthermocouples)whencalibrationatthe
the test specimen are 0.03 mm. Various radii of curvature may
testtemperaturerevealsanerrorof>2K.Itispreferabletouse
be used to adjust the gage section or change the mounting
fully sheathed thermocouples in order to minimize degrada-
configuration. Although these radii are expected to be larger,
tion.
resulting in a smaller stress concentration, wherever possible,
6.5.3 Pyrometers:
resort shall be made to a finite element analysis to determine
6.5.3.1 Calibration—Thepyrometer(s)shallbecalibratedin
thelocationsandintensitiesofstressconcentrationsinthenew
accordance with Test Method E639.
geometry.
6.5.3.2 Accuracy—The measurement of temperature shall
7.2 Test Specimen Preparation—Depending on the intended
be accurate to within 5 K. This shall include the error inherent
6,7
application of the data, use one of the following test specimen
tothepyrometerandanyerrorinthemeasuringinstruments.
preparation procedures:
6.6 Extensometers:
7.2.1 Application-Matched Machining—The test specimen
6.6.1 The strain measuring equipment shall be capable of
shall have the same surface preparation as that specified for a
being used at elevated temperatures. The sensitivity and
component. Unless the process is proprietary, the report shall
accuracyofthestrainmeasuringequipmentshallbesuitableto
be specified about the stages of material removal, wheel grits,
define the creep characteristics with the precision required for
wheel bonding, and the amount of material removed per pass.
the application of the data.
7.2.2 Customary Procedure—In instances where a custom-
6.6.2 Calibration—Extensometers shall be calibrated in ac-
arymachiningprocedurehasbeendevelopedthatiscompletely
cordance with Practice E83.
satisfactory for a class of materials (that is, it induces no
6.6.3 Accuracy—Extensometers with accuracies equivalent
unwanted surface damage or residual stresses), then this
to the B-1 classification of extensometer systems specified in
procedureshallbeused.Itshallbefullyspecifiedinthereport.
Practice E83 are suitable for use in high-temperature testing of
7.2.3 Standard Procedure—In instances where 7.2.1 or
ceramics. Results of analytical and empirical evaluations at
7.2.2 are not appropriate, then 7.2.3 will apply.This procedure
elevated temperatures show that mechanical extensometers
will serve as the minimum requirements, but a more stringent
(16) can meet these requirements. Optical extensometers using
procedure may be necessary.
flags have gage length uncertainties that will generally prevent
7.2.3.1 Grinding Process—All grinding using diamond grit
them from achieving class B-1 accuracy (17). Empirical
wheels shall be done with an ample supply of appropriate
evaluations at elevated temperature (18) show that these
filtered coolant to keep workpiece and wheel constantly
extensometerscanyieldhighlyrepeatablecreepdata,however.
floodedandparticlesflushed.Grindingshallbedoneinatleast
6.7 Timing Apparatus—For creep rupture tests, a timing
two stages, ranging from coarse to fine rates of material
apparatus capable of measuring the elapsed time between
removal. All machining shall be done in the surface grinding
mode, and be parallel to the test specimen long axis (several
Resolutions shall not be confused with accuracy. Beware of instruments that
test specimens are shown in the appendix). Do not use
readoutto1°C(resolution),buthaveanaccuracyofonly10Kor ⁄2%offullscale
Blanchard or rotary grinding.
( ⁄2%of1200Kis6K).
7.2.3.2 Material Removal Rate—The material removal rate
Temperature measuring instruments typically approximate the temperature-
EMF tables, but with a few degrees of error. shall not exceed 0.03 mm (0.001 in.) per pass to the last
C1291 − 18
0.06mm (0.002 in.) per face. Final and intermediate finishing 7.3 Test Specimen Sampling and Number—Samples of the
shall be performed with a resinoid-bonded diamond grit wheel material to provide test specimens shall be taken from such
locations so as to be representative of the billet or lot from
thatisbetween320and600grit.Nolessthan0.06mmperface
shall be removed during the final finishing phase, and at a rate which it was taken. Although each testing scenario will vary,
generally, a minimum of 24 test specimens is required for the
of not more than 0.002 mm (0.0001 in.) per pass. Remove
purposeofcompletelydeterminingthecreepandcreeprupture
approximately equal stock from opposite faces.
behavior across a significant temperature and stress range.
7.2.3.3 Precaution—Materials with low fracture toughness
Typically, six test specimens are run at each temperature of
and a high susceptibility to grinding damage may require finer
interest over the entire range of applied stresses of interest.
grinding wheels at very low removal rates.
Initial tests are used to define the range of temperature where
7.2.3.4 Chamfers—Chamfers on the edges of the gage
creepisthedominantdeformationmechanism,andtheremain-
section are preferred in order to minimize premature failures
der are used to acquire more precise creep and creep time-to-
due to stress concentrations or slow crack growth. The use of
failure data. Variations from this number are permitted as
chamfers and their geometry shall be clearly indicated in the
necessary to meet limitations on the amount of material or
test report (see 10.1.1).
other mitigating factors.Asmaller number of test specimens is
7.2.4 Button-Head Test Specimen-Specific Procedure—
permissible in cases where the ranges of applied stress or
Because of the axial symmetry of the button-head tensile test
temperature, or both, are more narrow.
specimen, fabrication of the test specimens is generally con-
ducted on a lathe-type apparatus. The bulk of the material is
8. Procedures
removed in a circumferential grinding operation with a final,
8.1 General:
longitudinal grinding operation performed in the gage section
8.1.1 Test Specimen Dimensions—Determine the thickness,
to ensure that any residual grinding marks are parallel to the
diameter, and width of the gage section of each test specimen
appliedstress.Beyondtheguidelinesstatedhere,morespecific
towithin1%ofitsabsolutevalue.Inordertoavoiddamagein
details of recommended fabrication methods for cylindrical
a critical area, carefully make the measurement using a flat,
tensile test specimens can be found elsewhere (4).
anvil-type micrometre. Ball-tipped or sharp anvil micrometres
7.2.4.1 Computer Numerical Control (CNC) Precaution—
are not recommended because they can cause localized crack-
Generally, CNC fabrication methods are necessary to obtain
ing. Use the measured dimensions to calculate the force
consistent test specimens with the proper dimensions within
required to achieve the desired stress in the gage section.
the required tolerances. A necessary condition for this consis-
8.1.2 Determination of Gage Length—Determine the gage
tency is the complete fabrication of the test specimen without
length of the test specimen by points of attachment of the
removing it from the grinding apparatus, thereby avoiding
extensometer system being used. It shall be as close to the
building unacceptable tolerances into the finished test speci-
length of the uniform cross section of the test specimen as
men.
possiblewithinthetemperaturevariationsstatedin6.4.2.Itcan
7.2.4.2 Grinding Wheels—Formed, resinoid-bonded,
be determined by any suitable optical or contact extensometry
diamond-impregnated wheels (minimum 320 grit in a resinoid
method.Anumberofsuchsystemsareavailablecommercially.
bond) are necessary to fabricate critical shapes (for example,
Make calibrations according to the appropriate manufacturer’s
button-head radius) and to minimize grinding vibrations and
instructions and check periodically using independent means.
subsurface damage in the test material. The formed, resin-
8.1.2.1 Mounting Flags to the Test Specimen:
bonded wheels require periodic dressing and shaping (truing),
(1)Optical Method—Attach two or more flags of dimen-
which can be done dynamically, to maintain the cutting and
sions suitable for the gage width and thickness chosen, to the
dimensional integrity.
test specimen gage length. Fig. 1 shows typical flags used for
7.2.4.3 Subsurface Damage—The most serious concern is
the test specimens shown in Fig. X1.2 of Appendix X1. They
not necessarily the surface finish (on the order of R = 0.2 to
a can be made from the test material itself or sintered SiC. The
0.4 µm) which is the result of the final machining steps.
depth of the flag (dimension d in Fig. 1) shall be kept as small
Instead, the subsurface damage is critically important although
as possible.
thisdamageisnotreadilyobservedormeasured,andtherefore,
(2)Contacting Method—Setting of the initial gage length
shall be inferred as the result of the grinding history. More
for a contacting extensometer depends on the extension mea-
details of this aspect have been discussed in Ref. (4). In all
surementmethod(capacitancebasedorstraingagebased),and
cases, the final grinding operation (“spark out”) performed in
the manufacturer’s procedures for setup shall be followed.
the gage section shall be along the longitudinal axis of the test
Position the extensometer probes with rounded knife-edge tips
specimen to ensure that any residual grinding marks are
in contact with the test specimen and hold in place with a light
parallel to the applied stress.
(0.1 to 1.0 N) contact force. A schematic of a contacting
7.2.5 Handling Precautions—Care shall be exercised in extensometer system is shown in Fig. 2. At elevated
temperatures, oxidation at the probe/test specimen interface
storingandhandlingoftestspecimenstoavoidtheintroduction
of random and severe flaws, such as might occur if the test minimizes slippage.
specimens were allowed to impact or scratch each other. Test 8.1.2.2 Mounting the Test Specimen in the Furnace—Mount
specimensshallbestoredseparatelyincushionedcontainersto
test specimens in the load train prior to heating the furnace.
minimize the occurrence of these problems. After the test specimens are mounted in the load train, apply a
C1291 − 18
NOTE 1—Dimensions shown are in millimetres.
FIG. 1 Schematic of Flags for Flat Dogbone Test Specimens of Dimensions as Shown in Appendix X1
specimentothetesttemperatureatarateofapproximately300
to 500 K/h, but may be as fast as 1000 K/h. The soak time at
the test temperature is determined experimentally, and shall be
long enough to allow the entire system to reach thermal
equilibrium. The total time for heating and soaking shall be
less than 24 h. State heating rates and soak times in the report.
(2)Test Specimens Using Contacting Extensometers—Test
specimens that utilize contact extensometry may be either
heated from room temperature to the test temperature in a
singlestageandconstantheatingrateofupto1000K/hormay
be heated from a preheat furnace temperature to the final test
temperature. If the furnace is heated from room temperature to
FIG. 2 Schematic of High-Temperature Contacting Extensometer
the test temperature, a soak time shall be determined
System
experimentally, and shall be long enough to allow the entire
systemtoreachthermalequilibrium.Thetotaltimeforheating
andsoakingshallbelessthan24h.Stateheatingratesandsoak
small preload to maintain the load train alignment during
times in the report.
subsequent heat-up to the test temperature. The preload shall
8.1.2.4 Use of Thermocouples:
introduce a stress of no more than 5 MPa in the gage section.
(1)Monitor test specimen temperature using a thermo-
For test specimens using contacting extensometry, make the
couplewithitstiplocatednomorethan2mmfromthesurface
extensometry settings prior to heating the furnace. The con-
midpoint of the tensile test specimen. Use either a fully
tacting probes may be left in contact with the test specimen
sheathed or exposed bead junction. If a sheathed tip is used,
during heat-up or brought into contact with the test specimen
verify that there is negligible error associated with the
after it has reached the test temperature, depending on the
8,9
covering.
testing setup.
8.1.2.3 Heating to the Test Temperature:
(1)Test Specimens with Flags—Test specimens with flags
Exposed thermocouple beads will exhibit greater sensitivity, but may be
may be heated to the test temperature in stages.The first stage,
exposed to vapors that can react with the thermocouple materials. (For example,
if required, takes the temperature to approximately 700 K to
silica vapors will react with platinum.) Be aware that the use of heavy-gage
burn off the room temperature cement. The soak time at this
thermocouplewire,thermalgradientsalongthethermocouplelength,orexcessively
temperature is about 1 h. The second stage takes the test heavy-walled insulators can lead to erroneous temperature readings.
C1291 − 18
(2)A separate thermocouple may be used to control the failure occurred at the extensometer contact points, use frac-
temperature of the furnace chamber if needed, but the test tography to determine whether the test specimen was affected
specimen temperature shall be the reported temperature of the bythecontact.Ifitwasaffected,discardthetestresult.Ifitwas
test. not affected, use the result.
(3)For longer gage sections where spatial temperature
9. Calculation
variation may be of concern, take additional thermocouple
temperature measurements at the top and bottom of the gage 9.1 Formulae:
9.1.1 The formulae for determining the force applied to the
section.
8.1.2.5 Calculating, Applying, and Recording the Force— test specimen are, for rectangular cross sections:
Based on the dimensions measured in 8.1.1, compute the
F 5σ wt (1)
a
preload force, F , needed to achieve the recommended 5 MPa
p
where σ is the applied stress, t is the thickness of the gage
a
stress in the gage section (see 8.1.2.2). Apply the preload in
section,andwisthewidthofthegagesection,andforcircular
force control to accommodate the dimensional changes ex-
cross sections:
pected in the test specimen and fixtures during heating.
Compute the force, F, needed to achieve the desired applied
F 5πσ d /4 (2)
a
stress,σ ,inaccordancewith9.1.1.Afterthetestspecimenhas
a
where d is the diameter of the gage section.
stabilized at the desired temperature, apply the desired creep
9.1.2 The standard formulae for the gage section stresses in
force over a period of approximately 30 to 120 s to prevent
tensile test specimens are stated in the following: For rectan-
premature failure. Measure and record the force at regular
gular cross sections,
intervals during the test to ensure compliance with the require-
F
ments of 6.1.
σ 5 (3)
a
wt
8.1.2.6 Recording of Displacement Data—Record the dis-
placement determined by the extensometry system at appro-
whereσ istheappliedstress, Fistheappliedforce, wisthe
a
priate intervals using an appropriate data logger. The number
width of the gage section, and t is the thickness of the gage
of intervals shall be at least 100, and be appropriate to the
section.
expected duration of the test. It may be necessary to record
For circular cross sections,
displacementdatamorefrequentlyatthestartofthetest,when
F
the creep rate is often higher, than later into the test when the
σ 5 (4)
a
πd /4
creep rate has decreased.
8.1.2.7 Use of Strain-Gaged Test Specimens—The occa-
where σ is the applied stress, F is the applied force, and d
a
sional use of a strain-gaged test specimen at room temperature
is the diameter of the gage section.
is recommended to verify that there is negligible error due to
9.1.3 The creep strain of the test specimen at any time is
bending. Do not leave strain gages on the test specimen when
determined from:
the system is heated up, since they will melt or burn incom-
ε 5 l 2 l /l (5)
~ !
0 0
pletely with the residue contaminating the test specimen or
fixture, or both. where l is the measured gage length and l is the initial gage
8.1.2.8 End-of-Test Criteria—The end of a given test has length under force and at temperature at the start of the creep
occurred when any of the following conditions has been met: measurement. l and l shall not include the elastic extension
(1) the test specimen fractures, (2) the test specimen reaches a thatoccurswhenthetestspecimenisfirstloaded.Alternatively,
predeterminedlevelofstrain,or(3)thetestspecimenhascrept the true creep strain curve can be obtained from a plot of lnl
for a predetermined length of time. In Case 1, examine the versustimewherethe y-axisisshiftedtogivezerocreepstrain
fracturesurfacestodeterminewhetherornotthetestspecimen at time zero, using
failed in the gage section. Generally, an invalid test is one in
ε 5 ln l/l (6)
~ !
which fracture occurs outside the uniform cross section of the
9.1.4 The creep strain rate of the test specimen at any point
gage section. In the event failure occurred outside of the
in time is determined by taking the tangent of the creep-strain-
measured gage section but within the uniform cross-sectional
versus-time curve. Creep strain rate can be calculated numeri-
area, the test may still be valid. Use fractography, along with
cally using a suitable method such as a seven-point secant
the knowledge of the testing apparatus and conditions, to
algorithm or as the derivative of the polynomial fit of the
determine what occurred at the failure point and make a
strain-time data (report the degree of polynomial fit and
determination of validity. In the event failure occurred outside
correlation coefficient).
of the measured gage section and outside of the uniform
cross-sectional area, discard the test result. In the event that
10. Report of Test Results
10.1 Report the following information:
10.1.1 Test configuration and test specimen geometry along
The thermocouple tip may contact the tensile test specimen, but only if it is
certain that the thermocouple tip or sheathing material will not chemically interact
with test specimen dimensions.
with the test specimen. Thermocouples are prone to breakage if they are in contact
10.1.2 The number of test specimens (n) used.
with the test specimen.
10.1.3 The relevant material data including vintage,
Tensiletestsaresometimesconductedinfurnacesthathavethermalgradients.
It is essential to monitor the temperature at the test specimen. component,orbilletidentificationdata.(Didalltestspecimens
C1291 − 18
comefromonecomponentorpla
...

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: C1291 − 16 C1291 − 18
Standard Test Method for
Elevated Temperature Tensile Creep Strain, Creep Strain
Rate, and Creep Time-to-Failure Time to Failure for
Monolithic Advanced Ceramics
This standard is issued under the fixed designation C1291; 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 tensile creep strain, creep strain rate, and creep time-to-failure time to failure
for advanced monolithic ceramics at elevated temperatures, typically between 1073 and 2073 K. A variety of test specimen
geometries are included. The creep strain at a fixed temperature is evaluated from direct measurements of the gage length extension
over the time of the test. The minimum creep strain rate, which may be invariant with time, is evaluated as a function of
temperature and applied stress. Creep time-to-failure time to failure is also included in this test method.
1.2 This test method is for use with advanced ceramics that behave as macroscopically isotropic, homogeneous, continuous
materials. While this test method is intended for use on monolithic ceramics, whisker- or particle-reinforced composite ceramics
as well as low-volume-fraction discontinuous fiber-reinforced composite ceramics may also meet these macroscopic behavior
assumptions. Continuous fiber-reinforced ceramic composites (CFCCs) do not behave as macroscopically isotropic, homogeneous,
continuous materials, and application of this test method to these materials is not recommended.
1.3 The values in SI units are to be regarded as the standard (see IEEE/ASTM SI 10). The values given in parentheses are
mathematical conversions to inch-pound units that are provided for information only and are not considered standard.
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 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
C1273 Test Method for Tensile Strength of Monolithic Advanced Ceramics at Ambient Temperatures
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
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E220 Test Method for Calibration of Thermocouples By Comparison Techniques
E230 Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples
E639 Test Method for Measuring Total-Radiance Temperature of Heated Surfaces Using a Radiation Pyrometer (Withdrawn
2011)
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
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 Sept. 1, 2016Jan. 1, 2018. Published October 2016January 2018. Originally approved in 1995. Last previous edition approved in 20102016 as
C1291 – 00a (2010). 16. DOI: 10.1520/C1291-16.10.1520/C1291-18.
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.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1291 − 18
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—The definitions of terms relating to creep testing,testing which appear in Section E of Terminology E6 shall
apply to the terms used in this test method. For the purpose of this test method only, some of the more general terms are used with
the restricted meanings given as follows.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 axial strain, ε , [L/L], n—average of the strain measured on diametrically opposed sides and equally distant from the test
a
specimen axis.
3.2.2 bending strain, ε [L/L], n—difference between the strain at the surface and the axial strain.
b
3.2.2.1 Discussion—
In general, it varies from point to point around and along the gage length of the test specimen. (E1012)
3.2.3 creep-rupture creep rupture test, n—test in which progressive test specimen deformation and the time-to-failure time to
failure are measured. In general, deformation is greater than that developed during a creep test.
3.2.4 creep strain, ε, [L/L], n—time dependent time-dependent strain that occurs after the application of force which is
thereafter maintained constant. Also known as engineering creep strain.
3.2.5 creep test, n—test that has as its objective the measurement of creep and creep rates occurring at stresses usually well
below those that would result in fast fracture.
3.2.5.1 Discussion—
Since the maximum deformation is only a few percent, a sensitive extensometer is required.
3.2.6 creep time-to-failure, time to failure, t , [T], n—time required for a test specimen to fracture under constant force as a result
f
of creep.
3.2.6.1 Discussion—
This is also known as creep rupture time.
3.2.7 gage length, l, [L], n—original distance between fiducial markers on or attached to the test specimen for determining
elongation.
3.2.8 maximum bending strain, ε , [L/L], n—largest value of bending strain along the gage length. It can be calculated from
bmax
measurements of strain at three circumferential positions at each of two different longitudinal positions.
−1
3.2.9 minimum creep strain rate, ε , [T ], n—minimum value of the strain rate prior to test specimen failure as measured
min
from the strain-time curve. The minimum creep strain rate may not necessarily correspond to the steady-state creep strain rate.
3.2.10 slow crack growth, ν, [L/T], (SCG), n—subcritical crack growth (extension) which may result from, but is not restricted
to, such mechanisms as environmentally assisted stress corrosion, diffusive crack growth, or other mechanisms. (C1145)
3.2.11 steady-state creep, ε , [L/L], n—stage of creep wherein the creep rate is constant with time.
ss
3.2.11.1 Discussion—
Also known as secondary creep.
3.2.12 stress corrosion, n—environmentally induced degradation that initiates from the exposed surface.
3.2.12.1 Discussion—
Such environmental effects commonly include the action of moisture, as well as other corrosive species, often with a strong
temperature dependence.
3.2.13 tensile creep strain, ε , [L/L], n—creep strain that occurs as a result of a uniaxial tensile-applied stress.
t
C1291 − 18
4. Significance and Use
4.1 Creep tests measure the time-dependent deformation under force at a given temperature, and, by implication, the
force-carrying capability of the material for limited deformations. Creep-rupture Creep rupture tests, properly interpreted, provide
a measure of the force-carrying capability of the material as a function of time and temperature. The two tests complement each
other in defining the force-carrying capability of a material for a given period of time. In selecting materials and designing parts
for service at elevated temperatures, the type of test data used will depend on the criteria for force-carrying capability that best
defines the service usefulness of the material.
4.2 This test method may be used for material development, quality assurance, characterization, and design data generation.
4.3 High-strength, monolithic ceramic materials, generally characterized by small grain sizes (<50 μm) and bulk densities near
their theoretical density, are candidates for load-bearing structural applications at elevated temperatures. These applications involve
components such as turbine blades which are subjected to stress gradients and multiaxial stresses.
4.4 Data obtained for design and predictive purposes shall be obtained using any appropriate combination of test methods that
provide the most relevant information for the applications being considered. It is noted here that ceramic materials tend to creep
more rapidly in tension than in compression (11-3, 2, 3). This difference results in time-dependent changes in the stress
distribution and the position of the neutral axis when tests are conducted in flexure. As a consequence, deconvolution of flexural
creep data to obtain the constitutive equations needed for design cannot be achieved without some degree of uncertainty concerning
the form of the creep equations, and the magnitude of the creep rate in tension vis-a-vis the creep rate in compression. Therefore,
creep data for design and life prediction shall be obtained in both tension and compression, as well as the expected service stress
state.
5. Interferences
5.1 Time-Dependent Phenomena—Other time-dependent phenomena, such as stress corrosion and slow crack growth, can
interfere with determination of the creep behavior.
5.2 Chemical Interactions with the Testing Environment—The test environment (vacuum, inert gas, ambient air, etc.) including
moisture content (for example, % relative humidity (RH)) may have a strong influence on both creep strain rate and creep rupture
life. In particular, materials susceptible to slow crack growth failure will be strongly influenced by the test environment. Surface
oxidation may be either active or passive and thus will have a direct effect on creep behavior by changing the material’s properties.
Testing shall be conducted in environments that are either representative of service conditions or inert to the materials being tested
depending on the performance being evaluated. A controlled gas environment with suitable effluent controls shall be provided for
any material that evolves toxic vapors.
5.3 Test Specimen Surfaces—Surface preparation of test specimens can introduce machining flaws that may affect the test
results. Machining damage imposed during test specimen preparation will most likely result in premature failure of the test
specimen but may also introduce flaws that can grow by slow crack growth. Surface preparation can also lead to residual stresses
which can be released during the test. Universal or standardized methods of surface preparation do not exist. It shall be understood
that final machining steps may or may not negate machining damage introduced during earlier phases of machining, which tend
to be rougher.
5.4 Test Specimen/Extensometer Chemical Incompatibility—The strain measurement techniques described herein generally rely
on physical contact between extensometer components (contacting probes or optical method flags) and the test specimen so as to
measure changes in the gage section as a function of time. Flag attachment methods and extensometer contact materials shall be
chosen with care to ensure that no adverse chemical reactions occur during testing. Normally, this is not a problem if test
specimen/probe materials that are mutually chemically inert are employed (for example, SiC probes on Si N test specimens). The
3 4
user must be aware that impurities or second phases in the flags or test specimens may be mutually chemically reactive and could
influence the results.
5.5 Test Specimen Bending—Bending in uniaxial tensile tests can cause extraneous strains or promote accelerated rupture times.
Since maximum or minimum stresses will occur at the surface where strain measurements are made, bending may introduce either
an over or under measurement of axial strain, if the measurement is made only on one side of the tensile test specimen. Similarly,
bending stresses may accentuate surface oxidation and may also accentuate the severity of surface flaws.
5.6 Temperature Variations—Creep strain is often related to temperature through an exponential function. Thus fluctuations in
test temperature or change in temperature profile along the length of the test specimen in real time can cause fluctuations in strain
measurements or changes in creep rate.
6. Apparatus
6.1 Force Test Machine:
The boldface numbers in parentheses refer to the list of references at the end of this test method.
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6.1.1 Test specimens may be loaded in any suitable test machine provided that uniform, direct loading can be maintained. The
test machine must maintain the desired constant force on the test specimen regardless of test specimen deformation with time,
either through dead-weight loading or through active force control. The force measuring system can be equipped with a means for
retaining readout of the force, or the force can be recorded manually. The accuracy of the test machine shall be in accordance with
Practices E4.
6.1.2 Allowable Bending—Allowable bending, as defined in Practice E1012, shall not exceed 5 %. This is based on the same
assumptions as those for tensile strength testing (see Ref. (4), for example). It shall be noted that unless percent bending is
monitored until the end-of-test condition has been reached, there will be no record of percent bending for each test specimen. The
testing system alignment including the test machine, gripping devices (as described in 6.2), and load-train load train couplers (as
described in 6.3), must be verified using the procedure detailed in the appendix such that the percent bending does not exceed 5five
at a mean stress equal to one half one-half the anticipated test stress. This verification shall be conducted at a minimum at the
beginning and the end of each test series. An additional verification of alignment is recommended, although not required, at the
middle of the test series. Either a dummy or actual test specimen may be used. Tensile test specimens used for alignment
verification shall be equipped with a recommended eight separate longitudinal strain gages to determine bending contributions
from both eccentric and angular misalignment of the grip heads. (Although it is possible to use a minimum of six separate
longitudinal strain gages for test specimens with circular cross sections, eight strain gages are recommended here for simplicity
and consistency in describing the technique for both circular and rectangular cross sections.) If dummy test specimens are used
for alignment verification, they shall have the same geometry and dimensions as the actual test specimens as well as an elastic
modulus that closely matches that of the test material to ensure similar axial and bending stiffness characteristics.
6.2 Gripping Devices:
6.2.1 Various types of gripping devices may be used to transmit the measured force applied by the test machine to the test
specimens. The brittle nature of advanced ceramics requires a uniform interface between the grip components and the gripped
section of the test 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 classed generally as those employing
active and those employing passive grip interfaces as discussed in the following sections. Regardless of the type of gripping device
chosen, it shall be consistent with the thermal requirements imposed on it by the elevated temperature nature of creep testing. This
requirement may preclude the use of some material combinations and gripping designs.
6.2.1.1 Active Grip Interfaces—Active grip interfaces require a continuous application of a mechanical, hydraulic, or pneumatic
force to transmit the force applied by the test machine to the test specimen. Generally, these types of grip interfaces cause a force
to be applied normal to the surface of the gripped section of the test specimen. Transmission of the uniaxial force applied by the
test machine is then accomplished by friction between the test specimen and the grip faces. Thus, important aspects of active grip
interfaces are uniform contact between the gripped section of the test specimen and the grip faces, and constant coefficient of
friction over the interface between the test specimen and grip.
(1) For cylindrical test specimens, a one-piece split collet arrangement acts as the grip interface (4, 5). Generally, close
tolerances are required for concentricity of both the grip and test specimen diameters. In addition, the diameter of the gripped
section of the test specimen and the unclamped, open diameter of the grip faces shall be within similarly close tolerances to
promote uniform contact at the test specimen/grip interface. Tolerances will vary depending on the exact configuration used.
(2) For flat test specimens, flat-face, wedge-grip faces act as the grip interface. Generally, close tolerances are required for the
flatness and parallelism as well as wedge angle of the grip faces. In addition, the thickness, flatness, and parallelism of the gripped
section of the test specimen shall be within similarly close tolerances to promote uniform contact at the test specimen/grip
interface. Tolerances will vary depending on the exact configuration used.
6.2.1.2 Passive Grip Interfaces—Passive grip interfaces transmit the force applied by the test machine to the test specimen
through a direct mechanical link. Generally, these mechanical links transmit the test forces to the test specimen by means of
geometrical features of the test specimens such as button-head fillets, shank shoulders, or holes in the gripped head. Thus, the
important aspect of passive grip interfaces is uniform contact between the gripped section of the test specimen and the grip faces.
(1) For cylindrical test specimens, a multi-piece split collet arrangement acts as the grip interface at button-head fillets of the
test specimen (6). Because of the limited contact area at the test specimen/grip interface, soft, deformable metallic collets may be
used to transfer the axial force to the exact geometry of the test specimen. In some cases, tapered collets may be used to transfer
the axial force to the shank of the test specimen rather than into the button-head radius (6). Generally, moderate tolerances on the
collet height shall be maintained to promote uniform axial-loading at the test specimen/grip interface. Tolerances will vary
depending on the exact configuration used.
(2) For flat test specimens, pins or pivots act as grip interfaces at either the shoulders of the test specimen shank (7, 8) or at
holes in the gripped test specimen head (9, 10). Generally, close tolerances of shoulder radii and grip interfaces are required to
promote uniform contact along the entire test specimen/grip interface as well as to provide for non-eccentric loading. Generally,
very close tolerances are required for longitudinal coincidence of the pin and the hole centerlines.
6.3 Load-Train Load Train Couplers:
6.3.1 Various types of devices (load-train (load train couplers) may be used to attach the active or passive grip interface
assemblies to the test machine as discussed in Test Method C1273. The load-train load train couplers, in conjunction with the type
C1291 − 18
of gripping device, play major roles in the alignment of the load-train load train and thus subsequent bending imposed on the test
specimen. Load-train Load train couplers can be classified generally as fixed or non-fixed as discussed in the following sections.
Note that the use of well-aligned fixed or self-aligned non-fixed couplers does not automatically guarantee low bending in the gage
section of the tensile test specimen. Generally, well-aligned fixed or self-aligning non-fixed couplers provide for well-aligned
load-trains, load trains, but the type and operation of grip interfaces as well as the as-fabricated dimensions of the tensile test
specimen can add significantly to the final bending imposed on the gage section of the test specimen. Regardless of the type of
load couplers chosen, they shall be consistent with the thermal requirements imposed on them by the elevated temperature nature
of creep testing. These requirements may preclude the use of some material combinations and load-train load train designs.
6.3.2 Fixed Load-Train Load Train Couplers—Fixed couplers may incorporate devices that require either a one-time, pretest
alignment adjustment of the load-train load train which remains constant for all subsequent tests or an in situ, pretest alignment
of the load-train load train which is conducted separately for each test specimen and each test. Such devices (11, 12) usually
employ angularity and concentricity adjusters to accommodate inherent load-train load train misalignments. Regardless of which
method is used, alignment verification shall be performed as discussed in 6.1.2.
6.3.3 Non-Fixed Load-Train Load Train Couplers—Non-fixed couplers may incorporate devices that promote self-alignment of
the load-train load train during the movement of the crosshead or actuator. Generally, such devices rely upon freely moving
linkages to eliminate applied moments as the load-train load train components are loaded. Knife edges, universal joints, hydraulic
couplers, and air bearings are examples (7, 11, 1313-15, 14, 15) of such devices. Although non-fixed load couplers are intended
to be self-aligning and thus eliminate the need to evaluate the bending in the test specimen for each test, the operation of the
couplers shall be verified as discussed in 6.1.2.
6.4 Heating Apparatus:
6.4.1 The apparatus for and method of heating the test specimens shall provide the temperature control necessary to satisfy the
requirements specified in 6.4.2 without manual adjustments more frequent than once in each 24-h period after force application.
It shall also satisfy the requirements of the testing environment in 6.4.3.
6.4.2 Temperature—The furnace shall be capable of maintaining the tensile test specimen temperature constant with time to 2
K. The temperature readout device shall have a resolution of 1 K or less. The furnace system shall be such that thermal gradients
are minimal in the tensile test specimen so that no more than a 5-K differential exists in the test specimen gage length at
temperatures up to 1773 K.
6.4.3 Environment—The furnace may have an air, inert, or vacuum environment as required. If an inert or vacuum chamber is
used, and it is necessary to direct force through bellows, fittings, or seal, then it shall be verified that force losses or errors do not
exceed 1 % of the applied force.
6.5 Temperature Measuring Devices:
6.5.1 The method of temperature measurement shall be sufficiently sensitive and reliable to ensure that the temperature of the
test specimen is within the limits specified in 6.4.2. Depending on the temperature range being used, this can be accomplished with
either calibrated thermocouples or pyrometers.
6.5.2 Thermocouples:
6.5.2.1 Calibration—The thermocouple(s) shall be calibrated in accordance with Test Method E220 and Specification and
Tables E230. For longer tests at higher temperatures, this shall be done both before the test is initiated and after the test is
completed in order to determine the extent of thermocouple degradation and possible thermal drift during the test.
6.5.2.2 Accuracy—The measurement of temperature shall be accurate to within 5 K. This includes the error inherent to the
6,7
thermocouple and any error in the measuring instruments.
6.5.2.3 Extension Wire—The appropriate thermocouple extension wire shall be used to connect a thermocouple to the furnace
controller or temperature readout device, or both. Special attention shall be accorded to connecting the extension wire with the
correct polarity.
6.5.2.4 Degradation—The integrity and degree of degradation of used bare thermocouples shall be verified before each test. At
certain temperatures, oxidation and elemental diffusion of the thermocouple alloys will affect the electromotive force (EMF) of the
thermocouple junctions. As a consequence, the EMF of a bare, used thermocouple will no longer correspond to the calibration
values determined in the pristine condition. The indicated temperature will therefore be less than the actual temperature. This is
a particular problem when the same thermocouple is used for both monitoring and control of temperature. Previously used bare
thermocouples shall be replaced (with newly welded and annealed, or cut-back, rewelded, and annealed thermocouples) when
calibration at the test temperature reveals an error of >2K.>2 K. It is preferable to use fully sheathed thermocouples in order to
minimize degradation.
6.5.3 Pyrometers:
6.5.3.1 Calibration—The pyrometer(s) shall be calibrated in accordance with Test Method E639.
Thermocouples shall be periodically checked since calibration may drift with usage or contamination.
6 1
Resolutions shall not be confused with accuracy. Beware of instruments that readout to 1°C1 °C (resolution), but have an accuracy of only 10 K or ⁄2 % of full scale
( ⁄2 % of 1200 K is 6 K).
Temperature measuring instruments typically approximate the temperature-EMF tables, but with a few degrees of error.
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6.5.3.2 Accuracy—The measurement of temperature shall be accurate to within 5 K. This shall include the error inherent to the
6,7
pyrometer and any error in the measuring instruments.
6.6 Extensometers:
6.6.1 The strain measuring equipment shall be capable of being used at elevated temperatures. The sensitivity and accuracy of
the strain-measuring strain measuring equipment shall be suitable to define the creep characteristics with the precision required for
the application of the data.
6.6.2 Calibration—Extensometers shall be calibrated in accordance with Practice E83.
6.6.3 Accuracy—Extensometers with accuracies equivalent to the B-1 classification of extensometer systems specified in
Practice E83 are suitable for use in high-temperature testing of ceramics. Results of analytical and empirical evaluations at elevated
temperatures show that mechanical extensometers (16) can meet these requirements. Optical extensometers using flags have gage
length uncertainties that will generally prevent them from achieving class B-1 accuracy (17). Empirical evaluations at elevated
temperature (18) show that these extensometers can yield highly repeatable creep data, however.
6.7 Timing Apparatus—For creep rupture tests, a timing apparatus capable of measuring the elapsed time between complete
application of the force and the time at which fracture of the test specimen occurs to within 1 % of the elapsed time shall be
employed.
7. Test Specimens and Sample
7.1 Test Specimen Size:
7.1.1 Description—The size and shape of test specimens shall be based on the requirements necessary to obtain representative
samples of the material being investigated as discussed in Test Method C1273. The test specimen geometry shall be such that there
is no more than a 5 % elastic stress concentration at the ends of the gage section. Typical shapes include square or rectangular
cross-section dogbones and cylindrical button-head geometries, and are shown in Appendix X1. It is recommended, in accordance
with Test Methods E139 and in the absence of additional information to the contrary, that the grip section be at least four times
larger than the larger dimension of either width or thickness of the gage section.
7.1.2 Dimensions—Suggested dimensions for tensile creep test specimens that have been successfully used in previous
investigations are given in Appendix X1. Cross-sectional tolerances are 0.05 mm. Parallelism tolerances on the faces of the test
specimen are 0.03 mm. Various radii of curvature may be used to adjust the gage section or change the mounting configuration.
Although these radii are expected to be larger, resulting in a smaller stress concentration, wherever possible, resort shall be made
to a finite element analysis to determine the locations and intensities of stress concentrations in the new geometry.
7.2 Test Specimen Preparation—Depending on the intended application of the data, use one of the following test specimen
preparation procedures:
7.2.1 Application-matchedApplication-Matched Machining—The test specimen shall have the same surface preparation as that
specified for a component. Unless the process is proprietary, the report shall be specified about the stages of material removal,
wheel grits, wheel bonding, and the amount of material removed per pass.
7.2.2 Customary Procedure—In instances where a customary machining procedure has been developed that is completely
satisfactory for a class of materials (that is, it induces no unwanted surface damage or residual stresses), then this procedure shall
be used. It shall be fully specified in the report.
7.2.3 Standard Procedure—In instances where 7.2.1 or 7.2.2 are not appropriate, then 7.2.3 will apply. This procedure will serve
as the minimum requirements, but a more stringent procedure may be necessary.
7.2.3.1 Grinding Process—All grinding using diamond-grit diamond grit wheels shall be done with an ample supply of
appropriate filtered coolant to keep workpiece and wheel constantly flooded and particles flushed. Grinding shall be done in at least
two stages, ranging from coarse to fine rates of material removal. All machining shall be done in the surface grinding mode, and
be parallel to the test specimen long axis (several test specimens are shown in the appendix). Do not use Blanchard or rotary
grinding.
7.2.3.2 Material Removal Rate—The material removal rate shall not exceed 0.03 mm (0.001 in.) per pass to the last 0.06 mm
0.06 mm (0.002 in.) per face. Final and intermediate finishing shall be performed with a resinoid-bonded diamond grit wheel that
is between 320 and 600 grit. No less than 0.06 mm per face shall be removed during the final finishing phase, and at a rate of not
more than 0.002 mm (0.0001 in.) per pass. Remove approximately equal stock from opposite faces.
7.2.3.3 Precaution—Materials with low fracture toughness and a high susceptibility to grinding damage may require finer
grinding wheels at very low removal rates.
7.2.3.4 Chamfers—Chamfers on the edges of the gage section are preferred in order to minimize premature failures due to stress
concentrations or slow crack growth. The use of chamfers and their geometry shall be clearly indicated in the test report (see
10.1.1).
7.2.4 Button-Head Test Specimen-Specific Procedure—Because of the axial symmetry of the button-head tensile test specimen,
fabrication of the test specimens is generally conducted on a lathe-type apparatus. The bulk of the material is removed in a
circumferential grinding operation with a final, longitudinal grinding operation performed in the gage section to ensure that any
C1291 − 18
residual grinding marks are parallel to the applied stress. Beyond the guidelines stated here, more specific details of recommended
fabrication methods for cylindrical tensile test specimens can be found elsewhere (4).
7.2.4.1 Computer Numerical Control (CNC) Precaution—Generally, CNC fabrication methods are necessary to obtain
consistent test specimens with the proper dimensions within the required tolerances. A necessary condition for this consistency is
the complete fabrication of the test specimen without removing it from the grinding apparatus, thereby avoiding building
unacceptable tolerances into the finished test specimen.
7.2.4.2 Grinding Wheels—Formed, resinoid-bonded, diamond-impregnated wheels (minimum 320 grit in a resinoid bond) are
necessary to fabricate critical shapes (for example, button-head radius) and to minimize grinding vibrations and subsurface damage
in the test material. The formed, resin-bonded wheels require periodic dressing and shaping (truing), which can be done
dynamically, to maintain the cutting and dimensional integrity.
7.2.4.3 Subsurface Damage—The most serious concern is not necessarily the surface finish (on the order of R = 0.2 to 0.4 μm)
a
which is the result of the final machining steps. Instead, the subsurface damage is critically important although this damage is not
readily observed or measured, and therefore, shall be inferred as the result of the grinding history. More details of this aspect have
been discussed in Ref. (4). In all cases, the final grinding operation (“spark out”) performed in the gage section shall be along the
longitudinal axis of the test specimen to ensure that any residual grinding marks are parallel to the applied stress.
7.2.5 Handling Precautions—Care shall be exercised in storing and handling of test specimens to avoid the introduction of
random and severe flaws, such as might occur if the test specimens were allowed to impact or scratch each other. Test specimens
shall be stored separately in cushioned containers to minimize the occurrence of these problems.
7.3 Test Specimen Sampling and Number—Samples of the material to provide test specimens shall be taken from such locations
so as to be representative of the billet or lot from which it was taken. Although each testing scenario will vary, generally, a
minimum of 24 test specimens is required for the purpose of completely determining the creep and creep rupture behavior across
a significant temperature and stress range. Typically, six test specimens are run at each temperature of interest over the entire range
of applied stresses of interest. Initial tests are used to define the range of temperature where creep is the dominant deformation
mechanism, and the remainder are used to acquire more precise creep and creep-time-to-failure creep time-to-failure data.
Variations from this number are permitted as necessary to meet limitations on the amount of material or other mitigating factors.
A smaller number of test specimens is permissible in cases where the ranges of applied stress or temperature, or both, are more
narrow.
8. Procedures
8.1 General:
8.1.1 Test Specimen Dimensions—Determine the thickness, diameter, and width of the gage section of each test specimen to
within 1 % of its absolute value. In order to avoid damage in a critical area, carefully make the measurement using a flat, anvil-type
micrometre. Ball-tipped or sharp anvil micrometres are not recommended because they can cause localized cracking. Use the
measured dimensions to calculate the force required to achieve the desired stress in the gage section.
8.1.2 Determination of Gage Length—Determine the gage length of the test specimen by points of attachment of the
extensometer system being used. It shall be as close to the length of the uniform cross section of the test specimen as possible
within the temperature variations stated in 6.4.2. It can be determined by any suitable optical or contact extensometry method. A
number of such systems are available commercially. Make calibrations according to the appropriate manufacturer’s instructions
and check periodically using independent means.
8.1.2.1 Mounting Flags to the Test Specimen:
(1) Optical Method—Attach two or more flags of dimensions suitable for the gage width and thickness chosen, to the test
specimen gage length. Fig. 1 shows typical flags used for the test specimens shown in Fig. X1.2 of Appendix X1the Appendix.
. They can be made from the test material itself or sintered SiC. The depth of the flag (dimension d in Fig. 1) shall be kept as small
as possible.
(2) Contacting Method—Setting of the initial gage length for a contacting extensometer depends on the extension measurement
method (capacitance-based (capacitance based or strain gage-based), gage based), and the manufacturer’smanufacturer’s
procedures for setup shall be followed. Position the extensometer probes with rounded knife-edge tips in contact with the test
specimen and hold in place with a light (0.1 to 1.0 N) contact force. A schematic of a contacting extensometer system is shown
in Fig. 2. At elevated temperatures, oxidation at the probe/test specimen interface minimizes slippage.
8.1.2.2 Mounting the Test Specimen in the Furnace—Mount test specimens in the load-train load train prior to heating the
furnace. After the test specimens are mounted in the load-train, load train, apply a small preload to maintain the load-train load
train alignment during subsequent heat-up to the test temperature. The preload shall introduce a stress of no more than 5 MPa in
the gage section. For test specimens using contacting extensometry, make the extensometry settings prior to heating the furnace.
The contacting probes may be left in contact with the test specimen during heat-up or brought into contact with the test specimen
after it has reached the test temperature, depending on the testing setup.
8.1.2.3 Heating to the Test Temperature:
(1) Test Specimens with Flags—Test specimens with flags may be heated to the test temperature in stages. The first stage, if
required, takes the temperature to approximately 700 K to burn off the room temperature cement. The soak time at this temperature
C1291 − 18
NOTE 1—Dimensions shown are in millimetres.
FIG. 1 Schematic of Flags for Flat Dogbone Test Specimens of Dimensions as Shown in theAppendix X1 Appendix
FIG. 2 Schematic of High-Temperature Contacting Extensometer System
is about 1 h. The second stage takes the test specimen to the test temperature at a rate of approximately 300 to 500 K/h, but may
be as fast as 1000 K/h. The soak time at the test temperature is determined experimentally, and shall be long enough to allow the
entire system to reach thermal equilibrium. The total time for heating and soaking shall be less than 24 h. State heating rates and
soak times in the report.
(2) Test Specimens Using Contacting Extensometers—Test specimens that utilize contact extensometry may be either heated
from room temperature to the test temperature in a single stage and constant heating rate of up to 1000 K/h or may be heated from
a preheat furnace temperature to the final test temperature. If the furnace is heated from room temperature to the test temperature,
a soak time shall be determined experimentally, and shall be long enough to allow the entire system to reach thermal equilibrium.
The total time for heating and soaking shall be less than 24 h. State heating rates and soak times in the report.
8.1.2.4 Use of Thermocouples:
C1291 − 18
(1) Monitor test specimen temperature using a thermocouple with its tip located no more than 2 mm from the surface midpoint
of the tensile test specimen. Use either a fully sheathed or exposed bead junction. If a sheathed tip is used, verify that there is
8,9
negligible error associated with the covering.
(2) A separate thermocouple may be used to control the temperature of the furnace chamber if needed, but the test specimen
temperature shall be the reported temperature of the test.
(3) For longer gage sections where spatial temperature variation may be of concern, take additional thermocouple temperature
measurements at the top and bottom of the gage section.
8.1.2.5 Calculating, Applying, and Recording the Force—Based on the dimensions measured in 8.1.1, compute the preload
force, F , needed to achieve the recommended 5 MPa stress in the gage section (see 8.1.2.2). Apply the preload in force control
p
to accommodate the dimensional changes expected in the test specimen and fixtures during heating. Compute the force, F, needed
to achieve the desired applied stress, σ , in accordance with 9.1.1. After the test specimen has stabilized at the desired temperature,
a
apply the desired creep force over a period of approximately 30 to 120 s to prevent premature failure. Measure and record the force
at regular intervals during the test to ensure compliance with the requirements of 6.1.
8.1.2.6 Recording of Displacement Data—Record the displacement determined by the extensometry system at appropriate
intervals using an appropriate data logger. The number of intervals shall be at least 100, and be appropriate to the expected duration
of the test. It may be necessary to record displacement data more frequently at the start of the test, when the creep rate is often
higher, than later into the test when the creep rate has decreased.
8.1.2.7 Use of Strain-Gaged Test Specimens—The occasional use of a strain-gaged test specimen at room temperature is
recommended to verify that there is negligible error due to bending. Do not leave strain gages on the test specimen when the system
is heated up, since they will melt or burn incompletely with the residue contaminating the test specimen or fixture, or both.
8.1.2.8 End of Test End-of-Test Criteria—The end of a given test has occurred when any of the following conditions has been
met: (1) the test specimen fractures, (2) the test specimen reaches a predetermined level of strain, or (3) the test specimen has crept
for a predetermined length of time. In Case 1, examine the fracture surfaces to determine whether or not the test specimen failed
in the gage section. Generally, an invalid test is one in which fracture occurs outside the uniform cross section of the gage section.
In the event failure occurred outside of the measured gage section but within the uniform cross-sectional area, the test may still
be valid. Use fractography, along with the knowledge of the testing apparatus and conditions, to determine what occurred at the
failure point and make a determination of validity. In the event failure occurred outside of the measured gage section and outside
of the uniform cross-sectional area, discard the test result. In the event that failure occurred at the extensometer contact points, use
fractography to determine whether the test specimen was affected by the contact. If it was affected, discard the test result. If it was
not affected, use the result.
9. Calculation
9.1 Formulae:
9.1.1 The formulae for determining the force applied to the test specimen are, for rectangular cross sections:
F 5 σ wt (1)
a
where σ is the applied stress, t is the thickness of the gage section, and w is the width of the gage section, and for circular cross
a
sections:
F 5 πσ d /4 (2)
a
where d is the diameter of the gage section.
9.1.2 The standard formulae for the gage section stresses in tensile test specimens are stated in the following: For rectangular
cross sections,
F
σ 5 (3)
a
wt
where σ is the applied stress, F is the applied force, w is the width of the gage section, and t is the thickness of the gage section.
a
For circular cross sections,
F
σ 5 (4)
a
πd /4
where σ is the applied stress, F is the applied force, and d is the diameter of the gage section.
a
9.1.3 The creep strain of the test specimen at any time is determined from:
Exposed thermocouple beads will exhibit greater sensitivity, but may be exposed to vapors that can react with the thermocouple materials. (For example, silica vapors
will react with platinum.) Be aware that the use of heavy-gage thermocouple wire, thermal gradients along the thermocouple length, or excessively heavy-walled insulators
can lead to erroneous temperature readings.
The thermocouple tip may contact the tensile test specimen, but only if it is certain that the thermocouple tip or sheathing material will not chemically interact with the
...