ASTM E2760-19e2

Standard Test Method for Creep-Fatigue Crack Growth Testing

SIGNIFICANCE AND USE
4.1 Creep-fatigue crack growth testing is typically performed at elevated temperatures over a range of frequencies and hold-times and involves the sequential or simultaneous application of the loading conditions necessary to generate crack tip cyclic deformation/damage enhanced by creep deformation/damage or vice versa. Unless such tests are performed in vacuum or an inert environment, oxidation can also be responsible for important interaction effects relating to damage accumulation. The purpose of creep-fatigue crack growth tests can be to determine material property data for (a) assessment input data for the damage condition analysis of engineering structures operating at elevated temperatures, (b) material characterization, or (c)  development and verification of rules for design and life assessment of high-temperature components subject to cyclic service with low frequencies or with periods of steady operation, or a combination thereof.  
4.2 In every case, it is advisable to have complementary continuous cycling fatigue data (gathered at the same loading/unloading rate), creep crack growth data for the same material and test temperature(s) as per Test Method E1457, and creep-fatigue crack formation data as per Test Method E2714. Aggressive environments at high temperatures can significantly affect the creep-fatigue crack growth behavior. Attention must be given to the proper selection and control of temperature and environment in research studies and in generation of design data.  
4.3 Results from this test method can be used as follows:  
4.3.1 Establish material selection criteria and inspection requirements for damage tolerant applications where cyclic loading at elevated temperature is present.  
4.3.2 Establish, in quantitative terms, the individual and combined effects of metallurgical, fabrication, operating temperature, and loading variables on creep-fatigue crack growth life.  
4.4 The results obtained from this test method are designed for crac...
SCOPE
1.1 This test method covers the determination of creep-fatigue crack growth properties of nominally homogeneous materials by use of pre-cracked compact type, C(T), test specimens subjected to uniaxial cyclic forces. It concerns fatigue cycling with sufficiently long loading/unloading rates or hold-times, or both, to cause creep deformation at the crack tip and the creep deformation be responsible for enhanced crack growth per loading cycle. It is intended as a guide for creep-fatigue testing performed in support of such activities as materials research and development, mechanical design, process and quality control, product performance, and failure analysis. Therefore, this method requires testing of at least two specimens that yield overlapping crack growth rate data. The cyclic conditions responsible for creep-fatigue deformation and enhanced crack growth vary with material and with temperature for a given material. The effects of environment such as time-dependent oxidation in enhancing the crack growth rates are assumed to be included in the test results; it is thus essential to conduct testing in an environment that is representative of the intended application.  
1.2 Two types of crack growth mechanisms are observed during creep/fatigue tests: (1) time-dependent intergranular creep and (2) cycle dependent transgranular fatigue. The interaction between the two cracking mechanisms is complex and depends on the material, frequency of applied force cycles and the shape of the force cycle. When tests are planned, the loading frequency and waveform that simulate or replicate service loading must be selected.  
1.3 Two types of creep behavior are generally observed in materials during creep-fatigue crack growth tests: creep-ductile and creep-brittle (1)2. For highly creep-ductile materials that have rupture ductility of 10 % or higher, creep strains dominate and creep-fatigue crack growth is accompanied by substantial t...

General Information

• Status: Published
• Publication Date: 31-Oct-2019
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.06 - Crack Growth Behavior

Relations

• Replaces: ASTM E2760-19e1 - Standard Test Method for Creep-Fatigue Crack Growth Testing
• Effective Date: 01-Nov-2019
• Refers: ASTM E1457-23 - Standard Test Method for Measurement of Creep Crack Growth Times in Metals
• Effective Date: 15-Nov-2023
• Refers: ASTM E1457-23e1 - Standard Test Method for Measurement of Creep Crack Growth Times in Metals
• Effective Date: 15-Nov-2023
• Referred By: ASTM E531-23 - Standard Practice for Surveillance Testing of High-Temperature Nuclear Component Materials
• Effective Date: 01-Nov-2019
• Referred By: ASTM F3122-14(2022) - Standard Guide for Evaluating Mechanical Properties of Metal Materials Made via Additive Manufacturing Processes
• Effective Date: 01-Nov-2019
• Referred By: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 01-Nov-2019

Overview

ASTM E2760-19e2 is an internationally recognized standard developed by ASTM for the reliable determination of creep-fatigue crack growth properties of nominally homogeneous materials. This standard outlines a comprehensive test method designed to evaluate how cracks propagate under the combined influence of cyclic loading (fatigue) and sustained loading (creep) at elevated temperatures. The method uses pre-cracked compact tension [C(T)] test specimens subjected to controlled uniaxial cyclic forces, considering the impact of both time-dependent creep and cycle-dependent fatigue mechanisms.

Understanding creep-fatigue crack growth is crucial for the mechanical integrity of high-temperature engineering structures, such as components in power generation, aerospace, and petrochemical industries. The data derived from this method support material characterization, structural integrity assessments, mechanical design, process and quality control, and failure analysis under service conditions that induce both cyclic stress and prolonged loading at high temperatures.

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Key Topics

• Creep-Fatigue Interactions: Evaluates the synergistic effects of cyclic and sustained loads at high temperatures, simulating real-world service environments.
• Crack Growth Mechanisms: Addresses both time-dependent (intergranular creep) and cycle-dependent (transgranular fatigue) crack propagation mechanisms, and their complex interactions influenced by loading frequency, waveform, and material properties.
• Test Specimen Design: Recommends pre-cracked C(T) specimens and emphasizes strict control of specimen geometry, crack orientation, and test environment for data reliability.
• Environmental Effects: Acknowledges the impact of oxidation and other environmental factors, underscoring the need to replicate service environments during testing.
• Measurement and Monitoring: Specifies calibration, instrumentation (e.g., displacement gages, thermocouples), and data acquisition requirements for accurate force, displacement, temperature, and crack size measurements.

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Applications

ASTM E2760-19e2 is widely applied in a variety of high-performance industrial sectors:

• Materials Research and Development: Enables quantitative comparison of material resistance to creep-fatigue crack propagation, supporting the development or qualification of advanced materials.
• Mechanical Design and Life Assessment: Furnishes critical material property data for the design and life prediction of high-temperature, cyclically loaded components such as turbine blades, pressure vessels, and heat exchangers.
• Quality and Process Control: Assists manufacturers in verifying material performance according to design specifications, especially for products intended for cyclic, high-temperature service.
• Structural Integrity Assessment: Provides input for the evaluation and mitigation of crack-like defects in infrastructure, helping establish inspection intervals and maintenance strategies.
• Failure Analysis: Supports investigations into component failures by characterizing the relevant crack growth behavior due to combined fatigue and creep loading.

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Related Standards

To ensure comprehensive and accurate assessment, ASTM E2760-19e2 references and aligns with several related standards:

• ASTM E1457: Measurement of creep crack growth times in metals, providing complementary data.
• ASTM E2714: Creep-fatigue crack formation testing for additional material characterization.
• ASTM E1823: Terminology relating to fatigue and fracture testing, ensuring consistency in definitions.
• ASTM E647: Measurement of fatigue crack growth rates.
• ASTM E83: Extensometer verification.
• ASTM E220: Thermocouple calibration.
• Other Relevant Standards: E4 (force calibration), E139 (creep, creep-rupture, and stress-rupture testing), E399 (plane-strain fracture toughness), and E467 (dynamic forces in fatigue testing systems).

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ASTM E2760-19e2 delivers a robust, standardized approach to creep-fatigue crack growth testing, supporting safer and more efficient design, evaluation, and maintenance of components in high-temperature, cyclic service environments. By adhering to this test method, organizations enhance the reliability of their materials data and the integrity of their critical engineering assets.

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.
´2
Designation: E2760 − 19
Standard Test Method for
Creep-Fatigue Crack Growth Testing
This standard is issued under the fixed designation E2760; 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.
ε NOTE—Section 3.2.18.4 was editorially corrected in July 2020.
ε NOTE—Sections 3.2.3 and 3.2.14 were editorially corrected in April 2024.
1. Scope have rupture ductility of 10 % or higher, creep strains dominate
and creep-fatigue crack growth is accompanied by substantial
1.1 This test method covers the determination of creep-
time-dependent creep strains near the crack tip. In creep-brittle
fatigue crack growth properties of nominally homogeneous
materials, creep-fatigue crack growth occurs at low creep
materials by use of pre-cracked compact type, C(T), test
ductility. Consequently, the time-dependent creep strains are
specimens subjected to uniaxial cyclic forces. It concerns
comparable to or less than the accompanying elastic strains
fatigue cycling with sufficiently long loading/unloading rates
near the crack tip.
or hold-times, or both, to cause creep deformation at the crack
1.3.1 In creep-brittle materials, creep-fatigue crack growth
tip and the creep deformation be responsible for enhanced
rates per cycle or da/dN, are expressed in terms of the
crack growth per loading cycle. It is intended as a guide for
magnitude of the cyclic stress intensity parameter, ∆K. These
creep-fatigue testing performed in support of such activities as
crack growth rates depend on the loading/unloading rates and
materials research and development, mechanical design, pro-
hold-time at maximum load, the force ratio, R, and the test
cess and quality control, product performance, and failure
temperature (see Annex A1 for additional details).
analysis. Therefore, this method requires testing of at least two
1.3.2 In creep-ductile materials, the average time rates of
specimens that yield overlapping crack growth rate data. The
crack growth during a loading cycle, (da/dt) , are expressed
avg
cyclic conditions responsible for creep-fatigue deformation and
as a function of the average magnitude of the C parameter,
enhanced crack growth vary with material and with tempera- t
(C ) (2).
t avg
ture for a given material. The effects of environment such as
time-dependent oxidation in enhancing the crack growth rates
NOTE 1—The correlations between (da/dt) and (C ) have been
avg t avg
are assumed to be included in the test results; it is thus essential shown to be independent of hold-times (2, 3) for highly creep-ductile
materials that have rupture ductility of 10 percent or higher.
to conduct testing in an environment that is representative of
the intended application.
1.4 The crack growth rates derived in this manner and
expressed as a function of the relevant crack tip parameter(s)
1.2 Two types of crack growth mechanisms are observed
are identified as a material property which can be used in
during creep/fatigue tests: (1) time-dependent intergranular
integrity assessment of structural components subjected to
creep and (2) cycle dependent transgranular fatigue. The
similar loading conditions during service and life assessment
interaction between the two cracking mechanisms is complex
methods.
and depends on the material, frequency of applied force cycles
and the shape of the force cycle. When tests are planned, the
1.5 The use of this practice is limited to specimens and does
loading frequency and waveform that simulate or replicate
not cover testing of full-scale components, structures, or
service loading must be selected.
consumer products.
1.3 Two types of creep behavior are generally observed in
1.6 This practice is primarily aimed at providing the mate-
materials during creep-fatigue crack growth tests: creep-ductile
rial properties required for assessment of crack-like defects in
and creep-brittle (1) . For highly creep-ductile materials that
engineering structures operated at elevated temperatures where
creep deformation and damage is a design concern and are
subjected to cyclic loading involving slow loading/unloading
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue
rates or hold-times, or both, at maximum loads.
and Fracture and is the direct responsibility of Subcommittee E08.06 on Crack
1.7 This practice is applicable to the determination of crack
Growth Behavior.
Current edition approved Nov. 1, 2019. Published January 2020. Originally
growth rate properties as a consequence of constant-amplitude
approved in 2010. Last previous edition approved in 2016 as E2760–16. DOI:
load-controlled tests with controlled loading/unloading rates or
10.1520/E2760-19E02.
hold-times at the maximum load, or both. It is primarily
The boldface numbers in parentheses refer to the list of references at the end of
this standard. concerned with the testing of C(T) specimens subjected to
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´2
E2760 − 19
uniaxial loading in load control mode. The focus of the E1823 Terminology Relating to Fatigue and Fracture Testing
procedure is on tests in which creep and fatigue deformation E2714 Test Method for Creep-Fatigue Testing
and damage is generated simultaneously within a given cycle.
3. Terminology
It does not cover block cycle testing in which creep and fatigue
damage is generated sequentially. Data which may be deter-
3.1 Terminology related to fatigue and fracture testing
mined from tests performed under such conditions may char-
contained in Terminology E1823 is applicable to this test
acterize the creep-fatigue crack growth behavior of the tested
method. Additional terminology specific to this standard is
materials.
detailed in section 3.3. For clarity and easier access within this
document some of the terminology in Terminology E1823
1.8 This practice is applicable to temperatures and hold-
relevant to this standard is repeated below (see Terminology
times for which the magnitudes of time-dependent inelastic
E1823, for further discussion and details).
strains at the crack tip are significant in comparison to the
time-independent inelastic strains. No restrictions are placed
3.2 Definitions:
on environmental factors such as temperature, pressure,
3.2.1 crack-plane orientation—direction of fracture or crack
humidity, medium and others, provided they are controlled
extension relation to product configuration. This identification
throughout the test and are detailed in the data report.
is designated by a hyphenated code with the first letter(s)
NOTE 2—The term inelastic is used herein to refer to all nonelastic
representing the direction normal to the crack plane and the
strains. The term plastic is used herein to refer only to time-independent
second letter(s) designating the expected direction of crack
(that is non-creep) component of inelastic strain.
propagation.
1.9 The values stated in SI units are to be regarded as
3.2.2 crack size, a [L]—principal lineal dimension used in
standard. The values given in parentheses after SI units are
the calculation of fracture mechanics parameters for through-
provided for information only and are not considered standard.
thickness cracks.
1.10 This standard does not purport to address all of the
3.2.2.1 Discussion—In the C(T) specimen, a is the average
safety concerns, if any, associated with its use. It is the
measurement from the line connecting the bearing points of
responsibility of the user of this standard to establish appro-
force application. This is the same as the physical crack size, a
p
priate safety, health, and environmental practices and deter-
where the subscript p is always implied.
mine the applicability of regulatory limitations prior to use.
3.2.2.1 original crack size, a [L]—the physical crack size
o
1.11 This international standard was developed in accor-
at the start of testing.
dance with internationally recognized principles on standard-
3.2.3 specimen thickness, B [L]—the distance between the
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- parallel sides of a test specimen.
mendations issued by the World Trade Organization Technical
3.2.4 net thickness, B [L]—the distance between the roots
N
Barriers to Trade (TBT) Committee.
of the side-grooves in side-grooved specimens.
3.2.5 specimen width, W [L]—the distance from a reference
2. Referenced Documents
position (for example, the front edge of a bend specimen or the
2.1 ASTM Standards:
force line of a compact specimen) to the rear surface of the
E4 Practices for Force Calibration and Verification of Test-
specimen.
ing Machines
3.2.6 force, P [F]—the force applied to a test specimen or to
E83 Practice for Verification and Classification of Exten-
a component.
someter Systems
3.2.7 maximum force, P [F]—in fatigue, the highest
E139 Test Methods for Conducting Creep, Creep-Rupture, max
algebraic value of applied force in a cycle. By convention,
and Stress-Rupture Tests of Metallic Materials
tensile forces are positive and compressive forces are negative.
E177 Practice for Use of the Terms Precision and Bias in
ASTM Test Methods 3.2.8 minimum force, P [F]—in fatigue, the lowest alge-
min
E220 Test Method for Calibration of Thermocouples By
braic value of applied force in a cycle. By convention, tensile
Comparison Techniques forces are positive and compressive forces are negative.
E399 Test Method for Linear-Elastic Plane-Strain Fracture
3.2.9 force ratio (also stress ratio), R— in fatigue, the
Toughness of Metallic Materials
algebraic ratio of the two loading parameters of a cycle. The
E467 Practice for Verification of Constant Amplitude Dy-
most widely used ratio is as follows:
namic Forces in an Axial Fatigue Testing System
minimum load P
min
E647 Test Method for Measurement of Fatigue Crack
R 5 5 (1)
maximum load P
max
Growth Rates
3.2.10 force range, ΔP [F]—in fatigue loading, the alge-
E1457 Test Method for Measurement of Creep Crack
braic difference between the successive valley and peak forces
Growth Times in Metals
(positive range or increasing force range) or between succes-
sive peak and valley forces (negative or decreasing force
For referenced ASTM standards, visit the ASTM website, www.astm.org, or range). In constant amplitude loading, the range is given as
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
follows:
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ∆P 5 P 2 P (2)
max min
´2
E2760 − 19
3.2.11 stress-intensity factor, K, K , K , K , K , K , K 3.2.18.2 Discussion—The value of C*(t) from this equation
1 2 3 I II III
-3/2
[FL ]—the magnitude of the mathematically ideal crack tip is path-independent for materials that deform according to a
stress field (a stress-field singularity) for a particular mode in a constitutive law that may be separated into single-value time
homogeneous, linear-elastic body. and stress functions or strain and stress functions of the forms
3.2.11.1 Discussion—For a C(T) specimen subjected to (1):
Mode I loading, K is calculated by the following equation:
ε˙ 5 f t f σ (7)
~ ! ~ !
1 2
P
ε˙ 5 f ε f σ (8)
~ ! ~ !
3 4
K 5 f~a/W! (3)
1/2 1/2
BB W
~ !
N where, f –f represent functions of elapsed time, t, strain, ε
1 4
21a/W and applied stress, σ, respectively and ε˙ is the strain rate.
2 3
f 5 ~0.88614.64~a/W! 2 13.32~a/W! 114.72~a/W!
F G
3/2
3.2.18.3 Discussion—For materials exhibiting creep defor-
~1 2 a/W!
4 mation for which the above equation is path-independent, the
2 5.6~a/W! ! (4)
C*(t)-integral is equal to the value obtained from two, stressed,
-3/2
3.2.12 maximum stress-intensity factor, K [FL ]—in
max
identical bodies with infinitesimally differing crack areas. This
fatigue, the maximum value of the stress intensity factor in a
value is the difference in the stress-power per unit difference in
cycle. This value corresponds to P .
max
crack area at a fixed value of time and displacement rate, or at
-3/2
3.2.13 minimum stress-intensity factor, K [FL ]—in
min
a fixed value of time and applied force.
fatigue, the minimum value of the stress intensity factor in a
3.2.18.4 Discussion—The value of C*(t) corresponding to
cycle. This value corresponds to P when R > 0 and is taken
min
the steady-state conditions is called C*. Steady-state is said to
to be 0 when R ≤ 0.
have been achieved when a fully developed creep stress
-3/2
3.2.14 stress-intensity factor range, ΔK [FL ]—in fatigue, distribution has been produced around the crack tip. This
the variation in the stress-intensity factor in a cycle, that is:
occurs when secondary creep deformation characterized by Eq
9 dominates the behavior of the specimen.
∆K 5 K 2 K (5)
max min
n
-2
ε˙ 5 Aσ (9)
3.2.15 yield strength, σ [FL ]—the stress at which the
ss
YS
3.2.18.5 Discussion—This steady state in C* does not nec-
material exhibits a deviation from the proportionality of stress
essarily mean steady state crack growth rate. The latter occurs
to strain at the test temperature. This deviation is expressed in
when steady state damage develops at the crack tip.
terms of strain.
3.2.15.1 Discussion—For the purposes of this standard, the
3.2.19 force-line displacement due to creep, elastic, and
value of strain deviation from proportionality used for defining
plastic strain V [L] —the total displacement measured at the
LD
yield strength is 0.2 %.
loading pins (V ) due to the initial force placed on the
specimen at any instant and due to subsequent crack extension
3.2.16 cycle—in fatigue, one complete sequence of values
that is associated with the accumulation of creep, elastic, and
of force that is repeated under constant amplitude loading. The
plastic strains in the specimen.
symbol N used to indicate the number of cycles.
3.2.19.1 Discussion—The force-line displacement associ-
3.2.17 hold-time (t )—in fatigue, the amount of time in the
h
ated with just the creep strains is expressed as V .
c
cycle where the controlled test variable (for example, force,
3.2.19.2 Discussion—In creeping bodies, the total displace-
strain, displacement) remains constant with time.
FLD
ment at the force-line, V , can be partitioned into an
-1 -1
3.2.18 C*(t)—integral, C*(t) [FL T ], a mathematical
instantaneous elastic part V , a plastic part, V , and a time-
e p
expression, a line or surface integral that encloses the crack
dependent creep part, V (6).
c
front from one crack surface to the other, used to characterize
V'V 1V 1V (10)
e p c
the local stress- strain rate fields at any instant around the crack
The corresponding symbols for the rates of force-line
front in a body subjected to extensive creep conditions.
displacement components shown in Eq 10 are given respec-
3.2.18.1 Discussion—The C*(t) expression for a two-
˙ ˙ ˙ ˙
tively as V, V , V , V . This information is used to derive the
e p c
dimensional crack, in the x-z plane with the crack front parallel
parameters C* and C .
t
to the z-axis, is the line integral (4, 5).
-1 -1
3.2.20 C parameter, C [FL T ]—parameter equal to the
t t
] u˙
C* t 5 W* t dy 2 T· ds (6)
~ ! *S ~ ! D value obtained from two identical bodies with infinitesimally
Γ
] x
differing crack areas, each subjected to stress, as the difference
where:
in stress power per unit difference in crack area at a fixed value
W*(t) = instantaneous stress-power or energy rate per unit of time and displacement rate or at a fixed value of time and
volume, applied force for an arbitrary constitutive law (5).
Γ = path of the integral, that encloses (that is, contains) 3.2.20.1 Discussion—The value of C is path-independent
t
the crack tip contour,
and is identical to C*(t) for extensive creep conditions when
ds = increment in the contour path,
the constitutive law described in section 3.1.18.2 of C*(t)-
T = outward traction vector on ds,
integral definition applies.
u˙ = displacement rate vector at ds,
3.2.20.2 Discussion—Under small-scale creep conditions,
x, y, z = rectangular coordinate system, and
C*(t) is not path-independent and is related to the crack tip
]u˙
= the rate of stress-power input into the area enclosed
T· ds stress and strain fields only for paths local to the crack tip and
]x
by Γ across the elemental length ds.
well within the creep zone boundary (7). Under these
´2
E2760 − 19
circumstances, C , is related uniquely to the rate of expansion specimens, this is typically the time required for the creep
t
of the creep zone size (7). There is considerable experimental deformation zone to spread through a substantial portion of the
evidence that the C parameter which extends the C* (t)- uncracked ligament, or in the region that is under the influence
t
integral concept into small-scale and the transition creep of a crack in the case of a finite crack in a semi-infinite
regime, correlates uniquely with creep crack growth rate in the medium.
entire regime ranging from small-scale to extensive creep 3.3.3.1 Discussion—An estimate of transition time for ma-
regimes (5).
terials that creep according to the power-law can be obtained
3.2.20.3 Discussion—For a specimen with a crack subject to from the following equation(9):
constant force, P and under a small-scale-creep (5): 2 2
K 1 2 ν
~ !
t 5 (15)
T
˙ E~n11!C*
PV
c
C 5 f'/f (11)
~ !
t
BW
where:
and
ν = Poisson’s ratio, and
df
n = secondary creep exponent as in Eq 9.
f' 5 (12)
d~a/W!
3.3.4 force-line compliance (C )—the elastic displacement
FL
3.2.21 creep zone boundary—the locus of points ahead of
in the specimen along the force-line divided by the force. This
the crack tip where the equivalent strain caused by the creep
value is uniquely related to the normalized crack size of the
deformation equals 0.002 (0.2 %) (8).
specimen.
3.2.21.1 Discussion—Under small-scale creep conditions, 1
˙
3.3.5 force line displacement rate due to creep, V [LT ]—
c
the creep zone expansion with time occurs under self-similar
rate of increase of the force-line displacement due to creep
manner for planar bodies (9), thus, the creep zone size, r , can
c
strains.
be defined as the distance of the creep zone boundary from the
crack tip at a fixed angle, θ, with respect to the crack plane. The
4. Significance and Use
rate of expansion of the creep zone size is designated as r˙ (θ).
c
4.1 Creep-fatigue crack growth testing is typically per-
3.3 Definitions of Terms Specific to This Standard:
formed at elevated temperatures over a range of frequencies
-1 -1
3.3.1 (C ) parameter, (C ) [FL T ]—the average
t avg t avg
and hold-times and involves the sequential or simultaneous
value of the C parameter during the hold-time of the cycle and
t
application of the loading conditions necessary to generate
is given by (1, 2):
crack tip cyclic deformation/damage enhanced by creep
1 t
deformation/damage or vice versa. Unless such tests are
h
~C ! 5 * C dt (13)
t avg t
t
performed in vacuum or an inert environment, oxidation can
h
also be responsible for important interaction effects relating to
where:
damage accumulation. The purpose of creep-fatigue crack
t = hold-time at maximum load measured from the start of
h
growth tests can be to determine material property data for (a)
the hold period.
assessment input data for the damage condition analysis of
Eq 13 can also be written as:
engineering structures operating at elevated temperatures, (b)
material characterization, or (c) development and verification
∆V
P ~ !
max c
C 5 f'/f (14)
~ ! ~ !
t 1/2
avg of rules for design and life assessment of high-temperature
BB Wt
~ !
N h
components subject to cyclic service with low frequencies or
where:
with periods of steady operation, or a combination thereof.
ΔV = the difference in the force-line displacement between
c
4.2 In every case, it is advisable to have complementary
the end and the start of the hold-time during a cycle
continuous cycling fatigue data (gathered at the same loading/
(1).
unloading rate), creep crack growth data for the same material
and test temperature(s) as per Test Method E1457, and creep-
3.3.1.1 Discussion—The value of (C ) from Eq 14 is
t avg
appropriate for small-scale creep regime but it’s value is fatigue crack formation data as per Test Method E2714.
Aggressive environments at high temperatures can signifi-
identical to the value of C*(t) for extensive creep conditions
when the constitutive law described in section 3.2.18 is cantly affect the creep-fatigue crack growth behavior. Attention
must be given to the proper selection and control of tempera-
applicable.
ture and environment in research studies and in generation of
3.3.2 creep-fatigue crack growth rate behavior (CFCGR):
design data.
for creep-ductile materials, this is a plot of the incremental,
average time rate of crack growth, (da/dt) , as a function of 4.3 Results from this test method can be used as follows:
avg
(C ) . 4.3.1 Establish material selection criteria and inspection
t avg
for creep-brittle materials, this is a plot of incremental requirements for damage tolerant applications where cyclic
crack growth rate per loading cycle, da/dN, as a function of the loading at elevated temperature is present.
cyclic stress intensity factor, ∆K, for constant temperature,
4.3.2 Establish, in quantitative terms, the individual and
hold-time, and force ratio, R. combined effects of metallurgical, fabrication, operating
3.3.3 transition time, t [T]—time required for extensive temperature, and loading variables on creep-fatigue crack
T
creep conditions to develop in a cracked body (9). For growth life.
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4.4 The results obtained from this test method are designed should incorporate cooling arrangements to limit heat transfer
for crack dominant regimes of creep-fatigue failure and should from the hot zone to the testing machine and in particular the
not be applied to cracks in structures with wide-spread creep force transducer.
damage. Localized damage in a small zone around the crack tip
6.4 Heating Apparatus:
is permissible, but not in a zone that is comparable in size to
6.4.1 The apparatus for, and method of, heating the speci-
the crack size or the remaining ligament size.
mens should provide the temperature control necessary to
satisfy the requirements in section 9.6.4, without manual
5. Functional Relationships
adjustments more frequently than once in each 24-h period
after force application.
5.1 Empirical relationships that have been commonly used
6.4.2 Heating shall be by an electric resistance or radiation
for description of creep-fatigue crack growth data are given in
furnace with the specimen in air at atmospheric pressure unless
Annex A1. These relationships typically have limitations with
other media are specifically agreed upon in advance.
respect to material types such as high temperature ferritic and
austenitic steels (creep-ductile materials) versus nickel base
6.5 Displacement Gage for the Measurement of the Force
alloys (typically creep-brittle materials). Therefore, original
Line Displacement During the Test:
data should be reported to the greatest extent possible. Data
6.5.1 Continuous force-line displacement measurement is
reduction methods should be detailed along with assumptions.
needed to evaluate the magnitude of (C ) as a function of
t avg
Sufficient information should be recorded and reported to
creep-fatigue cycles during the test in creep-ductile materials.
permit analysis, interpretation, and comparison with results for
Displacement measurements must be made on the force-line.
other materials analyzed using currently popular methods.
As a guide, the displacement gage should have a working range
no more than twice the displacement expected during the test.
6. Apparatus
Accuracy of the gage should be within 61 % of the full
working range of the gage. In calibration, the maximum
6.1 Testing Machine—Tests shall be conducted using a
deviation of the individual data points from the fit to the data
servo-controlled tension-compression fatigue machine that has
shall not exceed 61 % of the working range.
been verified in accordance with Practices E4 and Practice
E467. Hydraulic and electromechanical machines are accept-
NOTE 3—Thermal effects, particularly thermal gradients, can change
able. The complete loading system comprising force
extensometer output and must be minimized. It is good practice to keep
transducer, specimen clevises and test specimen shall have the body of the extensometer outside the furnace unless it is designed to
withstand the test temperature.
lateral rigidity and be capable of executing the prescribed cycle
in force control. It shall be possible to measure the response
6.5.2 Knife edges are recommended for friction-free seating
variable, extension, to the required tolerances. Further, auxil-
of the gage. Parallel alignment of the knife edges must be
iary equipment for measuring crack size as a function of cycles
maintained to within 61°.
to the required tolerances shall be available as part of the
6.5.3 The displacement along the force-line may be directly
apparatus.
measured by attaching the entire clip gage assembly to the
specimen and placing the whole assembly in the furnace.
6.2 Force Transducer:
Alternatively, the displacements can be transferred outside the
6.2.1 The force transducer shall be designed for tension-
furnace with ceramic rods. In the latter procedure, the trans-
compression fatigue testing and shall have high axial and
ducer is placed outside the furnace. Other designs that can
lateral rigidity. Its capacity shall be sufficient to measure the
measure displacements to the same levels of accuracy may also
axial forces applied during the test to an accuracy better than
be used.
1 % of the reading. The force transducer and its associated
6.5.4 The extensometer used shall be suitable for measuring
electronics shall comply with Practices E4 and Practice E467.
force-line displacements over long periods during which there
6.2.2 The force transducer shall be temperature compen-
shall be minimal drift, slippage and instrument hysteresis.
sated and not have zero drift nor sensitivity variation greater
Extensometers used for measurement shall be suitable for
than 0.002 % of the full scale per degree Celsius. During test,
dynamic measurements over periods of time, i.e. should have a
the force transducer shall be maintained at a temperature within
rapid response and with a low hysteresis (not greater than
its temperature compensation range. Force transducers are
0.1 % of extensometer output). Strain gauge, capacitance
subject to thermal drift in zero point and sensitivity and may be
gauge, DCDT or LVDT type transducers are generally used
permanently damaged by temperatures in excess of 50°C.
and should be calibrated according to Practice E83. The
Suitable cooling arrangements include forced air cooling of
extensometer should meet the requirements of Grade B2 or
fins at the outer ends of the loading bars or water cooling coils
better as specified by Practice E83.
or jackets. Care should be taken to ensure that force transducer
calibration and force train alignment are not affected by the 6.6 Crack Monitoring:
6.6.1 A direct current (DCPD) or alternating current
presence of the cooling devices.
(ACPD) electrical potential-drop crack monitoring system
6.3 Alignment of Grips—It is important that attention be
must be used. Further details on the attachment of the input and
given to achieving good alignment in the force-line through
output electrical leads and measurement procedures are given
careful machining of all gripping fixtures. The length of the
in Annex A2.
force train should be chosen with proper attention to the height
of the furnace for heating the test specimen. The loading train NOTE 4—It is good practice to electrically insulate the test specimen (or
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loading grips) from the test machine loading frame and ancillary equip-
6.9 Data Recording—An automatic digital recording system
ment in order to avoid unstable potential drop recordings associated with
should be used which is capable of collecting and simultane-
earth loops. However, it is not essential to do so. The contact resistance
ously processing the force, force-line displacement, DCPD or
between the loading pin holes and the pins can provide sufficient electrical
ACPD and temperature data as a function of time and cycles.
insulation.
The sampling frequency of the data shall be sufficient to ensure
6.6.2 The DCPD or ACPD system should be capable of
correct definition of the loading cycle. In particular, it should
reliably resolving crack extensions of at least 60.1 mm at the
be sufficient to identify values of load and extension at taming
test temperature.
points in the loading diagram, e.g. at cycle maxima and
6.7 Temperature Measurement and Control—Test specimen
minima, and start and end of hold-time values.
temperature shall be measured using Class 1 thermocouples in
NOTE 6—At least 200 data points should be collected to define the
loading and unloading segments of the cycle and an additional 100–200
contact with the test specimen surface in the region near the
data points should be collected to fully characterize hold-time duration.
crack plane. In all cases involving the use of thermocouples, it
NOTE 7—The simultaneous recording of servo position is also recom-
is essential to ensure that intimate thermal contact is achieved
mended to assist in the retrospective diagnosis of disturbances during test,
between test specimen and thermocouple without affecting the
e.g. extensometer slippage.
properties of the test specimen. When using furnace heating,
thermocouple beads shall be shielded from direct radiation.
7. Test Specimen
NOTE 5—For long duration creep-fatigue tests, the use of Type K
7.1 The schematic and dimension of the C(T) specimen is
thermocouples above 400°C is not recommended. Their use for short
shown in Fig. 1.
duration tests (<500 h) at temperatures up to 600°C is possible, but their
re-use is not recommended in these circumstances. Similarly, Type N
NOTE 8—The crack mouth geometry and dimensions and the machine
thermocouples may be used for short duration tests (<500 h) at tempera-
notch and knife edge configuration may be varied from the one in Fig. 1
tures up to 800°C, with their re-use not being recommended without
to adapt to the clip gage chosen for measuring force-line displacements.
recalibration.
7.2 The width-to-thickness ratio W/B for the C(T) specimen
6.8 Cycle Counter—Standard practice should be to record
is recommended to be 4, nominally. Other W/B ratios, up to 8,
all cycles in a data acquisition system. As a minimum, a digital
may be used for thickness effect characterization or to reduce
device should be used to record the number of cycles applied
forces during the test; it is however important to note that the
to the test specimen. Five digits are required. For tests lasting
stress state may vary with thickness.
less than 10 000 cycles, individual cycles shall be counted. For
longer tests, the device shall have a resolution better than 1 % 7.3 The initial crack size, a (including a sharp starter notch
of the actual life. or pre-crack), shall be at least 0.25 times the width, W, but no
FIG. 1 Drawing of a C(T) Specimen Recommended for Creep-Fatigue Crack Growth Testing and the Details of the Machined Notch
and the Knife-Edges for Securely Attaching the Extensometer
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greater than 0.35W. This may be varied within the stated 7.7.4 To facilitate fatigue pre-cracking at low stress ratios,
interval depending on the selected force level for testing and the machined notch root radius can be approximately 0.075
the desired test duration. mm (0.003 in.). It may at times be expedient to have an EDM
notch of 0.1 mm width to enhance the fatigue crack growth. A
7.4 Specimen Size—Specimen size must be chosen with
chevron form of machined notch as described in Test Method
consideration to the material availability, capacity of the
E399 or pre-compression of the straight through notch as
loading system, being able to fit the specimen into the heating
described in Test Method E399 may be helpful when control of
furnace with sufficient room for attaching the necessary
crack shape is a problem.
extensometers, and providing sufficient ligament size for grow-
7.7.5 Pre-cracking is to be done with the material in the
ing the crack in a stable fashion to permit collection of crack
same heat-treated condition as that in which it will be tested for
growth data. Specimen size requirements to maintain domi-
creep-fatigue crack growth behavior. No intermediate heat
nantly elastic conditions in the specimen to validate the
treatments between pre-cracking and testing are allowed.
creep-fatigue crack growth data are addressed in section
7.7.6 The size of the pre-crack extension from the machined
10.3.2.
notch shall be no less than 0.05 a/W.
7.5 Specimen Measurements—Specimen dimensions are
7.8 Specimen Preparation for Electric Potential
given in Fig. 1. They shall be machined within the machining
Measurement—The potential drop could be AC or DC pow-
tolerances specified and the dimensions should be measured
ered. The input should be remote from the crack and welded to
before commencing the test.
the specimen. The specific recommendations for the C(T)
7.6 Notch Preparation—The machined notch for the test
specimen is presented in Annex A2. For gripping fixtures and
specimens may be made by electrical-discharge machining
wire selection and attachment also refer to the Annex in Test
(EDM), milling, broaching, or saw cutting. It is recommended
Method E647.
that the last 0.1 a/W of the crack be machined using electro-
7.9 Attachment of Thermocouples and Input Leads:
discharge machining (EDM) of a width of 0.1 mm. This will
7.9.1 A thermocouple must be attached to the specimen for
allow easier pre-cracking or further crack tip sharpening by
measuring the specimen temperature. The thermocouple
EDM to the final crack starter size prior to testing.
should be located in the uncracked ligament region of the
7.7 Pre-cracking—Fatigue pre-cracking is used to introduce
specimen 2 to 5 mm (0.08 to 0.2 in.) above or below the crack
a sharp starter crack; it is recommended that a narrow slit (of
plane. Multiple thermocouples are recommended for speci-
0.1 mm width) ahead of the machined notch be introduced
mens wider than 50 mm (2 in.). These thermocouples must be
using electro-discharge machining (EDM) prior to fatigue
evenly spaced over the uncracked ligament region above or
pre-cracking. This ensures that the final crack front is straight
below the crack plane as stated above.
and flat and does not deviate from the crack plane. In
7.9.2 In attaching thermocouples to a specimen, the junction
creep-brittle materials, EDM notch itself may be used as the
must be kept in intimate contact with the specimen and
pre-crack due to difficulties in growing cracks with straight
shielded from radiation, if necessary. Shielding is not necessary
fronts. If there are indications that the mode of pre-cracking
if the difference in indicated temperature from an unshielded
has affected the initial CFCG data, such data must be excluded
bead and a bead inserted in a hole in the specimen has been
from being reported as valid data.
shown to be less than one half the permitted variations in
NOTE 9—If unusual crack growth trends are observed during the first
section 9.6.4. The bead should be as small as possible and there
0.25 mm of crack extension, the data could be excluded as being invalid
should be no shorting of the circuit (such as could occur from
CFCG rate or at the very least flagged as being suspect due to possible
twisted wires behind the bead). Ceramic insulators should be
transient effects.
used in the hot zone to prevent such shorting.
7.7.1 Care must be exercised during fatigue pre-cracking to
7.9.3 Specifications in Test Methods E139 identify the type
avoid excessive damage at the notch root. Hereafter, the
of thermocouples that may be used in different temperature
method for pre-cracking is described.
regimes. It is important to note that creep-fatigue crack growth
7.7.2 Fatigue pre-cracking:
test durations are invariably long. Thus, a stable temperature
7.7.2.1 Specimens may also be pre-cracked at room tem-
measurement method should be used to reduce experimental
perature or at a temperature between ambient and test tempera-
error.
ture under fatigue forces to be estimated from the following
equation:
8. Calibration and Standardization
∆K
23 23
8.1 Performance of the electric potential system, the force
# 0.08 × 10 =m~0.5 × 10 =in.! (16)
E
measuring system, the temperature measurement systems and
7.7.2.2 Fatigue pre-cracking is conducted at a load ratio, R, the displacement gage must be verified. Calibration of these
of 0.1 or higher using any convenient loading frequency. The devices should be as frequent as necessary to ensure that the
accuracy of the fatigue force value shall be within 65 %. The errors for each test are less than the permissible indicated
stress intensity factor range, ∆K, may be calculated using Eq 3 variations cited in this standard. The testing machine should be
and Eq 4. calibrated at least annually or, for tests that last for more than
7.7.3 The maximum force during the last 0.5 mm (0.02 in.) a year, after each test. Instruments in constant (or nearly
of crack extension must not exceed the maximum force used constant) use should be calibrated more frequently; those used
during creep-fatigue crack growth testing. occasionally must be calibrated before each use.
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E2760 − 19
8.1.1 Calibrate the force measuring system according to the current source and voltmeter, respectively. Attach the
Practices E4. displacement gage to the specimen and the thermocouple to the
8.1.2 Calibrate the displacement gage according to Practice appropriate potentiometer. Bring furnace into position and start
E83. heating the specimen.
8.1.3 Verify electric potential system according to guide-
9.4 Heating the Test Specimen—The test specimen shall be
lines in and recommendation in Annex A2.
heated to the specified temperature and shall be maintained at
8.1.4 Calibrate the thermocouples according to Test Method
that temperature for at least 30 minutes before loading. During
E220.
heating, the temperature of the test specimen shall not exceed
the specified temperature within its tolerances. A small pre-
9. Procedure
load equal to about 10 % of the maximum test load should be
9.1 Plans for a Test Matrix—A test matrix should be set up
applied to the specimen during heating to ensure that the
identifying, as far as possible, the goals for the tests such as the
loading train remains under tension at all times.
planned test times, available specimens, number of tests and
9.5 Cycle Shape—The cycle shape that shall be used for
the force levels that may be needed for the tests. At least one
creep-fatigue crack growth testing include (a) low frequency
duplicate test shall be conducted such that all test conditions
triangular wave forms with low control parameter ramp rates,
are nominally the same except the applied force ranges. The
(b) saw-tooth wave forms in which the ramp rate of the
differences in the applied force ranges between the two tests
tensile-going transient is significantly different to that of the
shall be such that the crack growth ranges are extended with
unloading portion, and (c) cyclic/hold forms comprising a
respect to each other and the overlap in the crack growth rates
series of ramps with hold-time(s) at the maximum load (the
between the tests is no more than one-third of the combined
ramp rates may not always be the same). Example creep-
crack growth rate range covered by the two tests. Availability
fatigue cycle shapes are shown in Fig. 2. There are many other
of spare specimens is essential as repeat tests may be required
possibilities depending on the practical application for which
in some instances.
the creep-fatigue data are required.
9.2 Number of Tests—Creep-fatigue crack growth rate data
9.6 Starting and Conducting the Test:
exhibit scatter. The (da/dt) values at a given value of (C )
avg t avg
9.6.1 The extensometer output should be brought to a null
for creep-ductile materials and da/dN versus ∆K for creep-
value with no force on the test specimen. A force to not exceed
brittle materials can vary by as much as a factor of 2 to 3 if all
0.5P should be applied in increments and the displacement
max
other variables such as geometry, specimen size, crack size,
and the PD should be monitored to ensure that the extensom-
loading method and temperature are kept constant. This scatter
eter is properly seated and the PD system is working well and
may increase further by variables such as microstructural
the information is available for post test analysis. The time for
differences, force precision, environmental control, and data
application of the force should be as short as possible within
processing techniques. Therefore, it is good practice to conduct
these limitations.
replicate tests whenever practical. Confidence in the inferences
9.6.2 The compliance of the specimen should be recorded
drawn from the data will increase with the number of tests and
by manually applying loads that do not exceed 0.5P . Three
max
the number of tests will depend on the end use of the data.
compliance measurements should be made and the average of
9.3 Specimen Installation—Install the specimen on the ma- the three readings should be within 15 % of the theoretical
chine by inserting both pins, then apply a small force (approxi- value for the specimen. The relationship between compliance
mately 10 % of the intended test force) to remove slack from and crack size for measurements made at the load-line are
the loading train. Connect the current input and voltage leads to given by the following equation (10):
FIG. 2 Example Creep-Fatigue Cycle Shapes
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E2760 − 19
V 1 W1a 9.6.6 Begin the test by applying the minimum force on to
Fl
C 5 5 @12.1630112.219~a/W!
F G
FL 1/2
P E~BB ! W 2 a the specimen and then subjecting it to the desired cyclic forces.
N
2 3 4
2 20.065 a/W 2 0.9925 a/W 120.609 a/W
~ ! ~ ! ~ !
9.7 Measurements During the Test:
2 9.9314 a/W (17) 9.7.1 The electric potential voltage, force, force-line
~ ! #
displacement, and test temperature should be recorded continu-
9.6.3 Choose the appropriate cyclic force range that will
ously during the test. The force and temperature records are
give the desired crack growth rate range. This estimate can be
retained to ensure that these control parameters remain within
made from previous tests under similar conditions if available
their prescribed limits at all times during the test. At the start of
or estimated from available data in the literature on similar
test, a continuous recording shall be made of the initial values
materials. If none is available the first test should be tested with
of the electric potential voltage and the displacement. During
incremental force increases to identify the appropriate force
the course of test, periodic recording is sufficient. The fre-
levels.
quency of these recordings shall be chosen appropriately for
9.6.4 Before the test force is applied and for the duration of
the expected overall duration of the test.
the test, do not permit the difference between the indicated
NOTE 12—It is common to continuously record the data from the first
temperature and the nominal test temperature to exceed the
5 cycles and then for cycles at logarithmic intervals (that is, 16, 25, 40, 63,
following limits: Up to and including 1000 °C (1832 °F) 6
100, 160, etc.). If data acquisition is automated, the acquisition of electric
2 °C (6 3 °F) above 1000 °C (1832 °F) 6 3 °C (6 5 °F). The
potential and displacement output as a function of time may be pro-
term “indicated temperature” means the temperature indicated grammed either with a predefined interval or as a function of the
progression of each of the two parameters (force and e
...