ASTM E1457-23e1

Standard Test Method for Measurement of Creep Crack Growth Times in Metals

SIGNIFICANCE AND USE
6.1 Creep crack growth rate expressed as a function of the steady state C* or K characterizes the resistance of a material to crack growth under conditions of extensive creep deformation or under brittle creep conditions. Background information on the rationale for employing the fracture mechanics approach in the analyses of creep crack growth data is given in (11, 13, 30-35).  
6.2 Aggressive environments at high temperatures can significantly affect the creep 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.  
6.2.1 Expressing CCI time, t0.2 and CCG rate, da/dt as a function of an appropriate fracture mechanics related parameter generally provides results that are independent of specimen size and planar geometry for the same stress state at the crack tip for the range of geometries and sizes presented in this document (see Annex A1). Thus, the appropriate correlation will enable exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables creep crack growth data to be utilized in the design and evaluation of engineering structures operated at elevated temperatures where creep deformation is a concern. The concept of similitude is assumed, implying that cracks of differing sizes subjected to the same nominal C*(t), Ct, or K will advance by equal increments of crack extension per unit time, provided the conditions for the validity for the specific crack growth rate relating parameter are met. See 11.7 for details.  
6.2.2 The effects of crack tip constraint arising from variations in specimen size, geometry and material ductility can influence t0.2 and da/dt. For example, crack growth rates at the same value of C*(t), Ct in creep-ductile materials generally increases with increasing thickness. It is therefore necessary to keep the component dimensions in mind when selecti...
SCOPE
1.1 This test method covers the determination of creep crack initiation (CCI) and creep crack growth (CCG) in metals at elevated temperatures using pre-cracked specimens subjected to static or quasi-static loading conditions. The solutions presented in this test method are validated for base material (that is, homogenous properties) and mixed base/weld material with inhomogeneous microstructures and creep properties. The CCI time, t0.2, which is the time required to reach an initial crack extension of δai = 0.2 mm to occur from the onset of first applied force, and CCG rate, a˙  or da/dt are expressed in terms of the magnitude of creep crack growth correlated by fracture mechanics parameters, C* or K, with C* defined as the steady state determination of the crack tip stresses derived in principal from C*(t) and Ct   (1-17).2 The crack growth derived in this manner is identified as a material property which can be used in modeling and life assessment methods (17-28).  
1.1.1 The choice of the crack growth correlating parameter C*, C*(t), Ct, or K depends on the material creep properties, geometry and size of the specimen. Two types of material behavior are generally observed during creep crack growth tests; creep-ductile (1-17) and creep-brittle (29-44). In creep ductile materials, where creep strains dominate and creep crack growth is accompanied by substantial time-dependent creep strains at the crack tip, the crack growth rate is correlated by the steady state definitions of Ct or C*(t) , defined as C* (see 1.1.4). In creep-brittle materials, creep crack growth occurs at low creep ductility. Consequently, the time-dependent creep strains are comparable to or dominated by accompanying elastic strains local to the crack tip. Under such steady state creep-brittle conditions, Ct or K could be chosen as the correlating parameter (8-14).  
1.1.2 In any one test, two regions of crack growth behavior may be present (12, 13)....

General Information

• Status: Published
• Publication Date: 14-Nov-2023
• ICS: 77.040.10 - Mechanical testing of metals
• Technical Committee: E08 - Fatigue and Fracture
• Drafting Committee: E08.06 - Crack Growth Behavior

Relations

• Replaces: ASTM E1457-23 - Standard Test Method for Measurement of Creep Crack Growth Times in Metals
• Effective Date: 15-Nov-2023
• Referred By: ASTM E2760-19e2 - Standard Test Method for Creep-Fatigue Crack Growth Testing
• Effective Date: 15-Nov-2023
• Referred By: ASTM E1823-24a - Standard Terminology Relating to Fatigue and Fracture Testing
• Effective Date: 15-Nov-2023

Overview

ASTM E1457-23e1 is the internationally recognized standard for the measurement of creep crack growth times in metals. Developed by ASTM International, this test method provides a systematic approach for determining both creep crack initiation (CCI) time and creep crack growth (CCG) rates in metallic materials exposed to elevated temperatures and subjected to static or quasi-static loading. The standard is applicable to tests performed on base materials with homogeneous properties as well as welded materials featuring inhomogeneous microstructures.

Creep deformation presents critical challenges in components and structures operating at high temperatures. Understanding creep crack growth behavior is essential for ensuring the safety and reliability of engineering components, especially in industries such as power generation, petrochemicals, and aerospace, where long-term high-temperature service is common.

Key Topics

• Creep Crack Growth (CCG): The standard outlines procedures to measure the time-dependent propagation of cracks in metals under sustained load at elevated temperatures.
• Creep Crack Initiation (CCI): Defines methodologies for determining the time required to initiate crack extension, typically measuring when the crack advances by 0.2 mm from initial loading.
• Fracture Mechanics Parameters: CCG and CCI results are correlated using fracture mechanics parameters, primarily the steady-state C*-integral, time-dependent C*(t), Ct, and the stress intensity factor K.
• Material Behavior: Differentiates between creep-ductile and creep-brittle materials and prescribes appropriate correlating parameters based on ductility, geometry, and specimen size.
• Geometry and Specimen Selection: Recommends standard test geometries such as compact tension (C(T)), single edge notched, and double edge notched specimens, ensuring data consistency across configurations.
• Environmental and Temperature Effects: Highlights the importance of controlling testing environment and temperature, as aggressive high-temperature conditions can significantly alter creep crack growth behavior.

Applications

• Life Assessment and Predictive Modeling: The material properties data derived from ASTM E1457-23e1 are vital for modeling crack growth and assessing the remaining life of components subjected to creep conditions. This information is instrumental in failure analysis and in the development of life prediction methodologies for critical structures.
• Component Design: Engineers utilize CCG rate data and CCI times to inform the design of components that must endure prolonged exposure to high-stress and high-temperature conditions, ensuring safety margins and compliance with industry codes.
• Material Selection and Inspection Planning: The standard enables comparative assessment of different alloys and welded joints, aiding in the selection of materials with superior creep resistance. It also supports the development of inspection intervals and maintenance schedules for in-service components.
• Cross-Industry Data Exchange: The methods provided allow for comparison and exchange of creep crack growth data across different laboratories and industries by minimizing the influence of specimen size and geometry.

Related Standards

• ASTM E139: Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials.
• ASTM E399: Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials.
• ASTM E647: Test Method for Measurement of Fatigue Crack Growth Rates.
• ASTM E1823: Terminology Relating to Fatigue and Fracture Testing.
• ASTM E83: Practice for Verification and Classification of Extensometer Systems.
• ASTM E220: Test Method for Calibration of Thermocouples.
• ASTM E74: Practices for Calibration and Verification for Force-Measuring Instruments.

Adherence to ASTM E1457-23e1 assures reliable, repeatable measurement of creep crack growth, supporting structural integrity, safety, and service life extension in high-temperature applications. Implementing this standard enhances confidence in material performance and supports compliance with international engineering best practices.

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.
´1
Designation: E1457 − 23
Standard Test Method for
Measurement of Creep Crack Growth Times in Metals
This standard is issued under the fixed designation E1457; 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 4.2.11 was editorially corrected in April 2024.
1. Scope elastic strains dominate and creep damage develops and in the
steady state region where crack grows proportionally to time.
1.1 This test method covers the determination of creep crack
Steady-state creep crack growth rate behavior is covered by
initiation (CCI) and creep crack growth (CCG) in metals at
this standard. In addition, specific recommendations are made
elevated temperatures using pre-cracked specimens subjected
in 11.7 as to how the transient region should be treated in terms
to static or quasi-static loading conditions. The solutions
of an initial crack growth period. During steady state, a unique
presented in this test method are validated for base material
correlation exists between da/dt and the appropriate crack
(that is, homogenous properties) and mixed base/weld material
growth rate relating parameter.
with inhomogeneous microstructures and creep properties. The
1.1.3 In creep ductile materials, extensive creep occurs
CCI time, t , which is the time required to reach an initial
0.2
when the entire un-cracked ligament undergoes creep defor-
crack extension of δa = 0.2 mm to occur from the onset of first
i
mation. Such conditions are distinct from the conditions of
applied force, and CCG rate, a˙ or da/dt are expressed in terms
small-scale creep and transition creep (1-10). In the case of
of the magnitude of creep crack growth correlated by fracture
extensive creep, the region dominated by creep deformation is
mechanics parameters, C* or K, with C* defined as the steady
significant in size in comparison to both the crack length and
state determination of the crack tip stresses derived in principal
the uncracked ligament sizes. In small-scale-creep only a small
from C*(t) and C (1-17). The crack growth derived in this
t
region of the un-cracked ligament local to the crack tip
manner is identified as a material property which can be used
experiences creep deformation.
in modeling and life assessment methods (17-28).
1.1.1 The choice of the crack growth correlating parameter
1.1.4 The creep crack growth rate in the extensive creep
C*, C*(t), C , or K depends on the material creep properties, region is correlated by the C*(t)-integral. The C parameter
t t
geometry and size of the specimen. Two types of material
correlates the creep crack growth rate in the small-scale creep
behavior are generally observed during creep crack growth
and the transition creep regions and reduces, by definition, to
tests; creep-ductile (1-17) and creep-brittle (29-44). In creep
C*(t) in the extensive creep region (5). Hence in this document
ductile materials, where creep strains dominate and creep crack
the definition C* is used as the relevant parameter in the steady
growth is accompanied by substantial time-dependent creep
state extensive creep regime whereas C*(t) and/or C are the
t
strains at the crack tip, the crack growth rate is correlated by
parameters which describe the instantaneous stress state from
the steady state definitions of C or C*(t), defined as C* (see
the small scale creep, transient and the steady state regimes in
t
1.1.4). In creep-brittle materials, creep crack growth occurs at
creep. The recommended functions to derive C* for the
low creep ductility. Consequently, the time-dependent creep different geometries shown in Annex A1 is described in Annex
strains are comparable to or dominated by accompanying
A2.
elastic strains local to the crack tip. Under such steady state
1.1.5 An engineering definition of an initial crack extension
creep-brittle conditions, C or K could be chosen as the
t
size δa is used in order to quantify the initial period of crack
i
correlating parameter (8-14).
development. This distance is given as 0.2 mm. It has been
1.1.2 In any one test, two regions of crack growth behavior
shown (41-44) that this initial period which exists at the start of
may be present (12, 13). The initial transient region where
the test could be a substantial period of the test time. During
this early period the crack tip undergoes damage development
as well as redistribution of stresses prior reaching steady state.
This test method is under the jurisdiction of ASTM Committee E08 on Fatigue
Recommendation is made to correlate this initial crack growth
and Fracture and is the direct responsibility of Subcommittee E08.06 on Crack
period defined as t at δa = 0.2 mm with the steady state C*
Growth Behavior.
0.2 i
Current edition approved Nov. 15, 2023. Published December 2023. Originally
when the crack tip is under extensive creep and with K for
ɛ1
approved in 1992. Last previous edition approved in 2019 as E1457 – 19 . DOI:
creep brittle conditions. The values for C* and K should be
10.1520/E1457-23E01.
calculated at the final specified crack size defined as a + δa
The boldface numbers in parentheses refer to the list of references at the end of o i
this standard. where a is initial size of the starter crack.
o
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
E1457 − 23
1.1.6 The recommended specimens for CCI and CCG test- on externally applied forces or displacements. Not taking the
ing is the standard compact tension specimen C(T) (see Fig. tensile residual stress effect into account will produce C*
values lower than expected effectively producing a faster
A1.1) which is pin-loaded in tension under constant loading
conditions. The clevis setup is shown in Fig. A1.2 (see 7.2.1 for cracking rate with respect to a constant C*. This would produce
conservative estimates for life assessment and non-
details). Additional geometries which are valid for testing in
this procedure are shown in Fig. A1.3. These are the C-ring in conservative calculations for design purposes. It should also be
noted that distortion during specimen machining can also
tension CS(T), middle crack specimen in tension M(T), single
edge notched tension SEN(T), single edge notched bend indicate the presence of residual stresses.
SEN(B), and double edge notched tension DEN(T). In Fig. 1.1.9 Stress relaxation of the residual stresses due to creep
and crack extension should also be taken into consideration.
A1.3, the specimens’ side-grooving-position for measuring
displacement at the force-line displacement (FLD) and crack No specific allowance is included in this standard for dealing
with these variations. However the method of calculating C*
mouth opening displacement (CMOD) and positions for the
electric potential drop (EPD) input and output leads are shown. presented in this document which used the specimen’s creep
displacement rate to estimate C* inherently takes into account
Recommended loading for the tension specimens is pin-
loading. The configurations, size range are given in Table A1.1 the effects described above as reflected by the instantaneous
creep strains that have been measured. However extra caution
of Annex A1, (43-47). Specimen selection will be discussed in
should still be observed with the analysis of these types of tests
5.9.
as the correlating parameters K and C* shown in Annex A2
1.1.7 The state-of-stress at the crack tip may have an
even though it is expected that stress relaxation at high
influence on the creep crack growth behavior and can cause
temperatures could in part negate the effects due to residual
crack-front tunneling in plane-sided specimens. Specimen size,
stresses. Annex A4 presents the correct calculations needed to
geometry, crack length, test duration and creep properties will
derive J and C* for weldment tests where a mismatch factor
affect the state-of-stress at the crack tip and are important
needs to be taken into account.
factors in determining crack growth rate. A recommended size
1.1.10 Specimen configurations and sizes other than those
range of test specimens and their side-grooving are given in
listed in Table A1.1 which are tested under constant force will
Table A1.1 in Annex A1. It has been shown that for this range
involve further validity requirements. This is done by compar-
the cracking rates do not vary for a range of materials and
ing data from recommended test configurations. Nevertheless,
loading conditions (43-47). Suggesting that the level of
use of other geometries are applicable by this method provided
constraint, for the relatively short term test durations (less than
data are compared to data obtained from standard specimens
one year), does not vary within the range of normal data scatter
(as identified in Table A1.1) and the appropriate correlating
observed in tests of these geometries. However, it is recom-
parameters have been validated.
mended that, within the limitations imposed on the laboratory,
that tests are performed on different geometries, specimen size,
1.2 The values stated in SI units are to be regarded as
dimensions and crack size starters. In all cases a comparison of
standard. The values given in parentheses after SI units are
the data from the above should be made by testing the standard
provided for information only and are not considered standard.
C(T) specimen where possible. It is clear that increased
1.3 This standard does not purport to address all of the
confidence in the materials crack growth data can be produced
safety concerns, if any, associated with its use. It is the
by testing a wider range of specimen types and conditions as
responsibility of the user of this standard to establish appro-
described above.
priate safety, health, and environmental practices and deter-
1.1.8 Material inhomogeneity, residual stresses and material
mine the applicability of regulatory limitations prior to use.
degradation at temperature, specimen geometry and low-force
1.4 This international standard was developed in accor-
long duration tests (mainly greater that one year) can influence
dance with internationally recognized principles on standard-
the rate of crack initiation and growth properties (42-50). In
ization established in the Decision on Principles for the
cases where residual stresses exist, the effect can be significant
Development of International Standards, Guides and Recom-
when test specimens are taken from material that characteris-
mendations issued by the World Trade Organization Technical
tically embodies residual stress fields or the damaged material,
Barriers to Trade (TBT) Committee.
or both. For example, weldments, or thick cast, forged,
extruded, components, plastically bent components and com-
2. Scope of Material Properties Data Resulting from This
plex component shapes, or a combination thereof, where full
Standard
stress relief is impractical. Specimens taken from such com-
2.1 This test method covers the determination of initial
ponent that contain residual stresses may likewise contain
creep crack extension (CCI) times and growth (CCG) in metals
residual stresses which may have altered in their extent and
at elevated temperature using pre-cracked specimens subjected
distribution due to specimen fabrication. Extraction of speci-
to static or quasi-static loading conditions. The metallic mate-
mens in itself partially relieves and redistributes the residual
rials investigated range from creep-ductile to creep-brittle
stress pattern; however, the remaining magnitude could still
conditions.
cause significant effects in the ensuing test unless post-weld
heat treatment (PWHT) is performed. Otherwise residual 2.2 The crack growth rate a˙ or da/dt is expressed in terms of
stresses are superimposed on applied stress and results in the magnitude of CCG rate relating parameters, C*(t), C or K.
t
crack-tip stress intensity that is different from that based solely The resulting output derived as a˙vC* (as the steady state
´1
E1457 − 23
formulation of C*(t)), or C for creep-ductile materials or as
Γ = path of the integral, that encloses (that is, contains)
t
a˙vK (for creep-brittle materials) is deemed as material property
the crack tip contour,
for CCG.
ds = increment in the contour path,
T = outward traction vector on ds,
2.3 In addition for CCI derivation of crack extension time
u˙ = displacement rate vector at ds,
t v C* (for creep-ductile materials) or t vK (for creep-brittle
0.2 0.2
x, y, z = rectangular coordinate system, and
materials) can also be used as a material property for the
]u˙
= rate of stress-power input into the area enclosed by
T· ds
purpose of modeling and remaining life assessment.
]x
Γ across the elemental length ds.
2.4 The output from these results can be used as ‘Bench-
4.2.1.2 Discussion—The value of C*(t) from this equation is
mark’ material properties data which can subsequently be used
path-independent for materials that deform according to con-
in crack growth numerical modeling, in component design and
stitutive law that may be separated into single-value time and
remaining life assessment methods.
stress functions or strain and stress functions of the forms:
3. Referenced Documents
ε˙ 5 f ~t!f ~σ! (2)
3 1 2
3.1 ASTM Standards:
or,
E4 Practices for Force Calibration and Verification of Test-
ε˙ 5 f ε f σ (3)
~ ! ~ !
3 4
ing Machines
E74 Practices for Calibration and Verification for Force-
where f -f represent functions of elapsed time, t, strain, ε
1 4
Measuring Instruments
and applied stress, σ, respectively and ε is the strain rate.
˙
E83 Practice for Verification and Classification of Exten-
4.2.1.3 Discussion—For materials exhibiting creep defor-
someter Systems
mation for which the above equation is path-independent, the
E139 Test Methods for Conducting Creep, Creep-Rupture,
C*(t)-integral is equal to the value obtained from two, stressed,
and Stress-Rupture Tests of Metallic Materials
identical bodies with infinitesimally differing crack areas. This
E220 Test Method for Calibration of Thermocouples By
value is the difference in the stress-power per unit difference in
Comparison Techniques
crack area at a fixed value of time and displacement rate, or at
E399 Test Method for Linear-Elastic Plane-Strain Fracture
a fixed value of time and applied force.
Toughness of Metallic Materials
4.2.1.4 Discussion—The value of C*(t) corresponding to the
E647 Test Method for Measurement of Fatigue Crack
steady-state conditions is called C*. Steady-state is said to have
Growth Rates
been achieved when a fully developed creep stress distribution
E1823 Terminology Relating to Fatigue and Fracture Testing
has been produced around the crack tip. This occurs when the
secondary creep deformation characterized by the following
4. Terminology
equation dominates the behavior of the specimen.
4.1 Terminology related to fracture testing contained in
n
ε˙ 5 Aσ (4)
Terminology E1823 is applicable to this test method. Addi-
ss
4.2.1.5 Discussion—This steady state in C* does not neces-
tional terminology specific to this standard is detailed in 4.2
and 4.3. For clarity and easier access within this document sarily mean steady state crack growth rate. The latter occurs
when steady state damage develops at the crack tip. In this test
some of the terminology in E1823 relevant to this standard is
repeated below (see Terminology E1823, for further discussion method, this behavior is observed as ‘tails’ at the early stages
of crack growth. This standard deals with this region as the
and details).
initial crack extension period defined as time, t , measured for
0.2
4.2 Definitions:
-1 -1 an initial crack growth of 0.2 mm after first loading (see 11.8.8
4.2.1 C*(t)-integral, C*(t) [FL T ]—a mathematical
for further details).
expression, a line or surface integral that encloses the crack
-1 -1
front from one crack surface to the other, used to characterize
4.2.2 C parameter, C [FL T ]—a parameter equal to the
t t
the local stress-strain rate fields at any instant around the crack value obtained from two identical bodies with infinitesimally
front in a body subjected to extensive creep conditions
differing crack areas, each subjected to stress, as the difference
4.2.1.1 Discussion—The C*(t) expression for a two- in stress-power per unit difference in crack area at a fixed value
dimensional crack, in the x-z plane with the crack front parallel of time and displacement rate, or at a fixed value of time
to the z-axis, is the line integral: applied force for an arbitrary constitutive law.
4.2.2.1 Discussion—The value of C is path-independent
t
] u˙
*
and is identical to C*(t) for extensive creep conditions when
C*~t! 5 W*~t!dy 2 T· ds (1)
S D
] x
Γ
the constitutive law described in 4.2.1 applies.
where: 4.2.2.2 Discussion—Under small-scale creep conditions,
C*(t) is not path-independent and is related to the crack tip
W*(t) = instantaneous stress-power or energy rate per unit
stress and strain fields only for paths local to the crack tip and
volume,
well within the creep zone boundary. Under these
circumstances, C is related uniquely to the rate of expansion of
t
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
the creep zone size (13-15). There is considerable experimental
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
evidence that the C parameter (5, 11, 13) which extends the
Standards volume information, refer to the standard’s Document Summary page on t
the ASTM website. C*(t)-integral concept into small-scale creep and the transition
´1
E1457 − 23
creep regime, correlates uniquely with creep crack growth rate Table A1.1 for creep ductile material where creep strains
in the entire regime ranging from small-scale to extensive dominate and in which test times are longer (usually >1000
creep regime. hours), the elastic and plastic displacement rate components
4.2.2.3 Discussion—For a specimen with a crack subject to are small compared to the creep and therefore it is recom-
˙
mended to use the total displacement rate, V assuming that,
constant force, P:
˙ ˙
V ≈ V to derive the steady state C*. See Section 11 for detailed
c
˙
PV
c
'
discussion.
C 5 f ⁄ f
~ !
t
BW
4.2.7.3 Discussion—The force-line displacement associated
and
with just the creep strains is expressed as V .
c
df
' -1
f 5
4.2.8 J-integral, J [FL ]—a mathematical expression, a line
d~a ⁄ W!
or surface integral that encloses the crack front from one crack
4.2.3 crack-plane orientation—an identification of the plane
surface to the other, used to characterize the local stress-strain
and direction of fracture or crack extension in relation to
field around the crack front.
product configuration. This identification is designated by a
4.2.9 net thickness, B [L]—distance between the roots of
hyphenated code with the first letter(s) representing the direc- N
the side grooves in side-grooved specimens.
tion normal to the crack plane and the second letter(s)
designating the expected direction of crack propagation.
4.2.10 original crack size, a [L] —the physical crack size
o
at the start of testing.
4.2.4 crack size, a [L]—principal lineal dimension used in
the calculation of fracture mechanics parameters for throught-
4.2.11 specimen thickness, B [L]—the distance between the
hickness cracks as defined in the applicable standard.
parallel sides of a test specimen.
4.2.4.1 Discussion—In practice, the value of a is obtained
4.2.12 specimen width, W [L]—the distance from a refer-
from procedures for measurement of physical crack size, a ,
p
ence position (for example, the front edge of a bend specimen
original crack size, a , and effective crack size, a , as appro-
o e
or the force line of a compact specimen) to the rear surface of
priate to the situation being considered.
the specimen.
4.2.4.2 Discussion—In this test method, the physical crack
-3/2
4.2.13 stress intensity factor, K [FL ]—the magnitude of
size is represented as a . The subscript, p, is everywhere
p
the mathematically ideal crack-tip stress field (a stress-field
implied.
singularity) for Mode 1 in a homogeneous, linear-elastic body.
4.2.5 creep crack growth (CCG) rate, da/dt, Δa/Δt [L/t]—
4.2.14 transition time, t [T]—time required for extensive
the rate of crack extension caused by creep damage and
T
expressed in terms of average crack extension per unit time. creep conditions to develop in a cracked body under sustained
[E1823] loading. For specimens, this is typically the time required for
the creep deformation zone to spread through a substantial
4.2.6 creep zone boundary—the locus of points ahead of the
portion of the uncracked ligament, or in the region that is under
crack front where the equivalent strain caused by the creep
the influence of a crack in the case of a finite crack in a
deformation equals 0.002 (0.2 %) (16).
semi-infinite medium. This limit is employed to validate the
4.2.6.1 Discussion—Under small-scale creep conditions, the
steady state correlating parameter C*. An estimate of transition
creep zone expansion with time occurs in a self-similar manner
time for materials that creep according to the power-law can be
for planar bodies, (10) thus, the creep zone size, r , can be
obtained from the following equation:
c
defined as the distance to the creep zone boundary from the
2 2
K ~1 2 v !
crack tip at a fixed angle, θ, with respect to the crack plane. The t 5
T
E n 1 1 C*
~ !
rate of expansion of the creep zone size is designated as
where:
~0!r˙ (θ).
c
v = Poisson’s ratio, and
4.2.7 force-line displacement due to creep, elastic and
n = secondary creep exponent.
plastic strain, V [L]—the total displacement measured at the
FLD
-2
loading pins (V ) due to the force placed on the specimen at
4.2.15 yield strength, σ [FL ]—the stress at which a
YS
any instant and the subsequent crack extension that is associ-
material exhibits a specific limiting deviation from the propor-
ated with the accumulation of creep, elastic and plastic strains
tionality of stress to strain at the test temperature. This
in the specimen.
deviation is expressed in terms of strain.
4.2.7.1 Discussion—In creeping bodies, the total displace-
FLD
4.2.15.1 Discussion— In this test method, yield strength is
ment at the force-line V can be partitioned into an instan-
determined by the Offset Method (at a strain of 0.2 %).
taneous elastic part V , a plastic part, V , and a time-dependent
e p
4.3 Definitions of Terms Specific to This Standard:
creep part V where:
c
4.3.1 C*-integral, C* [FL-1T-1]—The parameter relevant
V ; V 1V 1V (5)
e p c
to correlating creep crack growth in this document is given as
The corresponding symbols for the rates of force-line dis-
the C*-integral which is defined as the steady state definition of
placement components shown in Eq 5 are given respectively
˙ ˙ ˙ ˙
C*(t). C* is used to characterize the local steady state,
as V , V , V , V . This information is used to derive the pa-
e p c
rameter C* and C . See Section 11.
stress-strain rate fields at any instant around the crack front in
t
4.2.7.2 Discussion—For the set of specimens in Annex A1, a body subject to extensive creep conditions.
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4.3.1.1 Discussion—See A2.4 for further discussion and 5.5 Data scatter that is usually present in creep crack growth
equations to calculate C*. experiments (43, 45, 51, 52). This will indicate that more than
one test should be performed to gain confidence in the results.
4.3.2 initial crack extension increment (CCI) after full
The number of specimens to be tested is dependent on a
force-up, δa [L]—the recommended time taken to crack ex-
i
number of factors (52) such as the number of test variables
tension of δa = 0.2 mm after first application of force for
i
(specimen type, size, dimension, crack size, force, CCI and
defining a crack growth period t in hours as a function of
0.2
CCG range and material batches) being considered. In general
C*(t), C , or K value taken at crack length a + δa 0.2 mm.
t o i
it is recommended for the range of conditions that a minimum
4.3.3 initial crack time to 0.2 mm, t [T]—the time to
0.2 of five tests at different forces should be performed to produce
δa = 0.2 mm (0.008 in.) of crack extension δ a by creep after
i overlapping crack growth data over the region of CCG rate of
full loading. This size is chosen as the limit of accuracy set for
interest. Additional repeat tests would be preferable, but not
crack extension measurements in laboratory geometries.
compulsory, to improve confidence in the derived data range.
5.6 If the material exhibits such factors as irregular grain
5. Summary of Test Method
sizes and voids, weld (X-weld, HAZ) and other inhomogeneity
5.1 The main objective of creep crack growth testing is the
the minimum number of tests should be increased (see 5.5).
determination of the relationship between the time and rate of
Also, more tests should be performed if the material creep
crack growth, da/dt, due to creep and the applied value of the
crack growth behavior exhibits increased scatter regardless of
appropriate crack growth rate relating parameter. In addition
the reason for the variability. If there is insufficient material
results for time to crack extension of 0.2 mm at force-up (CCI)
available or if there are other reasons which would restrict
as defined in 1.1.5 are also correlated from the experimental
multiple testing then the results should be considered with
data. This test method involves loading of sharply notched by
increased caution.
means of EDM or fatigue pre-cracked specimens (see 8.8),
5.7 In some cases crack growth information is needed for
using the recommended geometries, heated to the test tempera-
the initial start of the test where steady state cracking has not
ture by means of a suitable furnace. The applied force is either
been reached. Also this period coincides with the limit of
held constant with time or is changed slowly enough to be
accuracy in crack growth measurement (recommended as 0.2
considered quasi-static. The temperature must be constantly
mm see 1.1.2 (35). The data produced for (CCI) will therefore
monitored to ensure that it remains at the specified level within
be one point per test (similar to uniaxial rupture tests). Hence
allowable limits during the test. If servo-mechanical loading
more tests would be needed to accommodate the variability in
systems are used to maintain constant force, or if tests are
the results. The minimum number of tests recommended will
conducted under conditions other than constant force, a record
depend on the level of scatter, but should not be less than 5
of force versus time also must be maintained.
tests which should also uniformly cover test times of interest.
5.2 Three different loading methods are available for creep
5.8 Specimen Selection—For all cases attention must be
crack growth testing. Dead weight loading is the recommended
given to the proper selection of specimen. The C(T) is always
method and is the most commonly used method for loading
the primary choice as there is ample reference in the literature
specimens. In addition, constant displacement (29) and con-
to the testing and analysis of this geometry.
stant displacement rate (1-4, 39) loading may also be used but
are only recommended when working with extremely brittle
5.9 The choice of specimen should reflect a number of
materials. For tests conducted under conditions other than factors. These priorities can be listed as follows:
dead-weight loading, the user must compare the results and
5.9.1 Availability and the size of material prepared for
verify the analysis from tests performed under dead-weight
testing indicates the number of specimens that can be tested.
loading conditions.
5.9.2 Material creep ductility and stress sensitivity; for
creep brittle specimens the C(T) is recommended.
5.3 It is recommended to carry out long term tests (at least
5.9.3 Capacity of the test rig; the 3-point bend specimens
>1000 h and usually, if possible, between 5000 to 10 000 h) in
order to reduce crack tip plasticity which would occur at higher and the C(T) specimens will typically take lower forces.
forces and allow for steady state creep cracking to take place.
5.9.4 Type of loading (tension, bending, tension/bending)
Large forces should be avoided since this will induce either fast
should be taken into consideration.
fracture or extensive deformation due to creep or plastic
5.9.5 Compatibility with size and stress state of the speci-
collapse and/or rupture, thus rendering the crack growth test as
men with the component under investigation.
void. Data from fast test are usually not appropriate for life
5.9.6 Following a test if the crack front is substantially
assessment purposes as they may not reflect the stress state of
leading in the centre the indications are that constraint should
the component at the crack-tip.
be increased. If the crack front is substantially receding at the
centre the opposite applies-This can be remedied by changing
5.4 The crack size and force-line displacements are continu-
the size, thickness, side-grooving or the geometry of the
ously recorded, digitally or autographically on strip-chart
specimen used for testing. See 8.3.
recorders, as a function of time. The force, force-line displace-
ment and crack size data are numerically processed as dis- 5.9.7 The length of time and temperature of testing; this will
cussed later to obtain the crack growth rate versus C*(t), C or dictate the size, the applied force, initial crack size and
t
K relationship. side-grooving of the specimen.
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5.9.8 Discussion—It is unlikely that all conditions for ma- data obtained should, if possible, be compared against test data
terial selection can be satisfied at any one time. The main derived from the standard C(T) tests in order to validate the
priority is to produce a test environment for stable crack data.
growth to occur under steady state conditions. Therefore 6.2.4 Creep cracks have been observed to grow at different
compromises may need to be made. This document goes part of rates at the beginning of tests compared with the rates at
the way to assist the user in this choice by identifying specific equivalent C*(t), C or K values for cracks that have sustained
t
detail of a number of geometries. The appropriate decision previous creep crack extension (12, 13). This region is identi-
may, however, need expert advice in the relevant field or fied as ‘tail’. The duration of this transient condition, ‘tail’,
industry. varies with material and initially applied force level. These
transients are due to rapid changes in the crack tip stress fields
6. Significance and Use after initial elastic loading and/or due to an initial period during
which a creep damage zone evolves at the crack tip and
6.1 Creep crack growth rate expressed as a function of the
propagates in a self-similar fashion with further crack exten-
steady state C* or K characterizes the resistance of a material
sion (12, 13). This region is separated from the steady-state
to crack growth under conditions of extensive creep deforma-
crack extension which follows this period and is characterized
tion or under brittle creep conditions. Background information
by a unique da/dt versus C*(t), C or K relationship. This
t
on the rationale for employing the fracture mechanics approach
transient region, especially in creep-brittle materials, can be
in the analyses of creep crack growth data is given in (11, 13,
present for a substantial fraction of the overall life (35).
30-35).
Criteria are provided in this standard to quantify this region as
an initial crack growth period (see 1.1.5) and to use it in
6.2 Aggressive environments at high temperatures can sig-
parallel with the steady state crack growth rate data. See 11.8.8
nificantly affect the creep crack growth behavior. Attention
for further details.
must be given to the proper selection and control of tempera-
ture and environment in research studies and in generation of
6.3 Results from this test method can be used as follows:
design data.
6.3.1 Establish predictive models for crack incubation peri-
6.2.1 Expressing CCI time, t and CCG rate, da/dt as a
ods and growth using analytical and numerical techniques
0.2
function of an appropriate fracture mechanics related param-
(18-21).
eter generally provides results that are independent of speci-
6.3.2 Establish the influence of creep crack development
men size and planar geometry for the same stress state at the
and growth on remaining component life under conditions of
crack tip for the range of geometries and sizes presented in this
sustained loading at elevated temperatures wherein creeps
document (see Annex A1). Thus, the appropriate correlation
deformation might occur (23-28).
will enable exchange and comparison of data obtained from a
NOTE 1—For such cases, the experimental data must be generated under
variety of specimen configurations and loading conditions.
representative loading and stress-state conditions and combined with
Moreover, this feature enables creep crack growth data to be
appropriate fracture or plastic collapse criterion, defect characterization
utilized in the design and evaluation of engineering structures
data, and stress analysis information.
operated at elevated temperatures where creep deformation is a
6.3.3 Establish material selection criteria and inspection
concern. The concept of similitude is assumed, implying that
requirements for damage tolerant applications.
cracks of differing sizes subjected to the same nominal C*(t),
6.3.4 Establish, in quantitative terms, the individual and
C , or K will advance by equal increments of crack extension
t
combined effects of metallurgical, fabrication, operating
per unit time, provided the conditions for the validity for the
temperature, and loading variables on creep crack growth life.
specific crack growth rate relating parameter are met. See 11.7
6.4 The results obtained from this test method are designed
for details.
for crack dominant regimes of creep failure and should not be
6.2.2 The effects of crack tip constraint arising from varia-
applied to cracks in structures with wide-spread creep damage
tions in specimen size, geometry and material ductility can
which effectively reduces the crack extension to a collective
influence t and da/dt. For example, crack growth rates at the
0.2
damage region. Localized damage in a small zone around the
same value of C*(t), C in creep-ductile materials generally
t
crack tip is permissible, but not in a zone that is comparable in
increases with increasing thickness. It is therefore necessary to
size to the crack size or the remaining ligament size. Creep
keep the component dimensions in mind when selecting
damage for the purposes here is defined by the presence of
specimen thickness, geometry and size for laboratory testing.
grain boundary cavitation. Creep crack growth is defined
6.2.3 Different geometries as mentioned in 1.1.6 may have
primarily by the growth of intergranular time-dependent
different size requirements for obtaining geometry and size
cracks. Crack tip branching and deviation of the crack growth
independent creep crack growth rate data. It is therefore
directions can occur if the wrong choice of specimen size,
necessary to account for these factors when comparing da/dt
side-grooving and geometry is made (see 8.3). The criteria for
data for different geometries or when predicting component life
geometry selection are discussed in 5.8.
using laboratory data. For these reasons, the scope of this
standard is restricted to the use of specimens shown in Annex
7. Apparatus
A1 and the validation criteria for these specimens are specified
in 11.7. However if specimens other than the C(T) geometry 7.1 Testing Machine—This standard does not recommend a
are used for generating creep crack growth data, then the da/dt specific type of testing equipment. It does however specify
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accuracy limits for the test equipment and suggestions for the 7.4 Heating Apparatus:
types of equipment that could be used to achieve the accuracy 7.4.1 The apparatus for, and method of, heating the speci-
limits specified. mens should provide the temperature control necessary to
7.1.1 Dead-weight or servo-mechanical loading machines satisfy the requirements in 10.3, without manual adjustments
capable of maintaining a constant force or maintaining constant more frequent than once in each 24-h period after force
-5
displacement rates in the range of 10 to 1 mm/h can be used application.
for creep crack growth testing. If servo-hydraulic machines are 7.4.2 Heating shall be by an electric resistance or radiation
used under constant force conditions, the force must be furnace with the specimen in air at atmospheric pressure unless
monitored continuously and the variations in the indicated other media are specifically agreed upon in advance.
force must not exceed 61.0 % of the nominal value at any time
NOTE 2—The test conditions in which tests are performed may have a
during the test. If either constant displacement rate or constant
considerable effect on the results. This is particularly true when properties
displacement is used, the indicated displacement must be
are influenced by plasticity, environmental effects, oxidation or other types
of corrosion.
within 1 % of the nominal value at any given time during the
test.
7.5 Temperature-Measurement Apparatus—The method of
7.1.2 The accuracy of the testing machine shall be within
temperature measurement must be sufficiently sensitive and
the permissible variation specified in Practice E4.
reliable to ensure that the specimen temperature is within the
7.1.3 If lever-type, dead-weight creep machines are used, it
limits specified in 10.3. For details of types of apparatus used
is preferable that they automatically maintain the lever arm in
see Specification E139.
a horizontal position. If such a device is not available, the lever
7.6 Displacement Gage—For the measurement of the FLD
arm should be manually adjusted at such intervals so that the
or CMOD displacement during the test.
arm position at any time does not deviate from the horizontal
7.6.1 Continuous displacement measurement is needed to
by an amount leading to 1 %, variation of force on the
evaluate the magnitude of C*(t) and C at any time during the
t
specimen.
test. Displacement measurements must be made on the force-
7.1.4 Precautions should be taken to ensure that the force on
line.
the specimen is applied as nearly axial as possible.
7.6.2 As a guide, the displacement gage should have a
7.2 Grips and Fixtures for specimens listed in Annex A1: It
working range no more than twice the displacement expected
is allowed to deviate from the recommended testing apparatus
during the test. Accuracy of the gage should be within 61 % of
as long as the relevant accuracies and loading conditions are
the full working range of the gage. In calibration, the maximum
adhered to.
deviation of the individual data points from the fit to the data
7.2.1 Clevis assemblies shall be incorporated in the force
shall not exceed 61 % of the working range.
train at both the top and bottom of the specimen to allow
7.6.3 Knife edges are recommended for friction-free seating
in-plane rotation as the specimen is loaded. Fig. A1.2 shows an
of the gage. Parallel alignment of the knife edges must be
example for the clevis setup for the tension specimens shown
maintained to within 61°.
in Fig. A1.3. The bend specimen will be simply a 3-point bend
7.6.4 The displacement along the force-line may be directly
loading assembly.
measured by attaching the entire clip gage assembly to the
7.2.2 Suggested proportions and critical tolerances of the
specimen and placing the whole assembly in the furnace.
fixtures shall be within the specified variation shown in Fig. Alternatively, the displacements can be transferred outside the
A1.2. Note that surface finish does not have a major effect on
furnace with a rod and tube assembly such as that shown in
creep crack growth and therefore a normal smooth finish to the
Figs. A1.4 and A1.5.
specimen is sufficient.
7.6.5 In the latter procedure, the transducer is placed outside
7.2.3 The pin-to-hole clearances are designed to minimize the furnace. It is important to make the tube and rod from
friction thereby eliminating unacceptable end-movements that
materials that are thermally stable and are from the same
would invalidate the specimen calibrations for determining K, material to avoid erroneous readings caused by differences in
J, and C*(t).
thermal expansion coefficients. Other designs that can measure
7.2.4 The material for the grips and pull rods should be displacements to the same levels of accuracy may also be used.
chosen with due regard to test temperature and force level to be
7.7 Apparatus for Crack Size Measurement—A crack size
employed. Some elevated temperature materials currently be-
monitoring technique capable of reliably resolving crack ex-
ing used include American Iron and Steel Institute (AISI)
tensions of at least 60.1 mm at test temperature is recom-
Grade 304 and 316 stainless steel, Grade A286 steel, nickel-
mended for creep crack growth measurements. Since crack
based superalloys like alloy 718 or alloy X750. The loading
extension across the thickness of the specimen is not always
pins are machined from A286 steel (or equivalent or better
uniform, surface crack size measurements by optical means are
temperature resistant steel) and are heat treated such that they
not considered reliable as a primary method. Optical observa-
develop a high resistance to creep deformation and rupture.
tion may be used as an auxiliary measurement method. The
7.3 Alignment of Grips—It is important that attention be selected crack size measurement technique must be capable of
given to achieving good alignment in the force-line through measuring the average crack size across the thickness. The
careful machining of all gripping fixtures. The length of the most commonly used technique for crack size measurement
force train should be chosen with proper attention to the height during creep crack growth testing is the electric potential
of the furnace for heating the test specimen. technique that is described in Annex A4.
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NOTE 3—The crack size measurement precision is herein defined as the
size for growing the crack in a stable fashion to permit
standard deviation of the mean value of crack size determined for a set of
collection of crack growth data (see also 1.1.7, 1.1.10, 5.9).
replicate measurements.
8.5 Specimen Measurements—The specimen dimensions are
7.8 Room Temperature Control—The ambient temperature
given in Fig. A1.3 and Table A1.1. They shall be machined
in the room should be sufficiently constant so that the specimen
within the machining tolerances given in Fig. A1.1 and the
temperature variations do not exceed the limits stated in 10.3.5.
dimensions should be measured before and after the test.
7.9 Timing Apparatus—Suitable means for recording and
8.6 Notch Preparation—The machined notch for the test
measuring elapsed time to within 1 % of the elapsed time
specimens (see 8.2.1) may be made by electrical-discharge
should be provided.
machining (EDM), milling, broaching, or saw cutting. It is
recommended that the last 0.1 a/W of the crack be machined
8. Specimen Configuration
using electro discharge machining (EDM) of a width of 0.1
8.1 The schematic and dimension of the standard C(T)
mm. This will allow easier pre-cracking or further crack tip
specimen and the additional specimens are shown in Fig. A1.3.
sharpening by EDM to the final crack starter size prior testing.
See Note in Fig. A1.3.
8.2 The configurations and size range of all the geometries
are given in Table A1.1.
8.7 Associated pre-cracking requirements are discussed in
8.2.1 Crack opening slot is the machined crack width. For
8.8.
C(T) specimens it can be as much as 0.1 a/W. For the rest of
8.8 Pre-Cracking—EDM or Fatigue pre-cracking are two
the geometries, which have shorter crack starters it is recom-
methods used to int
...