ASTM E1457-07e3
(Test Method)Standard Test Method for Measurement of Creep Crack Growth Times in Metals
Standard Test Method for Measurement of Creep Crack Growth Times in Metals
SIGNIFICANCE AND USE
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 (8, 10, 27-32).
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.
Expressing CCI time, t0.2 and CCG rate, da/dt as a function of an appropriate fracture mechanics related parameter, as discussed in 11.8 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.
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 selecting...
SCOPE
1.1 This test method covers the determination of creep crack growth (CCG) in metals at elevated temperatures using pre-cracked specimens subjected to static or quasi-static loading conditions. The time (CCI), t0.2 to an initial crack extension δai = 0.2 mm from the onset of first applied force and creep crack growth rate, ˙a or da/dt is expressed in terms of the magnitude of creep crack growth relating 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 (1-14). The crack growth derived in this manner is identified as a material property which can be used in modeling and life assessment methods (15-25).
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 (1-14) and creep-brittle (26-37). 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 (9, 10). The initial transient region where elastic strains dominate and creep damage develops and in the steady state region where crack grows proportionally to time. Steady-state creep crack growth rate behavior is covered by this standard. In addition specific recommendations are made in 11.7 a...
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´3
Designation: E1457 – 07
Standard Test Method for
1
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.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1
´ NOTE—Equation 6 was editorially corrected in August 2008.
2
´ NOTE—Equation A2.3 was editorially corrected in October 2009.
3
´ NOTE—4.2.1 and Eq 8 were editorially revised in May 2011.
1. Scope 1.1.2 In any one test, two regions of crack growth behavior
may be present (9, 10). The initial transient region where
1.1 Thistestmethodcoversthedeterminationofcreepcrack
elastic strains dominate and creep damage develops and in the
growth (CCG) in metals at elevated temperatures using pre-
steady state region where crack grows proportionally to time.
cracked specimens subjected to static or quasi-static loading
Steady-state creep crack growth rate behavior is covered by
conditions. The time (CCI), t to an initial crack extension
0.2
this standard. In addition specific recommendations are made
da =0.2 mm from the onset of first applied force and creep
i
in11.7astohowthetransientregionshouldbetreatedinterms
crack growth rate, a˙ or da/dt is expressed in terms of the
of an initial crack growth period. During steady state, a unique
magnitude of creep crack growth relating parameters, C*or K.
correlation exists between da/dt and the appropriate crack
With C* defined as the steady state determination of the crack
2
growth rate relating parameter.
tip stresses derived in principal from C*(t) and C (1-14). The
t
1.1.3 In creep ductile materials, extensive creep occurs
crack growth derived in this manner is identified as a material
when the entire uncracked ligament undergoes creep deforma-
property which can be used in modeling and life assessment
tion. Such conditions are distinct from the conditions of
methods (15-25).
small-scale creep and transition creep (1-7). In the case of
1.1.1 The choice of the crack growth correlating parameter
extensive creep, the region dominated by creep deformation is
C*, C*(t), C,or K depends on the material creep properties,
t
significant in size in comparison to both the crack length and
geometry and size of the specimen. Two types of material
theuncrackedligamentsizes.Insmall-scale-creeponlyasmall
behavior are generally observed during creep crack growth
region of the uncracked ligament local to the crack tip
tests; creep-ductile (1-14) and creep-brittle (26-37). In creep
experiences creep deformation.
ductilematerials,wherecreepstrainsdominateandcreepcrack
1.1.4 The creep crack growth rate in the extensive creep
growth is accompanied by substantial time-dependent creep
region is correlated by the C*(t)-integral. The C parameter
strains at the crack tip, the crack growth rate is correlated by t
correlates the creep crack growth rate in the small-scale creep
the steady state definitions of C or C*(t), defined as C* (see
t
and the transition creep regions and reduces, by definition, to
1.1.4). In creep-brittle materials, creep crack growth occurs at
C*(t)intheextensivecreepregion (5).Henceinthisdocument
low creep ductility. Consequently, the time-dependent creep
thedefinitionC*isusedastherelevantparameterinthesteady
strains are comparable to or dominated by accompanying
state extensive creep regime whereas C*(t) and/or C are the
t
elastic strains local to the crack tip. Under such steady state
parameters which describe the instantaneous stress state from
creep-brittle conditions, C or K could be chosen as the
t
the small scale creep, transient and the steady state regimes in
correlating parameter (8-14).
creep. The recommended functions to derive C* for the
different geometries is shown in Annex A1 is described in
Annex A2.
1
This test method is under the jurisdiction ofASTM Committee E08 on Fatigue
1.1.5 An engineering definition of an initial crack extension
and Fracture and is the direct responsibility of Subcommittee E08.06 on Crack
size da is used in order to quantify the initial period of crack
Growth Behavior.
i
Current edition approved March 15, 2007. Published April 2007. Originally
development. This distance is given as 0.2 mm. It has been
approved in 1992. Last previous edition approved in 2000 as E1457–00. DOI:
shown (38-40) that this period which exists at the start of the
10.1520/E1457-07E01.
2
test could be a substantial period of the test time. During this
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. early period the crack tip undergoes damage development as
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