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 (i.e. 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).
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).
In any one test, two regions of crack growth behavior may be
present (12, 13). 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 as
to how the transient region should be treated in terms of an
initial crack growth period. During steady state, a unique
correlation exists between da/dt and the appropriate crack
growth rate relating parameter.
In creep ductile materials, extensive creep occurs when the
entire un-cracked ligament undergoes creep deformation. Such
conditions are distinct from the conditions of small-scale creep
and transition creep (1-10). In the case of
extensive creep, the region dominated by creep deformation is
significant in size in comparison to both the crack length and the
uncracked ligament sizes. In small-scale-creep only a small region
of the un-cracked ligament local to the crack tip experiences creep
deformation.
The creep crack growth rate in the extensive creep region is
correlated by the C*(t)-integral. The
Ct
parameter correlates the creep crack growth rate in the small-scale
creep and the transition creep regions and reduces, by definition,
to C*(t) in the extensive creep region
(5). Hence in this document the definition C* is
used as the relevant parameter in the steady state extensive creep
regime whereas C*(t) and/or
Ct are
the parameters which describe the instantaneous stress state from
the small scale creep, transient and the steady state regimes in
creep. The recommended functions to derive C* for the
different geometries shown in Annex A1 is described in Annex
A2.
An engineering definition of an initial crack extension size δai
is used in order to quantify the initial period of crack
development. This distance is given as 0.2 mm. It has been shown
(41-44) that this initial period which exists at
the start of 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. Recommendation is made to correlate this initial
crack growth period defined as
t0.2 at δai =
0.2 mm with the steady state C* when the crack tip is
under extensive creep and with K for creep brittle
conditions. The values for C* and K should
be calculated at the final specified crack size defined as
ao + δai where
ao is initial size of the starter crack.
The recommended specimens for CCI and CCG testing is the
standard compact tension specimen C(T) (see Fig. 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 details). Additional
geometries which are valid for testing in this procedure are shown
in Fig. A1.3. These are the C-ring in tension CS(T), middle crack
specimen in tension M(T), single edge notched tension SEN(T),
single edge notched bend SEN(B), and double edge notched tension
DEN(T). In Fig. A1.3, the specimens’ side-grooving-position for
measuring displacement at the force-line displacement (FLD) and
crack mouth opening displacement (CMOD) and also positions for the
potential drop (PD) input and output leads are shown. Recommended
loading for the tension specimens is pinloading. The
configurations, size range are given in Table A1.1 of Annex A1,
(43-47). Specimen selection will be discussed in
5.9.
The state-of-stress at the crack tip may have an influence on
the creep crack growth behavior and can cause crack-front tunneling
in plane-sided specimens. Specimen size, geometry, crack length,
test duration and creep properties will affect the state-of-stress
at the crack tip and are important factors in determining crack
growth rate. A recommended size range of test specimens and their
side-grooving are given in Table A1.1 in Annex A1. It has been
shown that for this range the cracking rates do not vary for a
range of materials and loading conditions (43-47).
Suggesting that the level of constraint, for the relatively short
term test durations (less than one year), does not vary within the
range of normal data scatter observed in tests of these geometries.
However it is recommended that, within the limitations imposed on
the laboratory, that tests are performed on different geometries,
specimen size, dimensions and crack size starters. In all cases a
comparison of the data from the above should be made by testing the
standard C(T) specimen where possible. It is clear that
increased confidence in the materials crack growth data can be
produced by testing a wider range of specimen types and conditions
as described above.
Material inhomogeneity, residual stresses and material
degradation at temperature, specimen geometry and low-force long
duration tests (mainly greater that one year) can influence the
rate of crack initiation and growth properties
(42-50). In cases where residual stresses exist,
the effect can be significant when test specimens are taken from
material that characteristically embodies residual stress fields or
the damaged material, or both. For example weldments, or thick
cast, forged, extruded, components, plastically bent components and
complex component shapes, or a combination thereof, where full
stress relief is impractical. Specimens taken from such component
that contain residual stresses may likewise contain residual
stresses which may have altered in their extent and distribution
due to specimen fabrication. Extraction of specimens in itself
partially relieves and redistributes the residual stress pattern;
however, the remaining magnitude could still cause significant
effects in the ensuing test unless post-weld heat treatment (PWHT)
is performed. Otherwise residual stresses are superimposed on
applied stress and results in crack-tip stress intensity that is
different from that based solely on externally applied forces or
displacements. Not taking the tensile residual stress effect into
account will produce C* values lower than expected
effectively producing a faster cracking rate with respect to a
constant C*. This would produce conservative estimates for
life assessment and nonconservative calculations for design
purposes. It should also be noted that distortion during specimen
machining can also indicate the presence of residual
stresses.
Stress relaxation of the residual stresses due to creep and
crack extension should also be taken into consideration. No
specific allowance is included in this standard for dealing with
these variations. However the method of calculating C*
presented in this document which used the specimen’s creep
displacement rate to estimate C* inherently takes into
account the effects described above as reflected by the
instantaneous creep strains that have been measured. However extra
caution should still be observed with the analysis of these types
of tests as the correlating parameters K and C*
shown in Annex A2 even though it is expected that stress relaxation
at high temperatures could in part negate the effects due to
residual stresses. Annex A4 presents the correct calculations
needed to derive J and C* for weldment tests
where a mis-match factor needs to be taken into account.
Specimen configurations and sizes other than those listed in
Table A1.1 which are tested under constant force will involve
further validity requirements. This is done by comparing data from
recommended test configurations. Nevertheless, use of other
geometries are applicable by this method provided data are compared
to data obtained from standard specimens (as identified in Table
A1.1) and the appropriate correlating parameters have been
validated.
The values stated in SI units are to be regarded as the
standard. The inch-pound units given in parentheses are for
information only.
This standard does not purport to address all of the safety
concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and
health practices and determine the applicability of regulatory
limitations prior to use.
2 The boldface
numbers in parentheses refer to the list of references at the end
of this standard.