ISO 22848:2021

INTERNATIONAL ISO
STANDARD 22848
First edition
2021-02
Corrosion of metals and alloys —
Test method for measuring the
stress corrosion crack growth rate
of steels and alloys under static-load
conditions in high-temperature water
Corrosion des métaux et des alliages — Méthode d'essai pour le
mesurage de la vitesse de propagation des fissures de corrosion sous
contrainte des aciers et des alliages dans des conditions de charge
statique dans de l'eau à haute température
Reference number
©
ISO 2021
© ISO 2021
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ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle of test . 2
5 Specimen . 3
5.1 Specimen orientation . 3
5.2 Specimen geometry . 3
5.3 Specimen finish . 4
5.4 Specimen size requirement . 4
5.5 Specimen dimensional measurement . 5
5.6 Stress intensity factor, K .
I 5
6 Test equipment. 5
7 Crack length measurement by potential drop method . 6
8 Corrosion potential measurement . 7
8.1 General . 7
8.2 Measurement method . 7
9 Test procedure . 7
9.1 General . 7
9.2 Installation in autoclave . 8
9.3 Adjustment of test environment . 9
9.4 Loading . 9
9.4.1 General. 9
9.4.2 Fatigue pre-cracking . 9
9.4.3 SCC transitioning . 9
9.4.4 Static loading .11
10 Evaluation of test results .11
11 Test report .14
Annex A (informative) CDCB specimen geometry and stress intensity factor calculation .16
Annex B (informative) Equipment for SCC growth testing .19
Annex C (informative) Water chemistry and monitoring items in simulated BWR and PWR
environments .22
Annex D (informative) Approach to determine crack growth rate .24
Bibliography .25
Foreword
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iv © ISO 2021 – All rights reserved

INTERNATIONAL STANDARD ISO 22848:2021(E)
Corrosion of metals and alloys — Test method for
measuring the stress corrosion crack growth rate of
steels and alloys under static-load conditions in high-
temperature water
1 Scope
This document specifies a test method for determining the stress corrosion crack (SCC) growth rate of
steels and alloys under static-load conditions in high-temperature water, such as the simulated water
environment of light water reactors. The crack length of the specimen is monitored by a potential drop
method (PDM) during the test in an autoclave.
The test method is applicable to stainless steels, nickel base alloys, low alloy steels, carbon steels and
other alloys.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 7539-6, Corrosion of metals and alloys — Stress corrosion testing — Part 6: Preparation and use of
precracked specimens for tests under constant load or constant displacement
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7539-6 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
potential drop method
PDM
non-destructive method for measuring a crack length based on the change in the electric potential as a
crack propagates in the presence of an applied DC or AC current
3.2
stress corrosion crack transitioning
SCC transitioning
use of cyclic loading at low frequency and with increasing hold time at maximum load in the test
environment to promote a transition in the fracture surface morphology from a transgranular
(TG) fatigue pre-crack to SCC, typically intergranular (IG) or interdendritic (ID) morphology for
austenitic alloys
3.3
crack-tip re-activation loading
use of loading cycles to re-activate the tip of crack when crack retardation is observed under a
static loading
3.4
initial crack length
a
distance from the load line to the initial crack tip
Note 1 to entry: It can refer to the machined notch tip or the air fatigue pre-crack front in the specimen.
Note 2 to entry: For other fracture mechanics geometries, refer to ISO 7539-6. The crack length (a) is often
expressed as a proportion of the distance from the load-line to the end of the specimen (W): a/W.
3.5
final crack length
a
f
distance from the load line to the final crack front at the end of the stress corrosion crack growth test,
where the crack length is measured on the fracture surface of the specimen
3.6
flow stress at test temperature
σ
flowT
algebraic average of the yield stress (σ ) and the ultimate tensile strength (σ ) at the test temperature:
yT uT
σσ =+ σ /2
()
flowTyTuT
3.7
crack engagement
specimen thickness B where the stress corrosion crack has advanced
Note 1 to entry: It is expressed as a percentage.
3.8
average crack extension
A
average crack extension across the specimen thickness using a crack growth area or many equally
spaced measurements of crack length (equally divided method)
3.9
average crack extension in crack engagement area
A
average crack extension based on the fraction of the specimen thickness where the stress corrosion
crack has occurred
3.10
minimum crack extension
A
min
minimum extension of the stress corrosion crack in the specimen
3.11
maximum crack extension
A
max
maximum extension of the stress corrosion crack in the specimen
4 Principle of test
Stress corrosion cracking is a phenomenon in which a crack grows in an environment when stress is
applied to a susceptible material. Thus, stress corrosion cracking is affected by three general factors:
the material, stress and environment. The SCC growth rate is affected by the stress intensity factor,
K . The SCC growth rate, da/dt, is defined as the time derivative of the crack length. While there is
I
often no clear distinction between static loading and some very slowly increasing monotonically or
cyclic loading, the primary interest in most SCC growth testing is the behaviour under static loading. By
2 © ISO 2021 – All rights reserved

applying a static load to a specimen with a crack at a known K and continuously measuring the crack
I
length (a) using the PDM, the crack growth rate (da/dt) can be continuously obtained. Often the best
insight into the effects of environment and temperature are obtained by making periodic changes while
continuously measuring the SCC growth response.
This document specifies the preparation of specimens, the control of the testing environment, the
method of transitioning from fatigue crack to SCC, and the determination of the growth rate of a crack
using fracture mechanics specimens in high-temperature water environments, with an emphasis on
light water reactors.
Although the minimum requirements and basic procedures of SCC growth rate testing in high-
temperature water are summarized in this document, it should be noted that there are complex inter-
dependencies of many influential parameters on stress corrosion cracking phenomena, and subtle
variations in test conditions can have a major impact on the reproducibility and credibility of the
data. Extensive efforts to obtain high-quality SCC growth rate data have been undertaken over the
last four decades and many key issues must be understood and implemented (see References [1] to
[3]). For example, reliable SCC transitioning prior to static loading is an essential element, and specific
procedures have been developed to help achieve well-behaved response.
5 Specimen
5.1 Specimen orientation
The specimen orientation in the test material is designed in accordance with ISO 7539-6.
The relative orientation of the crack plane and growth direction in the test material shall be specified
in relation to the product form (such as plate rolling direction or pipe longitudinal direction) and, if
applicable, also specified in relation to the weld direction and additional cold work (e.g. for rolling or
forging). When the specimen is taken in or near a weld, the location of the crack plane of the specimen
in relation to the weld fusion line shall be provided because a very pronounced effect on SCC behaviour
is expected when the crack in the specimen propagates in the heat affected zone or weld metal of the
test material, and the properties can vary from the weld root to the weld crown.
5.2 Specimen geometry
Many specimen geometries have been used for crack growth testing (see ISO 7539-6). The most
common specimen is a compact tension (CT) specimen with a side-groove design, shown in Figure 1.
−3 −3
The specimen thickness, B, is usually between 12,5 × 10 m and 25,4 × 10 m. Smaller or larger
specimens are sometimes used but shall be justified from K-size criteria (see 5.4). The specimen width,
W, is typically two times the specimen thickness (B).
Side grooves on both sides of the specimen are recommended to help maintain in-plane crack growth,
but are not obligatory. The depth of each side groove is typically 5 % of B, and they are typically
hemispherical.
The more complex contoured double cantilever beam (CDCB) specimen is also used because the stress
intensity factor is practically considered constant over a certain range of crack lengths under constant-
load conditions. However, note that the criteria for a CT specimen given in 5.4 is not applicable to a
CDCB specimen. Details for a CDCB specimen are given in Annex A.
Figure 1 — Typical dimensions and tolerances of a CT specimen
5.3 Specimen finish
The surface of the specimen is mechanically finished to remove the working layers and residual
stresses, and the recommended surface roughness is given in Figure 1. Surface condition generally has
the largest effect on the corrosion potential, which in turn can affect SCC growth rates.
5.4 Specimen size requirement
Specimen size shall be chosen based on the mechanical properties of the test material and K value
I
used in testing. There is no well-established K-size criterion for SCC growth testing, but the following
formula is commonly used and considered conservative (except for low work-hardening materials, such
as highly cold worked or irradiated materials). When load is applied to a sharp crack, some plasticity
occurs, and linear elastic fracture mechanics (LEFM) requires that the plasticity (e.g. plastic zone size)
be limited to small scale yielding. Different values of β are specified in various standards, for example,
based on the magnitude of cyclic loading, which produces a smaller, fatigue hardened plastic zone.
There are some studies on specimen size effects in SCC testing, including irradiated materials. Some
publications are helpful for determining specimen size (see References [4] to [7]).
BW,/ −>aK βσ  (1)
()
eI yT
where
1/2
B is the effective thickness of the specimen, (B·B ) , where B is the specimen thickness at the
e N N
base of the side grooves; if no side grooves are used, B = B (in m);
e
W is the specimen width (in m);
a is the crack length (in m);
4 © ISO 2021 – All rights reserved

β is the factor related to the extent of plasticity considered acceptable (β = 2,5 is sometimes used);
K
is the stress intensity factor (in MPa m );
I
σ is the yield stress at the test temperature (in MPa).
yT
However, in Formula (1), when σ / σ > 1,3 is satisfied, σ can be used instead of σ .
uT yT flowT yT
5.5 Specimen dimensional measurement
The dimensions of the specimen are measured and confirmed to be within the fabrication tolerance
shown in Figure 1.
5.6 Stress intensity factor, K
I
The stress intensity factor, K , for a CT specimen is defined as shown in Formula (2):
I
()2+α
P
23 4
K = (,0 886+−46,,41αα3321+−47,,25αα6 ) (2)
I
32/
BW
()1−α
e
where
K
is the stress intensity factor (in MPa m );
I
P is the load (in MN);
α a/W.
NOTE The accuracy of Formula (2) is ±0,5 % over the range 0,2 ≤ α ≤ 1,0. K can also be obtained by finite
I
element analysis.
6 Test equipment
A typical test system consists of a high-temperature, high-pressure autoclave with a loading machine
and a water circulation system that flows a simulated light water reactor environment through the
autoclave. While the volume and pressure of the autoclave depend on the size of the specimen and
target test temperature, a smaller volume is generally preferable to achieve better control of the test
parameters. An ion exchanger shall be incorporated in the circulation loop to remove ionic impurities
generated by corrosion of the test specimen or system materials. If dosing species (such as pH control
additives or boric acid) are added to the test environment for a simulated pressurized water reactor
(PWR) environment, the demineralizer shall be equilibrated to the desired chemistry (e.g. B, Li). A
once-through system can also be used to maintain the environment chemistry, although a circulating
system generally provides a better control of the water chemistry in the autoclave because of the higher
refresh rate. For relatively pure boiling water reactor (BWR) environments, a recirculating system is
necessary, and if impurities are desired (e.g. 30 µg/l sulfate), they are ideally continuously added to the
water flowing into the autoclave, with appropriate continuous monitoring of ionic species or solution
conductivity, and clean-up of return water from the autoclave.
Control of dissolved gases, especially oxygen or hydrogen, in the test environment is usually
accomplished by continuous bubbling in the reservoir, which ideally has a much smaller diameter than
height, and generally should be of limited volume (e.g. < 10 l). The solubility of oxygen at standard
temperature and pressure (STP) is about 43 mg/l (and varies with temperature), and the solubility
of hydrogen is about 1,6 mg/l (and varies less near room temperature). Conditions (dissolved gases,
autoclave volume, refresh rate and system materials) shall be chosen to ensure that the outlet gas
concentration is a high fraction of the inlet (e.g. > 80 %); this is especially an issue for dissolved oxygen
concentrations below about 100 µg/l, and a bigger issue if hydrogen is also present.
Test stability is important, and load, temperature, water purity and dissolved gas concentration shall
be kept constant during a given test segment. Stability of room temperature is also important, because
it can affect dissolved gas concentration, water purity and precision instrumentation used to control
temperature and measure crack length.
The test equipment includes devices to continuously monitor the crack length, temperature, water
quality parameters such as solution conductivity and dissolved gases, and corrosion potential. The
corrosion potential measurement is less important in a simulated PWR environment (with dissolved
hydrogen and no dissolved oxygen) than in a simulated BWR environment with dissolved oxygen.
Nevertheless, measuring corrosion potential is recommended to ensure that the corrosion potential
has reached an appropriate value prior to the testing and to detect changes in water chemistry.
Some examples of test equipment in BWR and PWR water environments are provided in Annex B,
together with a typical schematic diagram of the water loop. More detailed water chemistry and
monitoring items are provided in Annex C and Reference [3].
7 Crack length measurement by potential drop method
7.1.1 Both direct current (DC) and alternating current (AC) PDMs have been used. DC is most widely
used because it is less susceptible to electrical noise, and easier and less expensive to implement.
7.1.2 The PDM has many applications in the measurement of the fatigue crack growth rate and extensive
guidelines exist for measuring fatigue crack growth rates (see References [8] and [9]). While the majority
of the practices described in the standards are valid when applied to SCC growth testing, there are
additional factors specific to SCC growth tests in high-temperature water environments.
7.1.3 For high-quality PDM measurements using reversed DC current, a digital voltmeter capable
of integration over ≥ 1 power line cycle is essential, and many ± current readings shall be averaged
(typically 100 to 5 000, depending on the crack growth rate) to achieve good crack length resolution. If
PDM readings are taken during any unloading and reloading cycles, some bias downward in crack length
may be observed from crack closure.
7.1.4 The wires inside the high-temperature autoclave shall be insulated. Below about 300 °C,
polytetrafluoroethylene (PTFE) (standard tubing, or heat-shrinkable tubing) is commonly used. At higher
temperatures, pieces of partially stabilized zirconia (e.g. 3 % mass fraction MgO) are commonly used.
7.1.5 The electrical resistivity changes with time at temperature in many nickel base alloys, and can
create the false impression of crack advance when using PDM measurements. Higher temperature or
cold work produces a faster approach to saturation. The need for compensation for resistivity changes
depends on both the alloy and heat as well as the crack growth rate being measured. For example, at
−8
10 m/s, a resistivity change even early in the test would not affect the subsequent measured growth
−11
rate. But below 10 m/s, small changes can affect the measured growth rate. A “reference potential”
shall be measured on the same material and condition as the specimen because the alloy, heat, cold work
and prior temperature exposure affect the resistivity change versus time. The reference potential can be
measured on a separate rod or coupon, or on the back-face of the specimen. When the back-face potential
is used, the change in reference potential as the crack grows should be considered. The resistivity change
with time for stainless and ferritic steels is much smaller than nickel base alloys, and compensation
is rarely done. Historically, a reference potential measurement has been used to compensate for
temperature or DC current fluctuations, but eliminating these fluctuations is much more effective than
compensating for them.
7.1.6 The temperature fluctuation in the autoclave has a big effect on the accuracy of the crack length
measurement by the PDM. It is necessary to suppress the change of temperature as much as possible.
7.1.7 Uneven crack advance is more common in SCC growth tests in high-temperature water than in
fatigue tests, and it significantly affects the accuracy of all forms of crack length measurement, which
6 © ISO 2021 – All rights reserved

are strongly biased by the regions of least crack advance. The (commonly IG) crack morphology, crack
branching and less planar crack growth in SCC versus fatigue growth tests can produce current shorting
in the wake of the crack, causing a significant underestimation of crack size. In such circumstances,
where a large discrepancy is observed between the PDM and fractography measurements, a linear post-
test correction of crack depth, crack growth rate and K is recommended because it is not known how or
when the problems developed during the test.
The following issues should be addressed to reduce the noise of the PDM signal: wire routing of the
PDM cables, electrical insulation between the specimen(s) and the loading clevis, position and stability
of the current lead attachment.
The current applied to the specimen during PDM measurements can affect the measurement of
corrosion potential, although the effect is minimized by using continuous PTFE insulation of the
current leads from outside the autoclave to the specimen. It is recommended that the current applied to
the specimen be turned off for several seconds before corrosion potential measurements.
8 Corrosion potential measurement
8.1 General
Corrosion potential (E ) shall be measured in simulated BWR environment tests. It should be
corr
measured in simulated PWR environment tests.
8.2 Measurement method
A high-input-impedance (≥ 10 Ω is recommended) electrometer is used to measure the potential
between the CT specimen and a reference electrode.
Several types of reference electrodes are commonly used:
-
— Ag/AgCl/Cl electrodes (internal type and pressure balanced external type);
— Membrane, metal/metal oxide electrodes (Fe/Fe O , Cu/Cu O, etc.).
3 4 2
Also, to ensure the accuracy of the potential measurements, including a platinum electrode is
recommended because its potential is precisely known in hydrogen-only environments, and is quite
well defined in oxygen-only environments.
9 Test procedure
9.1 General
A flow chart of SCC growth testing is shown in Figure 2. After preparing the specimen and getting ready
for the PDM measurement and E measurement (leads, reference electrode, etc.), the crack growth
corr
test is performed using the following procedures.
Figure 2 — A flow chart of SCC growth test in simulated light water reactor environment
9.2 Installation in autoclave
9.2.1 After cleaning, the specimen is installed in the loading clevises, and leads for PDM are attached:
one set for the DC or AC current to the specimen and one for measuring the electrical potential drop
as the crack grows. A third set is needed if reference potentials are used. The loading linkage shall be
insulated at least at one location to eliminate an alternate current path. The current shall flow only
through the specimen (see 7.1.4). A ground isolated power supply is also essential to prevent “ground
loops” (current that could flow through the linkage and autoclave materials to ground, thereby creating
an alternative path to its flow through the specimen).
9.2.2 A reference electrode for the E measurement is installed in the autoclave near the specimen.
corr
Also, it is recommended that a platinum electrode is installed near the reference electrode. The wiring
and inter-connections (which are often a limiting factor) shall be carefully designed to maintain a high
impedance between the two leads.
8 © ISO 2021 – All rights reserved

9.3 Adjustment of test environment
9.3.1 The water chemistry in the autoclave is limited by the water quality in the reservoir, the autoclave
volume and the refresh rate. At a minimum, it shall be characterized by monitoring the autoclave inlet
and outlet solution conductivity and dissolved oxygen.
9.3.2 After preparing pure water using ionic exchange resins, the solution conductivity at room
temperature (25 °C) should be < 6 µS/m (which is affected by exposure to air). Then, the water chemistry
condition is adjusted to the test conditions, e.g. simulated BWR or PWR water. For simulated PWR
primary environments, the concentrations of boric acid and lithium (or potassium) hydroxide are
controlled. Examples of the water chemistry conditions for simulated BWR and PWR environment tests
are shown in Annex C.
Note that mixed bed demineralizers often initially contain high levels of organics, and most have no
effect on solution conductivity until they are heated to 300 °C and decompose, and thus affect the outlet
solution conductivity and the environment to which the specimen is exposed. Pre-cleaning of new
demineralizers for 4 to 8 days using a benchtop loop comprised of a closed beaker (to minimize air
exposure), a small pump, the demineralizer, a sub-micron filter and a UV light is recommended.
9.3.3 The dissolved oxygen and hydrogen concentrations are adjusted by bubbling gases (O , H , N ,
2 2 2
Ar, etc.) in the reservoir. During the test, the water chemistry parameters, such as gas concentration,
solution conductivity, ion concentration and temperature, shall be controlled within the desired ranges.
The refresh rate of water through the autoclave shall be sufficient to ensure that the water chemistry at
inlet and at outlet of the autoclave is high quality and stable during the test. The parameters typically
used in simulated BWR and PWR tests are shown in Annex C.
9.3.4 When multiple specimens are tested in an autoclave, the temperature difference between them
should be characterized and minimized (ideally < 1 °C).
9.3.5 Temperature fluctuations in the autoclave have a big effect on the accuracy and resolution of
crack length measurements by PDM, and they should be minimized (ideally < 0,2 °C).
9.4 Loading
9.4.1 General
The specimen is loaded after the PDM system is operating and the water chemistry is stable (as
described in 9.3) and sufficient time has passed for development of the oxide film (generally a few days).
In water with oxygen, the corrosion potential will usually continue to rise slowly for a week or two.
9.4.2 Fatigue pre-cracking
A fatigue pre-crack shall be introduced before SCC transitioning described in 9.4.3.
The final maximum load during pre-cracking shall not exceed the initial load used for the SCC growth
test because compressive plastic strain at the crack tip due to a decrease in applied load can have a
significant effect on the applied K for the SCC growth test.
I
The fatigue pre-crack can be introduced in air or in the test environment. Familiarity with
[10] [8]
ISO 11782-2 and ASTM E647-15e1 is recommended.
9.4.3 SCC transitioning
Starting SCC growth tests under a static loading condition immediately after the TG fatigue pre-crack
often results in no or little growth of IG or ID SCC from isolated multiple areas along the fatigue crack
front especially in relatively resistant material to stress corrosion cracking. IG/ID SCC usually propagate
unevenly, especially when areas of the fatigue crack front are pinned and no IG growth occurs in those
areas. This causes large uncertainties in the applied K and the DC potential drop indicated crack length
I
(and therefore growth rate).
Since a TG fatigue pre-crack essentially never exists in plant components, a complete transition to IG
or ID SCC (100 % SCC engagement) for austenitic alloys is very important for obtaining relevant and
reproducible SCC growth rates.
The transitioning to SCC is accomplished by obtaining sustaining crack advance while reducing cyclic
contribution by decreasing the frequency, then increasing the hold time at maximum load. A common
approach is to cycle at about 0,01 Hz at R = 0,5, then reduce the frequency to 0,001 Hz keeping the
maximum load (P ) or K (K ) at or near the load used for SCC growth testing. Trapezoidal loading
max I Imax
with increasing hold times at P or K is then used to transition to static loading. Hold times at P
max Imax max
or K commonly used are ~3 h, ~9 h and ~24 h, but high growth rate materials and environments
Imax
−9 −11
(e.g. > 10 m/s) may require only a ~9 h hold time, and low growth rate situations (e.g. < 10 m/s)
may require four or five hold time steps. The example of applying load sequence for SCC transitioning
is shown in Figure 3. It is important that the growth rate behaviour be well-behaved in each step, not
decaying versus time.
Key
X test time 4 step 2 (lower frequency, e.g. 0,001 Hz)
Y load (P) or stress intensity factor (K) 5 step 3 (multi steps cycle including hold time)
1 P or K 6 hold time, T (e.g. 3 h, 9 h or 24 h)
max Imax h
2 P or K 7 fall time, T
min Imin f
3 step 1 (higher frequency, e.g. 0,01 Hz) 8 rise time, T
r
Figure 3 — Example of applying load sequence for SCC transitioning
It is recommended that a load ratio of < 0,5 be used for lower K tests (e.g. < 20 MPa m ), and > 0,7
max
for high K tests (e.g. > 60 MPa m ). When the growth rate is decaying versus time, a lower load ratio
max
is often needed.
Since it is the rising part of cyclic waveform that enhances crack growth, an asymmetric waveform (e.g.
5 % fall, 95 % reload) is recommended during SCC transitioning.
References [3], [11], [12] and [13] provide more details on SCC transitioning in both unirradiated and
irradiated materials.
10 © ISO 2021 – All rights reserved

9.4.4 Static loading
9.4.4.1 Constant load
A constant load condition is commonly used in a static-load SCC growth test. When applying a constant
load to a CT specimen, the K of the specimen increases slowly with crack advance. As the crack
I
becomes longer, the rate of K increase is faster. When the amount of crack advance during the test is
large, it is necessary to take the increase in K and its increasing rate into consideration in the test result
evaluation.
9.4.4.2 Constant K
The effect of the increasing K during testing can be reduced by constant-K tests. A typical constant-K
test is performed by shedding the load applied to a CT specimen as the crack grows so that the applied
K calculated based on the PDM signal stays essentially constant. Although the error in the crack size
I
measurement by PDM results in some variation in K, the effect of the change in K is usually smaller than
constant load tests.
Very small changes in applied load are preferred, but this is controlled by crack length resolution. The
magnitude of each load correction shall be limited so that one unusual PDM reading does not produce
over-correction. It is recommended that apparently shortening of the crack not result in an increase in
load, to avoid hunting or load cycling.
An alternative way to achieve a constant K condition is to use a CDCB specimen. As noted in 5.2, the K
I
of a CDCB specimen remains almost constant as the crack advances with a fixed applied load.
9.4.4.3 Changes in loading condition
Lowering the applied load by more than ~1 % during testing is not recommended since it can leave a
compressive stress field at the crack tip. When increasing the applied load, small increments of a few per
cent are recommended. For both falling and rising load, additional changes can be made after sufficient
crack advance. It must be recognized that K changes in most structural components occur because the
crack has grown; that is, changes occur by dK/da, not by dK/dt or simply a sudden change in K.
When there is no significant increase in crack length over the duration of the SCC growth test segment,
crack-tip re-activation using a slow cyclic load is considered appropriate, with subsequent transitioning
as described in 9.4.3. When a test must be interrupted, the test should be restarted in accordance with
the procedures described in 9.4.1.
9.4.5 It should be noted that the crack front unevenness significantly affects the accuracy of PDM
measurement, with regions of least crack advance strongly biasing PDM. This in turn affects the indicated
crack length, crack growth rate and K values. While unevenness is common in components or specimens
with extensive SCC growth, the most reliable data are obtained when unevenness is small (e.g. < 10 or
20 % of the SCC growth), in part because PDM does not reflect an average crack advance (and there is
no way to know how the unevenness evolved during a test), and in part because regions of least crack
advance can begin to grow as the stress on those areas rises, which can give the impression of rapid growth
even though it is occurring in a relatively small, high stress area. Both test design and test management
are needed to minimize this concern: excellent SCC transitioning procedures are needed to achieve
100 % SCC engagement from the fatigue pre-crack, and consideration shall be given to the subsequent
development of unevenness after extensive SCC growth. For example, after several millimetres of crack
growth, cyclic loading can be used to straighten the crack front and permit evaluation of other areas of
the microstructure.
10 Evaluation of test results
10.1 When the test is complete, the specimen is fractured apart by fatigue in air. The fatigue pre-crack
and the SCC growth surface developed in the high-temperature high-pressure water environment can
be observed with an optical microscope (where the oxide formed in high-temperature water is usually
visible) and/or scanning electron microscope (SEM), and the images recorded. The detailed fracture
morphology in the SCC region should be observed by SEM and the image recorded.
10.2 The schematic fracture surface of the specimen of austenitic alloy is shown in Figure 4. The crack
extensions A (averaged over specimen thickness), A (averaged over SCC engaged area) and A / A
1 2 min max
(minimum and maximum crack extension) are determined from the fracture surface of the specimen as
shown in the figure, for example.
10.3 The average crack extension values can be determined by the area method or equally divided
method, as shown in Figures 4 and 5, respectively. When the unevenness of the crack is significant, a
higher number of divisions is necessary (see Figure 5).
For a highly uneven, ID crack morphology, such as often develops in nickel base weld metals, the
procedure to correct crack length and crack growth rate using crack engagement is described in the
literature (see Reference [14]). However, the objective should always be to achieve 100 % engagement.
Key
1 fatigue pre-crack (TG) b thickness
2 SCC transitioning (TG to IG) b crack width
c
3 SCC growth region under static loading (IG) A maximum crack extension
max
S SCC growth area under static loading A minimum crack extension
SCC min
S transitioning area for fatigue pre-cracking and A average crack extension: (S + S ) / b
Trans 1 SCC Trans 0
SCC transitioning (excludes fatigue pre-crack in air)
S transitioning area corresponding to SCC A average crack extension in crack engagement
Trans′ 2
area: (S + S ) / b
engaged width (excludes fatigue pre-crack in air) SCC Trans′ C
Figure 4 — Method for obtaining crack lengths on the fracture surface of specimen [area method]
12 © ISO 2021 – All rights reserved

Key
1 fatigue pre-crack (TG) n division number
2 SCC transitioning (TG to IG) Δa crack extension
i
3 SCC growth region under static loading (IG) A average crack extension: ΣΔa / n
1 i
b thickness
Figure 5 — Method for obtaining crack lengths on the fracture surface of a specimen [equally
divided method]
10.4 The crack lengths obtained throughout the test by PDM measurement are corrected based on the
initial crack length (a ) and the final crack length (a ) that are determined from A and A (or, in rare cases,
0 f 1 2
A ), as described in 10.2. A is most commonly used for correction because it is considered unlikely
max 2
that regions of higher susceptibility (reflected in the regions of deeper cracking) extend throughout a
structural component.
When the corrected crack length versus time response is linear with time in the evaluation period and
the slope is significant compared to the PDM data fluctuations (e.g. standard deviation, σ), the crack
growth rate, da/dt, is calculated.
The schematic figure of the method to determine the crack growth rate from the crack length versus
test time is provided in Annex D.
NOTE When the water chemistry or the test load is changed during the SCC growth test, the SCC growth rate
is evaluated from the PDM signal during which the test conditions are stable.
10.5 The stress intensity factor, K , during the SCC growth rate evaluation period is determined from
I
the corrected crack length and the appli
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