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Scripta Materialia 49 (2003) 711–716
www.actamat-journals.com
Effect of pitting corrosion on very high cycle fatigue behavior
Q.Y. Wang a,*, N. Kawagoishi b, Q. Chen b
a Department of Civil Engineering and Mechanics, Sichuan University, Chengdu 610065, Chinab Department of Mechanical Engineering, Kagoshima University, Kagoshima 890-0065, Japan
Received 9 December 2002; received in revised form 30 May 2003; accepted 17 June 2003
Abstract
In this study, the effect of pre-existing corrosion pits on the fatigue behavior of 7075/T6 aluminium alloy in very long
life range and in the near threshold regime was investigated by using piezoelectric accelerated fatigue test. The results
indicate that the presence of pre-existing corrosion pits, produced by 1-day, 4-day, and 7-day immersion in salt water
significantly reduces the fatigue life of the aluminum alloy by a factor of 10–100.
� 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Pitting corrosion; Very long life fatigue; 7075/T6 aluminum alloy; Near-threshold crack growth
1. Introduction
Pitting (localized) corrosion leading to fatigue
crack initiation and crack growth is considered tobe among the most significant damage mecha-
nisms in aging structures [1–3]. Prior-corrosion
related fatigue process consists mainly of pitting
nucleation, pit growth, transition from pitting to
fatigue crack initiation, short crack growth and
long crack growth [3–5]. Pits almost always initiate
at some chemical or physical heterogeneity at the
surface, such as inclusions, second-phase particles,flaws, mechanical damage, or dislocations. The
aluminum alloys contain numerous constituent
particles, which play an important role in corro-
* Corresponding author. Tel.: +86-28-85404890; fax: +86-28-
85405534.
E-mail addresses: wangqy@scu.edu.cn, wangqy2000@
hotmail.com (Q.Y. Wang).
1359-6462/$ - see front matter � 2003 Acta Materialia Inc. Published
doi:10.1016/S1359-6462(03)00365-8
sion pit formation [2]. It is well known that cor-
rosion pitting has a strong effect on fatigue life of
structural Al alloys [1–5]. Under the interaction of
cyclic load and the corrosive environment, cyclicloading facilitates the pitting process, and corro-
sion pits, acting as geometrical discontinuities,
lead to crack initiation and propagation and then
final failure. The presence of localized corrosion
pits modifies the local stress and may ultimately
shorten fatigue life and lower the threshold stress
for crack initiation and propagation.
Although much fatigue data of aluminium al-loys have been published, the most experimental
data in the literature have been limited to fatigue
lives up to 107 cycles. In many industries (such as
aircraft, automobile, railway and offshore struc-
tures), however, the required design lifetime of the
components often exceeds 109 cycles. In recent
years there has been a development of interest in
very long life fatigue between 107 and 1010 cyclesfor various high strength steels [6–10] and very
by Elsevier Ltd. All rights reserved.
Table 2
Chemical composition (wt%)
Si Mn Cu Mg Zn Cr Fe
0.10 0.03 1.47 2.56 5.46 0.20 0.25
712 Q.Y. Wang et al. / Scripta Materialia 49 (2003) 711–716
slow fatigue crack growth (FCG) between 10�9
and 10�12 m/cycle [11]. Moreover, it is well known
that many materials, including aluminium alloys,
do not show a conventional fatigue limit in S–Ncurves between 106 and 107 cycles [6–13]. Fatigue
failure could occur beyond 107 cycles. But only
limited studies of very long life fatigue and near
threshold FCG behavior of aluminium alloys have
been performed.
In this study, the effect of pre-existing corrosion
pits, produced by prior immersion in 3.5 wt%
NaCI solution for various durations, on fatiguebehavior of aluminium alloys in very long life
range and in the near threshold regime was in-
vestigated by using piezoelectric accelerated fa-
tigue test at 19.5 kHz.
Fig. 1. Typical appearance of the pits on the surface of the
specimens exposed to salt solution for various durations.
2. Experimental procedure
2.1. Material and specimen
The material used in this study was an extruded
Al alloy 7075/T6. The mechanical properties and
the chemical composition are shown in Tables 1
and 2, respectively.
Plate dog-bone fatigue specimens designed to
resonate longitudinally at 19.5 kHz were machined
with single edge notch having length of 1 mm,
radius of 0.5 mm and stress concentration factor of3.05. The resonance length of the specimen was
calculated using an analytical method [12]. All
specimens were 114.6 mm in length and 16 mm in
width, with a nominal gage thickness of 3.0 mm.
All the specimen surfaces were mechanically
polished with 300-, 400-, 600-, and 1200-grade
papers. For pitting corrosion studies, the polished
specimens were immersed in a physiological salinesolution (3.5% NaCl) for 1, 4, and 7 days, re-
spectively. The typical appearance of the pits on
the surface of the specimens after exposure for
various durations is shown in Fig. 1. The pits are
random in size and irregular in shape. The
Table 1
Mechanical properties
E (GPa) q (kg/m3) rT (MPa) rY (MPa)
72 2800 764 691
observed maximum pit sizes for specimens exposed
for 1, 4, and 7 days are about 30, 50, 60 lm, re-
spectively.
2.2. Fatigue testing
Fatigue testing was carried out in a piezoelectric
resonance system operating at 19.5 KHz with zero
mean stress (R ¼ �1). The testing facility has been
described in detail elsewhere [12,13]. The vibration
loading amplitude was controlled during the test.
FCG for constant- and variable-loading condi-
tions was monitored using a traveling microscope
(magnification of 200·), following by use of plasticreplicas and microscopy observations. A cellulose
acetate film having length of about 15 mm and
width of 5 mm was first wetted with acetone and
carefully put onto the mid surface of the specimen.
Following evaporation of the acetone, the film was
peeled off for further treatment and developing the
replicas. The replicas with the inverse copy of the
specimen surface were examined using micro-scopy.
8
10
m)
7-day4-day1-day
Q.Y. Wang et al. / Scripta Materialia 49 (2003) 711–716 713
Fatigue tests were interrupted at certain num-
bers of cycles in order to make replicas of the mid
surface of the specimens. For each specimen test,
more than 30 replicas were taken for detectingcrack initiation and monitoring crack growth. The
minimum detectable crack length is within 0.05
mm. Fig. 2 shows examples of optical micrographs
of replicas of the small cracks initiated from the
notch in the near-threshold FCG test.
Two series of fatigue tests were conducted in
this investigation. In order to understand how the
various exposure durations affect the fatigue life,the first test was carried out under a constant load
amplitude at an initial stress amplitude of 80 MPa
to obtain crack length against number of cycles
data. As the second, FCG experiments were per-
formed under variable stress intensity amplitude to
obtain crack growth behavior of small and large
cracks in the near threshold regime. The FCG
curves were obtained by shedding the load in smallsteps until the threshold crack growth rates (less
than 10�10 m/cycle) were reached. Then, the load
might be increased again to obtain higher crack
growth rates. The stress intensity factor at the
crack tip was approximated by the formula [14]:
K ¼ Eð1� m2Þ
ffiffiffipa
rU0f
aw
� �ð1Þ
where
faw
� �¼ 0:64
aw
� �þ 1:73
aw
� �2
� 3:98aw
� �3
þ 1:96aw
� �4
ð2Þ
E is the Young�s modulus, m is the Poisson ratio, U0
is the vibration amplitude, a and w denote the
crack length and the specimen width.
Fig. 2. Optical micrographs of small cracks in near-threshold
FCG tests.
3. Results and discussion
3.1. Effect of exposure durations on fatigue life
Fig. 3 shows the measured responses of crack
growth length a to the number of cycles N . The 7-
day, 4-day, and 1-day pre-corroded specimens
failed at an average of 7.2 · 106, 9.0 · 106, and
5.9 · 107 cycles, respectively. Under the same
loading amplitude, there is no crack growth de-
tected for the non-corroded specimens up to
5 · 108 cycles. The a–N curve indicates that thepresence of pre-existing corrosion pits, produced
by 1-day, 4-day, and 7-day immersion in salt water
significantly reduces the fatigue life of the alumi-
num alloy by a factor of 10–100. Sankaran et al.
[15] have shown that prior corrosion pitting can
shorten the fatigue life of 7075/T6 aluminum alloy
about 10 times when cycles to failure was <106
cycles. So, not surprisingly, the presence of cor-rosion pits can cause a 100 times decrease in the
fatigue life in super-long life range. As well known,
there is a prolonged period of fatigue crack initi-
ation life due to high cycle fatigue [7]. Moreover,
no significant difference is found in lifetime be-
tween 4-day and 7-day pitted specimens. The slight
difference is from the crack initiation life.
3.2. Near-threshold fatigue crack growth behavior
Usually, the threshold stress intensity factor
range, DKth, is defined as that the stress intensity
factor range at which the crack growth rate is less
0
2
4
6
1.E+05 1.E+06 1.E+07 1.E+08 1.E+09
Number of cycles
Cra
ck le
ngth
(m 0-day
Fig. 3. Effect of exposure durations on fatigue life.
Fig. 5. FCG behavior of small and large cracks in 4-day, and 7-
day corroded Al 7075/T6.
714 Q.Y. Wang et al. / Scripta Materialia 49 (2003) 711–716
than 10�10 m/cycle, i.e., crack grows 1 mm within
107 cycles. In case where fatigue crack propagation
life primarily depend on the early stages of crack
growth, it is critical to understand the nearthreshold FCG behavior. The crack growth be-
havior of small and large cracks in non-corroded
aluminum alloy 7075/T6 is shown in Fig. 4. It is
noticed that the crack growth rates of small cracks
(0.1–1.0 mm in length) are greater than those of
large cracks at almost the same stress intensity
factor range, DK, and some small cracks may grow
at DK values below the large crack threshold in thenear threshold FCG regime. For DK < 5
MPaffiffiffiffim
p, large cracks develop a FCG threshold
and small cracks accelerate crack growth rates.
Fig. 5 compares the results of FCG of small and
large cracks between 4-day- and 7-day corroded
specimens in aluminum alloy 7075/T6. The differ-
ence in FCG rates due to exposure durations is not
significant. The slight difference is from a localeffect of surface pitting [5]. At longer exposures,
more pits formed on the corroded surface and
sufficient pits could contact the subsurface con-
stituent particles causing further corrosion and
increase of pit size. FCG might not occur solely by
a single crack, but involved multiple-site pitting (or
crack) coalescence. The higher pit density (and the
Fig. 4. FCG behavior of small and large cracks in non-corroded
Al 7075/T6.
bigger pit size) on the corroded surface, the higher
stress concentration factor has increased because
pre-existing pits might act as small notches.It is noticed that the FCG rates is much lower
for the stress intensity factor range, DK, below 5
MPaffiffiffiffim
p. Slightly higher small crack growth rates
compared to the large crack are also observed. The
crack growth rates �DK curve shows a slope re-
gime between DK ¼ 5 and 10 MPaffiffiffiffim
p, in the near
threshold crack growth rates between 10�9 and
10�10 m/cycle, where there is a small–large cracktransition. As shown in Figs. 4 and 5, the threshold
stress intensity factor ranges found in pitted
specimens are lower than the threshold in non-
corroded conditions. The presence of corrosion
pits can decrease the threshold stress intensity by
about 20%.
3.3. Fractography
The fracture surfaces of tested specimens were
investigated by the scanning electron microscope
(SEM). Fig. 6 shows fatigue fracture surfaces of a
7-day pitted specimen during FCG. A transla-
mellar cleavage fracture mode was observed in
near-threshold crack growth (Fig. 6a). Large mi-crocleavage facets were formed during crack
Fig. 7. SEM micrographs showing intergranular corrosion in
the subsurface of the fatigue tested specimen with 7 days pre-
exposure to salt water.
Fig. 8. Fatigue crack initiation at a pre-existing pit in Al-alloy
7075/T6.
Fig. 6. SEM micrographs showing two fracture modes. (a)
Quasi-cleavage fracture in near-threshold crack growth, (b)
ductile fracture surface at high crack growth rate.
Q.Y. Wang et al. / Scripta Materialia 49 (2003) 711–716 715
propagation. However, at high crack growth rate,
the crack was found to propagate in a ductile
mode (Fig. 6b).
The corrosive attack can produce a network of
corrosion on the metal surface. As shown in Fig.
7a, it may also penetrate deep into the metal [16].Intergranular corrosion (Fig. 7b), preferential
corrosion along grain boundaries, was observed in
the subsurface of the FCG tested specimen with 7-
day prior corrosion.
One of the pre-existing pits, produced by 7 days
immersion in salt water, was shown in Fig. 8. The
pit located in the cleavage initiation area, and very
possibly played a role in fatigue crack initiation.The pit has a hemispherical shape with a size of
about 65 lm in length and 20 lm in depth.
4. Conclusions
1. Fatigue properties are significantly affected bythe pre-existing corrosion pits, specially, crack
initiation in the very long life range. Extensivelydeveloped corrosion pitting due to longer expo-
sures accelerates crack initiation and promotes
multiple-site damage. FCG rates increased slightly
with increasing surface corrosion pitting because
716 Q.Y. Wang et al. / Scripta Materialia 49 (2003) 711–716
surface pre-existing pits might act as stress con-
centrators [5,17]. Fatigue crack may initiate from
pre-existing pits followed by two (cleavage and
ductile) fracture modes during crack growth. Theexperiment results shows that FCG rates of small
cracks in the aluminum alloy are greater than those
of large cracks at almost the same stress intensity
factor range, DK, and some small cracks may grow
at DK values below the large crack threshold in the
near threshold crack growth regime.
2. Fatigue failure process under prior-corrosion
effects represented as the total number of cycles,comprises the cycles needed to form a critical pit–
crack transition size and the cycles needed to
propagate the crack to failure. The ongoing work
is aimed to develop a quantitative evaluation of
the formation and growth of pits, pit-crack tran-
sition, and crack growth processes.
Acknowledgements
The authors are grateful to the support of
Japan Society for the Promotion of Science under
the Grant-JSPS-P01042, and Chinese SRF forROCS.
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