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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