Volume 7 Preprint 32
Analysis of the Corrosion Damage Configuration and Its Effect to Fracture Behaviour of Pre-Corroded Aluminium Alloy
Y. H. Zhang and G. Z. Lv
Keywords: aluminium alloy, corrosion, fatigue, damage tolerance
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Volume 7 Paper 32
Analysis of the Corrosion Damage
Configuration and Its Effect to Fracture
Behaviour of Pre-Corroded Aluminium Alloy
Y. H. ZHANG and G. Z. LV
School of Aeronautics, Northwest Polytechnical University, Xi’an
710072, China, email@example.com
The corrosion damage of laboratory coupons of LY12-CZ aluminium
alloy subjected to constant amplitude loading was investigated. Under
the conditions of several temperatures and durations of corroding,
specimens were exposed to EXCO solutions and then the pre-corroded
specimens were fatigue tested. The SEM and optical microscope
analyses indicated that the two key parameters (surface corrosion
damage ratio and the average depth of corrosion pits) were the metric
for evaluating the effect of corrosion damage to fracture behaviour of
aged aluminium alloy structure. Corrosion damage decreased the
fatigue lives by a factor of about 1.25 to 2.38, and then the residual
strength curve of a corroded specimen was presented.
Keywords: aluminium alloy, corrosion, fatigue, damage tolerance
As aircraft aged and were kept in service, service lives of many aircraft
fleets were beyond their original design lives. The incidence of
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science
and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it
should not normally be referenced in published work. © UMIST 2004.
corrosion tended to increase with aircraft aging. Corrosion can create
a modified damage state that may promote fatigue crack formation
and growth during flight . There is a synergistic interaction between
pitting and the early stage of corrosion fatigue crack growth . Many
uncertainties hinder accurate rate and life prediction of operating
aircraft and there is still no generally applicable model available to
predict the service life of corroded aluminium structures . LY 12-CZ
aluminium alloy frequently have been used in the manufacture of
aircraft, however, in the inshore circumstance, LY 12-CZ is susceptible
to corrosion damage. Understanding the effect of corrosion damage to
fracture behaviour is helpful to life prediction, and subsequently life
extension for aging aircraft.
2.Pre-Corrosion and Fatigue Experiment
The LY 12-CZ aluminium alloy dog-bone type specimens were
machined in the T-L orientation. Specimens were exposed to EXCO
solution  for 10, 20 and 31days at 20, 40 and 60℃(Fig.1). After the
pre-corrosion procedure, excess corrosion products were removed by
cleaning them first with a water rinse, then submerging in 68% HNO 3
for 5 min, rinsing again in pure water, and air dried. The corroded
specimens were then fatigue tested under constant amplitude loading
in the laboratory environment. The fatigue testing parameters were,
maximum stress: 256.4MPa, R: 0.02, and f: 10Hz.
Fig.1 Equipments for producing corrosion damage
3. Corrosion Damage Configurations and the Effect on the
3.1 Discussion on corrosion damage configuration
Using SEM, the dominant pit can be identified by the cracking pattern
surrounding the nucleating pits (the blackest semi-elliptic parts shown
in Fig. 2). It can be found that most of the corrosion pits present semiellipse shape. There were multiple fatigue crack for almost every
specimen, but most crack nucleation and growth were governed by a
Fig.2 Fatigue fracture surface showing crack growth from corrosion
Using the UNION DZ3 Electronical Video Microscope, the surface
corrosion damage configuration of specimens can be observed. The
surface corrosion damage ratio (ratio of sum planar area of corrosion
pits to area of surface corrosion part) was used to reflect the
roughness of corroded surface. At 40℃, when the corrosion days were
10 days, 20days, 31days, the average corrosion damage ratio were
separately 11.25%, 13.91%, 17.37%. When the corrosion days were
20days, at 20, 40, 60℃, the average corrosion damage ratio were
separately 8.42%, 13.91%, 23.30%.
3.2 Discussion on the metric for evaluating corrosion damage
The metric for evaluating corrosion damage based on an “effect of
defects”  study was proposed. The damage induced by precorrosion was corrosion pits, so the average depth of corrosion pits
which revealed the damage degree in the direction of cross section
and the surface corrosion damage ratio that revealed the damage
severities in the direction of surface were two key parameters
charactering corrosion damage. These two parameters entirely
describe the corrosion pits from the view of 3-D. The two parameters
can character the configuration of corrosion pit also the distribution of
corrosion pits and they could be used the metric for evaluating the
effect of corrosion damage to fracture behaviour of aged aluminium
3.3 The effect of corrosion damage to fatigue lives
Comparing with the un-corroded specimens, the fatigue lives of precorroded specimens descended dramatically, the extent of descending
was up to the initial corrosion damage configuration. Corrosion
damage decreased the fatigue lives by a factor of about 1.25 to 2.38.
With rise of the temperature, the dimension of corrosion pits enlarged
and the average values of corrosion damage ratio of specimens
became larger (from table1).
Table 1: The fatigue lives and the corresponding corrosion damage
damage ratio （
3.4 The damage tolerance analysis of pre-corroded specimens
The residual strength of the specimen, which was pre-corroded at
40℃ for 20 days, was investigated as an example. Based on the LEFM
model of corrosion pit (Fig.3), the AFGROW code was used to carry out
the analysis of crack growth. In the AFGROW parameter settings ,
the crack shape of planar center semi-elliptic (a=0.02886mm,
c=0.03mm) model and the constant amplitude loading (R=0.02) were
used. The resulting residual strength curve of this specimen is shown
Fig.3 LEFM model of the corrosion pit.
Fig.4 Residual strength of a specimen (40℃, 20days).
1. Rise of temperature and prolonging of corrosion time, the
dimension of corrosion pits enlarge and the values of surface
corrosion damage ratio of specimens become larger. Corrosion
damage decrease the fatigue lives by a factor of about 1.25 to 2.38.
2. The two key parameters (surface corrosion damage ratio and the
average depth of corrosion pits) were proposed as the metric for
evaluating the effect of corrosion damage. The residual strength curve
of a specimen is presented.
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