7-l
CORROSION
FATIGUE
OF ALUMINUM
ALLOYS
TESTING
AND PREDICTION
J-M. Genkin*, B.G. Journet
AEROSPATIALE,
CCR Louis-Bleriot
12, rue Pasteur, BP 76
92 152 Suresnes, France
* formerly with AEROSPATIALE
ABSTRACT
Since the Aloha in-flight
failure,
the ageing
aircraft
issue has prompted some renewed
effort from the research community in the area
of corrosion
fatigue. This paper presents a
methodology which
deals with
corrosion
fatigue crack initiation in a pitting environment.
The
investigation
is carried out on an
aluminum lithium alloy. The obtained results
shed a new light on the understanding
of
corrosion
fatigue, Before corrosion
fatigue
cracks take over,
the propagating
flaw
is a
hybrid half pit / half corrosion fatigue crack.
Both pitting and corrosion fatigue contribute to
the flaw growth. A predictive model has been
derived.
The
validation
was made by
comparing
the predictions
to experimentally
measured pit depths and fatigue lives. A set of
guidelines
is given for the prediction of in-
service corrosion
fatigue.
It highlights
the
materials parameters to be evaluated and the
testing conditions to use.
l-INTRODUCTION
Ageing
aircrafts
are prone
to corrosive
degradations
that may result
in Multi Site
Damage (MSD). On the Aloha airplane which
lost a considerable part of its fuselage upper
skin in 1988 during its flight, the rupture was
caused by the coalescence of a series of
corrosion
initiated cracks at adjacent rivet
holes. Furthermore this MSD was not detected
during the last inspection. It means that the
cracks do not need to be too long for the MSD
to become an impending threat.
MSD
reduces the residual
strength of a
structural part in a much worse manner than a
single crack does. Nowadays
the fatigue
propagation and link-up of MSD is understood
and can be calculated (1,2).
In-house studies have shown that the effect of
saline environment
(A3 type 3.5% NaCL and
ASTM G69) on the crack growth
rates of
alclad 2024 and 6013