Abstract:
The
present study is concerned with the interaction of stress and
corrosion in
high strength aluminium a11oys in sheet form used in the
aircraft
industry. Many of these alloys are susceptible to exfoliation
corrosion under service conditions. This
type of corrosion was reproduced
in the
susceptible a11oys using the Exco test method.
The fracture
toughness of an A1 - Zn - Mg - Cu a11oy (7178 - T6) was
assessed
using R - curves. The interaction between exfoliation corrosion
and the
thinning effect of exfoliation on the fracture toughness are
discussed.
Fatigue crack growth behaviour in two naturally aged A1 - Cu ~ Mg
alloys, 2024 - T351 and 3L7O (2014 - T4) was evaluated under conditions
of low and
high humidity using constant amplitude loading. A similar
assessment of the effect of
humidity on the propagation rates in the higher
strength 7178 - T6 was carried out. Additionally, the effect of the
presence of exfoliation corrosion on crack growth in 2024 - T351 and 7178 --
T6 was examined. Both humidity and exfoliation corrosion enhanced fatigue
crack
growth rates in the alloys, though to differing extents, with 7178 -
T6
being affected more. The fatigue resistance of 7178 - T6 diminished
even further in the
presence of humidity and corrosia together.
Finally, a mechanism to explain the way by which exfoliation corrosion
affects the
fatigue and fracture behaviour of thin aluminium sheets is
proposed.
