Typically, the environment and choice of the structural
material will change the rates at which cracks nucleate and grow, and can cause
cracks to nucleate in locations where the risk for cracking damage without the
environment is negligible. As a result
of using military aircraft past their initial planned design life (about 20-25
years), new categories of structural integrity problems caused by environmental
attack have been identified. Developing
a damage tolerant design guidelines handbook that covers corrosion damage and
environmental attack requires a more systematic approach for presenting
approaches and methods that engineers can use to control the risk of structural
failure.
Stress corrosion cracking (SCC) is a particularly
deleterious form of environmental
attack that will create opportunities for cracks to nucleate and grow to
failure, even under limited fatigue loading conditions (mechanism requires
constant tensile stress conditions, and low material resistance to this kind of
attack). SCC has caused extensive (and
expensive) problems due to the limited resistance of older forging alloys
initially used in the C-141, KC-135, B-52, C-130 and C-5 aircraft. These
problems have been recognized and, as materials have been developed for service
in the newer KC-10 and C-17 aircraft, the problem has been controlled. Besides potentially causing SCC problems,
the environment frequently will accelerate or enhance the fatigue process by
creating corrosion sites (pits, exfoliation damage, surface roughness, etc.)
where fatigue cracks will develop, accelerate the crack nucleation process, and
then accelerate the rate at which these fatigue cracks grow. In fuselage lap joints, the crevice
corrosion that occurs will result in pressure build up between the layers,
sometimes to the point where rivet heads will pop off and the joint will look
pillowed, such as shown in Figure 1.4.1.
Figure 1.4.1. Photo of Lap Joint Illustrating the Localized Pillowing Caused
by Crevice Corrosion Occurring between the Two Layers
There are several different features of corrosion that can be
used to characterize the severity and extent of damage. These corrosion metrics include thickness
loss, pitting, surface roughness and pillowing, deformation in the metal caused
by the excess corrosion by product produced between layers (see Figure 1.4.1).
Corrosion can grow in exposed areas, under paint, around fasteners,
between layers of skin, and inside structural components. Depending on the type of corrosion, it can
grow in depth and area. It can grow
along grain boundaries. Growth rates
are influenced by environmental and load factors. The impact that each of these characteristics have on structural
integrity continues to be the subject of current research, but will depend upon
the structure within which the corrosion is located.
Stress corrosion cracking is another form of corrosion damage
found in aircraft structures. As stated
in Tiffany, et al. [1996], “Stress corrosion cracking (SCC) is an
environmentally induced, sustained-stress cracking mechanism.” Of particular interest is the identification
of the operational need to reevaluate, possibly during ASIP durability and
damage tolerant assessments, SCC-susceptible components to look for potential
safety risks.
Currently the ALCs are operating under a maintenance philosophy
that has been termed “find it – fix it”.
Under this mode of operation, when corrosion is detected, it must be
dealt with by either repairing the damage, or replacing the component with the
damage. Corrosion is considered an
economic issue at this time, but the costs associated with maintaining the
aircraft in accordance with this philosophy are escalating. Correspondingly, the readiness of the fleet
is adversely affected. To respond to
this trend, the Air Force is pursuing the technologies necessary to implement a
corrosion management maintenance philosophy.
This so-called “Anticipate and Manage” mode of operation attempts to
make disposition decisions based on the impact of the damage to structural
integrity. This requires knowing the
condition of the corrosion damage through nondestructive inspection,
understanding the corrosion growth rates as affected by the environment, and
predicting the future corrosion condition using models of corrosion
growth. The present and future states
of corrosion can then be used in structural integrity calculations to determine
remaining strength and life.
Disposition may now include flying the aircraft with known corrosion
present, among other alternatives.
Economical disposition can then be made while maintaining safety of the
aircraft.
Other areas of ongoing research include understanding corrosion
growth mechanisms, corrosion inhibition and arrest, coating technology and the
replacement of chromate in coating systems.