Then Fmax = 920 x √(2p/p), or Fmax =
1,301 psi. Still not enough to create
stress beyond the endurance limit.
This piece should still have a long
However, let the radius of that
crack become small, such as 0.001X
(and this is still a large radius of
curvature for the end of a small
crack), and there is a large increase in
the stress. Fmax = 29 ksi.
Now it is above the endurance
limit. Fatigue life is no longer long.
In fact, it is probably less than one
hour for our Safari control tube.
Learn From This Article
The purpose of this article is to
share some information that is both
illuminating and shocking. Not all of
the mechanical engineers I talked to
are aware of the problems created by
anodizing. Not all of the anodizing
experts are familiar with the stress
concentration as the primary vehicle
in the reduction of fatigue life due
to anodizing. Being a Safari pilot
strongly increased my interest in
why that control tube should fail in
fatigue at such light loading.
I now have a feeling why some
of the folks at the plating shops just
said, “Do not plate or coat flight-
critical components unless you really
know what you are doing and can
offset the problems created by these
I have a better understanding of the
processes involved in the secondary
shaft failures that have plagued other
experimental helicopters. Those
shafts were designed to handle the
loads with a safety factor, as was
the Safari control tube. The cracks
that formed raised the stress level to
well over the design limits with the
Stu Fields and his wife, Kathryn,
publish Experimental Helo magazine.
He can be reached at 760-377-4478 or
760-608-1299 and via e-mail at eh@