A before photo of the Safari, which received an Oshkosh Grand
Champion Rotorcraft award.
did not have a well-defined endurance limit and would
fail no matter the stress. However, for the purposes of
quantifying and establishing estimates, the Aluminum
Association Inc. published Aluminum Standards and
Data 1988, listing an endurance limit of 10,000 psi for
500 million cycles.
Various other sources reported, for wrought aluminum,
a limit of somewhere between 8,000 and 18,000 psi
depending on the alloy. This number should not be used as
an absolute limit; however, I am going to use it as a barrier
between shortened fatigue life and long fatigue life.
Back to the Question
“How did the control tube fail in fatigue when it was
stressed to a level much lower than the endurance limit?”
The anodizing experts said that anodizing would cause
this. But how did the process cause stress in the Safari
control tube to exceed the endurance limit? There was
no evidence of a pre-existing crack or a notable surface
discontinuity being present prior to the anodizing that
might have led to a stress riser.
No matter my opinion or yours, the fact is that an
anodized control tube, under a relatively low load, failed
in a helicopter, causing a crash. To my knowledge, no
other Safari with bare aluminum control tubes has had a
fatigue failure of that tube, and that includes helicopters
with many more flight hours.
I have heard of stress concentration and stress risers but
only believed the effect was caused by a decrease in the cross
section of the material. Not so. Consider the following.
The maximum stress felt near a crack occurs in the area
of the lowest radius of curvature (the sharper the point
of the crack, the higher the stress). In an elliptical crack
(elliptical was chosen for mathematical descriptive ease)
of length 2X, under an applied stress of F, the stress at the
pointy end of the crack is given by:
An after photo of the same Safari helicopter.
The control tubes found at the crash site. This tube is cleanly
broken, not bent to failure by the crash, but fatigued as a result of
the propagation of a crack energized by vibration.
The ends of the control tubes that a test lab determined to be
the result of a fatigue failure.
Fmax = 2F √(X/p)
Here, p is the radius of the curvature of the crack tip
and F is the stress normally computed due to a load on
a given cross section. In our Safari example, F=460 psi.
Note: The stress concentration factor is a function of the
crack and its sharpness, but not of its width.
Applying this equation to the control load experienced
in the Safari, we have: Fmax = 2(460) √ (X/p).
Now, for argument’s sake, let’s say we don’t have a
crack but a drill-stop hole, and it is a circle where X = 2p.