On Jun 24, 7:48*pm, wrote:

It is incorrect to say that all metallic materials have a finite
fatigue life.

There are indeed metals that have effectively infinite fatigue lives.
Many steel and titanium alloys have that property. Though even among
those there is some evidence that the stress/cycle graph never goes
entirely asymptotic, I agree that it is fair to say that their fatigue
lives are essentially infinite.

Aluminum, however, does not have the "knee" in the stress/cycle graph
that takes it effectively parallel with the X axis. With aluminum, the
curve heads inexorably towards the X axis.

Metal structures have been certified with an infinite
fatigue life.

That is almost always true for aircraft certified under the old CAR 3
regulations which did not address fatigue. It is even true for some
aircraft certified under the more modern Part 23 and JAR22
regulations. Unfortunately, the map is not the territory: Just because
the CAA or whoever certified that it is so doesn't mean it is actually
so.

There is an EXCELLENT, plain language discussion of the design
considerations and fatigue calculations in the Blanik repair and
overhaul manuals published in the mid 70's.

That sounds like valuable material, I would definitely like to read
it. How can I get a copy of that documentation?

Corrosion is the real intractable issue with metals, and causes much
more [cumulative] damage in aircraft structures than fatigue alone.

I completely agree there. Corrosion is and should be a far more
pressing concern that fatigue alone. Very often, failures that
initially appear to have resulted from fatigue are actually more
directly caused by corrosion that reduces the effective cross-
sectional area and causes stress risers that result in local yielding
and accelerated fatigue.

Thanks, Bob K.