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Date: Wed, 17 Aug 88 23:23:56 EDT
From: attcan!utzoo!henry@uunet.UU.NET
Subject: Can current CAD/simulation methods handle long-term fatigue analysis?

>...  Are current supercomputer simulation methods capable of
>handling the complexity of long-term stess, fatigue, corrosive environments,
>etc., all of which were (apparently) factors in the Aloha incident?  Also I am
>not at all sure that such things were handled any better before wide-spread use
>of CAD -- I am just asking the question.  Any aeronautical engineers out there?

Well, I'm not an aeronautical engineer, but maybe I'll do... :-)  As I
understand it, metal fatigue in general is poorly understood, and there
is really no way of calculating it.  The whole area is still very much
rule-of-thumb engineering plus empirical testing.  There are rules that
give a rough idea of the fatigue life of an airframe, after which a big
safety margin is added (we're talking factor of 2, not 10%).  Even this
is only a tentative number.  Most any large, volume-production aircraft
will have one of the early prototypes shunted off into a corner to be the
"fatigue test" specimen, which means that it spends years (literally)
having its wings bent back and forth and pressurization cycled up and down
and generally having stresses applied in a speeded-up, exaggerated simulation
of real service life.  The objective is to keep the fatigue-test aircraft
well ahead of all the real ones, while watching it carefully for signs
of fatigue cracks.

Even so, one still gets surprises.  Not just the occasional accidents, but
also more mundane discoveries of cracks; fatigue-life estimates are not
trusted very much, and aircraft are inspected regularly.  Now and then
such an inspection yields a surprise, and the manufacturer or the FAA sends
the other users of that aircraft a telegram saying "inspect area XXX right
away and let us know if you find cracks".  This seldom makes the news, but
it happens with some frequency.

I guess this is not a bad example of how to manage a poorly-known risk...

				Henry Spencer @ U of Toronto Zoology

Newsgroups: comp.risks
X-issue: 7.42
Date: Wed, 31 Aug 88 23:08:24 EDT
From: attcan!utzoo!henry@uunet.UU.NET
Subject: Can current CAD/simulation methods handle long-term fatigue analysis?
Re: RISKS-7.38 and 40

> Metal fatigue can be calculated with a reasonable amount of accuracy. 

It is possible that my information is out of date.  However, Aloha Airlines
might dispute the matter!  If fatigue calculation for real structures under
real conditions is indeed accurate and practical, it is not being used very
widely, for some reason.  I'd be interested to see references on this.

> Most aircraft design use a 10% to 20% safety factor. A safety factor of
> two would make an aircraft so heavy it would never leave the ground.

For structural weights, yes, 10-20% is normal.  But what I was thinking of
was fatigue life, which -- at least in the military aircraft that are the
ones I know most about -- is treated *very* conservatively.

Henry Spencer U.Toronto Zoology uunet!attcan!utzoo!

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