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Old 08-17-2010, 03:39 PM   #72
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Hmm, I realize this is probably not the forum to go too deep into the engineering of structures to resist fatigue, but some good points have been raised already. As someone who designs structural parts and machinery I'm concerned regarding the explanation given so far by CFM.

The control arms that failed appeared to do so after less than 4 months of use. That sounds consistent with a low cycle fatigue failure.

The discussion in the other FC thread was that the tube material was out of specification (high carbon instead of whatever grade they used since they are not saying, My guess is the planned to use ASTM A513 1020 or 1026). This would be a plausable reason since trying to use the same weld procedure with a high carbon content steel would cetainly give you very brittle welds prone to cracking and fatigue failure. (This is contrary to the statement made above that it was a "weak spot" in the metal.)

My question is:

If the issue is simply poor quality control at the manufacturer due to incorrectly identifying tubing material, why are CFM requiring gussets be added ? These won't help if the same material identification failure happens again.

So that leaves the question of whether there is some other issue ? Has any FEA and fatigue analysis been done ? Has the weld procedure been reviewed ?

Personally I'd like to see the proposed solution address the original cause of the faulure. Everything else is just fluff.
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