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Re: aluminum design

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On Jan 31, 2005, at 10:13 AM, Harold Sprague wrote:

When it comes to designing in aluminum, you must use first principles.
Couple of very important things to be careful of. Welding is very different than for steel. The 6000 series alloys lose strength when they're welded and the AA rules set out rules for handling the loss of strength. Unless you're very careful to place welded joints in low stress areas, you'll take a huge hit on member size. Also not everyone who says he can weld aluminum can actually weld aluminum. The 7xxx and 2xxx series aren't weldable in the structural sense.

Aluminum structures are usually a lot softer than steel structures which makes them susceptible to fatigue and fracture. The AA rules have provisions for fatigue whic may seem draconian, but don't assume they're too conservative. They aren't. Aluminum exhibits good elongation, but don't be fooled. Aluminum isn't as fracture resistant as steel by a long shot. Weld cracks will take off like a shot at stresses not far from the yield value, sometimes less.

One of the most interesting projects I ever worked on was an aluminum plate structure which had been morphed from an identically shaped steel assembly. The strength was fine, but stress concentration and weld details that were inconsequential in the steel version soon had the thing literally tearing itself apart under service loading.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw/


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