At 08:29 PM 9/23/2008, neil N wrote:
>Does anyone have some pointers on a better design for these parts?

Neil, that driver's side bracket is designed to fail right where it
did -- it's very stiff with the angle iron and the weld fillet, and
very stiff where it's bolted down, and flexible in between, with a
fairly abrupt transition from the weld fillet to the plain
strap.  You'd expect it to fail right at the edge of the weld, which
seems to be what it did.

Adding a concave triangular piece up the inner edge, along with
doubling the thickness of the bolt strap would equalize things
considerably, make it pretty stiff all along.  That way the stresses
would distribute themselves better.

Generally speaking any transition between stiff and flexible will
concentrate stresses at the transition point, and the more abrupt the
transition, the higher the stress.  That's how cracks in e.g. glass
propagate -- the transition at the bottom of the crack is extremely
rapid, and the stress is essentially infinite.  So for example a
concave fillet such as from brazing will distribute stresses much
better than a convex one, because of the more gradual transition.

If you were to make a clear plastic model of a fitting and stress it
while looking at it between two polarizing filters (i.e. one at the
light source and one at the eye) you could see the stresses directly.

Cheers,
David

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"