On Aug 24, 1:38 pm, Gunny wrote:
stress cycle amplitude. The reason some pre-tensioned bolted
connections (esp. shear) have better fatigue characteristics is
because the cyclic portion of the load is transfered via friction. The
bolt actually experiences a drastically reduced or eliminated cyclic
stress, thereby extending it's fatigue life even though the mean
stress of the bolt is much higher. Tension connections see improvement
through a different mechanism, but the result is the same - reduced
cyclic stress in the bolt and increased fatigue life.

Correct me if I am mistaken but here Matt is giving us an example
of a bolt that is preloaded in tension and then stressed (cyclically)
in tension.


Yes. Actually I touched on both situations - bolted plates that slide
past one another, and bolted plates that are pulling apart. They are
different with regard to how preload helps. I only brushed over the
different mechanism for tension connections because I anticipated the
direction the argument was going.

That is different from a bolt that is pre-loaded in tension to fasten
two surfaces and then subject to shear, right?


Yes.


Matt made it clear that bolted and riveted structures typically rely
on friction from the clamping force of the fasteners so that the
fasteners themselves typically see very little shear. I believe that
is correct. In particular, and Matt alluded to this problem, imagine
the precision required to evenly distribute the transverse shear
stress over many fasteners over a large surface, and then to
maintain that distribution over changing loads, thermal expansion,
etc.


Mmmm..details... In buildings, rivets are generally assumed to _not_
develop pretension, therefore a riveted (building) joint would not
have considered friction in its design. In practice, as the hot-driven
rivet shrank it would induce clamping in the joint. A little padding
of the safety factor sure didn't hurt. Only for certain bolted joints,
where we have good control over the important parameters and we
actually require the fixity of the joint, do we consider friction. My
only comment on rivets was to reference Chris Heintz's body of work.

I suggest that relying on the bolts to carry the entire load
without ANY load being carried by friction between the wing
attachment fittings and the wood will concentrate the stress
at the locations of the fasteners. This may well locally stress
the wood to failure, e.g. it may split. The clamping force of
the fitting distributes that load over a larger area reducing the
stress concentrations.

Well, this isn't anything that I disagree with Bud about. It is
definitely more conservative to assume that friction doesn't help you
out. I can certainly believe that aero designers don't normally factor
it in. It just doesn't make sense to me for a wood propeller due to
the body of research I've read, and the materials and magnitudes of
stresses involved.

I am far from clear on what constitutes 'proper' for
a metal piece bolted to wood. If the wood is clamped
too tightly, an increase is humidity may overstress it
causing it to split. If too loose, a decrease in humidity
may cause the joint to loosen too much.

Marc Zeitlin and Paul Lipps have posted some of their results in
Contact and on the web for using Belleville washers to combat that
problem as it applies to wood propellers. It's a great read.