On Aug 25, 7:28 am, wrote:
As to whether or not the engineers I talked to were aircraft
engineers, most definately they are.

Like I said, giving you the benefit of the doubt. If they had been
building engineers or bridge engineers I doubt they would have said
that friction isn't a oft used mechanism.

I stated in my first post that friction existed and carried load, but
simply that for aerospace structures it is never counted on to carry
load. You only consider friction when it works against you. That I
know is true. In your statements about why using friction in the wood
spar joint is not a good idea, I think you have begun to uncover some
of the reasons why it is true. Since most airframes are thin shell
material, most of these reasons apply just as well to metal as wood.

Yes, I was agreeing with you.

As to the statement that I clearly don't understand the factors
involved, you clearly do not understand what I said, the nature of
preloaded bolts, or even the S-n curves themselves. Improved fatigue
life due to preloading has nothing to do with friction. Friction may
improve fatigue life in the real world by spreading load over a larger
area, but the benefit of preloading on fatigue life is due primarily
to an effect that exists even if no friction is present at all.

That is true in tension splices, but not in shear splices.

Why
you think I need it pointed out that higher stress levels result in
shorter fatigue life is puzzling. Of course the higher the load you
place on a structure, the fewer cycles it will survive before failure.
What is hard to understand about that? What you apparently don't
understand is what constitutes a load cycle, how much is the load, and
what preload does to that. Preloading the bolt reduces the cyclic load
that it sees, since the load in a preloaded bolt only increases about
10% until the applied load exceeds the preload.

You said:

"It is also an elastic material (unless overloaded) and it also
will fatigue
more quickly when cycled back and forth from tension to
compression
than it will from repeated tension or compression alone."

Bud, that statement is wrong in so many cases that it had to be
pointed out. A member experiencing a 60 ksi swing from from -30 ksi to
30 ksi axial force vs a member experiencing a swing from 0 to 60 ksi
would meet the parameters laid out in your sentence. Both are
experiencing a 60 ksi cyclic load. However, the member all in tension
is due to fail first, completely contrary to what your statement says.

It sounded suspiciously like the guys who neglect to do fatigue checks
on a member because there wasn't a stress reversal. That's why I
jumped on it. If you had qualified that statement better, I could have
accepted it.

Depending on the elasticity and thicknesses of the materials being
fastened, my experience is that reduction to 10% of the original cycle
is not a given and would typically be very optimistic. This can be
especially true of a wood member clamped with a steel bolt.

When the prop bolts
are allowed to lose their preload, the full applied load becomes the
amount of cyclic load that causes fatigue. This is best demonstrated
by giving an example. Take two identical bolts, having a breaking
strength of 5,000 lbs each, and preload one to 2000 lbs, and none to
the other. If we now begin to subject both bolts to the same cyclic
loading of 1500 lbs, where the applied load is increased from 0 up to
1500 and then reduced to zero again, the bolt with the 2000 lb preload
will see a cyclic load of only about 150 lbs, whereas the un-preloaded
bolt will see a cyclic load of 1500 lbs, and will obviously fail much
sooner. Same bolts, same loads. The meaning of this is that if you
keep the prop bolts properly preloaded or torqued as it is, then BOTH
the bolts and the prop hub see a much smaller cyclic fatigue load than
if you allow them to become loose, thereby greatly increasing the
cyclic load that they see, and increasing likelyhood of failure.

You've described the preload mechanism behind a typical tension
splice. As I said above, the reduction in cyclic stress is dependent
on elasticity and thickness of the members being bolted together. I
alluded to that mechanism in my previous post. I didn't elaborate on
it, because I'm not convinced that it any bearing in a wood propeller
attachment, where the shear between prop and the hub faces is what is
causing the failure. If you ignore friction, then how else does pre-
loading the bolt help? The force in the bolt is effectively
perpendicular to the shear, until which time the bolt has bent over
substantially.

As for S-n curves, there are more than one type. The one
relating to what I am talking about are the ones that show S vs N for
different stress ratios. The stress ratio is the fraction equivalent
of the maximum to minimum load. For example, something that is loaded
in tension to 25000 psi, followed by being loaded in compression to
25000 psi back and forth, will have a ratio of -1.0 ( +25000 tension/
-25000 compression). Something loaded to 25000 psi tension that is
reduced to 10000 psi tension and back and forth will have a stress
ratio of .4 (10000 tension/ 25000 tension). The S-n curves show that
the amount of cyclic load that structure loaded with a ratio of -1
will fail far sooner than one with a ratio of .4, even though the
maximum stress level is the same. You can look in Mil-Hnbk-5 or
elsewhere for S-n curves to verify that.

These are precisely the diagrams to which I am referring. Your example
seems somewhat contrived, however. How would a bolt achieve a stress
ratio of -1 in axial loading (ie, as specified in your example above)?
It is also a stretch to say that the maximum stress would remain the
same. Both variables change, and maybe only one time in ten would pre-
load push it outside the gamut of acceptable values, but that is
enough to void any blanket statement such as above.

If your argument is that you were discussing +/- shear, then how
exactly does the axial pre-load (substantially) affect the cyclic
shear loading? We have frictionless mating surfaces in your examples
remember, and the pre-tensioning is perpendicular to the developed
shear.

The best book to explain all this is "Mechanical Engineering
Design" by Joseph Edward Shigley, Professor at the University of
Michigan, chapter 8, "Design of Screws, Fasteners, and Connections".
It is THE most widely used text on the subject in the top engineering
schools of the country, and has been for many years.

MTU alum. Got it.

Regards,
Bud
M.S. Aerospace Engineering