I did not know that Butch. Major gap in my self taught mechanical
engineering education. Why? What is the theory?

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Paul, materials as a rule require more Force to Pull apart (Tension), than

to

Guillotine (Shear)..........Here's the actual rule of thumb:
"One must multiply the Tensile Strength by 0.6 to obtain the estimate of the
Ultimate Shear strength of a material, multiply Tensile Yield strength

by .557

to obtain the Yield Strength in Shear. This of course is for Single Shear,

and

sans Contact Friction.

Unfortunately most people (Read Engineers), do not practice this and usually
learn about it the hard way.......Thank God that the average "Tinkerer - Eye
Baller" has the tendency to go the other way (read, over size) their joints.

                                     Hope this clarifies it.
                                            Butch

Isn't this a function of the hardness of the materials the joint
is constructed from? For example; aluminum being softer than
steel will locally yield to reduce the stress concentration.

Paul Lamar

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Hi Paul;
Didn't mean to leave you hanging, I meant to reply as soon as you returned from
your vacation but you returned earlier, my silicon reminder was set for the
10th.
Hence........You're correct Paul, the compressive loading of the material
(Aluminum) will be lower than the steel bolt and will yield at the
joint....however by doing so the role of the bolt will start to switch over
from Shear to Tensile loading which theoretically means it's strength capacity
INCREASES........Here's the kicker, this is providing the loading is the same,
constant, in uni-directional ONLY ie: holding a hanging load. Barring
environmental conditions, it will hold the load infinitely......On the other
hand if it's a cyclic load, bi-directional ie: actuator application, the
strength capacity DECREASES, since the bolt will be subjected to bending
stresses, the strength of the joint deteriorates, hence the joint is doomed for
rapid, catastrophic, failure.
The obvious sure fire solution to this is to introduce an appropriately
designed bushing to the joint and design for Shear loads.

                                                  Butch

 
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