combined stresses on fasteners
combined stresses on fasteners
(OP)
I have some design projects in which bolts will be subjected to shear and tension.
In order to determine the max. principle stress I've been using the formula:
Stress = [(tension stress)^2 + 3(shear stress)^2]^(1/2)
which was handed down to me from a (since retired) structural engineer.
Is this the best to use?
In order to determine the max. principle stress I've been using the formula:
Stress = [(tension stress)^2 + 3(shear stress)^2]^(1/2)
which was handed down to me from a (since retired) structural engineer.
Is this the best to use?





RE: combined stresses on fasteners
i've never seen your interaction equation ... it looks like a von mises equation.
i've seen Rt^2+Rs^3 = 1, but i prefer Rt^2+Rs^2 = 1 'cause it's way easier to use (you don't have to solve a cubic to get your RF) and it's slightly conservative.
RE: combined stresses on fasteners
rb1957, can you give me more info about your equations?
RE: combined stresses on fasteners
RE: combined stresses on fasteners
If your tension and shear on the bolts is created by external forces and not by tightening then they can be calculated by the methods shown on this site:-
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If your interested in the stresses during tightening then on the same site:-
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regards
desertfox
RE: combined stresses on fasteners
[(actual shear/allowable shear)+ (actual tension/allowable tension)]< 1.0
RE: combined stresses on fasteners
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In ductile metals, this is arguably the most accurate way to predict yielding due to combined loading. Now, just about everything is in some sort of combined loading, so it turns out that it's pretty useful.
Your bolts are likely seeing tensile loading (mostly from preload) and shear loading from external loads. So, in a sense the formula given to you is correct. However, it's a good idea to understand where it's coming from before you use it.
If the shear loading on your joint is significant (more than you can handle with friction), consider a rabbet fit. Due to tolerances, etc, bolted joints in shear have to slip & yield a bit before all of the bolts become loaded. A rabbet or close-fit pin in your joint will predictably carry all of the applied shear load in excess of the friction force.
RE: combined stresses on fasteners
I found the formula you are quoting or at least in a similar form to what you posted:-
Failure criteria: Refer to page Failure Modes
The notes on this page In order to estimate the design factors of safety it is necessary to consider the failure modes. The preferred failure criteria for ductile metals is the "Shear Strain Energy Theory" (Von Mises-Hencky theory). For a stress regime associated with a bolt i.e pure tensile stress sx combined with shear stress τ xy. The Factor of safety relative to the material tensile strength Sy..is calculated as follows
Factor of Safety = Sy / ( sx2 + 3 .τ xy2 ) 1/2
the above can be found at this site:-
htt
Scroll down to the title "failure critera"
desertfox
RE: combined stresses on fasteners