Crippling Cutoff Stress

[luket]

In all of the crippling calculations I've done over the years, I've always used Fcy as the cutoff stress. I'm trying to get a little more out of something, and noticed that in McCombs' Bruhn Supplement, at the end of Art. C7.30, he mentions people have used 3 different cutoffs: (1) Fcy, (2) Fco (per Bruhn Table C4.2) or (3) Ftu. Does anyone else have any experience or practice using either (2) or (3)? Or, is there any other data or guidance for using these? Thanks!
-Luke

 

[SWComposites]

What is the material?
What is the cross-section geometry? b/t value?
Fco is sometimes used for ductile metals.
Ftu is likely too high.
Have you checked Niu's or Peery's texts?

 

[rb1957]

Ftu would applicable for stable sections (which have a very high Fcc) under plastic bending, maybe also axial load. Obviously this is least conservative, maybe keep for multi-loadpath situations, and/or sections which you will test.

Fcy would be applicable for stable sections under elastic bending or axial load. Obviously less conservative than Bruhn's cut-offs (typical 75% to 85% of fty), maybe check other failure modes (like lateral buckling) which maybe Bruhn's factors give you some coverage for.

Maybe McCombs is "just" saying that Bruhn's cut-offs are very (overly?) conservative.

This might may for an interesting thesis project.

Maybe have a peak at what a modern manufacturer (like Airbus) does.

A key part of the answer is going to be "are you going to test the section ?", which allows you more tolerance in your analysis.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[luket]

Thanks all.
Material is 2XXX AL extrusions.
Relatively thick sections, so the cutoff is affecting the final section result substantially.
The load is an ultimate load case, so plastic effects are allowed.

I hadn't checked Niu, as I didn't remember anything about higher cutoffs in it. Interestingly, one of his charts Fig 10.7.10 (red book) shows graphs with Fcc/Fcy that goes greater than 1 and then says Fcc/Fcy = Ftu/Fcy. However, in his example using that figure, he cuts off at Fcy.

Peery is using fairly different methods, but it does look like he can exceed Fcy when considering plastic buckling. I need to look at it a little closer to make sure. Thanks for the suggestion, I hadn't thought to look at his text for crippling.

I'll have a look at some of the proprietary resources too and see what I find.

I won't be testing the section - analysis only.

 

[rb1957]

I would want b/t < 10 if I wanted to "rationalise" not using cut-offs.

I think McCombs doesn't use cut-offs, and has a method that's less conservative than Bruhn's (as I remember it).

I would want to use one analysis method consistently, sounds like McCombs approach is widely used.

But ... if your work is to be approved by a delegate, best to get his thoughts first (before you get into a food fight !).

If this is a "one off" ... kill it, b/t about 6 ... this thing ain't ever going to cripple ! A pound of Aluminium ain't worth anything.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Ng2020]

If you can show there are no local instabilities in the section's elements (eg all flanges are stable *beyond* ultimate) that might otherwise lead to a crippling mode, then you could push it a bit as crippling becomes redundant. Otherwise I'd cut it off at Fcy. Unless you have a really good idea of what's gping on in this situation and are confident it will be capable to ultimate, best to err on the side of conservatism.