Does spar cap often cripple? 1

[haseg]

I'm sizing spar cap and have some questions which concerns to crippling stress.

Does spar flange cripple often ?
Is the actual stress of spar cap needed not to exceed crippling stress ?
(Should I keep cap stress under crippling stress?)

<Backgrounds>
I've read Bruhn and NIU's pink book.
From these books I got impression that crippling analysis should be applied to relatively light structures such as corner flange of pressed rib.

My investigating spar is fully machined from upper flange to lower flange.The spar is attached to machined skins.
I feel it is very conservative to apply crippling analysis to this spar flange.

The internal load used for stress analysis is based on linear analysis.
In ultimate load condition flange load's re-distribution will occur.

Any help would be nice. Thanks

 

[GregLocock]

They don't fail often precisely /because/ people analyse them. You are in a conservative business, get used to it.





Cheers

Greg Locock

 

[Aconnell]

Based on what I've seen in some of the OEM stress reports that I have read the Spar Cap is checked for crippling. The critical load case usually seems to be the landing loads placining the lower Spar caps in compression. Keep in mind that I work mostly on the liaison side of the house so I am no expert when it comes to stress analysis.

Regards

A Connell

 

[tmoritz]

Pardon my naiveté. Could you describe what you mean by crippling?

 

[btrueblood]

Crippling refers to buckling of the flanges of a beam in bending.

 

[btrueblood]

As opposed to buckling of the entire section (Euler or column buckling)

 

[GregLocock]

As btb says, it is a local instability of the flange, it is why a commercial I beam has flanges of the proportions that they do. A simple beam bending analysis would tell you to create I beams with very wide flanges which are very thin. This is the most efficient use of material, but practically useless.





Cheers

Greg Locock

 

[haseg]

Thanks guys,
any comments were helpful and interesting for me.

My understandings about crippling are ;
>It's a kind of inelastic buckling.
>We must resort emperical formula or conduct tests to calculate crippring strength or hi-ended FEA(fine mesh and non-linear).

Anyway, since I posted here at first, i've been striving for spar cap sizing.
Keeping GregLock's advise in my mind, I've tried to be conservative and made sizing of compression members by crippling stress.
Then, as Aconnel mentioned, lower spar cap around gear bay was critical in landing load case. It was amazing but maybe ordinary for experts.

On the other hand, I regret for conservatism. And I hitted another question.

In my case, upper cap was critical for crippling in positive gust load case.
Spar is fully machined from 2000 series aluminum alloy plate.
Upper skin is machined from 7000 series alminum alloy plate.

This time, I've caluclated crippling stress, substituting Fcy of 2000 series alloy for crippling formula.

Generally known, Fcy of 2000 series alloy is considerably lower than that of 7000 series alloy.
This fact puts weight penalty on me.
If I were able to substitute 7000 series Fcy, the spar will be 10% lighter than current weight.

Does anyone know a resonable method for crippling analysis of flange element which is conected to stronger alloy skin?

Thanks.

 

[arto]

not quite aircraft, but the AISC steel book has a simple analysis of I-beams when web reinforcement is required.
Max load = 3/4*Sy*tweb*(N+2k)
N = length of load bearing on flange,
k = outer flg surface to web fillet toe

 

[GregLocock]

I'm sure Bruhn C7.7 could be adapted to solve that problem, although, again, a conservative approach is to ignore the skin material other than that joined directly to the flange.

Also it is not necessarily inelastic, you can have elastic buckling of the flange. FWIW I did a project on this at uni, but other than the phrase 'moment softening method' I'm afraid I can remember nothing relevant. The agreement between the calculations and the tests was good.





Cheers

Greg Locock

 

[SuperStress]

The best description of the difference between local buckling and crippling is presented in class notes from a stress analysis class taught by Bill McCombs:

"To understand what local buckling and crippling are, consider a zee section subject to compression load. It has two types of "plates", the flange elements which have one edge free, and the other edge supported by the web. The web element has no edges free, the edges being supported laterally by the flange elements.

"As the compressive load is increased, assuming the zee's length is too short to buckle as a column (general instability), the compressive stress, P/A, increases and at some point the flanges will buckle... in a long curved manner along their free edges. Then, as loading is increased further, the web will buckle, but in a different mode. Since both of its sides are supported (by the flanges) it has no free edges and buckles...(with) the wave length being very nearly equal to the web height. The above phenomena are called local buckling, which is not, in itself, a failure.

"As the load is increased further, the "corners" of the zee take the additional load (since the flanges and web have buckled and cannot take more load), and eventually they, too, buckle. When this happens, all elements have buckled and a crippling failure occurs. Therefore, crippling is a failure which occurs when all of the cross-sectional elements have buckled."

Yes, this is frequently the limiting factor in aircraft wing design. Many a full-scale static test has failed due to instability failure of wing upper surface structure.

SuperStress

 

[GregLocock]

That's a sensible differentiation in terminology, which I was not aware of before. Do you agree that even a crippling failure can be elastic? I imagine that it is rare.

Cheers

Greg Locock

 

[SWComposites]

SuperStress's description from McCombs is not quite correct. The corners of the section do not typically buckle, rather they yield (in the case of a ductile metal) or fail (in the case of a brittle material, such as a composite) thereby causing collapse of the column. Crippling is essentially failure of a post-buckled section.

Greg - Local buckling is elastic; Crippling is not elastic since it involves yielding or failure.

Haseg - you cannot safely use a higher Fcy value just because the spar is attached to the 7000 skin without having test data to justify such an assumption. The typical crippling analysis approach for metal structures where the Fcc values for the different elements of the cross-section are averaged is only really valid for cross-sections built with the same (or very similar) materials.

Steve W.