Does spar cap often cripple?
Does spar cap often cripple?
(OP)
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
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





RE: Does spar cap often cripple?
Cheers
Greg Locock
RE: Does spar cap often cripple?
Regards
A Connell
RE: Does spar cap often cripple?
RE: Does spar cap often cripple?
RE: Does spar cap often cripple?
RE: Does spar cap often cripple?
Cheers
Greg Locock
RE: Does spar cap often cripple?
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.
RE: Does spar cap often cripple?
Max load = 3/4*Sy*tweb*(N+2k)
N = length of load bearing on flange,
k = outer flg surface to web fillet toe
RE: Does spar cap often cripple?
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
RE: Does spar cap often cripple?
"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
RE: Does spar cap often cripple?
Cheers
Greg Locock
RE: Does spar cap often cripple?
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.