Stiffened panel- crippling stress under Uniaxial compresion vs Bending

[JSF10]

Hi all, I am looking into buckling of unflanged stiffened flat panels under BENDING.

There are many reference for the above problem under UNIAXIAL COMPRESSION but NOT due to BENDING. How does the buckling stress and crippling stress defers from UNIAXIAL COMPRESSION to BENDING?.

Is the compression stress to cause crippling nearly the same for both UNIAXIAL COMPRESSION loading and BENDING?

Thank you.

 

[rb1957]

how many stiffeners ? (ie how many separate sub-panels)

stiffener crippling, or panel buckling ?

"unflanged stiffener" = simple angle stiffener, or integrally machined panel ?

in-plane bending, or out-of-plane ?

 

[JSF10]

Hi, thanks for the reply. Say 6 sub-panels. I am looking for stiffener crippling due to out-of-plane bending of the panel and to see how it will compare with uniaxial compression of the panel and to see how the panel buckling stress and stiffener crippling stresses differs.

Also would like to know in-plane bending of the panel too.

I am looking for unflanged integral stiffeners but would also like to know how it differs with simple angle stiffener.

Thanks.

 

[rb1957]

so you're looking at compression failure of the stiffener flange (as opposed to panel failure).

if it's a simple angle stiffener (ie without a stiffening flange on the free edge) then i think you're looking at a panel (being the stiffener web) in compression rather than a true crippling failure. i'd plot the variation of stress across the web (ie through the bending stress field), and see if a "rational" value less than the maximum leaps out ... it'll either be mostly axially loaded or mostly bending.

 

[JSF10]

Thank you. I get your point.

But if one wants to calculate the crippling stress of the unflanged-stiffener due to BENDING will the results agree closely with crippling stress of the unflanged-stiffener due to UNIAXIAL COMPRESSION. Or will they both differ by considerable amount.

 

[SWComposites]

JSF10 - what is your specific stiffener cross section? Blade? I-section?

 

[JSF10]

Panel having unflanged integral stiffeners with the dimensions skin thickness (t) = 0.080 in, stiffener thickness (ts) = 0.150 in, stiffener pitch (b) = 2.500 in and stiffener depth (h) = 1.550 in. The panel is to be manufactured from D.T.D. 5020 aluminium alloy.

I can work out the crippling stress of the unflanged stiffened panel under uniaxial loading using ESDU 70003. However what would the results be for bending of the panel. Will both (uniaxial loading and bending) results agree closely with each other or differ greatly.

 

[SWComposites]

No, they will differ, likely greatly.

In the axial compression case, the stress on the stiffener is uniform.

In the bending case, the stress on the stiffener is not uniform (essentially a moment in the plane of the stiffener. The crippling (failure) of the panel will depend on whether the skin or stiffener side is in compression. If the skin side is in compression, then you can work out the crippling stress for the skin and from there the bending moment required to reach that stress. If the stiffener side is in compression, then you will need to figure out the portion of the stiffener that is in compression, then the affective area for crippling, and from that the bending moment to reach crippling of the stiffener.. There might be something in Bruhn for this case (I'm too lazy too look it up now).

SW

 

[rb1957]

a book i've used used b = 2/3 of the flange in bending, but really crippling requires an angle in compression, and in bending the lip is in uniform compression and the effective web has the varying bending stress, so 2/3 the distance to the NA sounds reasonable. with this geometry the compression allowbale is slightly higher in bending than in axial load.

if you don't have an angle in compression, and only have a standing leg, then that looks like a long, narrow panel loaded in varying compression, with a panel allowable like kc*pi^2*E/(1-v^2)*(t/b)^2

 

[JSF10]

Thank you very much for both of you. Much appreciated.