panel post buckled capacity

[40818]

Hi all,

I'm looking for some sort of confirmation regarding whether a panel web can support shear loads after it has buckled under a uniaxial compressive load..

Cheers in advance.

 

[rb1957]

if you're applying the loads sequentially, possibly not ... a panel that had buckled in compression probably isn't going to react much shear.

if the loads are applied concurrently (more typical), then this sounds much more like a diagonal tension problem, so the panel would have quite some shear ability.

 

[40818]

Its a bit of a tricky one,the panel has essentially a biaxial compression and shear load applied. However, it is stable under pure shear alone, but becomes unstable by the application of the main compressive load without any other loading. The loads are applied together. If it buckled under the shear but was initally stable under compression then it would be easy, but i'm not sure with this one.

 

[SWComposites]

The answer is yes, but the question is how much shear can be carried. For that, you will have to run a non-linear post-buckled FEA, and then figure out how to predict strength in the post-buckled regime. Probably need test data to validate any predictions.

 

[rb1957]

if the panel fails (buckles) under the compression loads alone, then it's buckled ... sure it may be able to redistribute load to the stiffeners supporting the panel.

if the loads are applied concurrently, i don't think your question arises ... at some ratio of loads the combined shear and compression loads buckle the panel and it may continue as a diagonal tension panel. uniaxial compression and shear is textbook diagonal tension, bi-axial compression and shear isn't ... drag out the FEA !

i suspect that in your case the panel is basically buckling in compression (rather than shear) and probably won't have much post-buckling strength ... it'll depend on redistribution.

 

[RPstress]

It's my belief that a panel can still carry shear loads after exceeding the compression buckling load, but only if collapse in compression is prevented. (E.g., the compression is applied by thermal forces, which is essentially an enforced displacement, or there is other structure which will prevent excessive axial deformation*.)

Post-compression buckling the elastic forces in the panel do not drop, and continue to increase for additional applied compression; however the compression stiffness becomes so low that small additional forces over the buckling load cause very large deflections which rapidly lead to material failure. However, a lack of compression stiffness does not markedly decrease shear stiffness. (Though any applied shear will hasten collapse in compression.)

I'm not aware of any classical references which address post-buckling under combined loads. (I'm pretty sure that Timoshenko and Southwell both present elastic behavior post-buckling only under uniaxial compression.) So, the only way to assess post-compression buckling capacity for shear is by large-deflection non-linear FEA (unless someone knows better). Because stiffness never goes negative the usual Newton methods can be used for NL convergence.

* The additional structure can be quite low stiffness and low strength.

 

[kellnerp]

This is a crippling problem IIRC. I am going to assume that the OP has some kind of stiffeners surrounding the panel. If the panel buckles under a compressive load and then a shear load is applied there should be a tensile stress along a diagonal in the buckled panel. It then becomes a question of how much load the surrounding stiffeners can take.

Just my 2 cents.

TOP
CSWP, BSSE

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[graemew]

Uniaxial compression with a superimposed shear is wing torsion box upper skin stuff. Provided that the panel is bounded by stiffeners (aka stringers and ribs) the panel will carry substantial shear even though the onset of buckling may occur at relatively low stress.
The mix of shear and uniaxial compression will change the angle of the diagonal tension buckles compared with straight shear.
The solution resides in classic diagonal tension buckling per NACA TN 2661 (about May 1952) available from the NASA web-site. It deals with both elastic and plastic behaviour.
FEA may produce pretty pictures but for accuracy the old way is still more accurate. Takes a while though.

 

[40818]

Thanks for all the replies regarding this. I think that the general consenus seems to be that for concurrent loading its simple DT analysis, but when the panel becomes unstable by uniaxial compression prior to be loaded in shear then the ability to carry the shear load is degraded, by how much though remains unclear.
Although my panel is part of a larger structure and is bounded by stiffeners (that may or may not offer simple support) that will have any redistributed load once it has become unstable, i'm still trying to determine the reduction in shear stiffness of the plain panel due to being buckled. My initial feeling is that the change in the shear stiffness of the panel will be related to its buckling ratio.

I have a simple piece of paper in front of me (and a glass of vino), and badly re-creating the DT tension field by hand movement. However, if i form the paper to a curved shape then apply the same movement, the paper forms a completely different shape. Not very accurate or even probably applicable, but it does make me think that the shear stiffness post compressive buckle is the key to the amount of shear it can support subsequently.

 

[blakmax]

Can I throw in a concern of mine about post-buckeld structures? How do you repair them? If you apply either a bonded or mechanically fastened repair, then the additional skin stiffness changes the buckling characteristics which probably means buckling occurs at a higher load. If it is a composite structure, then there is a risk of bearing failure at the fasteners adjacent to the repair. Is my assessment correct?

 

[SWComposites]

How do you repair them?

Very carefully! Often the repair doublers cover the entire bay (depends on the size, specific configuration). Can be much more difficult for composite structure. Tests of post-buckled panels with specific repairs should be conducted to validate the repair designs.

 

[blakmax]

Thanks SWC My assessment is that if we must apply such large repairs, then the advertised weight advantages of composite post-buckled structures would dissappear within a few years of operation if frequent repairs are required?

 

[SWComposites]

The typical bays are not that large, so the repairs may not be as large as you imagine.

Also, there will the option of bonded composite repairs.

And, hopefully, the composite primary structure is more damage resistant.

 

[kellnerp]

I suppose if you figured on patching an area as much as has been inferred over the life of the aircraft you might come to the conclusion that making it a little heavier from the get go would be in order to make it more resistant to what ever the cause of the defects is going to be. Oh, now there I've done it and said the heavy word.

TOP
CSWP, BSSE

http://www.engtran.com

http://www.niswug.org

"Node news is good news."