local buckling and composite sandwich materials

[jlrp]

Hello,

I am using Femap to check the local buckling behavior of a composite material tube of roughly elliptic shape under axial compression + bending moment.
I am using 2D shell elements with laminate properties
my layups are a mix of 2D orthotropic materials (the composite plies) and isotropic material (the foam core of the sandwich)

I did a first buckling analysis with a monolythic layup and got an Eigenvalue of roughly 3.00 for the first buckling mode which is local as expected.
my second step was to increase the number of plies in my layup where the local buckling was happening and again as expected the Eigenvalue has increased to 3.50
the third step was to modify the first layup by adding a foam core where the local buckling was happening, but in this case, the Eigenvalue has dropped to 0.99 when it should have increased.
I did another check by changing the core material from foam to another isotropic material (6061-t6 alloy) and in this case the Eigenvalue has increased to 6.70 as expected.

So obviously something is wrong with my foam core material but I can't figure out what. I am using the values from the suppliers data sheet, and it is common practice in my industry to use this solution against local buckling.

I am doing something wrong but I don't know what
any ideas ?
Thanks,
Jeremy

 

[jotunn]

The first thought that comes to mind is that you're seeing a shear crimping form of buckling. Shear crimping will be picked up by an Eigen buckling analysis... Thick facesheets with thin, low shear modulus core exacerbates shear crimping. Thinner facesheets (if possible within the confines of facesheet stress and other constraints), thicker core, and/or higher shear modulus core would address the problem.

Beyond shear crimping, I'd have to see the model to make any other guesses.

 

[rb1957]

I'd look at the material properties (foam and 6061) to see what's different. I suspect one property (in the foam) is zeroed out (or very small) as a modelling thing.

by "foam" you mean a continuous solid (with low density or small cell size) as opposed to honeycomb.

another day in paradise, or is paradise one day closer ?