Compression buckling of polymeric beam/column

Hello,

I have a problem with a stacked colum of elastomeric membranes (natural rubber or silicone).

I am looking for a compression buckling criterion that applies for hyperelastic materials in presence of large deformations (up to 400%).

I've read about Haringx formula, that should apply for these materials, but I suspect that it is valid for small strains only.

My question is:

- is there a suitable buckling criterion for this case?

- If Haringx formula applies, are the shear modulus, G, and the Young modulus, E, referred to the undeformed configuration or to the actual configuration? Because, for hyperelastic matrerials, G and E are not constant...what about the column cross section and length (actual or undeformed)?

 Thank you in advance!