Linear Elastic material behaves as Neo-Hookean in ANSYS?

I solve the static bending problem for thin plate under uniformly distributed load acting orthogonally to the plate. I

need the solution up to quite large values of deflection (100

thicknesses, which implies the strains about 100%), but I want to take

into account only geometric nonlinearity and not the material

nonlinearity. I use elements which support large strains (SHELL181,

SHELL281). The software is ANSYS Mechanical APDL 12 and 13. Also, I use "Large displacement static" option for

solution.

Although I use Linear Elastic Isotropic material, the value of

deflection for a given pressure coincides with the value, obtained for

Neo-Hookean material. That is, for not very high deflections (less

that 20 thicknesses), where Neo-Hookean law almost coincides with

linear Hooke's law, ANSYS results are close to those that I have from

analytical approach (with only geometrical nonlinearity). But at

higher deflections, where the difference between Neo-Hookean and

Hooke's law is significant, ANSYS seems to use Neo-Hookean material

model ignoring my indication to use Linear elasticity.

Could someone give me any hint, how I can obtain solution without material nonlinearity?