I'm having difficulty simulating the deflection of a thin disk. The actual phenomenon has a nonlinear relationship with edge displacement and looks like this,

I'm using a 5 degree slice of the disk with a symmetry constraint with fixed constraint on the inner diameter. OD = 24.4mm, load line diameter of interest is actually 23mm, ID = 10mm, thickness = 0.150mm, modulus of elasticy = 210 GPa. I made the mesh 3 elements thick, I've done 5 but it didn't seem to matter. I'm running the study as nonlinear as well. The force-deflection is completely linear though and doesn't match experimental at all.

Am I overlooking something? This should have some kind of geometric nonlinearity that increases stiffness correct?

Any help would be appreciated,

Thank you