Geometric nonlinearity

Hi,

I have used structural mechanics to simulate an indenting sphere on a half space polymer film. I am changing the elastic modulus of the polymer film to go from hard material to soft material.
I am trying to reach a certain force using weak constraints. When I use the elastic moduli from some GPa to around 80 MPa for indentation of 100- 400 nm, very well I can use linear elastic physics without selecting the box "consider geometry nonlinearity".

However, as long as I choose the elastic modulus 60 MPa or less with indention between 500 nm to 600 nm (I want to reach to a certain force and as the material is soft indentation is high), the simulation fails. It does not stop but a big triangular max pick appears at around 500 nm. However if I turn on "consider nonlinear geometry " and change the poission ratios, it works. but then I reach to a force at much lower indentation that I cannot compare the results with the previous calculations.

I would like to know why do that happens? and how can I fix it?


Yours,
Marjan