Setting up simple optimization problem2

Hi,

I'm trying to set up a simple optimization problem.
I have a half sphere at the limit between two distinct elastic layers.

I have first run my model using a successful parametric sweep to find the maximal value of P_over acting on the sphere boundary for either the Tang_Stress or solid.sphi to reach zero.

Now I'm trying to use the optimization module.

As a first step find the value of P_over for which Tang_stress will reach zero:
I have tried to define the objective function as: mod1.maxop1(mod1.Tang_Stress)==0 with the control variable P_over ranging from 0 to 300 MPa.
but it doesn't seem to be working that way.

I'd like to have some help on how to use the different operators, because my goal would be to find the value of P_over when either of the Tang_stress or the solid.sphi stress reaches zero.
and how to define a proper objective function.

Thank you

N.