Thank you very much for your reply.
Yes I read the instructions you pasted. Sorry i was dumb but now i kind of get what you meant. You were saying Hill model is only for isotropic elastic material and in this way BISO is certainly able to be added to with only one yield strength (sigma zero) needed to be defined. So the material has to be isotropic in elastic phase but anisotropic for plastic phase.
I am tring to simulate an orthotropic nonlinear material. I have defined an orthotropic elastic material and I am wondering how to use Hill and any one of the plasticity model to simulate its plastic behavior? If it is possible, I was asking in previous posts what should be sigma zero defined in Hill. Bc there are three orthotropic planes for orthotropic elastic material and thus there should be three tangent moduli and yield strengths.
Thank you for your time replying to my inquries!