Hi Mike, 

Thank you for your answers. It is a great pleasure to receive a reply from an Ansys expert.

I understand Hill is an anisotropic plastic strain model, which indicates different behavior in different material axes. However BISO (bilinear isotropic hardening) can only define only one yield stress (sigma zero), as shown in the figure below, while the anisotropic material has three different yield stresses along its anisotropic axes. That is why I am confused how the BISO or BKIN can add to Hill model.

I am thinking do you mean the yield stress (sigma zero) is the yield stress used in Hill model and it does not represent the yield stress of any axes? If it is so, then i get it. If it is the case, can you advise me how to assign a value for the yield stress (sigma zero) appropriately?

Really appreciate your time on this issue and your advice are really helping me out.