Comparison of isotropic elasto-plastic models for the plastic metric tensor C_p=F_p^T\, F_p

We discuss in detail existing isotropic elasto-plastic models based on 6-dimensional flow rules for the positive definite plastic metric tensor $C_p=F_p^T\, F_p$ and highlight their properties and interconnections. We show that seemingly different models are equivalent in the isotropic case.