in which c0 is the initial cohesion (similar to the initial yield stress), H is the hardening modulus, and εpˉ is the accumulated plastic strain.
If one sets ηy=ηf=0, the model effectively becomes the von Mises model with the associative plasticity. In a uniaxial loading case, the yield function is then
F(σ,c)=σ−3ξc.
This leads to a yield stress $\sigma_y=\sqrt{3}\xi{}c_0$. The plastic hardening modulus is 3ξ2H. In terms of total strain and stress, the hardening ratio is 3ξ2H/(E+3ξ2H).