Cast iron plasticity in abaqus

Hello all,

 I want to use Cast iron plasticity in Abaqus/Explicit, but the cast iron plasticity in abaqus can only be used in Abaqus/Standard, so  a vumat should be written to do it. But in the document, the formulation of flow potential in tension region  is not given explicitly, i stopped here.

Can anyone help me with this?

 if anyone have the ph.d. Thesis of Hjelm,H.E., please send me a copy.

thanks.