Modelling isotropic hardening

Hi

I am having difficulty with modelling isotropic hardening for sheet metal material in a simple spreadsheet or MathCAD program.

I can model the initial yield condition using an appropriate (plane stress) yield criterion. I have also fitted a hardening law to the experimental data for uniaxial tension. I am having difficulty with the effective stress-strain relations and how this relates back to yield surface expansion. 

Has anyone modelled experimental data in this way?

I would be graeful for a step in the right direction.

Thanks,

Nicola