I've started to look into the modeling of polyurethane.  So far I'v found one paper that specifically looks into how to model the mechanical behavior of PU. It describes a possible constitutive model for PU:

Stress-strain behavior of thermoplastic polyurethanes, Qi,H.J.; Boyce, M.C., Mech. Mater. 2005, 37, 8, 817-839

Does someone know of any other resources on the modeling of PU?  Thanks, Andreas