plastic nonlinear data

I have been doing some simulations with plastic parts and amwondering if there is a good resource somewhere to get nonlinearstress/strain curves for various materials. Matweb has a lot ofmaterial data, but I haven't seen any type of nonlinearinformation.

I am sure many others have done simulations with nonlinearmaterials... does a person always have to get their material testedin a lab?

SolidworksSimulation