To have an apt material model is necessary to get good simulation results. Thus, you perform a tensile test/compression test to fit a suitable material model.
So far, you have got a nice colored picture in your FE solver. But still you need to interpret these results! From tensile tests, you have a critical stress and a critical strain, where plastic deformation or failure occurs. However, in a real setup and its simulation, you don't have pure tensile stress. You have a mix of tensile/compression and shear.
For metal, you can use the von-mises criterion or tresca for the failure criterion, what is very common. For plastics, on the other hand, I don't know such failure criterion.
So, is there a reasonable failure criterion available for plastics (e.g. in PolyUMod)? Implemented in a software or the theory for it somewhere published?
I like the failure model that is part of the PolyUMod TNV model. See the following page more info: https://polymerfem.com/tnv-model/