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Viscoplasticity vs. Nonlinear Viscoelasticity

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Hello -

I am currently a final-year PhD student working on predicting the response of a rather complex polymeric material. For the past two days I have been working with the free 14 day trial license of PolyUMod and have gone through a few of the tutorials and tried to calibrate some of the models to my own test data with varying degrees of success.

My question is related to something I keep seeing in the documentation (including the very helpful polymer book) and something that I have heard referenced in tutorial webinars. I do not see a distinction drawn between a nonlinear viscoelastic material and a viscoplastic material. Models such as the Three Network Model appear to be structured such that, at infinite time, the material will fully recover and there will be no lasting permanent deformation or history dependence. Is my assessment correct?

I ask because I am quite specifically interested in the long term permanent deformation of the material but I notice that the (what I would refer to as) nonlinear viscoelastic models can capture the "permanent" deformation (at a specific time in the future... say 1 day) more easily than models such as the DNF model which (again, I would say) actually has a plasticity component incorporated.

Any comments or suggestions would be very appreciated. Thank you very much for your time.