Small plastic strains before yield point in datasheets

[cldine04]

Hello, I have a doubt about the yield point (stress and strain) I usually find in datasheets. Usually this point is located just before the dramatic increase of non linearity in the material curve and is interpreted as the limit beyond which start plastic strains (or 0.2% plastic strains has occurred) ; below this point the material should behave in a perfectly elastic way.
Now, when I have to insert data for FEM analysis (typically I deal with polymers), I follow the procedure described at the following link :

https://caeai.com/blog/using-stress-strain-data-finite-element-simulation

and I have seen that many analysts use it when they want to use a detailed model of the material, inserting as input to the software the couples TrueStress/TruePlasticStrains and allowing just one initial elastic segment linearly obtained through Young modulus (non linear elasticity is neglected in this model). What I noticed is that, according to this model, I am inserting very small plastic strains at stresses that are definitely lower with respect to the yield stress I see in datasheets: nevertheless it is true that from the yield stress of datasheet the slope of the curve changes dramatically and that I have larger plastic strains also in the curve I represent with this model. My questions are:
1) is this situation (very small plastic strains before the yield provided by datasheets) something that occurs also in reality or is it just a consequence due to the material model I am using?
2) in this model, I assume that elasticity is completely linear; is it a good assumption to neglect non linear elasticity in polymers? According to what I read, the non linear elasticity after the proportional limit of material curve should be a very narrow region.
Thank you very much for the help!

BR
C

 

[EdStainless]

0.2% is a finite amount of yielding, the material is not perfectly elastic up to this point.
Many very ductile materials (soft metals and many plastics) begin having measurable yielding at very low loads.
This is real but in many systems it is ignored. You need to look at the impact and decide how to handle it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[cldine04]

Ok, many thanks for your answer!
C

 

[MagBen]

Polymers have a big range of stress-strain behaviors. For brittle and elastomer (fully amorphous?), there is no yielding. The former exhibits literally no deformation, the latter can have a large amount of elastic deformation without breaking. Both of them show very limited plastic deform. Plastic polymers, say semi-crystalline, deformation is the combination of amorphous and crystalline, you won't see a single straight line before .2% yield. There may have two Elastic moduli.