Maximum stress is roughly 111 MPa. When I solve the model (nonlinear analysis, nnlin geom ON), I get maximum stress values (both Von Mises and principal maximum) higer then the highest value in the stress-strain curve (this affect a wide part of the model, it isn’t just a few elements). This sound strange to me, expecially because the material has a perfect plastic behaviour from a certain strain. Here the two images of the model: Principal stress

In the majority of cases, this is caused by the extrapolation of stresses from the integration points to nodes. You would have to check the stresses directly at the integration points (it can be done in the .dat file or with a script: Stresses at the integration points in ParaView - #2 by mkraska).

when magnify is high, it can be used as sign to do mesh convergence study. refinement is required in these areas. still discrepancy will exist due to extrapolation.

@synt thank you for your advice: as you suggested I adopted a finer mesh in the area of interest, going from an average el. size of 0.5mm down to 0.1mm: results didn’t change, I still have values around 140MPa instead of the maximum value in the curve, 111 MPa.
Moreover, I tried with another material (same model, same loads, just different material) less ductile than the first I used, again adoptin gelasto-plastic behaviour such as this:

Young’s modulus is roughly 3 times higher than the initial material.

@FEAnalyst I see your point and I would agree if for one reason: maximum stress in stress-strain curve is lower than the value I should find in any of the integration points. Interpolating integration points to the nodes, should give me at least not accurate stress, but I don’t expect this huge difference (110 vs 140MPa in wide areas of the model is roughly 25% of difference!!!)

It’s because temperature is plotted in the same graph so there are different quantities on the Y axis. But temperature can be ignored here so it’s just yield stress - plastic strain plot of the input data for *PLASTIC.

If you use the Field Output (on one of the most loaded elements), you will get the stress results in the integration points. Check this stress to see its real value.