von mises stress higher than yield stress

[Kiks]

Dear all, am performing a perfect plasticity analysis on an I-beam fixed at both ends with udl of 0.5 kN/mm. the yield stress is 0.25 kN/mm2, beam length is 3000mm. From the results the Von mises stress are greater than the yield stress. what could be the problem? any ideas please? does the value of loading affect the Von mises stress? I have attached a picture of my results to this message. thanks in advance.

 

[Mustaine3]

The issue comes from a coarse mesh and the fact, that the material calculations are done at the integration points and the stresses are then extrapolated to the nodes for visualization.

Use Tools -> Query -> Probe Values to check the Mises stress at the integration points.
Also refine the mesh and check the results again.

 

[NRP99]

In addition to what Mustaine3 said, there will be some computational(such as rounding up to certain decimal) error while calculating stress since stress is derived from strain.

Try plotting un-averaged von-Mises stress and see if gap is minimum.