Strange Post-Pro Results with Quadratic Mesh

Hello all, I`m starting to use Salome-Meca now and I could use some help now.

I`m trying to plot Von Mises stresses across a piece, but whenever I plot the results obtained from a quadratic tetrahedral mesh, I get a really odd results, from negative Von Mises stresses to strange patterns on the stress map. There is a picture for linear and quadratic results attached. Now I`m not arguing about the numerical differences, since linear and quadratic elements behave different, linear being more stiff, I`m trying to understand the low stress spaces between elements, making the results very ugly on quadratic meshes. Also, I`d like to know why is Von Mises stress negative on some parts. Since Von Mises Stress is the root of the sum of square stresses acting on a point [(Stress1²+Stress2²+...+StressN²)^(1/2)], I can`t see how they can have a negative value, unless they have a complex component.

I've tried Salome 6.3, 6.5 and now 5.1.5, all of them have the same output, that is, the strange stress map while using quadratic meshes

Can someone help me?

claws, thanks for replying!

I've read that I should use ELGA results rather than NOEU, and the plot looks very nice with ELGA, good transitions and such. But I still don't understand how a extrapolation on nodes between 2 positive values can result on a negative value. Isn't that a problem with the way Salome-Meca interprets the results? It doesn't seem right at all.

Thanks for your answer!

keeswouters, thanks for your reply!

I sincerely still can't understand how the software can interpret a negative stress values between 2 positively loaded elements/nodes. But since ELGA plots seem more reliable on that, I'll use them instead of ELNO. Still, that should be looked on.

Thanks for your help, it was much appreciated!