unexplainable results of an elasto-plastic material modell_ dotted tension

Hello,

I need help for my final thesis. I want to simulate a pull-force at a piston rod connected to a valve. I modelled a simplified modell.

The elasto-plastic material modell (second picture) put out some unexplainable results. Does someone already had this problem or can help me please?

In the 3rd picture you can see the results of the linear model

The displacement and forces:Displacement and forcesdotted elasto-plastic simulationlinear simulationThe same at another try of an elasto-plastic material modell simulation

thanks for your help!

kind regards

Daniel