Material curve in Femap with Nastran solver

[SuG89]

Hello,

I am a beginner in Femap. I have defined the following 'vs Stress' curve for the plastic material model:

Strain ,,, Stress
0 ,,, 0
0.002 ,,, 300 MPa
0.2 ,,, 500 MPa
1 ,,, 500 MPa

I expected that the von Mises stress results would in any case be less than (or equal to) 500 MPa for the above definition which is not the case. Does anyone know what could be the problem?

Thanks,
SG