Why is calculated stress higher than maximum stress in material stress-strain curve?

[mscpro]

I have a bracket modeled with shell elements which is constrained on one end and loaded on the other. I also have defined the elastic and plastic data for aluminum material, with the last two data points defined such that the slope is perfectly plastic after 4% true strain.

I noticed that when I examine the von-mises stress results the peak stress is at the peak defined stress level in my my material data. However, when I look at the S11 stress component (which is in the direction of the loading), the maximum stress is significantly higher than the peak stress defined in my material plastic stress-strain curve. I was expecting the calculated peak S11 stress to be at or below the peak stress in my material data (not higher!).

I understand that for linear statics analysis the computed strains are used with the constitutive material model to recover the stresses, and as such I was expecting something similar, with S11 to not be higher than the stress level defined in my material data.

Am I missing something? Can anyone shed some light on this?

Thanks.

 

[corus]

The criteria is for the Von Mises stress intensity to follow the stress strain data and not the component stress. Another factor to consider is that the extrapolated nodal stress values may be higher than the at the gauss points.

 

[mscpro]

Thanks for the response. What you said does make sense now. I was probing the high stress elements and using the values from the integration points of the element to avoid any extrapolating to the nodes.

Thanks again.