plane stress/strain thickness

Hello friends:

I  am new here, and i am also a beginner to ABAUQS. 

Now i met a problem that is in the section module: when i create a 
section i have to fill out the "plane stress/strain thickness" what 
is this meaning? and if i set up a cylinder model how can i choose ?

Thank you very much if you can give me some suggestion.

xiao 

 








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