Plane Stress: Understanding 2D Stress in Thin Objects

[chetzread]

Homework Statement

my notes gave that in a 3d cubic , plane stress is state of stress in which two surfaces are free of stress ...

Homework Equations

The Attempt at a Solution

Does it mean the we only consider the stresses in xy plane only ? because the object is too thin, so we assume it's 2 dimensional only ?

 

[chetzread]

if so , the statement in the notes should be rewrite as 4 surfaces of cubic element is free of stress , right ? Since we only consider the stresses in 2 planes out of 6 planes , right ?