Concept of plane stress:
Define the concept of plane stress. Sol.: In cubical element of strained material is acted on by stresses acting on only 2 pairs of parallel planes and the 3rd pair of parallel planes is free from any stress, it is said that element is under action of plane stresses. So, the plane stress condition can be called 2-dimensional stress condition. Let the cubical element ABCD taken from strained body be subjected to normal stresses σ1 and σ2 and shear stress.
The planes AD and BC are subjected to normal stress σ1 and shear stress, and planes AB and CD are subjected to the normal stress σ2 and shear stress. But no stress acts on 3rd pair (front face and rear face of ABCD) of parallel planes. Thus it is said that cubical element ABCD is under the action of plane stresses.