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 3^{rd} 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 σ_{1} and σ_{2} and shear stress.
The planes AD and BC are subjected to normal stress σ_{1} and shear stress, and planes AB and CD are subjected to the normal stress σ_{2} and shear stress. But no stress acts on 3^{rd} 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.