In a cubical element of a strained material is acted on by stresses acting on only two pairs of parallel planes and the third pair of parallel planes is free from any stress, it is said that the element is under the action of plane stresses. So, plane stress condition can be called twodimensional stress condition. Let a cubical element ABCD taken from a strained body be subjected to normal stresses σ1 and σ2 and shear stress ¹.
Planes AD and BC are subjected to normal stress σ1 and shear stress¹, and planes AB and CD are subjected to normal stress σ2 and shear stress¹. But no stress acts on the third pair (front face and rear face of ABCD) of parallel planes. Hence it is said that the cubical element ABCD is under the action of plane stresse
