Plane Stress

At any point in a solid body, six components of stress exist:

Normal stress 3 components
Shear stress components

(The subscript indicates the plane on which the shear stress exists)

This is a 3D stress system.

Fig. 2.1

However, in practical applications, a critical element at the outer boundary of a member is selected for analysis where it is free of significant surface stress but having the largest internal stresses.
Now assume that stresses on surface ABCD are zero i.e.:

, then the 3D stress system is reduced to a 2D stress system or PLANE STRESS.

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