Principal Plane

The plane which carries only normal stress without any shear stress is called as ‘Principal Plane’.

In fig. 1 the planes AB, BC, CD and AD are called as principal planes because of there is no any shear stress is acting on that planes. In fig. 2 AB, BC, CD and AD are not principal planes because of there is shear stress ‘q’ is acting on that planes.
1. Major Principal Plane
2. Minor Principal Plane