Introduction:
In the field of mechanics, the concept of shear stress is important to understand the behavior of materials under load. Shear stress refers to the force that acts parallel to a given surface, causing it to deform. In the case of principal planes, which are defined as the planes on which the normal stress is maximum or minimum, the shear stress has a specific behavior.

Explanation:
The statement "Shear stress on principal planes is zero" is true. This can be explained by considering the orientation of the principal planes in relation to the applied load.

1. Definition of principal planes:
The principal planes are defined as the planes on which the normal stress is maximum or minimum. These planes are perpendicular to the principal axes, which are the directions in which the normal stresses are maximum or minimum.

2. Shear stress on principal planes:
On the principal planes, the shear stress is zero. This means that the force acting parallel to the surface is not causing any deformation in the material along these planes. This can be visualized by imagining a cube-shaped material subjected to a shear stress. The principal planes are the faces of the cube, and the shear stress acts parallel to these faces. Since the faces are perpendicular to the direction of the shear stress, no deformation occurs along these planes.

3. Maximum shear stress:
The maximum shear stress occurs on planes that are inclined at 45 degrees to the principal planes. These planes are referred to as the "maximum shear planes." Along these planes, the shear stress is maximum and causes the material to deform.

Conclusion:
In summary, the shear stress on principal planes is zero. This means that the force acting parallel to the principal planes does not cause any deformation along these planes. The maximum shear stress occurs on planes that are inclined at 45 degrees to the principal planes. Understanding the behavior of shear stress on principal planes is important in analyzing the mechanical properties and behavior of materials under load.