Simple Bending or Pure Bending:

Let a cantilever subjected to a moment at the free end. The shear force is zero & the bending moment is constant at all of sections. This cantilever is under pure bending or simple bending. Thus a beam or cantilever is called to be subjected to simple bending or pure bending while it bends under the act of uniform bending moment, without any shear force.

In practice, while a beam is subjected to transverse loads, the bending moment at a section is accompanied by shear force. But generally it is observed that the shear force is zero where the bending moment is maximum. Thus, the condition of pure bending or simple bending is deemed to be satisfied at that section.

Assumptions

The supposition made in the theory of simple bending is as follows:

1. The material of the beam is perfectly homogeneous (that means of the same kind throughout) and isotropic (that means of same elastic properties in all of  directions).

2. The material is stressed in elastic limit & obeys Hooke's law.

3. The value of modulus of elasticity for the material is similar in tension and compression.

4. The beam is subjected to pure bending and thus bends in the form of an arc of a circle.

5. The radius of curvature of the bent axis of the beam is vast compared to the dimensions of the section of beam.

6. The transverse sections, which are plane & normal to the longitudinal axis before bending remain plane & normal to the longitudinal axis of the beam after bending.

7. The stresses are purely longitudinal & local effects of concentrated loads are neglected.