Absence of Warping  
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Torque T is applied on a shaft of length L at either ends in opposite directions. The radial and longitudinal lines are shown on the shaft. Each cross section on the shaft remains flat.

Notice that each cross section remains flat (i.e., planar) as the torsion member rotates.

This observation holds true for torsion members with a circular cross section. Torsion of non-circular shapes produces warping in which planar cross sections before application of the loading become non-planar or warped after the torque is applied.

Since warping does not occur in circular cross sections, the theory of torsion for members with circular cross sections is relatively simple. This theory, however, has application in a wide variety of situations in which shafts are used to transmit rotation.