In ORDER to make calculation of stresses due to torsion easy, we need to make certain simplifying assumptions on the deformation pattern which is realistic. It is has been found from the rigorous solution PROCEDURE (elasticity solution) and from the experiments that the circular cross SECTION members SUBJECTED to pure torsion in the elastic range satisfy very closely this condition of plane sections remain plane and rigid. 

By making this assumption, the solution procedure becomes SIMPLE as shown in the basic concepts section in the derivation for tensional stresses.

In order to make calculation of stresses due to torsion easy, we need to make certain simplifying assumptions on the deformation pattern which is realistic. It is has been found from the rigorous solution procedure (elasticity solution) and from the experiments that the circular cross section members subjected to pure torsion in the elastic range satisfy very closely this condition of plane sections remain plane and rigid. 

By making this assumption, the solution procedure becomes simple as shown in the basic concepts section in the derivation for tensional stresses.