Abstract: In this paper we deal with the second-order effect of an elastic circular shaft during torsion.The analysis is based on the method of co-moving coordinates and the strain-rotation decomposition theorem[1] in continuum mechanics.By using asymptotic expansion methods, we comfirm that the effect of axial elongation and distortion of plane cross-section exists in an elastic circular shaft during large torsion and give the expressions of the axial force and the torque.

Key words: torsion, second-order effect, large deformation, S-R decomposition theorem