A set of nonlinear equations that describe the nonlinear deformation of initially twisted bars under the influence of torsional moment and tension force, which act simultaneously, are derived. Special attention is devoted to the case of thin symmetrical cross sections and the equations appropriate to this case are shown. The linear terms of the equations, in the case of thin rectangular cross sections, are compared to solutions of the same problem, obtained by other researchers, who investigated the torsion and extension of helicoidal shells. It is shown that even for thin cross sections having large values of initial twist, the deviations between the two linear solutions are very small. To check the applicability of the theory to nonlinear regions, the theoretical results are compared to experimental results obtained during the course of the present research. The experiments include the torsion and extension of thin steel strips having rectangular cross sections. The agreement between both is very good, which proves the validity of the theory.

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