Analysis of thin-walled composite box beam under torsional load without external restraint

A procedure for analyzing the mechanical behavior of laminated thin-walled composite box beam under torsional load without external restraint is presented. The method is based on the theory of composite laminated plates and is deduced by means of the free torsion theory of thin-walled beams, which makes the procedure simple and practical. In the present theory, the stresses are considered distributing unequally along the wall thickness and various coupling effects are taken into account. The calculation formulas of torsional angle and stress given by this method are concise and easy to use. The present analysis results indicate that by reason of coupling effects, in general, the free torsion of composite box beams may not exist definitely, so a concept of torsion without external restraint is suggested. Finally, the examples are given and their numerical results are analyzed and discussed. The values of torsional angle, ply stresses (including their variation with the off-axis ply angle) obtained by this paper are compared with those obtained by model test or finite element method (FEM).