Multi-cell carbon fibre composite box beams subjected to torsion with variable twist

The structural performance of multi-cell carbon fibre composite box beams when subjected to constrained torsional loading is examined in this paper. A simplified analytical procedure for determining the constrained torsional response of a specific class of multi-cell carbon fibre composite box beams is outlined in some detail. The constrained condition analysed is that of the cantilevered multi-cell beam with torque applied at the free end of the beam. Overall elastic couplings in the beams between bending, torsion and axial effects are eliminated from the analysis process through the use of constituent laminates for the thin walls of the cross-sections which are symmetrically layed-up about their own mid-planes and in such a manner that they possess in-plane orthotropy. The analysis procedure employed makes use, essentially, of the existing theories of torsion developed for isotropic construction and these are then suitably modified to account for the non-isotropic nature of the typical carbon fibre composite material. The resulting approach is shown to be able to predict the structural response of the multi-cell composite beams with a considerable degree of accuracy and comparisons between the theory and the results from finite element numerical modelling are shown to give close agreement. The torsional and warping rigidities of the composite multi-cell beams are calculated in a procedure which makes use of the appropriate membrane engineering elastic constants of the individual thin composite walls and the constituent thin walls can have different lay-up configurations provided the stiffness distribution around the sections is of a symmetrically disposed nature in order to preclude the influence of overall elastic couplings.