The analysis of carbon fibre composite box beams subjected to torsion with variable twist Original Research Article

A simple analytical procedure for determining the constrained torsional response of a specific class of carbon fibre composite box beams is detailed in this paper. The analysis approach essentially makes use of the existing theories of torsion appropriate to isotropic construction and these are then suitably modified to account for the non-isotropic nature of typical carbon fibre composite material. The composite box beams are devoid of overall elastic couplings through the use of constituent laminates which are symmetrically layed-up about their own mid-planes and which possess in-plane orthotropy. The thin walls of the box sections can have different lay-up configurations provided the stiffness distribution around the sections is of a symmetrically disposed nature. The flanges of a box section can thus have a different lay-up configuration to that of the section webs. The constrained condition considered in the paper is that of the cantilevered box beam with torque applied at the free end and the torsional and warping rigidities of the composite box sections are determined through the use of the appropriate equivalent engineering elastic constants of the individual thin composite walls. Comparisons between theory and finite element and between theory and experiment are given in the paper and these are shown to give close agreement. It is also clear that the use of the appropriate equivalent engineering elastic material constants in the theory is able to closely predict actual behaviour.