The restrained torsional response of open section carbon fibre composite beams

Abstract

The analysis procedure outlined in this paper essentially makes use of the existing isotropic theories of torsion suitability modified to account for the non isotropic nature of typical carbon fibre composite material. The warping and St Venant torsional stiffnesses of the beams are determined using the appropriate equivalent engineering elastic constants of the composite material which correspond to the membrane and bending modes of action respectively. The differential equation governing the constrained torsional equilibrium of the open section beams is solved exactly in the paper for Z and channel sections with some emphasis being given to the influence of ply stacking sequence. Theoretical results are presented in graphical form and these depict the variations in warping displacement, warping shear flow and longitudinal or axial constraint force intensity with applied torque for the cantilever beam condition with torque applied at the free end. The paper also gives details of finite element studies of the composite beams and of an experimental programme of work pertaining solely to the behaviour of composite Z beams. Comparisons between theory, finite element and experiment are presented and these are seen to give exceptionally close agreement. It is clearly indicated that fibre orientation significantly influences the restrained torsional behaviour of thin-walled open-section composite beams. 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.