Linear and nonlinear torsional behavior of unidirectional CFRP and GFRP

The helicopter bearingless rotor flexbeam is usually made of glass-fiber reinforced plastic composite (GFRP). Carbon-fiber composites (CFRP) are candidate for future flexbeam materials due to their superior tensile fatigue strength. This research examines the feasibility of CFRP as a future flexbeam material. The torsion behaviors of unidirectional CFRP and GFRP with the same matrix resin were investigated. As a result, it was confirmed that the behavior of both CFRP and GFRP is comprised of linear/nonlinear domains. The initial torsional rigidity of CFRP was almost the same as that of GFRP. The torsional rigidities calculated from Lekhnitskii’s equations agreed with the experimental results, and they are mainly determined by the shear stiffness of the materials. The nonlinear torsional behavior was observed above 0.5% of the shear strain, and it is due to plastic deformation of the matrix resin. A 3D plasticity model proposed by Sun et al. was applied to the plasticity parameters obtained from off-axis tensile tests. The numerical curves agree with the experimental data below 1.5% of the shear strain. The experimental result suggests that GFRP can be replaced by CFRP as torsional elements of a helicopter flex beam without an increase in torsional rigidity.