Refined Structural Model for Thin- and Thick-Walled Composite Rotor Blades

Abstract

A refined structural model based on a mixed force and displacement method is proposed for the analysis of composite rotor blades with elastic couplings. The present formulation allows the modeling of either open-section or closed-section blades of arbitrary section shape, stacking sequence, and end restraint effects. The theory accounts for the effect of elastic couplings, shell wall thickness, section warping, warping restraint, and transverse shear deformations. A semicomplementary energy functional is used to derive, in a variationally consistent manner, the beam force-displacement relations. Bending and torsion related warpings and shear correction factors are obtained in closed form as part of the analysis. The resulting first-order shear deformation theory (Timoshenko) describes the beam kinematics in terms of the axial, flap and lag bending, flap and lag shear, twist, and torsion-warping deformations. The theory is validated against experimental data and other finite element results for graphite-epoxy composite beams of various cross sections such as I sections, box sections, and two-cell airfoils. Good correlation is achieved for all of the test examples. The influence of wall thickness and transverse shear on the static beam response is also investigated. Wall thickness effects are shown to become significant when the thickness-to-depth ratio of the beam reaches around 20%. The slenderness ratio has a significant effect on the transverse shear behavior of the beam, especially for beams with low slenderness ratios. It is also shown that the layup angle has a nonnegligible effect on the transverse shear behavior of the beam.