Study on structural dynamic characteristics of rotating composite blades using three dimensional finite elements

Abstract

In this thesis, an eighteen-node solid-shell finite element was used for structural dynamics modeling of rotating composite blades. The analysis model includes the effects of transverse shear deformation, Coriolis effect and elastic couplings due to the anisotropic material behavior. Also, the out of plane warping was included without complicated assumptions for specific beam, plate or shell theories. The incremental total-Lagrangian approach was adopted to allow estimation on arbitrarily large rotations and displacements. The equations of motion for the finite element model were derived using Hamiltons principle, and the resulting nonlinear equilibrium equations were solved by utilizing Newton-Raphson method combined with the load control. A modified stress-strain relation was adopted to avoid the transverse shear locking problem, and fairly reliable results were obtained with no sign of the locking phenomenon. In order to reduce the computational complexity of the problem, the Guyan and IRS reduction methods were adopted. Those model reduction methods gave not only reliable solutions, but also less computational effort for any geometric configurations and boundary conditions. The present numerical results were compared to the several benchmark problems, and the results show a good correlation with the experimental data and other finite element analysis results. The vibration characteristics of shell and beam type blades were investigated. For the case of shell type blades, blade curvature, pre-twist and geometric nonlinearity may significantly influence the dynamic characteristics, and only the geometric nonlinear analysis model can capture significant drops in frequency and frequency loci veering phenomena. For the case of beam type blades, one-dimensional beam and three-dimensional solid models give comparable prediction for the straight and large aspect ratio blade. As decreasing in the blade aspect ratio, considerable differences appear in bending and torsion modes. The tip sweep angle tends to decrease the flap bending frequencies, but the torsion frequency increases with the tip sweep angle. On the contrary, the tip anhedral enforces to decrease the torsion frequency.