Energy finite element method for high frequency vibration analysis of composite rotorcraft structures

This thesis is concerned with the formulation and validation of energy finite element analysis (EFEA) method for the vibroacoustic analysis of composite structures.;The governing energy equations for elastic waves in composite media are derived on the basis of the angle-average of group velocities and structural loss factors. Spectral finite element method is employed for the calculation of the angle-averaged quantities in order to account for the through-thickness material variation and transverse shear deformation of multi-layered composite plates.;Due consideration is given to the development of analytical tools for the calculation of power transfer coefficients for two representative structural components: (1) a thin composite cylinder stiffened by periodically spaced ring frames and axial stringers, and (2) coupled thick composite plates. The periodic structure theory in conjunction with classical lamination theory (CLT) is used to compute propagation constants in the axial and circumferential directions of periodically stiffened cylindrical shells. The propagation constants corresponding to different circumferential modes and/or half-wave numbers are combined to determine the vibrational energy ratios between two adjacent periodic elements. Then, the power transfer coefficients associated with an elastic wave propagating a periodic structure are evaluated from the vibrational energy ratios through the application of an iterative algorithm. For a system of coupled thick composite panels, the diffuse-field power transfer coefficients are calculated by combining the wave dynamic stiffness matrix method with first-order shear deformation theory (FSDT).;The governing energy equations and the analytical methods for computing power transfer coefficients are implemented into an EFEA formulation. High-frequency vibration analyses of several coupled-plate systems have been performed as a verification study of the analytical approaches presented in this thesis. Comparing CLT- and FSDT-based numerical results with those of dense finite element models, the discrepancy between computations with and without shear deformation effect is clearly demonstrated Vibroacoustic responses (normal shell velocity and interior acoustic pressure) of a cylindrical composite rotorcraft-like structure have also been measured and compared to the proposed EFEA method. The EFEA predictions are in good agreement with the experimental results in both structural and acoustic domains.