PEM Based Random Vibration Analysis Of Composite Structures And Its Applications In Aero/Astro-nautical Engineering

Fibre reinforced laminated composites are advanced structural materials which are widely used in the aerospace industry and launch vehicles because of their many advantages e.g. their: high stiffness- and strength-to-weight ratios; high damping effects; long fatigue life and; design versatility. However the coupling effects caused by their anisotropy and non-homogeneity make their dynamic analysis much more complicated than that of metallic structures, particularly when the atmospheric turbulence induced random vibration needs to be accurately analysed, which is the main cause of fatigue failure for aeronautical structures. The power spectral method is considered theoretically appropriate for dealing with such problems; however it has hardly been practically applied in the aerospace fields due to its computational complexity when using it in the conventional way. The situation has been the main thrust behind the present degree. The pseudo excitation method (PEM) is a ground-breaking random vibration method in that it provides a systematic package to resolve multi-excitation and multi-response problems in an efficient and accurate way. The success of PEM has been witnessed by the seisimic and structural wind engineering community. Therefore introducing the pseudo excitation method as a new algorithm into the aerospace fields is important in a time when atmospheric turbulence caused random vibration has become a distinguished issue in designing large commercial passenger aircrafts. The research of this doctoral thesis is aiming at introducing the pseudo-excitation method into random vibration analysis of composite laminated aeronautical structures. The combined PEM and the laminate material finite element method is implemented on the DDJ software system.In this paper, the main research work can be summarized as follows:1. Based on the first order shear deformation theory, a finite element model for the composite laminated plate is established, and the formalue of the finite element mthod are deduced. The algorithm is implemented on the DDJ software system for the natural frequencies and modes of composite laminated plate. The pseudo excitation method has formed a whole system in recent years. It is widely used in the seismic and buffeting analysis of the long-span bridges, but its use in aerospace engineering is still a blank. Different from other types of structure, aerospace structures are contantly excited by random excitations from their working environment. In some sense the coupled effects make the analysis for aerospace structures more difficult than those for civil engineering structures. The pseudo excitation method is developed for general analysis of random vibration and can be readily extended to the aerospace craft structures. Based on the finite element model, an efficient and accurate algorithm including all the coupling efficiency is porposed for the random vibration of aerospace craft.2. Viscoelastic materials are widely used in the vibration control. This thesis proposed a PEM based random vibration algorithm for the composite laminated structures installed with viscoelastic damping layers. Both the damping of the composite laminated structures and the attached frequency-dependent damping layers are included in the analysis. For the resultant non-classically damped and frequency-dependent complex system, in combination with the real-mode-based degree-reduction scheme, the pseudo excitation method is developed and the precise integration method is applied to solve the reduced non-stationary random vibration equation. A composite horizontal tail of the aircraft structure is computed and studied. The complexity of the problem and good results abtained show that the proposed method is quite useful for the analysis and desigh of complex composite structures.3. The power spectrum analysis is the main method for the turbulence analysis of aircraft wings. Its use becomes more and more important to analyse the response and the probability of failure of the wing structures. Base on the unsteady aerodynamic force induced by a harmonic vibratiing plate in two dimension flow, the influences of vertical gust are also considered. The aircraft wing model is built by composite laminate elements; the Dryden turbulence spectrum function is chosen as the input for excitation. It is shown that PEM is highly efficient for the atmospheric turbulence response analysis of the two-dimensional airfoil.4. The flutter derivatives of the NACA0012 airfoil are identified through solving the unsteady flow around moving airfoils using the RANS turbulence modeling. Subsequently the flutter derivatives are used into the turbulence analysis; the solution is compared with the results obtained by the Theodorsen method. Due to the molecular viscosity and the eddy viscosity, the combined effect is an increased damping to the structures analysed. With many advantages, computational fluid dynamics (CFD) has become a powerful tool for solving unsteady aerodynamic problems.