Nonlinear Finite Element Numerical Computation And Stability Analysis Of Thin-wall Structure

With the advantages of high strength, light weight, simple manufacture and easy maintainance, stiffened thin-wall structures have been widely applied in modern aircraft structure for making full use of the most advantageous stiffness of each member to improve the structural performances, reduce structural weight and enhance loading capacity.For high aspect-ratio aircraft wing, large deformation will be happened with its carrying severe flight loads, which causes nonlinear geometrical problem and may make more error with using linear elastic analysis. Therefore, it is necessary to use geometric nonlinear method and analysis for this situation. After reviewed the overseas and domestic technical literatures about structural stability and geometrical analysis, the thesis introduces in more detail the formulations of stress and strain for geometric nonlinear problems and finite element numerical solution formats of total Lagrange (T.L) formulation and updated Lagrange (U.L) formulation. Meanwhile, some state-of-art numerical algorithms to solve nonlinear equation group also are discussed.Based on the above mentioned theoretical analysis and application of nonlinear finite element technique, the thesis studies computationally the nonlinear features and stability characteristics of the simple plate and stiffened panels, with regard to the influences of different boundary conditions, loading types, etc. The buckling loads in linear and nonlinear analysis in each-type structure are compared. Furthermore, the influence of computational grid density on numerical results is also studied, which follows that the bulking load will sharply converge to a fixed value as the grid density increases to a relative small element dimension. For more detailed analysis and study of the nonlinear features of the compressed and sheared structures, the course curves of von.mises stresses against displacements of the different structural locations are showed to reflect the nonlinear evolution and internal force transformation. As a more complicated numerical example, a small stiffend flated metallic box subjected to shear loading on free-end is computed and analyzed, for its nonlinear and stable features of compressed panel and spar-web. Finally, an experimented high aspect ratio multi-walls simulated wing is computed numerically with the help of commercial software pacakage ANSYS, and its nonlinear and stable features are discussed, as well as the numerical results are compared with the experimental data.As the summary of the thesis resarech work, some significant conclusions are drawn and further study work to the nonlinear analysis of thin-wall stiffened structure is envisaged.