| Light thin-walled structures are widely used in many fields as energy absorbers to dissipate the kinetic energy during an emergency such as a collision. These fields include automobile, ship, aircraft, spacecraft and etc. To design light and efficient energy absorbing devices with excellent crashworthiness is a significant task for the sake of public safety, energy saving and environmental protection. In the present work, a series of research work is carried out in order to improve the structural crashworthiness and energy absorption characteristics of thin-walled metal structures. With a deep comprehension of the relationship between deformation modes and energy absorption, several novel types of structures with excellent performance are proposed and studied. Appropriate theoretical model, trustworthy numerical technology and suitable optimization method are main research tools adopted. The main works of the dissertation are as follows:1. Based on the free inversion of circular tubes, a new type of structure, called retractable tube, is introduced to overcome the drawbacks of the traditional two types of tube inversion. Without the requirement of clamps or dies, this structure can hold a long effective crush distance and is found to possess excellent energy absorption performance. The inversion processes of the proposed tubes under axial compression are simulated by using the explicit finite element technique and the energy absorption characteristics during the processes are investigated. A comparative study is conducted to compare the energy absorption characteristics of the new proposed tubes with the plain circular tubes and the results show that the whole efficiencies of the former are significantly higher than the latter. This comparison result indicates that the proposed retractable tubes are very possible and promising to be extensively adopted in the future industry applications. Furthermore, a parametric study is carried out to investigate the effects of geometric parameters on the behavior of retractable tubes. In addition, the feasibility of multi-level retractable tubes is validated by a numerical example and the result shows that the performance of multi-level retractable tubes can be better than two-level tubes. (See Chapter 2)2. A new method, by introduction of patterns, is suggested to improve the energy absorption characteristics of thin-walled structures. Two types of patterns are introduced to the surface of conventional thin-walled square tubes aiming at controlling and changing the deformation modes of it under axial compression: Type A pattern was aimed at triggering the extensional mode for relatively thin square tubes whereas Type B pattern was intended to develop new collapse mode capable of absorbing more energy during collapse. The results show that the two types of patterns successfully take effect as expected and significantly improve the energy absorption characteristics of square tubes. The dimensions of the tube and the arrangement of the patterns are believed to be important factors that affect the deformation modes and their effects are analyzed. Finally, an experimental study is conducted to investigate qualitatively the influence of patterns on thin-walled structures. (See Chapter 3)3. Based on the Super Folding Element theory, the energy absorption of multi-cell square tubes under axial crushing is theoretically analyzed. A theoretical expression of the mean crushing force of multi-cell sections is derived by dividing the profile into three parts: corner part,crisscross part and T-shape part. Numerical simulations of multi-cell square sections subjected to dynamic axial crushing were conducted and the analytical solutions show an excellent agreement with the numerical results. By analyzing a group of sections with same weight and width, the effect of cell size on the energy absorption efficiency of the multi-cell columns is studied. In addition, a type of pre-crushed trigger was introduced and found to be effective to eliminate the initial peak force and improve the efficiency of a multi-cell column. (See Chapter 4)4. A comparative study of the energy absorption characteristics between foam-filled square columns and multi-cell square columns was conducted by using aluminium as structural material. A series of foam-filled columns and multi-cell columns with same weight and different dimensions are studied numerically under axial and transverse loading. The research results indicate that under both loading conditions, the energy absorption efficiency of multi-cell columns is considerably higher than corresponding foam-filled columns. The reason that multi-cell columns are better than foam-filled columns under axial loading is analyzed from the aspect of deformation modes. Furthermore, the switch of the deformation modes of foam-filled and multi-cell columns under axial and transverse loading is investigated when the dimensions of the columns are changed. (See Chapter 5)5. Due to the good energy absorption characteristics of bitubal foam-filled columns, a new type of bitubal hexagonal columns with multi-cell core is presented. By selecting appropriate design variables, the energy absorption of the bitubal columns under axial crushing is optimized with the constraint of constant structural mass and outer dimension. The optimization problem is solved by adopting the successive response surface method based on Chebyshev polynomials and the axial compression is simulated by using the nonlinear explicit finite element code LS-DYNA. Finally, a comparison for bitubal columns with different configurations is conducted to investigate the influence of the multi-cell core on the bitubal columns and to present the gain in energy absorption by response surface optimization. (See Chapter 6)The research of this dissertation is supported by National Natural Science Foundation of China (No. 10332010), National Creative Research Team Program (No. 10421202) and National Basic Research Program of China (2006CB601205). The financial contributions are gratefully acknowledged.
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