| Autobody design is one of the core technologies in automobile industry. It is absolutely necessary for independent enterprises to improve the competitiveness of core products. Automotive industry, "Eleventh Five-Year Plan", targetedly put forward that:"Large Automotive Group must have a platform for independent product, assembly of R& D capabilities; Key enterprises must have the ability to match among autobody, powertrain and chassis; Parts enterprises must master the major powertrain components and core technologies and R& D capabilities with the platform synchronization and the development of own-brand cars was encouraged".2011 is the initial year of "Twelfth Five-Year Plan", and it is the key stage of coping with the significant changes at home and abroad. The independent automobile products must make further efforts on the energy conservation, emission reduction and safty demands to reach the advanced level. The development of technology progress and innovation as one of the fundamental principles, is the important support of accelerated transformation of the economic development mode. Moreover, it is the determinant factor of changing the automobile products from " Made in China" to "Create in China". Improve the initial and core technology innovation capability are the assurances of the production technology upgrade. Particularly, under the intense trade competition and cost containment, the risks and costs in the follow-up modification works can be enormously reduced by making the main efforts on the performance analysis and optimization during conceptual design stage. As the critical measurement factor of automobile passive safety, autobody crashworthiness must be effectively evaluated during the conceptual design stage. That explicity reveals the importance of autobody simplified modeling method and theory research. This thesis is based on the research of simplified body framework modeling, static analysis and optimization supported by National Natural Science Foundation of China (No. 50975121), Science and Technology Development Plan of Jilin province (No.20096004) and its assorted project, FAW Group Combined Action Plan (No.0846), and FAW Group Science and Technology Innovation Project (No.0837 and 093715). The thesis intensively investigates the modeling method and theory of simplified crash autobody structure, as well as the crashworthiness design and optimization method of the detailed structures. The main work includes:First, the bending collapse mechanism of the thin-walled beam with box section is analyzed. Then, the simplified numerical computational method, called CKW method of bending characteristics is put forward based on the energy conservation. This modified numerical computational method meets kinematically admissible. Bending resistance capability of the thin-walled beam decrease rapidly when the straight structure could not stand the additional loading. By that time, the wrinkle appears on the local surface and the plastic deformation is concentrated on a narrow plastic hinge area. And other non-deformation parts of the beam can be treated as rigid body. The energy dissipation paths during the bending collapse are approximately represented by the assumptive plastic hinge lines which constitute the buckling deformation area. Finally, the total rate of energy dissipation along the plastic hinges are obtained, and the curve of bending resistant is analyzed. The CKW method provides a basis for the bending characteristics derivation of thin-walled beams with other sectional shapes.Second, based on the proposed CKW bending theory of box section, the bending mechanism of different thin-walled beams with hat-like section are deeply investigated. And the wrinkle characteristics of the local plastic hinge areas are analyzed, respectively. The simplified computational method for predicting bending characteristics of three plastic bending modes, including bending along the short side of thin-walled beam with top-hat section, in-plane tensile and in-plane collapse modes on flange surface of channel section beam are put forward. This work is the foundation of modeling the simplified body frame which reduced from the initial comparative autobody structure.Third, the simplified modeling method and process of autobody structure with crashworthiness analysis capability are discussed. Those methods are based on the bending computational theory of thin-walled beam proposed previously. The main crash energy absorption components, the frontal rail and B pillar, as well as their different crash conditions with frontal crash and roof crush are choosen as research objects, then their bending resistant characteristics are abstracted. By using LS-DYNA, the simplified method and process are applied based on the computational characteristics. In the simplified model, by distributing the plastic hinge area, nonlinear beam elements and rotational springs with bending characteristics are used to simplify the existing detailed models by replacing the shell elements. The comparisons of the crashworthiness parameters, such as the crush deformation curve, energy absorption, and rigid-wall displacement between the detailed and simplified models show the validity and reasonability of the proposed bending computational theory of thin-walled beams and the simplified modeling method of autobody structure.Fourth, in order to modified the crashworthiness defects of the comparative autobody structure, so as to improve the performance and competitive power of body products, the main energy absorption component is taken to analyze and optimize the crashworthiness capability. Moreover, the better detailed model can supply better sample to the simplified modeling. Strain-energy-density method is proposed here to analyze the load bearing distribution among the frontal rail. Based on the calculated strain-energy-density results, the initial structure is divided into several domains. By using topology optimization technology based on variable density method, the optimal material distribution among different domains are obtained. For the purpose of improving deformation mode and energy absorption capability, the small size tiggers with square shape are distributed uniformly on the corners of the frontal straight part, and the reinforcement plates are picked and placed inside the weaker domains. Those modified strategies are taken the usual energy absorption structural design method, as well as configuration design and processablity into account. The final crash analysis indicates that the modified rail model has an extremely excellent collapse mode and energy absorption capability.Fifth, another effective method on solving nonlinear crashworthiness optimization problem as known as surrogate model technology is investigated here. By the surrogate method, the optimal design is proposed to improve the low-speed crashworthiness performance of crash box structure. By taking the advantages of the response surrogate modeling based on orthogonal-composite experimental design, the reasonable design samples are distributed in the design space. Then, the response surface models of total energy absorption and peak crash force with excellent statistical characteristics and precise fitting capability are obtained, respectively. The adaptive response surface optimization method is employed here to solve the crashworthiness optimization problem by considering the lightweight demand. The thickness of four key components are taken into account, and the optimal result convergences at the sixteen step in the optimization process. The final comparisons show that total energy absorption is raised nearly 5%, meanwhile, the mass and peak crash force are slightly decreased. The optimal method and results have successfully achieved the expected targets.Finally, the main content is summarized and prospects for the futher work is predicted.
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