Research On The Beam Sections And Joints Of Car Body Structure

Nowadays, unitary construction body is widely used in modern cars, so stiffness of body in white(BIW) have a significant impact on static and dynamic performance of modern cars. BIW consists of three parts: carrier beams, joints and sheet metal and structures of beams and joints are the direct factor to decide the stiffness of BIW. So a detailed design of beams and joints structure in conceptual design is important and can guide to ensure the stiffness of BIW, as well as the overall parts design and optimization of full vehicles.Firstly, in combination with a saloon car of an automobile company, the flow of extraction, processing and storage the information of main beam section of car body is introduced in detail and a data base of beam section of car body including 8 different cars is also established in this paper. Next, the basic performance of BIW is analyzed in this paper including free modal analysis, the calculation of bending and torsion stiffness and input point inertance. The results show that the first order torsional and bending modal frequency of BIW respectively are 30.8Hz and 43.9Hz,and torsion stiffness is 13689.95 N·m/°in this paper. The above results meet the industry requirements. But the bending stiffness is 11714.17 N/mm and this indicator fail to meet the industry requirement which is 12200 N/mm, so the bending stiffness need to be optimized in subsequent process.And then, the control method of section shape of steel beam has been discussed in this paper and the method of scale vector is selected to be the optimal method after general comparison. Based on analysis of body structure sensitivity, the minimum mass of BIW as optimal goal and the bending stiffness of the BIW as constrains, the section shape of steel beam is optimized and the section shape of steel beam which can satisfy the performance requirements can be deduced reversely according to the scale vector. The results show that the mass of BIW increases by 3.25 kg but the bending stiffness increase to 12310.52N/mm which succeed to meet industry requirement. Afterwards, the mechanical characteristics of body joint structure is researched in this paper. According to the mechanical analysis of joint model, stiffness coupling in joint has been found and explained by using transformation of orthogonal matrix. The calculation of joint stiffness value and flexibility value has been acquired after decoupling of joint structure.Finally, body joint structure is optimized in global and local two ways. The first method is to make shape optimization for stiffening plate of joint in B column and the optimization goal is to ensure the mass of the joint minimized and the constrain is the angular displacement in first branch. The optimization results show that the mass of joint in B column increases by 0.728 kg and the torsional stiffness of joint increase by 291.12%. the second method is to make topological optimization for all the stiffening plate thickness(14 joints and 40 plate thickness in all) in BIW and the optimization goal is to ensure the mass of BIW minimized and the constrain is bending stiffness of BIW. The optimization results show that the mass of BIW increase by 22.13 kg and the bending stiffness increase to 12381.35N/mm, which meet the industry requirements.Some valuable methods and proposals in mechanical properties and optimization of car body beam section and joint structure are presented in this paper which can help to vehicle structure concept design to a certain degree.