Research On The Key Issues Of Forward Conceptual Design Of Complex Thin-walled Automotive Body

The vehicle body is the main activity space of passengers,and the structural performance directly affects the life safety of passengers.The research shows that the influence of body stiffness on vehicle stiffness performance is more than 60%,so the body structure design plays an important role in vehicle development process.According to the body design stage,it can be divided into conceptual design stage and detailed design stage.Among them,the concept design stage is an important part of the body structure design process.In terms of time and cost,the conceptual design stage accounts for about 40% of the vehicle body development prosecc,but determines 70% cost of the vehicle body design.This stage mainly studies the static stiffness,dynamic stiffness and crashworthiness of the vehicle body structure.According to whether there are detailed three-dimensional models in the early stage of the design,the conceptual design can be divided into forward conceptual design and reverse conceptual design.Traditional body design is mainly based on reverse design and simple modification of existing body structure.However,for performance-oriented body structure design,the forward conceptual design has more advantages.As the limitation of body structure data in the conceptual design stage,it is hard to simulate and analyze the body structure quickly.Therefore,the research on structural analysis method applicable to the forward conceptual design and avoiding the limitation of body structure data are of great significance to improve the efficiency of body design and the development of new car body.In this paper,based on the body structure with complex cross section shape in the conceptual design stage,the key problems in the process of establishing the body mathematical model are studied.Based on the introduction of the research background and significance of forward conceptual design,the main methods of realizing the mathematical modeling of conceptual body structure are described.The existing mathematical modeling methods of beam element and joint element in both domestic and overseas are summarized.The important engineering value and theoretical significance of considering the flexibility characteristics of complex thin-walled beam and joint in conceptual design stage are pointed out.In order to improve the calculation method of mechanical properties of complex thin-walled cross section,the management platform of the mechanical properties calculation of cross section is developed,and the system platform of body conceptual design is further built to promote the efficiency of forward conceptual design of thin-walled body.The main contents of this paper are as follows:(1)The stiffness chain model of thin-walled frame structure of vehicle is established to realize the performance-oriented thin-walled body structure analysis.Based on the geometric data of simplified benchmarking vehivle body,considering the complex cross-section geometric characteristics of the thin-walled beam element,the simplified body model is transformed into a space frame structure composed of semi-rigid thin-walled beams by using the thin-walled beam transfer matrix method,so as to realize the performance analysis of the bending stiffness,torsional stiffness and first-order eigen-frequency of the conceptual design body.(2)The simplified method of joint model is improved to improve the calculation accuracy of body structure performance analysis.Based on the flexibility characteristics of the joint,the star spring model of three tension springs and three torsion springs is taken as the simplified model of the joint,and the joint mathematical model is constructed by the traditional beam theory.The response parameters of the joint model are substituted into the mathematical model as input conditions,so as to obtain the stiffness coefficient of each branch of the joint.And the surrogate model of the joint is further studied.Taking the mechanical properties of the section as the independent variable and the stiffness coefficient of the joint as the dependent variable,the surrogate model of the joint is constructed by the neural network fitting method,which promotes the full parameter design of the body joint performance.(3)The calculation method of mechanical properties of cross section is developed to improve the efficiency of mathematical modeling of stiffness chain of thin-walled body structure.The cross section properties include mechanical properties and geometric properties.The cross sections of the vehicle body are divided into open section,closed section and mixed section according to the geometric characteristics of the cross section.The calculation method of the mechanical properties of the cross section of the body is derived to improve the efficiency of the body stiffness chain modeling.Based on the hybrid programming of My SQL and Matlab,the mechanical property calculation platform and database management system of main section are established to provide data support of cross section in body structure design.(4)Develop vehicle body concept design system platform,and the effectiveness of the platform is verified by an optimization example.Based on the object-oriented software development platform provided by MATLAB,a vehicle body conceptual design system platform with user interaction function is developed,which includes three modules: pre-processing,calculation and post-processing.A unified system regulation is made for the input of vehicle body geometric parameter data,the establishment of vehicle body mathematical model and the addition of constraint/loading boundary conditions.It can be used to analyze the structural performance of the thin-walled vehicle body and optimize the shape of the cross section.In order to verify the effectiveness of the system platform,the static stiffness and first-order eigen-frequency of the structure are taken as constraints to complete the shape optimization design of the cross section of the benchmarking vehicle.