Methods For Lightweight Automotive Body Design Using Multi-material Construction

Vehicle weight reduction is one key way to achieving fuel efficiency, harmful emissions reduction and raw materials saving. One basic approach to reducing the weight of car bodies is using lightweight materials. Compared to single-material automotive bodies, multi-material automotive bodies allow exploitation of the performance and cost advantages of each material and optimal material selection for each structural component, therefore, multi-material automotive bodies can achieve weight reduction under cost and performance constraints. The principle of the multi-material construction is that proper materials are selected for the intended part functions. This dissertation presents methods for optimal design of lightweight automotive body using multi-material construction with respect to structural performances, mass and cost, based on the principle of the multi-material construction. The main works in this dissertation are summarized as follows:1. Study on auto-body structural performances analysis and sensitivity calculation. The simulation and analysis of auto-body static stiffness and frontal crashworthiness are performed using finite element method. Sensitivity analysis is carried out to study the effect of the parametric variables and identify the main components. According to the structural characters and functions, car bodies are divided into two main groups, large body panels and thin-walled beams. And then, appropriate approaches to materials selection for large body panels and thin-walled beams are presented.2. Materials selection and design of large body panels for weight reduction. Taking automotive door as example, approach to material selection and design is presented. Firstly, a method for optimizing automotive doors under multiple criteria involving the side impact, stiffness, natural frequency, and structure weight is presented. For each potential material, the structural optimization method is employed to determine main panels' optimum thicknesses. And then, cost analysis is performed. Finally, the value function is employed to determine the optimum design of the door with respect to mass and cost. The present approach is feasible and suitable for material selection for other large body panels.3. Materials selection and design of thin-walled beams based on material performance indices. Accurate material performance indices for thin-walled beams of automotive bodies are built. The lightweight effect of lightweight materials to traditional steel can be analysis using the performance indices. The role of performance indices in the optimal material selection is studied. The method for material selection using material performance indices is presented. Values of performance indices for thin-walled beams are calculated and validated. And then optimal materials are selected for each primary thin-walled beam to design the body as inexpensive and light as possible. Compared to the original auto body, the optimal multi-material body structure achieves weight reduction of 30.9kg with only $14.3 (7%) cost penalty.4. Method for combined structural and material optimization of automotive body. The method for combined structural and material optimization design is presented. The material type and the thickness of each independent panel are treated simultaneously as design variables. And then the mathematical model of the optimization problem is built. Neural network approximations are used to represent the relationships between constraints (structural performances) and design variables. The optimization problem is solved by a multi-objective genetic algorithm. The proposed method enables optimal materials selection for each independent component of an assembly while simultaneously determining sizing variables.From the results of the study, it can be found that if proper materials are selected for the appropriate parts, multi-material automotive bodies can achieve weight reduction with small cost penalty.