| With the rapid growth of automobile, it has brought a series of social problems such as excessive energy consumption, environmental pollution etc. Lightweight is an important mean to reduce energy consumption, emissions and save raw materials. BIW mass account for 30-40% of the total mass, and it account for about 60% of the whole vehicle cost. Under no-load conditions, about 70% of the fuel is consumed by BIW. Therefore, BIW lightweight is an important part of the vehicle lightweight.The probability of frontal collision, injury and lethal is very high. The safety performance of frontal collision is one of the important performances. The mass of frontal structure account for about 30% of the whole BIW, the energy absorption is about 80%. So, the lightweight design for frontal structure seems more important. Lightweight design for frontal structure is a complex engineering, which involve multiple performances such as static stiffness, dynamic stiffness, NVH, crashworthiness and cost etc. With the help of SFE-CONCEPT software, this paper establish implicit parametric frontal model of BIW, and coupling together with the finite element(FE) rear structure of BIW. Lightweight integration optimization and design is done based on the coupling BIW, considering the factors of thickness, materials, shape, part curvature etc. while ensuring the static bending and torsion stiffness, low order modes and 100% frontal crash safety performances without obvious reducing and the cost without obvious increasing. The main researches of this paper include the following aspects:(1) The static bending and torsion stiffness and low order modals of the initial FE BIW are analyzed through simulation, and the performances are compared with tests. Then the BIW connects with FE chassis, engine and closure etc. to form the whole vehicle. After corresponding setting, the safety performances of 100% front crash and side impact of the initial FE vehicle is analyzed through simulation, and the safety performances are comparing with test. The analysis of both simulations and tests are done according China “New Car Assessment Program”(CNCAP). The initial FE model is proven effective though these comparison and Parametric model can be established based on FE model.(2) Parametric model is established based on FE model and the coupling model is constructed based on frontal parametric BIW and rear FE BIW. The performances of static bending and torsion stiffness, low order modes front crash and side impact are analyzed based on coupling model. Then the performances of coupling model are compared with FE model. The coupling model is proven effective though these comparisons and the multi-objective lightweight integration optimization and design can be done based on coupling model.(3) In order to reduce the repeat work during DOE in the process of the multi-objective lightweight integration optimization and design, this paper adopts modular method. And the optimization and design is done using surrogate model. The parametric frontal BIW and FE rear BIW save as respective modular based on modular classification principle. The performances of static bending and torsion stiffness, low order modes and front crash of sample points can be analyzed quickly through modify the parametric frontal BIW and combine with each corresponding modular.(4) The optimization time can be reduced through reasonable choosing variables in the process of multi-objective lightweight integration optimization of the parametric frontal BIW. This paper analyzes the relation and difference between the conventional criterions. Based on these analyses, this paper propose "comprehensive sensitivity" criterion, and prove that this criterion has higher optimization efficiency. 14 design variables are selected from 23 initial design variables by using this criterion. Three conventional surrogate models of Quadratic Response Surface Methodology, Kriging and Radial Basis Functions Neural Network are constructed. The approximate responses of all performances are obtained through NSGA-II algorithm. Then the approximate responses are compared with actual response, and the results showed that the Kriging surrogate model has least relative error.(5) In order to further reducing weight of the parametric frontal BIW, the material of front anti-collision beam and crash box is substitute from steel to aluminum alloy. After material substitution, topology optimization is done and corresponding main section structure is got. Then the parametric front anti-collision beam and crash box are established, and connected with the rest parts of parametric frontal BIW. After these steps, the dissimilar materials parametric frontal BIW is established. The crash box and front rail is connected through two flanges, and the two flanges are connected through bolt. In order to get the size, thickness and material parameter of the front anti-collision beam, crash box and flange, this paper increases 12 variables on the basis of the earlier 14 variables. At the same time, the paper also puts forward the method of calculating the cost of the components, considering the cost changes brought by the integrated optimization design.(6) The multi-objective lightweight integration optimization is done through Kriging surrogate model. The optimization objectives are mass of frontal BIW, static torsion stiffness and cost, which expects minimum, maximum and minimum. The constraints are the static bending stiffness, first order torsional and bending modal frequencies and front crash safety performances, which are not reduced obviously. The optimal compromise solutions are obtained through NSGA-II algorithm. In the process of optimization, there are 26 variables, which contain thickness, materials, shape, and part curvature. This paper chooses the least mass compromise solution as the optimized solution. After simulate and compare with the coupling model. The results show that the mass is reduced 5.81Kg; the lightweight rate is as high as 7.01% with the static bending and torsional stiffness, low order modal frequencies and front crash performances not reducing obviously and the cost increasing only 3.30%. Otherwise the side impact nearly has no influence after optimization of frontal BIW. |
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