Lightweight Multi-objective Collaborative Optimization Design Of Hybrid-material BIW

The lightweight technology is one of the main ways to achieve energy-saving and emission-reduction for vehicle,the lightweight design of body in white(BIW)is an important part of the automotive lightweight technology,research shows that the weight of vehicle is reduced by 10%,the fuel consumption will be reduced by 6%-8%,the corresponding emissions will be reduced by 4.5%.Due to the BIW accounts for 20%-30% of the total weight of vehicle,and it consumes about 30% of the fuel under no-load case.Therefore,the lightweight design of BIW plays an important role in the lightweight technique of vehicle.The BIW is an important load-bearing part,bearing the excitation from the road and the weight of the occupants.The loading condition is complex,and is also an important guarantee for the living space of occupants.So the performance of the BIW is not only affects the NVH,handling stability,and fatigue reliability of the vehicle,but also directly affects the collision safety performance of the vehicle.Therefore the lightweight design of the BIW is a comprehensive and multidisciplinary design project.In this paper,taking “the suitable material in the right position” as the design concept,using the steel BIW as the design foundation,The hybrid-material BIW is designed using light-weight materials such as high-strength steel,aluminum alloy and carbon fiber composite materials,and the multi-objective optimization techniques are adopted to optimize the carbon fiber composite floor,the section dimensions of beam and some key variables of BIW.In this paper,the optimization design of BIW integrates successfully structure-material-process design,and establishs the development process of BIW,and accumulates some engineering experience.First of all,the implicit parametric BIW model is built by based on the steel BIW.By comparing the experiment and simulation results of the static bending stiffness and torsion stiffness performance,the characteristic of lower-modal and the impact performance of BIW,the accuracy of the parametric model of BIW is verified,providing reliable data for the later optimization design.Then based on the implicit parametric BIW model,the carbon fiber composite material floor is used to replace the steel floor and the hybrid material BIW is built.According to the theory of composite material mechanics and the principle of composite structure design,the T300/5208 and the Araldite 2015 is selected as the manufacturing material and adhesive material of composite floor,and their performance parameters is achieved by experiment.The composite floor integrates front,middle,and back modules,realizing the integration design.Through the free size and size optimization method,with the elements thickness as the design variables,the bending stiffness and low order modal frequency as constraint,the weight coefficient of BIW as the goal,the floor optimization model is built.Finally,the paving block shape of each layer and the number of each layer are determined,which provided model and data support for the subsequent optimization of the layering sequence.According to the characteristic of explosive combination of variables in the optimization of layering order,Discrete particle swarm algorithm is carried out based on the traveling salesman problem and the basic particle swarm algorithm,and the memory check strategy and escape strategy is introduced,which effectively avoid the phenomena of “invalid exchange” and “premature” in the paving order optimization problem,improving the efficiency and accuracy.The discretization strategy of continuous variables avoids the loss of the in-plane stiffness of the laminate,the continuous fiber plying strategy reduces the defect caused by the lost layer phenomenon and ensures the continuous mechanical properties of the laminate.The rationality of the optimal solution is verified by comparing the static dynamic performance of steel BIW and hybrid-material BIW.Finally,take the hybrid-material BIW as research object,34 implicit parametric design variables is defined.On this basis,26 optimization variables which have great influence to the performance of BIW are determined by using the contribution degree and the main effect screening method.The optimization model is built which takes the static dynamic and collision safety performance as constraints and objectives.According to the error analysis of the Kriging and RBF approximate model,determining the RBF approximation model is more suitable for the nonlinear collision problem.Then the BIW is optimized by using MNSGA-II optimization algorithm,and obtaining 258 groups of non-dominant solutions.To select the optimal compromise solution,the grey entropy method is proposed to rank the 258 group of non-dominant solutions and obtaining the optimal solution A.To validate the reliability of grey entropy method,the TOPSIS method is used to rank these non-dominant solutions,and obtaining solution B.According to the comparison result before and after optimization of BIW,the weight of the floor is reduced by 34.4% and the light weight coefficient of BIW is reduced by 11.5%,verifying the feasibility of grey entropy method and the solution A,and the accuracy of the approximate model is also further verified.