Research on lightweight design of electric vehicle lower body structures based on composite materials and multi-objective optimization

This paper proposes a hybrid material sill beam structure combining high-strength steel outer panels with carbon fiber reinforced plastics (CFRP) reinforcement plates to enhance the stiffness of the vehicle underbody. Through multi-objective optimization, the crashworthiness and lightweight performance of the underbody ring frame are improved. First, a finite element model for the white body and full vehicle of an electric vehicle is established for frontal impact testing, with model accuracy verified through experiments. A multi-level optimization approach for CFRP sill beam reinforcement plates based on T700/WP-R2300 composite material is proposed, encompassing free-dimension optimization, dimensional optimization, and layup sequence optimization. The optimized sill beam assembly is integrated with components such as the A-pillar and torsion box to form the lower body frame ring. A multi-objective optimization model based on the NSGA-II algorithm was established to optimize component thicknesses. The TOPSIS method was employed to obtain the optimal ranking of the Pareto solution set. Optimization results demonstrate that the optimized design achieves a 29.2% weight reduction in both side lower sill beams and a 10.73% weight reduction in the lower body frame ring. Concurrently, acceleration at the right B-pillar decreased by 6.67%, along with a 4.53% reduction in intrusion at the junction between the right lower A-pillar and lower sill beam. Significant lightweighting objectives were achieved while maintaining the frontal crash performance of both the body-in-white and the complete vehicle.