<b>Multi-level Optimization Strategy for </b><b>carbody</b><b> Modal Performance and Lightweight Design</b>

Structural damageof metro carbodystructures under extreme operating conditions significantly compromisesoperational safety. Addressingthe limitations of conventional optimization approaches that consider single load cases and employ oversimplified optimization methods, this study proposes a novel multi-level optimization strategy for metro carbodystructures. The methodology enables multi-objective optimization analysis incorporating stiffness, static strength, modal performance, mass, and fatigue damage, providing engineers with an efficient framework for carbodystructural optimization. The research consists of threekey phases: First, a hybrid multi-level optimization approach integrating both topology optimization and size optimization techniques was developed. Second, a finite element model (FEM) of the metro carbodywas established and experimentally validated. Subsequent evaluation of the principal mechanical performance metrics was conducted based on the verified FEM. Third, the proposed multi-level optimization method was implemented, performing topology optimization on cross-sections of floor profiles and primary load-bearing roof profiles. Finally,conducting size optimization on thickness parameters of highly sensitive components based on RBF neural network and MNSGA-Ⅱ. The optimized carbodystructure achieved a 141 kg mass reduction and increased the first-order vertical bending frequency from 9.52 Hz to 10.047 Hz, demonstrating the effectiveness and feasibility of the proposed methodology. The findings provide valuable insights for optimizing metro carbodyperformance under complex operational conditions, particularly regarding the balanced improvement of multiple competing performance indicators.