Section Optimization Design Of Car Body Based On NSGA-Ⅲ Under Complex Engineering Constraints

The load-bearing body structure is often simplified by thin-walled beams.The properties of the beam determine the mechanical properties of the entire vehicle structure to a large extent.Therefore,the beam section has a direct effect on the rigidity of the car body in white.The effective design of the beam structure in the conceptual design stage has very important guiding significance for improving the rigidity of the body,ensuring the performance of the body structure and the lightweight design of the entire vehicle.The cross-sectional shape of the multi-chamber thin-walled beam is extremely complex.Therefore,in the design process,its mechanical properties should not only be considered,but various technological constraints in the manufacturing process should be met.In this paper,a detailed analysis of the cross-sectional shape constraints is carried out,and the problems of assemblability and manufacturability of the current thin-walled beam sections of the car body are studied,A cross-sectional shape control method is proposed to be compatible with various engineering constraints and the problem of single-chamber and multi-chamber cross-sections is optimized.The major findings are as follows:(1)The paper analyzes various engineering constraints of the thin-walled beam section of the Body In White,and designs a multi-objective optimization model that can optimize the section shape and plate thickness variables at the same time.A coding method for cross-section control point coordinates is proposed to adapt to various cross-section engineering constraints,which defines the corresponding design variables,and a cross-section attribute calculation method based on this coordinate coding method is deduced.(2)According to the defined design variable form,the mathematical processing methods of various engineering constraints are derived and integrated in the software module.Due to the high dimensionality of the objective function of the beam section optimization problem,the NSGA-Ⅲ algorithm has strong adaptability in dealing with such optimization problems,especially the optimization problem of the combination of discrete variables and continuous variables,so the NSGA-Ⅲ genetic algorithm is introduced to optimize the cross section as the optimization algorithm.The NSGA-Ⅲ algorithm is integrated into the software module to realize the processing of multiple target optimization problems.(3)The beam section shape database was established,It regards the continuous-type section shape control point coordinate value,the discrete-type section plate thickness value as the design variables,and the section attribute values of the section moment of inertia and torsion constant as the optimization target,and at the same time reduces the weight of cross-section as much as possible to achieve lightweight body and establishes the multi-objective optimization model.The corresponding software module was implemented on the Hyper Mesh secondary development platform through the Tcl language.(4)It integrates the multi-objective thin-walled beam section optimization part with the existing fast body structure design system(FVO)to realize the integrated design of body structure section optimization,and realize the lightweight body under the conditions of the rigidity of the body and the constraints of process manufacturing.The test results of the calculation examples show that this optimization method can optimize the multiple section attribute parameters of the beam section under the premise of satisfying various complex engineering constraints when it conducts a optimization design of the car body beam section at the stage of the vehicle conceptual design.It is of great significance to the rapid development of the body structure in the conceptual design stage.