Research On Multi-objective Lightweight Of A Bus Body Frame Based On Rollover Safety Verification

The bus body structure occupies almost 30%-50% of the complete vehicle kerb mass.According to the research datum,if the bus weight decrease 10%,the fuel efficiency can increase 6%-8%,in no–load cases,about 70% of the energy consumes on the body structure mass.So,lightweight design is necessary without sacrificing the bus safety,especially for body structure lightweight.In this paper,a 12-metre bus developed by a company is taken as the research object.Multi-objective size optimization of bus body skeleton is carried out,and the reliability of this lightweight scheme is tested by comparing the performance of the body structure before and after optimization.The main contents of this paper include the following aspects:1)The finite element model is established in the HyperMesh according to model specification after simplifying the three-dimensional CATIA model of bus body provided by the enterprise.2)Based on the body finite element model,typical working conditions and modal analysis of the bus body skeleton are carried out.The strength and stiffness of bus body are analyzed by simulating 4 typical working conditions,including horizontal bending,extreme torsion,emergency braking and emergency turning.The frequency distribution and mode shapes of the first 6 free modes of the body are analyzed to verify whether the body structure meets the design requirements.3)Based on direct sensitivity analysis and relative sensitivity analysis,some skeleton structures for optimal design are selected.According to the symmetry and functional similarity,the body structure is grouped into groups,and the wall thickness parameters are taken as design variables.The results of direct sensitivity analysis and relative sensitivity analysis are combined to screen out the structures which are sensitive to the weight of the skeleton,but not sensitive to the performance of the body skeleton,so as to improve the optimization efficiency.4)Multi-objective size optimization of the body skeleton is carried out to determine the optimization scheme in the HyperStudy.Based on Multi-Objective Genetic Algorithm,multiobjective size optimization was done by choosing the thickness of the filtrated components as design variables,targeting the minimum mass of bus structure and the maximum of bending and torsional stiffness,and restraining the first-order free modal frequency of bus body skeleton.As a result,the bus body structure obviously lose 197 kg weight,loss weight ratio reach 8.9%.Then,the stiffness,strength and modal performance of the body skeleton before and after lightweight are compared and analyzed,and the reliability of the scheme is preliminarily verified.5)Based on the requirements of GB17578-2013 “The standard of the bus upper structure strength requirements and test methods”,a finite element model of rollover test is established.The rollover safety of the lightweight bus body is verified in the LS-DYNA.The results show that the lightweight body structure meets the requirements of the national standard,and the rollover safety of it has little difference from the original body,which further verifies the reliability of the lightweight scheme.