| Vehicle lightweight is an important measure to save energy and reduce emissions,and to achieve "carbon peak" and "carbon neutrality";it is also a key technical approach to improve vehicle acceleration and braking,NVH characteristics and crash safety.The mass of the body accounts for about 30%-40% of the car,and the lightweight of the body is an important part of the lightweight of the car.The mixed use of high-strength and lightweight materials represented by high-strength steel,aluminum alloy,and carbon fiber composite materials on the car body is the future direction of lightweight development.However,the forward development of lightweight multi-material body is a complex project involving the material selection of parts and components in different parts of the body,the conceptual design of the body beam frame and the connection of dissimilar materials,and a systematic forward development design method for multimaterial body has not yet been established.Therefore,multi-material body lightweight material selection,multi-material body beam skeleton conceptual design,and body beam skeleton structure-connection-performance optimization matching are the common key technologies for the forward development of multi-material body,and are also the research hotspots and difficulties of future automotive body lightweightA small automotive body is taken as the research carrier,this thesis focuses on the research on material selection decision-making method of multi-material body driven by structural mechanical performance,the conceptual design of multi-material body beam skeleton and multi-dimensional collaborative optimization research of multimaterial body beam skeleton structure-connection-performance.According to the lightweight material selection,conceptual design and multi-objective optimization results of the multi-material body beam skeleton,the front-end structure of the concept body beam skeleton was fabricated,and the frontal collision test was carried out.The main research contents of the paper are summarized as follows:Firstly,a small passenger car was used as the benchmark car to establish a benchmark body-in-white finite element model,and a 100% frontal collision and side collision finite element model of the benchmark vehicle was built.The accuracy of the finite element model was verified by a real vehicle collision test.The bending stiffness,torsional stiffness,first order bend and torsion modal frequency,frontal and side impact resistance performance of the benchmark body were simulated and analyzed,and the relevant evaluation indicators were extracted as the reference for the forward development of the subsequent multi-material concept body.Then,a material selection decision-making method of the multi-material body driven by structural mechanical performance was proposed.The proposed material selection decision-making method includes three subsystems: 1)Decision-making criteria for lightweight material selection for multi-material automotive body was established.Aiming at the mechanical properties of thin-walled body beams in the two categories of linear elastic small deformation and large plastic deformation,numerical analysis method was used to analyze the structural bending-torsional stiffness,bendingtorsional strength,axial average crushing force,transverse buckling moment or force.Based on the internal correlation mechanism among the mechanical properties,structural mass,cross-sectional shape,size parameters and material properties,six structural mechanical properties-driven lightweight material indices combined with material prices were established respectively as the material selection decision-making criteria.2)A quality function deployment weighting method for the lightweight body material selection was proposed,which is coupled with AHP and Fuzzy AHP.The proposed weighting method is used to determine the mechanical performance design requirements of the parts at different positions of the body,and then the weights of the decision criteria for the lightweight material selection of the multi-material body are solved.3)Grey relational analysis is used to make multi-criteria decision-making on the comprehensive performance of alternative materials for body parts.The lightweight material selection of the front longitudinal beam of the body was taken as an example,the specific application of the proposed method was discussed in detail and the material selection effect was verified.Secondly,a conceptual design method of multi-material automotive body beam skeleton was proposed.The finite element model of the conceptual body beam skeleton was established,and the lightweight material selection of different parts of the conceptual body beam skeleton was realized by using the proposed material selection decision method.Then,the complex cross-section design of thin-walled beams of multi-material concept body is focused on,and a multi-level matching optimization design method for thin-walled beams complex cross-sections of multi-material concept body is proposed.In the first level,the simplified cross-section of the thin-walled beam of the concept body was optimized,and the performance design requirements of the of the conceptual body were quickly met;in the second level,according to the forming process characteristics of different material structures,a complex section shapes database of thin-walled steel beams was established,the complex section shapes of aluminum thin-walled beams were designed through the joint topology of multiple working conditions.The mechanical properties of the complex section of the thinwalled beam were solved,and the mechanical properties of the section were used as a bridge to solve the problem of matching and converting the simplified section of the thin-walled beam to the complex section.In the third level,the precise optimization method of complex section based on proportional vector control was established,and a new way for the lightweight optimization design of the complex section was provided.Through the multi-level progressive section matching transformation and optimization,the multi-material complex section thin-wall beam design was completed,and then the conceptual body beam skeleton model was built and the performance analysis was carried out.Compared with the initial multi-material conceptual body beam skeleton,the mass of the established multi-material conceptual body beam skeleton with actual complex cross-section reduces by 8.40 kg.The bending-torsional stiffness,one-bendone-torsion modal frequency,the performance indexes of frontal collision and side collision of the multi-material conceptual body beam skeleton meet the design requirements,which verifies the effectiveness of the proposed method.Then,the multi-dimensional collaborative optimization of multi-material concept body beam skeleton structure-connection-performance was completed.The mechanical properties and failure mechanism of self-piercing riveting(SPR)joints of steel/aluminum dissimilar materials were studied by means of theoretical analysis,simulation analysis and experimental testing,and a simplification model of the SPR joint of steel/aluminum structure in the conceptual body beam skeleton was established.The rivet set coding technology was proposed to solve the problem that the parameters of the SPR joint cannot be updated automatically during the optimization process,and the parametric multi-objective optimization model of the conceptual body beam skeleton was constructed by comprehensively using the model parameterization and mesh deformation technology.The optimal design variables were screened out by the combination of contribution analysis and experimental design.The RBFNN-Kriging hybrid surrogate model of the conceptual body beam skeleton was constructed,and the NSGA-II algorithm was combined to complete the multi-dimensional automatic iterative optimization of the conceptual body beam skeleton structure-connectionperformance.The Pareto frontier was obtained.Finally,the AHP-TOPSIS method was used to perform data mining on the Pareto frontier,and the optimal optimization scheme was selected.The results show that the performance of the optimized conceptual body beam skeleton was significantly improved.Finally,the optimal design variable parameters after the multi-dimensional synergistic optimization of the concept body beam skeleton structure-connectionperformance were re-assigned to the concept body beam skeleton finite element model.The simulation analysis of basic static-dynamic performance and collision performance were carried out.According to material selection of the multi-material conceptual body beam skeleton,complex section design and multi-objective optimization results,the front-end structure of the conceptual body beam skeleton was fabricated and a frontal collision test was conducted.According to the simulation analysis results of the final optimized design scheme and the frontal collision test results of the front-end structure of the conceptual body beam skeleton,the effectiveness of the method proposed in this paper is verified. |
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