| Along with the rapid development of the bus industry of China,the increasing competition requires bus manufacturers to make more efforts in the development of new techniques and product quality.Body structural development is not only one significant part of the bus product development process,but also represents the independent development capability of the bus manufacturers.The innovation degree of body structures strongly depends on the concept design process,which has always been the weak link of the bus industry of China.Sequential design pattern,the main work pattern of the traditional body R&D system,is the main reason why structural engineers lack correct guidance of scientific approach and strongly depend on experience in the early design stage.Besides,due to the work pattern of sequential design,CAE analysis is not involved at the very beginning of body structural design and is only used for local modification and optimization in the later development stage.In a word,there is less communication between structural design and performance analysis,the work pattern of sequential design is short of efficiency.In hence,it is important to introduce concurrent design pattern into the development process of bus body structure to make CAE method play a full role in the structural design,which will greatly increase the efficiency of design process and the quality of body products.Based on the idea of concurrent design and analysis-driven design,this paper mainly conducts research on the matching and optimization issues during the body structural development of a real 12-meter bus body structure.A basic procedure of the top-down design method driven by CAE approach for integral bus body structures is proposed.Firstly,in order to obtain a body structure scheme matching the load distribution,structural matching design method based on bending stiffness is proposed and applied to the simple wireframe sketch during the concept design stage of body structure.Then a cross-sectional size optimization based on sectional library of avaliable profile steel is carried out to satisfy the requirements of both performance optimization and lightweight design.Lastly,to realize the intelligent transformation from CAE optimization results to 3D geometry models,a secondary development is implemented on the basis of MSC.Patran.The proposed techniques constitute the basic framework of top-down design method for integral bus body structures in the early design stage,by which the requirements of lightweight and performances can be both well considered and the time which was wasted for model transforming will be saved.Meanwhile,the method for side-wall joint structural design and optimization is discussed and the influence of weld spacing on local performance of joint structure is summarized,which lays a foundation for the joint structural design.The main contents of this paper follow as:(1)A matching method based on the idea of complementation of the properties for bus body structural design is proposed and applied to the structural concept design process of a real bus body.The proposed matching method for bus body structure aims to solve the contradictory issue between the asymmetry of left and right body side walls and the approximate symmetry of load distribution.Several aspects of studies are carried out in order to realize the proposed method.Firstly,the traditional method for bending stiffness has been improved by mathematical ways,including extraction of feature points,feature curve-fitting and statistical treatments.By several numerical case studies,it is verified that the advanced computational method can effectively reduce the influence caused by loading position and local deformation.Secondly,the definition of bending stiffness for several main body subassemblies and half the body structure is presented by utilizing the advanced bending stiffness computational method.Finally,structural schemes of a real bus body with different bending stiffness matching are established by implementing complementary design to the frame and side wall subassemblies.The impact trend of different bending stiffness ratio upon the performances of bus body structure shows that the body structure gets the best and balance performance while the bending stiffness ratio of the left half to the right half of bus body is close to 1.The result also validates that the bending stiffness matching method for body structural design is effective and usable.(2)A bi-level optimization for cross-sectional size based on the discrete sectional library of bus body is proposed and implemented to a real bus body during the concept design stage in order to satisfy the requirements of both light weighting and performance improvement.For the limited variety of profile steel which are used in production,discrete optimization approach is adopted to the optimization,by which the process of rounding optimal results up to available values is no longer necessary.On the first level of the optimization,the lightest structure under the strength condition is obtained via implementing line-search process in the local bar models of the body structure which are established by extracting internal forces from the overall strength analysis.Besides,the efficiency of the searching optimization is well improved by these local computational models.On the second level,the optimization result of the former level is considered as the initial structure and the design variables are screened by means of sensitivity analysis,then the main performances of the bus body structure are optimized by using NSGA-II,which is a type of multi-objective genetic algorithm.Meanwhile,a programme which aims to distinguish the character of the low-order modes of the bus bodies is written and embedded to the second stage of the optimization process.The application result indicates that the body structure obtained by the proposed optimization method has the advantages of lightweight and heigh performances,which lays a good foundation for the subsequent engineering design.(3)Extended development for MSC.Patran is carried out to realize intelligent modelling of bus body structures during the conept design stage.In the concept design stage,it is much easier to build or modify the finite element model than the 3D geometrical model because the structural analysis can be well performed by the simple beam element model in this very period.In that case,structural designers have to spend more time to read the optimization results from CAE analysis and rebuild the 3D geometrical models repeatedly.The secondary development based on MSC.Patran is to settle the issue mentioned above.First of all,the four basic statuses of property definition in the beam element model of bus body structures are concluded.Based on the conclusion,the subprogramme of bar center lines regeneration is realized.Then the common cross-sectional types of bars in the bus body structure are summarized and the subprogramme of data extraction and geometry creation for cross section is written.On these basics,the automatic module of solid modelling for the entire bus body structure and the modification module for single bar are created.Besides,a specific module for weld spacing design is established,by which the structural designers can easily implement the creating or rotating operations to the local bars according to the given value of joint weld spacing.Via the extended development for MSC.Patran,a bar element model of bus body structure can be easily and automatically transformed into a 3D geometrical model,and the workload of extracting information from optimization relusts and rebuilding geometrical models for bus bodies is efficiently simplified.(4)Free mode test,static strength test and stiffness test are carried out to a real bus body,which is obtained by using the proposed top-down design techniques.The result validates that the the modeling method and the matching and optimizing method for bus body structure is correct and efficient.(5)The optimizing method for detail design of sidewall joints of bus body is discussed.As the overall design process,optimal design for local structure during the concept design stage is an important determinant of the basic performances of body structures.Therefore,the influence of joint weld spacing,which is one of the significant details of the local joint structures,is preliminary studied in this paper.At first,the influence of weld spacing to the strength and fatigue performance of typical joint is obtained based on the simplified models of sidewalls.Then the detailed joint model from right sidewall of a real bus body structure is built and the influence of weld spacing to the strength and fatigue performance of the real joint is obtained.Lastly the influence rules obtained from simplified models and real bus models are comparied and summarized,which provides reference for further research on the top-down design of local structures. |
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