Research On The Stiffness Matching Method Improvement And Local Precise Design Of Bus Body Structure

Many social issues such as environmental pollution,energy exhaustion and road congestion has always been restricting the development of the national automotive industry.As one of the most common public conveyances,buses have been playing a very important role in relieving traffic congestion and saving energy and reducing emission.In recent years,the fiercely competitive market brings higher demand on research and development of products to bus enterprises.Bus body design is a key part during the product development process,which represents the shaping language of the bus and further reflects the level of independent research and development of an enterprise directly.The design process of bus body can be generally divided into four phases,including concept design,detailed design,prototype vehicle trial and test.Concept design pays more attention to the general configuration,while detailed design puts more emphasis on the fineness of the key locals.Compared with foreign development,the top-down design of autobody has always been a weakness in our national bus companies.Due to the lack of effective guidance by scientific methods,designers are over dependent on previous design experience.Some advanced design concept such as concurrent engineering(CE)and CAE analysis have not been utilized sufficiently,which further leads to unsatisfactory in designing autobody structures.Many structural defects and problems can only be remedied by localFor recent years,integral bus body has become popular because of its lightweight structure and great load-carrying properties.Compared with separated body-frame structure,integral bus body changes a lot in both structural types and load transfer,which brings some new research topics for product development aimed at integral body characteristics.With this background,combining research projects of several integral bus bodies,this paper improves the stiffness matching method during top-down bus body design process by CAE analysis,and researches some key technology such as precise design on local structures.At concept design phase,according to the structure characteristics of integral bus body,this paper takes bending stiffness and torsional stiffness as design objectives to reasonably divide body structure into several modules and make a stiffness matching design.And the body structure design method and process at the concept design phase are initially discussed and summarized.On the basis of the above,this paper will do further research on improving the parts of key joints in bus body structure,so that the bearing capacity of bus body can be further improved with lightweight requirement being satisfied at the same time.To do this,the cross joint is taken as the research carrier to measure and analyze the welding deformation of the local parts of the joints under different welding process schemes.Then the influence of welding deformation under different welding process on local strength of the joints can be studied by finite element analysis.Comparing the local strength with each other,the welding process of the joints can be improved.Finally,a joint with high stress is extracted from a certain bus model to initially improve the parameters of weld corner and summarize the design rules.Based on the above research route,this paper completes several aspects of content as follows:(1)A body structure design method based on stiffness matching is proposed.According to the structural characteristics of integral bus body,stiffness is taken as the objective to reasonably optimize the arrangement of main beams of bus body at the preliminary stage of design,so as to obtain the concept design scheme of bus body juggling properties and lightweight.Firstly,the existing stiffness analysis methods,including vertical bending stiffness and longitudinal torsional stiffness of bus body,is analyzed and improved.Through curve fitting,transformation of coordinates and mathematical statistics,the influence of load position and local stiffness on analysis results can be greatly reduced.And the improved analysis method is programmed by FORTRAN code.On this basis,a stiffness evaluation method for bus body local modules is initially proposed.The stiffness of each module is adjusted in proportion as the change of Young’s modulus of the material.Then considering that the body structure with small deformation is similar to a linear system,the contribution degree of each module to the vehicle can be evaluated by calculating the proportion of the stiffness variation of vehicle occupied by that of each module.Finally,combined with the above method,a bus model to be developed of a certain company is taken as the carrier to be divided into several modules and to do stiffness match.Then the detailed process of the matching method is summarized.(2)The welding deformation of bus framework joints under different welding process schemes is analyzed.Firstly,based on the existing researches,the typical cross joint of the bus structure is taken as the research objective.The dimension and material parameters of the joint specimen are determined in accordance with the structural characteristics of the cross joint.And also,the spot-welding fixture and measuring tools are respectively designed and made.On the basis of these,reasonable testing schemes are formulated referring to the existing welding specifications.Separately taking welding sequence and welding current as the test variable,the welding deformation of joint specimens in each direction is measured.Finally,the influence of different welding parameters on local welding deformation of the joint can be summarized by processing the test data,so as to provide reference for designing fixture and optimizing process parameters during welding process of bus framework joints.(3)A local strength verification method of bus framework joints under the influence of welding deformation is proposed,and the welding process optimization of joint part is studied.On the basis of deformation data,welding process and structure design are combined together to propose a local strength verification method of the joint initially.Firstly,the deformed local model of the joint is established based on the welding deformation data,and then the welding deformation stress of the joint can be simulated by reverse loading.Secondly,the welding deformation stress and the joint model simulation stress under actual working conditions are superposed by extracting the element stress components,and then the local strength of the joint can be verified by the combined stress after superposing.Based on the above verification method,the optimum welding process scheme is selected by comparing local strength of the joint under different welding process schemes.An optimization method of welding process of bus framework joints is initially proposed,and the detailed process of this optimization method is summarized.(4)The optimal design of joint weld corner is discussed,and the influence of side length,shape and thickness on joint local strength is investigated.Firstly,a joint with high stress is selected from the keel of a certain bus as the research objective,and weld corner models with different specifications are built respectively at the joint.Secondly,the local strength of joint structures with different specifications under three typical working conditions is calculated by simulation analysis.The three typical working conditions include full bending loads,left front wheel suspended,and right front wheel suspended condition.Finally,comparing the simulation data with each other,the influence of shape and dimension parameters of the weld corner on joint local strength is initially obtained,so as to provide reference for optimization design of the weld corner.