Research On Modeling And Simulation Of Simplified Car Body Based On A Passenger Car

The body structure design usually contains the conceptual design stage and detaileddesign stage. Taking the design cost into account, nearly70percent of the cost will be spentin the conceptual design phase. In time, in order to shorten the body design time and reducerepetitive design, the key is to complete the concept design efficiently. However, vehicleperformance analysis and study is still concentrated in the detailed design and manufacturingand product development stages, while very little work will be completed in the conceptualdesign phase. Once the body structure need to be modified, the entire development workshould be done again, which will increase the design cost greatly. This means, the work donein the conceptual design stage will have a great impact on the follow-up works.In this thesis, based on the detailed structure model of a passenger car, the modelingmethods of simplified passenger car body model were studied, which is used in conceptualdesign phase. By analyzing the body structure and the basic performance of the detailedmodel (torsion and bending stiffness, opening deformation, lower order modes, etc.), thesimplified body models used in conceptual stage were constructed using two methods. Infirst method, also called PBM method, a property-based and parameterized model was usedto describe beam structures of car body. In the second method, the simplified model of beamstructure was built based on its sections. The built model was called SBM, and this modelingmethod was called SBM method. By comparing the basic performances with those of thedetailed model, the simplified model was validated with good effects. By analyzing theadvantages and disadvantages of these two methods, a mixed modeling method was putforward, as well as the corresponding calculation and validation. In order that these twomethods of simplified car body modeling can be implemented rapidly on commercialsoftwares, based on HyperMesh secondary development function, the modeling automationof conceptual car body was also studied. In the study of the PBM method, a method was put forward and used on this thesis,whichbuilt the corresponding equivalent stiffness beam elements based on the classic formula ofmechanics of materials and the overall stiffness of the beam members. Other parameters ofthe cross-section was expressed by the following parameters:the cross-sectional area, twoprincipal moments of inertia and the torsion constant of the beam elements, and then theoverall stiffness of the beam elements was determined. In the solution process of the beamelement cross-section parameters, the method of determining the beam section moment ofinertia and its orientation was studied, and the cross-section parameters of the simplifiedbeam structure was calculate by using an appropriate loading position..The property-based and simplified model focused on expressing the performance of theoriginal structure from its mechanical properties and ignored the simulation of the mass, sothe mass of them was unequivalent, which led to the inaccuracy of the dynamic analysis ofthe simplified model. Therefore, the mass of the simplified model was made equivalent tothe detailed model in this thesis by adding non-structural mass to each beam element, and thedesired results were obtained by comparing the lower order modes of two models.By analyzing and validating the basic performances of the simplified model built byPBM method, it was found that the error between torsion and bending stiffness was not morethan9percent, the error of the opening deformation was less than1percent, and the error ofthe lower order modes was less than14percent. Such errors were able to meet therequirements of the conceptual design stage, so the simplified model could simulate the basicperformance of the original body structure very well.The equivalent models built by PBM method could not reflect the cross-sectiondimensions of beam structures. Therefore, the method of rapidly building the simplifiedmodel based on the section dimensions was researched in this thesis, which is called theSBM method. The actual cross-sections of the beam members were simplified in theequivalent model built by SBM method, but still maintained the same characteristics of thetrue cross-section. By validating the basic performances of the simplified model, it wasfound that the torsion and bending stiffness of the simplified model were both greater than that of the original structure。Therefore，The influence of the stiffness characteristics of thebeam members was studied as a result of using the closed chamber to simulate the weldingcross section. And the difference between the beam components were simulated by the shellelements and beam elements was studied.In order to better apply the two modeling methods, this thesis analyzed and comparedboth the advantages and disadvantages of the two modeling methods comprehensively andsummarized their respective cause of having errors. Another method of building thesimplified model of the load-bearing structures using the two methods was proposed in thisthesis for avoiding weaknesses as much as possible. It was shown in the studies that theoverall stiffness of the hybrid model based on the two methods was guaranteed; the scope ofapplication of the PBM model was extended and the error caused by the excessive SBMmodels was reduced. The interested parts could be optimized using the parameter of the wallthickness. It could extend the use of the benchmarking car and apply the mechanicalinformation to the development of the new model fully.HyperMesh provides a secondary development interface, and the modeling automationof conceptual car body was also studied in this thesis. As an example, the process automationof modeling was showed by extracting the detailed finite element model of joints. In theconceptual design stage, the modeling process was automated, which could greatly reducethe creation time of the simplified model.In summary, the simplified model built in this thesis could more accurately predict themechanical properties of the car body structures that would be developed，which provided amethod for the car body modeling in the conceptual design phase. Owing to the automationof the process, the establishment of the conceptual model was greatly simplified and thedesign time of the conceptual design phase was greatly reduced too.