On Lightweight Design And Reliability Analysis Of Key Car Components Based On Loadcase Simulations

Energy saving and environment protection are two constant topics in car developments.An important way to achieve both goals is through lightweight designs.Both theories and experiments show that vehicle energy consumption is proportional to total weight.Weight reductions in passenge r cars are particularly important since passenger cars corresponding to 2/3 of the total number of vehicles in the world.Two trends are very clear in current car development: one is the transfer to electrical power,the other is the transfer to intelligen t system.Vehicle lightweight technologies play an important role in both fronts.With the developments of material technologies,there are a lot of challenges in lightweight designs,especially when domestic forward car design technologies are under development.The objective of this thesis is to perform deep research work in the key areas of vehicle lightweight design.First,the research proposed the theory and methodology to simulate loads for lightweight design and reliability analysis of key components,by building full vehicle load simulation platform based on flexible structures.Then the focus was put on structural innovative design and optimization with lightweight materials.The research consists of lightweight design and reliability analyses of ke y components in car body,suspensions and drive-line system.Several innovative methods and theories were presented and the main innovative points in the thesis are:(1)Load simulation methodologies for key components in passenger cars were presented.The simulated loads provide basis of lightweight designs under reliability requirements.Three failure criteria: fracture(by equivalent strain),yielding(by yield strength)and fatigue(by SN curve and linear fracture mechanics)criteria were used to validate designs under load cycling.The methodologies fulfill both accuracy and simulation efficiency requirements,and the load simulation platform for key components in passenger cars was built accordingly.Based on parameter model,the platform can be used for different car models in lightweight design and reliability analyses.It includes test tracks for durability testing,pre-defined loads and boundary conditions.Simulations are based on implicit and explicit co-simulations which combine advantages from both simulation packages.Substructure technique was used to reduce the model size of implicit part.Increment control algorithm based on DOFs in wheel centers was presented to maintain the accuracy.The co-simulation method overcomes common assumptions in multibody simulations such as rigid body and linear analysis.(2)Built the lightweight design optimization flow based on iterative load cycling.A FE-model which reflecting actual load characters of components,such as tire /road interactions,was built.An optimization searching algorithm based on parameter designs,sensitivity analysis,and optimal direction searching method was developed to improve utilization factors of key components.The algorithm was implemented in suspension arms and compared with topology optimization.The comparison shows the advantages of the new method in the detail design.In the durability analysis of key components,iterative load cycling together with loads which are consistent with the verification testing were used.(3)Based on the reliability analysis of drive-line system,key factors that affect the components’ reliability were identified.A lightweight design method was proposed to optimize the weight of drive axel.The new design is based on thin wall structure,combined with internal pressure to increase the buckling strength.In some design application,the weight is reduced by 3/4,with the same stress level;Fatigue analysis,considering variations of multi-parameters,was performed on transmission components.The method overcomes the traditional fatigue life analysis based on average material strength and load level;Assembly tolerance effect on fatigue life of main axel of E-motor was evaluated,the tolerance and fatigue life relation was revealed.Based on the fact that lightweight designs affect the stiffness of transmission box,the sealing functionality of transmission box was studied and method was developed.Also seals in the input and out axles were analyzed,and the improved design chart was proposed;C ompared to design recommendations of seals in the international standard,the improved design method is more comprehensive.(4)An innovative lightweight design was developed by using wave plate(s)to reinforce anti-collision beams in car frame.Design principals are proposed for different structural parameters.Comparison was done between other lightweight designs,reinforced by flat plate and porous aluminum.The results show that the wave plate reinforced beam has more loading capacity with the same weight.Research was performed on box beam in car body structure.Three methods to compute ultimate capacities were compared.It shows that the method based on elastic perfect plastic material model is accurate and less affected by user factors.Different methods for analyzing weld structures were compared,fatigue life analysis based on linear fracture mechanics is more suitable for analysis of complicated weld structures.In the beam buckling analysis,imperfection combinations based on design tolerances were performed.The results show that different combinations of imperfections under the same tolerance gave around 1/5 difference on buckling load.Above conclusions provide basis and references for lightweight design with similar high strength steels.(5)Based on the load simulation platform,a full passenger car model was built with all necessary nonlinearities,such as braking load case simulation.Both K&C testing and full vehicle durability testing were designed and implemented.Key parameters such as vehicle velocities,accelerations,strains in suspension arms etc.were measured.Static and dynamic verifications show the accuracy of the full vehicle model.Fatigue loads in suspensions were converted to equivalent loads in tire/road interaction points,based on rain flow counting of strain signals and linear damage theory.The results provide reference fatigue loads for suspension analysis on the test track.The full vehicle durability testing was simulated and compared with experimental results.Good agreements were achieved.