Simulation Study Of Subway Vehicle Collision Based On Damage And Fracture Model

As for the last protection of passengers,the crashworthiness design of subway vehi-cles has been increasingly attracting attention from scientific research institutions and manufacturers.It is difficult to verify the simulation study by experimental data for subway vehicle crashworthiness design.And subway accidents have provided valuable research cases for the crashworthiness study.In this paper,based on the deformation and failure of the vehicle structure in a collision accident,the damage and fracture perfor-mance of the vehicle body material tested and modeled.The limitations of the constant fracture model are discussed by comparing the actual accidents and simulation analysis.The stress state and element size effect for the stainless steel fracture model simulation analysis are carried out.And there are five parts to this thesis:Firstly,this paper investigates the structural damage of vehicles,couplers,and cabs in a subway accident,which occurred in Beijing.The final state of cars is restored.The investigation provides objects for subway collision accident simulation and metal frac-ture verification.Secondly,the finite element model of the subway accident is established.By the simulation and accident investigation,the structural weaknesses of the cab in the acci-dent is exposed.An improvement scheme is proposed and tested by the numerical sim-ulation model.The comparison between the structural deformation of the accident and the calculation simulation results show that there is a deviation between the accident and prediction by the constant strain fracture model.The improvement of the fracture model is an important issue to further improve the accuracy of subway collision simula-tion.Thirdly,the 301L-HT stainless steel sheet is selected for the fracture model study,as it fractures in the accident.Based on MMC（Modified Mohr-Coulomb）fracture model,the standard uniaxial tensile test,Arcan type tensile test,and stretch bending test are employed to calibrate damage fracture model parameters.The constant strain fracture model underestimated the ability of the material to resist shear fracture,and it is impos-sible to simulate the ductile fracture of stainless steel sheets with complex stress states.While the MMC fracture model could simulate the fracture failure process under dif-ferent stress states by using fine solid elements（0.1mm）.However,the MMC fracture model relies on fine solid elements to simulate the fracture process.Once the element size or element type is changed,will be leading a large error.Fourthly,the application of MMC fracture criteria to shell elements yields invalid after the onset of necking and sensitivity to the element size.To fill this gap,this paper proposed that the fracture stress thresholdσ_f for the Mohr-Coulomb criterion can be as-sumed to be an element dimension effect function of the form:σ_f（l_e）?l_e^-β.It follows then that the Modified Mohr-Coulomb fracture model and post-initiation damage are related to the element size.The damage/fracture model with mesh size effect for shell elements is established.The comparison of the experimental and FE simulation results demonstrates that the new coupled mesh size effect damage/fracture model can predict stainless steel ductile fracture in different shell element size with reasonable accuracy.The damage/fracture model lays a foundation for the study of damage and fracture be-havior of thin-walled car body structure in the collision.Fifthly,the damage/fracture model apply to the subway collision simulation study in different scenarios.Compared with the constant strain failure model,the fracture model proposed in this paper can better predict the deformation and fracture of the car body under the accident simulation.According to requirement EN15227,The crash-worthiness of a middle subway car,at 12.5km/h and 15km/h,is analyzed by the finite element method.The simulation and similar accident cases show that:the subway in-termediate car meets the requirements.However,the impact scenario,according to EN15227,cannot consider the climbing-up factor,as the destruction of the car body is different from similar accident cases.A vertical offset collision scenario is proposed to analyze the crashworthiness of the middle car at the climbing-up accident.The finite element method is used to analyze the performance of the vehicle under different speeds of climbing-up collision.