Study On Ductile Fracture Criterion Of Metal Materials For Railway Vehicles

Rail vehicles mainly operate under the guidance of signal system,and the probability of accidents is much lower than other transportation tools.However,under the action of accident factors such as turnout failure,line failure,vehicle failure,there is still the possibility of derailment and collision accidents.Railway vehicles operate in multi-group connection mode.After the train derails due to the earthquake and other accidental factors,a series of collisions will occur between the carriages,resulting in crawling,overturning,extrusion and other damage between the vehicles,resulting in huge casualties and property losses.How to use the body material to protect passengers' safety in extreme derailment is worth further study.Using the finite element method to simulate the train collision process can reduce the research and development cost and shorten the research and development cycle.However,there are still many problems in the application of FEM software.The main reason for the difference between the results of FEM calculation and the actual results is the lack of constitutive relation describing the deformation behavior of materials and the damage model describing the failure and fracture of materials in FEM modeling.Based on this,a constitutive relation which can accurately describe the deformation of 301L-DLT austenitic cold-rolled stainless steel under different stress states is established through a large number of experimental studies and comparisons of fracture damage models.The evolution law of ductile damage of metal materials is studied by observing the micro-structural damage of fracture surfaces,which provides a basis for correctly using finite element method to simulate car body collision.A sufficient data base has been established.This paper is mainly based on the following aspects: Firstly,the necking mechanism of the internal ligament and the shear mechanism of the micro-voids causing the ductile fracture of metal materials are studied.The effects of initial void volume fraction,stress triaxiality and Lode angle coefficient on the ductile fracture are discussed,which provides a theoretical basis for the design of subsequent experiments.In order to study the effect of material anisotropy on the test and numerical simulation,the tensile test was carried out on standard specimens.Three kinds of specimens with tensile direction and rolling direction of 0,45 and 90 degrees were designed.Tensile tests were carried out respectively.According to the test results,the anisotropic effect of the material was obtained.The fracture morphology of the specimens was observed by electron microscopy.Based on the micro-damage,the fracture of the specimens was observed.Process analysis.It is found that the rolling direction has little effect on the elastic-plastic mechanical properties of 301L-DLT stainless steel,and the anisotropic effect of the material can be neglected in the finite element simulation,which provides a basis for the design and theoretical study of subsequent experiments.It can be seen from the observation of electron microscopy that the fracture mode of tensile specimens is caused by the shear fracture between dimples.In order to further study the fracture process of 301L-DLT austenitic cold-rolled stainless steel and its relationship with stress triaxiality and Lode angle coefficient,three specimens with notches and four specimens with double-sided grooves were designed,and tensile tests and finite element analysis were carried out respectively.The fracture surface is a typical ductile dimple fracture with high stress triaxiality.The finite element analysis of notched and grooved specimens is carried out by using G-T-N model and J-C model,respectively.The change process of high stress triaxiality and Lode angle coefficient in the fracture process and their influence on the fracture are studied.According to the comparison between the analysis results and the test data,both models can reproduce the ductile fracture process in the tensile test.It is proved that when the tensile test involves high stress triaxiality,the ideal fracture simulation results can be obtained by using J-C fracture model and G-T-N fracture model.By designing four kinds of double notch shear tests and piercing tests,and combining with finite element simulation,the relationship between fracture strain,stress triaxiality,Lode angle coefficient and material plastic strain at the starting point of fracture of shear specimens is studied,and the influence of different stress triaxiality on fracture is discussed.The results show that the stress triaxiality distribution near the fracture surface is 0-0.9 and the Lode angle coefficient varies greatly from 0.27 to 0.96.The numerical simulation based on J-C model and G-T-N model has larger errors than the experimental results.The main reason is the shear fracture caused by the aggregation of shear band holes and the fracture response assumed by J-C model.With the increase of stress triaxiality,dimple fracture is defined as the main form of fracture,and the prediction of dimple pore growth assumed by G-T-N model is obviously different.According to the inaccuracy of G-T-N model and J-C model in simulating shear fracture,a modified J-C fracture model is proposed,in which Lode angle coefficient is introduced into J-C model.The modified J-C model is used to simulate and compare several test specimens.It can be seen that the modified J-C model has high accuracy for the simulation of high stress triaxiality and for the fracture process of low triaxiality with shear as the main fracture form.Compared with G-T-N model based on dimple pore growth principle,the modified J-C model has obvious advantages in predicting shear fracture process.It is also more reasonable to explain the phenomenon of aggregate fracture of shear band in the process of shear fracture.In this paper,the damage and fracture behavior of 301L-DLT austenitic cold-rolled stainless steel under different stress states are studied,and the general process of fracture behavior characterization test modeling and parameter calibration for the tough metal of railway train body is explored.