Study On Size Effect Of Fracture Toughness Of Bogie Steel For Alpine EMU

The low-carbon steel used in EMU bogies has a body centered cubic crystal structure,which is easily affected by the low temperature conditions in the high cold area,resulting in the phenomenon of ductile brittle transition,reducing the fracture toughness and even brittle fracture.Changchun rail car limited company has developed a low-carbon steel for the manufacture of EMU bogies in high cold.Due to the welding defects of the bogies and the non-uniformity of the structure and other factors,the low-temperature sensitivity of low-carbon steel will be affected,and the possibility of low-temperature brittle fracture will be increased.Therefore,the fracture toughness of low-temperature steel will continue to be studied.In this paper,the size effect of low temperature fracture toughness of low carbon steel was studied.The fracture toughness of four different size compact tensile specimens was tested at different temperatures.The fracture toughness data were obtained and the fracture was analyzed by SEM.The fracture toughness dispersion and temperature dependence caused by the ductile brittle transition behavior at low temperature are discussed.Based on the theory of K-T,J-Q and J-A2 and the experimental data,the relationship between the fracture toughness and specimen size of low carbon steel at low temperature is established,and the following research results are obtained.Based on the two parameter K-T theory,the influence of size effect on fracture toughness under linear elasticity is studied by finite element method and experimental data.Finally,the influence law of size effect(expressed by constraint parameter T)on fracture toughness is revealed.Based on the theory of J-Q and J-A2,the influence of size effect on fracture toughness under elastoplasticity is studied by finite element simulation.Finally,the relationship between constraint parameters Q,A2 and fracture toughness value is obtained.Based on constraint parameters A2,the transformation model of fracture toughness of specimens with different sizes is established.Based on the modified Beremin model theory in micro fracture mechanics and the finite element simulation,the fracture failure probability of specimens with different sizes at low temperature is studied.The fracture failure probability results of different size specimens expressed by Weibull stress and cumulative failure probability are obtained,and the transformation model of fracture toughness of different size specimens is established.The research content of this paper can provide a certain theoretical research method for the change of fracture toughness of low-carbon steel at low temperature,and help to reduce the difficulty of low-carbon fracture toughness experiment,so as to develop cold resistant steel materials with better performance at low temperature.