Research On Fatigue Fracture Performance Analysis Method Of High-speed Train Carbody Underframe End Structure

With the increasing of the number and mileage of high-speed trains,it is necessary to evaluate the safety status of high-speed trains.Now a large number of EMUs have entered the fifth-level maintenance process.Therefore,the research on the crack growth rate and remaining life has certain practical significance.High-speed train carbody is a typical large welded forming parts,welding defects caused by the cracks-like continue to expand under the action of complex loads,which affect the safety of train carbody.The research on crack growth rate and prediction of crack growth life of welded high-speed carbody in this paper has certain reference value for the improvement of maintenance strategy of high-speed train.In this paper,the crack growth rate of CT samples of aluminum alloy commonly used in high-speed carbody was studied.According to the plate thickness of different parts of the carbody,several sets of thickness CT specimens were established,and the effects of stress ratio,plate thickness and other factors on the crack growth rate were discussed.On this basis,the welding simulation of the butt welded joint was carried out by numerical simulation to obtain the residual stress distribution,so as to carry out the research on the structural crack growth rate considering the residual stress.Finally,the calculation of crack growth rate based on IIW standard,CJP model and literature test data parameters was compared and analyzed.Secondly,the finite element model of the EMU carbody was established,and the submodel method was used to model the part of the chassis end of the high-speed EMU.Then,according to the design standard EN12663-1,the load and boundary conditions of the vehicle body were determined to introduce cracks of different lengths with defects in the traction beam.The stress intensity factor of the crack tip under different crack lengths was calculated,and the critical crack size was obtained by comparing it with the fracture toughness of the aluminum alloy material of the carbody,and the safety of the crack in the key parts of the carbody was evaluated.Finally,according to the vertical longitudinal and transverse load spectrum of the carbody,the dangerous parts of the carbody were selected as the focus,and the Box-Behnken matrix design method and polynomial fitting method were used to build the proxy model of the carbody,and the multi-axial load spectrum of the carbody was transformed into the dynamic structural stress of the carbody.A surrogate model was established to calculate the equivalent model of the crack growth life of the carbody.The obtained dynamic structural stress was reversed based on the fitting relationship between load and stress to obtain an equivalent load history.Loading the aluminum alloy butt joint to calculate the crack growth life.The final calculated results were compared with the crack growth life based on IIW standard and literature test data.The results show that by combining the sub-model with fracture mechanics,the critical crack size of the end structure of the chassis can be accurately calculated and the fatigue fracture performance can be analyzed.The comparison shows that the crack growth life obtained based on the fracture parameters of the CJP model is conservative,which is more instructive.In this paper,the surrogate model of the high-speed train car body is established by the polynomial fitting method,and the equivalent prediction of the crack propagation life at the end of the car body underframe is carried out through the relationship between the crack driving force and the load,which has a certain rationality while improving the calculation efficiency.