Study On Life Prediction And Crack Growth Of Type 17 Forged Coupler Yoke In Heavy Haul Train

The coupler yoke is an important component of the heavy haul train coupler buffer device,transmitting the longitudinal traction.With the extensive operation of the 10000-ton heavy hual train,the increase of the longitudinal traction leads to the possible fracture of the coupler yoke,which damages the safe operation of the heavy hual train.In order to ensure the safe operation of the heavy hual train,it is particularly important to investigate the fatigue behavior of the coupler yoke.With the improvement of manufacturing technology,the forged coupler yoke with a better performance has widely replaced the cast coupler yoke.Comparing with the cast coupler yoke,the forged coupler yoke has a better fatigue and fracture resistance.The prescribed inspection standard of the cast coupler yoke is too conservative if it is used for the forged coupler yoke.Thus,based on the material performance experiments and finite element simulations,the fatigue initiation life and fatigue crack propagation life of the forged coupler yoke were predicted and discussed in this work,in order to provide the scientific evidence and the theoretical support for the design of the coupler yoke and the setting of relevant inspection standards.The main contents of this dissertation are summarized as follows:（1）Monotonic tensile test,fatigue crack growth rate test,crack growth threshold test and fracture toughness test were carried out for the material of the coupler yoke,i.e.,forged Grade E steel.The relevant mechanical parameters of the material were obtained as follows:the elastic elastic modulus is 209 GPa,the yield strength is 850 MPa,the tensile strength is 950MPa,the fitting parameters in the Paris’s formula are C=2.86×10^-11and m=2.4,the crack growth threshold is 213.30 MPa·mm^1/2,and the fracture toughness is 4969 MPa·mm^1/2.（2）Considering the additional bending moment caused by the height difference between the centerlines of two adjacent couplers,the finite element model of the coupler yoke was established and the static strength analysis was carried out to determine the dangerous location with the highest stress concentration.The nominal stress method was used to predict the fatigue initiation life of the coupler yoke and the influence of the additional bending moment was discussed.The predicted results show that the stress concentration mainly occurs at the corners of the coupler yoke,and the height difference between the centerlines of two adjacent couplers has a significant influence on the maximum stress.Under a fairly bad working condition with a height difference of 25 mm and a maximum load of 1500 k N,the maximum nominal stress of the coupler yoke is obtained as 300 MPa,which is lower than the fatigue limit of 394 MPa in the modified structural S-N curve of coupler yoke.It means that the forged coupler yoke meets the design condition of infinite life.（3）The surface crack propagation of No.17 forging coupler yoke was simulated by using FRANC3D software.The results show that the shape of the elliptical surface crack in the coupler yoke changes during the propagation,and the shape of the crack becomes narrow and long with the increase of the crack length.The static stress intensity factor of the crack tip with a depth-length ratio of 1 is higher than that of dynamic crack growth to the same length.The residual life prediction of the coupler yoke with a narrow and long shallow crack was carried out,and the influence of initial crack size and crack location on the residual life of the coupler yoke was discussed.It is shown that the residual life of the coupler yoke decreases sharply with the increase of the initial crack depth and length;if an initial intermediate crack with a length of 4 mm and a depth of 0.5 mm occurs at the inner corner of the coupler yoke,the coupler yoke can be still used for 3.9 years about two prescribed repairing periods.Therefore,No.17 forged coupler yoke with a crack length not exceeding a certain limit can be allowed to operate normally till the next repairing period.