Fatigue Life Test Study Of Weathering Steel Welded Joints Based On The Main S-N Curve Method Considering Residual Stress

In the service process of high-speed trains with weathering steel material as the body,fatigue crack sources are easily formed at the weld joints,which greatly reduces the weld fatigue life of the rail train body joints,so the fatigue resistance of the welded structure needs to be evaluated to ensure the safe and stable operation of high-speed trains.In this paper,the weathering steel lap laser welded joints as the research object,based on fracture mechanics and thermo-elasticity theory,proposed a welding fatigue assessment method that takes into account the effect of residual stress,and the accuracy of this method was verified by fatigue test data.In order to obtain the mechanical properties parameters of the laser welded joint,the tensile test of the lap joint was performed first in this paper,and the average breaking force was 9.51 k N,and the tensile crack occurred at the base metal,indicating that the tensile strength of the weld was greater than that of the base metal.Then a total of five sets of fatigue tests were performed according to the tensile test data to obtain the fatigue life of the welded joint under different fatigue load levels,and fatigue fracture locations occurred at the weld toe.Through metallographic tests to observe the microstructure of the welded joint at the location of fatigue fracture,the results show that: the laser welding pool at the weld is close to the circular distribution,lap joint weld forming good no obvious welding defects,heat affected zone microstructure dense.According to the main S-N curve theory,joint use Abaqus and Fe-safe software to predict fatigue life of lap joints,the results show that the predicted life distribution in the main S-N curve 95% confidence interval,However,the results obtained by this theory still have a certain gap with the experimental values.The accuracy of welding fatigue assessment methods after considering residual stress has been improved.In this paper,the temperature field and residual stress field of laser welding are calculated by the finite element method,the temperature field results and metallurgical tests to obtain the melt pool shape comparison to verify the accuracy of the numerical simulation process.Simulation results show that the maximum equivalent residual stress of the lap joint is close to the yield strength of the base material,so it is necessary to consider the impact of residual stress on welding fatigue.The transverse and longitudinal residual stresses along the weld direction are distributed in the form of high in the middle and low on both sides,and the longitudinal residual stress is generally greater than the transverse residual stress.In order to accurately characterize the fatigue crack expansion drive at the weld toe,the distribution function of transverse residual stress at the weld toe is obtained by the numerical calculation tool Matlab fitting,and the residual stress intensity factor is calculated by weight function method,so as to quantify the effect of residual stress on fatigue life.In this paper,according to Barsoum et al.based on the effective stress ratio proposed under the influence of residual stress improved Forman formula,a welding fatigue evaluation process considering the influence of residual stress is established,and the method comprehensively considers the structural stress strength factor and residual stress strength factor,the prediction results and test values have a high degree of agreement,the error distribution in 4.6% ～ 39.1% between,proving that the assessment method is effective.