Research On Fatigue Properties Of Carbon Steel Joints By Strain Energy Density Method Considering Welding Residual Stress

Welded joints are susceptible to fatigue damage under working conditions due to stress concentration caused by welding residual stress.At this stage,the test method is mainly used for structural fatigue performance assessment,but the demand for specimens is large and the overall assessment period is long.Semi-empirical methods such as the nominal stress method and the hot spot stress method are sensitive to the geometry of the welded joint and cannot be evaluated for joints where nominal stresses are partially unavailable or where stresses are concentrated at the weld root location.In view of the limitations of the above assessment methods,this paper proposes a consideration of residual stress strain energy density method,through the strain energy density as a control parameter for fatigue assessment of welded joints,the fatigue life of carbon steel T-joints with different welding processes to assess the fatigue performance to verify the accuracy of the method.In this paper,the fatigue life of Q345NQR2 weathering steel single side T-joints with different welding processes is evaluated by strain energy density method,and the influence of welding residual stress on the fatigue life of welded joints is considered.Combined with mechanical and fatigue tests,the relevant properties of welded joints are analyzed and studied,and two strain energy-life fatigue curves of welded joints are obtained by finite element method.A comparative analysis with the nominal stress method is carried out to verify the accuracy of the strain energy density method considering residual stress.The main research contents and achievements are as follows.(1)Based on the strain energy density method,through the finite element analysis method to verify the overall strain energy density of the T-joints on the grid size insensitivity,based on sub-model technology to extract the strain energy density at the weld root and toe of the welded joint,the results show that different grid types and the overall grid size of the sub-model strain energy density has a certain degree of influence.By comparing the calculation results of the two-dimensional model and the three-dimensional model,it is found that the results of the two models are almost the same.(2)The microstructure of the carbon steel T-joint was observed,and the average maximum tensile force of the joint was obtained through the tensile test.Fatigue tests were conducted at different stress levels to obtain fatigue life data.Based on the microscopic morphology of the fatigue fractured joints,the crack propagation modes of different welded joints can be seen,and it can be observed from the microscopic angle that both kinds of welded joints fracture from the welding root.Comparative analysis of the fatigue life evaluation curves of carbon steel T-joints by different fatigue evaluation methods.The strain energy density method and the nominal stress method have the same trend of joint fatigue life evaluation.It is verified that the strain energy density method can be used to evaluate the fatigue life of carbon steel T-joints.(3)The thermal analysis model of the welded joint is established,and the welding simulation numerical analysis of different welded joints is carried out by adding different heat source models.Based on the finite element analysis method to calculate and analyze the residual stress in the welded joint,the residual stress mainly exists inside the welded joint as longitudinal residual stress,and the stress is mainly concentrated in the weld zone.The temperature field results show that with the movement of the welding heat source,the surface node temperature in the weld area rises sharply and then falls,and the rate of rise is higher than the fall.Fatigue life assessment of welded joints under consideration of residual stresses using the strain energy density method has a narrower scattering band compared to the assessment curve without consideration of residual stresses.The three fatigue assessment methods were compared and studied,and the strain energy density method,which takes into account residual stresses,was closer to the actual test values when the fatigue life was in the range of 10~5-10~6.