Numerical Analysis Of Welding Residual Stress And Structural Fatigue Life Of X80 Pipeline

Welding plays an important role in the process of pipeline installation and construction,and the quality of joints directly determines whether the pipeline can serve safely.When the pipeline is in service,the fluid pressure changes periodically with time,and the welded joint is prone to fatigue fracture under cyclic loading.Therefore,it is of great significance to study the residual stress of pipeline welding and to comprehensively consider the prediction of structural fatigue life under the combined action of residual stress and cyclic load.Based on the theoretical analysis,this paper used the finite element method to simulate the welding process of X80 pipeline steel plate and pipeline,and analyzed the distribution law of residual stresses.The stress-strain field at the crack tip under internal pressure was solved to explore the variation law of stress intensity factor under fluid pressure and different crack sizes.Finally,the welding residual stress was introduced into the fatigue crack propagation model to study the fatigue life of the pipeline under the combined action of residual stress and alternating load.The main conclusions are as follows:(1)After the butt welding of X80 pipeline steel plate,the maximum vertical stress and equivalent stress appears near the weld,and the peak value of transverse stress is concentrated in the fusion line,while the peak stress in the thickness direction is located inside the structure.The residual stress gradually decreases with the increase of the distance from the weld center,and the residual stress test results are basically consistent with the simulation results.The residual stress in all directions of the pipeline appears to be the maximum in the fusion zone,and the stress value gradually decreases to zero as the weld center moves along the axis to both ends of the pipeline.The tensile stress at the fusion line rapidly changes into compressive stress,and the residual stress on the inner and outer surfaces of the pipeline shows opposite manifestations and distribution laws.The axial,circumferential and radial stress on the outer surface of the pipeline are mainly tensile stress,while the stress distribution on the inner surface of the pipeline is opposite to the outer surface and the stress value is relatively small.(2)The maximum error between the simulation results of stress intensity factor of plate surface crack and the value of N-R formula is less than 1.2%.Cracks are established at the fusion line position on the outer surface of the pipeline,and the equivalent stress,effective strain and total deformation increase significantly with the increase of pressure load and crack size,but the distribution law remains constant.The stress and strain on both sides of the crack are "butterfly"symmetrical distribution,and the equivalent stress and strain decrease rapidly with the increase of the distance from the crack tip.When the pressure load increases,the stress intensity factors KⅠ,KⅡ and KⅢ increase,and the direction of KⅢ changes from left to right,and the curve remains the same change rule.The change rule of stress intensity factor is not positively correlated with the change of crack size.Under the same conditions,KⅠ is far greater than KⅡ and KⅢ,indicating that the open crack is dominant in the circumferential crack on the surface of the pipeline.(3)The mapping calculation of welding residual stress field is carried out by the inverse isoparametric element mapping,and the mapping results of equivalent stress are in good agreement with the stress distribution law in direct solution.In the calculation process of fatigue life,the fatigue life of pipeline decreases sharply with the decrease of load ratio R and the increase of initial crack size.When the crack depth alt and shape ratio c/a are small,the fatigue life of the pipeline can be improved with high load ratio.With the increase of alt and c/a,the influence of R on fatigue life gradually decreases.Under the same conditions,the influence of alt on the stress intensity factor is larger than that of c/a,that is,the possibility of crack propagation in the depth direction is greater.