Research On Crack Propagation In Heat Affected Zone Of Pressure Pipe Welding

Nowadays,most of the components are welding structure,and the long distance pressure pipeline is a typical welding structure.Meanwhile in the process of the operation of the pipeline,there will inevitably generate crack defects on the parent metal of pipe or around the welds.Under the action of internal working pressure,this kind of defect may expand into the parent metal with high speed.Meanwhile the welding residual stress can also affect the crack propagation.Crack propagation on welding heat affected zone was studied,and the research can provide theoretical basis and reference for pipeline safety assessment and anti-crack design.Based on the ABAQUS software and the programming with Python and Fortran,this paper studies the expansion law of pipeline crack under static load and fluctuating load,and analyzes the pipeline welding process and post-weld heat treatment.In the final,the simulation and analysis of the pipeline crack propagation under the residual stress of welding were carried out.The following several aspects has been carried out:(1)Based on the theory of fracture mechanics,the variation law of the stress intensity factor,crack tip position and the extension direction of the crack in the crack propagation process were calculated by Python programming.The finite element method was explained by comparing the stress intensity factor of theory.(2)The surface crack and penetrating crack were analyzed.Firstly,by compared with theoretical calculation and numerical simulation,the feasibility of computing model was proved.Secondly,the variation law of fracture toughness parameters and the law of crack propagation in different pipe surface cracks and penetrating cracks were analyzed.Finally,combined with the full-scale fatigue test of the pipeline,the fatigue crack growth formula of pipeline based on energy release rate was deduced by the Walker formula and the full-scale experimental results.The expansion theory and ABAQUS were combined by programming to calculate the fatigue of the pipeline under different crack shape parameters life,which provides a theoretical basis for guiding the actual pipeline safety assessment.(3)Based on thermal elastic-plastic theory,the double ellipsoid heat source was applied by programming DFLUX subroutines,and the welding simulation was analysed by building units of life and death using Python.Firstly,the feasibility of the finite element method was proved by comparing the simulation results with the experimental data.Secondly,the welding numerical simulation of X80 pipe in West-East Gas Pipeline was carried out.The distribution of welding temperature field and welding residual stress were analyzed.The reliability of the results was proved by comparing with the steel plate welding.Finally,the post-weld heat treatment was simulated.(4)Numerical simulation analysis was carried out by mapping the welding residual stress to the crack propagation model and the crack propagation law without residual stress was compared.Firstly,the variation of the toughness parameters and expansion of the different shape factors under welding residual stress were studied.Secondly,the redistribution of the residual stress on loading and unloading was studied.Finally,the fatigue life of the heat affected zone was analyzed.(5)The effects of different shape factor cracks on the ultimate load of cracked pipe in the heat affected zone were analyzed by numerical simulation.The criteria of plastic failure were compared,and the limitations of the criteria were described.According to the nonlinear regression analysis method and PCORRC formula,the limit load formula of pipeline which contains surface crack was modified by using MATLAB.