Ductile Fracture Study Of X80 Pipes With Girth Welds

Pipeline is an important engineering structure for transporting oil and gas,and is the main way of transporting oil and gas.With the increasing demand for oil and natural gas in society,the distance laid for oil and gas pipelines is also increasing,the environment experienced is also becoming more complex,and the safety test of pipelines is also increasing.If a pipeline ruptures and fails during transportation,the economic losses caused are enormous,so ensuring the safety of the pipeline under complex environmental conditions is very important.In this paper,the ductile fracture behavior of X80 pipeline steel and X80 pipeline girth weld is studied by using the Damage mechanics model for X80 pipeline.The main research contents and conclusions are as follows:(1)To study the fracture behavior of X80 pipeline steel,its basic mechanical properties were first determined through standard tensile specimens,and plastic constitutive mechanical parameters were calibrated.Secondly,a special-shaped tensile specimen was designed with a stress state range from negative stress triaxiality to high stress triaxiality;By using numerical calculation methods to establish elastic-plastic finite element models under various working conditions,fracture parameters under different stress states were obtained by combining finite element analysis with experimental results.Based on the obtained fracture parameters,a fracture criterion for X80 pipeline steel was established.The prediction error of the fracture model was verified to be within 10%;At the same time,the tensile strength of X80 pipeline steel increases with the stress triaxiality as a quadratic function.(2)For the ductile fracture of the X80 pipeline girth weld,a small compression specimen was first developed for the pipeline girth weld,and the plastic hardening curve of the girth weld was determined.Secondly,a special-shaped specimen with a notch at the weld seam was designed,and failure tests were conducted on the weld material.Numerical calculations were also conducted for various working conditions.Based on the results of finite element analysis and experiments,the average stress triaxiality,average Lode parameters,and fracture strain under various working conditions were obtained,and a two-dimensional fracture model similar to the base material was established;On the basis of the two-dimensional model,the influence of Lode parameters on the fracture of circumferential welds was further considered,and the undetermined parameters of the ERT and LSMCS three-dimensional fracture models were calibrated.And the two-dimensional model and three-dimensional fracture model were brought into the calculation model for verification,and it was found that the prediction accuracy of the three-dimensional model fracture was higher: the maximum prediction error of the two-dimensional fracture model reached 18%,while the prediction error of the threedimensional fracture model was all within 10%.(3)Based on the established constitutive model,the fracture response of circumferential weld defects in pipelines with a diameter of 1422 mm was studied under the combined action of internal pressure and bending moment.Parameterized research was conducted on the size of circumferential weld defects,and the influence of circumferential weld defect size on fracture under the combined action of internal pressure and bending moment was obtained.