Multi-Scale Study On Crack Propagation Of High-Grade Pipeline Steel Based On Cohesive Zone Model

In recent years,with the development of natural gas long-distance pipeline projects at home and abroad,the number of damage accidents of long-distance pipeline caused by the rapid growth of cracks in pipelines is increasing.Especially under the background of large-scale application of high-strength and toughness pipeline steel,it is necessary to study the crack propagation behavior of high-strength and toughness pipeline steel.Although scholars have conducted intensive studies on mechanical properties,external shock response and other fields of long-distance pipelines,and the results of experiments and simulations at macro-scale are remarkable,but "characteristics of the material depends on the atomic structure and microstructure of the material".It is difficult to explain the failure phenomena such as hole nucleation,atomic cohesion,dislocation generation and atomic vacancy defect in the process of crack propagation by macroscopic experiment,thus it gives the significant of multi-scale analysis.This paper based on cohesive zone model and combined with molecular dynamics method and finite element method of continuum medium to study the crack propagation behavior of high strength and toughness gas pipeline steel at macroscopic,mesoscopic and microscopic scales by fracture toughness experiment and numerical simulation.(1)Based on J integral and crack tip opening displacement theory of elastic-plastic fracture mechanics,the quasistatic fracture toughness test of compact tensile specimen has been carried out.And the experimental data are regularized by using load separation method of single specimen.After validity judgment of the obtained fracture toughness resistance curve,the fracture toughness parameters of X80 pipeline steel,including critical J integral JIC and critical crack tip opening displacement ?IC,are determined.(2)Based on cohesive zone model theory,the user-defined material subroutine VUMAT of the exponential and trilinear cohesive zone model(CZM)were compiled.Then,the bilinear CZM embedded in ABAQUS and the VUMAT subroutines compiled were called to simulate the experimental process of fracture toughness of X80 pipeline steel.By comparing the simulation results with the experimental loading point force and displacement response curve,a suitable CZM for base metal of X80 pipeline steel was established.(3)For the atomic scale,the propagation behavior of mode I central penetrating crack at the ferrite-pearlite interface with Bagaryatskii orientation is simulated by molecular dynamics.For different cementite terminal plane structure models,the traction-separation curves at the interface during crack propagation were extracted,and the cohesive parameters at micro-scale were obtained.At the same time,from the perspective of atomic behavior,the phenomena of interface separation during crack propagation caused by vacancy formation,hole growth and aggregation,and atomic cohesion are explained.(4)The cohesive parameters obtained at atomic scale are successfully transferred from bottom to top to macro and micro scales by serial multiscale analysis combined with the bilinear CZM.The critical J integral Jic of fracture toughness of ferrite-pearlite pipeline steel at different scales was calculated by numerical simulation of loading point reaction force and displacement response curve.At the same time,the size effect of the model under different scales is also analyzed simply.Through cross-scale exploration,the atomic scale fracture parameters,reflecting the deep fracture mechanism of high strength and toughness gas pipeline steel,are successfully transferred to macro and meso scale,which breaks the limitation of traditional fracture risk assessment only by means of macro observation,and provides a new idea for the research and development of crack arrest technology and quantitative fracture risk assessment of long-distance gas pipeline.