Research On Ductile Fracture Of X80 Pipeline Steel And Finite Element Simulation

Natural gas transporting project from the West to the East is a milestone in China's oil and gas industry, pipeline safety is a major issue closely related to its social, economic, political, people's lives and safety . Fracture control methods used in the project are Charpy impact energy and fiber rate in DWTT cross-section. For high toughness X80 steel , JIC and CTOD as indicators to measure fracture toughness have invalid test results , and determination of basic fracture toughness parameters in widely applied traditional Failure assessment diagram is a exist problem. This dissertation focused on charactering ductile fracture of TMCP X80 steel using micro-porous dilational model to describe the process of ductile fracture properties, looking for reliable method to characterize fracture toughness of X80 steel.Appling a serials of tensile test of different necking degrees, by metallographic observation and micro observations, the mechanism of void nucleation in X80 steel was proved that, the first stage of voids nucleation occurs around the composite calcium sulfide inclusions, in the early necking stage; the second stage of nucleation of voids occurs at the MA island sites, in a deep necking stage, it begins voids nucleation by MA island / matrix interface decohesion. the strength of calcium sulfide/ matrix interface and the MA island / matrix interface are measured. Numerical analysis of voids growth in X80 steel had performed by cell models, Combining observation of crack body fracture surface and identifying microstructure parameters. Comparative analysis of the meso-damage theorys such as RT model, Gurson model, GTN model and Wang Ziqiang model had made, the applied scopes of them were distinguished. The void evolution laws under various stress states were obtained by finite element calculations. Voids nucleation through MA islands was discussed, and found that suffering high stress triaxiality(>2), stress at MA/matrix interfaces is not enough to take them apart from matrix, while in low stress triaxiality(≤1), interface stress is so higher to make voids nucleation. Load carry capacities of micro voids trending to spheric are near to each other, at the same stress triaxiality. But oblate micro voids show low load carry capacities, decreasing fracture toughness of X80 steel.Considering stress distribution in the fracture process zone, prediction of crack growth in X80 specimens by finite element analysis was approximated to experimental results, and the predicted load - displacement curves were consistent to the experimental results. Well predictions of macroscopic mechanical properties using the micromechanical parameters by cell model calculations were shown, which can improve the reliability of the numerical analysis. Through domain integration, we obtained a characterization value of fracture toughness of X80.Fatigue crack growth rates of X80 steel was tested, propagation process through the wall thickness of the surface crack in X80 pipeline under operating conditions had been performed with different initial surface crack shape using finite element analysis. Fatigue growth analysis of sub-critical breakthrough crack in pipeline had also performed using meso-mechanics model. Leak-before-break criteria (LBB) safety assessment diagram was constructed according to BS7910 specification.