Research On Post-Buckling Ductile Fracture Of High-Grade Pipe Subjected To Reverse Fault

Driven by the Second West to East Gas Pipeline Project and Russia-China Gas Pipeline East Line Project,X80 grade steel has been widely applied to the pipeline projects in China.Yet,the increase of steel grade always leads to higher diameter thickness ratio of the pipe,making the pipe more prone to buckling failure subjected to reverse fault.At the meantime,the existing strain-based design theory,which takes the buckling of pipe as the criterion,is conservative.It is necessary to introduce the ductile damage criterion based on microscopic mechanism,so as to propose a more scientific strain-based pipe design method.In this dissertation,a finite element model of mechanical response of high-steel pipe under reverse fault is established based on the criterion of pipe buckling;and the variation patterns of axial compression strain,axial force and bending moment of pipe under different working conditions(fault dip angle,pipe internal pressure,pipe wall thickness,soil type,pipe buried depth,etc.)are compared in detail.The critical fault displacement to pipe buckling is calculated according to Canadian Standards Association(CSA)Standard Z662.To determine the failure criterion based on the ductile damage of X80 grade steel,this dissertation carrys out uniaxial tensile tests with different notched round bar specimens,so as to require the stress and strain of the specimens.Finite element method based on Mohr-Coulomb theory is applied in this process.Finally,this dissertation analysis the post-buckling behavior of X80 grade pipe under the axial force and bending moment,which equals the reverse fault load.Based on the criterion of ductile fracture;the thrust behavior of pipe in the process of pipe buckling and post-buckling burst is revealed.And the critical fault displacement to pipe ductile fracture is required according to the cumulative plastic damage.By comparing with the critical fault displacement to pipe buckling and that to ductile fracture,a conclusion can be drawn the criterion of ductile fracture can significantly increase the critical fault displacement in the strain-based pipe design method and thus can improve the utilization rate of pipe under large displacement conditions.