Research On The Tensile Strain Capacity Of X80 Steel Pipe Welded Girth Based On Constraint-Modified J-R Curves

The increasing demand for energy in industrial requires long pipelines to be placed in hazardous environments,resulting in these pipelines being subjected to large strain.In these cases,the traditional stress-based design method is not applicable,and a strain-based design method needs to be performed.Thus,various means such as experiments,numerical simulations,and mathematical analysis are adopted to develop a strain-based design method based on the constraint-modified J-R curves.Crack resistance curves measured by Single-edge notch tension specimens are sensitive to crack-tip constraints（specimen dimensions）.The constraint effect was quantified using Finite Element Analyses,and three different constraint parameter A₂、Q_m and Q_Zwere calculated.It is discovered that the value of A₂ and Q_m increase with rising crack lengths.Besides,the value of Q_Zare divided into two region with rising specimen thicknesses.Specimens with B/W≥2 showed a 50%reduction compared to specimens with B/W<2.Considering both in-plane and out-of-plane constraints,the constraint-quantified J-R curves are then formulated using the bending modified J-Q theory and the out-of-plane extension of bending modified J-Q theory to assess the fracture resistance of full-scale pipe welded girth.And through comparison with experiment results,it is proved that the error predicted by Q_m-Q_z is under 10%,which is lower than that predicted by the J-A₂constraint-modified resistance curve（30%）,and can be used for predicting the fracture response of full-scale pipe welded girth.Subsequently,the extrapolated resistance curves from SENT to full-scale pipes are used for strain-based fracture assessment of a flawed pipeline.Based on the tangency method,a new strain-based design method is proposed using the Q_m-Q_z constraint-modified J-R curves.Results show that the tensile strain capacity decreases as the crack length gets longer,the tensile strain capacity first decreases by 20%while the crack length 2c/πD rises for 0.05,and then slows down to 5%.A relatively good agreement in terms of tensile strain capacity between the present work and Exxon Mobil’s strain-based assessment approach can be achieved,indicating the accuracy of the proposed method and providing references for studies.