True Stress-strain Curve Acquisition And Mechanical Properties Analysis For High-grade Pipe Steel

The high-grade pipe steel material represented by Grade X80 is the primary choice for current oil and gas pipelines,but less research for the true constitutive behavior of high-grade pipeline steel under large deformation and large strain conditions.In this paper,Grade X65 and Grade X80 are taken as the research objects,and the basic mechanical properties of Grade X65 and Grade X80 materials under uniaxial tension are obtained.The plastic deformation behavior and real stress state after necking are studied,and the local stress-strain relationship of welded joints of pipelines is studied.A method was conducted to achieve a zoned performance test of the welded joint.Considering the limitations of conventional mechanical performance testing under large-strain measurement,a method based on digital image correlation(DIC)is proposed to obtain the completed stress-strain relationship of the material,and compares the true stress-strain of X65 and X80 curves.The finite element simulation analysis was performed on the experimental results.It was depicted that the local triaxial stress state after necking had a greater influence.The Bridgman formula was used to correct the true stress after necking to eliminate the influence of triaxial stress.Based on the obtained true constitutive relation of the materials,the strain hardening index of the materials under the Ramberg-Osgood model was obtained and the properties of the hardening were compared.The results showed that the plastic hardening capacity of Grade X65 was better than Grade X80.The DIC technique is used to measure the strain field of Grade X65 welded joints during uniaxial tension.The real time local strain field in different areas of the welded joint surface can be obtained.A method based on experimental data to fit the parameters of the hardening model is presented.It is possible to accurately obtain the complete true stress-strain curve from the zero to the necking point in the weld zone,the base material zone,and the heat affected zone of the pipe weld joint.The mechanical properties of the weld were compared and analyzed.It was found that although the welded joints using the test pieces in this experiment had higher yield strength,the strain hardening performance and tensile strength were lower than those of the base metal area,resulting in fractures occurring in the Weld area.It has a strong practical significance for obtaining the local real constitutive relation of the weld zone and the heat affected zone and realizing the zonal testing of the welded joint.