Fracture Toughness Assessment of Longitudinally Seam-Welded Gas Pipelines at Low Temperatures

American Petroleum Institute (API) 5L X65 and X70 steel pipelines are employed widely for natural gas transportation and typically experience internal high transmission pressures and low temperatures in cold regions. Investigations show that in spite of strict inspections and controls, welding joints are commonly subjected to the presence of defects, with cracking being the most important one. Due to the many geometrical, material, and manufacturing variables involved, the failure mechanisms and procedures presented in the welded gas pipelines are complex. In order to achieve the benefits of gas pipelines, a deep understanding of the failure behavior is needed, especially in mixed mode loading conditions and at low temperatures. In this work, fracture toughness evaluation of longitudinal seam-welded gas transportation pipes at low temperatures and under mixed mode loading was studied based on experimental and numerical analysis. The experimental procedure included preparing the API X65 steel pipes, making the butterfly shaped experimental specimens, and encapsulating and insulating specimens in order to control and stabilize the temperature of test. By utilizing the Arcan fixture, the experiments carried out under different loading conditions at low temperatures and fracture forces by using force-displacement diagrams were obtained. Stress intensity factors (SIFs) for tensile and shear loading were found by numerical analysis and geometry correction factors were determined. The results indicated that the fracture toughness of the seam-welded samples decreased at lower temperatures, and it is strong to the tensile mode loading but weak to the shearing mode loading. The tensile and shear values of fracture toughness of seam weld under consideration for the temperature of $-25°\mathrm{C}$ were found 203.3 and $11.8\mathrm{kJ}/{\mathrm{m}}^2$, respectively.