This paper presents the results of a preliminary study to establish an assessment method for the tensile strain limit against brittle fracture of pressurized gas pipelines subjected to axial tensile deformation. The basis of the assessment method is the Japan Welding Engineering Society standard WES2808-2003, which provides a procedure for evaluating the fracture limit using the CTOD design curve taking into account the deterioration of the fracture toughness of materials due to large cyclic straining and dynamic straining. Modifications of the procedure to enable evaluation of the fracture properties of high strength gas pipelines under biaxial loading conditions are studied. The applicability of the CTOD design curve is investigated and problems with the CTOD design curve are clarified.
INTRODUCTION In recent years, construction of pipelines has expanded to arctic, subarctic and seismic regions. Gas pipelines traversing discontinuous permafrost areas are subject to repeated frost heave and thaw settlement caused by variations in operating and surrounding temperatures. In this case, the pipelines may be subjected to large cyclic strain. Pipelines in seismic regions may also be subjected to large cyclic strain by landslides. In order to secure the integrity of pipelines subjected to large cyclic strain, a compressive strain limit for initiation of local buckling/wrinkling in the pipe wall and a tensile strain limit for initiation of fracture from girth weld flaws are important parameters (Zhou, Horsley and Rothwell, 2006). Therefore, numerous researchers have made significant efforts to predict these strain limits. It has been recognized that the bending compressive strain capacity of linepipes is dependent on the D/t ratio (Glover, 2002; Suzuki and Toyoda, 2002), the strain-hardening properties of the pipe material (Suzuki and Toyoda, 2002; Suzuki et al., 2001), internal pressure (Suzuki et al., 2004) and geometric imperfection (Suzuki et al., 2006).