The purpose of this study is to clarify crack initiation, crack growth resistance and crack path from the weld flaws and to discuss the effect of weld flaws and internal pressure on the tensile strain limit for leakage of pipelines subjected to uniaxial tension. Tensile tests were conducted using steel pipes welded circumferentially with artificially introduced flaws under no and high internal pressure conditions.
With the improvement of mechanical properties such as strength and toughness of structural steel in recent years, the number of accidents directly caused by brittle fracture in structural members and buildings using such materials has decreased (Yasuda et al., 2004; Ishikawa et al., 2005).
High-pressure gas pipelines to transport natural gas play a key role as critical infrastructure against the background of rapid change in the energy scene in Japan. High-pressure gas pipelines must be designed not to leak gas outside for their performance guarantee requirements even if earthquakes cause them large deformation (Kawaguchi, 2011; JGA, 2013).
A ductile crack might be initiated from flaws which exist potentially in the girth weld joints and grown towards to the thickness direction when a high-pressure gas pipeline deforms longitudinally by earthquakes. Some companies conduct tensile or bending tests using large width test pieces or actual pipes with notches on the surfaces to verify the safety of high-pressure gas pipelines against the fracture mode (Sakimoto et al., 2009; Onuki et al., 2017).
Generally, the welded joints consist of three different parts such as base metal (BM), weld metal (WM) and heat affected zone (HAZ), which has different properties. Thus, crack initiation and growth from the weld flaws might be influenced by heterogeneity of strength and ductility in the welds.
The purpose of this study is to clarify crack initiation, crack growth resistance and crack path from the weld flaws and to discuss the effect of weld flaws and internal pressure on the tensile strain limit for leakage of pipelines subjected to uniaxial tension. In order to achieve the purpose, tensile tests were conducted using steel pipes welded circumferentially with artificially introduced flaws. An API 5L X65 seamless steel pipe with outside diameter of 164 mm and thickness of 9 mm was welded by performing narrow groove multi-layer welding using a welding wire as specified in AWS A5.18 by gas shielded arc welding in the flat position. The artificial flaws in each pipe specimen were introduced from inner and outer surface by electric discharge machining (EDM) method and the tip of EDM notch was located in the center of WM and coarse grain HAZ with accuracy. The test was done under no internal pressure and high internal pressure conditions.
