This paper presents the influence of internal pressure on ductile fracture behavior conducting three pressurized and un-pressurized pipe tension tests with machined surface notches. Especially, it is focused on investigation of the ductile crack initiation from the aspects on effective opening displacement at a notch tip. The influence of internal pressure was negligible on ductile crack initiation from a surface notch. On the other hand, the crack driving force became higher as internal pressure increased. Ductile tearing resistance curves (R-curves) obtained from pressurized pipe tests and Single Edge Notched Tension (SENT) tests are also investigated. A R-curve from SENT tests shows good agreement with that from pipe tests. Therefore, it is possible to use the R-curve of a SENT specimen as that of a pressurized pipe specimen. According to these results, the critical strain on ductile crack initiation and leakage of the pressurized pipe was predicted, and it is verified that predicted critical strain was well in agreement with an experimental result.
In recent years, construction of natural gas pipelines is expanding to severe environmental area such as seismic or permafrost region (Glover, 2003). In these regions, it can be expected that pipelines will be subjected to large deformation due to large ground movement associated with liquefaction in seismic region or frost heave in permafrost. It is not possible to apply conventional stress-based design or integrity assessment methods in cases where a pipeline is subjected to large plastic strain that greatly exceeds the yield stress of the pipe material. A number of studies have been carried out on strain-based design and integrity assessment methods. Tensile strain limit of a pipeline is usually characterized using uni-axial tests such as the curved wide plate (CWP) test (Denys, 2004). Accordingly, the most critical fracture mode in pipelines is considered to ductile fracture originating from defects in girth welds, and prediction of ductile crack initiation of CWP test has been proposed using critical equivalent plastic strain at notch tip obtained from SENT tests (Sadasue, 2004).
