Significance of Splits and Pop-Ins Observed During Fracture Toughness Testing of Line Pipe Steel

The occurrence of pop-in on the test record of fracture toughness tests and appearance of splits on the fracture surface, coupled with failure to meet Charpy toughness requirements, raised doubts about the fracture integrity of a parent pipe to API 5L X65 for low temperature operation. The cause of pop-in was investigated using fractographic and metallographic methods and additional fracture mechanics testing was conducted using different notch orientations. In addition, fitness-for-service was assessed using a fracture mechanics assessment procedure. It is shown that for typical flaw orientations that might be present, the apparent fracture toughness at pop-in is not representative. Nevertheless, it is shown that for the stress levels considered, the pipeline is tolerant to flaws without risk of fracture initiation at low temperature.