A neutron diffraction study of local stress concentrations surrounding defects in pipeline steel

Abstract

Magnetic flux leakage (MFL) is the most commonly used technique for the in-service inspection of both gas and oil pipelines. However, MFL is sensitive to the stress state of the pipe wall and the area surrounding defects. MFL tools are often calibrated using pipe sections with defects artificially introduced into the pipe wall prior to the pipe being pressurized. Naturally occurring defects, however, occur while the pipe wall is under stress due to line pressure. In this study, neutron diffraction is used to map the strain surrounding the two defects that have been introduced into plates of pipeline grade steel, one into a plate prior to uniaxial loading, the second into a plate held at a constant background load high enough to ensure that stress at the edge of the defect exceeds the elastic limit of the material.