A NEUTRON-DIFFRACTION STUDY OF LOCAL STRESS-CONCENTRATIONS SURROUNDING DEFECTS IN PIPELINE STEEL

Magnetic flux leakage (MFL) is the most commonly used technique for th e in-service inspection of both gas and oil pipelines, However, MFL is sensitive to the stress state of the pipe wall and the area surroundi ng defects. MFL tools are often calibrated using pipe sections with de fects artificially introduced into the pipe wall prior to the pipe bei ng 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 tha t 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 co nstant background load high enough to ensure that stress at the edge o f the defect exceeds the elastic limit of the material. (C) 1998 Elsev ier Science B.V. All rights reserved.