MACHINING OF SIMULATED DEFECTS IN PIPELINE STEELS - A NEUTRON-DIFFRACTION STUDY OF LOCAL STRESS EFFECTS

The most commonly used in-line pipeline inspection tools utilize the m agnetic flux leakage (MFL) technique, which is sensitive to the stress state of the pipe wall. Calibration of these MEL, tools is often carr ied out using unstressed pipeline sections containing simulated defect s, usually produced by mechanical drilling. There is also strong inter est in creating simulated defects in stressed pipe walls, thus simulat ing corrosion in the field. In this study, neutron diffraction is used to map the local stresses surrounding mechanically and electrochemica lly drilled holes in a steel plate sample. Holes were drilled in unstr essed samples, as well as samples held at a constant 80 MPa uniaxial s tress. Mechanical drilling into a stressed sample was found to induce considerable residual stresses around the hole. Conversely, electroche mical drilling did not create a residual stress field in the vicinity of the hole.