EFFECTS OF LINE PRESSURE STRESS, MAGNETIC PROPERTIES AND TEST CONDITIONS ON MAGNETIC FLUX LEAKAGE SIGNALS. ANNUAL REPORT, MAY 1995-APRIL 1996

Stress, such as produced by line pressure or bending, can cause changes of up to 80% in the amplitude of Magnetic Flux Leakage (MFL) signals. Corrections should therefore be made to high resolution MFL data to enable defect sizes to be deduced accurately. The effects are complex and depend on the magnetic properties of the particular line pipe. Tests of the effects of bulk stress on MFL signals using samples of GRI PSF flow loop show that the effects depend on the direction of the magnetizing field with respect to the applied stress direction, whether near or far side defects are being examined and the flux density in the pipe wall.