Comparison of the effects of surface and uniform stresses on magnetic flux leakage patterns

Magnetic flux leakage (MFL) is a very common method of inspecting pipelines. To test the effects of stress on MFL signals in the laboratory some experiments have used bending of strips as the mechanism for stressing samples. If bending stress and uniform stress produce similar effects on MFL signals then this idea would be confirmed. However, if the response is different, then the use of bending stress would not be advisable as a method of simulating normal operating stress in pipelines.;The differences between through wall bending stress and uniform stress are examined using two separate arrangements. Test samples are long axial strips of pipeline steel cut from a section of pipe. One strip is bent as a single beam to produce bending stress or varying stress through the wall thickness while a pit machined in the pipe section was magnetized by a detector and scanned with a Hall probe. Another two strips were bonded together with a fibre-glass and wood core to produce a composite beam that was bent to produce near uniform stress in each strip. The radial flux leakage patterns over the area surrounding the defects were mapped and compared. Results showed that near uniform stress in the composite beam produced much greater MFL amplitude response with stress then the bending stress in the single beam. This refutes the idea that surface stress alone is sufficient for considering the effects of stress on MFL signal amplitudes. (Abstract shortened by UMI.)...