Oil and gas pipelines are inspected for corrosion using magnetic flux leakage (MFL) inspection tools. It has been found that varying line pressure, bending or residual stresses cause changes in the MFL signals. Changes in the amplitude are thought to be due to stress-induced changes in the bulk magnetic anisotropy or preferred easy axis of the steel pipe wall. This suggestion is supported by results presented here of finite element calculations of the effects of changes in anisotropy on MFL signal amplitudes.
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Leonard et al. (1996) studied this question.