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Analyses of the Generating Mechanisms of Standard Magnetic Flux Leakage Testing Signals

The standard magnetic flux leakage testing (ML) signals of tangential and normal components were used for evaluating discontinuities. Based on the analysis of ML signal features and their generation mechanisms, the widely used peak-to-peak magnetic flux leakage (MFLpp) value, consisting of “negative” sidelobe features hidden in the test signals actually bears not only on the magnetic flux leakage from discontinuities but also all the magnetic behaviors including the magnetic flow bypass and its relevant “negative” magnetic region. These behaviors need to be modified as evaluation parameters for discontinuities. Unlike MFLpp, a new evaluation parameter, MFLp0, calculated by the height difference between the bump peak and baseline is provided, which eliminates the “negative” sidelobe features and agrees quite well with the single discontinuity ML contribution. The comparison of the evaluation parameters between traditional MFLpp and MFLp0 further indicate that MFLp0 decreases more slowly with the increase of liftoff or offset distance compared to MFLpp, which is helpful in reducing the influence on ML signals caused by unsteady liftoff/offset especially during the course of magnetic sensor shake. Meanwhile, the finite element method and experiments were conducted to confirm these comparisons. The clarity of the ML signal features, their generating mechanisms, and the provided MFLp0 value for evaluating discontinuities are intended to get accurate or even high precision evaluation for tested discontinuities.

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