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In magnetic flux leakage (MFL) testing, the scraggly surface of the objects actually consists of "concave" and "bump" defects, however, MFL inspection mechanism is only explained based on magnetic field leakage in the vicinity of the "concave" defects. In order to richen the MFL theory and improve the detection signal analysis, 3D magnetic flux leakage for three typical shapes of "concave" and "bump" defects (i.e., the conical, spherical and rectangular defects) are investigated by building magnetic dipole models. It was found that "bump" defects generate a total different 3D MFL distribution from the "concave" defects, including x, y and z components of the MFL. Besides, numerical simulations by finite element method are conducted to validate the magnetic dipole models. [ABSTRACT FROM AUTHOR]

To improve the accuracy of the magnetic flux leakage (MFL) testing in practical application, we analyzed the signal characteristics of rectangular induction coil affected by the sensor arrangement and scanning direction. On the basis of magnetic dipole theory, the descriptions for the MFL field distributions of standard discontinuity were established. Further, based on Faraday's law of induction, the signal characteristics of rectangular induction coils were calculated and it was found that the signal waveform of the horizontal coil is similar with the x component of MFL; in contrast, the signal waveform of the vertical one has the same changing trend with the z component of MFL. Finally, the influence of scanning direction was analyzed and found that different scanning directions will generate different signal characteristics; and that with the angle β between the coil and discontinuity orientation changing from 0 to π/2, the signal amplitudes of both induction coils decrease, which was verified by relevant experiments. [ABSTRACT FROM AUTHOR]