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3.3.2. Magnetic field leakage
against probe velocity
We are also interested in the intensities and features of MFL signals from
the high-speed MFL inspection. The analysis on MFL signals directs not
only the selection of magnetic sensor (array) based on specification such
as sensitivity, bandwidth, and measuring range but also the
signal-feature-based defect characterisation. The trajectories of
horizontal component of magnetic field leakage (Bx) between two polarities
of ferrite core against probe velocities are shown in the Fig. 6.
Fig. 2. Distribution of eddy currents
(10 m/s).
Fig. 3. Distribution of eddy currents (30 m/s).
From the Fig. 6, it can be found
that:
† Compared to the velocity case of 0 m/s, the trajectory of magnetic field
in each high-speed case is asymmetric. It is noteworthy that the
asymmetric signals from high-speed MFL are analogous to those for angular
defects using static MFL. The two peaks of MFL signal occur around the
edge of the slot defect. The difference in amplitudes of the two peak
values also reflects the probe moving direction.
Fig. 4. Distribution of magnetic flux
lines (10 m/s).
Fig. 5. Distribution of magnetic flux lines (30 m/s).
Moreover, magnitude of the
differences between the two peak values increases with the probe speed
increased.
† In general, the magnetic field strength decreases when the probe speed
is increased. Therefore, eddy currents due to the probe movement not only
distort the profile of magnetic field but also decrease the intensity of
magnetic field.
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