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3.3.3. Magnetic field leakage
against defect depth
From the simulations above, it can be seen that the defect width can still
be determined from the MFL signal by choosing the signal peaks as
features. Subsequently, the relationship between the defect depth and the
MFL signal of the high-speed MFL inspection system was observed by
conducting simulations with various depths of surface defect while keeping
the probe velocity constant at 30 m/s. The defect depths are sampled into
4–8 mm. In particular, 8-mm defect is throughwall defect. The results are
presented in the Fig. 7.
From the Fig. 7, it can be seen that: similar to the static case, because
the permeability of the specimen is decreased with the defect depth
increased, the magnitude of the MFL signal of the high-speed MFL
inspection system has inverse ratio to the depth of surface defect, in
addition to that there is distortion in MFL signals. Consequently, the
defect characterisation on its depth should be integrated with the speed
at which the probe moves and the direction where it travels.
4. Proposed high-speed MFL inspection system
Based on the simulations and analysis, we propose a highspeed MFL
inspection system for defect detection and characterization. According to
Eq. (1) and simulations, the magnetic field generated by eddy currents
that flow circumferentially in the pipeline will oppose the change of
primary magnetic flux and its distribution will depend on the probe moving
speed. Thus, it is difficult to use a single magnetic sensor for
capturing the profile of the magnetic field distribution. Therefore,
magnetic sensor array can be exploited and its deployment is determined by
the investigation on relationship between magnetic field measured and
defect inspected.
Fig. 6. Magnitude of Bx
vs. X-axis against probe velocity.
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