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3.3. Simulation results and analysis
In simulations, the probe moves from the left side of the specimen and
halted at the right edge at the specified speed. The magnetic field was
measured while the probe moved right over the surface defect.
3.3.1. Distributions of eddy currents and magnetic flux lines
In conventional static MFL inspection systems with DC excitation, there is
no current in conductive specimen. In contrast, eddy currents are
generated within the specimen when dynamic MFL inspection systems are
employed. By using the simulation software, the distributions of eddy
currents under different moving speeds of 10 and 30 m/s are illustrated in
the Figs. 2 and 3, respectively. In each velocity case, the magnetic flux
lines representing magnetic field within the system are also detailed in
the Figs. 4 and 5.
From Figs. 2–5, it can be seen that:
† Compared to the velocity case of 0 m/s where the probe is static and
there is no eddy current in the specimen, eddy currents exist within the
specimen when the MFL probe has relative speed to the steel specimen, even
though the excitation current is DC. Moreover, the profile of eddy
currents is dependent on the probe speed. As a result, as illustrated in
the Figs. 2 and 3, when the probe speed is increased, eddy currents
concentrate more on the specimen surface and longitudinally stretch with
longer distance after the probe. It is understandable that such skin
effect is also applied in high-speed MFL inspection. Consequently, it is
practicable to arrange electromagnetic sensor arrays behind the probe to
measure the magnetic field for defect detection indicated by eddy currents
in this region.
† From the Figs. 4 and 5, it is noticeable that the profile of magnetic
field is distorted because of the eddy currents generated in the specimen
due to high-speed-moving probe and asymmetric with respect to the
rectangular slot defect. Moreover, the distortion of magnetic field has
direct proportion to the probe velocity.
Table 2 The dimension and properties
of the conductive specimen
Length (mm) Thickness (mm)
Cross-section shape
Material
500
8, 9, 10
Rectangular
Steel (conductivityZ2e6 S/m, having B–H curve)
Table 3 The dimension and properties of the defect
Length (mm) Depth (mm) Cross-section
shape
Material
Flaw type
10
4, 5, 6, 7, 8 Rectangular
Air slot (PermeabilityZ1.0 Mu; conductivityZ0 S/m) Surface
artificial defecta
a Through-wall defect, No currents flow through the defect.
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