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Numerical simulation on magnetic flux leakage evaluation at high speed
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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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