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3. Simulation on MFL evaluation at
high speed
3.1. ANSOFT maxwell EM for solution to MFL application
Magnetic flux leakage is applied for detection and reconstruction of
corrosions on pipes and tubes. The material of specimen is always
ferromagnetic metal which is non-linear. Therefore, the numerical methods
which cannot deal with nonlinear problems (for example, BEM, FDM) are not
suitable for MFL simulation. Furthermore, in dynamic MFL systems, the
probe moves above the surface of ferromagnetic specimen. As a result,
dynamic simulation with consideration of moving components gives accurate
results for practical problems. Although time-stepping solver is
introduced in meshless method, the moving conduct or is prohibited, even
though it is applied in simulations on fluid mechanism. Consequently, FEM
is preferable in solving the equations governing electromagnetic field in
MFL system.
ANSOFT Maxwell EM is commercial software for numerical simulation on
frequency and time domain electromagnetic fields in complex structures. It
implements FEM whilst allowing BEM codes and has strongly-coupled
electromagnetic, drive circuit and mechanical formulations. It also
integrates several numerical modules for solving specific problems, such
as electrostatic, magnetostatic, quasi-static, transient problems as well
as those involving eddy currents and axial eddy currents. In the light of
features of ANSOFT Maxwell EM, the high-speed MFL inspection system was
simulated with 2D modeling in ANSOFT Maxwell EM V10 [19].
3.2. Simulation setups
The simulation on the high-speed MFL inspection system involves
investigation on eddy currents due to the movement of probe and their
characterisations of MFL signals with variations of defect properties. As
illustrated in Fig. 1, the simulation model built in 2D with X–Y
coordinates represents the cross-section of the moving MFL probe and the
steel specimen. The distributions of magnetic flux lines and eddy currents
in specimen are observed, and the magnetic field leakage is measured in
each case of probe velocity varying from 0 to 30 m/s with variations of
defect depth. The geometric parameters of probe, specimen and surface
defect are listed in Tables 1–3. The stand-off between the probe and the
specimen is kept 1 mm while the lift-off between the sensor array and the
specimen is 0.5 mm.
Fig. 1. Simulation model for MFL at high speed.
Table 1 The dimension and properties of the coil
Length (mm)
Thickness (mm)
Turns
Cross-section shape
Material
Current source
20
10
400
Rectangular
Standard copper (permeability 10Aa (Strand;
total)
Z1 Mu; conductivityZ5.8e007
a DC excitation. The eddy currents and displacement currents in
coil are neglected.
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