Impedance Evaluation of a Probe-Coil’s Lift-off and Tilt Effect in Eddy-Current Nondestructive Inspection by 3D Finite Element Modeling

The probe-coil’s lift-off and tilt variations will cause noise signals that can obscure the valuable signals when the surface inspection is performed. Prevention of noise signals due to lift-off and tilt variations is an important element in most eddy current inspections. In this paper, the electromagnetic field and impedance of a cylindrical probe-coil have been calculated numerically for arbitrary lift-off and tilt angle above a conducting plate using three-dimensional Finite Element Modeling. The probe’s liftoff and tilt effect on impedance change produced by a crack have been also investigated. The results show that the tilt curve are almost straight line and is characteristics of same phase with lift-off curve when probe’s tilt angle is less than a certain angle. Moreover, the phase of impedance locus due to a crack is approximately vertical to lift-off and tilt curve at the optimum excitation frequency. Phase rotation method can be used to remove simultaneously the probe’s tilt disturbance to crack impedance change with lift-off noise. The numerical results are confirmed by experiment measurement and the proposed method can be effectively applied to an actual eddy current testing.