Electromagnetic inspection technique of thickness of nickel-layer on steel plate without influence of lift-off between steel and inspection probe

There is a need to inspect thickness of a nickel layer of nickel-coated steel plate in the production process to guarantee the quality. The conductivity of a nickel layer in the nickel-coated steel is larger than that of mother steel, and its permeability is smaller than that of mother steel. Therefore, the estimation of thickness of the nickel-layer is possible by using differences of these electromagnetic properties. However, the signal is also influenced by the change of distance (lift-off) between the nickel-coated steel and an electromagnetic sensor. In this paper, the inspection method for measuring the thickness of nickel layer taking account of the lift-off is proposed using the 3-D edge-based hexahedral nonlinear FEM.