A surface impedance boundary condition for 3-D non destructive testing modeling

In this paper, the authors present an application of the surface impedance boundary condition to eddy current non destructive testing (NDT) modeling. When the skin depth is small, the electromagnetic field problem is reduced to an exterior field problem taking into account the influence of a ferromagnetic material (permeability,conductivity) by means of the boundary condition on the interface air-metal. The surface impedance model is implemented in a three dimensional boundary integral equation package and is compared with an axisymmetrical calculation. The comparison shows a good agreement between the two calculations. The application of the method is for NDT on surface cracks scanned with a circular coil fed with high frequency alternating current. In this case, the skin depth is small and the problem is three dimensional so that 2-D programs can´t be used. For this special problem, the surface impedance boundary condition is particularly well adapted and the proposed method shows encouraging results.