The evaluation of the structural changes in conductive materials by nondestructive eddy current techniques is based on the measurements of electric conductivity as a structure sensitive para-meter. In this case surface layers only are evaluated due to the skin- effect influence. The main advantage of the eddy current method is the possibility to carry out the measurements without the direct contact with the inspected surface. In the presented paper the possibility to optimize the choice of eddy current inspection parameters for determination of the specific electric con¬ductivity variations, initiated by in-service degradation of aluminum alloys in aging long-term exploited aircraft was investigated by numerical simulation. The main emphasis was done on the choice of the operational frequencies possible to optimize the inspection reliability when the measurements were done through the dielectric protective coating without its removal. The in¬fluences of the specific electric conductivity and dielectric coating thickness (or lift-off) for dif¬ferent operational frequencies on the transformer type eddy current probe transfer impedance were evaluated. New methodology to ensure the maximal sensitivity to the variations of the specific electric conductivity, determined through the dielectric coating by the optimal opera¬tional frequency selection, was proposed. For this purpose, the mathematical expression for eddy current probe sensitivity to electrical conductivity changes for different dielectric coating thicknesses was formulated. It was shown that for any determined dielectric coating thickness it was possible to find the optimal operational frequency with maximal sensitivity to conductivity changes. The angles between the directions of electric conductivity and the influence of the protective coating thickness on different operational frequencies were also investigated. The results obtained are important for the development of the inspection techniques possible to mo¬nitor the aluminum alloys degradation processes in ageing aircraft by noncontact electric con¬ductivity measurements in in-service conditions.

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