Évaluation non-destructive quantitative de structures aéronautiques par la méthode des courants de Foucault

The eddy current (EC) method is widely used in the industry for the nondestructive evaluation (NDE) of electrically conductive structures, because it is sensitive, robust, cost effective and non polluting. However, in the general case, the quantitative evaluation of the parameters of a structure is a difficult problem, because the available EC data are generally incomplete, because it is necessary to have numerical models developed to account for the probe / structure interactions, and finally, because the inverse problem which consists in estimating the parameters of an inspected structure from the knowledge of those physical interactions and the available EC data is "ill-posed".In this work, we focus on the evaluation of air gaps between the parts of a metallic assembly. In order to cope with the difficulties associated with the implementation of an EC quantitative evaluation, we choose to use a multi-frequency approach so as to increase the data provided by the EC probe, and to build behavioral models from the statistical analysis of the probe / structure interactions. Experimental data and simulated data based on finite elements modeling are analyzed. A first approach behavioral model of the probe/structure interactions is deduced from these analyses, which is based on the linear relationship observed (in a particular excitation an frequency band depending on the dimensions of the studied structure) between the variations of the sensor normalized impedance and the air gap to evaluate. Moreover, we propose, implement and discuss the performances of several algorithms designed to solve the inverse problem dealing with the estimation of both the multilayered structure air gap and "hidden" plate thicknesses. Then, we extend the proposed multi-frequency behavioral approach thanks to artificial neural networks, and we apply the proposed behavioral method to a second problem : that of the characterization of real fatigue cracks in metallic massive parts. The obtained results let the generalization of the proposed approach to other EC NDE configurations to be envisaged.