Contrôle non destructif par courants de Foucault : expérience et modélisation pour la conception et l'optimisation de capteurs

For inspecting tubular steel subsea structures, the conventional non destructive testing techniques require the removal of the marine fouling and corrosion protection coatings. These costly cleaning operations down to bare metal can be avoided with a new contactless technique which has been developed by the Institut Francais de Recherche et d'Exploitation de la Mer. This technique consists in inducing a strong alternative electric current flow at several frequencies in the material. Corresponding localized perturbations of this current flow caused by defects such as inclusions or cracks are then detected. The current perturbation is sensed with a small non contacting probe to detect the associated magnetic flux signal at the surface of the structure. Exploratory experiments allow us to conclude that this technique is very promising. The improvements in sensitivity and in characterization of the defects need a better quantitative understanding of the way in which slots in conducting materials interact with electric currents. We propose a numerical model, using our software: the Trifou code. This code solves tridimensional electromagnetic problems, by computing the current density in massive conductors under the effect of a knwon and time varying electromagnetic excitation. By taking advantage of our experience in modelling the eddy current testing of the frech nuclear power plants, we have reproduced one of the experiments. Numerical results are presented and make the deflection of current lines at slot edges visible. The electric activity of the modeled slot has been pointed out and we suggest that the flaw may be interpreted as a back electromotive force source which creaes divergence free local currents. Then, we have established a numerical method based on a local approach, to obtain the defect signal with a low computing cost. Numerical results fit the shape of the experimental results. In particular, we can determine the position of the crack. A more accurate comparison, for example on the crack depth, is difficult because of the uncertainty on the physical properties of the materials