Semi-analytical modelling of induction thermography for inspection of delaminated planar pieces
Résumé
Induction thermography has become increasingly important for the inspection of stratified metallic structures thanks to its peculiarity of assembling a number of attractive features from the two techniques that it combines, namely the eddy-current testing (ECT) and the infrared thermography (IT). It is contactless, fast and it provides direct (infrared) images. In addition, eddy-current heating through the Joule effect is a very effective and a robust way to thermally excite the medium under consideration without the need of special equipment or couplants. It can be used to detect both surface and volumetric defects like cracks, disbond, delamination and corrosion. The proposed semi-analytical solution is based on the truncated region eigenfunction expansion (TREE) method. The problem is transformed in Laplace domain with respect to time, and the temperature distribution is approximated by its expansion on a tensor product modal basis. This approach, limited to canonical pieces geometry, yields very fast and accurate results in this domain of application. In the context of this work, we focus on stratified planar pieces comprising thin delamination patches. The excitation sources are air-cored coils located above the inspected piece. The flaws are modelled as thin air gaps between the piece layers, which consist in an additional resistance to heat flow if the laminate is thermally stimulated. Their effect is thus taken into account by local modification of the continuity relations across the corresponding interfaces and does not require additional discretization.