Modeling of magneto-optic eddy current inspection of hidden flaws in aircraft skin

Eddy current inspection of aging aircraft has currently achieved new developments with the introduction of magneto-optic imaging (MOI). Real time images of aircraft skin cracks and corrosion can be obtained using MOI instruments. The sensitivity of MOI inspection and hence the feasibility of its use for actual configuration depends' on instrument parameters, structure geometry, and flaw size and depth. Computational modeling of the three dimensional structure under test and its interaction with the eddy current source is inevitable to enhance the equipment design and determine the probability of detection of critical flaw size defects using MOI. Finite element modeling of eddy current magneto-optic imaging is invoked in this paper and applied to a typical configuration of interest in aircraft inspection. The configuration considers a flaw initiating under the cap of a fastener used to connect two aluminum layers of the aircraft skin. Two materials are considered for the fastener: titanium and steel. The results are presented in a field map of the normal magnetic flux field, similar to expected images using MOI inspection. Simulation results revealed that the change in the vertical component of the magnetic field is noticeable for this configuration, and that MOI can be used to detect hidden flaws covered by the fastener cap.