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A novel scanning multi-directional induction thermography system for non-destructive testing (NDT) is presented. Inhomogeneous Joule heating from Eddy currents circumventing defects is one of the main factors enabling induction thermography as an NDT technique. Eddy current density around defects depends on magnetic field orientation which is the basis of the proposed system to segment defects from sound area on a continuous seamless scan. The method consists of linear scan with a system capable of sequentially inducing Eddy currents on multiple orientations on a continuous basis, enabled by the selective activation of 4 coils wounded on a quadratic ferrite core inductor. The registered scan thermography is temporally splitted for each magnetic field orientation, yielding 4 directional thermographies with spatio-temporal discontinuities. A subsequent temporal pulse synchronization and thermal drift compensation is applied, decomposing the thermogram’s oscillating component for each orientation. This enables the usage of conventional spatio-temporal reductions, such as PCA and FFT, for each direction. Additionally, this paper fuses these directional reductions to segment defects based on the variance of the thermal signal associated with defects, compared to sound area. The method is demonstrated on a steel billet with delamination defect, and forged steel bolt with longitudinal surface cracking.
Marco et al. (Mon,) studied this question.