PSI - Issue 82

Valentyn Uchanin et al. / Procedia Structural Integrity 82 (2026) 281–287 Valentyn Uchanin et al. / Structural Integrity Procedia 00 (2026) 000–000

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upward from the balance point (0). Thus, it is possible to discriminate the signal from cracks and defect-free rivets by direction in the complex plane (Fig. 5b and Fig. 5c).

Fig. 5. (a) Detection of cracks located in the second layer of the AS: 1 and 2 – first and second layers of the inspected unit, respectively; 3 – rivet; 4 – transverse crack; 5 – EC probe, (b) EC signals generated by the fatigue crack in the second layer, and (c) signals from a defect-free rivet.

Fig. 6. (a) Side view of corner cracks 9 mm long (A) and 11 mm long (B) in riveted spar skin 2, and a defect-free rivet (C), and (b) corresponding EC signals in the complex plane (clockwise – defect-free rivet C; 11 mm long crack (B) and 9 mm long crack A). The next challenge involved detecting cracks originating from rivet holes in the spar through 5.0 mm thick AA skin in AS riveted with ferrous steel rivets. In this case, the ferrous steel rivet creates very high signals, and the EC technique presented above cannot be applied. The needed result was achieved with the new EC probe with a diameter of 15 mm, which is possible to operate at a very low (100 Hz) operating frequency (Fig. 6b). Related signals (Fig. 6b) show the possibility of discriminating the signals obtained for cracks of different lengths and a defect-free rivet. The presented EC techniques were applied at Leonardo Aircraft Division to detect second-layer cracks in the vicinity of AA and ferrous steel rivets located in spars riveted with 2 mm and 5 mm thick skins. 3. Conclusion The EC method is highly important for NDT of AS during their service life due to its high sensitivity to fatigue and corrosion defects. The design and peculiarities of EC probes of double-differential type are presented. Proposed EC probes were applied to solve many AS maintenance problems related to the detection of subsurface defects in multilayer AS, namely, the detection of cracks through skin in the vicinity of the rivets or the detection of fatigue cracks or corrosion pitting situated on the reverse side of the AS skin, etc. The last demand was related to detecting the corner-type cracks originating from ferrous steel rivets in the wing spar through a 5 mm thick AA skin. It was demonstrated that the signals obtained for cracks of different lengths and a defect-free rivet can be discriminated. The