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

283

3

(2023, 2024), Uchanin et al. (2023), and Uchanin and Nardoni (2019), and were applied to solve many AS maintenance problems related to: 1) detection of subsurface defects in multilayer AS (e.g. cracks in the internal layers in riveted AS or cracks initiated on the reverse skin surface; 2) detection of cracks through repair patches fabricated from AA or carbon fiber reinforced plastic; 3) detection of the local CD situated on the reverse side of AS skin, etc. Our efforts have been directed at improving the characteristics of these EC probes to increase their sensitivity at low operating frequency and the depth of the inspected layer (Uchanin, 2012). The final challenge involved detecting cracks originating from ferromagnetic steel rivets in the wing spar through 5 mm thick AA skin. To address this, a low-frequency EC probe with a 15 mm diameter, capable of operating at frequencies as low as 50 Hz, was developed (Fig. 1b).

Fig. 1. (a) Configuration of a double-differential EC probe; (b) low-frequency double-differential EC probe of the MDF-1501 type; (c) primary electromagnetic field generated by the drive coils (side view); (d) eddy currents (top view): 1 – drive coil, 2 – sensing coil, 3 – tested object, 4 – primary electromagnetic field, 5 – neutral plane, 6 – eddy currents. 2.2. Detection of the fatigue crack initiated on the side surface of the multilayer AS in the zone of the reinforcing belt edge The timely detection of fatigue cracks on the reverse, non-visible side of AS skin is crucial for maintaining flight performance. A similar scenario occurs, for example, in the skin of a Boeing 737 in the area of lap joints, where a reinforcing AA belt is located between the two skins, as shown in Fig. 2a. During operation, fatigue cracks tend to develop along the edge of the reinforcing belt, where the local stiffness of the AS is higher. The thickness of both the reinforcing belt and the skins is 0.9 mm. The inspection task requires detecting cracks with a depth of 0.45 mm originating on the inner side of the upper skin while accessing the structure from the outer side (Fig. 2). In other words, subsurface cracks located at a maximum depth of 0.45 mm need to be revealed. However, a challenge arises because, at low frequencies, the edge of the reinforcing belt produces a constant interference signal that must be distinguished from the signal of an actual crack. Therefore, a reliable method for signal interpretation is required to distinguish crack signals from false responses. The developed EC technique employs a double-differential EC probe of the MDF 1201 type, operating at a frequency of 26 kHz. For the interpretation of signals with the selection of useful signals from cracks, an indication in the complex plane of the flaw detector was used (Fig. 2b, c). The signal created by the reinforcing belt was oriented in the horizontal direction by rotating the complex plane. When the gain coefficients along the vertical (Y) and horizontal (X) axes of the device indicator are equal ( K Y = K X ), the signals from cracks and belts differ only slightly, making them difficult to distinguish, as shown in Fig. 2b. The signal selection can be improved by a 12 dB increase ( K Y = 4 K X ) of the gain along the vertical Y-axis compared to the gain along the horizontal (Fig. 2c). The presented methodology was implemented for inspection of Boeing 737 aircraft operated by Ukraine International Airlines.