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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2.4. Rotational EC inspection technique for the detection of fatigue cracks under the rivet head The highest sensitivity for detecting cracks near rivets is achieved with techniques based on the rotation of the EC probe around the rivet head. Such an approach is applicable when it is necessary to identify cracks whose length is no longer critical for the given structure. Let’s consider the results of the inspection provided by the use of rotating EC probes of double-differential type. The scheme of the double-differential EC probe installed on the riveted lap joint (top and cross-sectional view) is presented in Fig. 4a and Fig. 4b. The drive and sensing coils are connected following a double-differential configuration presented above in Fig. 1. To realize rotational inspection, the EC probe was installed coaxially to the rivet head and rotated as shown in Fig. 4b. The proposed rotational EC probe was examined with the application of a riveted two-layer AA specimens with a 6 mm diameter hole with artificial 0.1 mm thick slots with a length from 1.0 to 6.0 mm in the second layer. The operating frequency was 2 kHz. The specimen was covered with a defect-free 2 mm thick skin containing a 6 mm diameter central hole to simulate a real two-layer AS (Fig. 4a). The EC probe signals were recorded in the complex plane using an EDDYMAX-type EC board. All artificial slots with a length from 1.0 to 6.0 mm in the specimens were detected with a high signal-to-noise ratio. Fig. 4 presents the signal in the complex plane for 1.0 mm long cracks (Fig. 4c) through a 2 mm thick upper skin. For the comparison, the noise signals were also investigated to estimate the signal-to-noise ratio. Structural noise was obtained by EC probe rotation around a defect-free rivet (Fig. 4d). It can be noted that the proposed rotational EC probe can detect as small as 1.0 mm long cracks under the rivet head and 2 mm thick upper AA skin with the signal to-noise level more than 6 dB. Therefore, fatigue cracks will be detected before they develop to a critical length.

Fig. 4. (a) Cross-sectional view of the two-layer riveted specimen; (b) top view of the double-differential EC probe installed on the rivet head: 1 and 2 – first (skin) and second (spar) layers, 3 – rivet; 4 – crack in the second layer, (c) EC probe signals in the complex plane for a 1 mm long subsurface crack, and (d) noise signals for the EC probe rotated around a defect-free rivet. 2.5. Sliding EC inspection technique for detection of fatigue cracks in riveted multilayer aircraft structures The rotational technique described above has the disadvantage of limited inspection speed. Higher inspection productivity can be achieved using the sliding EC technique, in which the EC probe moves along the rivet row at a certain distance from it. This technique is intended for detecting transverse (with respect to the rivet line) cracks originating in the rivet area of in-service AS. The proposed sliding technique employs a double-differential EC probe. To implement this approach, a double-differential EC probe with an operational diameter of 15 mm was developed. According to the proposed technique, the EC probe scans along a line parallel to the rivet row at a certain distance from it, as shown in Fig. 5a. Double-differential EC probes operated at the frequency of 2 MHz are possible to separate EC signals created by transverse cracks and defect-free AA rivets by different directions of signal responses in the complex plane, as it is presented in Fig. 5b and 5c. It is shown here that signals created by a 6 mm long crack in the second layer (Fig. 5b) and signals obtained from a defect-free rivet in the complex plane have different directions in the complex plane. By rotating the complex plane, the direction of the signal from the crack is set to be directed