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

282 2

1. Introduction Fatigue cracks and corrosion damage (CD) of the fuselage and wings are among the leading causes of aircraft accidents worldwide, as reported by Abolikhina and Molyar (2003), Campbell and Lahey (1984), and Goranson (1997). To minimize aircraft breakdowns, in-service non-destructive testing (NDT) based on various physical phenomena was implemented as an essential part of an aircraft maintenance program to ensure the structural integrity, safety, and durability (Hagemaier, 1991; Schmidt et al., 2000). NDT methods play a special role in implementing the concept of damage tolerance in aviation (Ball, 2003). Various NDT methods are commonly used to detect surface and subsurface defects in aircraft structures (AS), including those based on penetrant, eddy current (EC), radiographic, ultrasonic, and radiographic testing, among others. All NDT techniques have specific advantages and limitations related to the physical phenomena on which they are based, as reported by Ball (2003), Bossi (2014), Dalton et al. (2001), Hagemaier (1991), Qu et al. (2020), Riegert et al. (2006), Schmidt et al. (2000), and Toman et al. (2024). The EC NDT method is well established as a relatively low-cost technique (García-Martín et al., 2011; Hagemaier, 1991; Harrison, 1987; Khanz, 2000; Kim et al., 2014; Libby, 1971; Plotnikov et al., 2007; Thompson, 1993; Udpa and More, 2004). In practice, the EC method is frequently used not only for measuring geometrical parameters, but also for determining the specific electrical conductivity as a structure-sensitive parameter, i.e., a parameter sensitive to the structural state and the related mechanical properties of metallic alloys (Rummel, 1966; Tsai and Chuang, 1996). At the early stages of its development, the EC method was applied solely for surface defect detection due to the high operational frequencies used in the first EC flaw detectors and the limited penetration of eddy currents caused by a pronounced skin-effect (Hagemaier, 1985; Mottl, 1990). The depth of eddy current penetration can be increased by using lower operating frequencies (Dziczkowski and Zolkiewski, 2013; Majidnia et al., 2016; Stucky and Lord, 1992). At low operating frequencies, the EC method is considered most suitable for detecting subsurface or internal defects in multilayer AS without disassembling or removing rivets (Harrison, 1987; Khanz, 2000; Plotnikov et al., 2007; Thompson, 1993; Uchanin, 2023). While the EC method offers significant advantages for detecting cracks in internal layers of riveted multilayer AS, detecting cracks associated with rivets presents challenges due to the specific noise generated by defect free rivets. The purpose of this article is to provide an overview of EC NDT techniques that enable the detection of internal, hidden defects in multilayer AS without disassembly, using double-differential EC probes. 2. Results and discussion 2.1. Low-frequency eddy current probes of double-differential type The double-differential EC probes developed at the Karpenko Physico-Mechanical Institute consist of two drive coils (1) and two sensing coils (2). All coils are mounted on the ferrite core and situated in the tetragon corners (Fig. 1). Two identical drive coils (1) are connected in series and oriented to produce equal but opposite primary electromagnetic fields (Fig. 1c). Due to this connection of the drive coils, in the zone between them there is a so-called neutral plane, where the primary fields created by the drive coils are subtracted, and total primary field is equal to zero. At the same time, in the neutral plane, eddy currents are summed (Fig. 1d). This is clearly visible in the distribution of eddy currents created by both drive coils in the top view in Fig. 1d. Sensing coils (2, Fig. 1a) are oriented to detect the vertical component of the electromagnetic field and are positioned in a neutral plane, where this component is zero for defect-free isotropic TO (Fig. 1c). This feature results in a very low EC probe signal imbalance and reduced associated noise. The double-differential signal response is achieved through the opposite connection of the sensing coils (2, Fig. 1a). The developed EC probes exhibit high penetration into the material due to the spatial separation between the drive and sensing coils. The small coil diameters provide high spatial resolution. These EC probes are also characterized by high sensitivity to both elongated defects (such as cracks) and local defects (such as pitting or pores), as well as high sensitivity to surface and subsurface defects through dielectric protective coatings. A range of double-differential EC probes with diameters from 4 to 33 mm has been developed for various applications. These EC probes are compatible with modern flaw detectors produced by leading NDT companies (e.g., Rohmann, Olympus) and enable EC testing over a wide frequency range from 50 Hz to 10 MHz. Proposed EC probes can be characterized by an enhanced capability to detect subsurface fatigue cracks, as reported by Mook et al. (2007), Uchanin