Eddy Current Techniques for Detecting Hidden Subsurface Defects in Multilayer Aircraft Structures

Abstract

In-service non-destructive inspection (NDI) is a very important part of the aircraft maintenance program that minimizes aircraft breakdowns due to the fracture of critical components. The eddy current (EC) NDI method is one of the most applicable methods for this purpose, due to its high sensitivity to fatigue cracks and corrosion damage in the main structural materials. In this paper, selective double differential type EC probes characterized by the enhanced possibility of detecting subsurface cracks initiated by fatigue or stress corrosion phenomena are presented. For different applications, a family of double differential type EC probes was developed with different sizes (from 5 to 33 mm) and different spatial resolutions. These types of probes are characterized by different operational frequencies in a wide frequency range (from 0.2 kHz to 1.0 MHz), high penetration depth and unique sensitivity to subsurface defects of different types (like elongated fatigue cracks or local corrosive pitting), and a high level of specific noise suppression concerned with the scanning inspection procedures. The EC probes proposed were investigated as effective tools for characteristic aircraft applications concerned with subsurface defect detection in multilayer structures, such as the detection of cracks in the second layer of a riveted two-layer structure or cracks initiated on the side surface of a multilayer structure with the suppression of the reinforcing hoop influence; the detection of subsurface defects in arc welding with a rough surface; the detection of cracks through repair patches fabricated from aluminum alloy or carbon fiber reinforced plastic, etc. These techniques create remarkable possibilities for the well-timed detection of dangerous damage without disassembling the aircraft structure or removing protective coating.

Detecting and Estimating Local Corrosion Damages in Long-Service Aircraft Structures by the Eddy Current Method with Double-Differential Probes

“…In view of our review of the state of the art and our preliminary study, two lowfrequency double differential EC probes -of the MDF 0601 and MDF 0801 type -were selected for the detection of hidden local CDs in ASs [21][22][23]. Each of these EC probes consists of two excitation coils and two sensing coils, which are arranged according to the double differentiation scheme [22].…”

“…In view of our review of the state of the art and our preliminary study, two lowfrequency double differential EC probes -of the MDF 0601 and MDF 0801 type -were selected for the detection of hidden local CDs in ASs [21][22][23]. Each of these EC probes consists of two excitation coils and two sensing coils, which are arranged according to the double differentiation scheme [22]. The parameters of the probes are presented in Table 1, where D ECP is the diameter of the EC probe, D f is the diameter of the ferrite cores, W e and W s are the number of turns of the excitation and sensing coils, L e and L s are the inductances of the excitation and sensing coil, respectively.…”

Automated Eddy Current System for Aircraft Structure Inspection

“…(4) (5) Considering ( 4) and ( 5), the modulating functions associated with the TO parameters are determined by the equations: (6) (7) where L -the PSC expansion operator outside the interval.…”

“…Due to this, they are suitable for use in the same devices as traditional ones. Double differential eddy current probes (ECP) are effective tools for characteristic aircraft applications concerned with subsurface defect detection in multilayer structures, such as the detection of fatigue cracks in a riveted two-layer structure, the detection of subsurface defects in arc welding, the detection of cracks through repair patches fabricated from aluminium alloy or carbon fibre reinforced plastic composite [7].…”