Power generation plants, aerospace, pharmaceutical, and many other industries utilize a variety of stainless steel and other non-ferrous small and large piping, pressure vessels, and other critical components in their day-to-day operations. Many of these components are subjected to various chemicals, high temperatures, high pressures, vibration and other conditions which can lead to cracking and failure. Traditional inspection techniques often include visual and Dye Penetrant Testing (PT) for crack detection. In recent years, advancements in Surface Eddy Current Array (SECA) have proven to be a more sensitive, faster, clean, and a digitally encoded technique for in-depth analysis. This analysis includes accurate defect length and depth sizing, detailed reporting and archived digital records, all without the asset and area contamination introduced by the PT chemicals.

This presentation will focus on 2 case studies: The first on critical components of a Mars Lander spacecraft, and the second on critical welds in large tanks at a pharmaceutical manufacturing plant.

The durability of cable-supported structures depends critically on the corrosion status of the steel wires in cables. Reliable and portable techniques for in-situ corrosion evaluation are therefore of significant practical importance. In this study, we developed a compact eddy current testing (ECT) systems designed for the assessment of corrosion in steel cables.
In the compact ECT system, the excitation coil, the detection coil, the amplifier, the lock-in amplifier, and the AD converter were all integrated into a compact probe box (17 × 8.5 × 6 cm). A single USB cable connected the probe to a PC, providing both power and data transfer, while maintaining a power consumption of less than 1 W.
The ECT system was applied in laboratory and field experiments for the corrosion evaluation of the steel cables of cable-stayed bridge. The results confirmed that the compact design, low power consumption, and portability of the developed ECT systems made them promising tools for practical on-site corrosion evaluation of steel cables. This system can also be used to evaluate the corrosion of steel rebar in concrete structures.

Cracks can develop at the boundary between aluminium wing-skin layers, due to cyclic fatigue [1], corrosion [2], or a mixture of the two. The current methods for inspecting bolt holes in multi-layer aluminium wing skins rely on hand-held Eddy Current Testing (ECT) inspections [3]. Simple single coil probes are typically applied to the external surface, or bolts are removed to
allow for the insertion of bobbin or rotating probes, resulting in prolonged grounding periods and only offering discrete data points at inspection intervals. In this study, we explore the possibility of a permanently installed Structural Health Monitoring (SHM) Pulsed Eddy Current (PEC) system to minimise grounding times, thereby reducing costs, and enhance understanding of defect development throughout the wing’s load cycles.
PEC sensors are identical in physical design to standard ECT coils but excited with a square waveform, which contains multiple frequency components, allowing for data from different penetration depths [4], which may improve characterisation of certain conductivity discontinuities over single-frequency inspection [5]. Various authors [1, 6–9] have studied the use of PEC excitation in inspecting bolt-hole defects in aluminium, with experimental success found in detecting defects 2.8–5mm long and 0.24mm wide using a central driving coil and multiple coil differential receiver pairs [2]. However, this previous research has focused on error due to sensor displacement in handheld
applications and data analysis approaches such as PCA, with no attention given to optimal driver and receiver coil layout for maximising defect characterisation in SHM applications. We therefore aim to improve on these systems by designing and testing different sensor configurations for deployment as SHM systems.
Finite Element (FE) modelling was conducted in COMSOL 6.2 using a 2D axisymmetric setup to analyse axisymmetric defects. We used these models to identify regions where the induced electromotive force (EMF) in a coil differs most between defective and non-defective samples, assuming constant coil parameters. This is achieved by calculating the time gradient of
the magnetic flux density vector B field, and subtracting the field in the defect-free case from the defect case. As EMF generetated in a coil is proportional to the rate of change of flux through the area encircled by the coil, this results in a field showing the regions that would produce the strongest difference signal, were a coil to encircle that region. Since the vector field B has direction as well as magnitude, this also informs selection of coil orientation.
Using this numerical analysis approach to identify the most promising sensor configurations, an experimental study is now underway to test a variety of transmit/receiver coil configurations.
These include a single driver coil being testing in absolute mode to act as a benchmark; a transmit-receive pair with a receiver coil mounted coaxially to the driver coil and as close to the
surface as possible; and a pair of receiver coils, mounted at varying angles to the surface, and wired differentially to produce a self-referenced real-time signal suitable for SHM applications.
Through these experiments we aim to validate the numerical modelling approach, improve on the signal-to-noise ratio seen in previous literature, and provide real data to inform the design of
SHM systems for any multilayer, bolt-hole originating defect scenarios.

References
[1] D. R. Desjardins et al. “Advances in Transient (Pulsed) Eddy Current for Inspection of Multi-Layer Aluminum
Structures in the Presence of Ferrous Fasteners”. In: Review of Progress in Quantitative Nondestructive
Evaluation, Vols 31a and 31b 1430 (2012), pp. 400–407. ISSN: 0094-243x. DOI: 10.1063/1.4716256.
[2] V. K. Babbar et al. “Finite element modeling of second layer crack detection in aircraft bolt holes with ferrous
fasteners present”. In: Ndt E International 65 (2014), pp. 64–71. ISSN: 0963-8695. DOI: 10.1016/j.
ndteint.2014.03.005.
[3] Howard Garlick. Conversation about aerospace bolt inspection. Personal Communication. 27th November
2022.
[4] A. Sophian, G. Y. Tian, and M. B. Fan. “Pulsed Eddy Current Non-destructive Testing and Evaluation: A
Review (vol 30, pg 500, 2017)”. In: Chinese Journal of Mechanical Engineering 30.6 (2017), pp. 1474–1474.
ISSN: 1000-9345. DOI: 10.1007/s10033-017-0177-2.
[5] B. Lebrun, Y. Jayet, and J. C. Baboux. “Pulsed eddy current signal analysis: Application to the experimental
detection and characterization of deep flaws in highly conductive materials”. In: Ndt E International 30.3
(1997), pp. 163–170. ISSN: 0963-8695. DOI: Doi10.1016/S0963-8695(96)00072-2.
[6] D. P. R. Desjardins, T. W. Krause, and N. Gauthier. “Analytical modeling of the transient response of a coil
encircling a ferromagnetic conducting rod in pulsed eddy current testing”. In: Ndt E International 60 (2013),
pp. 127–131. ISSN: 0963-8695. DOI: 10.1016/j.ndteint.2013.07.007.
[7] V. K. Babbar et al. “Finite Element Modeling of Pulsed Eddy Current Signals from Conducting Cylinders
and Plates”. In: Review of Progress in Quantitative Nondestructive Evaluation, Vols 28a and 28b (2009),
pp. 311–318. ISSN: 0094-243x.
[8] V. K. Babbar, D. Harlley, and T. W. Krause. “Finite Element Modeling of Pulsed Eddy Current Signals from
Aluminum Plates Having Defects”. In: Review of Progress in Quantitative Nondestructive Evaluation, Vols
29a and 29b (2010), pp. 337–344. ISSN: 0094-243x. DOI: Doi10.1063/1.3362413.
[9] P. F. Horan V. K. Babbar P. R. Underhill and T. W. Krause. “Finite Element Modeling of Pulsed Eddy Current
Applied to Ferrous and Titanium Fasteners in F/A-18 Airplane Wing Structure”. In: COMSOL Conference.