## 2 Methods and Materials
### Non-Destructive Testing (NDT) Techniques
[46] evaluated Eddy Current Array (ECA) technology, focusing on aircraft specimens with artificially induced defects of various types, orientations, and sizes to simulate real-world conditions. The study aimed to assess how effective, sensitive, and reliable this technology was in detecting and characterising surface and subsurface defects in aircraft structures. The author also analysed the key factors impacting the performance of this technology, including advantages and limitations. A 360 mm by 120 mm aluminium alloy 31T5 (AD31T5) specimen was fabricated for the experiments, featuring multiple circular defects with varying cluster densities, each with a depth of 4 mm and a diameter of 0.5 mm. This alloy is widely used in the aviation construction industry. A key focus of the study was to assess the ability of ECA technology to detect small defects. For the experiment, the researchers used the Olympus Omniscan MX eddy current flaw detector with a matrix-type eddy current sensor. Due to its flexibility, this sensor could be attached to multiple geometry surfaces, flat or with curves. The results showed that the system could detect clusters of holes; however, it was challenging to detect single holes with such a small diameter of 0.5 mm. Careful calibration of the system, particularly by adjusting sensitivity levels, was crucial to enhance the visibility of defects. This process was challenging, as increasing sensitivity introduced more noise, leading to false positives. In other words, the system mistakenly identified non-defective areas as defective due to the added noise. Acoustic Emission (AE) [37; 38; 39] is a highly effective inspection technology capable of detecting a wide range of defects, including porosity, voids, delamination, debonding, foreign bodies, and cracks. It can identify both surface and internal defects, with particular effectiveness in monitoring active defects. However, AE has limitations, such as difficulty measuring the size and shape of detected defects. [PERSON] et al. [47] analysed the key benefits and limitations of using acoustic emission technology as a structural health monitoring system. Subsequently, they recommended an automated AE monitoring system capable of detecting fatigue fractures in noisy environments and complex structures, such as aircraft's metallic landing gear components. Starting with the advantages, AE technology could be highly capable of continuously inspecting large structures without limitations on defect size. However, some of the mentioned limitations included difficulties in signal characterisation and location accuracy due to complex geometries and background noise. To understand the complexity of this process, the author presented the image below to show the relationship between the source mechanism and digitised signal. The wave propagation path from the source to the sensor could affect the digitised signal. As presented in Figure 5, the type of material and geometry, the coupling method, the sensor itself, and the acquisition hardware could all impact the final result. Changing any of those factors could change the form of the recorded signal compared to the source signal. In addition, data analysis in AE technology is often complex, requiring considerable operator interpretation and time. A full-scale steel A320 landing gear component was subjected to fatigue loading, with multiple AE sensors carefully placed to monitor it. A fracture initiation was automatically detected after 49,000 fatigue cycles, well before the final failure, as confirmed by a dye penetrant inspection. The fracture's location was determined within 10 mm of its actual position. Infrared Thermography (IRT) [37; 38; 39] is a widely used, non-invasive aircraft inspection technology capable of detecting surface and near-surface defects, including delamination, debonding, corrosion, impact damage, and cracks. It is suitable for metallic and non-metallic materials and requires neither a coupling agent nor direct contact with the test piece, minimising contamination.