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I. Non-destructive testing methods: Eddy current testing technology. This method utilizes the principle of electromagnetic induction; by detecting changes in the induced eddy currents within the material being tested, it is possible to non-destructively assess certain properties of conductive materials and components, as well as to detect defects. Such non-destructive testing methods are referred to as non-destructive testing. In industrial production, eddy current testing is one of the main methods used to control the quality of various metal materials and a few non-metals (such as graphite, carbon fiber composites, etc.) and their products. Compared with other non-destructive testing methods, eddy current testing is easier to automate, and it offers excellent inspection results, especially for profiles such as pipes, bars, and wires. II. Eddy Current Testing: Eddy currents are generated when a conductor is placed in a changing magnetic field; an induced electric field with spiral patterns exists around this changing magnetic field, and this induced electric field acts on the free charges within the conductor, causing those charges to move and thus forming eddy currents. Eddy current testing (abbreviated as ET). Based on Faraday’s law of electromagnetic induction, alternating current is applied to the detection coil, generating an alternating magnetic field perpendicular to the workpiece. When the detection coil is brought close to the workpiece being inspected, eddy currents are induced on the surface of that workpiece, generating a magnetic field in the opposite direction to the original magnetic field. This magnetic field partially counteracts the original one, resulting in changes in the resistance and inductance of the detection coil. If a metal workpiece has defects, it will alter the intensity and distribution of the eddy current field, causing changes in the coil impedance; detecting these changes allows one to determine whether there are defects present. With the advancement of microelectronics and computer technology, as well as the adoption of various signal processing techniques, significant progress has been made in areas such as eddy current detection transducers, eddy current detection signal processing technologies, and eddy current detection instruments. III. Characteristics of eddy current testing 1. Advantages: During testing, the coil does not need to make contact with the workpiece, nor is a coupling medium required, so the testing speed is fast. It has a high sensitivity for detecting defects on or near the surface of workpieces, and exhibits good linear response within a certain range, making it suitable for quality management and control. Inspection can be carried out in high-temperature conditions, in narrow areas of workpieces, and on the walls of deep holes (including pipe walls). It can measure the thickness of metal coatings or non-metallic coatings. Non-metallic materials that can induce eddy currents, such as graphite, can be tested. The detection signal is an electrical signal that can be digitized, facilitating storage, reproduction, as well as data comparison and processing. 2. Disadvantages: The object must be a conductive material, and it is only suitable for detecting defects on metal surfaces. Detection depth and detection sensitivity are contradictory to each other. When performing ET on a material, it is necessary to take into account factors such as the material type, surface condition, and inspection standards before determining the detection scheme and technical parameters. When using through-coil ET, it is not possible to determine the exact position on the circumference where the defect lies. Rotating probe ET can be positioned, but the detection speed is slow. IV. Signal processing techniques for eddy current testing: It is necessary to improve the signal-to-noise ratio and interference resistance of the detection signals, as well as to enable the identification, analysis, and diagnosis of these signals, in order to obtain the best signal characteristics and detection results. 1. Signal feature extraction Common feature extraction methods include the Fourier description method, principal component analysis, and wavelet transform method. The Fourier description method is a common approach for extracting eigenvalues. Its advantage is that it is not affected by the probe speed, and the impedance profile can be reconstructed using this description method; the more sampling points there are, the closer the reconstructed curve is to the original one. However, this method is only sensitive to the shape of the curve, not to the zero point and gain of the eddy current detector, and it remains unchanged regardless of curve rotation, translation, size changes, or the choice of starting point. The method of using the eigenvalues and eigenvectors of the autocorrelation matrix of a test signal to describe the characteristics of the signal is called principal component analysis, and this method has strong resolution for similar defects. Wavelet transform is an advanced method for signal time-frequency analysis. Apply multi-resolution analysis in wavelet transformation to the analysis of eddy current detection signals, process different wavelet coefficients, and then reconstruct. The signal processed by wavelet transformation experiences a significant improvement in signal-to-noise ratio. 2. Signal analysis: Artificial neural networks – The input vectors of artificial neural networks are the characteristic parameters of a signal; the proper selection and extraction of these characteristic parameters is key to the successful use of neural networks for intelligent classification. The composite neural network model employs a hierarchical discrimination method to reduce the dimension of the network’s input variables from N2 to N, thereby significantly simplifying the network structure. This results in fast training times, along with a high defect detection rate and practical value. Neural networks can be used for defect classification, offering high accuracy in recognition, and they are also effective with incomplete or unclear data. Information fusion technology: Information fusion involves multiple stages of processing such as detection, correlation, association, estimation, and integration of data from different information sources, in order to obtain a unified and optimal estimate of the object under consideration. The fusion of eddy current scan images involves decomposing the images into multiple subband images, and then applying a fusion algorithm within the transformation area to combine the images. Ka Bartels et al. combined the eddy current signals using a signal-to-noise ratio optimization method, and applied a spatial frequency compensation technique to blur the high-frequency signals and sharpen the low-frequency signals before combination. Z Liu et al. used the maximum criterion to select the discrete wavelet transform coefficients of different signals, and adopted the maximum absolute value of the coefficients to be fused as the combined transform coefficient. Therefore, the fused signal can be reconstructed based on these coefficients using the inverse wavelet transform. Wavelet transformation can effectively extract significant features at different ratios. During the fusion of signals, all useful features of the signals are preserved, thereby enhancing the information on internal and surface defects. 3. Solution of the eddy current inverse problem: The signals detected by the transducer contain information such as the location, shape, size of defects, as well as the properties of the medium; by using these known signals, it is possible to infer the parameters of the medium (conductivity) or its shape (defects), which constitutes an inverse problem in electromagnetic field theory. To solve the eddy current inverse problem, it is first necessary to establish a mathematical model for defect identification, including models of artificially created defects with regular shapes, naturally occurring defects with complex boundaries, as well as single and multiple defects ； In terms of medium type, there are models such as composite materials and changes in the magnetic permeability of the surface of the component under test. With the development of computer technology, various numerical solutions for defect models have also made progress. Methods such as the finite element method, the moment method, and the boundary element method emerged. V. Development and Current Status of Eddy Current Testing Technology: In 1824, Gabay discovered the existence of eddy currents; in 1831, Faraday identified the phenomenon of electromagnetic induction; in 1873, Maxwell formulated the equations governing electromagnetic fields. In 1879, Hughes applied this technology to determine the properties of different metals and alloys. In 1926, the eddy current thickness gauge was invented; in 1935, the eddy current flaw detector was developed; in 1942, automated testing methods were introduced. In the 1950s, Foster developed the impedance analysis method, leading to further improvements in both theory and practice. In the 1960s, research on this technology began in China, with applications mainly in fields such as aerospace. The EM3300 and MIZ-20 in the United States are typical products that utilize impedance plane display technology, while the TM-128 eddy current detector was China’s first model equipped with a microcomputer and impedance plane display capabilities. The MFE-1 triple-frequency eddy current detector is the first multi-frequency eddy current testing device developed in China. Subsequently, various types of multi-frequency eddy current detectors were developed in China, such as EEC-35, EEC-36, EEC-38, EEC-39, and ET-355, ET-555, ET-556, etc. At present, our country has achieved research results in areas such as finite element numerical simulation, analysis of performance indicators for far-field eddy current probes, and the development of testing systems, and has introduced commercial far-field eddy current inspection instruments. Among them, ET-556H and EEC-39RFT have been used for in-service flaw detection of steel heat exchange tubes in chemical and refining equipment as well as steel tubes in power plant high-pressure heaters. VI. Applications of eddy current testing in high-end fields across various industries 1. Aerospace and aviation: Eddy current testing technology is widely used for inspecting metal components in the aerospace and aviation sectors. To ensure the safe flight of aircraft, regular in-service inspections of relevant components are necessary. Eddy current technology is commonly used to detect surface and subsurface defects such as cracks in aeroengine blades, cracks in bolts and screw holes, the multi-layer structure of aircraft, landing gears, hubs, and beneath aluminum skins, as well as defects in wing joint welds. During testing, it can effectively suppress interference signals caused by probe vibration and uneven material properties. Metal magnetic memory testing technology can be used for the diagnosis of stress concentration areas or early damage in the aforementioned components. 2. In the power and petrochemical industries, eddy current testing technology is used for the in-service and pre-service inspection of non-ferrous and ferrous metal pipes (such as copper pipes, titanium pipes, stainless steel pipes, and boiler tubes) in facilities such as power plants (thermal and nuclear power plants) and oil and chemical plants (oil fields, refineries, chemical plants). It can reliably detect intergranular corrosion, wall thickness reduction, and outer wall wear in pipes, and it effectively eliminates interference signals from the support plates and tube sheets during detection. In addition, the eddy current method is also used to inspect components such as the central hole of turbine shafts, engine blades, oil extraction rods, drilling rods, bolts, and screw holes. Acoustic pulse detection technology can be used for the rapid inspection of various metal or non-metal pipes ； Metal magnetic memory technology is used for the early detection of damage in ferromagnetic components of in-service equipment. 3. Metallurgy and machinery: Eddy current testing technology is used for online and offline flaw detection of various metal tubes, bars, wires, and filaments. During the flaw detection process, it is possible to take into account both long defects such as through-cracks and gradually changing defects, as well as short defects like pores. It can effectively suppress certain interference signals during online and offline inspection of pipelines (such as uneven material composition and vibrations), and exhibits high sensitivity in detecting defects on both the inner and outer walls of metal pipelines. It can also be used for mixing and sorting mechanical parts, evaluating carburization depth and heat treatment conditions, as well as measuring hardness. 4. Nuclear energy and military industries: Eddy current testing technology is used for inspecting metal pipes such as nuclear fuel rods, titanium tubes, and threaded tubes. Used for pre-service and in-service inspection of gun barrels, missile launchers, shell bases, and cartridge cases for military weapons, as well as engine blades, wings, landing gears, and hubs for fighter jets. Metal magnetic memory technology is used for the early diagnosis of metal structural components such as armored vehicles and ships ； Low-frequency electromagnetic fields and magnetic flux leakage technology are used for the quality control of ferromagnetic materials such as decks and oil tanks, as well as welds. Future research and development in eddy current testing technology will include: improving the theory behind transducer design, and developing eddy current testing transducers with better performance ； Eddy current positioning techniques and 3D imaging techniques for studying the size, shape, position, and depth of defects ； Research and promote far-field eddy current testing technology ； Further research on the propagation of fatigue cracks on metal material surfaces, cracking, mechanical machining grinding burns, and residual stress eddy current testing techniques. The use of this technology for non-destructive testing is bound to see widespread application.