Eddy Current Testing Basics and Innovation

Eddy Current Array Inspection of Zlin 142 Fuselage Riveted Joints

This paper describes the use of an eddy current testing (ECT) method for the inspection and detection of impact damage in carbon fiber-reinforced composites (CFRP). ECT method is a nondestructive testing (NDT) method where electric induction is used. This method is widely used for detecting cracks and corrosion in metals, or checking their electric conductivity. Because carbon fiber in CFRP has electric conductivity, ECT method has a potential to inspect the defects. However, electric conductivity of CFRP is much smaller than that of metals. Moreover, from the view of the eddy current probe, CFRP to be checked looks as a inhomogeneous conductive materials where conductive fibers are bundled and laid up, and this is completely different situation comparing with metal samples that are homogeneous. Therefore, there are several problems to be solved for applying ECT method to CFRP such as proper selection of test frequency, shape of probe, or signal processing.

Sensors’ signal conditioning and digital interface are important for sensor technologies, digital transformation, and the Internet of Things (IoT) in particular. Frequency output sensors and inductance-to-digital converters (LDC) offer distinct advantages over Analog-to-Digital Converter (ADC) interfaces. To miniaturize the eddy current testing system and achieve multi-parameter measurements, this paper reviews eddy current signal conditioning and digital interfaces in the field of displacement sensors and non-destructive tests & evaluation (NDT&E). After review and comparison of different signal conditioning and digital interfaces, the LDC chip is used in eddy current testing for multiple parameter measurements including lift-off distance and defect detection. An LDC-based integrative eddy current non-destructive testing method is proposed. The LDC chip provides AC to drive the LC resonator composed of the sensor coil and the parallel capacitor. By measuring the resonance frequency and parallel resistance (Rp) of the LC resonator, the simultaneous measurement of the lift-off distance and the defect is achieved. Based on the method, the testing experiments of two kinds of specimens made of aluminium and steel were carried out under different lift-offs. Experimental results show that both parameters of frequency and Rp of the LC resonator can intuitively reflect the defect depth and lift-off. Frequency- Rp curves can be used to quantify defect depth and lift-off distance and differentiate the material. This work also illustrates an integrated approach of signal conditioning and digital interface for multiple parameters or feature measurement for integrated eddy current NDT&E. The work opens a new way for addressing multiple parameter measurements of eddy current NDT & E including lift-off estimation and defect characterization via digital interfaces. It has the potential to bridge the gaps in metrology of geometry, displacement measurement, and NDT&E for defect detection and material characterization.

Steam generator tube in nuclear power plant is a boundary between primary side and secondary side. Nondestructive test for the steam generator tube, eddy current testing (ECT) method has been carried out. In case of bobbin-type ECT probe, transverse defect, defect with ferro-phase, outside of tube having ferrite sludge were very difficult to detect. To overcome these problems, a motorized rotating probe coil (MRPC) probe was developed but scan speed is slow and not effective method in time. In this work we have developed a new sensor having U-shape of yoke to measure permeability variation of test specimen, which can detect ferro-phase generated as well as normal defects in the tube material of Inconel 6oo. Electronics for signal processing, 2-phase lock-in amplifier per sensor, ADC, and embedded controller were employed in one probe and measured digital data were transmitted to the PC data acquisition software using RS232C interface. Using the developed sensors we have applied to detect defects in the tube outside with ferrite sludge which is very difficult to detect conventional bobbin-type ECT probe. Developed sensor could measure defect size of 0.2 mm times 10 mm times 0.44 mm (width times length times depth) for the normal defect and defect outside of tube having ferrite sludge. We expect the developed sensor could detect defects like as MRPC probe and scan sensor speed like as bobbin-type ECT probe.

Duplex stainless steel (DSS) presents a good combination of mechanical properties and high corrosion resistance. Its microstructure is composed of two phases with different physical properties: delta ferrite and gamma austenite. The best properties of DSS for in service applications are achieved when these phases are in equal proportions. However, exposition to high temperatures, may entail undesired consequences impairing the mechanical and corrosion properties of the material. Eddy current testing (ECT) has already been used for DSS inspection and microstructure characterization, mainly to detect the presence of deleterious phases, such as sigma. However, conventional ECT was not able to quantify different amounts of austenite and ferrite in the absence of deleterious phases. The present study focuses on the experimental and computational problem of DSS inspection using ECT in saturated magnetic conditions in order to identify different amounts of austenite and ferrite within the DSS microstructure. Samples containing ferrite to austenite ratio equal to 50/50, 70/30 and 80/20 respectively were brought to saturation as designed in the simulations and the ECT was applied presenting a very good result. Saturated low frequency ECT condition proved to be a reliable methodology in quantifying delta ferrite and gamma austenite phases in DSS.

A large number of carbon fiber reinforced polymers have been applied to aircraft and automobiles, and many nondestructive testing methods have been studied to detect their defects. Eddy current magneto-optical imaging nondestructive testing technology has been widely used in the detection of metal materials such as aircraft skin, but it usually requires a large excitation current and, at present, can only detect metal materials with high conductivity. In order to take full advantage of the innate benefits and efficiency of eddy current magneto-optic imaging and enable it to detect defects in carbon fiber reinforced polymers with weak conductivity, it is necessary to improve the magnetic field response of the eddy current magneto-optic imaging system and explore suitable excitation and detection methods. The scanning eddy current magneto-optical imaging nondestructive testing device built in this study has improved the magnetic field response of the system, and the eddy current magneto-optical phase imaging testing method has been proposed to detect the crack defects of carbon fiber reinforced polymers. The effectiveness of the method has been verified by simulation and experiment.

Nuclear steam generators (SGs) are a critical component for ensuring safe and efficient operation of a reactor. Life management strategies are implemented in which SG tubes are regularly inspected by conventional eddy current testing (ECT) and ultrasonic testing (UT) technologies to size flaws, and safe operating life of SGs is predicted based on growth models. ECT, the more commonly used technique, due to the rapidity with which full SG tube wall inspection can be performed, is challenged when inspecting ferromagnetic support structure materials in the presence of magnetite sludge and multiple overlapping degradation modes. In this work, an emerging inspection method, pulsed eddy current (PEC), is being investigated to address some of these particular inspection conditions. Time-domain signals were collected by an 8 coil array PEC probe in which ferromagnetic drilled support hole diameter, depth of rectangular tube frets and 2D tube off-centering were varied. Data sets were analyzed with a modified principal components analysis (MPCA) to extract dominant signal features. Multiple linear regression models were applied to MPCA scores to size hole diameter as well as size rectangular outer diameter tube frets. Models were improved through exploratory factor analysis, which was applied to MPCA scores to refine selection for regression models inputs by removing nonessential information.

Non-destructive eddy current testing (ECT) is widely used to examine structural defects in ferromagnetic pipe in the oil and gas industry. Implementation of giant magnetoresistance (GMR) sensors as magnetic field sensors to detect the changes of magnetic field continuity have increased the sensitivity of eddy current techniques in detecting the material defect profile. However, not many researchers have described in detail the structure and issues of GMR sensors and their application in eddy current techniques for nondestructive testing. This paper will describe the implementation of GMR sensors in non-destructive testing eddy current testing. The first part of this paper will describe the structure and principles of GMR sensors. The second part outlines the principles and types of eddy current testing probe that have been studied and developed by previous researchers. The influence of various parameters on the GMR measurement and a factor affecting in eddy current testing will be described in detail in the third part of this paper. Finally, this paper will discuss the limitations of coil probe and compensation techniques that researchers have applied in eddy current testing probes. A comprehensive review of previous studies on the application of GMR sensors in non-destructive eddy current testing also be given at the end of this paper.

Eddy current testing is an important and widely used method in non-destructive testing and evaluation. In this paper, an impedance analysis model of eddy current testing based on mutual inductance equivalent circuit is established using transmission line theory, and an analytical expression of impedance of the testing workpiece is obtained. Then the changes of impedance plane of ECT sensor affected by such as the thickness, the conductivity of the tested workpiece and the lift-off are studied. Experiment results are consistent with the theoretical analysis, which proves the validity of the model. All the conclusions derivated by the model can be helpful for eddy current testing optimization. This electronic document is a “live” template. The various components of your paper [title, text, heads, etc.] are already defined on the style sheet, as illustrated by the portions given in this document.

Eddy current and Barkhausen noise nondestructive testing techniques were considered to evaluate the magnetic properties of the decarburised steels as a function of microstructure. To make changes in decarburising depth, carbon steel samples were austenitised at 890 °C for 120–270 min. Considering different decarburised depths, height, position and width of the noise profiles were extracted in order to analyse the magnetic Barkhausen noise measurements. Next, the eddy current test was performed to detect the changes in the microstructure through decarburising of the steel taking into account the impedance variations. According to the results, both techniques allow us to detect changes in the magnetic properties of the decarburised steels and link them with their microstructural changes, nondestructively.

A scheme to apply signals of pulsed eddy current testing (PECT) to reconstruct a deep stress corrosion crack (SCC) is proposed on the basis of a multi-layer and multi-frequency reconstruction strategy. First, a numerical method is introduced to extract conventional eddy current testing (ECT) signals of different frequencies from the PECT responses at different scanning points, which are necessary for multi-frequency ECT inversion. Second, the conventional fast forward solver for ECT signal simulation is upgraded to calculate the single-frequency pickup signal of a magnetic field by introducing a strategy that employs a tiny search coil. Using the multiple-frequency ECT signals and the upgraded fast signal simulator, we reconstructed the shape profiles and conductivity of an SCC at different depths layer-by-layer with a hybrid inversion scheme of the conjugate gradient and particle swarm optimisation. Several modelled SCCs of rectangular or stepwise shape in an SUS304 plate are reconstructed from simulated PECT signals with artificial noise. The reconstruction results show better precision in crack depth than the conventional ECT inversion method, which demonstrates the validity and efficiency of the proposed PECT inversion scheme.

Metal surfaces on industrial equipment are susceptible to fracture when subjected to extreme pressure and speed conditions. The safety of the parts is at risk and their service life is decreased by these fractures. One of the primary difficulties in developing eddy current non-destructive testing (NDT) sensors is locating fractures in any direction. The limitation is that fractures that are not perpendicular to the current flows produced cannot be detected by eddy current-based approaches. To locate arbitrary orientation fractures in conductive non-ferrous materials, this work explores the use of a rotating electromagnetic field technique and provides a novel rotating uniform eddy current (RUEC) system. This novel approach has greatly improved the detection of fractures in any orientation. The impedance signature of the fracture is investigated using a three-dimensional finite element (FE) simulation model of the new RUEC system. The rotating alternating current field measurement (RACFM) approach, which was designed to detect artificial cracks at any angle in underwater structures, is used in[1] to validate the model proposed in this study. The efficacy of the RUEC system and its potential in the detection of different fracture orientations in the aluminium specimen are demonstrated by the simulation results.

The battery tabs are critical components connecting the battery to the circuit, and their welding quality directly influences the safety of electrical usage in batteries. Ultrasonic welding often induces thermoplastic deformation in the multilayer copper foil structure of the battery tabs, resulting in varying effective solid-phase bonding depths at the welding interface. However, conventional single-frequency eddy current testing (ECT) technique faces significant challenges in effectively characterising variations in welding quality. To address this issue, a Multi-Frequency Eddy Current ResNet (MFEC-ResNet) model is proposed. Firstly, the equivalent impedance variations at multiple frequencies are used as inputs to capture the complex relationships between multi-frequency features and welding quality. Secondly, multi-class focal loss is employed to tackle the class imbalance issue, enhancing the model’s ability to detect minority classes. Thirdly, Shapley Additive Explanations (SHAP) are used to interpret the contributions of individual features in the black-box model, providing insights into the model’s decision-making process. Experimental results show that the proposed multi-frequency ECT method achieves an accuracy of 95%, outperforming single-frequency ECT by nearly 10%.

Nondestructive Testing (NDT) methods provide scientists a way to understand the properties of samples without causing them damage. For an electrochemical device, it is crucial to investigate the behavior of it before, during, and after its operation. Therefore, NDT methods are indispensable for studying such devices.Literature highlights the effectiveness of elastic-wave-based testing methods in the evaluation of electrochemical devices. The Acoustic Emission (AE) method was applied to respectively study the behavior of Solid Oxide Fuel Cells (SOFCs) and Ni-MH batteries, while the Ultrasonic Testing (UT) method was applied to study Li-ion batteries. In our previous research, the signals representing cracking and delamination in SOFCs were successfully distinguished by their peak amplitude, wave shape, and AE energy [1]. The same technique was further used for the evaluation of mechanical damages in SOFCs during the operation [2]. In another study, similarly, the pulverization of the Ni-MH battery electrodes was observed by the AE technique, and frequency peak property was included in the discussion to classify the signals [3]. The UT method was employed to study the relationship between the state of charge of Li-ion batteries and the profiles of ultrasonic waves at different frequencies. The evolution of wave velocity, attenuation, and amplitude were discussed [4]. Based on these researches, elastic-wave-based testing methods are adopted by us.In our present study, the multi-frequencies in-situ ultrasonic monitoring technique was employed to analyze the behavior of the commercial Ni-MH batteries during the operation. Continuous ultrasonic waves at three different frequencies were separately sent into the specimens during charge cycles, and their waveforms were recorded and compared. Specific wave patterns were found near the end of the charge and discharge state, especially from the wave profiles at 750 KHz. The result showed that the evolution of waveform is frequency-dependent, which was also suggested in the literature [4]. Potential explanations for these wave patterns were inferred, including phenomena such as crystal structure evolution, internal pressure shift, and reverse of electrodes.Based on this finding, a new ultrasonic testing/monitoring method considering frequency analysis is also designed. In this new method, signals consisting of elastic waves at a wide range of frequencies with equal intensities are designed to be applied to various electrochemical devices. The application of this method will be included, while the frequency-dependent property of the testing of electrochemical devices will be assessed in combination with the present findings in this study.

NON-DESTRUCTIVE TESTING (NDT) utilizes the physical properties of a material for detection, location and evaluation of flaws/defects and judging of the microstructure and mechanical properties without damaging the functionality of component being inspected. Flaws or imperfections that are detected by inspection of the component may be obtained by several reasons. It may be due to originality of the material caused by the process used created by some human error or defects initiated during the operating life of components etc. NDT method ranges from the simple to the tangled. NDT is also used for condition monitoring of various components during operation to forecast and assess the rest of the life of components while retaining its structural integrity. Based on availability, accessibility, suitability, analysis and experience the choice of specific method varies. The end result is the saving of time, money and development in precision and reliability of results obtained. There are a vast number of techniques in NDT. Visual inspection testing, liquid penetrate testing, ultrasonic testing, magnetic particle testing, eddy current testing, thermal infrared testing, acoustic emission, leak testing, thermography, instinct metallographic examination. The industrial applications of NDT are mostly found in railways, nuclear, non – nuclear and chemical industries, aircraft and petroleum projects, coal mining industry. This paper presents the knowledge for selection of specific NDT detecting surface flaws and requisite applications. It presents vital aspects in each method along with their advantages and disadvantages.