Eddy Current Testing System Research Articles

Normalization and Processing of Rotational Eddy Current Scans for Layup Characterization of CFRP Laminates

ABSTRACT The verification of fiber orientation and layup in carbon fiber-reinforced polymer (CFRP) composite laminates is essential to guarantee the performance of the material. This work reports the background, methodology, and results of a rotational eddy current testing (ECT) system for determining fiber orientation and ply layup of unidirectional CFRP components. The system presented mechanically rotates a 15 MHz directional transmit-receive ECT probe, which poses several speed and consistency advantages. The paper presents a theoretical discussion of how these scans are produced, a unique preprocessing normalization method, and an optimization process for determining the layup from the captured datasets. Results show a high level of accuracy for identifying the unique directions present in a laminate, and the theory-supported model enables the extraction of layup order information of multilayer parts, even where repeated lamina are present within the laminate.

Development of Material Sensors Made of Metastable Austenitic Stainless Steel for Load Monitoring

AbstractMetastable stainless steels can be used as a load-sensitive sensor. In combination with an eddy current testing system, mechanical overloads of a component can be detected directly during operation. Material sensors were prepared by shot peening fatigue specimen of metastable austenitic steel to obtain a martensitic surface layer and a local heating by a laser beam to obtain an austenitic area in the layer. In order to investigate the response of the material sensor to overload and achieve different trigger thresholds, the thermal energy applied to create the sensor material and the geometry of the material sensors were varied. It is shown that the austenitized volume and the martensite fraction in the material sensor correlate with the phase of the eddy current signals. Starting from the martensitic surface layer, the phase decreases as the austenitized volume increases. If martensite formation takes place due to an overload, the phase increases as a result. To determine the threshold stress needed to trigger the material sensor, cyclic rotating bending tests were carried out on austenitic stainless steel 1.4301 (AISI 304). In step tests, the bending stress was gradually increased and subsequently ex-situ eddy current testing was carried out. The potential for predicting and classifying an overload is significantly greater with a higher applied thermal energy. Three different sensor geometries (rhombus, cross and ring) were employed in tests. In comparison, the rhombus-shaped material sensor provided the greatest potential for load history interpretation due to the significant phase change.

Numerical simulation and experimental studies of rotational eddy current detection of cracks around rivet holes

Multi-layered aeronautical structures, such as fuselage panels and wings, are composed of several layers of materials such as metal and composites. These structures are prone to damage such as corrosion, cracks and delamination between layers, which can compromise their structural integrity. Eddy current sensors are an essential tool for the inspection and preventive maintenance of multi-layered aeronautical structures, contributing to the safety and reliability of these crucial components. Differential-mode eddy current sensors are particularly well-suited for the inspection of multi-layer aerospace structures around dished-head rivets. The differential measurement improves the detection and characterization of defects in these complex geometries, which are prone to fatigue, corrosion, and other types of damage. The advantages of differential mode are improved sensitivity to small defects, elimination of lift-off variations (distance between the sensor and the surface), reduced electromagnetic interference and detection of defects around rivets, which are critical and difficult to access areas. This work deals with a study of the rotational differential sensor signal according to geometrical parameters of fastener holes defect. It is carried out by an eddy current nondestructive testing system implemented under COMSOL Multiphysics. Indeed, despite its rapidity, the results have shown that the sensor is sensitive to the hole defects when the distances Sensor/Rivet and Lift-off are reduced. As predicted, the experimental results have shown the presence of unsuitable signals caused by the additional Lift-off and the distance Sensor/Rivet. As a solution, these tow parameters should be reduced and kept constant in order to get a better defect signal.

Enhancing corrosion detection in pulsed eddy current testing systems through autoencoder-based unsupervised learning

Pulsed Eddy Current Testing (PECT) stands out as an advanced method in Non-Destructive Testing due to its extensive spectrum characteristics in comparison to traditional ECT techniques, making it exceptionally suitable for identifying corrosion. Nonetheless, the analysis of PECT signals for corrosion detection poses a challenge due to the transient nature of these signals and the impact of sensor lift-off effects. As a result, conventional methods are facing hurdles in dealing with corrosion signals of poor quality. In this study, the challenge is addressed by employing unsupervised learning methods utilizing an autoencoder neural network. This autoencoder integrates Long Short-Term Memory and 1D convolutional layers, acquiring the underlying features of normal PECT signals from non-corrosive regions. Significantly, the model is trained exclusively on this normal data, thereby obviating the necessity for pre-existing corrosion information. Through learning the inherent structure of normal signals, the model can detect anomalies in unseen data, potentially indicating corrosion. The unsupervised framework presents several advantages, such as reducing reliance on prior corrosion knowledge, mitigating inherent noise, and addressing sensor lift-off effects. Experimental results were conducted to compare with traditional methods like the lift-off of intersection and lift-off compensation methods. This approach resulted in a significant improvement in SNR, ranging from 100 % to 200 %, thus facilitating more robust NDT applications employing smart PECT sensors empowered by unsupervised learning techniques.

Development of an energy-autonomous eddy current sensor system for in-situ component monitoring

Energy-autonomous non-destructive testing (NDT) systems provide new opportunities for structural health monitoring. An NDT system that is energy-autonomous and transmits the recorded data wirelessly can be used in places that are not easily accessible for inspection. In addition, it can provide for continuous monitoring, which can improve the operational safety of various components and structures. This study presents a concept that enables energy-autonomous and wireless structural health monitoring using a splined shaft as a demonstrator. To monitor mechanical overloads, a material sensor is applied to critical areas of these components by a laser heat treatment. The structural change that occurs as a result of an overload can be monitored by the developed smart sensor system. This system consists of a material sensor, a low-cost eddy current sensor, an energy-efficient evaluation unit with an ultra-low-power microcontroller unit (MCU) and a Long-Range Wide Area Network (LoRaWAN) data transmission unit. Wireless data transmission via LoRaWAN provides access to the Industrial Internet of Things network, enabling the collection of NDT data from multiple smart sensors in a cloud and the processing of these data to assess the condition of the component from any location. It is shown that the MCU-based eddy current testing system is able to generate a similar signal quality for material characterisation compared to an industrial eddy current testing system, despite a much more compact design and a significantly higher energy efficiency. The energy autonomy of the smart sensor system is achieved by an energy harvesting system integrated into the splined shaft. In addition, three different testing scenarios are presented, which differ mainly in the scope of data processing and evaluation on the MCU, and thus also in the energy required.

Efficient MAPoD via Least Angle Regression based Polynomial Chaos Expansion Metamodel for Eddy Current NDT

In this article, a metamodeling approach based on non-intrusive polynomial chaos expansion (PCE) with least angle regression (LAR) method is used in boundary element analysis for a model-assisted probability of detection (MAPoD) study of eddy current nondestructive testing (NDT) systems. The LAR-PCE metamodel represents the NDT system model responses by a set of coefficients with the polynomial basis functions in lieu of pure kernel degeneration accelerated boundary element method (KD-BEM) based physical model. Both the computational accuracy and efficiency of the LAR-PCE metamodel over the ordinary least squares (OLS) based PCE metamodel are demonstrated by testing the 3D eddy current NDT benchmarks with different system setups, flaw lengths and widths. The simulation results show two digits accuracy of the PoD metrics compared with the ones achieved by the KD-BEM based physical model as the benchmark. The LAR-PCE metamodel has remarkable improvements in computational efficiency over the OLS-PCE metamodel and accelerates the MAPoD study.

Qualification of Austenitic Stainless Steels for the Development of Load-Sensitive Material Sensors

To detect mechanical overloads on the component directly in operation, a metastable material can be used as a load-sensitive sensor when combined with an eddy current testing system. In order to find a suitable metastable sensor material that exhibits microstructural changes at an early stage before fatigue failure, quasi-static tensile tests and cyclic rotating bending tests were carried out with the austenitic stainless steels 1.4301 (2 batches), 1.4305, 1.4541 and 1.4550. For the detection of microstructural changes, electromagnetic testing was used in-situ in the tensile test and ex-situ between the rotating bending test after a pre-defined number of cycles. The investigated materials 1.4301 batch2 and 1.4550 showed the largest signal changes and the lowest austenite stability both in the tensile test and under cyclic bending load. Due to the better mechanical properties, 1.4301 batch2 should be preferred. The order of the austenitic stainless steels tested was similar in terms of transformation behavior in both tests. Thus, the tensile test combined with in-situ electromagnetic testing appears to have potential as a suitable benchmark test for austenite stability. With regard to the cyclic bending stress, an overload of the specimens could be detected for the materials 1.4301 batch2, 1.4305, 1.4541 and for the 1.4550 on the basis of a significant amplitude change. At low bending stresses, uncritical for structural integrity, no increase in amplitude was measured. The results have shown that an early detection of overloads is possible with several materials, however, the potential for detecting overloads varies between materials and also between individual batches. In addition, it has been observed that as the bending stress increases, the gradient of the change in amplitude over the number of cycles increases as well. Thus, with a known number of cycles, it could be possible to classify the previous load spectrum based on the difference in amplitude between two measurements.

APPLICATION OF THE RED PITAYA PLATFORM IN THE TECHNOLOGY OF PROTOTYPING AUTOMATED EDDY CURRENT TESTING SYSTEMS

Automated eddy current non-destructive testing has an important role in the quality inspection system for products made of conductive materials both at the production stage and during their operation. The appearance of novel alloys, the complexity of product geometry and inspection conditions, and the increased requirements for the reliability and accuracy of inspection results stimulate the acceleration process of creating new eddy current inspection tools. The technology of prototyping makes it possible to solve this problem. The purpose of the article is to review the functionality and technical characteristics of Red Pitaya for use in the technology of prototyping eddy current inspection, as well as to create a prototype architecture of an automated eddy current inspection system based on it.The choice of the Red Pitaya platform as a convenient tool for prototyping automated systems was made due to its functionality, reliability, and availability. In general, the article highlights the advantages and prospects of using the Red Pitaya platform in the prototyping of automated eddy current non-destructive testing systems. The aspects of the Red Pitaya hardware and its capabilities for expanding the functionality, which allows to create highly efficient and flexible control systems, are considered in detail. The compatibility of Red Pitaya with other devices and accessories is considered, which significantly increases its versatility and adaptability to various requirements and applications.The article highlights the importance of using the Jupyter Notebook application in combination with Red Pitaya as a convenient tool for software development. This helps to facilitate the process of creating and programmatically implementing complex signal processing algorithms in eddy current control systems, and provides flexibility in creating new software products.Modern trends and prospects for the development of eddy current testing, market needs, and the ability to adapt to different application scenarios are taken into account in the proposed architecture of the prototype automated eddy current testing system.

Detection of Surface and Back-Surface Defects on Metal Plate via Rectangular Wave Eddy Current Testing Using Magnetoresistive Sensor

Eddy current testing (ECT) at different frequencies can identify surface and back-surface defects caused by the skin effect. Traditional ECT uses a pickup coil to measure the magnetic-field signal caused by eddy current. However, according to Faraday’s law of electromagnetic induction, the voltage signal detected at low frequencies is weak; thus, the signal is susceptible to interference from noise. By contrast, magnetoresistive (MR) sensors have high sensitivity at low frequencies. Therefore, we developed a rectangular wave eddy current testing system using an MR sensor. The system uses rectangular wave excitation, which can generate both low- and high-frequency signals, and the MR sensor detects the magnetic-field components. A 12-mm-thick aluminum plate with an artificial circular defect 10 mm in diameter and 4 mm in height was used as a specimen. The measurement results indicated that the developed system can accurately and quickly distinguish between surface and back-surface defects.

Directly-digitized pulsed eddy current based quantification of aluminum sheet thickness

Direct interfacing technique (DIT) eradicates additional circuit requirements for sensor-embedded system interface and digitization of analog signals. This technique provides advantages in designing an efficient, portable, and low-cost sensor system. Pulsed eddy current testing (PECT) systems are used for thickness and defect measurements of conductive materials. Circuitous sensor-interfacing methods and tedious data interpretation processes make PECT systems inapt for miniaturization and portable applications. In this work, DIT is used in conjunction with PECT for thickness estimation of conductive material. Change in the de-energizing time of a single coil probe with sample thickness, with respect to air, is used as a signal. The curve fitting method yields a maximum relative error of ≈2% in the thickness estimation. Effects of temperature and liftoff on system accuracy are also investigated. A liftoff compensation method using a 3-signal data group is proposed. It is shown that for thicknesses in the range of 0.508 mm–3.175 mm and liftoffs up to 3.000 mm (step size: 0.500 mm), the proposed scheme produces a maximum relative error of 5.2%. The DIT and PECT combination can be applied for different structural eddy current testing in the future.

Identification of overloads on splined shafts by means of eddy current testing technology

For the development of resource-efficient machines, the availability of high-quality information about the actual loads that occur is a key prerequisite. By continuously monitoring the loads on the component, critical events can be detected, and thus component failure prevented. Future product generations can also be designed in a resource-saving way depending on the loads that actually occur. The information is most useful when it is obtained directly from the highly loaded areas of the component. In this study, a concept is presented that enables in-situ detection of mechanical overloads on splined shafts by eddy current techniques. Splined shaft connections are among the most heavily stressed machine elements in the drive train and are usually located centrally in the powertrain. To monitor mechanical overloads, a material-integrated sensor will be developed. The structural change that takes place in the sensor as a result of overloads can be monitored with the help of eddy current testing technology. To ensure permanent monitoring of the sensor area, a compact eddy current testing system has to be integrated into the component. This consists of a printed circuit board coil, an evaluation unit, a data transmission module and an energy-harvesting module. The measurement principle for the stainless steels employed is based on the microstructural transformation from metastable austenite to martensite when the material is stressed beyond a threshold value. The threshold at which the structural transformation occurs can individually be adjusted in the sensor area by a local laser heat treatment.

Theoretical and Numerical Study of Eddy Current Pulsed Thermography to Detect Damage of Deep-Sea Manned Pressure Hull

At present, research on pressure hull safety is mainly focused on the constitutive model of material properties and the evaluation model of structural parameters aiming at fatigue life prediction. The damage identification and quantitative evaluation methods of pressure hulls have not been studied. In this study, an eddy current thermal imaging method is introduced to detect micro-cracks in a deep-sea spherical pressure hull. In the detection method, temperature is used as a parameter to identify and quantify cracks. The temperature distribution around the cracks is studied using theoretical analysis and finite element simulation. A theoretical model is established using electromagnetic theory and heat transfer theory. Moreover, the temperature difference between the cracked area and the non-cracked area can be obtained by solving the heat conduction equation. A pulsed eddy current thermal imaging testing system is established, and a defective titanium alloy specimen is tested. At the same time, the temperature around the cracks in the specimens is simulated. The specimens have the same material and welding as a deep-sea spherical pressure hull. This paper discusses the possibility of its use in a pressure hull, which will provide a reference for micro-crack damage identification and quantitative evaluation of a deep-sea spherical pressure hull.

Inspection of aircraft parts by eddy current method

Aviation technologies are developing due to the need for advancements at military field and the increase in commercial air transportation. The development of aviation technology also forces the development of Non-Destructive Testing (NDT) Standards. The accuracy of these inspections, their fast results and their applicability on many materials is one of the important issues to be studied. Eddy current testing, which has a significant advantage in terms of test speed and accuracy, is one of the main methods used in the non-destructive testing of aircraft parts. The test is used not only in metal and metal alloy materials, but also in composite materials with high conductivity such as carbon fiber to detect failures. Also, improvements at the eddy current test system and probes play a major role in the failure assessment of aircraft parts. In this article, comprehensive technical information about the eddy current testing method is given and the case studies are presented.

Investigation of rolling contact fatigue cracks using the transmitter-receiver eddy current testing under moving conditions

ABSTRACT Rolling contact fatigue (RCF) cracks online detection using eddy current testing (ECT) is in urgent demand. However, RCF cracks detection and evaluation in this way under moving conditions remains challenging due to the velocity effect in ECT. This paper aims to study the response of a transmitter-receiver eddy current probe to cracks under moving conditions and evaluate the depth and inclination angle of RCF cracks. In this paper, a high-speed eddy current testing system is developed to experimentally investigate the influence of coil gap, detection speed, and the lift-off on the eddy current probe’s response under moving conditions. In addition, the temporal and amplitude features of the eddy current signal are extracted to characterise the depth and inclination angle of the RCF cracks. The experimental results indicate the eddy current probe’s response can be improved by increasing the coil gap (coil centre distance) suitably, which can be done to compensate for the attenuation of the eddy current signal caused by detection speed and lift-off. The probe’s response hardly changes with an increase in detection speed when the driver and pick-up coils of the eddy current probe completely overlap. The crack depth and inclination angle can be evaluated under moving conditions.

Inductance-to-digital converters (LDC) based integrative multi-parameter eddy current testing sensors for NDT&E

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.

Non-Destructive Evaluation of In-Plane Waviness in Carbon Fiber Laminates Using Eddy Current Testing

Non-destructive detection of the in-plane waviness of carbon fiber-reinforced plastic (CFRP) laminates is of interest in a wide variety of industries, as wrinkles and other fiber alignment defects significantly impact the mechanical performance of the composites. This work demonstrates a method to detect in-plane wrinkles on a 5-ply unidirectional CFRP laminate with a customized eddy current testing (ECT) system. The results show that the ECT system is effective in detecting and quantifying in-plane waviness, and the results are compared to conventional X-ray computed tomography (CT) and ultrasonic testing (UT) methods. Using the anisotropic conductive nature of the aligned CFRP lamina, the ECT system was able to clearly detect throughout the part changes in the local fiber orientation, wave tangent angle, and wrinkle width.

Nondestructive Examination of Carbon Fiber-Reinforced Composites Using the Eddy Current Method.

This paper presents the results of experiments using the eddy current system designated for nondestructive inspection of carbon fiber-reinforced composites. For this purpose, the eddy current testing system with a differential transducer with two pairs of excitation coils oriented perpendicularly and a central pick-up coil was utilized. The transducer measures the magnetic flux difference flowing through the pick-up coil. The transducer of this design has already been successfully utilized to inspect isotropic metal structures. However, the anisotropy of the composites and their lower conductivity compared to metal components made the transducer parameters adjustment essential. Thus, various excitation frequencies were considered and investigated. The system was evaluated using a sample made of orthogonally woven carbon fiber-reinforced composites with two artificial flaws (the notches with a maximum relative depth of 30% and 70%, respectively, thickness of 0.4 mm, and a length of 5 mm). The main goal was to find a configuration suitable for detecting hidden flaws in such materials.

Classification of Corrosion Flaws on Front and/or Back Surfaces of Non-Magnetic Metal Plates Using a Rectangular Wave Eddy Current Testing System

If a rectangular wave excitation current is used for eddy current testing (ECT) instead of a sinusoidal wave, multifrequency testing can be performed directly because rectangular waves contain a fundamental wave as well as harmonic waves. Furthermore, considering the skin effect, it is possible to determine whether the specimen has a corrosion flaw on the front surface, back surface, or flaws on both surfaces. In this study, we first evaluate a method to classify these flaws on non-magnetic metal plates using rectangular wave ECT (RECT) systems with a linear amplifier. Here, we present an indicator in which each harmonic amplitude is divided by its fundamental amplitude. The results indicate that the amount of attenuation of the indicator in relation to the frequency depends on the type of flaw; therefore, we can classify the type of flaw using the indicator. Subsequently, we developed a RECT system with an inverter and conducted the same experiment. The results indicate that the tendency is the same as that when using the RECT system with a linear amplifier, which contributes to high efficiency, low cost, and weight reduction.

A Digital Lock‐in Amplifier Based on Adaptive Kalman Filter for Rail Defect Detection

In this paper, an eddy current testing system equipped with low‐performance processor is designed for rail defect detection. A digital lock‐in amplifier (DLIA) based on adaptive Kalman filter (AKF) is presented to detect the weak voltage induced in two differential coils. The proposed method can fast demodulate the amplitude of a signal with a randomized phase using one cycle of the sinusoidal signal, and simultaneously improve signal‐to‐noise ratio. This DLIA has made use of the sinusoidal orthogonality and concurrently utilized AKF to track on the amplitude variation of voltage, which is numerically simulated and analyzed in MATLAB. The simulated results indicate that increasing excitation frequency or sampling points in one cycle has an effective suppression in low‐frequency noise. Furthermore, the experiment is carried out to verify the performance of the system for rail defect detection. Finally, the experimental result shows that the proposed lock‐in amplifier has a root mean squared error of 1.3 mV and performs well in terms of memory consumption and precision. Moreover, the amplitude variation in differential coils is linearly tracked, and the surface defects on rail specimens can be detected.

Design of pulsed eddy current testing system for CFRP material damage

Design of pulsed eddy current testing system for CFRP material damage