Eddy Current Testing System Research Articles

In-situ monitoring of powder bed fusion of metals using eddy current testing

Powder bed fusion of metals (PBF-LB/M) is the most commonly used additive manufacturing process for the layerwise production of metal parts. Although the technology has developed rapidly in recent years, manufactured parts still lack consistent quality primarily owing to process-inherent variability, and the lack of effective sensing technologies enabling the ability to control the process during part production. Thus, there are high costs caused by rigorous post-process part inspection steps required to provide compliant part certificates. In contrast to typically deployed in-situ sensing technologies, eddy current testing (ECT) is a standardized nondestructive testing (NDT) technique able to provide compliant part certificates during post-process inspection according to existing standards. This study investigates the potential of ECT as an in-situ process monitoring technology for PBF-LB/M. Parts made from AlSi10Mg were manufactured on a PBF-LB/M machine using different process parameters yielding different relative densities ranging from 99%– 99.7%. During the build cycle, the parts were measured layer-by-layer with an ECT system mounted on the machine recoater. Signal analysis methods were developed which effectively separate and calibrate the electrical conductivity component (relative electrical conductivity) and the distance component (lift-off) of the ECT signals. The relative electrical conductivity was then compared to X-ray micro-computed tomography ( μ CT) measurement data demonstrating that layer-to-layer differences in relative density of about 0.1% can be successfully detected via ECT. In addition, the lift-off was used to monitor the thickness of the consolidated layers and the layer-to-layer part height. The results show that ECT is an effective technology for in-situ monitoring of the relative part density paving the way for deploying ECT for in-situ NDT of PBF-LB/M-manufactured parts.

An autonomous robot for shell and tube heat exchanger inspection

AbstractShell and tube heat exchangers (STHEs) are critical to energy conversion efficiency of power plants. Eddy current examination is a way to evaluate working conditions of these tubes. However, the current testing apparatus requires human to manually insert an eddy current testing (ECT) probe into and extract it out of individual tubes, and meanwhile monitor measurement results for diagnosis. It is a time‐consuming and labor‐intensive procedure even for an experienced technician. To tackle this challenge, in this study, we developed a robot enabled ECT system for autonomous inspection of STHEs. The robotic platform employs Mecanum wheeled chassis for high mobility, machine vision to locate tube bundle and tube inlets, a rotational Cartesian mechanism to operate at planes with all possible inclinations, and a task‐specific mechanism for ECT probe delivery. Machine vision locates tube bundle and tube inlets by an April tag detection algorithm and a Circle Hough Transform algorithm, respectively. Assisted by a guiding cone, the ECT probe is continuously fed into the tubes with a fill factor of 0.819. During this process, the eddy current data are automatically collected and real‐time analyzed by convolutional neural networks, showing accuracy of nearly 100% for identifying defective and nondefective tubes and 85% for four types of defective tubes and nondefective tubes.

A self-frequency-conversion eddy current testing method

Eddy current testing (ECT) technology is currently one of the effective methods for non-destructive testing of aviation materials. In the application of ECT technology, a lower operating frequency is generally used for aviation metal materials, while carbon fiber reinforced plastics (CFRP) often require higher operating frequencies due to the low conductivity characteristic. However, multiple materials are often used simultaneously in aircraft manufacturing, including metallic materials and CFRP. Therefore, due to the difference in materials, the traditional ECT technology needs to be carried out in the ultra-large frequency range (1 kHz to 100 MHz) during the ECT inspection process of the aircraft, which will bring great difficulties to instrument development and actual operation. Concerning this issue, a self-frequency-conversion ECT method is proposed in this paper, and the corresponding ECT system was developed. The proposed method has two detection signals simultaneously, the current signal and the frequency signal, which are used to characterize the defects of CFRP and metal materials, respectively. Experimental studies on CFRP and aluminum alloys were carried out using the proposed method. The experimental results show that the proposed method can realize the defect detection of metal materials and CFRP in the conventional frequency range of 1 kHz to 1 MHz. Compared with the traditional ECT technique, the proposed method greatly reduces the operating frequency range and can provide better detection sensitivity, which is an ECT technology with great engineering application value.

Inspecting Power Network Analysis on Eddy Current Test of SMAW Joints

Eddy Current Test (ECT) is proven to be the most reliable non-destructive assessment for electrically conductive materials. This assessment works based on the emerging Eddy Current when an alternating current is induced into conductive materials. The ECT system consists of three elements, i.e. current injection, the pick-up, and the conductive target. This system is then treated as an altered wireless power network between aforementioned elements. The use of alternating current (A/C) in this network provides a wide variety of power spectra for various analysis techniques. This paper introduces novel technique of subsurface crack assessment on SMAW welded joint of stainless steel 316 L by applying network altered response to ECT system. The analysis of the network is done by constructing mathematical models of the frequency domain in each component in the network to achieve the gain of the system. The mathematical model was constructed either for the defectless and the defected speciments, and then both result was compared to investigate and extract the information about the crack. The experiment also performed to confirm the simulation from the modeling. The experiment succeeded to differentiate the size of the subsurface crack on certain working frequencies.

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

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

Research of oscillation mode in automated pulsed eddy current testing systems

The formation and analysis of eddy current probe signals obtained in pulsed excitation mode is considered. The proposed method of implementing pulsed eddy current testing with the formation of attenuating harmonic oscillations is more resistant to the effects of noise and interference that accompany the process of inspected object parameters evaluation. The equivalent scheme of the system “test object–eddy current probe” is developed and analyzed. The obtained mathematical model of the eddy current probe signals allowed proposing the natural frequency and the attenuation as informative signals parameters, which are determined from signals phase and amplitude characteristics. Developed algorithm and the proposed methodology was implemented for evaluation of eddy current signals parameters and related characteristics of testing objects. This method was experimentally verified on a series of different test specimens. The obtained results confirm the possibility to apply the proposed informative signals to solve some problems concerned with automated eddy current testing. The formation and analysis of eddy current probe signals obtained in pulsed excitation mode are considered. The proposed method of implementing pulsed eddy current testing with the formation of attenuating harmonic oscillations is more resistant to the effects of noise and interference that accompany the process of automated eddy current testing. The equivalent scheme of the system “test object–eddy current probe” is developed and analyzed. The obtained mathematical model of the eddy current probe signals allows proposing the natural frequency and the attenuation as informative signals parameters, which are determined from signal phase and amplitude characteristics. Methods of increasing the accuracy of determining the eddy current probe signals attenuation and frequency using trends of signals phase and amplitude characteristics are considered. The proposed signal processing method was verified by modeling the process of determining the eddy current probe signals attenuation and the frequency from the signal with Gaussian noise. Algorithmic and software were developed based on the simulation results and the proposed improved methodology was implemented for determining signals parameters and related parameters and characteristics of testing objects.

Flexible ECT probe with front-end differential setting for inspection of curved CFRP structures

Carbon-fiber-reinforced plastic (CFRP) structures with curved surfaces are widely used. However, an efficient tool for curved CFRP inspection is still lacking. This paper proposes a flexible eddy current testing (ECT) probe with front-end differential setting designed specifically for curved CFRP structures inspection. The probe contains a pair of coils that are fabricated on a flexible printed circuit board . It can be bent to fit a curved surface without the need to redesign the probe. The two coils of the probe work simultaneously and highlight a defect's signal by the front-end differential configuration. It effectively suppresses common mode interference , which is generally evitable in the experiment of CFRP structures inspection using a high frequency ECT system. A finite element method (FEM) model is established to study the inspection principle in viewing of the anisotropic electrical conductivity of CFRP. A prototype probe is fabricated and tested, with which a flat and three curved CFRP samples with machined and natural impact cracks are imaged. A signal processing algorithm including detrending, gradient calculation and nonlinear transformation is developed to improve the probe's defect detectability . Experiment results show that a small defect located on the curved surface of a CFRP sample with dimensions: length 2 mm, width 1 mm and depth 0.4 mm can be detected by the probe. In addition, the effects of the operating frequency and orientation of the defect are discussed.

Detecting the defects of warm-sprayed Ti-6Al-4V coating using Eddy current testing method

Thermal spay coatings have been widely used to provide structural components with protection from harsh environments such as corrosion, heat, and wear resistance. However, delamination, pores or peeling defects may exist in an as-fabricated coating which influence the mechanical properties of the coating. We developed an eddy current testing (ECT) system using a special ECT probe. The ECT probe was composed of one detection coil and two excitation coils. The lower excitation coil was used to produce the excitation magnetic field in the specimen and the upper excitation coil was used to compensate the background signal. In this study, the feasibility of our ECT system to inspect the Ti-6Al-4V warm spray (WS) coatings were investigated. The artificial defects embedded between the Ti-6Al-4V coating and the substrate were successfully detected using the ECT system.

Multi-Channel Eddy Current Detector Based on Virtual Instrument Technology and Self-Balancing Technology

In order to reduce the complexity of software and hardware design in multi-channel eddy current testing, a multi-channel real-time eddy current testing system based on lock-in amplifier technology and self- balancing technology is designed. The system uses graphical programming language platform (LabVIEW), Myrio1900 data acquisition control card and lock-in amplifier as key components. The lock-in amplifier adopts the lock-in technology to detect the amplitude and phase of the weak eddy current signal. The Myrio1900 card outputs multiple analog switch control signals and collects the amplitude component of the weak eddy current signal. The LabVIEW platform uses self-balancing technology to eliminate the imbalance of the bridge itself. The detection system realizes the real-time and reliable display of the amplitude and XY components of the 6-channel eddy current signal. Experimental results show that the designed system has a detection width of 56.00 mm at one time, and can detect artificial grooves with a covering layer of 2.90 mm. The artificial grooves have a width of 1.00 mm, a depth of 1.00 mm and a length of 50.00 mm. The system design process described in the article can be used as a reference for equipment developers and can also be used for educational purposes.

Developing Gradiometer to Reduce the Low Frequency Noise of Magnetic Sensor with Amorphous Wire

Pico-Tesla level high sensitivity magnetic sensor has been developed using FeCoSiB amorphous wire as the core of a coil, but the low frequency noise of the magnetic sensor is somewhat high, and some applications of magnetic sensors, such as magnetic microscope or deep defect detection, require that the sensors have low noise at low frequency. To reduce the noise, it is necessary to know the noise sources of the magnetic sensor. By measuring the noise spectrum of each part of the magnetic sensor with FeCoSiB amorphous wire was measured, it was found that the white noise of the magnetic sensor was mainly determined by the noise of the preamplifier and the low frequency noise was mainly determined by the low frequency noises of the DC bias current and AC bias current of the FeCoSiB magnetic sensor. To reduce the low frequency noise of the magnetic sensor caused by the bias current noise, an electrical gradiometer composed of two magnetic sensors sharing the same DC bias current and AC bias current was developed. The results proved that the gradiometer was not only effective to reduce the low frequency noise caused the bias current, but also effective to reduce the environmental noise caused by the 50 Hz line interference. Using the gradiometer, an eddy current testing (ECT) system were constructed, and the deep defect with the depth of about 8 mm in an aluminum plate was successfully detected by the ECT system.

Self-resonance eddy-current testing method in noncontact detection of carbon fiber reinforced plastics

Eddy-current testing (ECT) is relatively mature and has gradually become one of the main non-destructive testing technologies of carbon fiber reinforced plastics (CFRP). From the perspective of low conductivity of CFRP, high-frequency ECT technology is considered to be more suitable for such materials with less conductivity. However, high-frequency ECT method is more susceptible to various interference factors and has higher background noise, which will cause great inconvenience to practical applications. Concerning this issue, a self-resonance ECT method in noncontact detection of CFRP was proposed in this paper, and the corresponding ECT system that can detect the defects of CFRP at a low-frequency and keep a relatively high signal-to-noise ratio was developed. The system is mainly composed of a resonant eddy-current probe, a special excitation circuit, and an ECT signal pickup module. Three CFRP samples (orthogonal woven laminate, cross-ply laminate, four-directional angle-ply laminate) were scanned by using the self-resonance ETC system developed in this paper. The scanning results show that the detection effect of the self-resonant ECT system at 500 kHz is similar to that of the traditional ECT system at 10 MHz, and 500KHz is enough to provide good detection results for the above CFRP samples. Therefore, the proposed research is a valuable ECT technology, which has played a guiding role in the application of low-frequency ECT method in CFRP.

Rail crack detection and evaluation at high speed based on differential ECT system

In this paper, the effects of detection speed on eddy current testing (ECT) signals are studied, and the quantitative evaluation method of rail surface cracks at different speeds is proposed. According to the finite element simulation analysis, the excitation frequency and coil parameters are optimized. A differential ECT system is established for rail crack detection experiments in a lift-off range of 1–6 mm and a speed range of 20–300 km/h. The experiment results show that the frequency of defect signal can be estimated by the coil diameter and the detection speed. The decrease trend of the defect signal amplitude with the increase of lift-off remains the same under different speeds. The defect signal amplitude decreases approximately linearly as the speed increases, and the decline slope is related to the defect depth. The relationship between the defect signal amplitude, the defect depth, and the detection speed is further obtained, which is used to evaluate the defect depth at different speed.

Eddy Current Characterization of 3D Crack by Analyzing Probe Signal and Using a Fast Algorithm Search

The study of 3D eddy current non destructive testing system for cracks characterization using finite element method requires a great amount of computing time and memory space. In this article, we have validated the developed model and then determined directly the crack length by analyzing the complete signal. Afterwards, we have extracted from the complete sensor sweep signal the maximal amplitude that we have exploited to estimate the crack depth.

PT-Level High-Sensitivity Magnetic Sensor with Amorphous Wire.

A picotesla (PT) level high-sensitivity magnetic sensor with amorphous wire was developed. The magnetic sensor was composed of a (Fe0.06Co0.94)72.5Si2.5B15 (FeCoSiB) amorphous wire with a coil wound around it. The amorphous wire had a diameter of 0.1 mm and a length of 5 mm. The coil was 30 turns. There was no electrical connection with the amorphous wire. The sensor was biased by an alternating current (AC) of about 1 MHz and a direct current (DC). To increase the sensitivity, a resonant circuit was used, and the signal amplitude of the magnetic sensor was increased 10 times from 10 mV/Gauss to about 100 mV/Gauss. The magnetic field resolution was improved 5 times from 30 pT/√Hz to 6 pT/√Hz. An eddy current testing system with a magnetic sensor was developed, and the artificial defects in an aluminum plate were evaluated.

A Feedback Method to Improve the Dynamic Range and the Linearity of Magnetoimpedance Magnetic Sensor

We developed a high-sensitivity magnetoimpedance magnetic field sensor using a FeCoSiB amorphous wire and a coil wound around it. The amorphous wire had the diameter of 0.1 mm and the length of 5 mm. The magnetic field resolution of about 20 pT/√Hz was achieved. But the dynamic range of the magnetoimpedance magnetic field sensor was only about ±0.7 Gauss, which was not enough for some applications, such as the defect evaluation of steel plate. The linearity of the system was also not good when big magnetic field was applied, which will cause some noise when the system is used in unshielded environment. We developed a feedback method to improve the dynamic range and the linearity of the magnetic field sensor. The operation point of the magnetic field sensor was fixed by sending a feedback current to the coil. Using the feedback method, the dynamic range was improved from ±0.7 Gauss to ±10 Gauss and the linearity was also improved over 100 times better. An eddy current testing system using the magnetic sensor was developed, and the crack defects in steel plate and in 3D-printed titanium alloy plate were evaluated.

Analytical solution for magnetic field of cylindrical defect in eddy current nondestructive testing

In metal components, a defect frequently causes the failure of the components and results in serious accidents consequently. Eddy current nondestructive testing method is one of the most effective testing approaches to quantitatively monitor the defects both in non-ferrite metal and in ferrite metal. Therefore, knowledge of the magnetic field is necessary to deeply understand the mechanism of eddy current nondestructive technique in defect characterization. In this paper, an analytical expression for the magnetic field for a cylindrical defect in metal in eddy current testing system is proposed by solving the partial differential equations. Firstly, a conductive metal plate with a cylindrical defect is modeled for the cylindrical coil with rectangular cross-section, and the analytical expression to the magnetic potential vector is obtained. Then, the series form expressions are extended to calculate the axial component of magnetic induction intensity Bz and is computed on the MATLAB platform. Finally, a finite element model is setup to validate the proposed analytical expression for the magnetic field. The results show that the magnetic field calculated by the analytical method is in good agreement with that calculated by finite element method, and the calculation efficiency of the analytical method is higher than that of FEM. The proposed expressions not only can be readily used to investigate the quantitative relationship of the cylindrical defects in a conductive metal, but also it is expected to be extended to seek the theoretical expression for the elliptic defect and also for the natural defect in key structures (e.g. oil pipe, rail track, iron bridge, oil tank, ocean ship) by using (pulsed) eddy current nondestructive technique.

Semi-analytical method for characterization slit defects in conducting metal by Eddy current nondestructive technique

Defects in the metal components is a typical cause of component failure in engineering, with a lack of timely detection and elimination of these defects usually causing serious accidents. A nondestructive testing method using eddy current, however, exhibits remarkable capabilities in quantitatively monitoring the formation of these artificial defects in metal materials. Owing that the commonly appeared defect species in metal structures is slit defect instead of cylindrical defect, a deeper understanding of the magnetic testing signal correlated to slit defects in eddy current nondestructive method is required. In this paper, based on pervious investigation on cylindrical defect, a semi-analytical method for the magnetic field of a slit defect in eddy current testing system is proposed. Firstly, an analytical expression to the magnetic field for a cylindrical defect in metal is briefly introduced. Then, a model with an elliptical cylindrical defect is studied, and a semi-analytical method is deduced in details. Next, a semi-analytical formula is proposed to calculate the magnetic signals generated by slit defect. Finally, with a case study, the proposed semi-analytical method is applied to calculate the slit defect magnetic testing signal for slit defect in eddy current technique. The results indicated that the maximal relative errors between the semi-analytical solution and FEM are less than 2 % and the computation efficiency is higher than 10 times. With the support of these results, not only our proposed semi-analytical method can be readily used to investigate the forward problem due to the slit defects in eddy current technique, but also it can be further extended to seek the theoretical expression for defects with complex shapes, including those natural defects in conductive metal components (e.g. defects in oil pipe, rail track, iron bridge, oil tank, ocean ship).

Development of Three-Channel HTS-SQUID Inspection System for Orthotropic Steel Decks of Expressway Bridges

We have developed a three-channel high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) eddy current testing (ECT) system for detection of a fatigue crack which occurs in a steel deck plate used in expressway bridges. To avoid physical interference between adjacent channels, a dipole coil with compact design is employed. A field test using the three channel ECT system was performed on an expressway bridge having an orthotropic steel deck structure under traffic lane restrictions. Although a signal corresponding to existence of a fatigue crack was not detected due to a low occurrence rate of fatigue crack, we could demonstrate a stable and long-time operation in an urban area during the daytime and acquisition of correct data corresponding to some structural features of the expressway bridge.

RESEARCH OF MEASUREMENT SIGNAL PARAMETERS DEFINITION ERRORS IN CASE OF IRREGULAR SAMPLING

Reviewed influence of an irregular sampling of informative cyclic signals in measuring testing computerized systems on an example of eddy current robotized testing system.It was found that in case of ADC clock signal forming in processor block, there is an irregular sampling of informative signals. Vibrations of sampling period appears because of interrupt processing in a computerized system and asynchrony of a clock frequency of internal bus and signal sampling frequency. Irregular sampling of a cyclic informative signals makes additional fault of signals characteristics measurement to appear. In testing-measuring systems such fault can cause decrease of testing reliability.In this article real signals of robotized eddy current testing system were analyzed. The distribution of sampling intervals for computerized eddy current flaw detector was obtained. Was performed evaluation of mean squared error of amplitude and phase harmonic test signal characteristics determination in case when signal is set on irregular time grid. Was obtained graphs of the mean squared error of harmonic signal phase and amplitude characteristics dependences with its irregular sampling. Experimental proved that in computerized eddy current testing systems with average reliability in purpose of costs minimization it is permissive to use informative signals irregular sampling, which have place in test signals generation in process block of the system. Although, in case of creation of precise systems with increased reliability level it is necessary to build system with different blocks for data collecting with regular signal sampling, which allows to collect some amount of data in autonomous mode and transmit it to computerized system for its further processing and visualization of flaw detection process results.

Application of ECT technology with a miniature PCB-based spiral coil in the visualization of CFRP laminates

ABSTRACTAn eddy-current testing (ECT) method with miniature-printed circuit board (PCB)-based planar spiral coils for carbon fibre-reinforced plastics (CFRP) is proposed in this paper. First, the electromagnetic model of the rectangular spiral coil is discussed, thus the analytical expressions of the impedance in free-space, and impedance change due to the eddy-current are provided. Next, the intensity areas of impedance change of a certain PCB-based planar spiral coil for metal and CFRP are theoretically given with different frequencies. Then, an ECT system for CFRP is established, and a resonant probe with a PCB-coil is designed by coupling to the parasitic capacitance of the cables. Finally, a series of ECT experiments for unidirectional, four-directional angle-ply, and orthogonal woven CFRP laminates are conducted. The results show that the minimum defect depth that can be detected on the unidirectional CFRP laminate reaches 0.1 mm. This study is an effective attempt for CFRP ECT based on planar spiral coils fabricated by PCB technology and lays the foundation for further ECT large-scale array scanners manufactured by PCB technology.