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

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.

This article presents a model-based eddy current (EC) testing method for the reconstruction of geometrical features, that may be hidden, through, or blind cavity in a large electrically conductive plate. Based on a distributed current source (DCS) modeling method, a perturbation model is formulated to convert the feature reconstruction problem into finding a possibility vector using a finite set of measured magnetic flux density (MFD) data and solved with the Tikhonov regularization method. The proposed method is numerically and experimentally validated and demonstrated by using the specimens with compound conductive/nonconductive geometrical features and comparing them with two published feature detection methods. Parametric studies are conducted to analyze the effects of feature shape, sensor configuration, and multi-coil excitation on reconstruction accuracy.

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.

The occurrence of surface and sub-surface local damage, such as squats and studs, primarily attributed to rail-wheel contact within rail tracks, underscores the necessity for developing non-destructive testing methods. Such methods should ideally be high-speed, non-contact, and readily adaptable. Initial studies were undertaken to ascertain the optimal configurations of directional transmit and differential receiver eddy-current testing (ECT) sensors for detecting artificially induced rail defects. Discrepancies were observed in the response of the developed ECT sensors when compared to real rail damage. Consequently, this research evaluates the real-time efficacy of the original figure-8 transmit configuration with non-unity aspect ratio differential receivers against a novel dual figure-8 transmit design paired with unity aspect ratio receivers. The comparison focuses on their respective capabilities in detecting actual defects induced within rail steel samples, aiming to provide crucial insights for future in-line measurements using a trolley on a live rail network. ECT achieved a signal-to-noise ratio (SNR) exceeding 47 for rail studs, while μ -CT scans validated these results by confirming an approximate defect depth of 7.9 mm and width of 36 mm. Quantitative characterization was achieved through an evaluation of the SNR, indicating the desirability of lower frequencies for facilitating electronic data acquisition and processing. Additionally, lift-off sensitivity analysis demonstrated that the optimized probe design, operating at the optimum frequency, could detect targeted defects with up to a 10 mm lift-off distance, achieving an SNR of 5. • Directional EC probes optimized for local damage detection in rail samples. • Dual figure-8 transmit with unity receiver reduces sensitivity to steel variation. • Optimal 0.3 and 2.2 MHz frequencies identified for single and dual figure-8 transmitters paired with non-unity and unity aspect ratio receivers. • SNR > 5 at 10 mm lift-off–outperforming prior STUD scan–using same figure-8 and non-unity receiver. • μ -CT validation confirms ECT accuracy in measuring STUD defect dimensions.

ECT benchmark results for 3D defect signatures in industrial applications

In traditional eddy current testing, crack orientation and direction of the eddy current are closely related. This becomes particularly problematic when cracks are oriented at specific angles, as the fixed transmitter–receiver mode creates an eddy current field that often results in signal blind spots, thereby reducing detection performance. To address this issue, this paper proposes a time-division multiplexing (TDM)-based multi-mode eddy current testing (ECT) method specifically designed for detecting directional defects in metallic materials. By combining multiple excitation modes, this approach gathers more comprehensive multi-angle, multi-frequency, multi-frequency and multi-mode information, which strengthens directional-crack detectability and improves accuracy and reliability. The study compares the variation in induced voltage amplitudes under single and mixed excitation conditions, then their correlation with various directional defects is analysed. Finite element simulations are used to evaluate the impact of key parameters such as coil excitation frequency, centre-to-centre distance and outer radius on detection sensitivity as well. Subsequently, the effects of defect length, width, depth and orientation on the induced voltage signal are investigated. Finally, an experimental platform based on time-division multiplexing is constructed to thoroughly validate the method with various directional defects in metal components.

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

The manufacturing process of carbon fiber reinforced polymer (CFRP) composite structures can introduce many characteristic defects and flaws such as fiber misorientation, fiber waviness and wrinkling. Therefore, it becomes increasingly important to detect the presence of these defects at the earliest stages of development. Eddy current testing (ECT) is a nondestructive inspection (NDI) technique which has been proven quite effective in metallic structures. However, NDI of composite structures has mainly relied on other methods such as ultrasonics and X-ray to name a few, and not much on ECT. In this paper, we explore the possibility of using ECT in NDI of CFRP composites. We base our research on the fact that the CFRP displays some low-level electric conductivity due to the inherent conductivity of the carbon fibers. This low-level conductivity may permit eddy-current pathways that can be exploited for NDI detection. An invention disclosure describing our high-frequency ECT method is being progressed. We use multiphysics FEM simulation to simulate the detection of various types of manufacturing flaws and operational damage in CFRP composites such as fiber misorientation; waviness; wrinkling, etc. ECT experiments were conducted on CFRP specimens with manufacturing flaws using the Eddyfi Reddy eddy current array (ECA) system.

Titanium (Ti) alloy sheets are important mechanical and structural components. However, thickness deviations may occur during the production of Ti-alloy sheets, significantly compromising product quality and structural safety. Eddy current testing (ECT) is a common method for measuring the thickness deviation of metal sheets. Nevertheless, conventional ECT methods often rely on complex calibration procedures or iterative inversion algorithms, thereby limiting their applicability. It was found that when low-frequency ECT excitation is used, such that the eddy current penetration depth exceeds three times the maximum target thickness of the Ti-alloy sheet, the tangent of the ECT coil impedance phase exhibits a linear relationship with the thickness. Based on this observation, by analyzing the low-frequency ECT response of Ti-alloys and separating the real and imaginary parts of the impedance under approximate conditions, a phase feature model was developed. The model effectively describes the linear dependence of the phase tangent on the thickness of the Ti-alloy sheet, offering a succinct characterization. The measurement method based on this model thereby allows for direct thickness calculation from the measured coil impedance without requiring master-curve calibration or iterative computation. Experiments were conducted using a custom-designed ECT coil and impedance analyzer to measure different Ti-alloy specimens. The results indicate that the measurement error was less than 3.5%. This research provides a theoretical foundation as well as a straightforward engineering solution for online, high-speed thickness measurement of Ti-alloy sheets.

New developments in eddy current testing of hot wire rod and tubes

The lay length is one of the important parameters for the wire rope and its detection can indirectly characterize its stress and surface damage state. The eddy current testing (ECT) method is advantageous because of its easy operation, single-side detection, low requirement of surface preparation, applicability in harsh environment, and other properties. Hence the lay length of the wire ropes is measured by ECT in this paper. The eddy current (EC) response to the wire rope is simulated. The EC on the surface of the strand below the pancake coil forms a vortex, and the distribution of the EC on the curved surfaces on both sides of the strand is symmetrical. The spacing between the adjacent peaks or troughs of the scanning signal is the distance between the adjacent strands, and the number of peaks or troughs of the scanning signal in a range of the lay length is related to the number of strands of the wire rope. Finally, the experimental validation is performed. The relative error of the lay length of the wire rope assessed by the EC method is less than 0.28%.

A new noncontact method for measuring the electrical conductivity of metals based on eddy current testing is proposed for the physics that the logarithm of phase signals of impedance change is linear to that of electrical conductivity. A finite element model for simulations was built, and the mathematical relationship between the logarithm of phase signals and that of electrical conductivity was established with linear least-squares fitting. The effects of the excitation frequency, lift-off distance, and relative permeability were investigated and analyzed. The investigations of lift-off effect enable us to reduce it by substituting the impedance of the coil at certain lift-off distance for that of the coil in air as the reference. Finally, the experimental platform was developed, and the experimental results matched well with the simulated results. The relative errors fall within 3%, which demonstrates the feasibility of the presented method for non-ferrous metals.

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.

The reliability of an eddy current testing (ECT) inspection system depends upon the inspection technique and quality of analyst. In evaluating the integrity of a steam generator (SG) tube, degradation detection and sizing accuracy are considered performance measures of the nondestructive evaluation (NDE) system. A probability of detection (POD) model serves as a functional measure of the ability of an NDE system to detect degradation. It is one of the inputs in the operational assessment, and it is used to estimate the degradation during service via ECT of the SG tube. In this study, the POD functions of the inspection technique and analyst were obtained to quantitatively analyze the ECT bobbin probe for axial outside diameter stress corrosion cracks in SG tubes. This should serve to evaluate the integrity of the SG tubes. The depth and amplitude of defects were used as parameters of the POD model. Hit (detection) and miss (no detection) binary data obtained from destructive and nondestructive inspection of cracked tubes were also used.

Eddy Current Array Inspection of Zlin 142 Fuselage Riveted Joints