Multi-frequency Eddy Current Testing Research Articles

Rapid Identification of Material Defects Based on Pulsed Multifrequency Eddy Current Testing and the k-Nearest Neighbor Method.

The article discusses the utilization of Pulsed Multifrequency Excitation and Spectrogram Eddy Current Testing (PMFES-ECT) in conjunction with the supervised learning method for the purpose of estimating defect parameters in conductive materials. To obtain estimates for these parameters, a three-dimensional finite element method model was developed for the sensor and specimen containing defects. The outcomes obtained from the simulation were employed as training data for the k-Nearest Neighbors (k-NN) algorithm. Subsequently, the k-NN algorithm was employed to determine the defect parameters by leveraging the available measurement outcomes. The evaluation of classification accuracy for different combinations of predictors derived from measured data is also presented in this study.

Ill-posedness of reconstruction of stress corrosion cracking and a regularization method using multifrequency eddy current signals

Stress corrosion cracking (SCC) widely occurs in the key structures of large-scale mechanical systems and may cause catastrophic accidents without evident macroscopic deformation. A sufficiently accurate quantitative evaluation of SCC is important to guarantee the safety and efficient operation of large mechanical systems. However, the reconstruction accuracy of SCC shape parameters using eddy current testing (ECT) signals remains a problem, given the complex microstructure of cracks and the properties of the ECT inverse problem. In this study, a two-dimensional numerical model of crack with conductivity distribution is used to calculate the ECT signals due to SCC. The relation between crack parameters and ECT signals is investigated through simulation and experiment. Results show that the ill-posedness of SCC shape reconstruction is mainly manifested in the nonuniqueness of the solution of crack shape parameters. Therefore, a regularization method based on the features of multifrequency ECT signals is proposed to solve ill-posedness by adding prior information to the solution space of the optimization problem. The reconstruction results of the simulated and measured ECT signals from the conductive crack models verify that the proposed strategy can effectively improve the precision of SCC shape reconstruction.

Lift-off invariant inductance of steels in multi-frequency eddy-current testing

Eddy current testing can be used to interrogate steels but it is hampered by the lift-off distance of the sensor. Previously, the lift-off point of intersection (LOI) feature has been found for the pulsed eddy current (PEC) testing. In this paper, a lift-off invariant inductance (LII) feature is proposed for the multi-frequency eddy current (MEC) testing, which merely targets the ferromagnetic steels. That is, at a certain working frequency, the measured inductance signal is found nearly immune to the lift-off distance of the sensor. Such working frequency and inductance are termed as the lift-off invariant frequency (LIF) and LII. Through simulations and experimental measurements of different steels under the multi-frequency manner, the LII has been verified to be merely related to the sensor parameters and independent of different steels. By referring to the LIF of the test piece and using an iterative inverse solver, one of the steel properties (either the electrical conductivity or magnetic permeability) can be reconstructed with a high accuracy.

Frequency component mixing of pulsed or multi-frequency eddy current testing for nonferromagnetic plate thickness measurement using a multi-gene genetic programming algorithm

For the efficient use of frequency components, a frequency mixed feature for pulsed eddy current testing (PECT) or multi-frequency eddy current testing (MultiECT) was proposed for nonferromagnetic plate thickness measurement. An evolutionary algorithm multigene genetic programming was employed to mix the frequency components using the best linearity as a target. Time domain and frequency domain finite element simulations of PECT and MultiECT were conducted. The simulation results revealed that, in terms of thickness measurement, a mixed feature comprising two or three frequencies was more linear and accurate than the traditional peak time and decay coefficient of PECT. Experiments were conducted to validate the results of the simulations and to test the mixed feature in aluminum plate thickness evaluations. The experimental results also revealed that the use of more frequencies did not always increase the accuracy of thickness evaluations. Proper frequency component selection was more efficient than blindly increasing frequency numbers.

Measuring Coaxial Hole Size of Finite-Size Metallic Disk Based on a Dual-Constraint Integration Feature Using Multifrequency Eddy Current Testing

This article presents a new approach of eddy current methods for determining the size of the coaxial hole in the metallic circular disk. In recent decades, for the air-cored sensor probe, the impedance change due to the presence of an infinite metal plate can be calculated by the Dodd–Deeds model. However, in practical measurements, the sample cannot match with the condition required—“infinite”; thus, the Dodd–Deeds model could not be applied to the disk with finite size and certainly not a coaxial hole in the center. In this article, a dual-constraint analytical method is proposed. That is, the upper and lower limits of the integration are substituted with specific values instead of the original 0 and $\infty $ . Besides, it is found that, once the outer radius of the disk is fixed (i.e., the lower limit of integration is fixed), the upper limit reduces linearly as the size of the coaxial hole increases. Both the FEM simulation and experiments have been carried out to validate this method. The radius of the hole can be estimated based on the dual-constraint integration feature.

Analysis for Hydrogen Concentration in Titanium Alloys Using Multifrequency Eddy Current

In this article, multifrequency eddy current testing is proposed to detect hydrogen concentration in alloy titanium. The effect of hydrogen on the titanium alloy was explored across the surface layer of the material. The sample has a fixed surface area, and the samples with the same thickness and different hydrogen concentrations are analyzed. Output voltage amplitudes and dielectric loss tangent in hydrogen-filled titanium VT1-0 were studied in the range of hydrogen concentration from 0.019 to 0.290 wt.% using the instrument Micromagnetic Multiparameter Microstructure and Stress Analysis (3MA-II) at the frequency from 200 to 1000 kHz. This article proposes a method of analyzing the internal conductivity of metals by eliminating surface effects. The linear relationship between hydrogen concentration and conductivity is obtained, and the experimental results that are closer to the theory than the four-probe method and the inclusion surface are obtained. The results show that the output signal amplitudes decreased with an increase of the hydrogen concentration and frequency, and in addition, the dielectric loss tangent obtains a similar result at the hydrogen concentra- tion >0.100 wt.%. It is shown that this method can be used to evaluate the concentration of hydrogen in metal.

A Class D Power Amplifier for Multifrequency Eddy Current Testing Based on Multisimultaneous-Frequency Selective Harmonic Elimination Pulsewidth Modulation

Efficiency and multisimultaneous-frequency (MSF) output capability are two major criteria characterizing the performance of a power amplifier in the application of multifrequency eddy current testing (MECT). Switch-mode power amplifiers are known to have a very high efficiency, yet they have rarely been adopted in the instrumental development of MECT. In addition, switch-mode power amplifiers themselves are lacking in the research literature for MSF capability. In this article, a Class D power amplifier is designed so as to address the two issues. An MSF selective harmonic elimination pulsewidth modulation method is proposed to generate alternating magnetic fields, which are rich in selected harmonics. A field-programmable-gate-array-based experimental system has been developed to verify the design. Results show that the proposed methodology is capable of generating high MSF currents in the transmitting coil with a low distortion of signal.

Experimental Study of Microstructures in Bias Weld of Coiled Tubing Steel Strip With Multi-Frequency Eddy Current Testing

Coiled tubing technology is widely used in drilling and repairing operations, and the quality of bias weld directly determines the fatigue life of the coiled tubing. The microstructures consistency between the bias weld and the base metal determines that whether the fatigue resistance of the weld reaches the level of the base metal. In this paper, multi-frequency eddy current testing technique is used to detect the reactance of the bias weld zone and the base material zone within coiled tubing steel strip, which can be used to determine the microstructure consistency between the base metal area and the weld area, and in turn to improve the welding technology and heat treatment process. In this work, CT100 and CT90 materials with the bias weld for coiled tubing were used as the test samples, and the on-line real-time non-destructive testing technology was used to judge whether welding technology and heat treatment technology have been improved. The basis for determination is whether the mechanical properties and fatigue properties of the welding site reach the base material level. The weld zone and base metal zone can be identified by analyzing the characteristic cloud map of the detection signal, and then the quality of partial weld can be evaluated according to the difference of microstructure between them, which is of guiding significance for quality control and improvement of coiled tubing production process.

Sizing of natural crack using multi-output support vector regression method from multi-frequency eddy current testing signals

In the nondestructive evaluation for components of key equipment, sizing of natural crack is important in order to guarantee both the safety and efficient operation for large mechanical systems. Natural cracks have complex boundary and there may be electric current flowing through crack faces. If a simple model of artificial notch is used to simulate it, errors often occur in crack depth reconstruction from eddy current testing (ECT) signals. However, if a complex crack conductivity model is used, quantitative evaluation of natural crack will be transformed into a multivariable nonlinear optimization problem and the solution is difficult. In this paper, based on the relationship between crack parameters and features of multi-frequency ECT signals, a multi-output support vector regression algorithm using domain decomposition for parameters was proposed. The algorithm realized the quantitative evaluation of multiple parameters of crack in turn. Numerical examples with simulated and measured ECT signals were presented to verify the efficiency of the proposed strategy.

Imaging a weld cross-section using a novel frequency feature in multi-frequency eddy current testing

In this paper, a novel frequency feature (the frequency at which the real part of the inductance reaches a specified value) is proposed for imaging an X70 welding cross-section. It is derived that this frequency feature is related to the permeability of the sample and therefore the imaging results obtained from this feature are a reflection of the electromagnetic properties (in particular permeability) map of the sample. The resulting images are then compared with the weld macrograph and the hardness map. The shape and contour of three weld zones (fusion zone, heat-affected zone and base metal) can be identified. This novel imaging method has a high sensitivity to changes in microstructure and, by adjusting the inductance value it crosses, the frequency range can be adjusted to suit the capability of a particular instrument.

Auxiliary Particle Filter–Based Multisensor Data Fusion for Quantification of Corrosion in Aerostructures

The identification of structural flaws and subsequent management is significant for health monitoring of aviation structures. Wall thickness loss can occur because of corrosion in exposed aerostructures, thereby reducing the strength of the structure. This may impose prolonged downtimes for repair and replacement activities if not detected at early stages. Eddy current testing (ECT) is a viable technique to detect such anomalies. Fusing multifrequency data produces better results because of complementary features of each contributing mode. Particle filter–based methods have already been proposed to detect and quantify flaw profiles of corrosion damage to an aircraft structure using a multifrequency ECT data fusion technique. However, to address the associated drawbacks, an auxiliary particle filter (APF) method is proposed here. In APF, the proposed importance density is a mixture of the prior state and the latest measurements, yielding focused estimation into a high-likelihood region. Simulation results are presented to demonstrate effectiveness of the proposed technique to quantify damages in aerostructures.

Simulation and Experimental Investigation of Pulsed Eddy Current Technique for Defect Evaluation

Pulsed eddy current (PEC) is an advanced Non-Destructive Testing (NDT) technique that uses transient waveform for their coil excitation. It has the advantage of gathering different depth information in a single excitation process, thus provide the solution towards the drawbacks by both single and multi-frequency eddy current testing. In this work, experimental and simulation investigations on PEC were conducted to establish the correlation between PEC signals and different defect i.e. crack, depths. Initial FEM simulation work has provided understanding of the underlying phenomena of the PEC results through the visualisation of the induced eddy current and defect interaction in the SS 304 sample. Features from the differential magnetic field transient profiles have provided information of the crack in terms of its depth and location. The understanding of the PEC responses and results can be extended for future work in quantitative evaluation of defect in conductive samples by PEC.

Simulation and Experimental Investigation of Pulsed Eddy Current Technique for Defect Evaluation

Pulsed eddy current (PEC) is an advanced Non-Destructive Testing (NDT) technique that uses transient waveform for their coil excitation. It has the advantage of gathering different depth information in a single excitation process, thus provide the solution towards the drawbacks by both single and multi-frequency eddy current testing. In this work, experimental and simulation investigations on PEC were conducted to establish the correlation between PEC signals and different defect i.e. crack, depths. Initial FEM simulation work has provided understanding of the underlying phenomena of the PEC results through the visualisation of the induced eddy current and defect interaction in the SS 304 sample. Features from the differential magnetic field transient profiles have provided information of the crack in terms of its depth and location. The understanding of the PEC responses and results can be extended for future work in quantitative evaluation of defect in conductive samples by PEC.

Eddy Current Testing of Metallic Materials Using Projection Methods

Using the example of nonmagnetic metallic materials, the projection methods of multidimensional analysis of multifrequency eddy-current testing measurement data presented for processing are considered. Based on the principal components method, the effects of electrical conductivity and sensor–material-surface gap on the results of testing have been separated. Using regression on principal components, the numerical values of the electrical conductivity and gap for manganese, copper, bronze, aluminum, and an aluminum alloy have been obtained. The results make it possible to extend the possibilities of nondestructive eddy-current evaluation of materials.

Surface Profiling and Core Evaluation of Aluminum Honeycomb Sandwich Aircraft Panels Using Multi-Frequency Eddy Current Testing.

Surface damage on honeycomb aircraft panels is often measured manually, and is therefore subject to variation based on inspection personnel. Eddy current testing (ECT) is sensitive to variations in probe-to-specimen spacing, or lift-off, and is thus promising for high-resolution profiling of surface damage on aluminum panels. Lower frequency testing also allows inspection through the face sheet, an advantage over optical 3D scanning methods. This paper presents results from the ECT inspection of surface damage on an approximately flat aluminum honeycomb aircraft panel, and compares the measurements to those taken using optical 3D scanning technology. An ECT C-Scan of the dented panel surface was obtained by attaching the probe to a robotic scanning apparatus. Data was taken simultaneously at four frequencies of 25, 100, 400 and 1600 kHz. A reference surface was then defined that approximated the original, undamaged panel surface, which also compensated for the effects of specimen tilt and thermal drift within the ECT instrument. Data was converted to lift-off using height calibration curves developed for each probe frequency. A damage region of 22,550 mm2 area with dents ranging in depth from 0.13–1.01 mm was analyzed. The method was accurate at 1600 kHz to within 0.05 mm (2σ) when compared with 231 measurements taken via optical 3D scanning. Testing at 25 kHz revealed a 3.2 mm cell size within the honeycomb core, which was confirmed via destructive evaluation. As a result, ECT demonstrates potential for implementation as a method for rapid in-field aircraft panel surface damage assessment.

Using signals of special form in multi-frequency eddy current testing

An eddy current testing method based on the use of an exciting signal of special shape is proposed. Subsequent digital processing of the detected output signal makes it possible to calculate the measuring sensor parameters at different frequencies and to construct an experimental hodograph of the sensor–test object system. Experiments performed for materials with different physical and geometrical characteristics of constructing hodographs have shown that the proposed method provides a reliable separation of the main factors affecting the results of multi-frequency eddy current measurement. The results can be used in automated systems for inspection and non-destructive testing of materials and products.

Separation of Surface Treatment on Carbon Steel by Using Multi-Frequency Eddy-Current Testing

Separation of Surface Treatment on Carbon Steel by Using Multi-Frequency Eddy-Current Testing

Principal Components Analysis of Multifrequency Eddy Current Data Used to Measure Pressure Tube to Calandria Tube Gap

Principal components analysis (PCA) involves transforming a set of correlated observations into a set of linearly uncorrelated variables, which can reveal simplified trends in data. Multifrequency eddy current testing contains correlations across different test frequencies. In this paper, PCA is used to extract unique information from multifrequency eddy current data sets, used to measure the pressure tube to calandria tube gap, in CANada Deuterium Uranium fuel channels. Advantages include compressed data acquisition, allowing for increased inspection speed, and monitoring for variation in physical parameters using a reduced number of variables. PCA employing analytical input model data is validated against PCA employing data from physical experiments.

Developments of non-destructive test method of jacket section for ITER Poloidal Field (PF) coils

The ITER magnet system is one of the largest and most complex superconducting installations ever undertaken. Therefore, all subcomponents have very demanding requirements, including the conductors which are of cable-in-conduit conductor type. Based on the structural analysis and fatigue life requirements of the coil systems, the maximum acceptable defect sizes inside the jacket material were defined. This paper presents the nondestructive test (NDT) method developed for the poloidal field (PF) coil conductor jackets made of 316L stainless steel. The difficulty in the development of the PF jacket NDT arises from the circular-in-square shape, i.e., extruded bars of a square cross section with an inner circular bore. Furthermore, in addition to volumetric defects, the inspection procedure has to detect planar defects oriented longitudinal (i.e., along the jacket) and transversal on the surfaces and inside the jacket material. After a series of experiments and optimization, the phased array ultrasonic test (PAUT) combined with eddy current testing (ECT) was developed for the PF jacket inspection. The outer surface and volume are inspected by PAUT consisting of 11 separate runs per jacket surface and using four different probes. The inner bore surface up to a depth of 2–3 mm is inspected by multifrequency ECT. The characteristics and limits of all inspections runs are discussed, particularly for the most difficult one with detecting transversal planar defects.

The application of principal-component analysis during eddy-current testing of aluminum alloys

The application of finite-element methods of multidimensional analysis in the measurement data processing of multi-frequency eddy-current testing has been considered based on the example of aluminum alloys. The results show that the suggested approach allows one to separate the effects of various factors and thus provides a reliable classification of materials according to their physicomechanical characteristics for the problems of incoming control.