The Pulsed Eddy Current Differential Probe to Detect a Thickness Variation in an Insulated Stainless Steel

Non-destructive testing (NDT) plays an important role in the safety and integrity of the large industrial structures such as pipelines in nuclear power plants (NPPs). The pulsed eddy current (PEC) is an electromagnetic NDT approach which is principally developed for the detection of surface and sub surface flaws. In this study a differential probe for the PEC system has been fabricated to detect the wall thinning in insulated steel pipelines. The differential probe contains an excitation coil with two hall-sensors. A stainless steel test sample was prepared with a thickness that varied from 1 mm to 5 mm and was laminated by plastic insulation with uniform thickness to represent the insulated pipelines in the NPPs. Excitation coil in the probe is driven by a rectangular current pulse, the resultant PEC response which is the difference of the two hall sensors is detected. The discriminating features of the detected pulse, peak value and the time to zero were used to describe the wall thinning in the tested sample. A signal processing technique such as power spectrum density (PSD) is devised to infer the PEC response. The results shows that the differential PEC probe has the potential to detect the wall thinning in an insulated pipeline of the nuclear power plants (NPPs).     

Analysis of an open-ended coaxial probe with lift-off fornondestructive testing

The open-ended coaxial probe with lift-off is studied using a full-wave analysis, and an uncertainty analysis is presented. The field equations for the following terminations are worked out: (1) the sample extends to ∞ in the positive axial direction, (2) the sample is backed by a well-characterized material, and (3) the sample is backed by a short-circuit termination. The equations are valid for both dielectric and magnetic materials. The model allows the study of the open-ended coaxial probe as a nondestructive testing tool. The analysis allows a study of the effects of air gaps on probe measurements. The reflection coefficient and phase are studied as a function of lift-off, coaxial line size, permittivity, permeability, and frequency. Numerical results indicate that the probe is very sensitive to lift-off. For medium to high permittivity values and electrically small probes, gaps on the order of fractions of a millimeter strongly influence the reflection coefficient. In order for the field to penetrate through the air gap, larger size coaxial line or higher frequencies need to be used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included     

Independent Component Analysis–Based Feature Extraction Technique for Defect Classification Applied for Pulsed Eddy Current NDE

With the development of nondestructive detection, the emerging testing techniques provide new challenges to signal analysis and interpretation approach applied to the inspection evaluation. Some researchers have developed the methods that focus on feature analysis of detected signals. This article presents a new feature analysis by the Independent Component Analysis (ICA) approach to evaluate the defects tested by the pulsed eddy current (PEC) technique. ICA is a high-order statistics technique used to separate multi-unknown sources, which has been successfully applied to facial image identification and separation of the components of 1D signal. In this article, the ICA approach is utilized to project the response signals of various defects into the independent components (ICs) feature subspace by signal representation model. Dependent on the selected ICs, each defect is represented by different projected coefficients, which are proposed to discriminate and classify the defects that belong to three categories. The improved ICA model is proposed to improve the classification of two similar categories of single defects: metal loss and subsurface defects. The evaluation using the series of experimental data has validated the classification of single defects and the defects with lift-off effect by our ICA approach. The comparison with Principal component analysis (PCA)–based approach further verified the better performance of the ICA-based model.     

飞机结构腐蚀检测中的脉冲涡流无损检测技术

对脉冲涡流无损检测技术的工作原理进行了分析，建立了脉冲涡流的检测系统对加工的模拟飞机多层结构的试件进行了测试，提取其时域瞬态感应信号的峰值和过零时间作为特征量，对出现在飞机多层结构第二层中的腐蚀缺陷进行了定量检测，试验结果和理论分析相一致，证明了脉冲涡流检测方法的有效性，由于其具有快速和定量化的优点，因而在飞机结构的腐蚀检测中具有广阔的应用前景．     

Accurate probe-response calculation in eddy current NDE by finite element method

A method is presented to evaluate flaw signals in eddy current NDE using the finite element technique. The analysis of the electromagnetic field is based on a three-dimensional finite element scheme that computes directly the electromagnetic field distortions due to defects. This direct field-distortion calculation together with an accurate unflawed field calculation provides accurate total field values in general three-dimensional geometries. The paper shows that the application of the reaction concept and the reciprocity theory allows computations of the probe responses by performing integrals over the flaw region only, even if the analysis is performed by a finite element scheme. Two benchmark problems—a plate with rectangular slot scanned by a differential probe and a tube with axial and circumferential slots scanned by an absolute probe—have been solved to demonstrate the validity and the efficiency of the method. The calculated probe responses show good agreement with the measured trajectories. In order to reach better quantitative agreement, a calibration algorithm that adjusts the parameters of the cylindrical coil model and the lift-off within the range of the geometrical tolerances has been developed.     

Optimized Eddy Current Detection of Small Cracks in Steam Generator Tubing

A complete, computer based design methodology is described, aiming to develop an eddy current sensor with increased sensitivity to flaws, and reduced sensitivity to probe lift-off. The first part of the paper contains an analysis performed in order to establish detailed criteria for an effective design. Numerical investigations have been carried out and their results are discussed, regarding various problems of detectability and lift-off noise level. Based on these results, in the second part two probe arrangements are proposed, and it is shown how their performance parameters could be further improved.     

A New Approach for Restoration of Eddy Current Images

Eddy current images of defects are blurred due to convolution of point spread function of eddy current probe with defects. Disturbing variables such as lift-off, surface roughness, and material property variations influence the eddy current images. In order to restore the length, width, depth, and orientation of surface-breaking defects in the presence of disturbing variables, a new and comprehensive approach has been developed. This approach uses artificial neural network and image processing methods. Studies on austenitic steel plates confirm that through this approach it is possible to restore the spatial information of surface-breaking defects of uniform or slowly varying depth and also to form their accurate three-dimensional pictures. This approach is fast as well as amenable for automation.     

Electromagnetic crack detection in ferritic steel

The detection and sizing of fatigue cracks in ferritic materials, especially in offshore structures, is of major importance. This paper describes an eddy current system to detect such cracks, operating in an offshore environment with a search probe to electronic instrument separation of up to 600 m. An analysis of the various factors influencing the search coil behavior is given in terms of a mutually coupled primary and secondary electrical circuit; the component values of which are influenced by the coil design, induced electrical currents in the material, and any defects it contains. It is further shown that, with proper coil parameters, the phase variation in the phase angle of the coil complex impedance can be kept almost constant compared with the rapid variation occurring during lift-off. This behavior can be exploited in the instrumentation to give a clear vector display indication of the crack presence on less than ideal surfaces.     

The universal impedance diagram of the ferrite pot core eddycurrent transducer

Experimental results indicate that the shapes of normalized impedance diagrams of ferrite pot core eddy current transducers are independent of transducer design parameters, lift-off, and material resistivity. When reduced with respect to coupling coefficient, normalized impedance diagrams of this class of transducers are represented by a universal curve. This curve is functionally described by two relationships. The first is that between reactance and the lift-off angle (angle whose tangent is the ratio of the change in the imaginary component of the impedance to the change in the real component). The second is the linear relationship between the tangent of the lift-off angle and the dimensionless ratio of the mean core radius to the skin depth. The lift-off angle is independent of the coupling coefficient. The independence of the functional relationships from the design variables is significant in that an analytic model of this class of eddy current transducers would be greatly simplified by the omission of these variables     

Multiphysics Modeling and Analysis of the Photoinductive Imaging Effect for Crack Detection

Numerical multiphysics modeling of the photoinductive imaging (PI) effect was performed with a 2-D transient to characterize corner cracks at the edge of a specimen with a bolt hole. We present how the finite-element method (FEM) can be utilized to model the PI effect and observe the influence of critical factors on the coil probe impedance for a rectangular crack in the Ti-6Al-4V specimen. As anticipated, the proposed model can show that the PI method has a higher spatial resolution in the defect in 2-D models compared to the conventional eddy current testing method. The FEM simulation results for 0.25-, 0.50-, and 0.75-mm rectangular notches are shown and discussed. The effects of coil current frequency, laser-point temperature, and lift-off distance on the PI signal are also examined and analyzed. We demonstrate that the PI effect is a novel sensing method for characterizing the geometric shape of cracks and that the enhanced output signals of the coil probe can also be obtained given an appropriate quantity of factors.