Eddy Current Testing Technology Research Articles

Defect identification method for stainless steel welded pipes based on eddy current testing

Abstract A defect identification and classification method for 316 stainless steel welded pipes is proposed based on eddy current testing technology. Initially, this approach acquires eddy current signals from steel pipes, applies Empirical Mode Decomposition (EMD) to derive Intrinsic Mode Functions (IMFs), chooses the principal IMF based on interrelationships, and extracts time-frequency domain characteristic parameters from the selected IMF. To enhance model recognition efficiency, Principal Component Analysis (PCA) is employed to reduce the dimensionality of the feature vector set. Ultimately, a Support Vector Machine (SVM) is utilized to identify and classify weld defects. The results indicate that this method is highly accurate in identifying defects in 316 stainless steel welded pipes.

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.

Coating thickness measurement of multilayer ferromagnetic samples based on pulsed eddy current testing technology

Pulsed eddy current (PEC) technology serves as a popular testing method for multilayer structures and is widely used in coating and substrate thickness measurement. However, in the coating thickness measurement of a multilayer structure, the substrate thickness effect is a disturbance that needs to be eliminated urgently. In order to reduce the substrate thickness effect, in this paper a twice difference normalisation method is proposed to obtain a signal feature independently of the substrate thickness effect for measuring the coating thickness of a multilayer ferromagnetic structure. The simulation and experimental results demonstrate that a fitting line of the peak value of twice difference normalisation signals can be obtained by using the twice difference normalisation method when only the coating thickness changes. The normalisation fitting line is immune to the substrate thickness effect and can be used to measure the coating thickness of a multilayer ferromagnetic structure, which means that the twice difference normalisation method is feasible for high-precision evaluation of the coating thickness of a multilayer ferromagnetic structure when the substrate thickness changes. This study will improve the coating thickness measurement accuracy of multilayer ferromagnetic structures when the substrate thickness changes in the PEC testing.

Study on Remote Field Eddy Current Testing Technology for Crack-like Defects in Long Truss Structure of Aircraft.

Detection of hidden defects of aircraft long truss structures (aluminum alloy) is a challenging problem. The shape of the aircraft truss structure is complex, and the crack defects are buried in a large depth. Without the restriction of skin effect, remote field eddy current (RFEC) has great advantages in detecting buried depth defects. In this paper, in order to detect the hidden defects of the aluminum alloy aircraft long truss structure, the remote field eddy current probe is improved from two aspects of magnetic field enhancement and near-field signal suppression using the finite element method. The results show that indirect coupling energy is greatly enhanced when the connected magnetic circuit is added to the excitation coil. By adding a composite shielding structure outside the excitation coil and the detection coil, respectively, the direct coupling energy is effectively restrained. As a result, the size of the probe is reduced. By optimizing the coil spacing and probe placement position, the detection sensitivity of the probe is improved. The simulation is verified by experiments, and the experimental results are consistent with the simulation conclusions.

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.

Quality reliability analysis of flash welding joint of cold-rolled 50JW800 silicon steel strip

In this study, the quality reliability of the flash butt welding (FBW) joint of a 50JW800 silicon steel strip is analyzed through metallographic observation, cupping tests, and tensile tests. It is found that the recrystallization in some areas of the joint is insufficient, as is the mechanically forced connection, owing to the plastic deformation and uneven heat distribution of the joint during the process of FBW. Insufficient recrystallization is the root cause of strip breakage during cold rolling. The quality reliability of the 50JW800 silicon steel strip FBW joint is also analyzed using eddy current nondestructive testing technology. It is found that the eddy current testing signal has a good response to different structural characteristics of the joint. Therefore, eddy current test technology can be used to ensure the quality reliability of silicon steel strip FBW joints in actual production.

Quantitative analysis of wall thinning of bimetallic clad steel tube based on pulsed eddy current

AbstractBimetallic clad steel tube (BCST) has been widely adopted in onshore and offshore oil and gas transportation due to its excellent corrosion resistance and mechanical properties. Aged BCSTs are susceptible to the initiation of various types of defects, including corrosion, dents, cracking/leakage, and wall‐thinning defects. However, due to the complex structure of BCST and the different materials of inner and outer tubes, it is difficult to implement conventional nondestructive testing. In light of this, a finite element simulation model is established based on the pulsed eddy current testing technology in this paper. Through a series of simulation analyses, the influence of tubing material, defect location, and other factors on the detection results is intensively investigated. By establishing the mapping relationship between the detection signal characteristics and the wall thickness reduction ratio, it is confirmed that there is a quadratic relationship between the differential voltage peak value and the wall thickness reduction, and the defect identification and quantitative evaluation are carried out. The validity of the proposed defect identification and quantitative evaluation method is verified by experiments, which provides a theoretical basis for BCST nondestructive testing and state maintenance. This work facilitates BCSTs structural integrity management and state maintenance decisions, which are critical to the safe operation of the oil and gas 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.

Simulation on Far Field Eddy Current Detection on Condenser Tube Groups in Fossil Power Plants (I)

Condenser tube tubes are widely applied in condenser in thermal power plants. Leakage often occurs in condenser tube groups due to the quality of condenser media or high serving stress. Condenser tube leakage not only brings economic losses to power stations, but even influences the safety operation of the power generation units. This paper simulates and experimentally studies the ferromagnetic heat exchanger tubes based on the detection mechanism of far field eddy current. The results show that the position of the far field is with the increase of the wall thickness and the inner diameter of the tubes, and the detection sensitivity of the far field is high for the same size defect, but the difference of the magnetic flux intensity varies equally when the defects with the same size are located at the inner and outer walls. This is consistent with the experimental results of real tube defect detection and meets the principle of far field eddy current detection. It is concluded that far field eddy current testing technology can better determine and identify ferromagnetic tube defects, and has a good application prospect for online nondestructive testing and safety performance evaluation of service tubes

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.

Classification and Quantitative Evaluation of Eddy Current Based on Kernel-PCA and ELM for Defects in Metal Component

Eddy current testing technology is widely used in the defect detection of metal components and the integrity evaluation of critical components. However, at present, the evaluation and analysis of defect signals are still mostly based on artificial evaluation. Therefore, the evaluation of defects is often subjectively affected by human factors, which may lead to a lack in objectivity, accuracy, and reliability. In this paper, the feature extraction of non-linear signals is carried out. First, using the kernel-based principal component analysis (KPCA) algorithm. Secondly, based on the feature vectors of defects, the classification of an extreme learning machine (ELM) for different defects is studied. Compared with traditional classifiers, such as artificial neural network (ANN) and support vector machine (SVM), the accuracy and rapidity of ELM are more advantageous. Based on the accurate classification of defects, the linear least-squares fitting is used to further quantitatively evaluate the defects. Finally, the experimental results have verified the effectiveness of the proposed method, which involves automatic defect classification and quantitative analysis.

Estimation of stress distribution in ferromagnetic tensile specimens using low cost eddy current stress measurement system and BP neural network.

Estimation of the stress distribution in ferromagnetic components is very important for evaluating the working status of mechanical equipment and implementing preventive maintenance. Eddy current testing technology is a promising method in this field because of its advantages of safety, no need of coupling agent, etc. In order to reduce the cost of eddy current stress measurement system, and obtain the stress distribution in ferromagnetic materials without scanning, a low cost eddy current stress measurement system based on Archimedes spiral planar coil was established, and a method based on BP neural network to obtain the stress distribution using the stress of several discrete test points was proposed. To verify the performance of the developed test system and the validity of the proposed method, experiment was implemented using structural steel (Q235) specimens. Standard curves of sensors at each test point were achieved, the calibrated data were used to establish the BP neural network model for approximating the stress variation on the specimen surface, and the stress distribution curve of the specimen was obtained by interpolating with the established model. The results show that there is a good linear relationship between the change of signal modulus and the stress in most elastic range of the specimen, and the established system can detect the change in stress with a theoretical average sensitivity of -0.4228 mV/MPa. The obtained stress distribution curve is well consonant with the theoretical analysis result. At last, possible causes and improving methods of problems appeared in the results were discussed. This research has important significance for reducing the cost of eddy current stress measurement system, and advancing the engineering application of eddy current stress testing.

Research and Application of Pulsed Eddy Current Testing Technology for Pressure Equipment

Research and Application of Pulsed Eddy Current Testing Technology for Pressure Equipment

A Thickness Nondestructive Evaluation Strategy for Coating Located in Truncated Conical Steel Shells of Various Sizes Based on Eddy Current Testing Technology

The anti-corrosive coating in oil tanker is a crucial part. Its thickness needs a robust non-destructive evaluation (NDE) tool to estimate. To get satisfied results from eddy current signals based on a limited number of samples, the influence of test-piece sizes on evaluation precision of liftoff is analyzed by using a FEM–BEM hybrid program, and a NDE strategy based on sample database is proposed. Its applicability is verified by simulation results.

プレーナ形渦電流探傷プローブの欠陥検出特性

This paper describes the detective characteristics of a planar-type eddy-current testing (ECT) probe for use in advanced ECT technology. The probe has a sandwich structure of meander and mesh coils. The key operation depends on the patterns of both meander and mesh coils for high-frequency excitation and sensing. The particular functions of the probe are the detection of (1)crack size and (2) its direction on the conducting plate. Experimental results confirm that the probe has detective characteristics, and sufficient sensitivity to detect cracks.