Pulsed Eddy Current Research Articles

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.

Traditional Eddy Current⁻Pulsed Eddy Current Fusion Diagnostic Technique for Multiple Micro-Cracks in Metals.

Due to a harsh working environment, micro-cracks in metal structures (e.g., airplane, oil/gas pipeline, hydro-turbine) often lead to serious accidents, so health monitoring of the metals is of great significance to ensure their safe operation. However, it is hard to perform quantitative detection of multiple micro-cracks by a single nondestructive testing (NDT) technique because of their limits. To monitor for multiple micro-cracks in metals, a Traditional Eddy Current (TEC) and Pulsed Eddy Current (PEC) fusion NDT technique is proposed in this paper. In the proposed technique, the TEC technique is adopted to seek the locations of the micro-cracks in the whole of the metal, while the PEC technique is adopted to acquire information on the depth of micro-cracks automatically according to the location information by the TEC. The experiments indicate that the TEC–PEC fusion NDT system can localize the micro-cracks as well as detect the micro-cracks quantitatively and automatically; therefore, it can be applied in structural health monitoring of metal equipment or in picking candidate components in re-manufacturing engineering.

Study on Detection Methods of Residual Wall Thickness for Process Pipeline Corrosion in Oil and Gas Gathering and Transportation Station

In this paper, it is proposed to adopt Pulsed Eddy Current (PEC) testing of coaxial double coil structure to implement the detection on residual wall thickness of pipeline corrosion in oil and gas gathering and transportation station, in the view of reduction of wall thickness caused by corrosion. The electromagnetic field theory is applied to conduct theoretical analysis on PEC detection. On such basis, a theoretical model of coaxial double coil structure for PEC detection is built, and the relation of voltage phase of the detected signal varying with the wall thickness of the pipeline is derived. Then a method for evaluating the residual wall thickness of pipelines based on the phase trough time of PEC signals is proposed. Moreover, the parameters of the PEC probe are optimized by finite element method, and the detection system is designed accordingly. As a result, the practicability of the detection system and the correctness of the theoretical model are verified by experiments, proved that the model can realize the detection of the residua wall thickness under the condition of different media transported by the pipeline.

A new approach to increase the subsurface flaw detection capability of pulsed eddy current technique

In this paper, the detection of subsurface flaws by pulsed eddy current (PEC) technique has been studied. Experiments have been carried out on an 8.0 mm thick stainless steel plate with subsurface flaws located at different depths below the surface by varying the excitation rise time (Er). The conventional PEC signal parameters like peak amplitude (Vp), time-to-peak (Tp) and rise time (tr) are influenced by noise and could not be effectively used to detect flaws located beyond 4.0 mm below surface. In this context, a new approach based on fitting the PEC response pulse to a modified inductor current equation has been proposed. Based on this approach, two parameters viz. time constant (τ) and voltage parameter (V1/V0) have been derived and used for detection and separation of flaws. The study shows that the two parameters derived from the proposed approach can be used to detect flaws located 6.0 mm below the surface with improved sensitivity. The advantage of the proposed approach is that, it does not require subtraction from reference signal to detect flaws. The derived parameters are also able to differentiate the subsurface and surface flaws.

Wall Thinning Characterization of Composite Reinforced Steel Tube Using Frequency-Domain PEC Technique and Neural Networks

Resin fiber composites reinforcement is used to recover the original mechanical properties of steel tubes subjected to corrosion wall thinning. Pulsed Eddy Current (PEC) technique can perform nondestructive evaluation of this kind of component, due to its capability to penetrate nonmagnetic insulation. Despite the evaluation capability, distinguishing inner surface from outer surface defects is not an easy task for time-domain PEC technique. In this paper, Fast Fourier transform (FFT) in combination with multilayer perceptron (MLP) neural network classifiers are applied to PEC signals and used to detect defects (wall thinning) and also to indicate their position. The tested sample is a carbon steel tube, with 17 mm of composite reinforcement, where two defects were manufactured, one at the inner and another at the outer surface. An automated scanner system is used to obtain C-scan maps, showing the thinning areas. Two feature extraction methods are used to produce the input features for the neural network classifier: the coefficients of the FFT; and the parameters of an exponential curve fitted to the FFT coefficients. The results indicate that the MLP neural network correctly recognized the presence of wall thinning and its location with detection efficiencies of 97.4 and 97.0%, respectively. The PEC technique analysis in frequency-domain associated with a neural network classifier seems to be a promising alternative to identify the position of defects in composite reinforced steel tubes.

A Temperature Drift Compensation Method for Pulsed Eddy Current Technology.

Pulsed eddy current (PEC) technology is another important non-contact nondestructive testing technology for defect detection. However, the temperature drift of the exciting coil has a considerable influence on the precision of PEC testing. The objective of this study is to investigate the temperature drift effect and reduce its impact. The temperature drift effect is analyzed theoretically and experimentally. The temperature drift effect on the peak-to-peak values of the output signal is investigated, and a temperature compensation method is proposed to reduce the effect of temperature variation. The results show that temperature drift has a negative impact on PEC testing and the temperature compensation method can effectively reduce the effect of temperature drift.

Lift-Off Point of Intersection in Spectral Pulsed Eddy Current Signals for Thickness Measurement

Lift-off invariant strategies play an important role in pulsed eddy current (PEC) testing. As an effective signal feature immune to lift-off effect, lift-off point of intersection (LOI) has been applied to determine thickness or evaluate defects. Typically, an LOI feature is extracted from time domain PEC signals. In this article, we observed a novel LOI feature in real and imaginary part of frequency domain PEC responses via simulation and experiment. An analytical model is formulated to investigate the novel LOI feature. It is found that the proposed LOI feature varies with sample thickness increasing, which indicates that the LOI feature from spectral PEC signals could be used for thickness measurement.

Pulsed Eddy-Current Detection of Loose Parts in Steam Generators

Flow-induced vibration of loose parts located on steam generator (SG) tube sheets can result in SG tube damage and eventual loss of SG efficiency. Regular inspection of SG tubes at support structures can help extend SG life by detecting and monitoring flaws as well as identifying accumulation of loose parts. Application of conventional eddy-current technologies in this case, however, is limited when magnetite fouling is present or when loose parts are resting directly on the SG tube sheet. Pulsed eddy current (PEC) combined with a modified principal components analysis (MPCA) was investigated as a method of detecting and identifying a variety of loose part materials in close proximity to 15.9-mm outer diameter Alloy-800 SG tubes and surrounding steel support structures. Loose parts were found to be detectable for varying axial positions along the SG tube and when in contact with the tube sheet. When offset radially from the SG tube, 1.6-mm-diameter carbon steel parts with a strong magnetic component allowed detection up to 3.2 mm away, while nonferromagnetic stainless steel parts of the same size could only be detected up to 1.5 mm away. Signal variation in the presence of loose parts was attributed to differences in amplitude of transient response and in relaxation times for diffusion of transient magnetic fields through various materials. The ability to accurately detect and identify loose parts located in close proximity to SG tubes establishes PEC combined with MPCA as a potential candidate for SG inspection.

The metal thickness detection using pulsed eddy-current computation and detection method

Through detecting method of pulsed eddy-current (PEC) and according to thickness testing model of multi-layer metal, infer the mathematical model of eddy current testing method in single-layer metal. Use Maxwell field equations to find solutions of vector magnetic potential in its boundary conditions thus infer the impedance of hollow coil placed right above metal. Analyze the variation relationship between induced current and metal thickness in theory level. (1) Obtain three characteristic signals of induced current: altitude of current peak value (ACPV), time of reaching current peak (TRCP) and time of traversing zero point (TTZP). The thickness of the metal and the distance between the metal and coil were obtained. (2) Introduce the concept of the lift-off intersection point (LOI) and puts forward a new method of metal thickness detection based on the LOI. By analyzing the results of the experiment, it confirms the feasibility of the LOI of measurement in the metal thickness.

Lift-off nulling and internal state inspection of multi-layer conductive structures by combined signal features in pulsed eddy current testing

Pulsed eddy current (PEC) testing, as an emerging technique of eddy current testing, has been used for detection and analysis of internal states of multi-layer conductive structures. However, the lift-off noise, caused by irregular sample surface, varying coating thicknesses or movement of probes, may seriously influence the accuracy. Therefore, in order to improve the detection accuracy under lift-off noise, an algorithm is proposed. Firstly, the response signals with lift-off noise are analysed theoretically and experimentally; secondly, according to analysis of the multi-collinearity between the lift-off and the response signal, a model to predict the lift-off is constructed by Partial Least Square (PLS); then, based on the prediction value of the lift-off by the PLS model, an approach to detect the internal states of multi-layer conductive structures is proposed. In this approach, the internal states are determined by the classification model, which is constructed by Support Vector Machine (SVM) based on combined features. The combined signal features are composed of the lift-off feature and principal component features. Finally, a bimetallic thermostat is taken as our experimental subject to validate the proposed method. The experimental results indicate that the proposed method can reduce the effect of lift-off fluctuations and improve detection accuracy.

Characterization of pulsed eddy current signals to discriminate cladding change over wall thinning of ferromagnetic pipes

Pulsed eddy current (PEC) testing technique is regarded as the most effective nondestructive evaluation method used to measure wall thinning through multiple layer and irregular surface objects. When the test specimen is covered with insulation and shielded with thin aluminum, PEC technique became advantageous to completely illustrate structural variations. However, the interpretation of PEC signal seems complex when the specimen is insulated and externally cladded with aluminum sheet, since variations in aluminum thickness and wall thinning results into similar effect on PEC signal. In this study, the focal point is to detect the pipe wall-thinning through insulation and aluminum cladding, another important objective is to characterize the cladding and test specimen thickness change.

Pulsed Eddy Current Nondestructive Testing for Defect Evaluation and Imaging of Automotive Lightweight Alloy Materials

Rapid and accurate damage detection of magnesium-aluminum alloy, which is an important material for automotive lightweight, is of great significance. Pulsed eddy current (PEC) is an effective electromagnetic nondestructive testing and evaluation (NDT&E) technique for metal materials. Metal loss evaluation and imaging are one of the most important steps in quality control and maintenance of key components of automobiles. A PEC method based on a rectangular excitation coil and an axial parallel pickup coil is proposed and investigated for the purpose of metal loss evaluation and imaging. Metal loss type of defects with different sections is designed and detected using line scanning technique and C-scan imaging in two scanning directions. Experimental results have illustrated that metal loss depth can be estimated effectively by the peak amplitude of PEC A-scan response. Then, the quantification information of metal loss depth is preliminarily obtained based on the linear fitting equation. Consequently, metal loss evaluation is realized by line scanning peak waves and C-scan pseudo 3D images. At last, the sensitivity comparison shows that the metal loss can be detected in both directions. The proposed method is an effective approach to evaluate the image surface-breaking metal loss in automotive lightweight alloy materials.

Detection of tension force reduction in a post-tensioning tendon using pulsed-eddy-current measurement

Post-tensioning (PT) tendons are commonly used for the assembly of modularized concrete members, and tension is applied to the tendons during construction to facilitate the integrated behavior of the members. However, the tension in a PT tendon decreases over time due to steel corrosion and concrete creep, and consequently, the stress on the anchor head that secures the PT tendon also diminishes. This study proposes an automatic detection system to identify tension reduction in a PT tendon using pulsed-eddy-current (PEC) measurement. An eddy-current sensor is installed on the surface of the steel anchor head. The sensor creates a pulsed excitation to the driving coil and measures the resulting PEC response using the pick-up coil. The basic premise is that the tension reduction of a PT tendon results in stress reduction on the anchor head surface and a change in the PEC intensity measured by the pick-up coil. Thus, PEC measurement is used to detect the reduction of the anchor head stress and consequently the reduction of the PT tendon force below a certain threshold value. The advantages of the proposed PEC-based tension-reduction-detection (PTRD) system are (1) a low-cost (< $ 30), low-power (< 2 Watts) sensor, (2) a short inspection time (< 10 seconds), (3) high reliability and (4) the potential for embedded sensing. A 3.3 m long full-scale monostrand PT tendon was used to evaluate the performance of the proposed PTRD system. The PT tendon was tensioned to 180 kN using a custom universal tensile machine, and the tension was decreased to 0 kN at 20 kN intervals. At each tension, the PEC responses were measured, and tension reduction was successfully detected.

Damage detection of cracks in carbon fibre reinforced composites by pulsed eddy-current testing

Non-destructive evaluation techniques that are based on electromagnetic methods are commonly used for inspection of metallic and carbon fibre reinforced plastics parts. Some of these inspection methods are based on eddycurrents which are generated by the electromagnetic induction phenomenon occurring under variable magnetic induction field. In eddy-currents testing, the depth of penetration into the material is controlled by the conductivity of the tested material and also the work frequency. Transient eddy-currents, also called pulsed eddy-currents, is an emerging non destructive technique that employs a pulsed excitation to induce a transient electromagnetic response from defects lying deep within a conducting structure. Such defects are difficult to inspect by conventional techniques, such as harmonic eddy-currents or ultrasonics. A large number of recent scientific publications have dealt with the theoretical understanding of the pulsed eddy-currents phenomenon and have also undertaken the design feature of appropriate probes. Finite element solution of the governing equations has been used to simulate the output signals as function of the input electrical excitation signal. Considering a B-scan strategy, simulation of a pulsed eddy-currents based probe is performed in this work with the objective to assess detectability of small defects through monitoring impedance changes of a detection probe.

Through coating imaging and nondestructive visualization evaluation of early marine corrosion using electromagnetic induction thermography

Early marine corrosion especially under coating is difficult to be detected, sized and predicted. This work proposed through coating imaging (TCI) based on high frequency electromagnetic induction, thermal wave propagation and infrared temperature field measurement. The proposed method is called electromagnetic induction pulsed phase thermography (EMIPPT), whose advantages include being nondestructive, non-contact, fast speed, large-area, high resolution and visualization. By phase analysis, the complex influence of parameter variation on temperature can be eliminated and then relative variation of corrosion height can be characterized. The mild steel (S275) samples providing coated real marine atmosphere corrosion with exposure from 1, 3 and 6 months were tested. The experimental results illustrate that EMIPPT has greater potential for in-service corrosion detection, sizing and monitoring than laser profilometry, pulsed eddy current, and microwave waveguide.

Effects of Coil Diameter in Thickness Measurement Using Pulsed Eddy Current Non-destructive Testing

Non-destructive testing (NDT) techniques are used in industry to evaluate the properties of a material, component or structure without causing any permanent damage. Among the techniques, pulsed eddy current (PEC) NDT is regarded as a new technique where a broadband pulse excitation is used, as opposed to single frequencies employed in conventional eddy current NDT. In this study, a 2D axisymmetric electromagnetic model of a PEC probe has been developed and it has been used to study the effects of the excitation coil diameter on the performance of PEC probes in sample thickness measurement. A PEC system has also been built to validate the model. Aluminium plates are used as the sample and they can be stacked up to replicate thickness from 1 mm to 10 mm. The results show that there is a very good correlation between the simulation and experimental results, with an average error of less than 10%. The results also suggest that the larger the diameter of the excitation coil, the deeper the penetration and therefore the larger the thickness measurement range. It has also been shown that although the larger diameters have deeper penetration, the smallest diameter has the highest sensitivity if normalization is not used. These conclusions indicate that coil diameter is an important parameter in a PEC probe design for thickness measurement applications.

Pulsed Eddy Current Sensing for Critical Pipe Condition Assessment.

Pulsed Eddy Current (PEC) sensing is used for Non-Destructive Evaluation (NDE) of the structural integrity of metallic structures in the aircraft, railway, oil and gas sectors. Urban water utilities also have extensive large ferromagnetic structures in the form of critical pressure pipe systems made of grey cast iron, ductile cast iron and mild steel. The associated material properties render NDE of these pipes by means of electromagnetic sensing a necessity. In recent years PEC sensing has established itself as a state-of-the-art NDE technique in the critical water pipe sector. This paper presents advancements to PEC inspection in view of the specific information demanded from water utilities along with the challenges encountered in this sector. Operating principles of the sensor architecture suitable for application on critical pipes are presented with the associated sensor design and calibration strategy. A Gaussian process-based approach is applied to model a functional relationship between a PEC signal feature and critical pipe wall thickness. A case study demonstrates the sensor’s behaviour on a grey cast iron pipe and discusses the implications of the observed results and challenges relating to this application.

ENSURE THE INTEGRITY OF OFFSHORE RISERS AT THE SPLASH ZONE AREA &amp; OPTIMIZE THEIR EXTERNAL INSPECTION REGIME

At present time with oil prices continuing to fall, most operators work hard to enforce the cost reduction philosophy and the synergy operations in their maintenance regimes to reduce their daily operational expense (OPEX), This starts from decreasing the gap between the maintenance costs and the applicability of this in the industry this led to a rapid development in the unconventional technologies and produced new advanced inspection technology. In light of the above, the use of the traditionally inspection techniques applied at the splash zone area such as general visual inspection (GVI) and local thickness measurement (UTM) are not practical as they require the removals of the marine growth, armwrap, concrete coating and in some cases the marine painting system. This require a massive preparations activities prior completion till reach the final acceptable conditions and risers reinstatements are reached. Some of the inspection areas where most of the current developments in offshore risers are being seen are in splash zone area that poses some of the biggest challenges to smooth operation in harsh environment. Where the corrosion at splash zones of offshore risers can be severe reading corrosion rates up to 1mm/Year, due to the lack of effectiveness of cathodic protection and coating damages caused either by disbandment or object impact. So it’s highly recommended that this zone have a specific inspection plan combined with a special inspection program for preventing the occurrence of failure as well as following up the evaluation of any failure mechanism which eventually might be present. This paper highlights the challenges to overcome these maintenance costs related to the periodical external inspection of insulated offshore risers at splash zone area. Also the objective of this paper is optimizing the offshore riser’s inspection regimes via evaluating the effectiveness of the unconventional non-destructive testing (NDT) methods for in-service inspection of insulated offshore risers at the splash zone area. The assessment will be completed using two unconventional advanced inspection techniques these are the Ultrasonic Guided Wave Technique (wavemaker GW) and the Pulsed Eddy Current – (PEC). Subsea PEC and GW have been already achieved with considerable success and encouraging results during inspections carried out in the Mediterranean Sea, This technology allows complete mapping of the pipeline corrosion status without production interruptions since no destruction or pre-treatment of the protective coatings is required. The state of the art of these nonintrusive technological solutions for inspecting the corroded areas of splash zones of offshore risers using the two inspection techniques (PEC & GW) are demonstrated through an actual case study. Keywords— offshore riser, splash zone, External corrosion, Pulsed eddy current (PEC),Guided wave (GW),Armawrapm, General visual inspection (GVI).

Pulsed eddy current signal analysis of ferrous and non-ferrous metals under thermal and corrosion solicitations

Most metals endure in contact with ambient air, liquids or other metals the phenomena of oxidation. The kinetics of the corrosion can be significant and destroy the metal. The marine corrosion or soil affects many materials, more than ever if it touches the pipeline, ship hulls or buried pipes. The investigation of NDT methods to predict corrosion behavior can be a rational solution to prevent from this kind of industrials problems.Eddy current and pulsed eddy current (PEC) are proposed as a powerful Nondestructive Testing and Evaluation (NDT&E) technique. The application of this kind of technique in the industrial domain is used in detection of the defects, particularly in the determination of corrosions information contained in the received signal gives the possibility allow to use the PEC in the Non Destructive Evaluation of materials that can be analysed. The work in this papers explain the behaviour of the eddy current, pulsed Eddy Currents and various electromagnetic parameters in the analysed samples. We have showed in this work that all microstructure modifications of the samples were detected and can be quantified by Pulsed eddy current measurements. Modifications of the microstructure obtained by air cooled, quenched and corroded samples in aluminium, heat treatment changement in mild steel were evaluated by PEC.

LOI-Based Pulsed Eddy Current Evaluation of Hidden Material Degradation in Coated Nonmagnetic Conductors

Abstract Coated nonmagnetic conductors are subject to material degradation hidden under nonmetallic coating. It is formidable to evaluate the degradation degree and thickness of degradation region without removing the non-conductive coating. In light of this, in this paper the feasibility of pulsed eddy current (PEC) probes for nondestructive assessment of hidden material degradation in coated nonmagnetic conductors was investigated. The characteristics of lift-off intersection (LOI) of signals from PEC probes were utilized in a bid to cancel out the influence of coating thickness on signals. A fast inverse scheme based on LOI was proposed, along with the probe to implement simultaneous evaluation of the degradation degree and thickness. A series of experiments revealed that the proposed probe is high-efficiency and capable of assessing hidden material degradation without much loss of accuracy.