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•A funnel-shaped probe of pulse-modulation eddy current technique is investigated.•Closed-form expressions of field quantities and testing signals are formulated.•Sensitivity of the ...funnel-shaped probe to subsurface flaws is analyzed.•Advantage of the funnel-shaped probe in evaluation of subsurface defects is affirmed.
The subsurface flaw has been taken as one of the most severe threats to integrity and safety of conductive structures, leading to structural failure and catastrophic accidents. The nonintrusive detection and evaluation of subsurface flaws via effective Non-destructive Evaluation (NDE) techniques are the pressing concern, which essentially relies on the high testing sensitivity to a subsurface flaw. Following the proposition of Pulse-modulation Eddy Current technique (PMEC) for high-sensitivity evaluation and imaging of subsurface defects in conductors with pancake-shaped probes, in this paper the funnel-shaped PMEC probe is proposed in a bid to further enhance the testing sensitivity to the subsurface flaw by geometrically extending the cross-section of the excitation coil from rectangle to parallelogram whilst keeping the inner and outer diameters of the bottom winding fixed. Closed-form expressions of field quantities and testing signals for the proposed probe are formulated, based on which the theoretical investigation of the probe and its comparison with the pancake-shaped probe is intensively conducted. In parallel, a series of experiments for evaluation and imaging of localised subsurface corrosion via PMEC are carried out together with intensive assessment regarding testing sensitivities of the funnel-shaped and pancake-shaped probes. Through theoretical simulations and experiments, it has been revealed that the proposed funnel-shaped probe is advantageous over the pre-designed pancake-shaped probe, giving higher sensitivity to hidden flaws in conductors. This is beneficial to detection, evaluation and imaging of subsurface defects in conductive structures.
A corrosive environment leaves in-service conductive structures prone to subsurface corrosion which poses a severe threat to the structural integrity. It is indispensable to detect and quantitatively ...evaluate subsurface corrosion via non-destructive evaluation techniques. Although the gradient-field pulsed eddy current technique (GPEC) has been found to be superior in the evaluation of corrosion in conductors, it suffers from a technical drawback resulting from the non-uniform field excited by the conventional pancake coil. In light of this, a new GPEC probe with uniform field excitation for the imaging of subsurface corrosion is proposed in this paper. The excited uniform field makes the GPEC signal correspond only to the field perturbation due to the presence of subsurface corrosion, which benefits the corrosion profiling and sizing. A 3D analytical model of GPEC is established to analyze the characteristics of the uniform field induced within a conductor. Following this, experiments regarding the imaging of subsurface corrosion via GPEC have been carried out. It has been found from the results that the proposed GPEC probe with uniform field excitation not only applies to the imaging of subsurface corrosion in conductive structures, but provides high-sensitivity imaging results regarding the corrosion profile and opening size.
The cladded conductor is broadly utilized in engineering fields, such as aerospace, energy, and petrochemical; however, it is vulnerable to thickness loss occurring in the clad layer and ...nonconductive protection coating due to abrasive and corrosive environments. Such a flaw severely undermines the integrity and safety of the mechanical structures. Therefore, evaluating the thickness loss hidden inside cladded conductors via reliable nondestructive evaluation techniques is imperative. This paper intensively investigates the pulse-modulation eddy current technique (PMEC) for the assessment of thickness loss in a cladded conductor. An analytical model of the ferrite-cored probe is established for analyzing PMEC signals and characteristics of lift-off intersection (LOI) in testing signals. Experiments are conducted for evaluation of the thickness loss in cladded conductors. An inverse scheme based on LOI for estimation of the thickness-loss depth is proposed and further verified. Through simulations and experiments, it is found that the influences of the thickness loss in the clad layer and protective coating on the PMEC signals can be decoupled in virtue of the LOI characteristics. Based on LOI, the hidden thickness loss can be efficiently evaluated without much of a reduction in accuracy by using the PMEC probe for dedicated inspection of the cladded conductor.
Thermal Barrier Coating (TBC), as the insulation between high-temperature gas and the alloying bodies of gas turbine blades, is adopted for protection and long-term service of gas turbines. It is ...imperative to non-destructively evaluate TBC of blades, since the TBC with thin thickness or delamination will lead to catastrophic accidents of gas turbines. In this paper, Eddy Current inspection (EC) is proposed for quantitative evaluation of TBC, which involves estimation of: (1) thickness of the top coating to identify possible thinning and (2) thickness and conductivity of bond coating to detect delamination and degradation. In an effort to implement the fast inversion, the closed-form expressions of the Jacobian matrix are derived based on Truncated Region Eigenfunction Expansion (TREE) modeling. The Levenberg–Marquardt (LM) algorithm is also adopted in the inversion for the parameters. The proposed inverse algorithm is verified by hybrid numerical modeling and experiments, which indicates that the quantitative evaluation of TBC by using EC in conjunction with the proposed inverse algorithm is valid and applicable to the engineering application.
► LM-based inversion for retrieving the parameters of TBC has been realized. ► Closed-form expressions of the Jacobian matrix in inversion have been formulated. ► Proposed inversion was proved efficient and applicable.
Corrosion is a central problem for the oil and gas and nuclear industries, affecting pipelines and nuclear waste storage tanks. Reliable methods for timely corrosion detection and integrity ...evaluation are needed for a range of industrial structures where corrosion under insulation manifests as wall loss. Commercially available detection systems cannot be used in harsh nuclear-storage environments. Moreover, these systems are not reliable for detecting the inner wall thickness of industrial structures made of multiple layers of material, including insulation layers. A novel nondestructive testing method based on low-frequency eddy-current induction was developed and tested on a multilayered steel structure, including an insulating gap. The method enables through-wall thickness measurements between 4 and 16 mm, with a measurement error of 1 mm. The technique is quantitative, nondestructive, and allows the sensor coil to be positioned on the outside of the composite steel structure, which is useful when the interior of an industrial container or pipeline is not accessible.
Structures of nonmagnetic materials are broadly used in engineering fields such as aerospace, energy, etc. Due to corrosive and hostile environments, they are vulnerable to the Subsurface Pitting ...Corrosion (SPC) leading to structural failure. Therefore, it is imperative to conduct periodical inspection and comprehensive evaluation of SPC using reliable nondestructive evaluation techniques. Extended from the conventional Pulsed eddy current method (PEC), Gradient-field Pulsed Eddy Current technique (GPEC) has been proposed and found to be advantageous over PEC in terms of enhanced inspection sensitivity and accuracy in evaluation and imaging of subsurface defects in nonmagnetic conductors. In this paper two GPEC probes for uniform field excitation are intensively analyzed and compared. Their capabilities in SPC evaluation and imaging are explored through simulations and experiments. The optimal position for deployment of the magnetic field sensor is determined by scrutinizing the field uniformity and inspection sensitivity to SPC based on finite element simulations. After the optimal probe structure is chosen, quantitative evaluation and imaging of SPC are investigated. Signal/image processing algorithms for SPC evaluation are proposed. Through simulations and experiments, it has been found that the T-shaped probe together with the proposed processing algorithms is advantageous and preferable for profile recognition and depth evaluation of SPC.
An unsupervised approach to classify surface defects in wire rod manufacturing is developed in this paper. The defects are extracted from an eddy current signal and classified using a clustering ...technique that uses the dynamic time warping distance as the dissimilarity measure. The new approach has been successfully tested using industrial data. It is shown that it outperforms other classification alternatives, such as the modified Fourier descriptors.
Coated conductive structures play an important role in such industrial fields as aerospace, energy, chemical engineering, etc. Because of the corrosive and hostile environments, the corrosion which ...is normally hidden under the conductor coating, occurs in the surface of the in-service structure. It poses a severe threat to the integrity and safety of the structure. Therefore, it is imperative to effectively detect and quantitatively evaluate the hidden corrosion via Non-destructive Evaluation (NDE) techniques. Pulse-modulation Eddy Current technique (PMEC) has been identified to be superior to other electromagnetic NDE methods, particularly regarding evaluation and imaging of corrosion. Whereas, the acquired corrosion images hardly indicate the true opening profile of the corrosion. This hinders the quantitative evaluation of the corrosion. In light of this, in this paper PMEC along with the image processing technique for imaging and evaluation of corrosion in coated conductors is intensively investigated. The image processing technique is proposed based on Theta Map (TM) which is a processing method for quantifying the source parameters of magnetic data that describe the anomalous structure. The validity of PMEC combined with modified TM for profile recognition and assessment of hidden corrosion in nonmagnetic structures is identified via experiments.
Efficient algorithms for pre-processing of eddy-current signals that are typical when inspecting the condition of metal in the heat-exchanger pipes of steam generators of NPPs with VVÉR-440 and ...VVÉR-1000 reactor facilities are described. Flaws in these pipes can be reliably detected and classified, with their geometrical parameters evaluated, if one can tune out from multiple interfering factors and disturbances that affect the signal and emerge due to a number of structural irregularities in the test object (geometrical noise (pilger noise), spacer and antivibration grids, bends), specific detector-design features, and noises in the measuring tract of eddy-current apparatus. The tuning-out algorithms have been tested with experimental signals and incorporated in the PIRATE (Program for Identification & Recognition of defects through signal Analysis in Testing by Eddy currents) software intended for analyzing multifrequency signals.
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