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Eddy current sensors can be used to test the characteristics and measure the parameters of the conductive samples. As the main obstacle of the multifrequency eddy current (MEC) sensor, the liftoff distance affects the effectiveness and accuracy of the measurement. In this article, a material-independent algorithm has been proposed for the restoration of the liftoff distance when using the MEC sensor, which is based on the approximation under the thin-skin effect. Experiment testing on the performance of the proposed method is presented. The results show that from the dual-frequency inductance, the liftoff distance could be restored with a maximum error of 0.24 mm for the distance up to 12 mm. Besides, the derived liftoff distance is used for the inversion of the magnetic permeability. Based on a liftoff insensitive inductance (LII) feature, the magnetic permeability of steels can be inversed in an iterative manner, with an error of less than 0.6 % for the liftoff distance up to 12 mm. [ABSTRACT FROM AUTHOR]

Detecting defects in metal structures quickly and robustly is critical to maintaining safe operation in industry. This work discloses an eddy current (EC) testing probe with coils carrying three-phase currents as excitation and integrated array tunnel magnetoresistance (TMR) sensors measuring the magnetic field as receiver. The three-phase currents in the planar layout coils induce EC that migrates electrically in the material under test. This excitation method does not require multiplexing of the coils and is sensitive to defects of any orientations. The TMR array contains 64 sensors, which are microfabricated and packaged in a line. Within single probe pass, the sensors generate magnetic field image with high sensitivity ($\text{1.99}\;\text{nT}/\sqrt {\text{Hz}} @\text{30}\;\text{kHz}$) and fine spatial resolution (0.5 mm pitch). By analyzing the output image, defects are identified and localized. The operating principle of the probe was investigated based on a finite-element method model. A prototype probe was developed and tested, with which machined defects in aluminum samples were inspected. Experimental results show that the probe has comparable sensitivity for horizontal and vertical defects, and it can recognize a small defect with length × width × depth = 1 × 0.2 × 1 mm3. The defects are located within 1 mm with an artificial neural network. [ABSTRACT FROM AUTHOR]

Eddy current testing (ET) and eddy current thermography (ECT) are both important nondestructive testing methods that have been widely used in the field of conductive materials evaluation. Conventional ECT systems have often employed to test static specimens even though they are inefficient when the specimen is large. In addition, the requirement of high-power excitation sources tends to result in bulky detection systems. To mitigate these problems, in this article, a moving detection mode of multiphysics structured ET and ECT is proposed in which a novel L-shape ferrite magnetic yoke circumambulated with array coils is designed. The theoretical derivation model of the proposed method is developed which is shown to improve the detection efficiency without compromising the excitation current by ECT. The specimens can be speedily evaluated by scanning at a speed of 50–250 mm/s while reducing the power of the excitation current due to the supplement of ET. The unique design of the excitation-receiving structure has also enhanced the detectability of omnidirectional cracks. Moreover, it does not block the normal direction visual capture of the specimens. Both numerical simulations and experimental studies on different defects have been carried out and the obtained results have shown the reliability and detection efficiency of the proposed system. [ABSTRACT FROM AUTHOR]

Nondestructive testing (NDT) for ferromagnetic material under motion condition is a challenging topic, because the distribution of the magnetic fields inside the ferromagnetic material is more complicated due to motion-induced eddy current (MIEC) and ferromagnetic material magnetization. Therefore, a fast, accurate, and automatic NDT technique for quantitative detection of crack in the moving ferromagnetic material is an urgent issue. Currently, the pulsed eddy current (PEC) technique is widely applied in quantitative crack characterization in metals; however, the literature focus on the investigation of the velocity influence on PEC is rare. This article performs a deep investigation on the velocity effect of PEC technique by numerical simulation and experiments, and the relationships of speed and PEC signal are discovered. The results show that the velocity effect can change the baseline value of the detection signal in the PEC detection system. Moreover, the crack location, width, and depth can be characterized by PEC with high speed when crack appears in the high-level stage of the excitation signal, and in contrast to static PEC testing, motion detection has a stronger capability to characterize the crack depths. Based on the results, it is beneficial to propose a new high-speed inspection technique for crack characterization in ferromagnetic material components, such as rotating metal components, pipelines, and rail tracks. [ABSTRACT FROM AUTHOR]

The development of eddy current nondestructive techniques and instruments is moving toward new challenges in detecting buried and smaller defects and minimizing the amplitude of excitation currents. The smaller the size of the detected defect, the greater is the diagnostic capability of these techniques. Moreover, the lower the excitation current required, the longer the battery discharge time of the portable instruments, the greater the possibility of implementing broadband excitation strategies with reliable results. This feature is also important when portable instruments with array of probes have to operate for a large amount of time. Following the actual trends based on the use of double-coil excitations, this paper proposes the investigation of the performance of these kinds of probes, when small and buried defects are involved, and low amplitudes of the excitation current are adopted. To allow these activities a suitable double-coil-based excitation probe has been assembled. The obtained performance has been also compared with a traditional single-coil excitation probe based on the same sensing element. The experimental analysis demonstrated the appreciable performance of the proposed solution in a number of test cases. [ABSTRACT FROM AUTHOR]

In order to solve the problems of low efficiency and small area of existing eddy current thermography industrial nondestructive testing, this work realizes in-field-of-view (FOV) dynamic scanning eddy current pulsed thermography (DSECPT) with the help of blind source separation (BSS) algorithms for continuous detection of large-scaled industrial components. The principle of FOV-DSECPT, including finite-length inductive heating of mobile coil, the reconstruction of transient temperature response, and feature extraction, is investigated. The original thermal images cannot be used for depth analysis; thus, new data reconstruction method with good adaptivity for manual movement was proposed. The emerging BSS algorithms, including independent component analysis and nonnegative matrix factorization, are employed to process the reconstructed data. Through experimental studies, images using various features from classical and BSS algorithms were compared. The proposed FOV-DSECPT could provide a visualized and effective means for quality control and inspection of large-scaled key components in both manufacturing and service processes. [ABSTRACT FROM AUTHOR]

We present a new approach for depth detection of hidden defects based on the analysis of the frequency spectrum of the output waveform in a nondestructive testing (NDT) system. In our eddy current Superconducting quantum interference device (SQUID) based NDT system, we apply multiple frequencies to its single excitation coil in a magnetically unshielded environment. The excitation coil is a planar D-shaped printed circuit board coil, and a high-Tc gradiometer YBCO rf-SQUID is used as the electromagnetic sensor in this system. An automated two-dimensional nonmagnetic scanning robot is used to test samples with intentional defects at different depths. In this approach, a diagram labeled “FFT relative changes” is assigned to each test sample. The optimum points of the FFT relative changes diagram occur at the optimum frequencies directly related to the depth of the corresponding defects. The relation between the optimum frequency and the corresponding depth in our system is obtained both experimentally and by simulation. An excellent agreement between the practical and the simulation results confirmed the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a new portable Lorentz force eddy current testing system using a diametrically magnetized cylindrical magnet rotating around its mass centroid axis. In order to compare the electromagnetic torque caused by the Lorentz force and the induced voltage generated by the secondary magnetic field, three different model approaches are considered: the weak reaction approach and the quasi-stationary approach as approximating approaches and the transient simulation as a reference one. The experimental results of the induced voltage in the coil surrounding the permanent magnet are compared with the results of the transient solution. In an additional simulation a possible imperfection of the real system and its influence is studied. [ABSTRACT FROM AUTHOR]

This paper presents a new type of an electromagnetic sensor for nondestructive testing of carbon-fiber-reinforced polymer composites. The sensor utilizes coupled planar spiral inductors operating typically in the range of 10–500 MHz. The method proposed here shows some similarity to the eddy current technique, but as will be shown, the principles of operation are different as the sensitivity to defects is mostly due to the magnetic field components tangential to the surface of a material. It is shown that the method is applicable to 3-D inspection of carbon-fiber-reinforced composites widely employed in the aerospace industry. [ABSTRACT FROM PUBLISHER]

An enhanced and innovative laser-based magneto-optic (MO) imaging (LMOI) system is presented in this paper to detect buried subsurface flaws in metallic structures for structural nondestructive evaluation. Several key improvements for the new imaging device have been discussed, including the following: the optimization of the MO sensor, the design of the magnetic excitation device, and the development of the image processing approaches, which result in the enhanced MO image quality comparing to the first generation of LMOI. Experimental data have also been obtained that clearly demonstrated the improvement in imaging results. [ABSTRACT FROM AUTHOR]

The paper proposes a fast and accurate modeling technique for calculating the electromagnetic fields around a planar surface crack with arbitrary depth profile in a ferromagnetic metal, induced by a high-frequency ac current-carrying wire of arbitrary shape. The proposed technique first reduces the governing 3-D problem into two new equivalent 2-D electrostatic Dirichlet problems. Then, the method-of-moments is used to solve the new resultant problems. The main feature of the proposed technique is its remarkable high computation speed as compared to the conventional methods. The efficiency of the proposed technique is demonstrated by comparing the experimental and theoretical results. [ABSTRACT FROM AUTHOR]

International standards for testing soft magnetic materials require that the magnetic flux density remain sinusoidal with respect to time. Traditionally, this has been achieved via control of the magnetic flux using either analog feedback, which provides real-time control, or iterative digital feedback, which yields more accurate solutions with the cost of increased convergence time. In certain applications, such as magnetic nondestructive testing, rapid convergence of multiple interacting flux controlled channels is required. In this paper, a multichannel flux controller design is presented that combines the real-time performance of analog feedback with an iterative digital feedback algorithm to reduce error. The system demonstrates a 95% reduction in convergence time at power line frequencies over a comparable system using only digital feedback. Several examples of the system's ability to control arbitrary periodic waveforms are presented over the frequency range from 0.735 Hz to 100 Hz. Sinusoidal form factor errors are shown to be 0.1% from 2 Hz to 100 Hz across four strongly coupled channels with highly nonlinear magnetizing conditions. A detailed description of both analog circuit and digital algorithms is provided. [ABSTRACT FROM PUBLISHER]

In electromagnetic nondestructive testing (E-NDT), it is possible not only to detect cracks, but also the local variations of resistivity which are correlated with fatigue. We have developed methods to this end. A case when it is not possible to detect a crack directly, but through the accompanying resistivity variation, is described. [ABSTRACT FROM AUTHOR]

Numerical modeling of nondestructive testing (NDT) by eddy currents has been applied for qualification of two monitoring devices. Experimental data have enabled us to validate reliable and relevant finite-element method models. The encountered difficulties due to geometry (size and topology), the nature of the control signal (weak flux differences), and the movement accounting have been success- fully overcome by evaluating the A - φ and T - ℧ dual formulations with appropriate source fields and applying classical method to suppress the mesh numerical error and choosing the well-known lock-step technique. [ABSTRACT FROM AUTHOR]