Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review

Chin. J. Mech. Eng. (2017) 30:500–514 DOI 10.1007/s10033-017-0122-4 REVIEW ARTICLE Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review Ali Sophian1 • Guiyun Tian2,3 • Mengbao Fan4 Received: 2 November 2016 / Revised: 18 January 2017 / Accepted: 28 March 2017 / Published online: 17 April 2017 Ó Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2017 Abstract Pulsed eddy current (PEC) non-destructive testing and evaluation (NDT&E) has been around for some time and it is still attracting extensive attention from researchers around the globe, which can be witnessed through the reports reviewed in this paper. Thanks to its richness of spectral components, various applications of this technique have been proposed and reported in the literature covering both structural integrity inspection and material characterization in various industrial sectors. To support its development and for better understanding of the phenomena around the transient induced eddy currents, attempts for its modelling both analytically and numerically have been made by researchers around the world. This review is an attempt to capture the state-of-the-art development and applications of PEC, especially in the last 15 years and it is not intended to be exhaustive. Future challenges and opportunities for PEC NDT&E are also presented. Keywords Non-destructive testing  Pulsed eddy currents  Material characterization  Structural integrity  Non-destructive evaluation & Guiyun Tian 1 Faculty of Engineering, International Islamic University Malaysia, Kuala Lumpur, Malaysia 2 School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu, China 3 School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK 4 School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China 123 1 Introduction Despite its approximately-five-decade-long history, PEC is still considered by many as a new emerging eddy current NDT&E technique. Compared to other eddy current testing (ECT) techniques this view can be true. Literature shows that PEC has been attracting the attention of researchers from around the globe, including countries, such as China, UK, Canada, Portugal, USA, South Korea, Japan, France, Slovakia, Poland, and Italy. The amount of attention that PEC NDT&E has been receiving owes to the key potential benefits that it offers. The first and main advantage is that, compared to single frequency ECT, PEC inherently has a broadband of frequencies [1], which is advantageous for any eddy-currentbased NDT&E techniques due to the frequency-dependant skin effect. Another benefit is that PEC signals are relatively easier to interpret, while it requires a special skill of the operators for interpreting conventional ECT signals which are presented in the impedance plane trajectory. Conventional ECT only applies a single frequency for excitation which makes it unable to detect both surface and sub-surface defects reliably. The improved technique is the multi-frequency ECT which applies different excitation frequencies, one after another. Compared to multi-frequency ECT, PEC can potentially be applied in shorter time for inspection of different depths as PEC applies a wideband of frequencies in a single pulse. This allows to reduce the measurement time to the minimum one depending on the sample characteristics. Fig. 1 provides the illustration of the excitation waveforms of each of the methods. Similar to other ECT techniques, in general PEC requires no surface preparation which leads to reduction of inspection time and costs efficiency is improved. The Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review 501 Conventional ECT 1 0 -1 0 5 1 10 15 Multi-frequency ECT 1 1 0 0 -1 0 10 20 -1 0 20 0 10 20 -1 0 10 20 Fig. 2 Illustration of the working principle of ECT PEC 1 rffiffiffiffiffiffiffiffi 2 d¼ ; 0.5 0 -lr 0 5 10 Time 15 20 Fig. 1 Illustration of excitation waveforms for different ECT techniques inspection can also be done without interrupting the operation or service of the structure being tested, unlike for example X-ray testing. In many applications where the sample is coated, no removal of the coating is required when ECT NDT&E is used. Any eddy-current systems are relatively cost-effective and reliable. In the following sections, the concept of PEC is briefly discussed which is then followed by the review in systems, modelling, signal processing and applications. A conclusion completes this review paper. 2 Concept of Pulsed Eddy Current In eddy current NDT, an AC-driven excitation coil induces eddy current in the sample through electromagnetic coupling. In turn, the circulation of the eddy current induces a secondary magnetic field as illustrated Fig. 2. This field will vary if flaw that impedes the eddy currents is present or there is a change in the electrical conductivity, magnetic permeability or thickness of the sample. The change in the field will be picked up by a sensing device, which is typically either a coil or a magnetic sensor. The penetration and the density of the eddy current in the sample is an important issue in any ECT. The penetration is limited due to the skin effect, which causes its density to decrease exponentially with depth. The depth at which the density has reduced to 1/e of the density at the surface is termed the skin depth d and defined by ð1Þ where d is skin depth (m), l is magnetic permeability (H/ m), r is electrical conductivity (S/m) and x is angular frequency (rad/s). The equation shows that the depth of penetration depends on the excitation frequency. The lower the frequency, the deeper the penetration and vice versa. In contrast to conventional sinusoidal eddy current technique, where the excitation is limited to one frequency component, pulsed eddy current techniques excite the induction coil with a pulse waveform. The frequency components of pulse waveform can be demonstrated using Fourier Transform. If the excitation waveform is defined as 8 T T > < A;   t  ; 2 2 f ðtÞ ¼ ð2Þ > : 0; jtj [ T ; 2 where A is the amplitude of the pulse and T is the pulse width, then using the amplitude spectrum of the excitation is defined as F ðx Þ ¼ 2 sin xT=2 : x ð3Þ Fig. 3 shows examples of the pulses with two different widths and their power spectra, which shows that the excitation has a series of frequency components, which has given the technique the potential to inspect different depths simultaneously and therefore it will be able to offer more information compared to the conventional approach. 3 PEC Systems Despite variations that exist, a typical PEC system will look like the illustration shown in Fig. 4. A pulse signal at a chosen frequency and pulse width is generated which is then power-amplified to drive an excitation coil. In turn, a 123 502 Ali Sophian et al. (a) (...truncated)