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Chin. J. Mech. Eng. (2017) 30:500–514
DOI 10.1007/s10033-017-0122-4
REVIEWARTICLE
Pulsed Eddy Current Non-destructive Testing and Evaluation:
AReview
Ali Sophian1 • Guiyun Tian2,3 • Mengbao Fan4
Received: 2 November 2016/Revised: 18 January 2017/Accepted: 28 March 2017/Published online: 17 April 2017
Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2017
Abstract Pulsed eddy current (PEC) non-destructive test- 1 Introduction
ing and evaluation (NDT&E) has been around for some
time and it is still attracting extensive attention from Despite its approximately-five-decade-long history, PEC is
researchers around the globe, which can be witnessed still considered by many as a new emerging eddy current
through the reports reviewed in this paper. Thanks to its NDT&Etechnique.Comparedtoothereddycurrent testing
richness of spectral components, various applications of (ECT) techniques this view can be true. Literature shows
this technique have been proposed and reported in the lit- that PEC has been attracting the attention of researchers
erature covering both structural integrity inspection and from around the globe, including countries, such as China,
material characterization in various industrial sectors. To UK, Canada, Portugal, USA, South Korea, Japan, France,
support its development and for better understanding of the Slovakia, Poland, and Italy.
phenomena around the transient induced eddy currents, The amount of attention that PEC NDT&E has been
attempts for its modelling both analytically and numeri- receiving owes to the key potential benefits that it offers.
cally have been made by researchers around the world. The first and main advantage is that, compared to single
This review is an attempt to capture the state-of-the-art frequency ECT, PEC inherently has a broadband of fre-
development and applications of PEC, especially in the last quencies [1], which is advantageous for any eddy-current-
15 years and it is not intended to be exhaustive. Future based NDT&E techniques due to the frequency-dependant
challenges and opportunities for PEC NDT&E are also skin effect. Another benefit is that PEC signals are rela-
presented. tively easier to interpret, while it requires a special skill of
the operators for interpreting conventional ECT signals
Keywords Non-destructive testing Pulsed eddy currents which are presented in the impedance plane trajectory.
Material characterization Structural integrity Conventional ECT only applies a single frequency for
Non-destructive evaluation 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-fre-
& Guiyun Tian quency ECT, PEC can potentially be applied in shorter
g.y.tian@uestc.edu.cn time for inspection of different depths as PEC applies a
1 Faculty of Engineering, International Islamic University wideband of frequencies in a single pulse. This allows to
Malaysia, Kuala Lumpur, Malaysia reduce the measurement time to the minimum one
2 School of Automation Engineering, University of Electronic depending on the sample characteristics. Fig. 1 provides
Science and Technology of China, Chengdu, China the illustration of the excitation waveforms of each of the
3 School of Electrical and Electronic Engineering, Newcastle methods.
University, Newcastle upon Tyne NE1 7RU, UK Similar to other ECT techniques, in general PEC
4 School of Mechatronic Engineering, China University of requires no surface preparation which leads to reduction of
Mining and Technology, Xuzhou 221116, Jiangsu, China inspection time and costs efficiency is improved. The
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Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review 501
Conventional ECT
1
0
-1
0 5 10 15 20
Multi-frequency ECT
1 1 1
0 0 0
-1 -1 -1
0 10 20 0 10 20 0 10 20
PEC Fig. 2 Illustration of the working principle of ECT
1
rffiffiffiffiffiffiffiffi
0.5 d ¼ 2 ; ð1Þ
-lr
00 5 10 15 20 where d is skin depth (m), l is magnetic permeability (H/
Time m), r is electrical conductivity (S/m) and x is angular
frequency (rad/s). The equation shows that the depth of
Fig. 1 Illustration of excitation waveforms for different ECT penetration depends on the excitation frequency. The lower
techniques the frequency, the deeper the penetration and vice versa. In
contrast to conventional sinusoidal eddy current technique,
inspection can also be done without interrupting the oper- where the excitation is limited to one frequency compo-
ation or service of the structure being tested, unlike for nent, pulsed eddy current techniques excite the induction
example X-ray testing. In many applications where the coil with a pulse waveform. The frequency components of
sample is coated, no removal of the coating is required pulse waveform can be demonstrated using Fourier
when ECT NDT&E is used. Any eddy-current systems are Transform. If the excitation waveform is defined as
relatively cost-effective and reliable. 8 T T
>
In the following sections, the concept of PEC is briefly
:0; t [ ;
modelling, signal processing and applications. A conclu- jj 2
sion completes this review paper. where A is the amplitude of the pulse and T is the pulse
width, then using the amplitude spectrum of the excitation
2 Concept of Pulsed Eddy Current is defined as
2sinxT=2
In eddy current NDT, an AC-driven excitation coil induces FðÞx ¼ x : ð3Þ
eddy current in the sample through electromagnetic cou- Fig. 3 shows examples of the pulses with two different
pling. In turn, the circulation of the eddy current induces a widths and their power spectra, which shows that the
secondary magnetic field as illustrated Fig. 2. This field excitation has a series of frequency components, which has
will vary if flaw that impedes the eddy currents is present given the technique the potential to inspect different depths
or there is a change in the electrical conductivity, magnetic simultaneously and therefore it will be able to offer more
permeability or thickness of the sample. The change in the information compared to the conventional approach.
field will be picked up by a sensing device, which is typ-
ically either a coil or a magnetic sensor.
The penetration and the density of the eddy current in 3 PEC Systems
the sample is an important issue in any ECT. The pene-
tration is limited due to the skin effect, which causes its Despite variations that exist, a typical PEC system will
density to decrease exponentially with depth. The depth at look like the illustration shown in Fig. 4. A pulse signal at
which the density has reduced to 1/e of the density at the a chosen frequency and pulse width is generated which is
surface is termed the skin depth d and defined by then power-amplified to drive an excitation coil. In turn, a
123
502 Ali Sophian et al.
(a) 1 Pulse width = 2 ms (b) 1
Pulse width = 5 ms Pulse width = 2 ms
0.8 0.8 Pulse width = 5 ms
0.6 0.6
Amplitude (V)0.4 Amplitude0.4
0.2 0.2
0 00 50 100 150 200
-20 -15 -10 -5 0 5 10 15 20
Time (ms) Frequency (Hz)
Fig. 3 (a) Examples of pulses with different widths, (b) Power spectra of the pulses
Fig. 4 Generic configuration of
a PEC NDT system
time-varying magnetic field is induced by the current in the duration in order not to overheat the coil and the driver
excitation coil. The magnetic field, which is called the pri- electronics. And there are also other shapes of excitation
mary field, induces eddy current in the sample. Consecu- signal that have also been used and proposed by
tively, a secondary magnetic field is induced by the eddy researchers. A study on different excitation waveforms,
current and it opposes the primary field. This secondary field namely square, half-sine and ramp, shows a favour for
is then detected by a sensing device, which typically can be the square waveform [2]. A variable pulse width exci-
either a magnetic sensor or a coil. The output signal of the tation has also been proposed [3], which was used in the
sensing device is then passed to the next stage to be con- inspection of subsurface corrosion in conductive struc-
ditioned and processed where eventually features are tures [4]. Pulse width modulation, as illustrated in Fig. 5,
extracted in order to infer the desired parameters, such as provides different frequency spectra and is suggested of
wall thickness and lift-off, from the testing. being able to eliminate the need for reference sample
From one implementation to another, the systems vary signal [5]. PEC has also been implemented by using the
primarily because of the differences in the excitation sig- decaying part of the step signal, rather than the raising
nal, excitation system, sensing device and the signal pro- part, after the power supplied to the excitation coil is
cessing and feature extraction techniques. These variations disconnected [6–8].
are discussed below.
3.2 Probes
3.1 Excitation Signals
Typically, a PEC probe would contain one excitation coil
In many implementations, the excitation current or and one or more sensing devices. An excitation coil gen-
voltage is a square waveform. In some other applica- erates primary transient excitation field, while one or more
tions, the excitation is of rectangular waveform which sensing devices picks up secondary eddy current field due
allows a very high power to be delivered in a limited to a sample. Probe designs are usually optimized in terms
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Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review 503
and therefore no reference signals are required. This type
of probe can be implemented by using two pick-up coils or
two magnetic sensors with the output signal being the
difference between the two output signals from the sensing
devices. A differential double-D probe using two Hall
devices has been investigated by Park, et al [13] which
shows a potential for detection and sizing of sub-surface
cracks in stainless-steel structures. Differential probes have
also been studied for crack detection near a fastener in
aircraft structures [14], [15].
Less common probe configurations have also been used,
such as a planar matrix probe that can generate a color map
that is useful in the identification of defects [16]. Their
work shows the use of an 8-by-8 array of sensors, as shown
in Fig. 7(a), successfully maps the surface defects that have
Fig. 5 Excitation currents with varied pulsed widths [5] been artificially made on the sample, which justifies the
complexity of the excitation and sensing circuits used in
of its structure, the type of sensing elements and the use of the probe. The application of independent excitations lead
cores based on the specific applications in which they will to a more uniform excitation field which, in turn, leads to a
be deployed. simpler interpretation of the detected signals. Another
Based on the electromagnetic coupling between the interesting example of the use of sensor array in ECT, is
excitation coil and the sample, eddy current probe’s excita- shown in Fig. 7(b), where the printed array is flexible and
tion coils can generally be categorized into one of the fol- can be used to produce a color map of surface corrosion
lowingthreetypes:surface(orpancake)coil,encirclingcoil [16]. Another unique example is a symmetric excitation
(or OD for outer diameter) and internal coil (also called coil introduced by Yang et al, which is expected to gen-
bobbinorIDforinnerdiameter)[9].Thethreetypesofcoils erate linear eddy currents with the benefit of virtually no
are illustrated in Fig. 6. Surface or pancake coils may be field will be detected by the pick-up sensor when no defect
orientated either parallel or normal to the surface of the is present [18].
sample and they are used for both flat and curved samples. Another differentiating feature is the shape of the coil.
Encircling coils are generally used in the inspection of Rather than being circular, which is the most common
cylindrical elongated structures, such as hollow pipes and shape, the coil may be rectangular or racetrack. This non-
solid rods [10], [11]. Coils of this type form a circle around circular type of a coil is also referred as directional as
the diameter of the test object coaxially. The specimen opposed to non-directional or isotropic for circular coils.
maybeinsulated or coated. The bobbin-typed coils are usu- With directional probes, the paths of the induced eddy
ally used to inspect hollow cylindrical structures, such as currents are not circular, and, therefore, they are more
pipes and bore holes, from the inside. In PEC NDT, a coil of sensitive to changes in a particular direction. One example
thistypewhichisusedinremotefieldmodehasbeenusedfor of the use of directional rectangular coil is the work done
measurement of wall thickness of ferromagnetic tubes [12]. by He, et al [19], [20]. They also state that a more uniform
Differential probes, as opposed to absolute probes, are eddy current distribution is being an advantage gained by
also used with the advantage of the self-nulling features using such a coil.
Fig. 6 Coil types used in ECT: (a) surface coil, (b) encircling coil, and (c) internal coil
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