on Pulsed Eddy Current
(adapted from Blitz, 1997)
The use of pulsed eddy currents has long been considered for
testing metals (Libby, 1971) and it has been applied to operations
in specialized areas, such as in the nuclear energy industry,
where testing equipment is often constructed to order. However,
significant progress in this direction has taken place only recently
after appropriate advances in technology (Krzwosz et al. 1985;
Sather, 1981; Waidelich, 1981; Wittig and Thomas 1981), but at
the time of writing, commercial equipment was not yet available.
The method has the potential advantages of greater penetration,
the ability to locate discontinuities from time-of-flight determinations,
and a ready means of multi-frequency measurement. At present,
it does not generally have the precision of the conventional methods.
The apparatus is somewhat complicated in design and not readily
usable by the average operator who is experienced with the conventional
eddy current equipment. Its main successes are in the testing
of thin metal tubes and sheets, as well as metal cladding for
measuring thickness and for the location and sizing of internal
When comparing the pulsed method with the conventional eddy current
technique, the conventional technique must be regarded as a continuous
wave method for which propagation takes place at a single frequency
or, more correctly, over a very narrow frequency bandwidth. With
pulse methods, the frequencies are excited over a wide band, the
extent of which varies inversely with the pulse length; this allows
multi-frequency operation. As found with ultrasonic testing, the
total amount of energy dissipated within a given period of time
is considerably less for pulsed waves than for continuous waves
having the same intensity. For example, with pulses containing
only one or two wavelengths and generated 1000 times per second,
the energy produced is only about 0.002 of that for continuous
waves having the same amplitude. Thus, considerably higher input
voltages can be applied to the exciting coil for pulsed operation
than for continuous wave operation.
Pulsed waves can reasonably be expected to allow penetration
of measurable currents through a metal sample to a depth of about
10 times the standard penetration depth, provided a suitable
probe is used (i.e. a shielded ferrite-cored coil, see section 5.3).
Therefore, penetration is possible through a 2 mm thick plate
at frequencies of 1-3 kHz for non-ferromagnetic metals having corresponding
electrical conductivities ranging from 60 down to 20MS/m. However,
with an unmagnetized steel plate 2 mm thick, where sigma = 5 MS/m
and µr = 100, the maximum frequency for through-penetration
is only 100 Hz.
Pulsed eddy currents may be generated by a thyratron connected
in series with the exciting coil through a capacitor (e.g. Waidelich,
1981). A direct voltage, on the order of 1200 V, slowly charges
the capacitance and when the thyratron conducts there is an abrupt
discharge through the coil in which free-damped harmonic oscillations
occur. This is repeated periodically (i.e. at 1 kHz), so as to
propagate the eddy current pulses through the metal.
The currents are detected by a receiving probe located either
adjacent to or on the opposite side of the metal sample from the
exciting probe when access is possible. The range of propagated
frequencies depends on the logarithmic decrement of the exciting
circuit, and because the speed of the waves is a function of frequency,
dispersion takes place and the pulse changes in shape as it progresses
through the metal. As one would expect, the height of the peak
and its time delay can be related to the thickness of the metal.
Waidelich reports a maximum penetration of 90 mm for aluminum
sheet and 10 mm for steel. For 6 mm thick sheets, the peak value
of the received pulse voltage was 13 V for aluminum but only 20
mV for steel. Krzwosz et al. (1985) has shown how pulses that
result from the presence of internal simulated defects produce
broadening with an increase in depth.
The frequency content of the pulses depends on their lengths,
and in the extreme, contains continuous spectra ranging from less
than 100 Hz to 1 or 2 kHz. By performing a Fourier transformation,
the pulse obtained by the receiving probe can be displayed in
the form of the variation of amplitude (or phase) with frequency.
By sampling different delay times within a pulse, different parts
of the spectrum can be evaluated (Sather, 1981). If both amplitude
and phase are measured, two parameters (i.e. presence of defects,
variations in tube thickness, and changes in fill-factor or liftoff)
can be evaluated for each frequency selected in the same way as
with the multi-frequency method, although, at present, with a lower
degree of precision.
Dodd et al.(1988) has designed and developed a pulsed magnetic
saturation method for the eddy current testing of ferromagnetic
metals. The DC field pulses are generated by passing a high-current
pulse through an electromagnet so as to produce saturation in
the metal object; the pulse length is made equal to the thickness
of the object, thus ensuring complete eddy current penetration
where feasible. The DC pulse, on the order of 1 ms duration, simultaneously
produces an eddy current pulse, which is detected by a probe;
the output of the probe is characteristic of the material being
This technique has the advantage of producing high magnetic peak
powers with low average powers, thus keeping any heating of the
test sample down to an acceptable level. It has been applied successfully
to the internal testing of the walls of steel steam generator
tubes, and tubes of diameter 10.9 mm and wall thickness 5 mm have
been examined with peak powers of 500 kW. Small defects close
to the external surfaces can be detected, and by taking advantage
of the multi-frequency properties of pulsed eddy currents, their
indications can be resolved from those that originate from other
characteristics of the tubes.
More recent work on the use of pulsed eddy currents has been
reported by Gibbs and Campbell (1991), who inspected cracks under
fasteners in aluminum aircraft structures. Here, a Hall element
was used as a receiver. Radial position, approximate depth, and
relative size of defects hidden under fastener heads could be
determined in countersunk areas for defect depths of up to 7 mm
for nonferrous fasteners and 14 mm for ferrous fasteners.
Lebrun et al. (1975) reported the detection of deep cracks in
ferromagnetic samples using an emission coil excited by square
pulses of high intensity and employing highly sensitive magneto-resistive
sensors to measure the resultant magnetic fields. Defects of 1
mm x 1 mm could be detected at a depth of 5 mm and 3 mm x 4 mm at a depth of 20 mm.