Instrumentation & Measurement Magazine 25-2 - 7

balance states for each pixel, and used it to correct the transient
response. In this way, the possibility of false alarms caused by
uneven surface emissivity is greatly reduced (Fig. 2c).
Configuration of ECPT Equipment
ECPT equipment usually consists of two parts (Fig. 3): the excitation
module and the temperature recording module. The
excitation module includes the excitation source and coils, and
the temperature recording module includes infrared sensor
and data capture equipment. Moreover, some additional devices
can be added, if necessary, such as water-cooling devices,
shading devices, etc.
Excitation Module
The excitation source generates the alternating current in the
coil to induce eddy current on or in the specimen. Table 1 shows
the representative materials/structures that can be tested by
the ECPT and the corresponding excitation information. According
to the mechanism of ECPT, the typical materials are
conductive materials in which electrical conductivity varies
from the order of 103
S/m (Carbon Fiber) to 107
S/m (Steel, Aluminum,
and Copper) or even more. The structures include the
plate, wire, blade and so on, common in industry. The defect
types include cracks, pitting, thermal fatigue, etc. The figures
of different materials are cited and used with permission from
[4], [5], [1], [6] and [7], respectively. Electrical conductivity is
taken as the laboratory value at 20 °C. Excitation parameters
are excitation current amplitude, excitation frequency, and excitation
duration, respectively. Steel includes pure iron, carbon
steel, stainless steel and other iron-based alloys.
Table 1 indicates that, in most cases, the excitation for ECPT
has the following three characteristics: high frequency, high
amplitude and short heating duration. In practice, due to the
skin effect, that is, the skin depth of eddy current is inversely
proportional to the square root of the frequency, high frequency
excitation is required to ensure the energy would be
deposited at the surface, so as to acquire higher quality ECPT
sequences. Higher current induces higher intensity of eddy
current, which enlarges the temperature difference between
defect and non-defective area and improves the contrast of
ECPT sequences. Short duration is beneficial for the reduction
of the negative influences by thermal diffusion on the surface
during heating phase (heat flows from the defective area to the
non-defective area, and the non-defective area gradually heats
up). Taking the laboratory where the authors work as an example,
most of the experiments were performed under high
frequency (256 kHz) sinusoidal current excitation of about
150 A with a duration of 0.1 to 0.2 s.
In addition, ECPT can adopt different excitation modes,
such as sine excitation, square wave excitation, frequency
modulation excitation, etc., and the eddy current changes under
the different excitation modes. In most papers, sinusoidal
alternating current excitation is used due to its good periodicity
and stability.
The coil is another key point of ECPT. Different coils generate
different magnetic fields, and the amplitude and direction
of the eddy currents are also significantly different. In addition,
the placement also varies, depending on the shape of the
coils. In most cases, the helix and line coils are vertically set
1 mm to 2 mm above the surface of the specimen. The height
between the coils and specimen surface is called the lift-off
height. The closer the lift height is, the stronger the eddy current
that is induced. The yoke coil will attach the specimen to
the yoke, and the yoke will generate a uniform magnetic field
near the surface of the specimen, so as to induce the eddy current
on specimen. The advantage of yoke coil is that the coil
does not cover the specimen's surface, so that the heat information
of can be observed more completely. However,
circular and line coils also have their advantages in industry.
Since there is no magnetic yoke, helix and line coils have
smaller size, simpler structure, and can work in narrower
spaces.
Fig. 3. (a) Schematic diagram of ECPT equipment; (b) ECPT equipment setup. (From [3], ©2020, IEEE).
April 2022
IEEE Instrumentation & Measurement Magazine
7

Instrumentation & Measurement Magazine 25-2

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