FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The impedance (or the phase lag) of each coil is measured and if
there is a sufficiently large deviation from the measurement
above a good test object a 1 is assigned to the location of the
coil in question. Another approach has the coils acting
simultaneously- as driver and receiver. The induced eddy currents
would be similar around each probe. If the eddy current signal
could be analysed at the coil location for depth (by measuring
amplitude and phase at frequencies), an immediate profile of
the defect could be shown. A holographic method of imaging is
also possible in the AS mode. It is assumed that the response of
a coil is not altered by the presence of neighbours. If no
reasonable pictures can be developed from the static case (AS),
it will be difficult to develop these from the dynamic case (AD).
There are similarities with early radiographie techniques
indicating the presence of something but no indication of the
depth. Therefore a binary picture will not give the necessary
information to fully evaluate the importance of a defect.
However, it suffices for the detection of anomalies.
In our approach, the response of each individual coil is
processed, resulting in a depth profile. The experiments
addressed the questions below.
- Are all the coils identical or can their response be made
identical?
- Are neighbours altering the coil response?
- Can a calibration curve be drawn for all probes?
EXPERIMENTAL
Design of Coils and Building of the Array
We used the Hocking Phasec D6A Eddy Current instrument, which is
a multi-frequency device (ranging from 1 kHz to 1 MHz), highly
flexible and easy to operate. The Hocking Phasec D6A prefers
driver coils with impedances larger than 50 ohms. This puts
certain limitations on the choice of coils and the working
frequency.
Two tools assisted in coil design. The first is a computer
program AIRCO, developed by Dodd et al.,(9) which allows for the
calculation of the impedance of coils with air cores, taking
into account geometry and wire gage. This program was adapted
for the microcomputer in C BASIC. Ferrite cores were also
introduced to increase the inductance of the coils.
Probes were manufactured in-house or were commissioned from a
reputable manufacturer for the final array. An HP 4192A
impedance meter was used to measure the impedance and inductance
of the coil in air. The resonance frequency of the coils was over
600 kHz.