FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

INTRODUCTION
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The inspection for defects by Eddy Currents is based on an indepth
analysis of the impedance changes experienced by a single
coil (1,2). This can, after analysis, yield the location of the
defect and an estimate of its depth. However, the evaluation of
the defect type and depth from the impedance plane image or chart
recording is tedious (Figure 1). Special calibration is required
to obtain semi-quantitative data on flaw size. Our work proposes
to take a different approach: developing an eddy current system
capable of imaging flaws directly, with the goal of designing a
real time eddy current imaging system to detect and image flaws.
EDDY CURRENT IMAGING
A breakdown of eddy current methods for imaging is based on two
characteristics:
1) the movement of the testing system in relation with
the test object.
2) the number of coils used.
One method of eddy current imaging is eddy current holography,
described in detail by B.P. Hildebrand (3). This technique is
based on the observation that phase is dependent on the distance
from the receiving coil and uses the backward wave reconstruction
algorithm of Van Rooy (A). The results reported are encouraging
but there are some distinct problems with focus and resolution.
The experimental set-up is of the SD (single coil dynamic) type,
scanning a single coil, preprocessing the signal in a multifrequency
eddy current instrument to minimize the noise due to
variations in lift-off, and using a digital reconstruction
algorithm developed originally for ultrasonics.
More sophisticated inversion procedures are discussed in the
1 itérât - e (5) based on the convolution of some parameters of the
test object, and a response function. C-scan type pictures have
been made for eddy currents (6). This practice was also used
extensively to distinguish between types of defects (pits,
cracks).
The AS (array-static) type of test is quite different in nature
from the previous one. It consists of N coils arranged in an
array which are then placed on top of the test object to produce
an image.
There are several approaches to array imaging. One can separate
the driver and receiver function of the coils; the array of
receiving coils is placed within a large driver coil. This has
the advantage that phase lags can be measured with respect to one
driver coil. The signal information can be processed in a binary
fashion. This method is being used by DREP (7)»