FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

- 363 -
the right of coils 1 to 6, with the opposite being the case for
coils 7 to 12. These flux lines were constrained by the presence
of the neighbours.
Tests were performed with single coils over the source plate and
symmetrical curves were observed as shown in Figure 10. The
asymmetries observed in the array studied were due solely to the
effect of neighbours and / or asymmetries in the induced field.
The coils had to be individually balanced, and lift-off
directions and multipliers had to be adjusted for each coil.
The eddy current signal was influenced by individual coil
parameters and to a small degree by the location of the coil. It
was acknowledged that coil parameters such as impedance were
affected by the location of the coil in the array. Also, for our
array, a simple signal processing method was not directly
apparent. However, a novel inversion protocol was devised which
would lead to advanced true flaw imaging.
CONCLUSIONS
Eddy Current imaging is feasible with arrays of coils. Coils,
when arranged in an array, influence each other. Interactions
between neighbours must be considered. The changes in impedance
due to the presence of defects are very small (Az = 10" 3 - 10" 2 ).
With present technology there is a need for individual treatment
of each coil in the areas of balancing, lift-off orientation and
calibration. We believe the balancing problem can be solved by
means of software.
The test notch (,1mm X 2 mm) is detected with no difficulties in
the coil design used in the array. The coils should be made
small, identical and powerful. Printed circuit coils can result
in identical elements in the array. However these coils operate
at higher frequencies (250 KHz and above) and would be useful for
the inspection of open shallow cracks.
We have developed a novel inversion procedure to size flaws. A
hexagonal stacking of identical coils is advocated and the
summing of coil responses in groups of six is shown to have
significant signal to noise improvement. Utilizing arrays to
produce images of defects yields improved S/N and prompt imaging,
speeds the inspection and provides superior flaw
characterization.
The signal processing and three dimensional colour graphics
display techniques developed elsewhere can be easily applied to
this problem (10).