FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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When this happens, the instrument subtracts the second frequency from the
first and, of course, if they are identical, the resulting signal is a zero
(Figure 6). So by making the support plate signal identical in both
channels, we can effectively eliminate it.
For those of you who are interested in the mathematics, they are given in
Figure 7.
Taking the example of a titanium tube with a mild steel support, we can show
how this occurs.
If a defect such as a 1 mm diameter hole is introduced into the tube, we can
show the resultant signals before and after mixing, with and without the
support plate (Figure 8).
We can extend this application into the aerospace industry for the
examination of cracks extending from bolt or fastener holes which have steel
bolts or fasteners inserted (Figure 9), and again show the resultant signal
before and after mixing (Figure 10).
My next example still, keeps us in the heat exchanger tubing industry, but
this time deals with a probe design.
In the development of heat exchangr tubing, the major criterion is thermal
performance. Improvements in the heat transfer characteristics can
frequently be gained by increasing the surface area of the tube relative to
its length and also by altering the flow characteristics of the fluid inside
the tube. This has led to tubes which are thinned, roped or studded on both
the inside and the outside as illustrated in Figure 11.
A further complication has been the presence of skip-finned areas which occur
at the support plate regions (Figure 12).
Even with dual frequency techniques, saturation of the instrument occurs at
fairly low gain levels, therefore the sensitivity to defects in a
conventional test is poor. A standard differential type probe will also not
satisfactorily detect the long splitting type of defect, commonly called
"zipper" cracks found in these tubes.
One attempted solution to the problem of this type of defect developed here
.in the USA is the cross axis probe which has absolute type detection
characteristics with a differential response.
The latest design is a special dual differential probe in which signals from
opposing sides of the tube are cancelled. This enables the cylindrically
symmetrical signals to be balanced out and gives good sensitivity to metal
loss type defects.
A comparison of the relative sensitivities to a hole defect and defects in
the skip-fin area for the differential probes is illustrated in Figure 13.
Finally, my last example involves the problem of eddy current instrument
calibration. There is a requirement by both user and manufacturer to have an