FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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After washing, drying and developing, the examination is another problem.
Often a number of rows of teeth have to be examined and accessibility is limited;
an operator has to work with tiny pieces of mirror attached to popsickle
sticks which he tries to position in the bottom groove, shine the black light
onto them and look at the same time for defects.
Even if defects were found, the familiar relationship between the crack depth
and the amount of penetrant-bleeding is often not there because of the very
tight nature of this type of defect. The cracks may be intergranular and
tightly filled with combustion or corrosion products which did not wash away
in the cleaning operation. The small indication of Figure 3 proved to be
from a substantial flaw (Figure 4).
Operators with less sophisticated cleaning facilities would find even less
cracks. This becomes a problem where no cracks are found in a disc which
does contain them. Fortunately, small cracks are not instantly causing a catastrophic
failure; most cracks grow slowly and are found during the next overhaul
at which time the disc can be replaced. Nevertheless, a flawed disc
which was given a clean bill of health because the testing method was inadequate,
may fail before the next overhaul and cause expensive damage. The test
therefore is inadequate and better testing means ought to be found.
3. EDDY CURRENTS
An obvious solution to the testing of disc teeth seems to be an Eddy Current
test. However, these tests are not so obvious when one actually tries them.
Rotors become slightly scaled and the rough surface so far has caused problems
with the Eddy Current test. Descaling by means of glass-beading has been
practised before an Eddy Current test was attempted and leaves a sort of rough
surface.
Figure 5 shows an Eddy Current signal of a defect-free tooth. There is some
horizontal wandering and an exit signal veering off to the upper left. Figure
6 shows the signal when the probe passes over a defect close to the exit. The
display shows loops which betray defects. The sensitivity setting of the
instrument was relatively low and when increased to find small defects, the
displays would grow much larger than the screen and could no longer be interpreted
(Figure 7).
The Eddy Current tester was adjusted such that the lift-off signal as well as
the wandering of the dot while the probe travels were horizontal on the screen.
The vertical component is then always related with defects and end-of-groove
effects. The vertical component has an output available on the instrument
which was connected to the Y direction of an ordinary storage osciloscope. A
relatively simple fixture was made to sense the position of the probe and fed
to the x-direction of the scope. Figure 8 shows how a differential probe senses
a crack halfway along the tooth. The tooth is 91 mm long, the crack 13 mm
but it shows 20 mm on the base line which means that the probe sees 4 mm more