FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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on the fringes. (The sensitivity setting on the instrument was very low, hence
the defect would show even larger with the sensitivity increased to normal
testing levels.)
4. VERIFICATION
ïhe position of the cracks depends on the design of the turbine. Depending on
the model, the experienced operator could tell ahead where the cracks will be
found. That means that a probe can be designed with the highest sensitivity
at the location of the anticipated cracks. Nevertheless, one needs a probe
which covers a reasonably wide swath, in case defects with another orientation
are present which ought to be detected.
Test blocks were manufactured from homogeneous teeth by Electro Discharge
Machining small artificial defects in the grooves (Figure 9)• One groove contained
notches of different depths for calibration purposes, another groove
contained identical notches which were offset to determine field-of-view of
the ET probes.
It was found before, that absolute probes have advantages when only one phenomenon
is sought. 1 An absolute probe was made with both a wide and a limited-length
field of view. A once-through test with this probe on the reference
notches with respect to their position is shown in Figure 10.
By changing the frequency, the optimum response was found, and it was not surprising
that it was close to the design frequency. It was expected that a
change in ratio of signals from small to large reference notches would also be
found with a change of frequency but it was not (Figure 11).
Since notch 2 is supposed to be exactly 0.2 mm deep and notch 3, 0.4 mm deep,
the ideal signal ratio would be two. From 800 to 900 kHz the ratio was 2.3,
considered the best obtainable. It was also decided that it was probably better
to use a firm signal as a set-up rather than a smaller but noisier one and
the 0.4 mm deep notch was a tentative choice.
At 800 kHz a series of measurements with different instrument sensitivities
was made (Figure 12). This showed that without increasing the noise level,
signals could be boosted to show the 0.1 mm notch (No. 4, Figure 9) clearly.
With this level of sensitivity, any dangerous crack would cause the indication
at the lower sensitivity. The latter appears better for testing since too
many indications would go off-screen if the sensitivity were too high. With a
reduced sensitivity test may be more realistic and could be used to as close
as 5 mm from the end. (Notches are at 20, 40, 60 and 78 mm from the left
edge, Figure 13.) With this setting, the real crack in Figure 13 showed a
start in steps but is fully established at 50 mm and carries on to the end.
(The microscope showed it to fade at 4 mm from the end or 87 mm from the left
which is revealed in the picture although not clearly.)
1. G. Van Drunen and V.S. Cecco, CSNDT Journal, Vol. 5 No. 1, 1983 October