FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

ACOUSTIC LENSES
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Ten acoustic lenses were designed to cover the range of wall thicknesses
and weld preparations suggested for inspection. Selection of defects for
inclusion in the program finalized the inspection requirements (inspection
angle, defect location), and four lens/wedge/transducer combinations were
then chosen to be used. Each assembly was calibrated by signal amplitude
and time—of-flight using a calibration block containing 1.5 ram diameter
side drilled holes spaced 2.5 and 4.0 mm apart (Figure 2a). The high
resolution capability of focused beams is readily capable of
differentiating between such small targets (Figure 2c) in contrast to the
broad beam results for a standard transducer (Figure 2b). The calibration
data was used to determine the beam inspection angle and time-of-flight of
sound in the lens/wedge assembly.
DEFECT SIZING
Each weld specimen was examined ultrasonically from both sides of the outer
surface weld bead using a simple scanning mechanism (Figure 1). The
pulse-echo technique, which is commonly used in the field, was used
throughout in contact mode with a light oil as couplant. The inspection
data was recorded as multiple A-scans. In this technique, the probe is
incrementally moved axlally away from the weld bead. The gate of the
ultrasonic instrument is swept across the CRT screen, and the signal
amplitude versus time display (shown on the CRT) is plotted for each probe
position (see Figure 3). As the path length to the defect increases, the
time-of-flight increases and the defect response on the A-scan moves to the
right on the time axis. Conversely, any extraneous signal (e.g. from the
lens/wedge assembly) remains at constant time and is readily
distinguishable on the multiple A-scan record. The through-wall depth of
the defect can then be calculated based on the extent of the defect
response along the time axis.
Metallurgical Examination
After ultrasonic examination, each specimen was broken open by cooling it
in liquid nitrogen and using three point impact loading to fracture along
the centre of the weld. Where samples did not break through the defect
(primarily for the smaller buried defects) the sample was sectioned and
metallographically prepared to reveal the defect size.
RESULTS
A typical inspection record (Figure 4a) and specimen cross-section (Figure
4b) are shown for a planar root crack giving a comparison of predicted
versus actual defect size. These results are representative, as in all
cases size predictions were in good agreement with the actual defect
depths. This excellent correlation is further illustrated in Figure 5
where the majority of data points fall within a band +1.5 mm of the line as
determined by regression analysis. The 95% confidence limits for the
predicted mean flaw depths are shown. There was little distinction in
sizing accuracy between volumetric and planar defects. The technique is
conservative in that defects tend to be slightly oversized (average error
was 0.37 mm).