TPT November 2011

95 / 108

November 2011 93 ›
Article
make the image, while echoes from other depths were ignored. In
this image, this technique was selected in order to permit reflected
ultrasound to shadow the top of each crack and make it appear
dark. The echoes to be used in an image can be selected by “gating”
the echoes according to their arrival times at the transducer. Gating
is important because it permits the system operator to image only a
desired depth rather than the whole thickness of the sample, which
might result in overlapping features.
The vertical lines in Figure 1 are machining marks running around
the circumference of the pipe. The more or less horizontal features,
four of which are marked by arrows, are the tops of the cracks
through which water escaped from the interior of pipe during testing.
The cracks are aligned with the long axis of the pipe. They are all of
similar length, and are rather evenly spaced in more or less straight
rows running the length of the pipe. The reason for this somewhat
orderly organisation of the cracks at the surface is unknown, but
it seems to explain why the pressurised pipe was observed to be
leaking water over its entire surface.
The pipe was then imaged acoustically below the surface using a
fairly narrow gate. The same pattern of cracks is evident (Figure
2). In any single crack, some portions are bright white (meaning
that there is a gap that has an unobstructed path by which to send
high-amplitude echoes to the transducer), and other portions are
less clear. The cracks seem to be irregular rather than planar, and
analysts suspected that they were somewhat slanted rather than
vertical. This acoustic image also suggests that there are no other
internal anomalies in the pipe. Note that the region between the two
vertical rows of cracks in Figure 2 is featureless, just as one would
expect from a homogeneous material without anomalies.
It would be possible to image the cracks by making planar acoustic
images at increasing depths encompassing the entire wall thickness,
but researchers chose instead to use a different form of C-SAM
imaging in order to view the actual vertical structure of the cracks.
One reason for selecting a different method: truly vertical cracks
can be very difficult
to image acoustically
because the internal
feature from which
ultrasound should be
reflected has more or
less the aspect of a
knife blade. In some
ceramic samples such
as chip capacitors,
vertical cracks are
both fairly frequent
and significant in their
impact on reliability.
They can be imaged,
though, by a method
that causes the vertical
crack to cast a dark
acoustic shadow.
The alternate method
selected here, known is
B-scan imaging, is also
called acoustic cross
sectioning, and involves
pulsing ultrasound
into the sample along
a straight line marking
a vertical plane. The
resulting acoustic
image will look much
like an optical image
made after physical
cross sectioning. If, for
example, a small sample
thought to contain
defects is imaged by
the B-scan method
and then physically
sectioned through the
same vertical plane, the
two images will show the same features in the same locations.
In a flat sample, a B-scan image is made by selecting a specific
vertical plane defined by a straight line along the surface and
making one transducer scan along that line. The transducer is then
gated at a slightly deeper point and scanned again. Multiple scans
produce the acoustic echoes needed to make the cross-sectional
B-scan image.
In the case of the bismuth alloy pipe, the surface is not flat, but
the rotational fixturing makes it possible to select a plane for data
collection. Researchers also considered an additional factor: the
cracks are unlikely to be truly vertical, and may therefore deviate
from the selected vertical plane as the gated depth increases. To
compensate for this deviation, it was decided to rotate the sample
very slightly after each line was scanned.
The result of this innovative imaging method is shown in Figure 3,
where arrows indicate a single crack imaged at two depths arbitrarily
designated #3 and #4. The outer diameter of the pipe is at the top.
The irregular white features near the bottom are the inside diameter.
The horizontal lines above the cracks are artifacts of imaging, and
not features in the pipe.
Figure 4 shows the arrangement of the cracks in diagrammatic
end view. One row of cracks is shown in section, while two rows
are shown at the surface. The widths of the cracks are greatly
exaggerated in this diagram to make them visible.
Acoustic imaging is often used to image tubes and pipes, generally
to look for cracks or other anomalies within the wall, or to image
the inside diameter for anomalies. In this instance acoustic imaging
revealed the cracks that could not be seen optically, and used the
B-scan mode to track their vertical extent. The actual thickness of
these cracks might be a small fraction of a micron, but they were
still visible acoustically.
Acoustic micro imaging systems can handle cylindrical, spherical
and even conical samples. Two factors made the analysis of this
unusual sample possible: the high sensitivity of very high frequency
ultrasound to gaps 1µ or less in thickness, and the data-collecting
flexibility gained from multiple acoustic imaging modes.
Figure 4: End view and surface view of cracks.
The thickness of the cracks is greatly enhanced
in this diagram
Sonoscan, Inc
2149 E Pratt Blvd, Elk Grove Village, IL 60007, USA
Tel: +1 847 437 6400
Fax: +1 847 437 1550
Email:
Website:
Figure 3: Progressive B-Scan images
(non-destructive cross sections) were used to
follow the progress of individual cracks.
The cracks were typically slanted, like this crack