TPT November 2011

A rticle

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

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

Figure 4 : End view and surface view of cracks. The thickness of the cracks is greatly enhanced in this diagram

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.

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

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

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