TPT November 2011

A rticle Acoustic microscope finds “invisible” leaks by Tom Adams, consultant, Sonoscan, Inc. Tested under pressure, a bronze alloy pipe that was part of a pump assembly appeared to be leaking at numerous locations, but close visual inspection found no cracks. An acoustic microscope revealed not only the cracks but also their unusual arrangement. The sample of pipe described here measured 2.5 inches long, 1.25 inch in OD and 1.00 in in ID. Since it was designed to carry potable water, the bronze alloy from which it was fabricated had been modified to remove lead (Pb) and replace it with non-toxic bismuth (Bi), a requirement of both state and federal regulators in the US. This sample was one of a small number in one of the earliest bismuth alloy lots to exhibit an unusual leakage pattern. As part of routine testing, the pipe was filled with water under pressure and observed for leaks. Water soon appeared on the outer surface of the pipe, but it could not be traced to a single leakage point. Instead, it appeared that water was leaking through the wall of the pipe at numerous locations, and the pipe appeared to be sweating. Pipes that leak during testing typically leak along a single definable crack. The next step was to stop pressure testing and examine the outer surface of the pipe for leakage points. None were found, even at very high optical magnification, although it is possible that the texture of the machined surface might help to conceal very fine cracks. The presumed cracks didn’t seem to be good subjects for X-ray, so C-SAM ® acoustic micro imaging was used because of its reputation for being able to identify and image exceedingly thin internal gaps, cracks, delaminations and the like. For acoustic imaging, the sample was sent to Sonoscan’s (www.sonoscan.com) headquarters laboratory in Elk Grove Village, Illinois, USA. A C-SAM system uses a highly focused ultrasonic transducer that raster-scans the surface of the sample at speeds up to 40 inches per second. Each second, it sends thousands of pulses of very high frequency ultrasound into the sample, and receives the return

except cracks which, if present, will appear bright white (indicating the highest possible amplitude) in the acoustic image. Suppose that a metal to ceramic interface has over part of its area a crack (or disbond, or delamination) that is 1mm thick. More than 99.99% of the ultrasonic pulse will be reflected when it strikes the solid-to-air interface. If the crack is less than 1µ thick, it will still reflect the same 99.99%+ of the pulse. What matters is the interface between the solid material (metal, in this case) and the air in the crack. The thickness of the crack doesn’t matter because essentially no ultrasound crosses the crack to be reflected from the interface at the bottom of the crack. Most samples for acoustic imaging have at least one flat surface, and the internal features, whether good bonds or gaps, are also often flat. The transducer scans in a single x-y plane and creates images of planar defects. This sample, however, was cylindrical. Cylindrical samples are not unusual, and Sonoscan has developed a fixture that permits the transducer to scan along a single longitudinal line, pulsing and receiving return echoes, and then pausing at the end of the line. During the pause, the sample is rotated a fraction of a degree. The transducer then scans back along the length of the sample, which is then rotated again. Although reasonably fast, rotational imaging is too slow for production environments and is best suited to laboratory analysis. The transducer typically scans slightly more than the entire circumference of the cylindrical sample – 365°, for example – to give evidence in the acoustic image that the entire sample has been examined. The acoustic image of a small portion of the sample surface and several cracks is shown in Figure 1. The sample was imaged using an ultrasonic transducer with a frequency of 50MHz, selected to give sufficient penetration and good spatial resolution.The ultrasonic echoes that were used to make this image were “gated” on a very shallow depth immediately below the sample surface. This means that echoes only from this narrowly defined depth were used to

echoes a few millionths of a second later. The ultrasonic echoes come only from material interfaces. If the sample is homogeneous (as the wall of the pipe sample should be), there will be no echoes from the interior. If the sample contains two or more bonded materials, echoes will be sent back from the material interfaces, such as a metal to ceramic interface. The highest amplitude echoes are reflected from an interface between a solid and a non- solid; the non-solid is nearly always the air inside a gap of some type. A homogeneous solid material such as the pipe sample would be expected to have no internally visible features

Figure 1 : Acoustic image, focused and gated just below the pipe surface, reveals cracks that are invisible optically

Figure 2 : Deeper acoustic image shows the cracks within the pipe wall

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N ovember 2011

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