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evaluation: a visual image. When an area suspected of defect or damage is
systematically tapped and the results are displayed as a visual image, the inspector
can identify the defect or damage, as well as the normal substructures. The size,
shape and location of the damages can be assessed much more easily with the aid
of an image. An image-capable tap test device would also produce an electronic
archivable record for later reference.
So the question is: what should be the quantity to display in a tap test image? The
answer lies in an important attribute of tap test: a tap test samples the local
mechanical property of the structure, specifically the contact stiffness. This is quite
different from a global test such as listening to the “ring” of a railroad wheel after it
is struck with a hammer. In a tap test the time of contact (impact duration) between
the impactor mass and the structure depends on the local stiffness of the structure.
An impactor would bounce off quickly from a solid, un-damaged structure (short
contact time), but would ride with the surface of a disbonded or broken structure
and produce a long contact time. The duration of the impact is therefore the most
fundamental quantity to measure and record in a tap test. The common practice of
measuring the impact duration is to use an accelerometer that contains a
piezoelectric crystal. As an example, figure 1 shows a 8-inch diameter repaired
region on a honeycomb composite sandwich with 7-ply CFRP facesheet. The image
shows short contact time (blue color) attributed to increased stiffness around the
scarfed circle and potting at the center, and long contact time (orange to black) due
to imbedded flaws and induced damages.
Page 1
Story of the Tapper
Figure 1 Tap test image of a repair and engineered flaws on a CFRP honeycomb sandwich
The next issue in producing a tap test image is position encoding. To generate an
image, one needs to acquire contact time data as a function of position over the
area of inspection. Although there are numerous motorized, computer-controlled
scanning systems that are capable of acquiring such data, the experience of the
CNDE composite group has been that the airline inspection community tends to shy
away from complex automated systems due to cost, speed and operator training
considerations. The composite group has therefore opted for a simple method of
position encoding: a thin plastic sheet with a printed square grid taped over the area
of inspection. To take data, each grid box was tapped once by hand. The tapping
mass was an accelerometer fitted with a hemispherical tup. The output voltage
signal of the accelerometer was processed in an accelerometer-computer interface
circuit and the contact time data were then fed into a laptop PC for generating the
tap test images. A considerable and sustained effort was made to develop versatile
software for data acquisition and processing. To initiate a scan, the operator would
specify the width and length of the scan area and the row and column convention on
the PC and then proceed with the tapping. Figure 2 shows a manual tap test
conducted on the wing of an MD-80 over an aluminum honeycomb structure known
as a heater blanket. The scan image shows small disbonds in a 2-foot by 3-foot
inspection area. Although the manual mode of tap test is intrinsically tedious, it still
remains a valuable technique for inspecting curved, irregular parts and in hard to
reach places.
Page 2
Story of the Tapper
Figure 2 Grid overlay used as a position encoder in manual tap test on an MD80 wing. The resulting image shows a 2-foot by
3-foot area.
Although the manual tap test system using the grid overlay was successfully applied
to on-aircraft inspections and produced useful images, the slow speed and tedious
data taking clearly needed improvements. To make the tapping faster and more
uniform, Dan Barnard and John Peters, two engineers in the FAA-funded project,
invented an elegant “magnetic cam” that used the repulsive force between small but
strong permanent magnets to throw the accelerometer toward the part surface. As
shown in figure 3 and in the linked movie, magnets embedded in the
circumference of a wheel pushed a magnet in a rocker arm, and hence the
accelerometer, up and down as the wheel rolls over the surface. Because the
magnetic cam is based on action-at-a-distance, it is largely free from wear and
alignment problems and hence superior to a mechanical tapper. The magnetic cam
makes uniform, equally spaced taps along the rolling direction and only requires an
overlay with a set of parallel lines to guide its indexing motion as it scans an area.
　
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