In some samples, it is important to seek out very subtle features at the interface between two materials. The feature of interest might be, for example, greater or lesser degrees of bonding at various spots on the interface. Each x-y coordinate at the interface will reflect ultrasound at many different frequencies, as mentioned above. In C-Mode imaging, only the single highest-amplitude echo is used, and the others (there may be 20 or 30) are ignored. One type of software regime accepts all of the echoes of all frequencies from an x-y location and produces a series of planar images, one for each frequency. The process is known as frequency-domain imaging, to distinguish it from time-domain (C-Mode) imaging, where signals are classified by their arrival time. If the range of frequencies is from 80 to 115 MHz, software may produce twenty or so acoustic images, each providing a different view of the same interface at a single frequency. A feature that is absent or ambiguous at 92 MHz may be crisp and sharp at 87 MHz (see Figure 4).
For some samples the most useful type of acoustic image is a non-destructive cross section. Visually, this acoustic image is the equivalent of viewing a physical cross section, but is done without destroying the part. Often it is preceded by a planar C-Mode acoustic image. The planar images display the whole area of the part, which might be a ceramic substrate containing internal traces. The planar image can locate in x and y specific anomalies or defects that can then be non-destructively cross sectioned.
Scanning proceeds along a single line in the planar image that marks the sectioning plane that intersects the anomaly or defect. The transducer scans back and forth along this line, going deeper at each pass. The return echoes are collected by the transducer for processing. The key parameters are x-y location, elapsed time, amplitude, and polarity. Software assigns gray-scale values and arranges the resulting pixels into a display showing the cross-section.
The non-destructive cross section and the planar image of the composite are shown in Figure 5. The sectioning plane passes vertically through damaged areas but not through the milled slit. In the cross section at bottom, irregular white features form an inverted U shape; these are delaminations and cracks caused by drilling and milling. The U shape seen here is generally considered acceptable because delaminations travel along fiber lengths where the material is strongest. The smaller, more regularly arranged features in the top layers are minor delaminations, probably caused by lack of wetting between the fiber and the epoxy.
There are numerous other modes in which hardware can collect and software can manipulate acoustic signals to solve specific problems, such as displaying the 3-dimensional structure of an internal feature like a crack, or preserving the entire acoustic content of a sample to permit comparison to its original condition after failure in testing or in service. These modes, most of which were developed and patented by Sonoscan, have greatly broadened the scope of problems that can be solved.
This article was written by Tom Adams, consultant at Sonoscan, Inc. (Elk Grove Village, IL). For more information, visit http://info.hotims.com/34455-145.