Software Controls the Imaging Process
Figure 2 is an acoustic image known as a C-Mode image, produced by raster-scanning a transducer pulsing 30 MHz ultrasound over the surface of the composite sample and collecting echoes from a depth of interest. Three features are immediately evident:
- The cross-hatch pattern of the graphite fibers is visible.
- The dark diagonal line at center is the slit milled into the composite.
- The white features adjacent to the slit are delaminations between the layers of the composite.
This image does not encompass the full thickness of the composite. In this sample, echoes can be reflected from each of the 27 layers, which would produce a very complex image. For this reason, a C-Mode image is typically “gated” on a narrow time window — only echoes arriving within that window are used to make the image. Here the gated depth is the top few layers of the composite.
Software Takes Over
The ultrasonic echoes in Figure 1 travel through the lens of the transducer and into the piezoelectric element, and the mechanical waves are converted into RF electrical signals. These analog signals are then converted into digital signals; at this point software takes over from hardware. Each signal is first analyzed to determine its polarity — positive if the interface was from lower to higher acoustic impedance, and negative if the opposite.
From this point there are two general functions that software can carry out with the millions of incoming signals. First, it can classify and sort them to make one or more of the many types of acoustic images. Second, it can analyze the results of classifying and sorting.
Software operations begin by assigning a gray-scale value to some attribute of the signal — often its amplitude, but for some purposes, its frequency or location or polarity. At a single x-y coordinate within a single gate, not one but many echoes are returned. Suppose, for example, that a 100 MHz transducer is being used. The ultrasound in a single pulse from this transducer will actually contain ultrasonic frequencies from perhaps 70 to 120 MHz. In producing a C-Mode acoustic image, software selects the single echo at that x-y location that has the highest amplitude, and then assigns to that echo a gray-sale value between 1 and 256. This value can then, if desired, be converted into a color by any one of numerous color maps that assign colors to gray-scale values. The other signals from this x-y coordinate are discarded. White areas in Figure 2, for example, have the highest amplitude and indicate gaps. Black areas have very low amplitude, or returned no signal at all. Many areas, especially among the fibers, are some shade of gray.
One newly developed technique deviates sharply from C-Mode imaging by discarding both amplitude and polarity of echoes. Instead software assigns grayscale values based only on the echo arrival time, to measure the precise distance from the transducer to the surface of the part at each x-y coordinate. The result is a contour map of the surface (see Figure 3). Magenta indicates the highest points on the surface, while green indicates the lowest points. The color map in Figure 3 shows the relative altitude of each coordinate. The area around the drilled hole is raised, a feature that can also be seen in the cross-sections at the right and bottom.