Ct C

0 10 20 30 40 50 60 70 80 Distance from edge of Si3N4 specimen, mm

FIGURE 13.18 Sensitivity of ultrasonic velocity to distributed creep damage.

0 10 20 30 40 50 60 70 80 Distance from edge of Si3N4 specimen, mm

FIGURE 13.18 Sensitivity of ultrasonic velocity to distributed creep damage.

decisions can be made based on these data. One such example is in monolithic rigid hot-gas filters [38, 39]. Rigid ceramic hot-gas filters with about 15% porosity are being developed to clean particulate matter in hot-gas streams of various coal-fired plants. One principle application is in combined cycle power plants where the hot-gas is used for fuel in a gas turbine. In some plants, over 3000 such filters can be used and thus reuse/replace decisions are costly. In recent work [39], filters studied were made of clay-bonded SiC, recrystallized SiC, and alumina-mullite.

These filters were studied using an NDE technique referred to as acousto-ultrasound [40, 41]. An acousto-ultrasonic (AU) system (see Fig. 13.19) is a hybrid combination of classical ultrasonic techniques coupled with acoustic emission technology. A typical system consists of (a) two acoustic transducers placed in contact with the component under investigation, (b) supporting signal conditioning and detection electronics, (c) a computer for signal generation and detection, and (d) software packages for digital signal processing. When one transducer is pulsed, an acoustic wave travels along the wall of the specimen and is then detected by the second transducer. Previous research [41] has shown that the AU technique is applicable to a variety of materials. Plotting the stress-wave factor, obtained from the detected signal, versus strength of the materials yielded

FIGURE 13.19 Schematic diagram of an automated acousto-ultrasonic method used to characterize accumulated damage in rigid-ceramic hot-gas filters.

300 o 250

0 0

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