1346 Resonant Frequency and Damping

It was noted in Section that, for rotating machinery components such as bladed turbine wheels [59], it is important to know the resonant frequencies and damping characteristics. Knowledge of the uniformity of resonant frequencies helps to assure that the structure will not go into an unstable resonant condition thereby developing a potentially catastrophic condition. The NDE methods to measure resonant frequencies and damping capacity of ceramic composites were described in Section, Spohnholtz [53] and Bemis et al. [54] have described the various details involved in making these measurements.

In a recent set of experiments, a blisk (an integrally bladed disk) made of C(f)/SiC was examined [59]. Resonant frequency measurements were conducted using the setup described in Section Figure 13.40 shows a photograph of the blisk understudy. Figure 13.41 shows a plot of normalized resonant frequency for each blade prior to spin testing. Note that blade number 21 was shown to have a significantly lower resonant frequency. Subsequent X-ray tomographic imaging revealed that a void about 1 mm in size was present.

FIGURE 13.40 Photograph of bladed disk (blisk) made of C(f)/SiC and studied by impact acoustic resonance and X-ray tomographic imaging.
FIGURE 13.41 Resonant frequency of each blade of blisk shown in Fig. 13.40 and X-ray CT image showing the void in blade 21.
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