### ASA 124th Meeting New Orleans 1992 October

## 4pSA9. Acoustic monitoring of metallic beams by evaluation of resonant
frequency decays.

**Pierre Y. Robin
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*Dept. of Elec. Eng., Univ. of Houston, 4800 Calhoun Rd., Houston, TX
77204-4793
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The resonant frequencies of intact metallic beams, impacted with a hammer,
were observed to decay at different rates than for slotted beams. A new
technique, based on time-dependent spectral analysis of acoustic vibrations,
has been developed to measure these decays. The method involves computing the
Fourier transform from short-time windows of 25 ms, passed over 0.5 s of
recorded vibrations. As a result, the amplitude of the beam's resonant
frequencies can be observed as a function of time. In particular, the
amplitude, on a logarithmic scale, is shown to decay linearly with time, so
that the resonant frequency decay rates can be quantified in terms of the slope
of a linear regression. An experimental study was conducted with 36-in.-long
steel beams with a rectangular cross section of 0.75x2.0 in. A slot,
approximating a real crack, was gradually cut to 1.25 in., in the center of one
beam, so as to evaluate the sensitivity of the decay rates to the crack size.
The results show that these decay rates are discriminative features of the beam
integrity and can be used in a pattern recognition scheme classifying intact
and defective beams. [Work supported by NSF Grant No. MSS-9024224.]