Vibration monitoring of steel beams by evaluation of resonance frequency decay rates

Defects in steel structures cause relatively small changes in the resonance frequencies, necessitating long observation intervals or the use of powerful shaking mechanisms. Much simpler monitoring procedures could be devised if the rate of decay of the resonance frequencies contains information about the structural integrity. The study presented here was aimed at determining which decay rates are most sensitive to small structural defects in steel beams. A technique, based on time-dependent spectral analysis of acoustic vibrations, has been developed to measure the decay rates for modes 2 through 6. An experimental study was conducted with 36-in. (0.9 m)-long steel beams with a rectangular cross section of 0.75 (19 mm) by 2.0 in. (51 mm). A slot, approximating a real crack, was cut to 0.7 in. (18 mm) in increments, in the center of one beam, so as to evaluate the sensitivity of the decay rates to the crack size. It was found that the logarithm of the amplitude of the resonance frequencies decays linearly with time, so that the resonance frequency decay rates can be quantified in terms of the slope of a linear regression. Significant differences in decay rates from intact and slotted beams were found for slots as small as 0.3 in. (7.6 mm).