Methodologies for quantifying changes in diffuse ultrasonic signals with applications to structural health monitoring [5768-14]

Changes in diffuse ultrasonic signals recorded from permanently mounted sensors can be correlated to initiation and growth of structural damage, offering hope that sparse sensor arrays can be utilized for monitoring large areas. It is well-known that benign environmental changes also have significant effects on diffuse ultrasonic signals that are of comparable magnitude to the effects of damage. Several methodologies are investigated for quantifying differences in diffuse ultrasonic signals by computing parameters that can be used to discriminate damage from environmental changes. The methodologies considered are waveform differencing, spectrogram differencing, change in local temporal coherence, and temperature compensated differencing. For all four methods, a set of baseline waveforms are first recorded from the undamaged specimen at a range of temperatures, and subsequently recorded waveforms are compared to those of the baseline set. Experimental data from aluminum plate specimens with artificial defects are analyzed. Results show that the local coherence method is the most effective for discriminating damage from temperature changes whereas waveform differencing is the least effective. Both the spectrogram differencing method and the temperature compensated differencing method offer intermediate performance. As expected, the efficacy of all four methods improves as the number of waveforms in the baseline set increases.