The Long Term Performance of a Guided Wave SHM System on a Steel Tank

A guided wave structural health monitoring system has been attached to a steel water tank situated outside in a sheltered location. It consists of an array of PZT disk transducers which preferentially excite the S0 mode. The system has been operating for a period of 3 years. The first arrival time, maximum signal amplitude and bandwidth of the received signals was found and they exhibited no long term changes over the collection period. The centre frequency decreased by about 5 kHz for the last year of operation but this decrease is less than the range of earlier measurements. The dataset was processed using different baseline subtraction based approaches and in all cases the system performance degraded quickly to a point where damage was unlikely to be detected in a period of days. This was not suggested by the measurement of single parameters. These results highlight the need for better understanding of how transducers perform over long periods of time and signal processing techniques to compensate for these changes.