ABSTRACT
Ultrasonic monitoring, making use of the sensitivity of the coda of repeated transmission measurements to changes in stress, temperature, moisture, as well as localized or distributed damage, has gotten attention in structural health monitoring (SHM) research recently. Analysis methods such as coda wave interferometry (CWI), including its nonlinear extension, have been shown to be able to measure ultrasonic wave velocity changes with a 1∙10-5 resolution, while indicators such as cross-correlation or cross-coherence have been used to distinguish between reversible and irreversible changes. Several small- and large-scale laboratory experiments have demonstrated that stress changes in structures can be captured or damage detected in a very early stage. The use of this technique for pre-warning before failure are currently under investigation, as well as detailed research on the physical causes and the connection between ultrasonic wave properties and material/structural behavior. Recently, several of large-scale laboratory and real structures have been instrumented with embedded ultrasonic transducers to gather experience and evidence on how to use this technology in real-world applications. Preliminary results from installations on a new bridge, an existing bridge, a tunnel, a laboratory earthquake test as well as a historic stadium in Germany, Poland, and the United States, respectively, are presented. Environmental influences (mainly temperature) and validation by load tests are discussed.
