Railroad bridge monitoring using Dragonfly® piezoelectric strain gauge

Structural Health Monitoring (SHM) applied to civil structures such as bridges, high-rise towers, and electricity pylons can help plan their maintenance, detect damages or un-expected events, and extend their remaining operating lifetime. Metallic foil strain gauge and vibrating wire are the historic sensors used to measure strain on civil structures. However, civil structures are designed to undergo little deformation in service (0.1-50 µdef). Both SG and vibrating wire are limited by their resolution around 1 µdef. Moreover, they are known for their DC drift over time, which complexifies long term monitoring. There is therefore a need for more sensitive and less prone to drift sensors in the time domain. Dragonfly new piezoelectric strain gauges, with a resolution three order of magnitude over classic sensors, solve these issues and enable new applications. Dragonfly sensors consist in an extremely thin piezoelectric ceramic packaged in a flexible PCB and pre-wired with a SMA connector. The flexible sensor is glued with cyanoacrylate glue to the object to be studied. This paper presents the implementation of Dragonfly sensors on a railroad bridge. With a single sensor, it has been possible to identify the bridge girder first two resonance using ambient noise, train passage loading for fatigue-life analysis, train passage reproducibility, and pedestrian passage on the walkway. A complete comparison with classic metallic foil strain gauge is detailed.