Novel fiber Bragg Grating-based strain gauges for monitoring dynamic responses of Celtis sinensis under typhoon conditions

In recent decades, conventional electric instruments have been widely adopted to monitor rupture failure of trees by measuring the longitudinal strains of tree trunks. However, the good measurement accuracy is compromised by the significant difference in stiffness of the sensing element and tree trunks. Besides, the reliability of electric instruments under harsh environments, especially extreme weathers, such as thunderstorms and typhoons, is also doubtful. In this study, a novel strain gauge based on fiber Bragg grating (FBG) sensing technology was developed specifically for measuring the strain distribution of tree trunks under static or dynamic loading. The main principle of the design of the strain gauges is presented in detail. The laboratory calibration proves that the FBG-based strain gauges in polyoxymethylene (POM) and polylactic acid (PLA) backings show a better performance than those made of metal. To test the performance of this novel FBG-based strain gauge, a set of transducers were installed at different heights of a Celtis sinensis. Firstly, a pull test on this tree trunk was conducted to validate the good performance of the novel strain gauge when the tree is subjected to static loading. Secondly, the good dynamic performance of the novel strain gauge is proved by successfully recording the dynamic motion of a tree trunk during a typhoon. Furthermore, a monitor system relied on the FBG-based strain gauges is conceived to assess the resilience of urban ecosystems formed by trees to extreme weather events.