Challenges and Opportunities offered by FBG Sensors for Reference-free Guided Waves based Structural Health Monitoring

Guided waves (GW) based structural health monitoring (SHM) is considered the most promising technique for thin-walled structures. Traditionally, the piezo-electric sensors have been used for the actuation and sensing of the GW in structures. But the piezo-electric sensors (PZT) cannot be embedded and are electrically and magnetically sensitive which limits their applications. A recent trend is to use the fiber Bragg grating sensors for GW based SHM. The FBG sensors are light weight, can be embedded, are immune to electric and magnetic changes, may be multiplexed and hence are seen as a replacement for the PZT sensors. The key drawback of the FBG sensors is the large equipment cost, the passive nature and the directional sensitivity. In the last few years some research has been carried out to overcome these shortcoming and employ the sensors for SHM. This paper outlines the challenges and the unique opportunities that the FBG sensors offers for the SHM in structures- more specifically with focus on reference-free detection. The special abilities of FBG sensors such as self-referencing, ability for mode separation, ability for acoustic coupling, ability for specialty optical fibers such as polarization maintaining FBG are discussed. At the same time, the challenges associated with the use of FBG sensors such as sensitivity to ambient conditions, ambient loads etc. are discussed and techniques to overcome these are proposed.