Deflection monitoring of thin-walled wing spar subjected to bending load using multi-element FBG sensors

This paper presents a measurement technology based on the multi-element fiber Bragg grating (FBG) sensors for monitoring the deflection of thin-walled wing spar. A finite element model of the tested thin-walled beam structure subjected to bending load is developed in order to analyze stress status and to derive the convenient position of the optical fiber sensors for a correct monitoring of the wing spar beam. FBG rosette has been designed between the shear web and spar cap as a new installation position of bonding optical fiber sensors for estimation the deflection. An experimental campaign is carried out in order to verify the proper behavior of the deflection of wing spar beam and sensing strategy. The loading test results revealed that multi-element FBG rosette was successfully implemented on the tested thin-walled wing spar beam as well as providing reliable strain and deflection monitoring data from the FBG sensors. The results show that a good correspondence between the input load and output strain of the FBG sensing system with good reproducibility, and the comparative analysis between FBG and resistance strain gage have been achieved, which measurement results of both are consistent. These data and monitoring method are useful for fatigue life prediction of the potential thin-walled I-beam structure.