Investigation of FBG based optical NDT for aircraft structures

Abstract

There have been many innovative inventions of fiber optic sensors introduced in the past century. A good candidate for smart structure applications is the optical Fiber Bragg Grating (FBG) sensors. It is able to perform strain measurement withought signigicant compromise to the host structure. In addition, the FBG wavelength is directly related to the fiber strain, making it possible to obtain absolute measurements of the strain. To date, FBG has become one of the most prominent sensors for structural health monitoring and defects detection. Further development and researches have been ongoing to explore its capability of detecting defect. The purpose of this report is to configure an optical Fiber Bragg Grating based sensor system as a structural health monitoring system and to investigate the feasibility of wavelength shift as an interrogation factor for defects detection. The experiments carried out included firstly tensile testing which was done to analyze structural properties of [0°,0°] and [0°, 90°] CFRP composite configurations. Secondly, three point bending tests were carried out to investigate the Bragg wavelength response for FBG surface mounted CFRP specimens. The effect of delamination of different geometrical characteristics was explored, including establishing a linkage between wavelength shift and load. Lastly, much effort was contributed to perform vibration analysis for the real time structural health monitoring on the CFRP specimen. A linear trend and significant difference of gradient change was observed in the Bragg wavelength shift vs. load graphs for CFRP specimens with different delamination. The results showed good sensitivity of the FBG optical sensor system as a structural health monitoring system and verified the feasibility of Bragg wavelength shift as an interrogation factor for defects detection. Last but not least, as a recommendation to future works, embedded FBG was recommended since it offers better sensitivity and results.