Damage detection in FRP structures using fiber Bragg grating dynamic strain sensing systems

Fiber optic sensors have become widely used for structural health monitoring in recent decades. The aim of this research is to characterize the dynamic failure signals emitted in fiber reinforced polymer (FRP) stay cable and specimens using Fiber Bragg Gratings (FBGs) and two types of interferometric demodulation systems, namely Michelson interferometer (MI) and two-wave mixing interferometer (TWMI) for detection. Due to its one-dimensional form, only one FBG and the Michelson interferometer are used for damage monitoring in a carbon FRP stay cable under various types of loading. Michelson interferometer is capable of detecting frequency contents extending up to 500 kHz, where frequency contents below 250 kHz are categorized as matrix failure and those above 300 kHz corresponded to fiber failure. Two channels of FBGs are used with the TWM interferometer to track local damage in coupon-size FRP samples. Using TWM scheme, continuous and burst acoustic emission events are detected with frequency responses extending up to 125 kHz in coupon-size GFRP specimens, limited only by the sampling rate of the data acquisition system. The experimental results suggest that both types of FBG demodulation systems may be suitable for monitoring high frequency mechanical strains in civil structures, providing a tool for local structural damage detection.