A strain-insensitive method for matrix crack density detection in FRP composites using embedded FBG sensors

Fiber Bragg grating (FBG) sensors have been employed to detect matrix crack density, providing early warning for the structural health of fiber-reinforced polymer (FRP) composites. However, the spectral features commonly utilized for detection exhibit cross-sensitivity to multiple parameters, such as strain amplitude and strain gradient distribution under different external loading condition. This reduces the correlation between these spectral features and crack density, leading to misjudgment of crack density. Here, we proposed a novel spectral feature analysis method for detecting matrix crack density in FRP structures using embedded FBG sensors. The innovation of this method lies in the identification of strain-insensitive, newly emerged spectral peaks (NESPs) dominated by crack spacing, which exhibits insensitivity to strain amplitude and strain gradient distribution. The detection mechanism of this method is theoretically derived and analyzed, and its validity is confirmed through numerical simulations and experiments. Simulation and experimental results indicate that the wavelength difference of NESPs has an inverse proportional relationship with crack spacing. Experimental results demonstrate the strain insensitivity of the NESP wavelength difference, while the FWHM exhibits a distinct linear strain dependence. As crack density increases and crack spacing decreases, the NESP shifts away from the main peak and exhibits a distinct broadening trend in the spectral distribution. The resulting spectral pattern can be effectively utilized for detecting matrix crack density in FRP structures. This method provides a novel solution for in-situ health monitoring of FRP structures using embedded FBG sensors.