Identification of excitation and modal frequencies of wing rib beams using fiber bragg grating sensors: Experimental characterization and comparative simulation analysis

Vibration-induced fatigue failure is a prevalent and critical damage mechanism in wing rib-beam structures operating under complex loading and harsh service environments. Accurate characterization of their dynamic properties is essential for enhancing structural safety and reliability. Strain modal analysis, recognized as a powerful tool for structural health monitoring (SHM) and early damage detection, shows significant potential in high-performance aerospace applications. Fiber Bragg Grating (FBG) sensors, owing to their compact size, low weight, immunity to electromagnetic interference, and high multiplexing capability, are particularly advantageous for capturing dynamic strain responses in complex structural configurations. This work presents an integrated frequency identification approach that combines distributed FBG sensing with two complementary frequency-domain techniques: Fast Fourier Transform (FFT) and Multiple Signal Classification (MUSIC). FFT is applied to rapidly and intuitively identify dominant single-frequency excitation components, while MUSIC provides high-resolution separation of closely spaced modal frequencies and precise extraction of modal parameters. The methodology is first implemented on a full-scale wing rib-beam structure to simultaneously identify excitation and natural frequencies. It is further validated through controlled experiments and finite element simulations on a representative cantilever beam. Experimental results demonstrate that the combined FFT-MUSIC framework achieves accurate and efficient identification of both excitation and natural frequencies. The frequencies and strain mode shapes obtained from the FBG system exhibit excellent agreement with those measured by conventional accelerometers and predicted by numerical models, confirming high accuracy and repeatability. The proposed method offers a robust and scalable solution for dynamic performance monitoring and vibration fatigue assessment of aerospace structures under complex service conditions.