Adaptive Fiber Bragg Grating Sensor Network for Structural Health Monitoring: Applications to Impact Monitoring

Abstract

A passive structural health monitoring (SHM) system for locating foreign-object impact using a network of fiber Bragg grating (FBG) sensors that monitor high frequency dynamic strains is described. The FBG sensor signals are adaptively demodulated using a two-wave mixing (TWM) spectral demodulator. Strains applied on the FBG sensors are encoded as wavelength shifts of the light reflected by the FBG sensor which are then converted into phase shifts and demodulated by the TWM interferometer. The demodulator adaptively compensates for low frequency drifts caused by large quasi-static strain and temperature drift and allows only high frequency signals to pass through. The FBG sensor network is mounted on a plate, and the structure is subjected to artificial impacts generated by dropping small ball bearings. Owing to the directional sensitivity of the FBG sensors, an FBG sensor-pair configuration is used at each sensing location. The impact signals from multiple FBG sensors are simultaneously acquired at frequencies of up to 180kHz. Using time-frequency wavelet analysis, the group velocity dispersion curve of the detected Lamb wave modes is obtained from the measured transient responses of the sensors, and this is used to determine the location of the impact.