Damage Monitoring in Composite Structures through High Speed, Full-spectral Interrogation of Fiber Bragg Grating Sensors

This research validates a newly developed high speed, full-spectral interrogator for dynamic measurements of embedded fiber Bragg grating sensors and demonstrates new damage monitoring capabilities for composite structures based on this unique sensing capability. The new instrumentation enables rapid measurements of strain and vibration for structural health monitoring of composite structures and is capable of scanning speeds up to 300 kHz. Several tests were conducted to validate the new measurement system. Fiber Bragg grating sensors were embedded in composite laminates which were then exposed to low-velocity impacts. The system captured the impact in real time and revealed new information on the material being tested. Matrix cracking and delamination of the plies were observed through the new high speed rate capabilities of the measurement system. Birefringence of the optical fiber due to transverse compressive loading resulted in peak splitting in the FBG sensor response.;To further test the new dynamic measurement capabilities, a vibration platform was developed for lab testing to simulate a realistic in-flight aircraft environment. For characterization of the platform, testing was simplified by mounting FBG sensors on a thin aluminum plate near a notch tip. As non-uniform strain increased with tension, the forced-vibration response of the thin plate revealed changes in the sensor frequency response. Numerical finite-element simulations were conducted to validate the measurements. During.;static loading, simulations of the FBG sensor response were conducted by extracting axial strain from the elemental solution of the model to use as input into a modified transfer matrix. The simulations indicated increasing distortion in the full-spectral FBG response as the stress concentration impinged greater magnitudes of non-uniform strain at the location of the sensor.;The final step in this research embedded FBG sensors in composite lap joints to monitor cyclic fatigue damage during a realistic in-flight aircraft environment. Dynamic full-spectral measurements were conducted at 100 kHz to study the dynamic precursors to joint failure. The full-spectral information was used to avoid dynamic measurement errors encountered when interrogating a complex multi-peak spectrum using traditional peak following or edge filtering techniques. As damage accumulated, frequency analysis of the corrected peak wavelength indicated progressive nonlinear dynamic behavior marked by intermittent frequencies and amplitudes not associated with the external harmonic excitation. The broadband, nonlinear frequency response revealed a transition to a chaotic state of vibration attributed to accumulated damage. Numerical simulations were used to simulate the fatigue damage by introducing plastic deformation and geometric nonlinearities to the lap joint. The dynamic behavior was assessed by performing forced-vibration transient analyses. Comparable transitions in the dynamic behavior of the lap joint captured by the FBG sensor were found during the simulations and can be attributed to fatigue damage at the adhesive layer.