Guided wave-based system for real-time cure monitoring of composites using piezoelectric discs and phase-shifted fiber Bragg gratings

A real-time, in-process cure monitoring system employing a guided wave-based concept for carbon fiber reinforced polymer composites was developed. The system included a single piezoelectric disc that was bonded to the surface of the composite for excitation, and an embedded phase-shifted fiber Bragg grating for sensing. The phase-shifted fiber Bragg grating almost simultaneously measured both quasi-static strain and the ultrasonic guided wave-based signals throughout the cure cycle. A traditional FBG was also used as a base for evaluating the high sensitivity of the phase-shifted fiber Bragg grating sensor. Composite physical properties (degree of cure and glass transition temperature) were correlated to the amplitude and time of arrival of the guided wave-based measurements during the cure cycle. In addition, key state transitions (gelation and vitrification) were identified from the experimental data. The physical properties and state transitions were validated using cure process modeling software (e.g. RAVEN®). This system demonstrated the capability of using an embedded phase-shifted fiber Bragg grating to sense a wide bandwidth of signals during cure. The distinct advantages of a fiber optic-based system include multiplexing of multiple gratings along a single optical fiber, small size compared to piezoelectric sensors, ability to embed or surface mount, utilization in harsh environments, electrically passive operation, and electromagnetic interference (EMI) immunity. The embedded phase-shifted fiber Bragg grating fiber optic sensor can monitor the entire life-cycle of the composite structure from curing, post-cure/assembly, and in-service creating “smart structures”.