Humidity influence on embedded fiber Bragg grating strain sensors

Fiber Bragg grating strain sensors embedded in composite material are generally coated with polyimide which can operate at high temperature environment with large tensile stress. However, the volume of polyimide coating is influenced by humidity. The humidity influence on fiber Bragg grating strain sensors which were embedded in carbon fiber reinforced composite material was investigated by theoretical analysis, finite element simulation and experimental method. The partition of mesh was optimized to improve the precision of the simulation, and the simulated strain variation of fiber Bragg grating strain sensor caused by relative humidity change was less than 4 . The humidity experiment results were obtained from 10 days continuous tests, and the measured strain variation of fiber Bragg grating strain sensor in carbon composite material was less than 8 with temperature compensation. The simulation and experiment results reveal that the embedded fiber Bragg grating strain sensors coated with polyimide are not significantly influenced by humidity change. The characterizations of embedded fiber Bragg grating strain sensor, including repeatability and linearity were tested and analyzed in tension test. The embedded fiber Bragg grating strain sensors possess good repeatability and linearity.