Monitoring of Fabrication Strain and Temperature During Composite Cure Using Fiber Optic Sensor

Abstract

Fabrication strains during cure can lead to warpage or spring-back, which presents difficulties in the assembly of composite structures and residual stresses due to differences of thermal expansion coefficients between fiber and matrix in cooling stage can have intense effects on the mechanical properties of the composite product. The quality of the final composite part is directly affected by cure time, temperature, and pressure. The knowledge of cure process is very helpful to obtain high quality composites at reduced producing cost. Therefore, sensors capable of monitoring the cure process are desired and fiber optic sensors are the good candidate for cure monitoring of composite materials. In this paper, we present the simultaneous monitoring of the fabrication strain and temperature during the composite cure process by using fiber optic sensors. Fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors are used to monitor those measurands. The characteristic matrix of the sensor is analytically derived and measurements can be done without sensor calibration for each experiment. A wavelength-swept fiber laser is utilized as a light source. FBG/EFPI sensors are embedded in graphite/epoxy composite laminates at different direction and different location. We perform the real time monitoring of fabrication strains and temperatures at two points of composite laminates during cure process in an autoclave. Through this experiment, we can provide a basis for the efficient smart processing of composites.