Novel Optical-Fiber-Based Cure Monitoring Technique for Large-Scale Composite Structures by Hybrid Brillouin-Rayleigh System

Carbon fiber reinforced plastics (CFRP) have high specific strength and stiffness and have been used extensively in many industrial fields. Especially in a field of aircraft, their application extends to primary structures. Therefore, quality assurance of the composite materials is necessary and effective cure monitoring techniques of composite materials are becoming more and more important since quality of the materials is very dependent on their cure processes. Optical-fiber-based distributed sensing system is considered very useful for cure monitoring of composite structures due to their outstanding properties. Since they are light weight and small, they do not affect mechanical properties of composites in which optical fibers are embedded. However, since both strain and temperature changes affect outputs of the sensing systems, they cannot separate strain and temperature contributions. In order to overcome this limitation, a newly developed hybrid Brillouin-Rayleigh system is considered to be quite useful. This study demonstrates a new effective cure monitoring technique by applying the hybrid system to composite materials. One optical fiber for the system, an FBG sensor and a thermocouple were embedded in a CFRP specimen to measure thermal residual strain and temperature change in a cooling down period of cure process. The temperature and residual strain obtained by the hybrid system agreed well with the results by the FBG sensor and the thermocouple, confirming that the proposed cure monitoring technique is quite useful for separated strain/temperature measurements of composite materials. By applying the proposed technique to actual composite products, quality assurance for whole parts can be effectively accomplished.