Fiber optic sensor development for real-time in-situ epoxy cure monitoring

Analysis of a new fiber optic sensor based on the principle of Fresnel reflection is presented in order to develop a robust, flexible, readily embedded, high sensitivity and low-cost cure monitoring tool for intelligent control of composite manufacturing. This approach is distinct from previous work on on-line in-situ monitoring sensors in that the transducer is simply the fiber optic/epoxy interface. This leads to a more simple, less intrusive, and lower cost sensing system. The response of the sensor is a function of the mismatch in refractive index between the fiber optic end/resin interface. The refractive index of the resin is a nonlinear function of the temperature and the cure reaction of the resin. The sensing system detects the mismatch in the refractive index and generates a characteristic profile that gives the chemorheological information about the curing resin. Under isothermal cure conditions, the effect of the temperature on the refractive index mismatch can be eliminated. In this situation, the optical response of the sensor is only dependent on the cure kinetics and the state of the cross-linking in the material. A calibration method has been developed to interpret the optical response directly as the degree-of-cure of the resin. This work provides analysis of the fundamental sensor response and correlates it to the material state (degree-of-cure). These results provide a basis for extending the use of this sensor technology for on-line real-time cure monitoring and control.