The optical properties of curing epoxies and applications to the fiber-optic epoxy cure sensor

Abstract An in situ fiber-optic technique for determining the endpoint of cure of a thermoset polymer has previously been demonstrated and described. The technique requires no calibration and is readily used as a cure detector in epoxy-based composite materials. The sensor depends upon the relative difference in refractive index between a fully-cured epoxy fiber and the curing epoxy matrix of the composite. In this paper, we present data on the change of the index of refraction of typical aerospace thermoset resin during cure at several elevated temperatures. The measurements were made using a fiber-optic Fresnel reflection technique.