In-situ Measurement of Cure Index and Strain of FRP Laminates by Fiber Optic Sensors

Abstract

Recent sensing technologies allow us to monitor cure process in FRP materials at real time. Fiber optic sensors are most promising for measuring cure index, cure shrinkage and residual strain. In the present paper, we used refractive-index sensors and FBG (Fiber Bragg Grating) sensors in order to measure cure index of matrix and curing strain in FRP laminates during a hot-press molding. Refractive-index sensors, FBG sensors and thermocouples were embedded in the laminated prepregs. From the measured refractive-index variations and temperature, we estimated cure index of matrix resin from B-stage state to cure completion. The experimental results showed that the cure index curves obtained by the refractive-index sensors showed good repeatability. Comparing results of sensors embedded in 0° direction with 90° direction, it appeared that the both results agreed very well with each other. However, the cure index curves measured by the sensors embedded in resin rich region showed apparent delay from the curves by the sensors in FRP region. Strain profiles were obtained by the FBG sensors embedded in 0° and 90° of unidirectional FRP laminates. From the results, it was found that strain of laminates showed isotropy when the cure index was small