In-situ monitoring of carbon fiber/epoxy composite with FBG sensors under curing and thermal cycling conditions

Abstract

Residual strains from curing process as well as thermal expansion behaviors in application process are critical to structural design of composites. Here we demonstrate the applicability of fiber Bragg grating (FBG) sensors for in-situ measurements of strain evolution in epoxy resin and unidirectional (UD) laminate under curing and thermal cycling conditions. The validity of the FBG sensors was verified by the TMA tests. From the data recorded by the sensors, the information about real-time gel point, strain/temperature evolution and residual strains during curing can be obtained. The epoxy resin and UD laminates have distinct curing histories and characteristics under the same curing conditions because of the exothermic reaction. During thermal cycling, we found that the coefficient of thermal expansion (CTE) of the UD laminate is directional dependent as is the residual strain, i.e., it increases with fiber orientation angle. In addition, the relaxation of partial residual strains, attributed to the mismatch in CTE between the specimen and the mold, was captured by FBG sensors around the glass transition temperature during the first thermal cycle.