Determination of residual stresses in a single FBG fiber/epoxy composite system

A new approach to determine residual fiber and matrix stresses in single fiber Bragg grating (FBG) fiber polymer matrix composites is developed based on the earlier work by Hoffman et al., 2020 [( Hoffman et al., 2020) 1] and Khadka et al., 2020 [( Khadka et al., 2020) 2]. In this new study, our already measured fiber strains in the composites based on room temperature (RT) and higher temperature (HT) epoxies were used to determine the stresses in the fibers. The total manufacturing residual stresses in the fibers were also separated for the curing and cooling parts of the manufacturing cycles. The cooling stresses for both epoxy systems were then independently verified through single fiber thermal numerical models. Subsequently, using our new methodology based on the Eshelby misfit approach and the single fiber thermal models, the total residual stresses in the matrix were found and then separated into the curing and cooling stresses. The maximum axial stresses at the matrix/fiber interfaces were found to be increasing with an increase in the curing temperature of the RT epoxy. For the highest curing temperature of the RT resin (70 °C), the residual axial stresses in the matrix (20 MPa) were close to the stresses in the HT epoxy (25 MPa) at the curing temperature of 121 °C. The curing matrix stresses were observed to be significant for both epoxy systems with their contributions to the total stress ranging from 28% to 48% depending on the resin type and curing conditions.