Dependence of Fiber Stresses on Cyclic Changes of Temperature and Water Absorption in Carbon/Epoxy Single-Fiber Model Composites

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In this study, we examine the variation in residual stresses in polymer matrix composites with cyclic changes in hygrothermal environments by measuring the axial stresses in the fiber of single-carbon fiber/epoxy resin model composites using Raman microspectroscopy. To change the temperature and water absorption in the specimens separately and simultaneously, the specimens were (1) heated and cooled at atmospheric environment, (2) immersed in water and dried at room temperature, and (3) immersed in hot water (80°C) and cooled to room temperature in water and then dried. All specimens were thus restored to their initial states and then evaluated. In the cases of (1) and (2), the fiber stresses reverted to almost the same as their initial states, but in the case of (3) the axial stress after hygrothermal cyclic change was more compressive compared to the initial state. The differences among the results of the three cases are discussed based on the fiber stresses at elevated temperature, which were evaluated by Raman microspectroscopic measurement performed at temperatures ranging from room temperature to those above the glass-transition temperature of the epoxy matrix resin.