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.

The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change of surroundings, e.g., temperature and water, have been measured by micro-Raman spectroscopy. The specimens were soaked in hot air, water and hot water, and returned to the beginning state to cause cyclic changes of temperature and water absorption :(1) heating and cooling at atmospheric environment, (2) immersing in the water of the room temperature and drying, and (3) immersing in the hot water of 80 and drying at the room temperature. In the case of ( 1) and ( 2), the fiber axial stresses after the cyclic change of the surroundings recover almost the same before the heating or the immersion. On the other hand, the fiber axial stress after the drying is more compressive than that before the hygrothermal exposure in the case of (3). Hygrothermal effects on the residual stress are discussed based on fiber stresses caused at elevated temperature over the glass transition temperature of the epoxy resin.

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