In this study, effect of thermal cycling after water absorption on flexural property of CFRP(Carbon Fiber Reinforced Plastics) was examined. Water absorption tests were conducted for 24 hours, 100 hours and 500 hours at 90°C. Thermal cycling test was conducted by using water absorbed specimen. The temperature range of thermal cycling test was from room temperature to-196°C. The static three-point flexural test of CFRP with freezing after water absorption was conducted based on JIS(Japanese Industrial standard) K 7074. Following conclusions are obtained. In case of immersion for 24 hours, flexural strength and modulus of CFRP with freezing after water absorption decreased with an increase of the number of cycles. In case of immersion for 100 hours and 500 hours, flexural strength and modulus of CFRP with freezing after water absorption did not change. The water absorption of CFRP was the phenomenon that the water penetrated into fiber/resin interface. So, in the case of immersion for 24 hours that the absorption time is not so long, damage progresses gradually in thermal cycling test. On the other hand, in the case of immersion for 500 hours is long, the damage progresses at the early stage of the thermal cycling test, and after that the damage does not almost progress.

This study investigated the effect of water absorption on the residual flexural property of CFRP during thermal cycling. Water absorption tests of CFRP and epoxy resin were conducted for 3 hours and 100 hours at 90°C. Thermal cycling tests of water absorbed CFRP and epoxy resin were also conducted. The temperature of the thermal cycling test ranged from-196ºC to room temperature, and the maximum number of cycles was 10 3 cycles. A static three-point flexural test of water absorbed CFRP was conducted after the thermal cycling test. A damage observation of CFRP during thermal cycling was performed. A singleedge notched bending test of water absorbed epoxy resin was also conducted. As a result, the following conclusions were obtained. In the case where the water absorption rate was 0.3% (3 hours), the flexural strength and modulus of CFRP decreased with an increase in the number of cycles. In the case where the water absorption rate was 1.1% (100 hours), there was almost no change in the flexural strength and modulus of CFRP. From the damage observation, crack initiations in the cross section of 0.3% and 1.1% water absorbed CFRP were found during thermal cycling. The number of cracks in the cross section of 0.3% and 1.1% water absorbed CFRP increased with an increase in the number of cycles. The Mode I fracture toughness of 2.6% (100 hours) water absorbed epoxy resin increased compared with that of 0.7% (3 hours) water absorbed epoxy resin during thermal cycling. Therefore, the flexural property of CFRP was affected by the Mode I fracture toughness of water absorbed epoxy resin.