The Influence of Hot Water on the Fatigue Strength of Aramid/Epoxy Composites

Fatigue tests of 0° unidirectional and ±45° cross-ply composites of epoxy resin reinforced by two types of aramid fibers (HM50, Teijin and Kevlar 49, Dupont) were carried out in 71°C hot water. An emphasis was placed on the influences of hot water on the fatigue strength and fatigue mechanism, and the following was made clear. In case of ±45° cross-ply composites, the fatigue strength of both HM50 and Kevlar 49 composites decreased in 71°C hot water from that in 30°C water. It is considered that the fatigue strength of the resin and the interface between fiber and resin became weaker by the exposure to hot water. Both fiber/resin interfacial debonding and interlaminar delamination occurred more distinctly in 71°C hot water than those in 30°C water. In case of 0° unidirectionally reinforced composites, the fatigue strength of both composites became stronger in 71°C hot water than that in air and almost similar to that in 30°C water. It is considered that the resin absorbing water facilitated the local re-alignment of fibers under atensile stress. Kevlar 49 fibers splitted more easily than HM50 composites.