The Influence of Water and Test Frequency 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 an aramid fiber (HM50, Teijin and Kevlar 49, Dupont) were carried out in air and in water at a low test frequency of 1Hz, in order to investigate the influence of water absorption on the fatigue mechanism at such a low test frequency. The following was found. In the case of ±45° cross-ply composites in air, the fatigue strength increased at 1Hz from that of 16.7Hz due to the scarce generation of heat. More fibers were pulled out in water than in air because of the weakening of the resin and fiber/resin interface due to the long exposure in water. A large splitting of Kevlar 49 fibers was found in the fatigue test in water, which had a meaningful effect on the fatigue strength. In the case of 0° unidirectional composites in water, the fatigue strength at 1Hz increased from that of 30Hz because the plasticization of resin and local re-alignment of fibers were promoted more at 1Hz. Though both unidirectional HM50 and Kevlar 49 fibers always broke with a certain amount of splitting in water, the fiber splitting did not appear to have a significant effect on the fatigue strength.