Abstract
An investigation has been carried out concerning the influence of water absorption on the fatigue properties under completely reversed stress of 0° unidirectional and ±45° cross-ply composites of a Technola (HM50, Teijin)/epoxy and a Kevlar 49 (Dupont)/epoxy. The influence of water on the fracture mechanism was observed by a SEM.The ratio of fatigue strength under reversed tension and compression at N=106 in water to the one in air of the ±45° cross-ply composite was 0.61-0.66, and a remarkable decrease in fatigue strength was observed in water. In addition to the weakening of a fiber/matrix interface, local buckling of reinforcing fibers under compression due to the softening of water absorbed resin decreased the fatigue strength in water. The ratio of the fatigue strength in water to the one in air of the 0° unidirectional composite was 0.86-0.94, and the reduction of fatigue strength was moderate compared to that of the cross-ply one. This was due to the fact that the softening of resin not only increased the fatigue strength under tension but also decreased that under compression. Under reversed tension and compression, fiber splitting was decreased in water than in air in both types of composite. This was quite opposite to the case of pulsating tension, where the fiber splitting caused by water absorption reduced the fatigue strength in water.
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