An investigation has been carried out concerning the effect of water absorption on the tensile and fatigue properties of a Technora (HM50, Teijin)/epoxy and a Kevlar49 (Dupont)/epoxy composites. The effect of water on the fracture mechanism was observed using AE measurements and SEM microscopy.
The weight gain for the HM50 specimens immersed in distilled water at 70°C levels off at about 2% after two months. For the Kevlar49 this is about 3.5%. Moisture does not appear to have any influence on the tensile strength. For the HM50 the elastic moduli of the dry and wet specimens are the same at the start. After a certain point the dry specimen stiffens, whereas the wet one first yields, and then stiffens. The Kevlar49 specimens do not show any yielding behavior unlike the HM50. For both composites the specimens in water exhibit a longer fatigue life than in air, though the Kevlar49 has a slightly bigger increase in life than the HM50. In water the AE signal total is much lower than in air. The HM50 composite has a lot of matrix attached to the fiber after failure, while the Kevlar49 shows a very clean pullout of the fiber. The fracture of the fiber itself is also quite different in the two types of composites, and the HM50 fiber shows a more ductile behavior than the Kevlar49 fiber.