Effect of Saltwater Absorption/Desorption Cycling on Strength of Carbon Fibre/Epoxy Composites

The use of carbon/epoxy composites is ever increasing in the aerospace industry. The topic of saltwater absorption/desorption cycling effects is of interest as aircraft will experience significant moisture absorption/desorption and thermal cycles both during the short time scale of a sortie, as well as during the long time scale of service life. Coupled with this is exposure to elements, such as salt, from the environment. Carbon/epoxy materials are known to absorb moisture throughout their service life, and the extent of this absorption and its effects on the material may be estimated. It is generally assumed that the effects of fresh water absorption are reversible upon drying with respect to some basic characteristics such as glass transition temperature and associated mechanical properties. It is also acknowledged that repeated fresh water cycling does not have significant negative effects. However, the effect of a very common component of the environment, salt (NaCl), has had little attention in the research to-date. It has been seen in the few experiments conducted on carbon/epoxy, that saltwater/seawater conditioning degrades matrix-dominated properties to a greater degree than fresh water conditioning.The effects of repeated cycles of saltwater absorption/desorption on short-beam strength of carbon/epoxy composite materials are explored in this thesis. Experiments were conducted on Toray T700S/Cytec MTM57 plain weave carbon/epoxy composite material. The material was exposed to four cycles of immersion to saturation and drying and the gravimetric results obtained. The diffusion constants for absorption of fresh water and of salt water were obtained. Finally, the short-beam strength was tested for saltwater-cycled versus as-manufactured Toray T700S/Cytec MTM57 material. A relative reduction in strength of approximately 10% was observed.These results indicate that cycles of saltwater have irreversible effects. And further studies into the effects of the cyclic saltwater environment may be useful for the development of design allowables of composite materials employed in aircraft