Transverse corrosion fatigue tests were performed on a carbon-fiber epoxy-matrix composite in air, simulated seawater, and distilled water environments. Crack growth rates were determined for ambient and elevated pressure (up to 4000 psi) fluid environments and were compared to baseline values for the composite tested in air. In addition, the effects of water chemistry and loading frequency on fatigue crack propagation were investigated. Finally, composite specimens saturated with simulated seawater were fatigued in order to determine the effect that prior moisture content has on crack growth rates of specimens tested in either air or immersed environments.
In the coming years, exploration for offshore oil deposits will employ the use of tension-leg platforms (TLP) to drill at ocean depths of 10,000 feet where the hydrostatic pressure reaches 4400 psi. The high hydrostatic pressure, corrosive seawater, and cyclic loading induced by wave motion or ocean currents present a challenging environment for TLP components. A TLP is weight critical, and the potential for mass and cost savings afforded through the use of carbon or glass-reinforced epoxy composites make these composites ideal candidate materials for TLP components such as platform facilities, risers, and tethers (Kim, Hahn, and Williams, 1988). However, the effect of long term high pressure seawater exposure on these materials must be understood if they are to be used in structures with design-lives of 20 or more years. Previous research has identified both beneficial and deleterious effects of seawater and hydrostatic pressure on composite materials. These effects are dependent not only on the choice of the fiber and matrix composite system but also upon the composite laminate layup. In addition, the composite response to seawater and pressure exposure is highly dependent upon the mode of deformation imposed during testing.