Effect of hydrostatic pressure on the mechanical behavior of seawater-absorbed carbon/epoxy composite

In order to investigate compressive behavior of carbon/epoxy composite in the deep-sea environment, compressive tests were performed on seawater-absorbed unidirectional ([0°]₆₄) carbon/epoxy thick composite at various hydrostatic pressures up to 270 MPa. The specimens contained 64% fiber by volume. The tests were performed in a high pressure apparatus which was capable of containing pressures of up to 700 MPa. The pressures applied were 0.1 MPa (atmospheric pressure), 100, 200, and 270 MPa. The results showed that the seawater absorption of carbon/epoxy composite was approximately Fickian and the seawater content at saturation was about 1.2% of the specimen weight. The compressive properties of seawater-absorbed carbon/epoxy composite underwent significant changes with increasing pressure. When the hydrostatic pressure increased from 0.1 to 200 MPa, the compressive elastic modulus increased about 10%. Then, the modulus increased 2.3% more as the pressure increased to 270 MPa. Fracture strength and fracture strain increased with pressure in a linear fashion. Fracture strength increased about 28% and fracture strain increased about 8.5% as the hydrostatic pressure increased from 0.1 to 270 MPa. End-crushing and delamination cracks were primary fracture modes at all pressure levels. However, the number of delamination cracks was reduced with increasing hydrostatic pressure.