Delamination characteristics of multi-directional carbon fiber/epoxy composites under high pressure

It was shown in a previous study that for unidirectional (0-deg) graphite/epoxy composites, the fracture toughness under hydrostatic pressure increased 38% as hydrostatic pressure increased from 0.1 MPa to 200 MPa. This work investigates the compressive delamination behavior of multi-directional graphite/epoxy laminated composites subjected to various hydrostatic pressures. Compressive delamination tests were performed under four hydrostatic pressure levels: 0.1, 100, 200, and 300 MPa. Eighty-eight-ply dog-bone type specimens with a single delamination at the center of the specimen were used. The stacking sequence applied was [0°/±45°/90°]_11s. The compliance and fracture load were determined from load-displacement curves as a function of hydrostatic pressure. The results show that the compliance decreases with increasing pressure while fracture load increases with increasing pressure. The compressive delamination toughness, G_c, was determined from the compliance method as a function of applied hydrostatic pressure. The results also show that G_c is significantly affected by hydrostatic pressure and increases from 2.11 kJ/m²to 3.04 kJ/m² (44% increase) as hydrostatic pressure increased from 0.1 MPa to 300 MPa. Visual examination of the fractured surface revealed that the increase of G_c is due to the suppression of micro-cracks with increasing pressure. It was also found from SEM examination of delaminated surface that the G_c increase is due to more epoxy adhering to the fibers and more plastic deformation of epoxy material as applied hydrostatic pressure increases.