Effects of Hydrostatic Pressure on In-Plane Shear Properties of Graphite/Epoxy Composites

In order to determine effects of the fiber orientation on the pressure de pendent in-plane shear properties of continuous graphite fiber reinforced composites, mandrel-wrapped tubes are tested in torsion in the specially designed high pressure torsion test apparatus that is capable of containing pressure up to 7 Kbar. The shear properties that are determined include the in-plane shear modulus, shear yield strength and fracture strengths (maximum stress), and the strain to fracture as a function of hydrostatic pressure and fiber orientation. These results are then compared with those of the epoxy resin matrix without fibers and (0°) composite samples also tested under hydrostatic pressure. In all cases, the shear properties increase with increasing pressure but in different rates depending on the fiber orientation. The increase in the strengths and the toughness is manifested in the change in the modes of fracture. Visual and SEM examination are made to study fracture surface morphology and possible frac ture mechanisms.