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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. [ABSTRACT FROM PUBLISHER]

The torsional shear stress-strain behavior of a graphite/epoxy composite material and also the epoxy matrix material alone has been determined at various hydro static pressures up to 6 kbar, using a newly constructed high pressure torsion apparatus. The composite samples were machined into thin walled hollow cylinders from press molded and oven cured, laminated panels. Graphite fibers of the samples were continuous, (0°) uniaxial, and 60% by volume.The normally linear elastic shear stress-strain curve of the epoxy matrix material at at mospheric pressure shifted upwards with pressure and showed nonlinearity at 2 kbar and higher. The shear modulus (G ) and the fracture strength (τf) and strain (γ f) all increased markedly and bilinearly with a break occurring at 2 kbar. The shear stress-strain curves of the composite material showed dramatic changes from an almost linear curve with γf of 8% at atmospheric pressure to a nonlinear curve, exhibiting yielding, work-hardening, and extensive drawing with γf of 57 % at 6 kbar. The G increased bilinearly with pressure from 10.7 x 10 8 at atmospheric pressure to 12 x 108 N/m2 at 6 kbar with a break also occurring at 2 kbar. The work hardening parameter, x, determined at 1% offset strain in creased significantly with pressure. The τf and γf also increased linearly with an abrupt jump occurring between 4 and 5 kbar as the mode of fracture changed from delamination to shearing. [ABSTRACT FROM PUBLISHER]