EFFECTS OF HYDROSTATIC-PRESSURE ON THE TORSIONAL SHEAR BEHAVIOR OF GRAPHITE EPOXY COMPOSITES

The torsional shear stress-strain behavior of a graphite/epoxy composite material and also the epoxy matrix alone has been determined at various hydrostatic 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-degrees) uniaxial, and 60% by volume. The normally linear elastic shear stress-strain curve of the epoxy matrix material at atmospheric pressure shifted upwards with pressure and showed nonlinearity at 2 kbar and higher. The shear modulus (G) and the fracture strength (tau(f)) and strain (gamma(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 gamma(f) of 8% at atmospheric pressure to a nonlinear curve, exhibiting yielding, work-hardening, and extensive drawing with gamma(f) of 57% at 6 kbar. The G increased bilinearly with pressure from 10.7 x 10(8) at atmoshperic pressure to 12 x 10(8) N/m2 at 6 kbar with a break also occurring at 2 kbar. The work hardening parameter, kappa, determined at 1% offset strain increased significantly with pressure. The tau(f) and gamma(f) also increased linearly with an abrupt jump occurring between 4 and 5 kbar as the mode of fracture changed from delamination to shearing.