The tensile fatigue characteristics of graphite/glass/epoxy hybrid composites were investigated before and after exposure to various high humidity environments. Accelerated aging treatments of 98 percent RH and 120°F were applied both continuously for 500 and 1000 h and coupled with a 350°F thermal shock after −67°F low temperature exposure. This latter thermohumidity cycle was applied for 500 h total exposure time. Cyclic loading tests showed that degradation due to the moisture and thermohumidity conditioning treatments was in‐most cases small. Some bereficial behavior of the humidity exposure was also noted. The hybrid fatigue characteristics both before and after exposure were a blend of the nearly flat advanced fiber composite S‐N behavior and the highly curved glass fiber composite S‐N behavior. This was attributed to a mixture of failure modes for the hybrid system.

The static mechanical properties of S-glass/high-strength graphite/ epoxy hybrid composites were investigated before and after exposure to high humidity for 1000 h. Graphite-to-glass ratios of 0, ∞, 1, and 2 were studied. Both unidirectional (0 deg) and quasi-isotropic fiber orientations were studied. In general, the stress cycling caused a decline in the static tensile strength, an increase in the Poisson's ratio, and no change whatever in the elastic modulus of the base and hybrid composites. The addition of moisture did not result in a basic alteration in the behavior of the hybrid or base composites after stress cycling.

Natural weathering of selected graphite/epoxy composite materials has been studied primarily by measuring specific mechanical properties. Commercially available composite prepregs were fabricated into panels and exposed at Warminster, Pa., and Canal Zone, Panama. Principal mechanical property determinations used to assess degradation were tension, compression, and short-beam shear tests. The T400/2544 panels were severely degraded after 18 months of exposure in Panama. Pronounced warping, loose fibers, and loss of resin were very much evident with this material. Shear data taken from Panama exposures showed that the use of a coating system on the panels was extremely beneficial. To data, the Modulite 5208 composite system has shown the least variability in measured mechanical properties with increasing exposure time.

A comparison is made between ambient and accelerated aged panels and panels tested immediately after curing. Strength reductions as a function of time of aging and temperature for several orientations of laminates were determined. The effect of adding interlayers of glass scrim cloth between graphite plies was investigated. The high tensile strength graphite-epoxy material studied was Modmor® II graphite/Narmco® 5206 epoxy.

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