Tensile Behavior of Filament-Wound Composites in Medium Vacuum

In order to design the most efficient flywheel energy systems using fiber-reinforced polymer composite rotors, design allowables for the rotor material in a realistic environment and under realistic stresses must first be established. As a first step, this investigation characterizes the 0-deg tensile behavior of unidirectional T700 carbon and E-glass reinforced epoxy composites both as received and after conditioning at a pressure of 15 mtorr and a temperature of 20 to 25C. Constant-amplitude tension-tension cyclic fatigue tests were carried out on half the specimens in a similar medium vacuum environment. Weights of specimens were tracked with immeasurable changes noted due to vacuum conditioning for up to 300 days' duration. E-glass/epoxy specimens had a higher quasi-static strength following conditioning, but this difference was not evident on an absolute stress basis in the fatigue tests. At the baseline comparison point of 10⁵ cycles to failure, there was little difference in fatigue strength of as-received and conditioned E-glass/epoxy specimens. In the case of T700/epoxy, little difference between conditionings was noted in quasi-static tests, but as-received specimens had slightly higher fatigue strength at a lifetime of 105 cycles. Microscopic inspection of fracture surfaces following quasi-static and fatigue failures revealed no significant differences due to the type of loading or conditioning.