Tensile behavior of filament wound composites in vacuum

The tensile behavior of candidate filament wound composite materials for flywheel energy storage applications was evaluated. The materials were conditioned in a vacuum of 10-30 mtorr for at least 85 days prior to testing, and no appreciable weight losses were recorded during that period. Glass and carbon composites conditioned and fatigue tested in vacuum lost 10% to 12% of remaining strength per decade of logarithmic lifetime over the 10² to 10⁵ range of cyclic lifetime. The ZenTrom glass/epoxy and the S2-glass/epoxy composites were noted to have more overall fatigue sensitivity than either E-glass/epoxy or T700S carbon/epoxy due to a steeper slope of the S-N curve at very low cyclic lifetimes. Typical fatigue failures were characterized as a longitudinal splintering mode growing inward from the lateral edges of the specimen. No significant differences in the ultimate tensile strength, Young's modulus, and ultimate strain were noted when vacuum-conditioned specimen were compared to as-received specimens.