Flywheel energy storage offers an attractive alternative to the electrochemical battery systems currently used for space applications such as the International Space Station. Rotor designs utilizing the load carrying capacity of carbon fibers wound in the hoop direction are capable of high operational speeds and specific energies. However, the long-term durability of such rotors may be limited by the time-temperature dependent behavior of the epoxy matrix, which controls the uniaxial properties transverse to the fiber and the shear properties. This proposition was investigated for the prototypical rotor material, IM7/8552. Flat filament wound panels were made using the same process as for composite rotors. Coupon specimens, sectioned normal and parallel to the winding axis, were tested in compression and tension, at room temperature (RT), 95 °C and 135 °C for strain rates from 5 × 10-6/s to 5× 10-3/s. Creep and stress relaxation testing ran 72 hrs followed by a 72 hr recovery. Time, temperature and load sign dependent effects were significant transverse to the fiber. Under a fixed deformation of -0.5% strain for 72 hrs, compressive stresses relaxed 16.4% at 135 °C and 13% at 95 °C. Tensile stresses relaxed only 7% in 72 hrs at 135 °C for 0.5% strain. The postulate of a linear hereditary material response and the application of Boltzmann's principle of superposition to describe the behavior observed here are problematic if not intractable. Undoubtedly microstuctural analysis including the influence of residual stresses due to processing will be needed to resolve the observed paradoxes. Within the scope of these experiments, uniaxial compressive stress relaxation data may be used to bound the amount of relaxation with time of radial pre-load stresses in flywheel rotors. A more detailed appraisal of the design implications of these results is made in the analysis by Saleeb and co-workers .