Time-Temperature Dependent Response of Filament Wound Composites for Flywheel Rotors

Abstract

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 [1].

A Study of Time-dependent and Anisotropic Effects on the Deformation Response of Two Flywheel Designs

“…Consequently, the mere fact that most high performance flywheels will be manufactured using PMC materials does not automatically eliminate the need for accounting for irreversible behavior -thus viscoplastic constitutive representations. Furthermore, the present IM7/8552 system of interest, has exhibited significant irreversible deformations, in the range of 5-17%, at multiple temperatures, see reference [20], in relatively short periods of time (days) as compared to the desired 15 year flywheel mission cycle, thus further justifying the need to perform the current parametric study, to enhance our understanding of the ramifications of time dependent material behavior.…”

“…The majority of the present study will involve a highly anisotropic polymer matrix composite (PMC) material system known as IM7/8552 [20] and will be performed at one constant elevated temperature, i.e., 135 °C. This temperature is considered to be at the high end of the operational temperature range for this material, hence leading to significant time dependency.…”

“…Rapid characterization of the pertinent materials using the current material model is accomplished (given experimental test results see references [20,23]) using the constitutive parameter estimator known as (COMPARE, reference [24]) to estimate the material parameters. As alluded to earlier, two materials were used in the present analysis.…”

“…These experiments, which are described in detail in reference [20], include one constant-strain-rate experiment in the longitudinal direction, and in the transverse direction, three relaxation-with-unloading relaxation periods, three creep-with strain recovery, and a single cyclic (stress-controlled) test. All seven tests were conducted under compressive loading.…”

“…All seven tests were conducted under compressive loading. For convenience in the present study, identical behavior in tension and compression is assumed, even though some "small" differences in tension and compression were observed experimentally [20]. The PMC was characterized using two modeling approaches; a purely viscoelastic representation (using 5 viscoelastic mechanisms), and a generalized viscoelastoplastic model (using 1 viscoelastic combined with 2 viscoplastic mechanisms).…”