| An investigation of the creep behavior of polymer composites was performed in developing a closed-form structural model of advanced polymer matrix composite (PMC) flywheels. The model is of the linear-viscoelastic type, and is applicable to the design and analysis of multiple concentric-ring PMC flywheel rotors in a state of plane-stress. This model incorporates techniques for approximating the quasi-static response to general time-varying loads including rotation, temperature change, and interference-fits between adjacent rings. The model accounts for the effect of temperature on the material response using the time-temperature superposition principle. The quasi-elastic technique was used to discretize the linear viscoelastic constitutive law, allowing the derivation of approximate solutions for the stress and strain field variables. Experimental work performed in support of this model includes thermo-viscoelastic characterization of a unidirectional glass/epoxy composite. Experimental measurement of pressure loss and strain redistribution in interference-fitted filament-wound glass and carbon fiber PMC ring pairs was performed using moiré interferometry and electrical resistance strain gages. Good agreement between the data and the plane-stress model at locations away from the ring interfaces was obtained. With the purpose of making creep measurements through the radial thickness of high-speed rotating flywheels, a new optical displacement measurement method was developed. Notable improvements over a known related method include greater displacement sensitivity, the ability to measure rigid body vibrations and separate the associated vibration-induced displacement from the strain-induced displacement, and the ability to compensate for sensor drift during flywheel operation. Displacement measurements made on an aluminum rotor operating at a maximum speed of 16 krpm (255 m/s at the point of measurement) were made with ±1-micrometer accuracy. At this speed, hoop strains were found to be within 40 to 125 × 10−6 of theoretical predictions. Relative to the theoretical hoop strains, the measured hoop strains differed by 5–6% at 16 krpm. |
