Investigation of Failure Modes of Fiber Reinforced Polymer Composite Flywheel Rotors for Energy Storage Systems
Creators: Skinner, Miles A
Publisher: name: University of Alberta Library
Year: 2022

<p>High-velocity and long-lifetime operating conditions of modern high-speed energy storage flywheel rotors may create the necessary conditions for failure modes not included in current quasi-static failure analyses. The central hypothesis for this thesis research is that (i) viscoelastic effects and shear stress effects can cause creep rupture, matrix cracking, or hub rim interfacial failure in composite flywheel rotors, and that (ii) these failure modes can be prevented employing an appropriate modeling approach in the flywheel energy storage system (FESS) design process. In this thesis, a computational algorithm based on an accepted analytical model was developed. This model progresses in two phases. First, the viscoelastic behavior of fiber reinforced polymer composite (FRPC) flywheel rotors was investigated by simulating a 10-year operational lifetime. The simulations indicate that viscoelastic effects are likely to reduce peak stresses in the FRPC material and the hub-rim interface while also increasing stress in the metallic hub. Second, flywheel rotors also experience a large number of acceleration/deceleration cycles, which raises concerns regarding the effects of shear stresses on flywheel rotor reliability.  The computational models were used to describe the transient behavior of radial, circumferential, and shear stresses in FRPC flywheel rotors during constant power demands. This thesis discusses failure predictions using the maximum stress and Tsai-Wu failure criteria. The Tsai-Wu criterion predicted failure to occur at higher loadings compared to a maximum stress threshold. A strength ratio determined from the Tsai-Wu criterion indicates a changing peak stress location from the inner radius at the start of rotor acceleration to approximately the center of the rotor thickness at top speed. The results from this study indicate strong variability in the loading conditions, which may promote damage, crack initiation and crack propagation, and fatigue effects, posing possible risks to the long-term structural health of composite flywheel rotors.<br>
Predicting the behavior of the flywheel rotor over the lifetime of the system requires a thorough understanding of the evolution of the composite material properties. To assess the viscoelastic behavior of the FRPC, an effective experimental test platform and methodology was developed to conduct elevated temperature tensile creep testing of FRPC tube specimens. Using this methodology, the creep compliance of a glass fiber reinforced polymer composite (GFRP) was measured at various elevated temperatures. Then a time temperature superposition approach was applied to shift the compliance curves along the time axis to create a master curve. The resulting transverse master curve can accurately predict the material compliance over an approximate 67-year period. Additionally, the compliance values compare well with published values of other similar materials.<br>
The findings from this thesis research provide evidence to conclude that viscoelasticity can significantly affect the reliability of flywheel rotors over an average 10-year operational lifetime. Shear stress, however, was found to be minimally impactful on the evolution of internal stresses, and, in isolation, is unlikely to lead to rotor failure. Finally, viscoelastic creep testing of a common GFRP composite proved successful and a compliance master curve was developed.</p>