LIFETIME ANALYSIS OF A COMPOSITE FLYWHEEL ENERGY STORAGE SYSTEM.
Abstract
This thesis is concentrated on the long-term fracture of thick unidirectional glass and
carbon fibre composites subjected to transverse stress. The objective was to develop a
methodology for predicting the long term lifetime of a composite rotor used as part of
a flywheel based energy storage system. The flywheel design is based on
accommodating high hoop stresses induced during the high speed rotation. However,
the different Poisson's ratios of the constituent materials in the rotor result in a
complex stress distribution with significant stresses introduced in a direction
transverse to the fibres. The possibility has been raised that the lifetime of the rotor
will be limited by crack growth in this transverse direction, originating from defects
(pores, cracks etc) that can be introduced into the rotor during its manufacture.
The approach explored in this work has been to adopt a fracture mechanics based
methodology whereby the rate of crack growth in a thick composite is measured as a
function of an applied stress intensity. The basic fracture parameters for the material
were measured such that the time taken for a crack to grow to a size sufficient to cause
failure under an operating stress could be calculated. The materials were also
examined to characterise the nature, size and extent of inherent defects. The stress
distribution in the rotor under operating conditions was modelled using finite element
analysis. The combination of information on inherent defects, stress directions and
crack growth rates enable predictions to be made concerning the likely lifetime of the
composites. Proof stress diagrams were also constructed in order to demonstrate an
approach to product quality assurance testing.
The end point of the work was to identify critical manufacturing defect sizes that
could be tolerated under the specified operating conditions. The methodology
developed for lifetime predictions was critically assessed and considered to be
generally acceptable. The work did however raise some concerns regarding the
applicability of a conventional fracture mechanics approach applied to heterogeneous
composite systems where the size of the cracks are very small. It is recommended that
future work should concentrate on studying this area with an emphasis on crack
nucleation studies rather than on further crack propagation work.
Authors
Neumann, Robert J.Collections
- Theses [4122]