Fatigue Crack Growth of Filled Elastomers
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In the past, the use of a fracture mechanics approach to describe crack growth
in elastomers has been shown to work well for specimens of simple test geometry,
simply loaded. This has been the case because elastic strain energy density (e.s.e.d.)
functions could reliably be used to calculate both the magnitude of elastic stored
energy available to drive a crack and the magnitude of the rate of release of such
energy as the crack grows.
The aim of this thesis was to investigate the applicability of such a
methodology to situations of more complex loading. To this end two novel test-piece
geometries were developed. The first consisted of a pure shear geometry with the
sample having been pre-strained in the longitudinal direction to varying extents, hence
introducing a type of bi-axial deformation. The second consisted of a pure shear
geometry test-piece inclined at 30° to the horizontal and loaded in the vertical
direction, hence inducing simultaneously pure shear and simple shear loading. Both
types of test-piece were used to study the validity of the particular e.s.e.d. functions,
the energetics and mechanics of crack growth and crack growth geometries on a
macro and micro scale.
The constants in particular e.s.e.d. functions were determined by uniaxially
deforming in pure shear each of the carbon black reinforced materials used in this
study. The resulting functions became progressively less good at predicting the elastic
strain energy in the novel geometry test-pieces as the deformation modes became
more complex. Anisotropy induced by deforming specimens in one direction was not
easily removed even by an imposed large deformation in another direction.
Nevertheless, the functions were successfully used to predict crack growth directions
in the 30° inclined test-piece. However in the pre-strain pure shear test-pieces the
functions significantly underestimated the elastic strain energy. Hence the real
energies had to be determined from the forces and extensions measured during cyclic
crack growth tests. In these tests crack growth rates for a given tearing energy (elastic
energy release rate) increased as the magnitude of the pre-strain increased. This
significant weakening was associated with the development of a strain induced
molecular and carbon black anisotropy.
Authors
RATSIMBA, Christian H. H.Collections
- Theses [4235]