Fatigue Crack Growth of Filled Elastomers

Abstract

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.