The effects of particulate filters on the strain energy function and crack growth in rubbers
The thesis presents a wide range of studies on carbon black and silica particulate reinforced rubbers. These include stress-strain, strain energy function, static and cyclic stress relaxation (stress softening), trouser test piece tearing and cyclic crack growth studies. The novel features of the work include the development of a simple strain energy function which is shown to represent the stress-strain behaviour of carbon black and silica filled rubbers up to strains of 100%. The numerical values of the constants in this function are shown to vary in a meaningful and systematic manner with the fraction of reinforcing filler and with the crosslink density. The cyclic stress relaxation studies are the first of their kind and demonstrate a significantly increased relaxation rate resulting from cycling in filled rubbers. The trouser tearing studies give some insight as to the materials and experimental variables that determine the type of tear growth and regime of tearing. The process of stress whitening around the tear tip during steady tearing in silica filled compounds provide the first opportunity to quantitatively relate the tearing energy to the hysteresis energy loss in a known volume of rubber at the tear tip. The cyclic crack growth studies show for the first time a systematic decrease in crack growth per cycle (dc/dn) at a given tearing energy as the carbon black filler content is systematically increased and as the crosslink density is decreased. A novel feature of the work is the demonstration of the effect of pre-strain in one direction on the cyclic growth rate of a crack in this direction when cyclically strained in a direction at right angles. The very large increase in dc/dn with increasing pre-strain is discussed in terms of pre-orientation of the rubber/carbon black structure.