A study into initiation, evolution and propagation of cracks in elastomeric vulcanisates under dynamic loading using mechanical and analytical methods

Crack initiation and growth in rubber vulcanisates is a major problem when rubber is repeatedly stressed in service. Often, micro-cracks exist in rubber because of mold damage or dust particles which mix with rubber compound during various stages of processing. These imperfections act as crack initiation sites and when cracks grow under repeated stressing, rubber fails, causing poor performance and short service life of rubber article. In a simple term, flex life is defined to be the number of cycles to failure in a test. There are numerous factors which affect crack initiation and growth in rubber vulcanisates. Crosslink density variation and molecular structure of raw rubber can influence the flex life of rubber vulcanisates. This project firstly investigated three grades of polybutadiene rubber (BR) i.e., highly linear, linear and branched chain grades. The flex life or the number of cycles to failure was measured for each rubber vulcanisate. The results indicated that the rubber vulcanisate with highly linear chains had the longest flex life, followed by the one with linear chain and then the one with long branched chains. In above work the viscosities of different grades of Polybutadiene rubber grades were widely different, so another study in the second part of this thesis had been carried out in order to measure the flex life of rubber compounds based on similar viscosities. In a separate experiment, the crack initiation and propagation were measured. The results indicated that crack initiation covered 75% of the flex life of these rubber compounds.

To correlate the strain energy involved in flex life of the rubber vulcanisate with linear chains to the rate of crack growth in the rubber, further tests were performed. The results indicated a linear relation exist between the rate of crack growth to the strain energy release rate. This correlation was subsequently used to re-define an actual flex life for the rubber vulcanizate.In the next part of thesis , Kaolin filled polybutadiene rubber compounds were manufactured, tested for various properties and lastly, Kaolin and silica filled Natural rubber and Polybutadiene rubber compounds had been prepared in order of increasing loading of fillers and tested for curing characteristics, mechanical properties, crosslinking linking, crack propagation, dynamic mechanical properties and filler dispersion by SEM and these properties correlated in order to study the effect of fillers on the properties of these rubber compounds.