Swelling effects in dynamic equi-biaxial testing of EPDM elastomers by the bubble inflation method
Document Type Article
The thesis describes the effect of oil swelling on the fatigue life of EPDM under conditions of multi-axial fatigue using bubble inflation. The motivation for the research is outlined, along with a review of previous research conducted on the topic. Initial test results are presented and the evolution of the procedure for the final set of tests is described. One principal requirement identified when carrying out the fatigue tests was the need for constant stress control. Following development of the final test procedure, specimens were subjected to varying degrees of swelling in reference oils and cycled at constant engineering stress amplitudes at a frequency of 1Hz. The degree of rubber swell was controlled by using reference oils for the swelling tests. Sample swelling during cycling was minimised by using silicone based oil as the inflation fluid. Three specimen sets were used in the fatigue tests, one dry set and two other sets with different degrees of oil swelling. S-N (alternating stress versus cycles to failure) curves were generated for the dry and swollen specimens with the fatigue life of the EPDM reduced in proportion to the amount of swelling. The changes in complex elastic modulus E* and dynamic stored energy were analysed and a relationship between the stored energy and the cycles to failure for the dry EPDM was observed. However, the stored energy at failure is reduced as the degree of swelling increases. Moreover, it was found that if the residual, or specific moduli for the swollen test-pieces were considered against a datum of the initial conditioned modulus of the dry rubber at a similar stress amplitude, a limiting range of E*residual, was established for both the dry and swollen specimens. E* for different levels of swelling was also calculated using models generated from the experimental data. Additionally, the differences in the fracture surfaces of the specimens due to swelling were analysed using scanning electron microscopy showing the morphology of the failure surfaces are greatly influenced by the degree of oil swelling.