AbstractThis PhD-Thesis describes the fatigue life and dynamic crack propagation behaviour of elastomers, especially their dependency on test parameters. It, furthermore, presents concepts of data evaluation in order to develop criteria for more precise predictions of service life of elastomeric parts. Fatigue life was investigated using EPDM and SBR dumbbell specimens tested under load control at 1Hz until failure. Tests were made in order to create a common Wohler-(S-N)-curve while increasing stress amplitude. Additionally, the influence on fatigue properties of increasing minimum stress at constant stress amplitude was investigated. All testing were carried out in the Deutsches lnstitut fur Kautschuktechnologie e.V. (DIK). The results of these tests confirmed the well-known amplitude dependence of fatigue life in filled rubbers. An additional significant influence on fatigue life was found to be the level of minimum stress and consequently mean stress applied to these materials, which in turn to most other materials increase the fatigue life of rubbers. This additional influence on fatigue life is not dependent on strain crystallisation in Ethylene-Propylene-Diane Polymer (EPDM) or Styrene-Butadiene Polymer (SBR) as it is for Natural Rubber (NR). It could be proved that this effect rather specific to filled systems. The investigation shows that the fatigue behaviour of carbon black filled non strain crystallising rubbers can not be described with a maximum stress or a maximum strain criterion. It shows that an energy criterion should be considered.
Another important effect is highlighted in this research. Following the dependence on load cycles it can be seen that most mechanical properties, but particularly the stiffness of specimens change throughout the whole test, and never reach equilibrium. This behaviour is particularly observed in elastomers containing reinforcing fillers. It was found that all carbon black filled EPDM test specimen failed when they reached approximately 76% of their initial stiffness or complex modulus, despite the high variation of test conditions. The carbon black filled SBR test specimen failed when they reached 71% of their initial stiffness. These results indicate that there is a characteristic material parameter of loss in complex modulus that is reached at failure.
The results of the dynamic crack propagation tests on these unfilled .and carbon black filled non strain crystallising elastomers show a similar behaviour like the fatigue to failure tests with respect to their minimum load dependence. Increasing minimum loads at a constant strain (displacement) amplitude under pulsed as well as sinusoidal excitation decrease the crack growth rate of the carbon black filled rubber material and, thus can lead to a higher service life of parts produced from these materials. The unfilled materials showed the expected opposite effect on an increase of the minimum load at constant strain amplitude which confirms the findings of the fatigue to failure experiments. It should be noted that maximum stress and maximum strain criteria are no tools to uniquely describe the crack propagation properties of filled rubbers.
As the normal failure observed during the fatigue to failure experiments was a sudden catastrophic crack after reaching a certain level of remaining stiffness or modulus it is assumed that these tests characterise the initiation process. It is, thus, an important finding of this research that crack initiation and crack growth of rubber material show a similar dependence on test parameters. Furthermore, initiation and growth seem to be energy controlled processes in rubber materials.
|Date of Award
|Stephen Jerrams (Supervisor) & Thomas Alshuth (Supervisor)